Photo-Reaction of Copolymers with Pendent Benzophenone

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Photo-Reaction of Copolymers with Pendent Benzophenone Photo-Reaction of Copolymers with Pendent Benzophenone

Scott Kenneth Christensen University of Massachusetts Amherst

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Recommended Citation Recommended Citation Christensen, Scott Kenneth, "Photo-Reaction of Copolymers with Pendent Benzophenone" (2013). Open Access Dissertations. 730. https://doi.org/10.7275/69rm-4g21 https://scholarworks.umass.edu/open_access_dissertations/730

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PHOTO-REACTION OF COPOLYMERS WITH PENDENT BENZOPHENONE

A Dissertation Presented

by

SCOTT KENNETH CHRISTENSEN

Submitted to the Graduate School of the

University of Massachusetts Amherst in partial fulfillment

of the requirements for the degree of

DOCTOR OF PHILOSOPHY

May 2013

Polymer Science and Engineering

© Copyright by Scott Kenneth Christensen 2013

All Rights Reserved


A Dissertation Presented

by

SCOTT KENNETH CHRISTENSEN

Approved as to style and content by:

___________________________________________

Ryan C. Hayward, Chair

___________________________________________

Kenneth R. Carter, Member

___________________________________________

Dhandapani Venkataraman, Member

____________________________________

David Hoagland, Department Head

Polymer Science and Engineering

To my best friend, my wife Jen

v

ACKNOWLEDGMENTS

I feel that the list of individuals responsible for the completion of this work could

span its entirety, yet I will do my best to be concise. First and foremost, my sincerest

gratitude goes to Professor Ryan Hayward, who I have been lucky enough to work with

these past six years. Ryan I thank you for being patient and encouraging throughout my

time, yet assertive when needed. It has been a privilege to grow as a scientist with your

guidance, and if I had to repeat this process I would once more make the same decision. I

must also extend my thanks to Professors Ken Carter and Dhandapani Venkataraman.

Each of you has played a vital role in the completion of my PhD, and I truly appreciate

the time and effort you have expended at my request. Ken you are one of the reasons I

came to UMass PSE, not only due to your scientific achievements but also because when

I met you during recruitment I noticed the leg lamp in your office (A Christmas Story is a

personal favorite of mine…). DV you have personally affected me greatly as my

collaboration with you and your group have served to give me confidence in my scientific

knowledge and perspective on topics I would otherwise be ignorant to.

I have had the joy of working with many qualified people during my time here,

and it is through involvement with you all that I find myself completing this work. I find

myself reminiscing about simpler times, when the Hayward group consisted of only 7

people and group meeting would be over in an hour every week. Jintao, Ian, Felicia, Le,

and Jungwook each witnessed the growing pains of my earliest attempts and I thank you

for your patience and guidance in getting through them without too many scars. The

group has expanded greatly to this point to include so many individuals who contribute to

my everyday life, from discussing the frustrations of research (looking at you Dayong,

vi

Maria, Cheol, and Nakul), to helping me run experiments (hey Rachel), to enriching my

knowledge by discussing their work (Kyle and Daniel). It has been a learning experience

just understanding how I fit into such a large group of qualified scientists, and I truly

appreciate it. I have also had many opportunities for collaboration, and I would be remiss

not to mention the fantastic people involved. The many members of the Center for

Chemical Innovation allowed me to explore new fields which were both a challenge and

a treat, and solid scientific connections sprang from the open and willing environment

created by them. I must give special thanks to Dipankar Basak, from whom I learned the

most effective ways to work together toward a common goal, and two amazing

undergraduate students, each of whom I worked with for a summer, Kate Arpino and

Anna Melker. Thanks are also due to the PSE faculty who taught me in and out of the

classroom, and to the technical staff members who keep our instruments running and my

blood pressure a little lower: Lou Raboin, Sekar Thirunavukkarasu, Weiguo Hu, Steve

Eyles, and Wim de Jeu.

Focusing on the scientific side of my PhD is only possible because of the superb

administrative staff we have here in our department. Sophia Hsu has made my life

especially easy, as I have never had to deal directly with a Procard audit and not once

missed a pay period, and everything she does is with a friendly and willing smile! The

front office staff has worked with me through arriving at PSE, my time as PSE Club

president, and the completion of my PhD, and Lisa Groth, Maria Farrington, Ann

Brainerd, and Vivien Venskowski are deserving of their prominent placement welcoming

everyone in the door of Conte. Thank you all!

vii

I have been lucky to have an unbelievable support system in place ever since I

began here in Fall 2007, and I apologize in advance for forgetting you (clever, right?).

I’m going to start by thanking my incoming classmates from 2007 – it’s amazing how

you can forge such bonds with people after only months crammed together in a

classroom. I think I could meet any of you in a decade and be right at home. I want to

personally thank Bugra, Eric, Sam, Nick, Ruosty, Felicia, Katrina, Yuri, and my many

other roommates for being there when I needed you to be. Of course I would be ashamed

to forget the Valley Radicals, Spare Parts, and all the folks from Amherst Sandlot soccer

who helped me release tension these past few years.

Finally I must thank my family, who I owe for helping me to become the person I

am today. Mom and Dad you have pushed all the right buttons along the way and given

me opportunities for which I am forever grateful. My brothers Matt and Robb have been

my best friends for longer than I realized, and I thank you for providing me with such

wonderful sisters Ortal and Kate. I am also extremely lucky to have amazing in-laws, as

Rick, Mary Anne, RJ, and Brian have been welcoming from the start and never once

made me feel like I wasn’t family. Last but certainly not least I owe my wife Jen my

sincere gratitude. Over the past decade she has been loving, patient, supportive, and

understanding of my mania and without her I could not reach this point.

viii

ABSTRACT


MAY 2013

SCOTT KENNETH CHRISTENSEN, B.S.E., CASE WESTERN RESERVE UNIVERSITY

M.S., UNIVERSITY OF MASSACHUSETTS AMHERST

Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST

Directed by: Professor Ryan C. Hayward

This dissertation aims to both deepen and broaden our understanding of

copolymers with pendent benzophenone (BP) in relation to both established applications

and novel directions in materials science. Photo-reaction of these BP copolymers is

explored in attempts to achieve three distinct goals: (1) robust and efficiently photo-

crosslinkable solid polymer films, (2) photo-reacted polymer blends with disordered

bicontinuous nanostructures, and (3) photo-patterned hydrogel materials with

environmental UV stability. We begin by investigating the fundamental gelation behavior

of solid polymer films, finding BP copolymers to be particularly effective crosslinkable

materials. Gelation efficiency can be tuned according to comonomer chemistry, as BP

hydrogen abstraction on the main polymer chain increases chain scission, reducing

crosslinking efficiency. This knowledge is then applied in Chapter 3, wherein we discuss

two potential methods for preparing nanostructured polymer blends from these

copolymers, namely spinodal decomposition of a photo-crosslinked polymer blend and

solution-state photografting to create interfacially active species. While each technique

shows promise, the ultimate goal of a disordered bicontinuous morphology will require

further tuning of materials systems and protocols. Finally, chemical deactivation of BP

ix

photo-crosslinker in copolymers for use as photo-patternable and environmentally stable

hydrogel materials is investigated. Reduction of BP by sodium borohydride proves a

feasible route toward deactivating residual photo-crosslinker in patterned hydrogel films.

These results confirm the utility of copolymers with pendent benzophenone photo-

crosslinkers as useful tools for complex material systems.

x

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ...................................................................................................v

ABSTRACT ..................................................................................................................... viii

LIST OF TABLES ........................................................................................................... xiii

LIST OF SCHEMES........................................................................................................ xiv

LIST OF FIGURES ...........................................................................................................xv

CHAPTER

1. INTRODUCTION‡ ..........................................................................................................1

1.1 Benzophenone photo-reaction and applications ..........................................2

1.2 References ....................................................................................................8

2. GELATION OF COPOLYMERS WITH PENDENT BENZOPHENONE

PHOTO-CROSSLINKERS† ..................................................................................16

2.1 Abstract ......................................................................................................16

2.2 Motivation ..................................................................................................17

2.3 Experimental ..............................................................................................19

2.4 Results and Discussion ..............................................................................23

2.5 Conclusions ................................................................................................42

2.6 References ..................................................................................................43

3. ROUTES TOWARD NANOSTRUCTURED POLYMER BLENDS BY

BENZOPHENONE PHOTO-REACTION ............................................................49

3.1 Abstract ......................................................................................................49

3.2 Motivation ..................................................................................................50

xi

3.3 Arresting phase separation of LCST blends by BP photo-

crosslinking ................................................................................................51

3.3.1 Experimental ..................................................................................56

3.3.2 Results and Discussion ..................................................................59

3.3.3 Conclusion .....................................................................................63

3.3.4 Critical Blend Future Directions ....................................................63

3.4 Solution-state asymmetric photo-grafting of benzophenone

copolymers .................................................................................................65

3.4.1 Experimental ..................................................................................69

3.4.2 Results and Discussion ..................................................................72

3.4.3 Conclusion .....................................................................................81

3.4.4 Solution-state Photo-Grafting Future Directions ...........................82

3.5 Conclusions ................................................................................................83

3.6 References ..................................................................................................84

4. CHEMICAL DEACTIVATION OF BENZOPHENONE IN PHOTO-

PATTERNED HYDROGELS ...............................................................................94

4.1 Abstract ......................................................................................................94

4.2 Motivation ..................................................................................................95

4.3 Experimental ..............................................................................................97

4.4 Results and discussion .............................................................................101

4.5 Conclusion ...............................................................................................110

4.6 Future directions ......................................................................................110

4.7 References ................................................................................................113

xii

5. CONCLUSIONS..........................................................................................................116

BIBLIOGRAPHY ............................................................................................................119

xiii

LIST OF TABLES

Table Page

2.1: Properties of copolymers studied ................................................................................24

2.2: Results of Charlesby-Pinner Analysis; Number of crosslinks per unit AmBP

converted, ηc; Number of dislinks per crosslink, ηd/ηc (*value based on

SEC analysis of samples); Fraction of crosslinks formed by

benzopinacol formation, F/2ηc. ........................................................................31

2.3: Rate constants for hydrogen abstraction by benzophenone ........................................35

xiv

LIST OF SCHEMES

Scheme Page

1.1: Photo-reaction of benzophenone: Absorbance of ultraviolet irradiation

promotes BP from the ground state (S0) to the n-π* singlet (S1) which

undergoes rapid intersystem crossing to the diradicaloid n-π* triplet

(T1). This diradical then abstracts aliphatic hydrogen from its

environment, generating an aliphatic radical and a BP-centered radical

which then initiate further reaction. ...................................................................3

1.2: Possible triplet states for benzophenone. The n-π* triplet (left) is often

represented as a diradical and is the most reactive of excited state

species in BP. Possible π-π* triplets are shown in the bracket, with the

most common labeled as π-π* and π-π* CT (charge transfer). The CT

state is the least reactive of BP triplets. Schematic after reference 44

. ...............4

2.1: Several possible reaction pathways following hydrogen abstraction by triplet

BP: (a) Routes yielding a net cross-link via recombination of ketyl and

aliphatic- centered radicals; (b) One possible route yielding a net

dislink via β-scission of an aliphatic-centered radical .....................................19

2.2: Copolymer architecture and monomers used for copolymer synthesis (S:

styrene; pMS: p-methylstyrene; MAA: methacrylic acid; MMA:

methyl methacrylate; MA: methyl acrylate; iBMA: isobutyl

methacrylate; iBA: isobutyl acrylate; NiPAm: N-isopropyl

acrylamide); )( mnmxAmBP , where m and n are the mole fractions

of AmBP and comonomer, respectively. .........................................................22

xv

LIST OF FIGURES

Figure Page

1.1: Energy levels of p-aminobenzophenone in isopropanol and cyclohexane. The

more polar isopropanol causes inversion of the n-π* and CT states,

causing p-aminobenzophenone to be inert in this solvent. Solvation in

nonpolar cyclohexane permits reaction due to the n-π* triplet having

the lowest energy. Schematic after reference 38

. ................................................6

2.1: a) AmBP conversion is monitored via UV/Vis absorption of the π -π* peak

(~ 300 nm). Absorbance data have been normalized between 0 and 1

and fit with a single exponential decay (inset: spectra for PMMA (11K

Mw, xAmBP = 0.067)); b) Gel fraction is determined by measuring the

thickness reduction for surface-attached polymer films following

exposure to UV and extraction of the sol component. .....................................25

2.2: Charlesby-Pinner plot of PMMA (103K Mw, xAmBP = 0.033), used to

determine the number of crosslinks, ηc, and dislinks, ηd, per unit BP

converted according to Equation 6. .................................................................30

2.3: A comparison of the gel curves for PMMA copolymerized with AmBP (75K

Mw, xAmBP = 0.051, filled symbols), and doped with an equivalent

molar concentration of BP (55K Mw, open symbols), reveals a roughly

30-fold increase in gelation efficiency for the copolymer. ..............................34

2.4: a) SEC traces for PS (79K Mw, xAmBP = 0.05); b) reveal a nearly constant Mn

and increasing polydispersity with UV dose, indicating a system with

ηd/ηc ≈ 1. ...........................................................................................................38

2.5: SEC traces for PMAA (117K Mw, xAmBP = 0.030), show a decrease in Mn with

UV dose, indicating an overall ηd/ηc > 1. .........................................................40

2.6: Gel curves in both air and nitrogen for select copolymers reveal little

dependence on atmosphere, with the exception of PS which exhibits a

slightly higher ηeff under nitrogen; a) PMMA, 42K Mw, xAmBP = 0.048;

b) PS, 79K Mw, xAmBP = 0.05; c) PNiPAm, 43K Mw, xAmBP = 0.078; d)

PiBMA, 37K Mw, xAmBP = 0.055. .....................................................................41

xvi

3.1: Phase separation mechanisms of a binary polymer mixture. Nucleation and

growth (top) occurs when the homogeneous blend is quenched to a

metastable region of the phase diagram. Isolated particles of

equilibrium composition form and grow, forming a droplet in matrix

morphology. Spinodal decomposition (bottom) occurs when the

homogeneous mixture is quenched to a thermodynamically unstable

state. Concentration fluctuations vary with time, initially maintaining

constant size while increasing in amplitude. At later stages, both

amplitude and wavelength increase. Figure from Bates, F. S. Polymer-

Polymer Phase-Behavior. Science 251, 898–905 (1991). Reprinted

with permission from AAAS.21

........................................................................54

3.2: Sample turbidity data obtained from small angle light scattering. a)

Transmitted intensity versus temperature for a 25/75 PS-BP/PVME

sample exposed to 365 nm UV irradiation 30 s prior to heating at 5

°C/min through the cloud point. Inset images show light scattered prior

to and after clouding occurs. b) Cloud points determined for different

heating rates, showing the rate dependence on observed phase

separation. Reported cloud points have been extrapolated to a heating

rate of 0 °C/min. ...............................................................................................58

3.3: LCST Cloud points measured by SALS for PS/PVME58K Mn (black squares )

and PS-BP (55K Mn, xAmBP = 0.05)/PVME58K (red circles ) showing

little change in phase behavior upon incorporation of benzophenone

monomer. Inset is a representative bright field optical microscope

image of a 1/1 PS-BP/PVME blend after heating from 120 – 200 °C at

3 °K/min. Phase separation is observed. ..........................................................59

3.4: Cloud points measured by SALS for a 30/70 PS-BP (55K, xAmBP =

0.05)/PVME (58K Mn) blend exposed to 365 nm UV irradiation prior

to heating. a) Cloud points increase with increasing exposure energy

until 90 s, after which point the clouding behavior becomes difficult to

discern. b) Data plotted with inverse cloud point on the y-axis,

showing a dependence on radiation similar to that shown by Bauer and

Briber67

. ............................................................................................................61

xvii

3.5: Small angle X-ray scattering analysis of selected 50/50 PS-BP (55K, xAmBP =

0.05)/PVME (58K Mn) blends which have been pre-exposed to 365

nm UV irradiation for different times prior to phase separation at 175

°C. No discernible scattering peaks are observed in any sample from

30 – 240 s irradiation time, indicating a lack of resolvable features on

the 4 – 25 nm length scale. ...............................................................................62

3.6: Experimental technique for reactive blending. i) PS-BP copolymer and either

PEG or POEGMA (each polymer at 40 mg/mL) are dissolved in

benzene, and ii) are injected into a capillary tube, degassed, and

exposed to 365 nm UV for a prescribed irradiation time. After

exposure the solution is cast onto a glass slide and annealed prior to

characterization. ...............................................................................................71

3.7: UV/Visible absorption spectra of small molecule analogs at 45 mM

concentration in benzene. Benzophenone (black) shows the slowest

decay while incorporation of cumene with benzophenone (red) decays

faster and diethyl ether with benzophenone (blue) decays the most

rapidly, showing that benzophenone reacts most rapidly with the ether

species. Decay constants are extracted from single exponential fits. ..............74

3.8: SAXS traces for reactive blends of 1/1 PS-BP /PEG6K Mn exposed to 365 nm

UV for different durations. In the exposure range 65 – 90 minutes

annealed powder samples exhibit scattering peaks, indicating that

nanostructures are indeed present. ...................................................................75

3.9: Reactive blending results for 1/1 blends of PS-BP /PEG; a) Exposure times

required to acquire scattering peaks in SAXS for different molecular

weights of PEG, showing that with increased PEG molecular weight

the exposure time required is diminished. b) Length scales observed by

SAXS for blends with varying PEG molecular weights. .................................76

3.10: TEM image of a 1/1 PS-BP/PEG6K blend which has been exposed to 365

nm UV for 60 minutes. A hierarchical phase separation is observed,

with large domains on the order of 100 nm containing smaller features

on the 10 nm length scale, corresponding well with SAXS results

indicating 11.5 nm features. This sample has been cryo-microtomed

into 70 nm thick sections at -90 °C and stained with RuO4 vapor for 40

minutes prior to imaging. .................................................................................78

xviii

3.11: SEC traces of a 1/1 blend of PS-BP/PEG6K and the corresponding individual

polymers. The black and red curves are the PS-BP copolymer and PEG

homopolymer, respectively, while the green and blue curves represent

refractive index and ultraviolet signal from the polymer blend after 60

minutes of UV exposure. The refractive index detector peak (green

curve) at high retention times in the blend indicates a fraction of PEG

which has decreased molecular weight compared to the homopolymer.

This is indication of main chain scission during irradiation. Refractive

index curves magnified 25X. ...........................................................................79

3.12: SEC traces of a 1/1 blend of PS-BP/POEGMA and the corresponding

individual polymers. a) the black and red curves are the PS-BP and

POEGMA copolymers, respectively, while the green and black curves

represent refractive index and ultraviolet signals from the polymer

blend after 70 minutes of UV exposure. Unlike the PS/PEO blends,

this material clearly shows an increased molecular weight with UV

exposure. Refractive index curves have been magnified 25X. b)

Progression of molecular weight distribution (UV signal) with

exposure time for this blend signal. A peak at higher molecular weight

arises between 50 and 70 minutes of exposure, and disappears by 90

minutes, indicating the formation of insoluble species at high reaction

times. ................................................................................................................80

4.1: Floating hydrogel disc deactivation. i) PNiPAm-AAc-AmBP film is exposed

to UV through a mask and developed. ii) The sacrificial layer is

dissolved in an aqueous 1 mM phosphate buffer with 1 mM NaCl.

Deactivation is achieved through inclusion of solid NaBH4 to a

concentration of 0.15 M. iii) Concentration of NaBH4 is decreased

such that ionic strength of the buffer is at least 30X greater than that of

NaBH4. The samples are permitted to swell at room temperature and

are imaged, yielding discs with area A1. iv) Samples are heated

through the LCST to 60 °C, deswelling the discs. They are then

subjected to UV exposure overnight in the deswelled state. v) The

samples are swelled once more at room temperature, yielding discs

with area A2......................................................................................................99

xix

4.2: Deactivation of AmBP in PNiPAm-AAc-AmBP copolymer as studied by

UV/Vis and SEC. a) The inset shows spectra of polymer solution prior

to and post deactivation with 0.15 M NaBH4 (in 2:1 ethanol:water),

showing the disappearance of AmBP absorbance at 300 nm with a

corresponding rise of photo-product at 250 nm. Monitoring absorbance

at 300 nm during deactivation in we observe >90% conversion within

2500 s in ethanol solution, yet in a 2:1 ethanol:water mixture

deactivation is achieved within 300 s; b) SEC traces of polymer

solutions prior to and after treatment with NaBH4. The curves are

nearly identical, indicating that treatment does not significantly

influence molecular weight distribution of the copolymer. ...........................102

4.3: Swelling curve of PNiPAm-AAc-AmBP hydrogel discs after one exposure.

Areal swelling is calculated as A1/A0, with A0 the area of the initial

photo-mask. The minimal difference in swelling between samples with

and without NaBH4 treatment is indication that the deactivation

process does not disturb the existing hydrogel network. ...............................103

4.4: Final versus initial swelling for PNiPAm-AAc-AmBP hydrogel discs

subjected to the protocol depicted in Figure 4.1. By treating the

hydrogel discs with NaBH4 we are able to recover over 85% of the

initial swelling over the entire first exposure energy range. Conversely,

we see that the sample without NaBH4 treatment was only able to

achieve a fraction of the initial swelling. At high energy the curves

intersect due to conversion of all AmBP photo-crosslinker in the first

UV exposure step. ..........................................................................................104

4.5: Extent of reaction of AmBP photo-crosslinker in PNiPAm hydrogel discs

following each UV exposure. Values are calculated based upon

equilibrium swelling theory of a polyelectrolyte gel in the low salt

limit33

. a) After the first UV exposure, extent of reaction 1p ranges

from 0.59 to 1.0, and results are consistent between samples treated

with NaBH4 and controls; b) After deswelling and subsequent UV

irradiation, extent of reaction 2p for samples that have been treated

with NaBH4 are nearly identical to their 1p values, while controls

exhibit significantly higher values, further evidence that reduction with

NaBH4 prevents crosslinking upon subsequent UV irradiation in these

photo-patterned hydrogels. ............................................................................107

xx

4.6: Fluorescence microscopy images of PNiPAm-AAc-AmBP-RhBMA/PpMS-

AmBP bilayers. The untreated bilayer in both the deswelled (i) and

swelled (ii) states is nearly flat. Importantly, no distinction in swelling

contrast is observed between the halves of this sample. The bilayer that

has been treated with NaBH4 swells from the flat (iii) state to a curled

cylinder (iv) with a distinct difference between low and high swelling

areas, indicating successful deactivation of AmBP. ......................................109

1

CHAPTER 1

INTRODUCTION‡

Photochemistry, the study of reactions which proceed by the absorption of

photons, is a relatively new field when one considers that photo-reactions have been

utilized for millennia. Ancient Egyptians would soak linen in oil and place them in the

sun to dry, creating waterproof and robust materials which were applied to

mummification and boat building. What they did not fully appreciate at the time was that

the lavender oil used contained Syrian asphalt, which would effectively photo-crosslink

with radiation, and thus they were applying photochemistry over 4,000 years ago1,2

. More

recently, the advantages of photo-reactions have become apparent, as evidenced by the

vast attention they have received in the scientific and technological communities. The use

of photo-reactions allow processing of materials after their initial synthesis, and the

availability of low-cost and tunable radiation sources make photo-chemical techniques

exceedingly versatile3–5

. Furthermore, radiation processing permits scaling for high

throughput fabrication and spatial variation of intensity (i.e. lithography), with theoretical

resolution dependent on the wavelength of irradiation2,6–8

. As a result photo-induced

reactions have become essential to modern life, used in coatings, biosensors, inks,

photoresists, adhesives, and many other applications9–12

.

In polymer systems, photo-reaction generally proceeds by one of several

mechanisms including radical formation and cycloaddition. Radicals can be generated by

‡ Portions modified and reprinted (adapted) with permission from Christensen, S.K.,

Chiappelli, M.C., and Hayward, R.C. Macromolecules, 2012, 45 (12), pp 5237–5246.

Copyright 2012 American Chemical Society.

2

either Norrish Type I or Type II photo-initiation. Norrish Type I is achieved by α-

cleavage, whereby photon absorption is followed by homolytic cleavage of a bond alpha

to the excited carbonyl, producing a radical pair (as in acetophenone). Conversely,

Norrish Type II photo-initiation occurs by hydrogen abstraction to generate radicals13

.

Phenyl azides are also extensively used, as they will undergo one of several insertion

reactions after conversion to the highly reactive nitrene species when activated14–16

.

These processes are reasonably nonspecific, meaning that species so formed have the

potential to take part in any number of subsequent reactions and thus no additional

functionalization is necessary. A more selective set of photo-reactive materials are based

on self-cycloaddition, under which two identical unsaturated species dimerize to generate

a crosslink (examples include cinnamate esters, stilbazolium salts, coumarin,

styrylpyridinium, or maleimides)5,7,17–21

.

1.1 Benzophenone photo-reaction and applications

A set of photo-active molecules that have attracted a great deal of attention are

based on benzophenone (BP), and have been widely studied due to the photophore’s

numerous attractive properties. BP has proven to be extremely amenable to

experimentation, as it is more chemically robust than alternative molecules, stable to

ambient illumination, and active under wavelengths which do not damage the majority of

biomolecules10,11,22–24

. The modest molar absorbance of BP also permits photo-reaction

of reasonably thick samples with minimal gradient in illumination intensity3,25

. Coupling

this ease of use with high photochemical activity, broad reactivity, and low cost allows

BP to be applied in a diverse range of applications5,9,13,22,26–28

.

3

A Norrish Type II photo-initiator, BP is active under UV irradiation, where upon

absorption of a photon with wavelength of ~ 250 - 365 nm, ground state BP (S0) is

excited to a singlet state (S1) that rapidly and efficiently undergoes intersystem crossing

to yield the lowest energy reactive triplet13,26,29

(Scheme 1.1). The n-π* triplet (T1), which

can be represented as a diradical, is well known to abstract aliphatic hydrogens, with

especially high reactivity for those located alpha to electron donating heteroatoms (most

commonly nitrogen, sulfur, and oxygen)22,30

, thus converting the triplet BP to a ketyl

radical and also yielding an aliphatic carbon-centered radical. This process is known to

proceed with reasonable efficiency even in the presence of oxygen31–37

, making BP a

robust and broadly useful photo-chemically reactive species.

Scheme 1.1: Photo-reaction of benzophenone: Absorbance of ultraviolet irradiation

promotes BP from the ground state (S0) to the n-π* singlet (S1) which undergoes rapid

intersystem crossing to the diradicaloid n-π* triplet (T1). This diradical then abstracts

aliphatic hydrogen from its environment, generating an aliphatic radical and a BP-

centered radical which then initiate further reaction.

4

Reactivity of benzophenone derivatives can vary broadly depending upon

substituent groups and solvent conditions. In particular, reactivity depends on the lowest

energy triplet state in a given material, where the π-π* charge transfer (CT) state is least

reactive and the n-π* triplet is the most reactive (Scheme 1.2)38–40

. While unsubstituted

BP attains a lowest energy n-π* triplet, substitution with certain functionalities alters the

energy levels of these triplet states, even inverting them to make the unreactive CT state

lower energy26,41

. This inversion is caused by incorporation of an electron donating

species (i.e. amino, dimethylamino, hydroxy, methoxy, etc.) at the meta or para position

of BP, which serves to stabilize the CT state seen in Scheme 1.2, thereby lowering its

energy and making the BP species much less reactive38,42–44

. This effect can be made

more clear if we consider the hydrogen abstraction process in Scheme 1.1 as a reaction

between the electrophilic n-π* diradical and an electron-rich hydrogen donor substrate.

Including an electron donor on the aromatic ring serves to increase the electron density at

the carbonyl oxygen, making this group less electrophilic and thus less reactive. The

Scheme 1.2: Possible triplet states for benzophenone. The n-π* triplet (left) is often

represented as a diradical and is the most reactive of excited state species in BP. Possible

π-π* triplets are shown in the bracket, with the most common labeled as π-π* and π-π*

CT (charge transfer). The CT state is the least reactive of BP triplets. Schematic after

reference 44

.

5

opposite is true of electron withdrawing groups, which generally enhance reactivity of BP

by maintaining the n-π* as the lowest energy triplet13,39,40,44

. A special case is found when

substitution is at the ortho position of BP, which prevents reaction with substrate

materials due to the occurrence of intramolecular hydrogen abstraction26,38,41,44

.

Solvent conditions are also known to affect reactivity of benzophenone

derivatives, as changes in solvent polarity will change the relative energy levels of

substituted BP, sometimes inverting the n-π* and CT states when they are close in

energy. An example is shown in Figure 1.1 for p-aminobenzophenone, where in the more

polar isopropanol the π-π* CT state is the lowest triplet, yet in nonpolar cyclohexane the

n-π* is of lower energy38

. Generally, the energy of an excited state will decrease in polar

solvents if its polarity is higher than that of the ground state. As the π-π* CT state as

shown in Scheme 1.2 is more polar than the n-π* triplet, its stabilization in polar media is

greater, preventing reaction of some BP derivatives in polar solvents38,42–44

. It must be

said, however, that even when the lowest energy triplet is of π-π* (or CT) nature, some

reactivity will likely be observed as thermal fluctuation to an n-π* triplet will occur with

some frequency38,44

.

Due to its generally high reactivity, the scientific community has made extensive

use of BP, particularly in polymer systems where BP acts as both a photo-initiator and a

photo-grafting agent. As the n-π* triplet behaves as a diradical, many vinyl monomers

are susceptible to direct photo-polymerization by BP45,46

. Furthermore, when combined

with alkyl amine as a co-initiator, BP derivatives initiate polymerization of acrylate

monomers with very high efficiency, and as such are commonly used in

photoresists9,47–49

. Numerous polymers have also been successfully photo-grafted to

6

substrates by activation of BP dissolved in solution50,51

or anchored to a

surface11,23,45,52–57

, which allows coupling of radicals generated on polymer chains to

those on the substrate58

. These techniques have improved adhesion of polymer films54–56

,

increased solubility of nanoparticles in a polymer matrix57

, and allowed the photo-

patterning of biomolecule probes10,11,52

. Furthermore, addition of BP as a dopant has also

been used to achieve photo-crosslinking of a variety of polymers in the solid and melt

states3,4,6,25,28,59–61

. Despite their effectiveness in many arenas, the use of small-molecule

BP-species suffers from several intrinsic limitations, namely the possibility for the dopant

to macrophase separate, migrate preferentially to certain regions of the sample, and even

be volatilized at high temperature6,28,47,62–65

.

.

Figure 1.1: Energy levels of p-aminobenzophenone in isopropanol and cyclohexane. The

more polar isopropanol causes inversion of the n-π* and CT states, causing p-

aminobenzophenone to be inert in this solvent. Solvation in nonpolar cyclohexane

permits reaction due to the n-π* triplet having the lowest energy. Schematic after

reference 38

.

7

The use of a one-component material, i.e., a copolymer containing covalently

attached BP moieties, can potentially eliminate these issues. By including the BP

functionality in a macromolecule, phase separation and migration are prevented by

chemical compatibilization with the matrix material, and the vapor pressure of BP is

effectively decreased. What’s more, in several cases the photochemical activity of BP

copolymers has proven higher than their small molecule counterparts9,28,65

. It is thus no

surprise that copolymers containing covalently-bound BP as photo-crosslinkers and

photo-grafting agents have been described in the literature going back over fifty

years6,31,32,62,66–72

.

More recently, however, BP copolymers have gained attention as a powerful route

to prepare hydrogels as surface attached films23,52,54,55,73

, micro-devices8,12,74–76

,

biosensors5,77

, biomedical coatings56

, and patterned sheets78,79

. These versatile materials

have also been used to generate resist layers80

, pressure-sensitive adhesive coatings24,81

,

optical multilayers82

, and even to align liquid crystals83

. Therefore, despite having existed

for over half a century, it is clear that benzophenone copolymers are an increasingly

important tool for expanding the fundamental and practical applications of materials

science.

This dissertation aims to both deepen and broaden our understanding of

copolymers with pendent benzophenone in relation to both established applications and

novel directions in materials science. BP copolymers wherein the photo-crosslinker is

located pendent to the main polymer chain have been synthesized by conventional free

radical polymerization, and other materials are acquired from commercial sources or

facile chemical means in order to limit synthetic demands. Photo-reaction of these BP

8

copolymers is then explored in attempts to achieve three distinct goals: i) robust and

efficiently photo-crosslinkable solid polymer films, ii) photo-reacted polymer blends with

disordered bicontinuous nanostructures, and iii) UV stable photo-patterned hydrogels for

multilayer systems. In Chapter 2 we investigate the fundamental gelation behavior of

solid polymer films by a combination of UV/Visible absorption spectroscopy, gel fraction

measurements, and chromatography. Knowledge gained from this study is then applied in

Chapter 3, wherein we discuss two potential methods for preparing nanostructured

polymer blends from these materials. Finally, Chapter 4 investigates the chemical

deactivation of BP photo-crosslinker in copolymers for use as photo-patternable and

environmentally stable hydrogel materials. The combined results from this report serve to

define the capabilities of BP copolymers in material systems of the future.

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16

CHAPTER 2

GELATION OF COPOLYMERS WITH PENDENT BENZOPHENONE PHOTO-

CROSSLINKERS†

2.1 Abstract

A series of copolymers containing covalently attached benzophenone (BP) photo-

crosslinkers was synthesized and their UV-induced gelation monitored as a function of

the extent of BP conversion. For poly(methyl methacrylate) copolymers, the

recombination yield between BP- and aliphatic-centered radicals was estimated and

compared to that for dimerization of each species, directly confirming that the high

gelation efficiencies observed for these copolymers arises due to the additional

crosslinking pathways provided by covalently incorporated BP, as compared to doping

with a small-molecule crosslinker. The placement of the hydrogen species most

susceptible to abstraction by triplet benzophenone is found to greatly influence gelation

efficiency, since radical generation on the polymer backbone typically increases the

probability of dislinking events, while hydrogen abstraction pendent to the copolymer

backbone tends to enhance crosslinking. Finally, the presence of atmospheric oxygen

during photo-crosslinking was found to yield only modest changes in the gelation

behavior of these copolymers.

† Reprinted (adapted) with permission from Christensen, S.K., Chiappelli, M.C., and

Hayward, R.C. Macromolecules, 2012, 45 (12), pp 5237–5246. Copyright 2012

American Chemical Society.

17

2.2 Motivation

Benzophenone (BP) has long been used as a dopant to achieve photo-crosslinking

of a variety of polymers in the solid and melt states, including poly(ethylene)1–5

,

poly(ethylene oxide)6, poly(norbornene)

7, and poly(phosphazene)s

8. While this doping

with small-molecule BP-species is a simple route to achieve photo-crosslinking, it suffers

from several intrinsic limitations, including the possibility for small molecule

crosslinkers to macrophase separate, migrate preferentially to certain regions of the

sample, and even be volatilized at high temperature4,9,10

.

The use of a one-component material, i.e., a copolymer containing covalently

attached BP moieties, eliminates issues of phase separation and migration, and can also

significantly enhance gelation efficiency since it provides additional pathways to

crosslink formation. As shown in Scheme 2.1a, following hydrogen abstraction there are

three pathways by which radical recombination can yield crosslinks: (i) coupling of ketyl

and aliphatic radicals, (ii) dimerization of ketyl radicals to form a benzopinacol, and (iii)

dimerization of aliphatic radicals. By contrast, for samples doped with unattached BP

derivatives, only the third mechanism provides a crosslink. Given the possibility for

radicals placed on the polymer backbone to undergo competing reactions leading to chain

scission or “dislinking” (an example of which is shown in Scheme 2.1b), these additional

pathways can in many cases be critical to obtaining reasonable crosslinking yields.

Copolymers containing covalently-bound BP as photo-crosslinkers and photo-

grafting agents have been described in the patent literature going back nearly fifty

years9,11–15

, but have only more recently gained attention in the open scientific literature

as a powerful route to prepare hydrogels as surface attached films16–19

, micro-devices20,21

,

18

biosensors22,23

, biomedical coatings24

, and patterned sheets25,26

, as well as to generate

resist layers27

, pressure-sensitive adhesive coatings28,29

, and optical multilayers30

. Several

efforts have been made to investigate how the photoreaction of these copolymers depends

on BP concentration and mode of substitution29,31–34

, comonomer composition29,33,35,36

,

and reaction temperature28,37,38

. These studies on crosslinking are largely incomplete,

however, as they are complicated by the use of multi-component28,29,39

systems and

exotic monomers33,36

, or substitution effects due to incorporation of BP in the polymer

backbone32,37,40

, making the extension of these results to common material systems

difficult.

As copolymers containing covalently bound BP represent an increasingly

important tool for materials chemists, the current report seeks to better understand the

gelation of these polymers, in particular the efficiency of photo-crosslinking and its

dependence on comonomer chemistry. Here, a series of copolymers containing

acrylamidobenzophenone (AmBP) monomers, and thus pendent BP moieties, were

synthesized and crosslinked using 365 nm UV light, with the conversion of BP monitored

by UV/visible absorption spectroscopy and the gel fractions determined from film

thickness measurements following extraction of sol components. Effective crosslinking

efficiencies, and the number of crosslinks and dislinks per BP unit converted, were

estimated using mean-field and Charlesby-Pinner theories, respectively. Gelation of

select polymers was also examined under a nitrogen environment to quantify the effect of

oxygen on crosslinking.

19

Scheme 2.1: Several possible reaction pathways following hydrogen abstraction by

triplet BP: (a) Routes yielding a net cross-link via recombination of ketyl and aliphatic-

centered radicals; (b) One possible route yielding a net dislink via β-scission of an

aliphatic-centered radical

2.3 Experimental

Materials

Acryloyl chloride (97 %), N-isopropyl acrylamide (NiPAm), methacrylic acid

(MAA), isobutyl acrylate (iBA), and methyl acrylate (MA) were purchased from Sigma

20

Aldrich. 4-aminobenzophenone (98 %), triethylamine (99.7 %), p-methyl styrene (pMS)

and isobutyl methacrylate (iBMA) were obtained from Acros Organics. Methyl

methacrylate (MMA) was obtained from Fisher Scientific and styrene (S) from

Malinckrodt-Baker. Inhibitor was removed from monomers by extraction with KOH (aq)

and water, and subsequently drying over anhydrous MgSO4 (Fisher).

Azobisisobutyronitrile (AIBN, Aldrich) was recrystallized from methanol prior to use

while 1,4-dioxane (Aldrich), methanol (Fisher Scientific) and

methacryloxypropyltrichlorosilane (Gelest) were used without further purification.

Synthesis

Acrylamidobenzophenone (AmBP) was synthesized from acryloyl chloride and 4-

aminobenzophenone according to the method of Jia et al41

. Polymers were synthesized by

free radical polymerization under nitrogen at 80 °C in 1,4-dioxane with AIBN as initiator

(for methacrylic acid, a 3:1 methanol:1,4-dioxane by volume mixture was used).

Resulting polymers were then precipitated and dried under vacuum. 1H-NMR was

performed with a Bruker Avance 400 MHz or DPX 300 MHz spectrometer to determine

AmBP content. Molecular weights were determined via size exclusion chromatography

(SEC) with a Polymer Laboratories GPC-50 with a polystyrene column and THF as

eluent (0.5 mL/min). A refractive index detector was used, and molecular weights were

calibrated with poly(methyl methacrylate) standards, or poly(styrene) standards for

styrenic materials. Poly(methacrylic acid) molecular weight was characterized with a

Hewlett Packard Series 1050 aqueous SEC with 0.1M NaNO3 and 3 mM NaN3 as eluent

and a refractive index detector against poly(ethylene glycol) standards.

21

UV Exposure

Polymer film samples (on quartz or silicon substrates) were exposed to 365 nm

UV radiation using a UVGL-58 Handheld UV lamp (UVP; ~ 2 mW/cm2, spectral range

340 - 410 nm). For samples on quartz substrates, a silicon wafer was placed behind the

sample to mimic the reflection of light that occurs for samples on silicon substrates. The

lamp was allowed to equilibrate for at least 1 h prior to use, and power was frequently

measured with an X-Cite XR2100 Power Meter (EXFO) to ensure accuracy of the

reported dose. Exposures under inert atmosphere were performed by placing samples

inside an Instec HCS621V microscope hot stage purged with N2 gas. The sample was

maintained at a temperature of 35 °C, matching that of the surface of the UV source.

Unless otherwise specified, all exposures were performed in open atmospheric conditions

with the sample placed in contact with the window on the UV light source.

AmBP Conversion

Samples for UV/visible absorption spectroscopy (UV/vis) were cast by placing

~ 100 L of a filtered 10 mg/mL polymer solution (PMMA, PiBMA, PiBA, PS, PpMS in

toluene; PMA in chloroform; PNiPAm in ethanol) onto UV-ozone treated 1 x ½ in quartz

plates, yielding films of ~ 1 m thickness, which were subsequently annealed under

vacuum for at least 12 h at 30 K above the glass transition temperature (Tg) of each

material. Conversion of AmBP was monitored as a function of UV dose with a Hitachi

U-3010 spectrophotometer in absorbance mode over a wavelength range of 200 - 500 nm.

22

Sol/Gel Measurements

Silicon wafers were silanized with the adhesion promoter

methacryloxypropyltrichlorosilane and samples were subsequently spin coated from 40

mg/mL solutions (PMMA, PiBMA, PiBA, PS, PpMS in toluene; PMA in chloroform;

PNiPAm in ethanol; PMAA in methanol) at 2000 rpm for 50 s, yielding film thicknesses

of 150 - 400 nm upon annealing at Tg + 30 K. Each sample was exposed to UV for a

prescribed period of time, then immersed in a marginal solvent mixture for at least 5 h

which was sufficient to completely remove the sol fraction (solvent mixtures: v/v

Scheme 2.2: Copolymer architecture and monomers used for copolymer synthesis

(S: styrene; pMS: p-methylstyrene; MAA: methacrylic acid; MMA: methyl methacrylate;

MA: methyl acrylate; iBMA: isobutyl methacrylate; iBA: isobutyl acrylate; NiPAm: N-

isopropyl acrylamide); )( mnmxAmBP , where m and n are the mole fractions of

AmBP and comonomer, respectively.

23

THF/water: PMMA 80/20, PMA 73/27, PiBMA 84/16, PiBA 84/16, PS 78/12; v/v

toluene/hexanes: PpMS 75/25; v/v ethanol/water: PNiPAm 31/69; water: PMAA).

Samples were then annealed for at least 12 h (at Tg + 30 K) prior to thickness

measurement, which was performed with a Filmetrics F20 thin film measurement system

with a tungsten-halogen fiber light source over a wavelength range of 400 - 900 nm. Each

film was sampled four times, and uncertainties reported as plus or minus one standard

deviation.

2.4 Results and Discussion

A series of copolymers containing the photo-crosslinker AmBP and one of eight

other monomers were first prepared, as summarized in Scheme 2. The chemistries

chosen include the commonly-used materials poly(styrene), poly(methyl methacrylate),

and poly(N-isopropyl acrylamide), as well as five other (meth)acrylate or styrenic

monomers that allow for comparisons between materials with varying reactivity, density,

and location of abstractable hydrogens. The copolymers synthesized had molecular

weights (weight average, Mw) ranging from 11,000 to 117,000 g/mol and dispersities (Ð)

between 1.5 and 2.2, as indicated in Table 2.1. The mole fraction of photo-crosslinkers

xAmBP in these copolymers ranged from 0.03 to 0.125, corresponding to an average

number of AmBP units per chain between 3 and 21. For the case of PMMA, four

polymers with different molecular weights and crosslinker contents were prepared to

determine whether these parameters significantly influence our results.

24

Table 2.1: Properties of copolymers studied

Polymer M

w

kg/mol Ð x

AmBP AmBP/

chain Ec

J/cm2

pc η

eff

PMMA 103 1.8 0.033 18 7.8 ± 0.5 0.045 ± 0.007 0.7 ± 0.1 11 1.7 0.067 4.1 19.7 ± 3.9 0.15 ± 0.03 1.0 ± 0.2 42 1.5 0.048 13 11.1 ± 1.6 0.074 ± 0.016 0.7 ± 0.2 75 1.7 0.051 21 11.6 ± 1.4 0.016 ± 0.005 1.7 ± 0.6 PS 79 1.8 0.050 20 30.1 ± 6.3 - < 0.05 PpMS 32 1.9 0.125 16 11.8 ± 0.7 0.039 ± 0.012 0.9 ± 0.3 PNiPAm 43 1.8 0.078 15 5.2 ± 0.8 0.018 ± 0.006 2.1 ± 0.7 PMA 36 1.7 0.048 10.7 6.1 ± 1.0 0.086 ± 0.001 0.6 ± 0.1 PiBA 19 2.2 0.052 3.4 5.3 ± 0.4 0.25 ± 0.04 0.5 ± 0.1 PiBMA 37 1.6 0.055 8.6 6.0 ± 0.2 0.14 ± 0.02 0.5 ± 0.1 PMAA 117 1.8 0.030 21 - - < 0*

* Estimated from SEC analysis of PMAA, which exhibits a decreased Mn with UV dose

The conversion of BP photo-crosslinkers as a function of UV irradiation dose was

determined by monitoring the decay of the π -π* absorption peak at 300 nm using UV/vis

spectroscopy for samples supported on quartz substrates (absorption peak mislabeled as

n-π* in original publication, reference 42

). To correct for variations in film thickness,

each spectrum was normalized using the empirically determined isosbestic point (falling

between 265 and 275 nm, depending on the copolymer) arising due to the decay of the

π -π* absorption and corresponding rise of the benzopinacol peak at 250 nm, as seen in

the inset to Figure 2.1a. From a measurement of the molar absorption coefficient of

benzophenone in solid state PpMS at 365 nm (ε ~ 60-90 m2

mol-1

), we estimate that for

the thickest samples considered here (~ 5 μm) and the highest BP concentration (12.5

mol%) the intensity variation from the top to bottom surface of the film (including 100 %

reflectivity from the Si substrate) is only 15 %, thus we neglect the effects of gradients in

conversion of BP. Representative decay curves of π -π* absorbance vs. exposure energy

E are shown in Figure 2.1a for two polymers, PMMA and PMA, with data normalized

25

Figure 2.1: a) AmBP conversion is monitored via UV/Vis absorption of the π -π* peak

(~ 300 nm). Absorbance data have been normalized between 0 and 1 and fit with a single

exponential decay (inset: spectra for PMMA (11K Mw, xAmBP = 0.067)); b) Gel fraction is

determined by measuring the thickness reduction for surface-attached polymer films

following exposure to UV and extraction of the sol component.

between zero and one (after subtracting the constant absorbance at infinite energy K) for

clarity. In all cases, the data are well described by a single exponential decay

26

KeAA cEE

0 , with a characteristic dose for conversion of BP units Ec that is

related to the extent of conversion p by

cEE

ep

1 . (2.1)

As summarized in Table 2.1, values of Ec ranged from ~ 5 - 30 J/cm2

for the samples

considered here.

To approximate the yield of the photoproduct benzopinacol relative to

consumption of AmBP, the peak maxima were monitored as a function of UV dose. An

exponential background was subtracted from each spectrum, as a significant background

due to absorbance from the comonomers is present at low wavelengths. Combining the

initial and final absorbances of AmBP ( 0

AmBPA and f

AmBPA , respectively) and benzopinacol

( 0

bpA and f

bpA , respectively) in these copolymer systems allows the fraction of AmBP

converted to benzopinacol F to be calculated as

)(1

)(2

0

0

f

AmBPAmBP

AmBP

bp

f

bp

bp

AA

AA

F

(2.2)

where εAmBP (540 m2/mol) and εbp (1180 m

2/mol) are the peak molar absorption

coefficients of AmBP and benzopinacol, respectively, measured in methanol. Values of F

range from 0.15 to 0.58, indicating significant formation of benzopinacol through the

combination of AmBP-centered radicals.

For each copolymer, a corresponding set of films were prepared on silicon

substrates treated with an adhesion promoter to allow measurement of the gel fraction.

These films were exposed to UV and subsequently developed by immersion in a solvent

27

mixture of moderate quality to extract the sol fraction without extensively swelling the

gelled material. The gel fraction for each sample was calculated as the ratio of the (dry)

film thickness after solvent extraction to that of the initial film prior to UV exposure.

Figure 2.1b shows data on gel fraction vs. irradiation energy for the same PMMA and

PMA polymers as in Figure 2.1a. Each curve shows a clear gel point, as evidenced in the

jump from zero to finite gel fraction at a particular dose. Using Equation 1 to relate the

energy at the gel point to an extent of reaction, we convert this dose to a critical extent of

reaction pc for each sample. Experimental values are reported as the average of the last

point with zero gel fraction and the first point with non-zero gel fraction for each

copolymer. As summarized in Table 2.1, these values ranged from 0.016 to 0.25 for the

samples studied, with the exceptions of PS and PMAA, which showed zero gel fraction

even at full conversion (p = 1).

Mean-Field Model

A crude estimate of the efficiency of BP photo-crosslinkers is obtained by

comparison with the mean-field bond percolation model of gelation for lattice sites with

functionality f. For the case of a polymer with a weight-average43,44

degree of

polymerization N, if each monomer is capable of forming a crosslink and N >> 1, then

the mean field prediction for the critical extent of reaction is:

Nfp fm

c

1

1

1

, (2.3)

i.e., gelation occurs at an average of one crosslink per chain45,46

. For the copolymers

considered here, only a fraction xAmBP of the monomers are capable of forming crosslinks,

thus we modify Equation 3 to yield

28

AmBP

fm

cxN

p

1, (2.4)

as the mean field prediction for gelation47

. An overall efficiency of conversion of BP

units into crosslinks can then be calculated from the experimentally measured gel point pc

by

AmBPcc

fm

ceff

xNpp

p

1

, (2.5)

with values for each copolymer listed in Table 2.1. Importantly, the use of PS or PMMA-

equivalent molecular weights to determine N introduces a modest systematic error into

this analysis which varies depending on the copolymer being tested. However, we do not

correct for this since Mark-Houwink parameters are not readily available for all the

copolymers studied.

Charlesby-Pinner Analysis

While mean-field theory provides a simple and useful metric for crosslinking

efficiency, it suffers from the well-known limitation that it ignores cyclization, and also

cannot distinguish the influence of dislinking reactions from inefficient crosslinking.

While the former effect is difficult to correct for and will generally lead to an

underestimate (~5%) of crosslink formation from experimental pc values48

, dislinking

reactions can be taken into account by a Charlesby-Pinner analysis49

. While the original

theory is developed in terms of the radiation dose, this treatment cannot be directly used

as AmBP reactivity varies with different co-monomers. Further, since our experiments

reach BP conversions near unity, we rewrite the Charlesby-Pinner equation as:

pxNSS

AmBPcc

d

1

22

1

(2.6)

29

where S is the sol fraction, and c and d represent the number of crosslinks and dislinks

per unit of BP converted, respectively. Due to the presence of significant uncertainties in

our measured values of both p and S, we calculated effective error bars in S incorporating

both components for purposes of curve fitting. Figure 2.2 shows representative data for a

PMMA copolymer plotted according to the form of Equation 6. At low values of 1/p the

data are in good agreement with a linear relationship, allowing the determination of c

and d from the slope and intercept of the uncertainty-weighted least squares best-fit line.

Fits were also performed over the entire range of 1/p, and while this leads to changes in

the values of c and d determined, the qualitative trends reported remain intact. The

observed deviations from linearity at high p1 are presumably caused by the assumption

of a random molecular weight distribution ( 2nw NN ) in Charlesby-Pinner theory. As

the prepared copolymers have initially non-random molecular weight distributions, it is

only upon random crosslinking and dislinking (at low 1/p) that samples approach this

polydispersity value.

Crosslinking Efficiency by Mean-Field and Charlesby-Pinner Analyses

The values of effective crosslinking efficiency ηeff determined from mean-field

analysis, as summarized in Table 2.1, are at least 0.5 for all copolymer samples, with the

exception of PS and PMAA, which did not gel even at full AmBP conversion (ηeff <

0.05). Although this approach has significant limitations due to the inherent

simplifications in the mean field model, it suggests that pendent AmBP units are highly

efficient crosslinkers in all but these two cases. Only one polymer, PNiPAm, gave a

value of ηeff (2.1 ± 0.7) that with reasonable confidence can be said to exceed unity. As

we are not aware of reactions that would yield multiple crosslinks per AmBP converted,

30

this likely does not reflect a true efficiency above one, but rather a limitation of the mean

field analysis and/or a systematic error in our measurements. One such source of

systematic error is the use of PMMA-equivalent molecular weight determined by size

exclusion chromatography. Taking into account the Mark-Houwink parameters50

for

PMMA (K = 7.5x10-3

, a = 0.72) and PNiPAm (K = 9.59x10-3

, a = 0.65), Mw is more

accurately estimated as 58 kg/mol, bringing ηeff to a more physically reasonable, though

still high, value of 1.4 ± 0.5.

Figure 2.2: Charlesby-Pinner plot of PMMA (103K Mw, xAmBP = 0.033), used to

determine the number of crosslinks, ηc, and dislinks, ηd, per unit BP converted according

to Equation 6.

Notably, the samples of PMMA with different molecular weights and AmBP

contents yielded consistent values of ηeff, at least for three of the four copolymers (0.7 ±

0.1, 1.0 ± 0.2, and 0.7 ± 0.2), while the 75 kg/mol polymer gave rise to a slightly larger

value of 1.7 ± 0.6. The characteristic doses Ec also agree well for three of the four

31

PMMA copolymers, though the 75 kg/mol sample shows a value roughly twice as large

as the others. We note that differences in xAmBP and “blockiness” of the random

copolymers (i.e. due to differences in the degree of conversion of monomer used during

polymerization) could influence the ability of AmBP to locally aggregate, and therefore

lead to real differences in the photo-chemical activity of AmBP in copolymers of

otherwise identical chemistry. However, the reasonably good agreement obtained for

these PMMA samples provides an indication that the trends we report here indeed arise

due to chemistry of the predominant comonomer.

Table 2.2: Results of Charlesby-Pinner Analysis; Number of crosslinks per unit AmBP

converted, ηc; Number of dislinks per crosslink, ηd/ηc (*value based on SEC analysis of

samples); Fraction of crosslinks formed by benzopinacol formation, F/2ηc.

Polymer Mw

(kg/mol) c cd cF 2

PMMA 103 0.53±0.11 0.25±0.19 0.20±0.05

11 0.62±0.12 -0.18±0.14 0.12±0.02

42 0.50±0.13 -0.04±0.14 0.16±0.04

75 1.08±0.14 0.56±0.1 0.07±0.02

PS 79 - ~1* -

PpMS 32 0.85±0.06 0.61±0.09 0.34±0.02

PNiPAm 43 1.06±0.15 0.33± 0.1 0.14±0.03

PMA 36 0.43±0.06 0.22±0.08 0.21±0.12

PiBA 19 0.37±0.07 -0.50±0.3 0.33±0.06

PiBMA 37 0.35±0.05 0.01±0.2 0.51±0.08

PMAA 117 - >1* -

The results of Charlesby-Pinner analysis, summarized in Table 2.2, provide

information on both crosslinking and dislinking reactions. Importantly, these values are

dominated by the gelation behavior at high conversion, whereas the mean-field analysis

32

considers only the gel point, thus they provide a complementary picture of the activity of

AmBP in these copolymers. The number of crosslinks per unit AmBP converted c,

excluding the values for PS and PMAA, all lie between 0.25 and 1, and are in general

agreement with the efficiencies determined by mean-field analysis. Dislinking reactions,

expressed in terms of the ratio d/c, and again excluding PS and PMAA, range from

approximately 0 to 0.6, thus they appreciably reduce the net efficiency of crosslink

formation for some copolymers. Only one sample (PiBA) yielded a value of d/c that

was negative with reasonable confidence (-0.50 ± 0.3), which is an unphysical result that

again must reflect limitations in the model and/or systematic errors. As in the mean-field

analysis, three of the four PMMA copolymer samples show very good agreement, with ηc

values of 0.5 – 0.6, and d/c close to zero. Surprisingly, however, the 75 kg/mol PMMA

sample shows larger values of ηc = 1.08 ± 0.14 and d/c = 0.56 ± 0.1. The reason for this

discrepancy is not clear, but it does indicate that sample-to-sample variations can be

significant.

As a Charlesby-Pinner analysis yields the number of crosslinks formed per AmBP

converted ηc, a direct comparison can be made with the fraction of benzophenone

converted to the dimer benzopinacol, F, defined in Equation 2. Assuming that each

benzopinacol formed yields one elastically active crosslink (i.e., ignoring loops), an

estimate for the maximum fraction of crosslinks due to benzopinacol formation is

obtained from the quantity cF 2 . The factor of two is introduced since each AmBP

converted to benzopinacol yields half of a crosslink. Results are seen in Table 2.2, and

fall between 0.07 and 0.51, indicating a significant fraction of crosslinks formed due to

AmBP-centered radical recombination in these copolymers. Interestingly, PMMA

33

copolymers exhibit a relatively low overall fraction, ranging from 0.07 to 0.20, again

with three of the four copolymers falling close to one another (0.20 ± 0.05, 0.12 ± 0.02,

and 0.16 ± 0.04). The lower value of cF 2 for 75K Mw PMMA (0.07 ± 0.02) could

potentially be associated with a lower incidence of intramolecular AmBP recombination,

again suggesting a different compositional gradient compared to the other three PMMA

copolymers. Significantly higher values of cF 2 (0.33 – 0.51) are seen for PpMS, PiBA

and PiBMA, indicating a higher fraction of crosslinks due to benzopinacol formation.

Pendent vs. Doped Benzophenone

We next performed a direct comparison between a PMMA copolymer containing

AmBP (xAmBP = 0.051, Mw = 75 kg/mol), and a similar molecular weight PMMA

homopolymer (Mw = 55 kg/mol, Ð = 1.7) doped with an equivalent molar concentration

of free benzophenone. As shown in Figure 2.3, while the doped sample does gel, it

requires a conversion of approximately 0.6 to do so, in comparison to only 0.016 ± 0.005

for the copolymer. Despite variations seen among PMMA copolymers, this result

represents a significant decrease of crosslinking efficiency in the doped system, with a

roughly 30-fold lower value of eff. While we cannot rule out the possibility that BP

aggregation reduces the measured efficiency in the doped case, the samples were

similarly transparent, at least excluding the possibility of large-scale phase separation.

This result, combined with values of cF 2 between 0.07 and 0.20 for PMMA, provides

a clear picture of the contribution of each crosslink-forming photoproduct to the overall

gelation behavior (Scheme 2.1a). In polymer doped with BP, the only route for crosslink

formation is recombination of aliphatic radicals, and thus the 30-fold difference in eff

indicates that ~ 3 % of the total crosslinks in the copolymer are formed through aliphatic-

34

aliphatic radical recombination. Considering that only 7 – 20 % of the crosslinks formed

in the copolymer are due to benzopinacol formation, recombination of BP- and aliphatic-

centered radicals is revealed to be the dominant mechanism, accounting for 77 – 90 % of

crosslinks formed in PMMA copolymers.

Figure 2.3: A comparison of the gel curves for PMMA copolymerized with AmBP (75K

Mw, xAmBP = 0.051, filled symbols), and doped with an equivalent molar concentration of

BP (55K Mw, open symbols), reveals a roughly 30-fold increase in gelation efficiency for

the copolymer.

Reactivity of Hydrogen to BP Abstraction

As previously mentioned, the selected copolymers were chosen to provide

variations in reactivity, density, and location of abstractable hydrogens, and thus of the

radicals resulting from reaction with triplet BP. Numerous studies have detailed the

sensitivity of hydrogen abstraction by BP to chemical environment, and for reference,

rate constants from several sources are reported in Table 2.3. Photoinitiation studies have

35

indicated that aliphatic hydrogens alpha to certain heteroatoms (nitrogen, sulfur, and

oxygen in particular) are especially susceptible to BP abstraction, thus the reason that

amines are often used as coinitiators with BP. This increased reactivity is generally

attributed to charge transfer interactions between the BP triplet and electrons in the

heteroatom prior to abstraction. Of these heteroatoms, nitrogen-containing materials have

been found to be the most reactive compared to materials containing oxygen or

sulfur51–54

. Aliphatic sites lacking heteroatoms at the alpha position are less susceptible to

hydrogen abstraction by BP, with reactivity decreasing as the stability of the resulting

radical decreases55,56

. As radical stability generally increases with substitution, rate

constants for hydrogen abstraction increase in the order of primary (1°), secondary

Table 2.3: Rate constants for hydrogen abstraction by benzophenone

Species k [s-1

] k [M-1

s-1

] Relative to

primary Ref

Primarya 1.70x10

3 1 57

Secondarya 6.8x10

4 40 57

Tertiarya 5.1x10

5 300 57

Secondary

cyclohexyla

6.0x104 35 57

Primary benzylica 1.3x10

5 74 57

Benzene C-Ha 2.7x10

1 0.016 58

Tertiary benzylicd 1.9-5.0x10

5 59

t-C4H9NH2b 1.1x10

8 60

sec-C4H9NH2b 3.0x10

8 60

n-(C3H7)2NHc 3.4x10

9 60

C6H12e 3.3x10

6 3.6x10

5 55

CH3OHe 3.8x10

6 1.5x10

5 55

C2H5OHe 12.0x10

6 7.0x10

5 55

(CH3)2CHOHe 23.0x10

6 1.8x10

6 55

a: 25 °C in acetonitrile, rates per R-H hydrogen bond broken; b: 25 °C in acetonitrile; c:

25 °C in benzene; d: 25 °C in acetonitrile, estimated from ratio of primary benzylic to

tertiary benzylic photoproducts; e: values originally obtained at 20 °C in pure solvent

(s-1

), divided by molarity of pure solution to obtain M-1

s-1

36

cyclohexyl, secondary (2°), primary benzylic, and tertiary (3°). Aromatic hydrogen is by

far the least reactive, with the rate of hydrogen abstraction two orders of magnitude lower

than that of 1° hydrogen57

. While we note that amides and esters may have reduced

reactivity compared to amines or alcohols, we still expect these functional groups to

enhance susceptibility to abstraction relative to aliphatic sites lacking heteroatoms at the

alpha position. Therefore, by considering the results from mean field and Charlesby-

Pinner analyses in light of the reactivity of the different hydrogen species in each

material, we can rationalize the effect of comonomer chemistry on gelation behavior.

(Meth)acrylate copolymer gelation

We first compare acrylate- and methacrylate-based copolymers, which contain a

range of hydrogens with varying reactivity and location. Poly(methyl methacrylate)

(PMMA) contains 1° and 2° hydrogens, with three 1° located alpha to oxygen, and

exhibits a characteristic dose for AmBP conversion Ec between 7.8 and 19.7 J/cm2. The

presence of 3° hydrogens as in poly(methyl acrylate) (PMA), poly(isobutyl methacrylate)

(PiBMA), or poly(isobutyl acrylate) (PiBA) yields a corresponding decrease in Ec (5.3 –

6.1 J/cm2), consistent with the greater susceptibility of 3° hydrogen species to abstraction

by BP. Despite this lower reactivity for PMMA copolymers, however, ηeff is as high or

higher for PMMA than any other acrylate-based copolymer. For PMA, which contains 3°

hydrogens on the polymer main chain, a higher ηd/ηc value is found, indicating an

increased occurrence of dislinking in PMA than the corresponding (similar N*xAmBP) 42K

PMMA. This clearly suggests that hydrogen abstraction, and thus radical placement, on

the 3° position of the polymer main chain is unfavorable for gelation of these

copolymers. The increase in dislinking is likely caused by β-scission, as previously

37

suggested by Torikai et al (Scheme 2.1b)61

. For PiBA, in contrast to PMA, a negligible

occurrence of dislinking was observed, suggesting that the additional 3° hydrogen

pendent to the main chain in PiBA greatly decreases the probability of hydrogen

abstraction from the backbone. A comparison of PiBA and PiBMA shows the values of

Ec, ηeff, ηc, and ηd/ηc to be nearly identical for these copolymers, further suggesting that the

reactivity of 3° hydrogens located pendent to the polymer chain are enhanced compared

to those on the backbone, presumably due to steric effects. Together, these results on

acrylate and methacrylate copolymers suggest that hydrogen abstraction, and thus radical

placement, pendent to the polymer backbone is conducive to gelation, since radicals

placed on the main-chain have an increased probability of undergoing dislinking.

Styrenic Copolymer Gelation

We next consider the complementary case of styrenic copolymers. Poly(styrene)

(PS) contains a majority of relatively unreactive aromatic hydrogens, with two 2° and one

3°-benzylic hydrogen on the polymer backbone. The high Ec for PS (30.1 ± 6.3 J/cm2,

nearly three times larger than any other material) despite the presence of what should be a

quite reactive 3°-benzylic position suggests a steric effect here as well. While quenching

of AmBP triplet by the styrenic polymer62

could also contribute to lower reactivity, we

note that the incorporation of a 1°-benzylic position pendent to the polymer chain as in

poly(p-methyl styrene) (PpMS) lowers Ec considerably (11.8 ± 0.7 J/cm2). Together with

the aforementioned results for PiBA and PiBMA, these findings paint a consistent picture

that steric effects lead to generally reduced reactivity of backbone hydrogens. In addition

to this reduced reactivity, PS exhibits a very low ηeff (< 0.05), not gelling under

atmospheric conditions even at full conversion. To determine whether any crosslinking or

38

Figure 2.4: a) SEC traces for PS (79K Mw, xAmBP = 0.05); b) reveal a nearly constant Mn

and increasing polydispersity with UV dose, indicating a system with ηd/ηc ≈ 1.

dislinking had occurred, a series of irradiated samples were characterized by SEC (Figure

2.4), with results showing a constant number average molecular weight with increasing

polydispersity as exposure energy increases. A similar trend of increasing polydispersity

has been reported previously63

, and corresponds to a system approaching gelation46

with

a dislink to crosslink ratio ηd/ηc of approximately one. Much like acrylate materials, these

dislinking reactions likely occur via β-63–66

or oxidative-scission67

, which have previously

been reported for irradiated styrenic systems containing BP dopant. Despite the very low

39

effective crosslinking efficiency for PS, when compared to the results of a study of PS

doped with free BP65

, which yielded ηd/ηc ≈ 7.7 - 25, this provides further evidence that

polymers with covalently-attached BP provide superior crosslinking efficiency.

As PS yields a nearly equal rate of dislinking and crosslinking reactions, a second

styrenic copolymer (PpMS), was studied, which incorporates three additional 1°-benzylic

hydrogens pendent to the polymer main chain. The inclusion of these additional reactive

positions serves to increase ηeff to nearly one while decreasing ηd/ηc to 0.61 ± 0.09. This

result is similar to our findings for the acrylate copolymers, i.e. that shifting hydrogen

abstraction away from the polymer backbone to pendent 1°-benzylic hydrogens enhances

crosslinking, thus providing an effective means to gel even polymers that are susceptible

to degradation.

Gelation of poly(N-isopropyl acrylamide) and poly(methacrylic acid)

To further extend our understanding of these systems, two additional copolymers

were investigated, with each presenting a high degree of asymmetry between the lability

of hydrogens located pendent to the polymer main chain compared to those attached

directly on the backbone. The first of these materials is poly(N-isopropyl acrylamide)

(PNiPAm), which contains 3° hydrogens alpha to nitrogen and pendent to the polymer

backbone. As this position is anticipated to be highly reactive, the vast majority of

hydrogen abstraction should occur on the side chains. Consistent with this expectation, a

previous study has shown that BP-containing copolymers with pendent aliphatic amine

are extremely susceptible to photocrosslinking36

. Our results show PNiPAm to exhibit

among the lowest Ec of any material studied, confirming the high reactivity of these

hydrogens. Further, PNiPAm yields a high ηeff (1.4 ± 0.5, with the molecular weight

40

correction) and ηc (1.06 ± 0.15). While a reasonably high ηd/ηc (0.33 ± 0.1) is also

observed in this sample, these results reinforce that hydrogen abstraction pendent to the

main polymer chain is conducive to gelation.

The second material chosen is poly(methacrylic acid) (PMAA), whose most

reactive hydrogen (2°) is located on the polymer main-chain. As our copolymer systems

have, to this point, indicated an increased occurrence of dislinking with hydrogen

abstraction from the polymer backbone, this material should be inefficient in gelation.

This notion is confirmed as PMAA, much like PS, does not gel in air. SEC analysis of

this material (Figure 2.5) shows a decreasing molecular weight with increasing irradiation

energy, indicating ηd/ηc to be greater than one. While conversion of AmBP was not

monitored for this sample, the nearly constant molecular weight distributions seen for the

highest doses indicate that p in these samples has approached unity. This result further

confirms that hydrogen abstraction from the polymer backbone favors dislinking,

hindering gelation.

Figure 2.5: SEC traces for PMAA (117K Mw, xAmBP = 0.030), show a decrease in Mn with

UV dose, indicating an overall ηd/ηc > 1.

41

Effect of oxygen on copolymer gelation

Since an attractive feature of BP is that it retains high photochemical activity even

in air, an open atmosphere was used to this point in the study. However, to determine

whether oxygen does affect gelation of these copolymers, several samples were studied

under a nitrogen atmosphere (Figure 2.6, Table 2.4). With the exception of PS, only

minor variations in gelation behavior are observed, with PMMA, PNiPAm, and PiBMA

exhibiting ηeff values nearly identical to atmospheric conditions, in agreement with prior

results for dimethylaminostyrene-containing terpolymers68

. Small changes to the gel

curves for the latter three polymers indicate only a slight susceptibility of these materials

Figure 2.6: Gel curves in both air and nitrogen for select copolymers reveal little

dependence on atmosphere, with the exception of PS which exhibits a slightly higher ηeff

under nitrogen; a) PMMA, 42K Mw, xAmBP = 0.048; b) PS, 79K Mw, xAmBP = 0.05; c)

PNiPAm, 43K Mw, xAmBP = 0.078; d) PiBMA, 37K Mw, xAmBP = 0.055.

42

to oxygen quenching of triplet AmBP and oxidative scission. We note that the

characteristic time for oxygen diffusion69

across a 10 μm PMMA film at 35 °C is roughly

30 s, whereas our experimental Ec values indicate deactivation of AmBP on a time scale

about 80 times longer. Therefore, we conclude that insensitivity to oxygen in these solid-

state copolymers is not simply a matter of mass transport limitations, in agreement with

previous reports37,40

. Conversely, PS shows an increase in gelation efficiency from < 0.05

in air to 0.08 ± 0.02 under nitrogen, showing that oxygen plays a slight role in preventing

crosslinking in this case. These results agree with those for BP-doped polymer films,

wherein increased peroxy radical concentration in PS causes an increase in chain

scission67,70

. As PS in an open atmosphere yields a dislink to crosslink ratio ηd/ηc of

roughly one, preventing oxidative scission allows the material to gel, albeit with very

poor efficiency.

2.5 Conclusions

Copolymers containing pendent BP have been prepared, and their gelation

behavior examined via UV/Vis and sol-gel measurements. Compared to doping with BP,

covalent attachment of BP photo-crosslinker provides two additional routes by which

radical recombination can result in crosslinking, thus greatly increasing gelation

efficiency. The location of highly abstractable hydrogens is found to be the dominant

factor, with hydrogen abstraction on the main polymer chain tending to favor dislinking,

while abstraction from side chains promotes gelation. Finally, oxygen was found to exert

little influence on gelation of most copolymers, with the exception of PS, which exhibits

a slightly higher efficiency under a nitrogen atmosphere. The insights into crosslinking

43

efficiency provided by this study should help to guide the choice of appropriate

copolymer chemistries for fabrication of robust photo-crosslinkable materials.

Acknowledgements

We are grateful to Jungwook Kim for synthesis of the PNiPAm copolymer. This

work was funded by the US Army Research Office through grant W911NF-11-1-0080

and the Defense Threat Reduction Agency through grant HDTRA1-10-1-0099, and made

use of facilities supported by the National Science Foundation MRSEC on Polymers at

UMass (DMR-0820506).

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64. Ikeda, T., Kawaguchi, K., Yamaoka, H. & Okamura, S. Photoinduced and

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49

CHAPTER 3

ROUTES TOWARD NANOSTRUCTURED POLYMER BLENDS BY

BENZOPHENONE PHOTO-REACTION

3.1 Abstract

Photo-reaction of copolymers containing pendent benzophenones (BP) was

utilized in two approaches in attempts to prepare highly interconnected three-dimensional

nanostructured polymer blends. Arresting of spinodal decomposition by crosslinking was

first investigated by UV exposure in a poly(styrene-AmBP)/poly(vinyl methyl ether)

critical blend prior to phase separation. Photo-crosslinking elevated the cloud point of

blends due to A-B reaction which constrains phase separation. Morphological

characterization of this system was impossible, however, as the electron density contrast

between the two polymers is insufficient for SAXS. In the second technique we prepared

nanostructured polymer blends by UV-initiated solution-state photo-grafting of

poly(styrene-AmBP) to PEG-containing homopolymer. Nanoscale features were

generated upon UV exposure as evidenced by SAXS, yet clear signatures of bicontinuous

morphologies remain elusive. Refinement of each method is discussed in an effort to

provide successful realization of photochemically prepared bicontinuous polymer blends

in the future.

50

3.2 Motivation

Nanostructured polymer materials have garnered immense interest from

technological perspectives due to their potential to enhance material properties. For

instance, polymer blends with nanoscale domains have been shown to improve

thermomechanical properties, optical transparency, and toughness compared to

conventional blends1, often attributed to the high surface area of such nanoscale features

2.

This improvement of properties represents an opportunity to vastly increase performance

in a wide range of applications, from proton exchange3 and lithium ion

4 membranes to

peptide biosensors5. In fact, proton exchange membranes (PEMs) with nanoscale

domains have previously been found to exhibit conductivity orders of magnitude higher

than their homogeneous counterparts due to the connectivity of ionic domains promoted

by nanoscale phase separation6,7

. Furthermore, nanometer domain size in PEMs increases

selectivity and can even promote retention of water in hydrated films due to capillary

forces8, further improving function.

While nanoscale domains are important, equally important is how these domains

are arranged in space. For many applications (i.e. sound damping, gas transport, and

impact resistant materials) it is desirable for blends to demonstrate properties of both

components in a single volume, and as such a bicontinuous morphology is preferred. This

morphology is composed of two chemically distinct material networks that percolate in

three dimensions with high connectivity. Because each material occupies the entire three-

dimensional space, each component gives its maximum contribution to the blend,

improving physical properties (modulus, strain at break, toughness, and impact

strength)9,10

. Furthermore, because of the high interconnectivity of each domain, these

51

materials have found use in ion transport3,6,11–13

, charge dissipation10,14

, photovoltaic

technology15

, and templates for porous membranes which can be used for filtration or

controlled drug release16,17

.

Current routes toward generation of bicontinuous morphologies generally rely on

kinetic trapping during melt mixing or spinodal decomposition, or the use of block

copolymers18

. Spinodal decomposition and block copolymers are discussed in Sections

3.3 and 3.4, respectively. By incorporating compatibilizer into a polymer melt, it is

possible to achieve kinetically trapped cocontinuous morphologies, yet these features are

micron scale and will only ripen with time2,9,14,19

. In this chapter we discuss new

photochemical methods for the preparation of nanoscale bicontinuous morphologies

using copolymers with pendent benzophenones. We believe that these materials offer

several advantages to traditional approaches for the creation of three-dimensional

interconnected domains, and we explore two potential methods toward their creation.

First we describe attempts to arrest the phase separation of a lower critical polymer blend

by photo-crosslinking, and in Section 3.4 we discuss attempts to create nanostructures by

UV-initiated solution-state photo-grafting. While these approaches garner limited

success, significant refinement of the materials systems and processes would be

necessary to achieve the desired bicontinuous morphologies.

3.3 Arresting phase separation of LCST blends by BP photo-crosslinking

Polymer blends are known to develop nanoscale bicontinuous morphologies

during phase separation20,21

, and as such represent one potential route toward

cocontinuity for copolymers containing pendent benzophenones. For phase separation to

52

occur, polymers must first be miscible, meaning that at some condition there must exist a

negative free energy of mixing, mixG , defined as

mixmixmix STHG (3.1)

where mixH and mixS are the enthalpy and entropy of mixing, respectively. Adjusting

the free energy equation to split the combinatorial ( combS ) and noncombinatorial

entropies of mixing allows us to rewrite Equation 3.1 as

comb

E

mixmix STGG (3.2)

where E

mixG represents the excess free energy of mixing, or a combination of the

noncombinatorial entropy and enthalpy of mixing. This excess free energy of mixing

E

mixG encompasses three distinct effects: (1) interactions between the two polymers

( 0 mixH favors miscibility), (2) free volume effects caused by differences in thermal

expansion coefficient (a difference of >4% between the two polymers is generally

sufficient to yield phase separation22,23

), and (3) size effects from differences in

segmental size (usually minimal in polymers, favors phase separation)24

. Because combS

is small due to the long chain length of polymers, miscibility in a polymer blend requires

some favorable interaction between the two component materials. Therefore within some

temperature range the favorable interactions between component polymers cause

diminished free volume effects and enhanced size effects, rendering the blend

homogeneous22

. These favorable interactions and size effects generally diminish with

temperature while free volume effects are enhanced, and thus the blend will phase

separate above a given temperature, yielding a lower critical solution temperature (LCST)

blend. If upon cooling the size effects overcome the interaction term phase separation

53

may also occur, resulting in an upper critical solution temperature (UCST) blend24

. While

both phenomenon are thermodynamically possible for any polymer blend with favorable

segmental interactions, crystallization or glass transition temperatures often occur above

the UCST (see e.g. Briber25

or Isayeva26

), and consequently LCST blends are much more

common than UCST blends. There have been numerous experimentally realized LCST

blends27–42

, UCST blends38,43–51

, and even some systems exhibiting both UCST and

LCST behavior52–56

.

Quenching of these critical blends into the two-phase range induces phase

separation of the component polymers, which occurs by one of two mechanisms:

nucleation and growth or spinodal decomposition (Figure 3.1). If the blend is quenched

from a homogeneous to a metastable region, nucleation and growth will occur, whereby

isolated particles of equilibrium composition appear and grow, forming droplets of one

polymer in a matrix of the other57

. If instead the miscible blend crosses into the unstable

region, spinodal decomposition will occur whereby concentration fluctuations initially

vary with time, maintaining a constant size while increasing in amplitude58,59

. This

spinodal morphology is characterized by an almost uniform, yet random and highly

interconnected cocontinuous structure36

. At later stages of spinodal decomposition, both

amplitude and wavelength of this cocontinuous morphology increase with time until

finally interconnectivity is lost57

. Thus while it is possible to achieve a bicontinuous

morphology via spinodal decomposition of critical blends, in the absence of additional

stimuli the architectures will ripen to macroscopic length scales, resulting in fully phase

separated morphologies.

54

Figure 3.1: Phase separation mechanisms of a binary polymer mixture. Nucleation and

growth (top) occurs when the homogeneous blend is quenched to a metastable region of

the phase diagram. Isolated particles of equilibrium composition form and grow, forming

a droplet in matrix morphology. Spinodal decomposition (bottom) occurs when the

homogeneous mixture is quenched to a thermodynamically unstable state. Concentration

fluctuations vary with time, initially maintaining constant size while increasing in

amplitude. At later stages, both amplitude and wavelength increase. Figure from Bates, F.

S. Polymer-Polymer Phase-Behavior. Science 251, 898–905 (1991). Reprinted with

permission from AAAS.21

As a consequence of macrophase separation in these materials, many studies have

attempted to kinetically trap the bicontinuous spinodal morphology. The incorporation of

particles into polymer systems can effectively stem the phase separation process, as

evidenced by Fillipone60,61

, Li62

, and Gubbels63,64

, wherein nanoparticles preferentially

occupy one material and prevent the phases from coalescing. “Bijels” have also been

experimentally realized, wherein microparticles exist at the interface between two phases,

55

thus stabilizing the bicontinuous morphology, albeit on the micron scale65,66

.

Nanoparticle-loaded blends, however, require specific interactions be imparted into the

nanoparticles, and suffer limitations due to difficulty of achieving an initially

homogeneous distribution of particles, and the changing properties of said materials

through the incorporation of a high filler volume fraction.

A second method of trapping these structures involves the photo-crosslinking of

blends prior to phase separation. First theoretically described by deGennes in 1979 and

expanded upon by Briber in 1988, it was believed that by crosslinking the polymers,

concentration fluctuations can be constrained, thus limiting the wavelength of phase

separation to a finite size67–70

. Briber extended this theory to a poly(styrene)/poly(vinyl

methyl ether) (PS/PVME) blend, which upon exposure to gamma radiation increased the

single phase regime and exhibited scattering by features on a 60 nm scale upon phase

separation67

, in qualitative agreement with the theory. In years since, a number of studies

by Tran–Cong Miyata have investigated blends consisting of anthracene-functionalized-

PS and PVME, which upon UV exposure will undergo anthracene dimerization to yield a

crosslinked PS phase 20,71–80

. While in some cases this system has resulted in crosslinked

spinodal morphologies20,71,72,77

, the A-A self-crosslinking inherent to this photochemistry

consistently shifts the LCST phase boundary down due to increased PS molecular weight,

decreasing the window of miscibility and often inducing phase separation without any

thermal treatment20,73,75,76,78,80–83

. The use of A-A (anthracene) crosslinking in these

studies has therefore limited the usable range of the LCST blend, while crosslinking in

gamma irradiated blends is poorly defined and structure in these materials has never been

studied in detail.

56

Copolymers functionalized with benzophenone thus represent a versatile system

for the study of photo-crosslinkable critical polymer blends, and in this section we

investigate the possibility of achieving a bicontinuous morphology by photo-crosslinking

of a benzophenone-containing polystyrene (PS-BP)/PVME LCST blend prior to phase

separation. Benzophenone offers the capability of tuning crosslinking in the blend to be

preferentially A-A, B-B, or A-B depending on the asymmetry of hydrogen abstraction in

the chosen polymers and the material in which the photo-crosslinker resides. As

PS/PVME undergoes a well-described phase separation at a LCST falling above the glass

transition and below the decomposition temperature, this is the blend system of

choice84,85

. To promote A-B crosslinking, acrylamidobenzophenone (AmBP) will be

copolymerized into PS thus expanding the miscibility window and allowing phase

separation to be better controlled. Phase behavior of these blends is investigated by a

combination of scattering techniques, and the effect of UV irradiation is described.

Morphological characterization is incomplete, however, as this particular blend system

does not yield strong enough X-ray scattering for our purposes.

3.3.1 Experimental

Materials and Sample Preparation

Poly(styrene) (PS; Mn = 16,000 g/mol) and poly(styrene-AmBP) copolymer (PS-

BP; Mn = 55,000 g/mol, xAmBP = 0.05) were synthesized as in Chapter 2, while poly(vinyl

methyl ether) (PVME) with a molecular weight of 58,000 g/mol was purchased directly

from Polysciences, Inc. Water was removed from PVME by rotary evaporation and the

dried polymers were subsequently dissolved in toluene. Blend solutions were prepared at

57

20 mg/mL and filtered through a 0.45 μm PTFE syringe filter prior to drop casting onto

untreated glass cover slips. After evaporation at ambient conditions, the solid polymer

films were annealed under vacuum at 100 °C for at least 10 hours and cooled slowly,

forming clear blend films with thicknesses of 5 – 10 μm. UV exposures were performed

in the same way as in Chapter 2.

Small Angle Light Scattering

Small angle light scattering (SALS) was performed on an in-house setup

operating with a 632 nm HeNe laser. The beam passes through two pinholes prior to a hot

stage containing the sample of interest. Behind the sample plane is a 2-dimensional image

plate coupled to a CCD camera and a photodiode which captures the intensity of the

direct beam (turbidity measurement). Samples were placed in the hot stage at room

temperature and heated through the cloud point under a constant nitrogen purge, as

shown in Figure 3.2a. At low temperatures the sample is transparent, causing minimal

scattering of the incident beam and thus a high transmitted intensity. At high

temperatures the sample phase separates, becoming opaque and scattering the direct beam

away from the photodiode, causing a substantial decrease in transmission. The intercept

of the initial slope and the slope after phase separation defines the cloud point in these

experiments. It is important to note that the measured cloud point will depend on the

heating rate, and thus for each experiment several heating rates were utilized, with a

linear fit extrapolated to 0 °K/min to yield the reported cloud point for each blend

composition or exposure time (Figure 3.2b).

58

Small Angle X-Ray Scattering

X-Ray scattering was done using an in-house setup from Molecular Metrology Inc.

(presently sold as Rigaku S-Max3000). It uses a 30 W microsource (Bede) with a 30 x 30

μm2 spot size matched to a Maxflux® optical system (Osmic) leading to a low-

divergence beam of monochromatic CuKα radiation (wavelength λ = 0.1542 nm). After

passing beam defining and guard pinholes, the beam of about 0.4 mm diameter enters the

sample chamber. The whole system is evacuated, and the actual scattering angles are

calibrated using the accurately known reflections from silver behenate.

Figure 3.2: Sample turbidity data obtained from small angle light scattering. a)

Transmitted intensity versus temperature for a 25/75 PS-BP/PVME sample exposed to

365 nm UV irradiation 30 s prior to heating at 5 °C/min through the cloud point. Inset

images show light scattered prior to and after clouding occurs. b) Cloud points

determined for different heating rates, showing the rate dependence on observed phase

separation. Reported cloud points have been extrapolated to a heating rate of 0 °C/min.

59

3.3.2 Results and Discussion

Blend films were prepared as described and subjected to SALS experiments at

several heating rates (5, 10, and 30 °C/min) to determine cloud points extrapolated to 0

°K/min. Blends ranging from a polystyrene mass fraction (ΦPS) of 0.1 to 0.9 were first

investigated to define the phase behavior of our materials. As shown in Figure 3.3 (black

squares), a blend of poly(styrene) homopolymer with PVME yields a LCST blend with a

critical composition between ΦPS = 0.3 – 0.5 at 155 °C. This temperature is higher than

Figure 3.3: LCST Cloud points measured by SALS for PS/PVME58K Mn (black squares )

and PS-BP (55K Mn, xAmBP = 0.05)/PVME58K (red circles ) showing little change in

phase behavior upon incorporation of benzophenone monomer. Inset is a representative

bright field optical microscope image of a 1/1 PS-BP/PVME blend after heating from 120

– 200 °C at 3 °K/min. Phase separation is observed.

most reported in the literature, but it must be noted that the molecular weights

investigated in this study were significantly lower than in previous

60

publications20,38,68,74,80,82

. Replacing the poly(styrene) with a corresponding PS-BP is

shown to have little effect upon phase behavior, indicating that incorporation of

benzophenone in the poly(styrene) phase preserves the LCST nature of the blend (Figure

3.3, red circles). A slight lowering (Tc ≈ 150 °C) and shift (ΦPS-BP = 0.3 – 0.4) of the

critical point toward low PS-BP compositions is observed, however, and likely reflects an

increase in molecular weight of the PS-BP relative to neat poly(styrene). This molecular

weight change decreases the entropy of mixing and therefore results in a less miscible

system. The inset in Figure 3.3 shows a bright field optical microscope image of a

symmetric PS-BP/PVME blend film which has been heated to 200 °C at 3 °K/min. It is

clear from this image that phase separation has occurred in the sample, which exhibits

features characteristic of a nucleation and growth mechanism, which is to be expected at

an off-critical ΦPS-BP = 0.5.

After determining the phase diagram of the PS-BP/PVME blend system, blends

located near the critical point (ΦPS-BP = 0.3) were exposed to 365 nm UV irradiation for

various times to determine its effect on the cloud point of the system. As seen in Figure

3.4a, UV exposure up to 90 s raises the cloud point by 70 °C, after which point the data

varies significantly up to 240 s. Samples irradiated up to 90 s are consistent with previous

theoretical and experimental work, with the finding that the inverse critical temperature is

proportional to radiation dose (Figure 3.4b)67,70

, however at longer exposure times

clouding behavior is either absent or poorly defined. As the majority of crosslinking in

this system should occur between PS-BP and PVME (A-B crosslinking due to the high

reactivity of the methyl hydrogen alpha to the ether of PVME, which should minimize

chain scission in this polymer system by promoting abstraction on the side groups), the

61

size of the one-phase envelope is greater due to constraints on phase separation of the two

chemically unique polymers67,68

. This constraint causes the unpredictable clouding

behavior observed above 90 s, as the samples have gelled and thus cannot phase separate

on the length scale observed here (samples exposed for >300 s never yield a measurable

cloud point). Below 30 s, the deviation from linearity is likely due to breakdown of the

theoretical treatment afforded by deGennes near the sol-gel transition70

. Phase behavior

of samples irradiated between 15 and 90 s are strikingly similar to those seen by Briber

and Bauer, indicating a strong likelihood that these PS-BP/PVME blends will exhibit

phase separated morphologies with a characteristic length scale67

.

Figure 3.4: Cloud points measured by SALS for a 30/70 PS-BP (55K, xAmBP =

0.05)/PVME (58K Mn) blend exposed to 365 nm UV irradiation prior to heating. a)

Cloud points increase with increasing exposure energy until 90 s, after which point the

clouding behavior becomes difficult to discern. b) Data plotted with inverse cloud point

on the y-axis, showing a dependence on radiation similar to that shown by Bauer and

Briber67

.

62

In an effort to quantify any nanoscale morphology present in these materials,

ΦPS-BP = 0.5 blends were cast onto Kapton 300HN polyimide films and subjected to

small-angle X-ray scattering. Selected results are shown in Figure 3.5 for samples

irradiated for several exposure times, and unfortunately it is impossible to define any

scattering features from these results. Weak scattering intensity across the entire range of

UV exposures (30 - 240 s) provides no quantifiable data, and without some indication of

nanoscale ordering further characterization was not performed. It is important to note that

while these X-ray results do not definitively eliminate the possibility of nanoscale

features in these samples, it does bring to light some significant experimental

considerations when performing these tests. The first is that the electron density contrast

Figure 3.5: Small angle X-ray scattering analysis of selected 50/50 PS-BP (55K, xAmBP =

0.05)/PVME (58K Mn) blends which have been pre-exposed to 365 nm UV irradiation

for different times prior to phase separation at 175 °C. No discernible scattering peaks are

observed in any sample from 30 – 240 s irradiation time, indicating a lack of resolvable

features on the 4 – 25 nm length scale.

63

in this PS-BP/PVME blend system may in fact be too low to adequately scatter X-rays

within any accessible experimental time frame, as suggested by Tran-Cong et al81

. If this

is true, it would be impossible to obtain scattering results similar to those obtained by

Briber and Bauer without resorting to selective deuteration, as in their work67

. It may,

however, be possible to obtain evidence of phase separated morphologies via electron

microscopy or AFM, yet these techniques are very time consuming and would likely

require investigation of many samples.

3.3.3 Conclusion

Photo-crosslinking of the LCST blend poly(styrene-AmBP)/poly(vinyl methyl

ether) was investigated by a combination of light and X-ray scattering. Incorporation of

benzophenone preserves the LCST nature of the blend, and exposure with 365 nm UV

causes an increase in phase separation temperature as defined by the cloud point. This

elevation is caused by A-B crosslinking in the blend, which constrains phase separation

of the two components, even preventing it altogether at high UV doses (>300 s). While

results are consistent with phase separation, morphological characterization has proved

difficult as the electron density contrast between the two polymers is insufficient for

SAXS.

3.3.4 Critical Blend Future Directions

While this series of experiments was unsuccessful in demonstrating a

bicontinuous blend morphology, there are numerous routes that could be taken in future

experiments to achieve this goal. Several paths are possible if one does not wish to

64

change the material system significantly (PS/PVME with benzophenone photo-

crosslinker), each focusing on simplifying characterization of the materials. First and

foremost, deuteration of one component (i.e. deuterated PS, as in Briber and Bauer’s

work67

) would allow for neutron scattering of the UV-exposed polymer blends and thus

more effective morphological characterization. Since neutron scattering provides

characterization on the nano- and micro-scale, this would allow investigation of samples

fabricated with current methods, determining if spinodal morphologies have been

arrested and perhaps completing the project. It should be noted, however, that deuteration

is known to influence phase behavior of PS/PVME blends, raising the spinodal

temperature by 40 °C in one investigation86

. It would thus be necessary to repeat all

experiments in this study. Second, if sample size were increased characterization with

AFM could be performed. AFM is chosen over TEM due to the extensive sample

preparation required for TEM. Since AFM is a surface characterization technique, this

would require a uniform film where neither the PS-BP nor the PVME have a preference

for the substrate; otherwise it is possible that only one of the component polymers would

be present at the surface. If high throughput characterization is possible, the PS-

BP/PVME LCST blend system could yet result in the desired bicontinuous morphologies

on the nanoscale.

While PS-BP/PVME remains a suitable material system for these model

experiments, there are numerous other critical polymer blend systems that may be

utilized (see introduction). Rather than select one specific polymer blend, requirements

that should yield the most promising results will be discussed. Of primary importance is

accessibility of the temperature at which phase separation will occur. While most

65

polymer blends will exhibit some critical temperature, this transition is often impeded by

the glass transition, crystallization, or degradation of one or both polymers. Crosslinking

of the polymer blend will also alter the temperature at which phase separation occurs, and

this effect will vary depending upon the polymers and crosslinker selected. A-B

crosslinking is desired as this will increase the area of the miscible regime while A-A

crosslinking of a glassy polymer will generally shrink this window by increasing Tg and

decreasing TLCST82

. A-B crosslinking is further beneficial because A-A crosslinking can

require extremely high crosslink density to achieve nanostructured morphologies (for

example, 36 crosslinks per chain were required to create nanostructures in a PS-

anthracene/PVME blend20

, whereas 5 crosslinks per chain led to micron scale features in

the same material system72

). By chemically immobilizing the two component polymers

together as in A-B crosslinking, fewer junctions should be required to achieve

nanostructured materials. A final suggestion resulting directly from this work is that

characterization of materials should be fully considered prior to material selection, as

without scattering methods it is very difficult to demonstrate the desired goals.

3.4 Solution-state asymmetric photo-grafting of benzophenone copolymers

A second means to achieving nanoscale three-dimensional interconnected

polymer networks involves the blending of two homopolymers with corresponding A-B

diblock copolymer. Analogous to oil/water mixtures with surfactant87,88

, these polymeric

systems are often referred to as bicontinuous microemulsions (BμEs), wherein the

diblock copolymer acts as a polymer surfactant, stabilizing the interface between A and B

domains. These blends have been shown to form highly connected three-dimensional

66

morphologies in many polymer systems, and as such have received much attention in

recent years10,89–93

. This section investigates the possibility of creating

thermodynamically stable three-dimensional interconnected polymer networks by the

creation of interfacially active graft copolymers through solution-state photo-reaction of

copolymers with pendent benzophenone.

As the basis for BμEs, any discussion must begin with block copolymers. These

materials have been extensively studied and well characterized, with several

morphologies accessible by varying volume fraction and molecular weight of the

component polymers with a given χ interaction parameter94

. Bicontinuous morphologies

are usually found within phase diagrams of block copolymers, yet the range over which

they are accessible is very small, requiring a precise balance of experimental variables

and chemical incompatibility (i.e. gyroid morphology generally forms only with modest

segregation strenths91

). BμEs represent an attractive alternative route toward these three-

dimensional morphologies via dilution of the pure block copolymer with the

corresponding homopolymer. It has been shown that with equal volume fractions of

homopolymer, only 10 - 20% diblock copolymer is necessary for BμE generation in the

blend9,90,95

, which significantly increases synthetic throughput. A symmetric volume

fraction of components is necessary as it has been found that swelling of lamellar

domains with homopolymer causes fluctuation in the lamellar morphology, generating

this interconnected network morphology89,93

.

As defined above, BμEs utilize block copolymers with low polydispersity,

materials which generally require time consuming and costly synthetic routes. The

benzophenone copolymers in Chapter 2 are synthesized via conventional free radical

67

polymerization and are thus polydisperse, so we must now consider this effect on

morphology generation. Fortunately, this question has previously been raised and

polydispersity has been found to actually stabilize nanostructured morphology in block

copolymer systems (both di-93,96–98

and tri-blocks99,100

). This stabilization is attributed to

a reduced entropic cost of space filling by the polydisperse component, reducing packing

frustration and causing the polydisperse block to occupy a smaller volume than

expected96,97,99,101

. It has thus been found that a polydisperse minority phase is preferred.

Polydispersity also favors curved interfaces and larger domain spacing than a

corresponding monodisperse block copolymer, and as such reduces long range order97,102

.

While this may appear detrimental, decreased long range order is actually beneficial for

our purposes, as a disordered three dimensional BμE morphology is desired. In fact,

polydispersity may expand the range of polymer blends for which nanophase separation

occurs99,103

. It has even been theorized that polydisperse block copolymers will form

more flexible interfaces and suppress attractive copolymer interactions, allowing

formation of BμEs from highly immiscible polymers. Polydispersity is therefore a

potentially useful tool for the generation of highly interconnected morphologies.

While BμEs represent an effective route to highly interconnected three-

dimensional network morphologies, significant reduction of domain size is possible by

the formation of copolymers in-situ, which are known to stabilize interfaces more

effectively than premade block copolymers104

. This is commonly achieved by reactive

blending, whereby compatibilization is accomplished by thermal coupling reactions at the

interface between polymers105

. This process has the added benefit of eliminating the

transport limitation of block copolymer dispersion to the interface in the melt state10

. A

68

number of studies have created nanostructured polymer blends by this

technique1,16,106–109

, yet the inspiration for this section is derived from the method

developed by Pernot et al110

. This process consists of the polymerization of two separate

materials: poly(ethylene) with maleic anhydride groups placed randomly along the

polymer and polyimide end-functionalized with complementary amine. Extrusion of the

materials at high temperatures causes reaction of the functional groups and in-situ

formation of graft copolymers with various architectures. These interfacially active

species each have a preferred curvature defined by the relative volume fraction of each

component, and when combined with unreacted homopolymer, stable cocontinuous

morphologies are developed. The stability of these morphologies are attributed to several

factors: (1) the homogeneous stabilization afforded by randomly distributed functional

groups109

, (2) the relative difficulty of removing graft copolymers from the interface108

,

and (3) to the improved reduction of interfacial tension by graft copolymers than by

corresponding diblock copolymers104,111

. While this method is successful at preparing

bicontinuous morphologies, it still suffers from the necessity of functionalizing both

polymers with complementary moieties and the potentially harsh processing conditions

required.

While the previous section focused on the potential to arrest spinodal

decomposition in a polymer blend, this section instead discusses the possibility of

creating thermodynamically stable nanoscale bicontinuous morphologies by the

formation of graft copolymers through solution-state photo-reaction of copolymers with

pendent benzophenone. These materials offer an improvement in that they represent

polydisperse materials functionalized randomly with a photo-crosslinking group, much

69

like the poly(ethylene) in Pernot et al’s work. Unlike this work, however, BP offers the

added advantages in that existing hydrogen in the component polymers can be exploited

in place of additional chemical modification, and milder processing conditions can be

achieved through the use of photochemistry. The selected material systems consist of PS-

BP and PEG-containing homopolymer (both PEG and poly(oligoethylene glycol

methacrylate) (POEGMA)). After exposure in benzene solution the morphology of these

materials is interrogated by X-ray scattering and TEM, finding that nanoscale features

can be generated with this technique. Unfortunately, bicontinuous morphologies remain

elusive due to significant chain scission in PEG and unresolvable morphology in

POEGMA.

3.4.1 Experimental

Materials and Sample Preparation

Poly(styrene-co-acrylamidobenzophenone) (PS-BP) and poly(oligoethylene

glycol methacrylate) (POEGMA) were synthesized as discussed in Chapter 2. PS-BP

exhibited Mn = 55,000 g/mol (Ð = 1.3) and xAmBP = 0.05. POEGMA was prepared from

poly(ethylene glycol) methyl ether methacrylate (Aldrich, Average Mn = 475 g/mol) and

di(ethylene glycol) methyl ether methacrylate (Aldrich) in 1,4-dioxane yielding liquid

polymer with Mn = 13,400 g/mol (Ð = 1.69) and 8.6 mol% monomer with PEG side

chains. Poly(ethylene glycol) (PEG) with molecular weights ranging from Mn = 2,050 to

35,000 g/mol were purchased from Aldrich and used as received. Benzophenone (>99%)

was purchased from Sigma Aldrich, benzene was purchased from TCI America, while

70

cumene (99.9%) and extra dry ether (99.5%) over molecular sieves were purchased from

Acros Organics. All reagents were used without further purification.

Solution-state reactive blending was performed by preparing concentrated

solutions of PS-BP and PEG in benzene and mixing in an appropriate ratio such that both

PS-BP and PEG were present at 40 mg/mL (Figure 3.6). This solution was then filtered

through a 0.45 μm PTFE syringe filter and injected to a capillary tube fitted with a

needle. The capillary tube is then sealed with a Luer-lok three-way stopcock attached to

the needle and degassed by three freeze-pump-thaw cycles with liquid nitrogen followed

by a dry nitrogen purge. Samples were then exposed to 365 nm ultraviolet irradiation

(same source as Chapter 2, ~2 mW/cm2) at a distance of 1 cm from the lamp surface for a

given time. After exposure, capillary tubes were broken and the contents cast onto glass

slides, removing solvent. These samples were then annealed under vacuum at 130 °C for

at least 4 hours prior to characterization.

UV/Visible Absorption Spectroscopy

Benzophenone, cumene, and diethyl ether solutions were prepared in benzene and

diluted to a final 43 mM concentration of each species prior to injection into 2 mm path

length quartz cuvettes fitted with ground PTFE stoppers to limit evaporation of solvent.

Three samples (benzophenone alone, benzophenone with cumene, benzophenone with

ether) were exposed to UV (1.58 mW/cm2 at plane of exposure) for different times and

the benzophenone π-π* peak maximum at 336 nm was monitored as a function of UV

exposure time with a Hitachi U-3010 spectrophotometer in absorbance mode over a

wavelength range of 200 - 500 nm. Decays were then normalized to the same maximum

absorbance at 0 s.

71

Figure 3.6: Experimental technique for reactive blending. i) PS-BP copolymer and either

PEG or POEGMA (each polymer at 40 mg/mL) are dissolved in benzene, and ii) are

injected into a capillary tube, degassed, and exposed to 365 nm UV for a prescribed

irradiation time. After exposure the solution is cast onto a glass slide and annealed prior

to characterization.

Morphological Characterization

Annealed samples were embedded in epoxy and cured at room temperature prior

to preparing 70 nm sections with a Leica Ultracryomicrotome at -90 °C. Grids were then

treated with ruthenium tetroxide vapor for 40 min to provide sample contrast.

Transmission electron microscopy was performed using a JEOL 2000FX 200 kV

transmission electron microscope with a lanthanum hexaboride filament in bright field

mode. Small angle X-ray scattering was performed on powder samples with the same

setup as Section 3.3.

Size Exclusion Chromatography

Molecular weights were analyzed via size exclusion chromatography (SEC) with

a Polymer Laboratories GPC-50 with a polystyrene column and THF as eluent (0.5

72

mL/min). Refractive index and UV detectors were simultaneously used, and molecular

weights were calibrated with poly(styrene) standards. Refractive index signals have been

magnified by 25X in reported data.

3.4.2 Results and Discussion

Solution state asymmetric photo-grafting between PS-BP and five different PEG

homopolymers and a single POEGMA homopolymer were investigated by a combination

of scattering, microscopy, and chromatography techniques. Blends were exposed to 365

nm UV for between 10 and 100 min prior to casting and annealing, in most cases yielding

solid films which could then be either redissolved for SEC or scraped off as powder for

TEM and SAXS. In several cases, particularly at high NPEG samples gelled in capillary

tubes and could not be further analyzed. Benzophenone-containing poly(styrene) and

PEG-containing polymers were chosen because of the anticipated requirement for

successful preparation of nanostructures, namely that the preference for hydrogen

abstraction by benzophenone (BP) should be highly asymmetric to promote A-B grafting

and prepare interfacially active species. As discussed in Chapter 2, PS-BP exhibits the

highest characteristic dose for BP conversion, and as such represents the polymer that is

least likely to undergo self (A-A) reaction. Combining PS-BP with PEG-containing

homopolymers, each of which contain a large number of aliphatic hydrogen located alpha

to oxygen (highly reactive hydrogen species for BP abstraction112

), we should thus

promote A-B grafting from the PS-BP to the homopolymer. We must also consider

results from Chapter 2, which indicate that abstraction of hydrogen from PEG will

73

potentially result in chain scission. It is hoped that this will prove to be inconsequential

for our system, yet we investigate this effect in detail later in this section.

To ensure this desired reactivity, small molecule analogues for the polymers of

interest were first investigated by UV/Visible absorption spectroscopy. Cumene and

diethyl ether were selected for investigation, as each molecule should behave in a similar

fashion as a single repeat unit of the corresponding polymer. Based on reactivity of

functional groups to hydrogen abstraction by BP, cumene (containing a single tertiary

benzylic hydrogen with much less reactive primary and aromatic hydrogen) should be

close in reactivity to a styrene repeat unit while diethyl ether (with four hydrogen alpha to

an oxygen molecule with less reactive primary hydrogen) is analogous to an ethylene

glycol. Solutions were prepared in benzene, which should be relatively inert to hydrogen

abstraction by BP, and decay of the π-π* peak absorbance of BP was monitored as a

function of UV exposure time, as shown in Figure 3.7. Clearly evident from these results

is that conversion of BP is slowest with benzene as a hydrogen abstraction substrate,

increases with the addition of cumene, and is still more rapid in the presence of diethyl

ether. This result is consistent with a previous report stating that hydrogen abstraction

from benzene by BP is minimal113

. Considering that the abstraction rate for cumene has

earlier been shown to be equal to that of oligostyrenes in acetonitrile, and since

oligostyrenes were shown to quench BP triplet three times faster than cumene, it is

reasonable to believe that PS-BP will not readily react with itself114

. Therefore, it is likely

that reaction of PEG with PS-BP will be much faster than reaction of PS-BP with itself,

creating A-B grafted species and making this system a candidate for formation of blends

with nanoscale morphology.

74

Figure 3.7: UV/Visible absorption spectra of small molecule analogs at 45 mM

concentration in benzene. Benzophenone (black) shows the slowest decay while

incorporation of cumene with benzophenone (red) decays faster and diethyl ether with

benzophenone (blue) decays the most rapidly, showing that benzophenone reacts most

rapidly with the ether species. Decay constants are extracted from single exponential fits.

Polymer systems consisting of PS-BP and PEG in benzene with various molecular

weights were investigated following this initial small molecule study. Across the entire

range of samples, PS-BP concentration was maintained below the overlap concentration

at 40 mg/mL to prevent gelation of the PS-BP matrix115

. As an equal concentration of

PEG was included in each sample to maximize the opportunity for connectivity9 (i.e.

constant number of monomers per volume), the number of PEG chains was decreased

with increasing PEG molecular weight, causing gelation to more readily occur in samples

with higher NPEG (fewer reactions are required for one crosslink per chain). After UV

irradiation soluble samples were dried, placed into quartz capillary tubes and subjected to

SAXS as a first check to see if any nanoscale morphology was present. Examples of

azimuthally averaged 1-dimensional SAXS profiles exhibiting a scattering peak are

75

shown in Figure 3.8, each with a very poorly defined scattering peak located at d = 13 nm

for this particular PEG (Mn = 6,000 g/mol). Samples which exhibited any resolvable

scattering peak were considered to have nanoscale domains, with results plotted in Figure

3.9. As is plainly seen in Figure 3.9, the exposure time required to achieve nanoscale

features decreases while the length scale of these features increases with increasing NPEG.

The exposure time dependence is caused by a more rapid decrease in the amount of

ungrafted materials as PEG molecular weight increases. The observed increase of domain

spacing in Figure 3.9b is also expected, as the volume fraction of PEG in graft

Figure 3.8: SAXS traces for reactive blends of 1/1 PS-BP /PEG6K Mn exposed to 365 nm

UV for different durations. In the exposure range 65 – 90 minutes annealed powder

samples exhibit scattering peaks, indicating that nanostructures are indeed present.

76

copolymers will increase with higher molecular weights. Additionally, where

BPPSPEG NN , the domain spacing scales as 21

~ Nd , or with the radius of gyration of

the PEG polymer chains116

. Where BPPSPEG NN , the domain spacing is maintained at

19 nm, potentially indicating that at this high volume fraction, PEG homopolymer is

excluded from the nanoscale domains and some macrophase separation has occurred108

.

The absence of higher ordered peaks in SAXS indicates that long-range order is not

present in these samples and as such lamellae have not been formed. Further, because the

mass fraction of polymer in these samples is 5.0 PEGBPPS , it is possible that a

bicontinuous morphology has been prepared.

Figure 3.9: Reactive blending results for 1/1 blends of PS-BP /PEG; a) Exposure times

required to acquire scattering peaks in SAXS for different molecular weights of PEG,

showing that with increased PEG molecular weight the exposure time required is

diminished. b) Length scales observed by SAXS for blends with varying PEG molecular

weights.

77

To further interrogate the morphology of these photo-grafted PS-BP/PEG blends,

transmission electron microscopy was utilized, with a representative image of a blend

with 000,6, PEGnM g/mol shown in Figure 3.10. It is clear from this image that there

are two length scales of phase separation in this material, with structure on both the 10

nm and 100 nm length scales. The shorter of the two corresponds well with X-ray

scattering results which showed features at 5.11~d nm, however the resulting

morphology is certainly not bicontinuous, as the features are not present uniformly across

the sample.

To better understand this photo-grafting process, we next examined the

progression of molecular weight in this blend system using a combination of refractive

index (RI) and ultraviolet (UV) detection in size exclusion chromatography. Because

PEG will not provide signal from the UV detector, combining the two detectors allows us

to distinguish between polymers containing poly(styrene) and those without it. Figure

3.11 shows RI SEC traces for pure PS-BP (black) and 6,000 g/mol PEG (red), with both

RI and UV traces for a blend exposed to 365 nm UV for 60 minutes (in green and blue,

respectively). Evidence of graft copolymer formation is present via the slight increase in

molecular weight observed at low retention time in both RI and UV signals of the blend,

however there is also evidence of chain scission in this material system. Examining the

blend traces at high retention times, we see a significant signal from the RI detector

without a corresponding signal from the UV detector. This indicates that the fraction of

sample at this molecular weight does not contain poly(styrene), and thus must be PEG.

Comparing the position of this feature to the neat PEG homopolymer, one sees clearly

that PEG molecular weight is decreased by exposure of the blend to UV. This can be

78

Figure 3.10: TEM image of a 1/1 PS-BP/PEG6K blend which has been exposed to 365

nm UV for 60 minutes. A hierarchical phase separation is observed, with large domains

on the order of 100 nm containing smaller features on the 10 nm length scale,

corresponding well with SAXS results indicating 11.5 nm features. This sample has been

cryo-microtomed into 70 nm thick sections at -90 °C and stained with RuO4 vapor for 40

minutes prior to imaging.

explained by considering the results of Chapter 2, which stated that abstraction of

hydrogen from the main polymer chain will often lead to a dislinking event. As PEG

contains only main-chain hydrogen species, it comes as no surprise that a significant

79

fraction of photo-reaction in this blend system is main chain PEG scission. For effective

solution state photo-grafting with BP copolymers one should encourage hydrogen

abstraction on a side group of the polymer.

Figure 3.11: SEC traces of a 1/1 blend of PS-BP/PEG6K and the corresponding individual

polymers. The black and red curves are the PS-BP copolymer and PEG homopolymer,

respectively, while the green and blue curves represent refractive index and ultraviolet

signal from the polymer blend after 60 minutes of UV exposure. The refractive index

detector peak (green curve) at high retention times in the blend indicates a fraction of

PEG which has decreased molecular weight compared to the homopolymer. This is

indication of main chain scission during irradiation. Refractive index curves magnified

25X.

With this in mind we then alter the blend system to promote hydrogen abstraction

on the homopolymer side chain by incorporating PEG functional groups into the side

chains of a methacrylate based polymer, which we call poly(oligoethylene glycol

methacrylate) or POEGMA. Performing the same SEC analysis with a blend system of

80

PS-BP and POEGMA exposed to UV for 70 min, we see in Figure 3.12a that the

molecular weight of the blends exhibits a peak at lower retention time than the

corresponding homopolymers, indicating the presence of higher molecular weight

species. Because the RI and UV traces of the blend agree well with one another, it can be

concluded that these polymers contain both PS-BP and POEGMA, and thus graft

copolymers have been successfully prepared.

Figure 3.12: SEC traces of a 1/1 blend of PS-BP/POEGMA and the corresponding

individual polymers. a) the black and red curves are the PS-BP and POEGMA

copolymers, respectively, while the green and black curves represent refractive index and

ultraviolet signals from the polymer blend after 70 min of UV exposure. Unlike the

PS/PEO blends, this material clearly shows an increased molecular weight with UV

exposure. Refractive index curves have been magnified 25X. b) Progression of molecular

weight distribution (UV signal) with exposure time for this blend signal. A peak at higher

molecular weight arises between 50 and 70 min of exposure, and disappears by 90 min,

indicating the formation of insoluble species at high reaction times.

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More importantly, there is no significant signal at high retention time, and therefore the

POEGMA appears stable to hydrogen abstraction by PS-BP. Figure 3.12b shows the

progression of blend molecular weight monitored by UV detector from 50 to 90 min of

UV exposure. Slight variations in signal intensity of the primary peaks centered at 1200 s

are likely due to variation of solution concentration. The high molecular weight graft

peak is present in all samples, yet nearly disappears after 90 min of exposure. We believe

this to be due to further grafting of the polymers which creates species which are either

insoluble or filtered from the solution prior to sampling. SAXS investigation of these

blend systems did not yield any scattering, however, indicating that these blends do not

phase separate on the nanometer scale.

3.4.3 Conclusion

Graft copolymers have been formed in blends of PS-BP and either PEG and

POEGMA by a solution state photo-grafting process. While nanostructured materials

were observed by both SAXS and TEM in PEG blends, bicontinuous morphologies were

not achieved. SEC analysis revealed a significant amount of PEG chain scission due to

hydrogen abstraction by BP on the main polymer chain, an issue which appears to have

been remedied by POEGMA, whose abstractable hydrogen are on the side groups of the

polymer. This was encouraging for the PS-BP/POEGMA blends, however they

unfortunately did not exhibit any phase separation on the length scale probed by SAXS.

82

3.4.4 Solution-state Photo-Grafting Future Directions

While it is possible that increased reaction time may potentially increase the

number of grafted species formed in the PS-BP/PEG system, significant refinement of

reaction conditions would be required to overcome the chain scission and achieve the

desired morphology, with no guarantee of success. Much like in Section 3.3., there are

several different paths which could ultimately achieve the desired goal of a nanoscale

bicontinuous morphology by solution state photochemical reaction. The first possibility

involves utilizing a BP copolymer but altering the material system to give the greatest

possible chance of success. The same requirements must be met, namely that A-B

grafting is necessary and reactivity of BP with its parent polymer must be minimized,

therefore BP copolymers with very few (or no) abstractable hydrogen should be used.

Candidate materials could be poly(α-methyl styrene-BP), poly(methacrylonitrile-BP), or

more synthetically challenging copolymers which only contain aromatic or primary

hydrogen (i.e. BP copolymer prepared from α,β,β-trimethylstyrene). The corresponding

homopolymer should have an adequate number of abstractable hydrogens on the side

chains to encourage grafting and prevent scission of the main chain. Due to the

persistence of secondary hydrogen on the backbone of the majority of common polymers,

it is necessary that either tertiary hydrogen or hydrogen on carbon alpha to nitrogen or

oxygen be present on the side chains. Poly(N-isopropyl methacrylamide) or a more exotic

material containing alkylamine or alkanol groups could potentially yield the desired

results. Whenever deciding on a material system, care must be taken to ensure adequate

contrast for the characterization method of choice, preferably a high-throughput

scattering technique.

83

It may be of value to change the photochemistry used in this photo-grafting

process. Without a hydrogen abstracting or alpha-cleavage photo-reactive agent,

however, it will be necessary to functionalize both component materials of the blend,

eliminating the most attractive feature of the current technique – synthetic simplicity. If

one were to utilize photochemistries which undergo self cycloaddition (i.e. anthracene or

cinnamate esters), grafting would not occur and nanoscale domains are very unlikely to

be formed. Functionalizing both materials with complementary groups would make the

photo-grafting process much cleaner, as the reaction would be better defined than in the

case of BP copolymers. As an alternative photochemistry, UV initiated thiol-ene could be

an attractive option. This reaction is known to proceed quickly and with appropriate

concentration of polymer and functional group the desired bicontinuity could be

achieved.

3.5 Conclusions

The ability of benzophenone (BP) copolymers to form nanostructured

bicontinuous polymer blends was investigated, with limited success. Two approaches

were used in this study, and while nanostructured blends were realized, the ultimate goal

was not achieved. This is attributed to several factors, mainly generation of radicals on

the polymer backbone which induces chain scission, and the inability to characterize the

blends with a high throughput technique. Upon consideration of the above results,

copolymers containing pendent BP photo-crosslinkers are candidates for the formation of

nanoscale bicontinuous morphology, yet significant refinement for each method is

necessary. Benzophenone as a UV-active agent is certainly capable of providing A-B

84

crosslinking and grafting in a critical polymer blend and solution, respectively, but a

significant limitation exists: in order to prepare the desired blend morphology significant

synthetic challenges must first be overcome. Unfortunately this negates a very attractive

feature of the as-synthesized BP copolymers, in that they are very simple to prepare via

conventional free radical chemistry. If more complex chemistry is to be undertaken there

is no reasonable need to use benzophenone as the UV active chromophore, and much

more convenient photochemistries can (and probably should) be used.

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93

111. Sundararaj, U. & Macosko, C. W. Drop Breakup and Coalescence in Polymer

Blends: The Effects of Concentration and Compatibilization. Macromolecules 28,

2647–2657 (1995).

112. Topp, M. R. Activation-controlled hydrogen abstraction by benzophenone triplet.

Chem. Phys. Lett. 32, 144–149 (1975).

113. Dedinas, J. Photoreduction of benzophenone in benzene. I. Mechanism of

secondary reactions. J. Phys. Chem. 75, 181–186 (1971).

114. Horie, K. et al. Rates of Quenching and Hydrogen Abstraction of Benzophenone

Triplet with Cumene and Oligostyrenes. Polym. J. (Tokyo, Jpn.) 16, 887–893

(1984).

115. Hadgraft, J., Hyde, A. J. & Richards, R. W. Diffusion of Polystyrene in

Poly(Methyl Methacrylate)+Benzene Solutions Measured by Photon-Correlation

Spectroscopy. Trans. Faraday Soc. 75, 1495–1505 (1979).

116. Rubinstein, M. & Colby, R. H. Polymer Physics. (Oxford University Press, Inc:

New York, 2003).

94

CHAPTER 4

CHEMICAL DEACTIVATION OF BENZOPHENONE IN PHOTO-

PATTERNED HYDROGELS

4.1 Abstract

Benzophenone copolymers offer an attractive route to patterned hydrogels,

however residual unreacted photo-crosslinker present in these systems causes these

materials to be unstable to further UV radiation. To remedy this issue, deactivation of

residual benzophenone photo-crosslinker with sodium borohydride was monitored by

UV/Vis and hydrogel disc swelling. Nearly complete deactivation was seen within five

minutes in a mixture of ethanol and water, and treated hydrogels recovered on average

98% of initial swelling after full conversion of AmBP. These results indicate that UV-

stable hydrogels can be prepared from benzophenone based photo-crosslinkable hydrogel

materials by reduction with NaBH4. This opens up the possibility of preparing

independently photo-patterned multilayers, and as proof-of-concept a bilayer was

prepared from photo-crosslinkable poly(N-isopropylacrylamide) and poly(p-

methylstyrene), demonstrating promise for complex photo-patterned hydrogel systems.

95

4.2 Motivation

Hydrogels have been intensely studied for many years, due in large part to their

biocompatible, non-fouling, and non-cytotoxic nature. The high porosity present in these

aqueous gel networks allows for high loading of functional material, making hydrogels a

versatile basis for various applications1–5

. Furthermore, many hydrogels are known to be

stimulus responsive, changing material properties with variation of temperature, pH,

pressure, humidity, or electromagnetic stimulus1–3,6–9

, and as such hold great potential for

actuating and sensing devices, piezoelectric current generation10

, microfluidic devices9,

and the mimicking of muscle tissue11

, among many other applications12

.

While the majority of stimulus-responsive hydrogels rely upon critical phase

behavior or changes in osmotic pressure, hydrogel properties can also be tuned by

changing the density of crosslinks in the polymer network. This serves to shift the

equilibrium swelling, and is experimentally realized by either changing the fraction of

multifunctional monomer during hydrogel formation or by some post-synthetic method,

such as chemical exposure, thermal treatment, or photo-processing4. Of these options,

photo-reaction is the most attractive, as it is an accessible, scalable, and low cost

technique which allows fine tuning of crosslink density in hydrogels6,13–16

. An added

benefit of post-synthetic photo-crosslinking is that it allows for simple fabrication of thin

hydrogel films by either spin coating or drop casting prior to gelation of the network.

This reduction of thickness to the micron-scale greatly increases response time, as

swelling and deswelling are typically dominated by poroelastic relaxation, yielding

diffusive scaling6.

96

Post-synthetic photo-crosslinking has therefore been utilized in many hydrogel

studies by preparing linear copolymers containing photo-reactive monomers. The most

commonly used reagents are activated by ultraviolet radiation, and crosslink by

cycloaddition 6,17–19

, hydrogen abstraction1,3,5,7,14–16,20–22

, or reaction of phenyl azide23–25

.

Spatial variation of UV dose in these hydrogel materials has proven capable of producing

significant swelling variations in a single sample9,14,15

, yet in the absence of full

conversion these materials are susceptible to further reaction by additional radiation. In

the case of three-dimensionally morphing hydrogels, this excess reaction would

homogenize crosslinking in the sample, eliminating function by the removal of

differential swelling14,15

. It is therefore attractive to deactivate residual crosslinker that

remains after photo-patterning in order to obtain environmentally robust patterned

hydrogels onto which additional layers can be independently photo-reacted without

concern for the initial film.

In this work we investigate the possibility of preparing stable photo-patterned

hydrogel materials by selective deactivation of residual crosslinker. Poly(N-

isopropylacrylamide)-based hydrogel materials with copolymerized

acrylamidobenzophenone (AmBP) photo-crosslinker are studied, as their material

properties and UV crosslinking are well known. To selectively deactivate AmBP, we

utilize sodium borohydride, a mild and selective reducing agent for ketones and

aldehydes26–28

. Deactivation is monitored by UV/Visible absorption spectroscopy and

swelling changes of hydrogel discs. Finally, as a proof-of-concept that this technique can

be applied to multilayer fabrication methods, a bilayer photo-patterned polymer film is

97

fabricated from deactivated poly(N-isopropylacrylamide-AmBP) hydrogel and glassy

poly(p-methylstyrene-AmBP).

4.3 Experimental

Materials and sample preparation

Benzophenone copolymers were synthesized as in Chapter 2 and previous

publications14,15,29

. In brief, N-isopropylacrylamide (NiPAm), acrylic acid (AAc),

acrylamidobenzophenone (AmBP), and rhodamine B methacrylate (RhBMA) were

copolymerized via conventional free radical polymerization with AIBN initiator in

1,4-dioxane. After reaction overnight at 75 °C, copolymers were recovered by

precipitation in diethyl ether followed by drying under vacuum, yielding two copolymers

with NiPAm-AAc-AmBP-RhBMA molar compositions of 91.6 - 3.6 - 4.8 - 0 and

86.7 - 6.4 - 6.6 - 0.3, respectively. Poly(p-methylstyrene-AmBP) was prepared similarly,

and recovered by precipitation in cold methanol. Reactants were purchased from Sigma

Aldrich, with the exceptions of AmBP and RhBMA, which were synthesized according

to published protocols15

.

Silicon wafers were cleaned by sonication and subjected to UV-ozone treatment

for 30 minutes prior to coating with a Ca2+

-ion crosslinked poly(acrylic acid) sacrificial

layer (Sigma Aldrich, 000,30wM g/mol)30

. 100 μL of 1 wt% PNiPAm copolymer

solution was then drop cast onto coated substrates and solvent removed at 60 °C

overnight in a closed glass container to yield solid polymer films. These films were then

photo-crosslinked through a photo-mask with a Zeiss Axiovert 200 epi-fluorescence

microscope-mounted UV source (365 nm) for a prescribed radiation dose, as monitored

98

by an X-Cite XR2100 Power Meter (EXFO). Uncrosslinked material was then developed

in a 2:1 ethanol:water mixture, yielding surface-attached PNiPAm gels. Immersion of the

wafer in an aqueous 1 mM phosphate buffer containing 1 mM NaCl (hereafter

“phosphate buffer”) dissolved the poly(acrylic acid) sacrificial layer, permitting the

crosslinked gel material to release from the surface and swell.

UV/Visible monitoring of AmBP conversion

To ensure the ability of NaBH4 to convert AmBP, solid NaBH4 (Fisher Scientific)

was first added to a quartz cuvette followed by a 2 mg/mL solution of PNiPAm-AAc-BP

copolymer in either ethanol or a mixture of 2:1 ethanol:water. Conversion of AmBP was

monitored as a function of time by observing the absorbance of AmBP at 300 nm with a

Hitachi U-3010 spectrophotometer in absorbance mode. Spectral profiles were also

obtained over a wavelength range of 200 - 500 nm both prior to and after reaction.

Chemical deactivation of AmBP in hydrogel discs

After development of PNiPAm-AAc-AmBP discs with 400 μm diameter,

deactivation of AmBP was performed by adding solid NaBH4 to a poly(styrene) petri dish

prior to the addition of 250 μL of phosphate buffer (Figure 4.1, step ii). Samples were

then immersed, releasing the discs from the substrate, and allowed to react for 90 min. At

this time the solution was diluted with fresh phosphate buffer such that the ionic strength

of NaBH4 was less than 3% of phosphate ionic strength. Samples were then allowed to

swell at room temperature in the phosphate buffer at least 90 min and imaged on an

Axiovert 200 epi-fluorescence microscope in DIC mode (Figure 4.1, step iii). Subsequent

heating of the samples to 60 °C crosses the LCST of PNiPAm, expelling the aqueous

phase and causing the hydrogel discs to deswell. Samples were irradiated overnight (>8

99

h) with a low power UVGL-58 Handheld UV lamp operating at 365 nm (UVP; ~ 2

mW/cm2, spectral range 340 - 410 nm), reacting any remaining AmBP in the hydrogel

(Figure 4.1, step iv). Finally the hydrogel discs were cooled to room temperature and

swelled for at least 90 min prior to imaging their final dimensions in DIC mode (Figure

4.1, step v). Data points reported are for one exposure which yields anywhere from 5 - 9

hydrogel discs for imaging, and errors are reported as one standard deviation.

Figure 4.1: Floating hydrogel disc deactivation. i) PNiPAm-AAc-AmBP film is exposed

to UV through a mask and developed. ii) The sacrificial layer is dissolved in an aqueous

1 mM phosphate buffer with 1 mM NaCl. Deactivation is achieved through inclusion of

solid NaBH4 to a concentration of 0.15 M. iii) Concentration of NaBH4 is decreased such

that ionic strength of the buffer is at least 30X greater than that of NaBH4. The samples

are permitted to swell at room temperature and are imaged, yielding discs with area A1.

iv) Samples are heated through the LCST to 60 °C, deswelling the discs. They are then

subjected to UV exposure overnight in the deswelled state. v) The samples are swelled

once more at room temperature, yielding discs with area A2.

100

Preparation of multilayer film

PNiPAm-AAc-AmBP-RhBMA films were cast on a 1 cm2

wafer as described

above and photo-patterned with a series of two masks, yielding an exposed 800 x 100 μm

rectangle. One half (400 x 100 μm) of the rectangle was subjected to a UV dose of 100

J/cm2 while the other was given 10 J/cm

2. After development in 2:1 ethanol:water,

deactivation was performed by immersion in a series of solutions: (1) 1 M CaCl2 in 2:1

ethanol water, (2) 1 M CaCl2 + 0.15 M NaBH4 in 2:1 ethanol:water, and (3) 2:1

ethanol:water with no salt present. Calcium chloride solution served to stabilize the

underlying sacrificial layer to dissolution by sodium borohydride, and rinsing with clean

solvent prevented the deposition of salts on the sample. The sample was immersed in

each solution for 15 seconds followed by a 30 second drying step in between to prevent

excessive swelling of the sample. This process (3 immersions with 3 drying steps) was

repeated five times for a total of 75 seconds immersed in the NaBH4 deactivation

solution. After drying any remaining solvent, a poly(p-methylstyrene-AmBP) thin film

was spin-cast onto the sample (40 mg/mL in chloroform, 2000 rpm, 1000 ramp, 60 s). A

slightly larger rectangular area (900 x 200 μm) was aligned to the previously developed

fluorescent film and exposed to 100 J/cm2 UV dose and developed in a 1:0.65

toluene:hexanes developer, creating a rectangular bilayer consisting of PNiPAm-AAc-

AmBP-RhBMA and PpMS-AmBP. Immersing this sample in the phosphate buffer

permitted release from the substrate and subsequent fluorescence imaging.

101

4.4 Results and discussion

Chemical reduction has been utilized in an attempt to prepare photo-patterned

hydrogels which are stable to subsequent UV irradiation. To achieve this goal sodium

borohydride has been chosen as reducing agent, as it is well known to selectively reduce

ketones and aldehydes, including benzophenone. Since our copolymer system is a

complex combination of different functionalities (amide, carboxylic acid, etc.) and

benzophenone is a reasonably stable biaryl ketone with extended conjugation, we first

must confirm the selectivity and efficacy of NaBH4 in benzophenone reduction in our

copolymers. Primary investigation was made via UV/Visible absorption spectroscopy.

Solid NaBH4 was added to the quartz cuvette prior to injection of polymer solution to

limit borate formation26

(and thus neutralization of the reducing species) prior to

interaction with AmBP. Monitoring the decay of the π-π* transition of AmBP (300 nm)

as shown in Figure 4.2a, we observe a pronounced decrease in absorbance, indicating

effective reduction of the photo-crosslinker. In ethanol, this process nears completion

(>90 % absorbance decrease) after 2,500 s, while a mixture of 2:1 ethanol:water requires

only 300 s to achieve a similar conversion. This increased rate of reaction is due to

inclusion of water, which is more protic than ethanol and thus promotes NaBH4 reduction

by acting as a more effective hydrogen donor31

. The inset to Figure 4.2a shows UV

spectra of the copolymer solution in 2:1 ethanol:water both before and after reaction, and

it is clearly seen that the AmBP absorbance at 300 nm has disappeared after reaction with

a corresponding increase of a different species at 250 nm, likely the diphenyl methanol

product32

. This chemical reduction is not accompanied by any molecular weight

variation, as evidenced by the nearly identical SEC traces of both NaBH4-treated and

102

-untreated samples in Figure 4.2b. Reduction with NaBH4 therefore does not significantly

degrade the polymer chain, and when combined with the lack of any additional

absorbance peaks in the post-reaction UV spectrum it appears that NaBH4 is a selective

and effective reducing agent for AmBP in our acrylamide-based copolymer systems. We

note that while UV spectra would be insensitive to reduction of amide or carboxylic acid

in our materials, reaction of these functionalities with NaBH4 is rare26–28

.

Figure 4.2: Deactivation of AmBP in PNiPAm-AAc-AmBP copolymer as studied by

UV/Vis and SEC. a) The inset shows spectra of polymer solution prior to and post

deactivation with 0.15 M NaBH4 (in 2:1 ethanol:water), showing the disappearance of

AmBP absorbance at 300 nm with a corresponding rise of photo-product at 250 nm.

Monitoring absorbance at 300 nm during deactivation in we observe >90% conversion

within 2500 s in ethanol solution, yet in a 2:1 ethanol:water mixture deactivation is

achieved within 300 s; b) SEC traces of polymer solutions prior to and after treatment

with NaBH4. The curves are nearly identical, indicating that treatment does not

significantly influence molecular weight distribution of the copolymer.

103

PNiPAm-AAc-AmBP hydrogel discs were then prepared by photo-crosslinking

400 μm diameter discs with different UV doses, as described above. After development

these samples were released into phosphate buffer, and their swelling monitored after at

least 90 min of immersion. In samples that were treated with NaBH4, swelling was

observed at least 90 min after dilution with fresh phosphate buffer. Results for discs that

were UV exposed to a dose between 3 and 100 J/cm2 are found in Figure 4.3, with areal

swelling defined as the swelled area (A1) divided by the initial mask area (A0). Swelling

ratios between 2.5 and 6 are obtained for these samples, with little difference between

samples treated with NaBH4 and those without. The consistency of the two swelling

Figure 4.3: Swelling curve of PNiPAm-AAc-AmBP hydrogel discs after one exposure.

Areal swelling is calculated as A1/A0, with A0 the area of the initial photo-mask. The

minimal difference in swelling between samples with and without NaBH4 treatment is

indication that the deactivation process does not disturb the existing hydrogel network.

104

curves, when combined with the clean UV and SEC experiments performed above,

seemingly indicate that NaBH4 treatment does not have any adverse effect on the

PNiPam hydrogel network, at least insofar as it influences the equilibrium swelling

behavior of the material. We note that while it is possible to prepare hydrogels at 1 and 2

J/cm2 UV dose, these samples no longer retain the shape of the photo-mask and are

poorly defined, likely indicating that these doses are close to the gel point for the system.

Figure 4.4: Final versus initial swelling for PNiPAm-AAc-AmBP hydrogel discs

subjected to the protocol depicted in Figure 4.1. By treating the hydrogel discs with

NaBH4 we are able to recover over 85% of the initial swelling over the entire first

exposure energy range. Conversely, we see that the sample without NaBH4 treatment was

only able to achieve a fraction of the initial swelling. At high energy the curves intersect

due to conversion of all AmBP photo-crosslinker in the first UV exposure step.

105

After the initial swelling, hydrogel discs are then heated (deswelled) and exposed

to UV radiation overnight to react any remaining AmBP in the copolymer. Cooling the

discs to room temperature and reswelling for at least 90 min, we are then able to measure

the equilibrium swelling area A2 and compare to the initial swelling, with A2/A1 reported

in Figure 4.4. This quantity represents the fraction of initial swelling recovered after UV

exposure, and should be a good indicator of whether AmBP has been deactivated. It is

clear that samples which have not been treated with NaBH4 recover significantly less

swelling than those which have been treated. In fact, over 50 individual discs which have

been swelled in the presence of NaBH4 recover an average of 98% of their initial

swelling across the entire range of UV doses investigated, with a minimum average

recovery of 92% for any given initial UV dose. Conversely, untreated samples can

recover as little as 25% at low initial UV dose. The gap between treated and untreated

samples narrows with increasing first exposure dose because at high energy nearly all the

AmBP has been reacted in the initial crosslinking step, limiting crosslink formation

during the second exposure step. At lower than 3 J/cm2, significant sample to sample

variation was observed, again likely due to the fact that at this low dose the sample is

very near its gel point. This is strong evidence that sodium borohydride effectively

reduces AmBP photo-crosslinker to prepare UV-stable hydrogel films.

Estimation of extent of reaction after UV irradiation

To estimate the extent of AmBP reaction in these discs, we now consider the

theory for equilibrium swelling of a polyelectrolyte gel, as discussed by Rubinstein and

Colby33

. In this work, the equilibrium swelling of a polyelectrolyte gel without salt in the

preparation state scales as

106

KNQ (4.1)

where Q is the equilibrium volumetric swelling ratio, K encompasses material constants,

N is the degree of polymerization between crosslinks, and α an exponent which depends

upon the salt concentration of the swelling solution ( 23 and 53 in the low and high

salt limits, respectively). Our materials fall near the low salt limit as the concentration of

charged monomer is significantly higher than that of salt in the swelling medium. For our

AmBP copolymer gels, the degree of polymerization between crosslinks can be

calculated as

pxN

AmBP

1 (4.2)

with AmBPx the molar fraction of AmBP in the copolymer and p the extent of reaction of

the photo-crosslinker. By combining equations (4.1) and (4.2) we determine that in our

copolymer gels the extent of reaction can be estimated by the equilibrium swelling as

32

32

1

iAmBP

i

QKxp . (4.3)

The material constant K is calculated by considering that after 50 J/cm2 the material is

fully crosslinked ( 1p ), and the average areal swelling ratio for higher energies (Q for

50 – 100 J/cm2) is 2.64 ± 0.09. We then calculate the extents of reaction achieved after

the first ( 1p , Figure 4.1 step iii) and second ( 2p , Figure 4.1 step v) UV exposures,

reporting these results in Figure 4.5.

As observed in Figure 4.5a, extents of reaction after the first UV exposure range

from 0.58 to 1.0 as energy increases, with little difference between samples that have

been treated with NaBH4 and those that have not. Following the second UV exposure

107

step, extents of reaction 2p remain nearly identical for NaBH4 treated samples, while

values increase substantially for the control discs (Figure 4.5b). Extents of reaction for all

control discs are estimated as greater than one, which likely represents limitations of both

the experimental procedure and the theoretical model. At low first exposure energies, the

values of 2p are especially high, which we believe is due to the hydrogel discs having

Figure 4.5: Extent of reaction of AmBP photo-crosslinker in PNiPAm hydrogel discs

following each UV exposure. Values are calculated based upon equilibrium swelling

theory of a polyelectrolyte gel in the low salt limit33

. a) After the first UV exposure,

extent of reaction 1p ranges from 0.59 to 1.0, and results are consistent between samples

treated with NaBH4 and controls; b) After deswelling and subsequent UV irradiation,

extent of reaction 2p for samples that have been treated with NaBH4 are nearly identical

to their 1p values, while controls exhibit significantly higher values, further evidence that

reduction with NaBH4 prevents crosslinking upon subsequent UV irradiation in these

photo-patterned hydrogels.

108

some sol fraction which is extracted prior to the deswelled exposure. What is clear,

however, is that the observed difference in 2p between samples treated with and without

NaBH4 indicates that NaBH4 is an effective agent for deactivation of AmBP photo-

crosslinker, and may potentially enable the preparation of complex UV-stable hydrogel

materials composed of multiple, independently photo-patterned layers.

Finally, as proof-of-concept that NaBH4 chemical reduction can be integrated to a

multistory fabrication procedure, we prepared a multilayer strip containing two photo-

crosslinkable copolymers with the same AmBP photo-crosslinker. For this experiment,

fluorescent PNiPAm-AAc-AmBP-RhBMA was first photo-patterned, yielding a 800 x

100 μm rectangle with two distinct UV doses (each half subjected to either 100 or 10

J/cm2) within the uncrosslinked polymer film matrix. This substrate-bound film was then

subjected to repeated 15 s immersions in solutions prepared with 2:1 ethanol:water to

sequentially stabilize the sacrificial layer, deactivate AmBP, and remove residual salt

from the sample surface. A 30 s dry was included between each step to limit swelling of

the film. After this process, development was completed in a neat 2:1 ethanol:water

mixture to yield a rectangular hydrogel feature on the surface. It should be noted that

despite our efforts the portion of the hydrogel film with a lower crosslinking dose (10

J/cm2) underwent extensive swelling during development, as seen in Figure 4.6i and iii.

Poly(p-methylstyrene-AmBP) was then spin cast onto the wafer and a larger rectangular

photo-mask (900 x 200 μm) aligned to the fluorescent PNiPAm feature for an exposure

of 100 J/cm2 over the entire region. A larger mask was required in this step in order to

restrict swelling over the entire hydrogel surface. If photo-crosslinker in the underlying

film were not deactivated, this final UV exposure would fully crosslink the entire

109

PNiPAm film, removing any swelling contrast and yielding a bilayer with homogeneous

properties over its area. This is exactly what is seen in Figure 4.6i-ii, where upon swelling

very little change is noticed from 55 °C to 30 °C, indicating that photo-crosslinker in the

PNiPAm layer has fully crosslinked over the entire sample. If, however, the photo-

crosslinker has been effectively converted to a nonreactive product the swelling contrast

will remain in the hydrogel layer and the bilayer will demonstrate different properties in

each half of the rectangle. This is precisely what is shown in Figure 4.6iii-iv, as upon

cooling from 55 °C to 30 °C, one half of the strip curls significantly more than the other.

Figure 4.6: Fluorescence microscopy images of PNiPAm-AAc-AmBP-RhBMA/PpMS-

AmBP bilayers. The untreated bilayer in both the deswelled (i) and swelled (ii) states is

nearly flat. Importantly, no distinction in swelling contrast is observed between the halves

of this sample. The bilayer that has been treated with NaBH4 swells from the flat (iii)

state to a curled cylinder (iv) with a distinct difference between low and high swelling

areas, indicating successful deactivation of AmBP.

110

Shown in Figure 4.6v, one observes that the right side of the sample is nearly a complete

cylinder while the left side is mostly flat, indicating that a significant swelling contrast

remains in the sample. This is proof that chemical reduction of AmBP in the PNiPAm

hydrogel materials effectively prevents additional photo-chemical reaction due to

imparted UV, making these materials environmentally stable and permitting the

fabrication of multilayer materials with the same photo-chemistry.

4.5 Conclusion

Deactivation of benzophenone photo-crosslinker by reduction with sodium

borohydride was monitored by UV/Vis and hydrogel disc swelling. Nearly complete

deactivation was seen within five minutes in a mixture of ethanol and water, and treated

hydrogel recovered at least 85% of its initial swelling after full conversion of AmBP.

These results indicate that UV-stable hydrogels can be prepared by NaBH4 reduction of

residual unreacted AmBP groups in benzophenone based photo-crosslinkable hydrogel

materials. Finally, as proof-of-concept that this technique can be integrated to a

multilayer fabrication procedure, a bilayer was prepared from photo-crosslinkable

poly(N-isopropylacrylamide) and poly(p-methylstyrene), demonstrating promise for

complex photo-patterned hydrogel systems.

4.6 Future directions

While treatment with NaBH4 was moderately successful, this is certainly not an

ideal method for deactivation of benzophenone in these materials. First, it appears that

the NaBH4 deactivation process for photo-patterned hydrogels imparts some added stress

111

to the samples. This has been determined by first photo-patterning a hydrogel sample

which can be actuated from a target shape to a flat state by swelling and deswelling

through the LCST. Treatment with NaBH4 as in Figure 4.1 proved to affect this actuation,

as samples would not completely flatten upon deswelling. Additionally, the protocols

used to prepare multilayer materials are far from ideal, as swelling of a layer during

deactivation affects deposition and alignment of the next layer, and the sacrificial layer is

compromised by the presence of Na+ and the formation of diborane and hydrogen gases.

It is certainly a viable option for AmBP deactivation, yet practical use requires

modification of the sacrificial layer (preferably not ionically crosslinked) and

optimization of solvent conditions. NaBH4 is, however, still the best candidate for

chemically reducing AmBP as other reagents are likely too aggressive and would damage

the polymer network in addition to reducing benzophenone (treatment with KOH and

sec-butanol did just this).

If, however, one is willing to allow some amount of crosslinking to occur during

the deactivation process, incorporation of sacrificial hydrogen abstraction substrates may

be an attractive alternative to chemical reduction. This process consists of swelling the

crosslinked gel with a small molecule during exposure which will preferentially react

with the n-π* triplet state of AmBP. This converts residual AmBP, prevents formation of

an elastically active crosslink, and instead grafts a small molecule species to the gel

network. The table of hydrogen abstraction rates provided in Chapter 2 thus gives

guidance for selection of candidate materials for this process. Since PNiPAm contains

tertiary hydrogen alpha to an amide, we must find functional groups that will be more

reactive than this species, namely either secondary or primary hydrogen on the alpha

112

carbon to a nitrogen atom (the order of reactivity of hydrogen alpha to amine is reversed

from aliphatic groups: 1° > 2° > 3°). Small molecule amines are therefore attractive

candidate materials, and given the appropriate swelling of the hydrogel network could be

effective at benzophenone deactivation. In fact, preliminary experiments with

triethylamine show promise for AmBP deactivation. This process may not be completely

efficient, however, and it must be considered that amines with multiple hydrogen at the

alpha positions could lead to crosslinking in their own right by reaction with more than

one photo-crosslinker (assumedly concentration of these species would be high enough to

prevent crosslink formation).

Because this process would likely not be 100% efficient, it is also worth

considering the possibility of incorporating functionality to the small molecule that would

increase swelling in aqueous media. For instance, grafting of polar groups (i.e. carboxylic

or sulfonic acids) during benzophenone deactivation would serve to increase swelling of

the hydrogel, perhaps offsetting whatever extra crosslinks form. Amino acids are

especially attractive candidates for such reactions as they are water soluble and contain

both the polar group and candidate hydrogen for abstraction by benzophenone. One of the

simplest molecules to begin with is alanine, which has a single hydrogen alpha to amine,

which would prevent crosslink formation by multiple reaction. This hydrogen is also

located alpha to the acid group, however, and may not undergo the charge transfer

necessary for rapid abstraction. Instead, β-alanine may be more electronically favorable

for abstraction due to an ethyl spacer between amine and acid. The use of amino acids

would permit a systematic study of their influence in deactivation and also provide

113

evidence for the ability to tether different functionalities to previously crosslinked

hydrogels.

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116

CHAPTER 5

CONCLUSIONS

The goal of this dissertation was to both deepen and broaden our understanding of

copolymers with pendent benzophenone (BP) in relation to both established applications

and novel directions in materials science. Photo-reaction of these BP copolymers was

explored in attempts to achieve three distinct goals: (1) robust and efficiently photo-

crosslinkable solid polymer films, (2) photo-reacted polymer blends with disordered

bicontinuous nanostructures, and (3) photo-patterned hydrogel materials with

environmental UV stability. The work described herein makes it clear that copolymers

with pendent BP are a useful and versatile tool in materials science, however limitations

exist which restrict their efficacy in certain applications.

As described, copolymers with pendent BP are very synthetically accessible,

requiring only the preparation of a BP monomer and subsequent free radical

polymerization, yielding photo-reactive materials which abstract hydrogen from a

multitude of substrates. These represent the greatest benefits of BP copolymers: synthetic

ease and versatile reactivity, requiring no additional functionalization of complementary

materials for effective reaction. Photo-reaction proceeds via abstraction of hydrogen

followed by radical generation, which then initiates further reaction in multiple ways.

This nonspecific reactivity of BP is what largely serves to define practical and

impractical applications for copolymers with pendent BP, and this thesis demonstrates

both their advantages and limitations. In Chapters 2 and 4, wherein highly crosslinkable

films and photo-patterned hydrogels were prepared with BP copolymers, the reaction is

117

allowed to be somewhat undefined, and as long as an elastically active crosslink is

formed after radical generation, BP copolymers are an ideal tool. In Chapter 3, wherein

we attempt to crosslink or graft BP copolymer to a corresponding homopolymer, the

reaction must be more controlled and as such BP copolymers are not ideal tools.

In the case of photo-crosslinkable polymeric materials, copolymers with pendent

BP are a robust and attractive system. We have shown that their crosslinking is much

more efficient than a doped material, and as long as a sufficiently high concentration of

BP exists in the copolymers, gelation will proceed so long as hydrogen abstraction is not

limited solely to the polymer backbone. This work provides a much improved design

criteria for selection of copolymer chemistries, and this knowledge will facilitate future

studies. The one potential downside of using AmBP photochemistry for this application

lies in its relatively low reactivity to UV. If a rapidly photo-crosslinkable polymer film is

desired one may turn either to a more highly reactive substituted BP or an alternative

photochemistry such as a phenyl azide. With higher reactivity comes an entirely different

set of problems, however, and the materials will be significantly more sensitive to

ambient illumination making them challenging from a practical aspect.

The relatively low reactivity of AmBP to UV illumination has proven to be an

advantage for photo-patterning of hydrogel materials, as it allows facile spatial variation

in crosslinking and thus swelling, and BP copolymers have proven once more to be an

appealing material system for the preparation of complex hydrogels. Multilayer hydrogel

films are possible through treatment of a patterned hydrogel film with NaBH4, yet

modifications must be made to the procedure to prevent extensive swelling of the film

prior to deposition of the next layer. This is not a downfall of copolymers with pendent

118

BP, but rather a reflection of solvent conditions currently used, and alternative routes to

deactivation of BP photo-crosslinker are outlined in Chapter 4.

Copolymers with pendent BP are not ideal for every application, however, as

evidenced by our failed attempts to obtain bicontinuous nanostructures in Chapter 3. It

may yet be possible to obtain the desired results from critical polymer blends with BP

copolymers, yet it is clear that these materials are not ideal for solution-state photo-

grafting. Their failure lies in the aforementioned unspecific reaction of BP, which is

generally considered as an advantage yet in this case precludes the realization of our goal.

Because photo-reactive molecules which undergo self-cycloaddition will only react with

themselves, in order to achieve exclusively A-B grafting in these systems,

functionalization of both materials is necessary. This would result in a much more

specific photo-reaction, and an obvious candidate photochemistry is found in thiol-ene

reaction, which could promote graft copolymer formation much more efficiently than a

radical generating agent such as BP.

Copolymers with pendent BP photo-crosslinkers have therefore proved to be

useful tools for the achievement of complex materials systems. This work has expanded

knowledge of the benefits and downfalls of their use, providing guidance for materials

selection in future studies. Even though limitations do exist, they are far outweighed by

advantages, and thus BP copolymers remain powerful and extremely useful

photochemical tools for technological applications which will be increasingly used in the

future.

119

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Photo-Reaction of Copolymers with Pendent Benzophenone