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Novel polymer spheres and nanocomposites from the collapse of polyacrylic acid

Adernar de Luna dela Santa

A thesis submitted in conformity with the requirements for the degree of Master of Science

Graduate Department of Chemistry University of Toronto

@Copyright by Ademar dela Santa, 2000.

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Abstract

Novel Polymer Spheres and Nanocomposites from the Collapse of Polyacrylic Acid

Master of Science, 200 1

Ademar de Luna dela Santa Department of Chemistry, University of Toronto

Polyelectrolytes in very dilute solution exist as single chains. These single chains

undergo a change in size fiom an expanded coi1 to a collapsed globular state upon

addition of salt. The effects of NaCI, CuSo4 and AgN03 on the collapse transition of

polyacrylic acid were investigated. Gamma irradiation was used to permanently f o m the

collapsed coils and reduce the metai ions. Novel particles in the form of polyrner

spheres and polymer-metal nanocomposites were observed and characterized.

Acknowledgements

A lot of perçons other than the author make up this thesis. It is with very great appreciation and sincere gratitude to acknowledge the following persons/group of persons:

Prof: Cynthia Goh and Prof: James Guillet, my advisors, for the support, attention, help and al1 the things they taught me while undertaking this research.

Members of the research group for making my stay here gratiSing and stimulating.

Ttrntn Nugraha for being very accommodating when it comes to the use of the gamma cell.

Al1 my feachers - for instilling in me the value of excellence.

My farnily for being very supportive of al1 my endeavors.

Dinah Laderas, of course, my ever loving girlfiend.

U of T Open and OGSST for felIowshipslfinanciaI support.

Most of all, the Lord Almighty for al[ the things He has given me al1 these years and for making me appreciate what life is.

iii

Table of Contents

ACKNOWLEDGEMENTS ...................................................................................................................... ILI

INTRODUCTION ........................................................................................................................................ 1

3 POLYELECTROLY~ES ................................................................................................................................... - NANOSTRUCTURED MATERIALS ................................................................................................................ 4

........................................................................ POLYELEC~ROLYTES AND NANOSTRUCTURED MATERIALS 6 ATOMIC FORCE MICROSCOPE ......................,.................... ......................................................................... 7

........................................................................................................................................ EXPERIMENTAL 9

................................................................................................................................ SAMPLE PREPARATION 9 INSTRUMENTATI~N ..................................................................................................................................... 9

................................................................................................................. RESULTS AND DISCUSSION 11

POLYELECTROLYTE .................................................................................................................................. I I ....................................................................................................................... Po fyelecrroiyte Spheres 11 ..................................................................................................................... E'ecrs of various factors 13

Neckface configrrration ....................................................................................................................... 15 Laser Irradiation ................................................................................................................................. 16

.................................................................................................... METAL COLLO~DS ......................... ,.. 16 Copper CO floids ................................................................................................................................... 16 Sîfver Colloids ..................................................................................................................................... 20

SUMMARY AND CONCLUSIONS .......................... ., ......................................................................... 37

Introduction

Polyelectrolytes are a class of polymers having charged groups. They are mainIy

soluble in polar solvents and it is this property where they are largely utilized as materials.

They are used as binders, emulsion stabilizers, film fomers, viscosity controlling agents,

water absorbers, flocculants and some other applications. An important concept discovered

20 years ago by Tanaka (1) is the collapse of polyelectrolyte network in polyacrylamide

gels.

PartiaIly hydrolyzed acrylamide gel changes its volume discontinously when the

solvent composition is continously varied. Either a change in the pH of the solvent, addition

of salt or application of an electric field induces this transition. The abrupt contraction of

the weakly charged polyelectrolyte gel could be explained by the avalanche type counterion

condensation by the chains constituting the gel. Each chain undergoes the transition and as a

result the gel sample collapses as a whoIe (2).

M i l e the collapse transition of polyelectrolytes is well studied experimentally,

mostly by scattering and viscosity techniques, no one to Our knowledge has used Atomic

Force Microscope (AFM) to study the systern. One big challenge is how to see the

collapsed state. This was overcome by crosslinking the collapsed state through gamma

irradiation. In effect, pennanentry forrning novel polyelectrolyte spheres. Once the spheres

are forrned they can now be imaged by AFM and can be studied by some other techniques,

too. Investigation of these polyelectrolyte spheres, mainly by AFM and their utilization for

other applications are the focus of this thesis.

The prediction of Feynman (3) of having a lot of room at the bottom is already

becoming an ovemsed cliché in nanostructured materials. Over the past ten years, research

in this area - generally called nanoparticles is very active. In fact, journals devoted

especially to this topic alone have blossomed. Utilizing the collapse transition of

polyelectrolytes, metal colloids (oftentimes called metal nanoparticles) specifically - copper

and silver colloids. were synthesized and characterized. Formation of novel polymer- metal

nanocomposites were observed and described.

The remaining part of this section introduces polyelectrolytes and the theory behind

its collapse transition. The next part introduces nanostructured materials and some methods

of their synthesis. Then the two topics are merged. Finally, some very basic concepts in

AFM are given.

PoZyeZectro lytes The average dimension of a polymer chain in solution is dependent on the quality of

the solvent on which it is dissolved. As early as 1953, Flory (4) postulated that the better the

solvent the larger the size of the polymer coil and the poorer the solvent the smaller the

polymer coil. The dimension of the coil is a result of the interplay of monomer pair

interaction and entropic energy. In a good solvent, the monomer pair interaction is repulsive

swelling the coil and end-to-end distance, R - N~'' (where N = number of monomers). This

is the so-calied expanded coil state. In a poor solvent, the monomer pair interaction is

attractive collapsing the coil and R - N'! This state is termed as the collapsed globule

state. In a 0 - solvent, the attractive and repulsive forces balances each other, the chahs are

said to be ideal and R - N ' ~ . In effect from a good solvent to a poor solvent, a polymer

chain undergoes a transition fiom an expanded coil to a collapsed globular coil; a process

called the collapse transition of polymers. Solvent quality can be changed by varying either

the temperature, pressure or by mixing two or more solvents.

Polyelectrolytes are a class of polymers bearing charged groups either on the main

chain or on the substituents. The changes can be either positive or negative or both. They

act like other macromolecule when b e y are uncharged but exhibit distinct behavior when

charged. And like other electrolytes, they dissociate in solution into charged links and

counterions. Having charges are t h e defining characteristic of polyelectrolytes as their

properties and behaviour in solution are very well influenced by their charges plus the

counterions that balances these charg;es. But it is due to these charges also that they are

among the least understood systems in. macromolecular science (5).

Polyelectrolytes are classifred into strongly and weakly charged ones, based on the

number of charges they carry with t h e former containing a considerable fraction of charged

links. In strongly charged polyeiiectrolytes Iike DNA, Coulomb interaction between

charged monomers predominates awer moIecular interactions. Weakly charged

polyelectrolytes manifest appreciable non-Coulomb interactions.

A number of parameters a r e necessary to fully understand polymer solutions:

polymer concentration, solvent property, monomer structure-function, etc. In addition, salt

concentration has to be taken into account when considering polyelectrolytes. As a polymer,

the size of a polyelectrolyte is propmrtional to the number of monomers in each chain.

However, the charges play an importamt role and differences between strongly charged and

weakly charged polyelectrolytes arises. Since this thesis deals with weakly charged

polyelectrolyte, strongly charged polyelectrclyte will not be discussed. Numerous theoretical

and experimental reviews (5-7) exist oen this topic.

Khoklov (2, 8, 9) worked o.ut the theoretical formulations for weakly charged

polyelectrolytes. At dilute salt fiee smlution and 8 conditions with respect to non-Coulomb

interactions, weakly charged polyelectrolytes can be pictured as a chain of blobs. The blobs

represent consecutive charges of chains with the size depending on thermal energy. The

chain of blobs is valid in good and poor solvents though each of the blobs is in the globular

state for the latter case. The blobs in some conditions contain a fiaction of counterions

inside. This effectively diminishes its charge and a phenornenon called counterion

condensation (10) occurs. The process, however, occurs only when there is decrease in the

fractions of counterions in the region outside of the polymer chain, Le., there is an increase

in counterion concentration in the molecular vicinity of the polymer chains, leading not to a

decrease but rather to a growth of the linear charge density. This is due to the collapse of

the chain of blobs as the charge becomes neutralized. An avalanche like counterion

condensation occurs because the initial decrease induces additional influx of counterions.

And it stops only when almost al1 of the counterions are condensed ont0 the polyrner chain.

That is, the charges in this region become totally compensated effectively, destroying the

sequence of blob structure. A globule is formed where almost al1 of the counterions are

Iocated inside.

Nanostructured Materials Materials undergo a dramatic change in properties when their dimensions are

reduced to the nanometer size regime. At this size regime, electronic, magnetic and optical

properties different from both bulk and moIecular properties arise(l1). The particles in this

regime are called by many names such as Q-dots, Q-particles, nanoparticles, clusters,

nanocrystais, etc. Nanostructured matenals is the name most preferred by many researchers.

From the fundamental point of view, the transition kom bulk to molecular scale is bndged.

Quantum size effects, electron confinement, photon

fundamental topics to emerge. Some of the topics

confinement are just some of the

that have been studied before were

resurrected in light of new theories and new instrumentation. New uses and applications are

already envisioned by a lot of researchers. One of those topics is the metal colloid.

Metal colloids, in particular gold coIloids, were first systernatically studied by

Faraday (12) in mid 1 9 ~ century. He studied changes in the color of the gold colloid

solutions and correctly stated that they are small particles and they aggregate. A number of

people have worked on the problem afterwards. Over the years, researchers have noted that

these matenals have size dependent properties. It was Halperïn (13) in his article who stated

that the cause of this size dependence is due to quantum size effect. In a simple level,

particles at the nanometer size regime have energy levels that are not anymore band-like but

are discrete. The essential idea is that the spacing between conduction energy States

increases invenely with the volume of the particle. For metal colloids, the quantum size

effect is manifested through the surface plasmon resonance observed in visible

spectroscopy. This is due to the excitation of plasmon resonances in the confined electron

gas of the particles. The absorption maximum is dependent on the size, dielectric property

of the solvent, shape of the particles, proximity of the particles to each other, presence of

adsorbates and surface composition (14).

For these particles to be useful as an advanced material and for their fundamental

properties to be studied, they must be prepared and isolated in monodisperse form. There

should be a great deal of control over size, structure and surface composition. Physicists and

chemists have different perspectives in coming up with materials at this dimension.

Chemists were considered to work towards this goal fiom the atom up whereas physicist

tends to operate from the b u k down (15). Synthetic chemical techniques are more widely

used due to their straightforward nature and potential for producing industrial quantities.

Problems such as reproducible and controIlable synthesis and having narrow size

distribution were addressed in various ways.

Synthetic procedures can be into divided into large groups based on the

methods/techniques used. A review by Fendler (16) comprises the technique luiown as the

membrane mirnetic approach. Here the particles are synthesized inside confining structures

such as micelles, reverse micelles, vesicles, microphase separated block copolymers, porous

membranes, among others. Various groups utilized the direct reduction approach. The

reducing agent serves as a capping agent, directly or indirectly stopping the growth of the

particle at a particular stage according to nucleation and growth and coIIoidaI aggregation

theones. Physical techniques using light, heat, sound, electricity among others are quite

common too in preparing these materials. RecentIy, a book (17) was published detailing

some preparative schemes by Ieading groups.

In most of these techniques, there is always a species which either stabilizes the

particles at a particular size or confines them in a constricted space. lndeed that is the

common theme arnong al1 the chemical synthetic techniques.

PoZyelectroZytes and Nanosrructzrred Materials Polymers in the form of spheres are already well known. By rnicroemulsion

techniques, spheres with sizes from micrometers to tens of nanometers can be produced. In

this thesis, a novel method of producing polymer nanospheres is presented. The spheres

produced, in this case, are much smaller than the ones produced by the microemulsion

method.

Utilizing the collapse transition of polyelectrolytes, the collapsed globule state is

crosslinked with gamma radiation. Gamma radiation is welI known as a photochemical

crosslinking agent (18). Absorption of gamma radiation by molecules leads to the

production of various species. In polymeric systems, cation and anion radicaIs, cations,

anions and excited state species are formed. These species can either react with each other

forming bonds (crosslinking) or dissipate the excess energy by breaking bonds (scission).

Crosslinking occurs principally by the dirnerization of adjacent free radicals. This happens

when radicals are formed in pairs or are able to migrate through the chain or between

molecules by H-abstraction on a neighboring chain until they are close enough together to

react. This interna1 crosslinking process permanently stabilizes the spherical structure of the

globules. Laser irradiation can also be used as a crossIinking agent.

Using the same collapse transition concept, meta1 colloids can be synthesized inside

the spheres. The polyelectroIyie, as a stabilizing and capping agent restricts the size of the

colloids to the size of the spheres. In this case, novel nanocomposites of metal colIoids

inside polymer spheres are formed. Like nanoparticles, nanocomposites are of interest due

to their unique properties (1 9).

Atornic Force Microscope Atomic force microscopy is a member of a family of scanning probe techniques

invented almost 14 years ago by Binnig, Quate and Gerber(2O). It is a multifunctional

instrument used for the characterization of topography, mechanical, adhesion and other

properties of surfaces on scales from microns to nanometers. It can be applied to the study

of conducting and non-conducting samples in both air and fluid conditions.

In an AFM analysis, a probe consisting of a very sharp tip attached to a cantilever

scans an area of the sample. An image can be acquired by moving the sarnple in raster

pattern under the tip in the horizontal plane (x and y) with the vertical motion (z-direction)

controlled by a feedback mechanism and monitoring the response of the cantilever. The

cantilever moves in response to the attractive and repulsive forces between the tip and the

sample. This movement is monitored by an optical system composed of a laser reflected on

the cantilevers' surface to a four-quadrant photodetector. The variations of the z- position in

the sample during scanning are plotted as a function of the x, y position of the tip to create a

height image.

The tip is driven at its resonance fkequency by a small piezoelectric device and the

damping of its oscillation amplitude in response to the forces is monitored in the tapping

mode operation. The bending of the cantilever in response to the forces is monitored in

contact mode. Both modes can be done in air and under fluid conditions. There are other

modes of operation like the frictional mode, electrical mode, non-contact mode among

others but they are not used in this investigation. Numerous reviews and book chapters(21)

have been written about how the instrument operates and its application to vanous problems

including the analysis of polyrner surfaces (22).

Experimental

SampZe preparation Polyacrylic acid (Polysciences, Inc) with an average molecular weight of 1 000 000

was used. Dilute solutions were prepared by dissolving polyacrylic acid in water to make

1.0 mg/mL. The solution was magnetically stirred for at least four hours until complete

dissolution. The acidity of the solutions were adjusted accordingly with 0.10 N NaOH. 15

ml aliquots of this solution were taken and added to the pre-weighed salt corresponding to

the ratio (A) for each salt as mentioned in the discussion. The solution was s h e d

vigorously in a vial. A portion of this solution is transferred to plastic cuvets and covered

with an elastomeric septum (Aldrich). The solution is bubbled with nitrogen gas for at least

15 minutes before being subjected to laser or gamma irradiation.

hstmrnentation Laser irradiation was camed out using a frequency doubled picosecond pulsed Nd-

YAG laser at 266 nrn. The beam of the laser is unfocused and is about 0.5 cm in diameter.

The solution was placed in a specially fabricated glass cylinder measuring 15 cm in length

and 5 cm in diameter. The cylinder is covered with a quartz wall on one side and has an

inlet and an outlet covered with septa. Solutions for laser irradiation are directly transferred

and bubbled with nitrogen in this glass cylinder. The cylinder is placed along the beam of

the laser. The solution was magnetically stirred throughout the irradiation process.

Gamma irradiation of the solution was carried out by placing the already bubbled

60 solution in a plastic cuvet into the Co gamma cell. The gamma cell has an irradiation dose

rate of 0.5 Mradhr.

The visible spectra o f irradiated solutions were obtained using a Perkin - Elmer

(Lambda 11) spectrophotometer. The spectra were acquired directly against plastic cuvet

blank containing water only.

Atomic force microscope images were acquired using a Digital Instruments

Nanoscope III machine. The irradiated solutions are first dialyzed for at least 12 hours

against deioinized distilled water in SpectraPor (MW1244 000) dialysis bags. Microliter

amounts of the solution were transferred to fkeshly cleaved mica and the solvent evaporated

to dryness (for imaging under dried conditions). The dried sample is placed on a

magnetized sample holder and imaged. Silicon nitride tips with a spring constant of 14 N/m

were used for the tapping mode imaging. Fluid ce11 was utilized for the Ruid imaging

work. A large drop of the solution is placed on freshly cleaved mica encircled with a Teflon

tape as developed by Sattin (23).

Transmission electron microscope images were obtained through a Hitachi H-600

TEM machine. The solution was sprayed ont0 a carbon coated TEM grid, air dried and

imaged with a 75 kV operating power.

A Spex 1887C Triplemate Raman spectrometer with a CCD detector and an argon

ion (Lexel mode1 3000) laser was used for the acquisition of the Raman spectra.

Results and Discussion

PoZyelectroZyte Spheres Addition of salt to a very dilute solution of polyacrylic acid collapses the single

polyrner chains into smalI spherical balls. The chains once collapsed can then be

pennanently stabilized through crosslinking. In this case radiochemical crosslinking in the

form of gamma radiation was used. Figure l(a) shows such spheres fonned when NaCl (h

= 2.90) is used as an added salt. The crosslinked solution was placed on mica and air dried

before imaging with AFM. As seen in the cross section, a representative particle has a

diameter of 3.8 nrn. Shown in the same figure is the control solution (figure lb). This

image came from the same solution but was not subjected to gamma radiation. A drop of

this solution was placed on mica and imaged. The image is a polymer film with a thickness

of 0.5 nm. The area shown is a break in the polyrner film. At this concentration of NaCl,

spheres were formed. Other concentrations of NaCl were not investigated since the interest

is just on the formation of the spheres.

The average size of the spheres reported above corresponded to most of the imaged

samples. However, it is worthwhile to note that some images were obtained wherein the

diameter of the particles can be classified as outliers, statistically. Particles with diameters

of around 20 nm were seen for a number of images due presumably to the agglomeration of

particles. The concentration of the polyelectrolyte is important since at high concenh.ations,

the polyelectrolyte precipitates as a gel containing many polymer molecules. At low

concentrations, below the Cc - critical concentration, the polyelectrolyte precipitates as

individual polymer chains. The spherical shape is expected as it is the shape with the lowest

surface energy. Note that the particles are approximately the diameter predicted fiom the

molecular weight of the starting polyrner (1 000 000).

The spheres have a net negative charge and should repel one another. However, it is

interesting to note that some aggregation patterns were observed similar to the images

shown in figure 2. In one picture (figure 2a), the spheres seem to approach one another

forming larger aggregates. The aggregates observed did not form any specific structure but

just one large clump of spheres. In another instance, the polymer spheres were observed to

form a somewhat compact packing structure shown in figure 2b. This presented a simiIar

property to latex spheres where hexagonal packing structures are formed. However, for this

case, no hexagonal packing was observed. Figure 2c shows polyrner spheres on top of each

other. A fiactal like pattern is exhibited by the image in figure 2d. Figure 2e is an image

acquired using the laser as the crosslinker. In this case, the aggregates have a very different

structure. Whereas in the previous images the spheres do not form a clear structure, the

aggregates, in this image, show spheres forming a somewhat secondary structure. The

polyrner spheres form a "ring" with a flat center. A close-up of one of the structures is

shown in figure 2f.

Dilution, addition of acid or bases do not provide conclusive evidence as to whether

the particles can be shrunk or expanded based on M M measurements. Measurement of the

distribution of the sizes of the particles was hampered by the presence of salt deposits on the

mica surface due to salt formation from the acidhase and impurities on the solvent. It is hard

to identiw which are the particles and which are not. Also, most of the particles are circled

around large salt deposits indicating that thz particles are indeed charged.

Subjecting the particles to higher temperatures however, reduces their diameter.

Cornparisons of cross sections (figure 3) of several particles before and after heating showed

a general decrease in size. The particles were initially placed, dried and imaged in mica

before heating to 80 C for one hour. The previous spot was located with the help of TEM

grids underneath the mica film. One of the reasons for the decrease in size is possibly due to

the evaporation of trapped water inside the sphere.

With the characteristics observed for these polyrner nanospheres, fluid imaging was

considered. A method used by Sattin (23) was tried and a sample image is shown in figure

4. The spheres are clearly observed. There were no problems of the samples not adhering to

the surface as repeated imaging of the same area showed the same image. This mode of

imaging was not further explored but it is interesting to consider that with this mode various

properties of the spheres c m be studied. One of them is the formation of aggregated

structures seen in the dried solutions. Aggregates were not seen in images acquired under

fluid. This could be due to factors like concentration of the spheres and the ionic

environment. Theoretically, one could foIlow the aggregation of these spheres if one would

be able to adjust the solution conditions. And that is one nice way of following aggregation

kinetics in real time using AFM.

Effects of various factors

The dissolution of polyacrylic acid in water can be done at various pHs. Changing

the pH of the solution effectively alten the charges o f the polyelectrolyte. This presents

some complications in the collapse process. Previous studies have however suggested that

what matters is the ratio of the concentration of the added salt to the polyelectrolyte

concentration [C,,b,AC/C,lt] (5). Literature values of this ratio (called A) ranges £kom 1 - 5

for the collapse process to occur. Basically, various parameters can be varied to achieve the

desired effect.

Solutions with different initial acidities were prepared, collapsed and crosslinked.

Al1 the M M images showed the same characteristic spheres indicating that the initial pH of

the solution does not seem to have any effect in the formation of the polymer spheres.

Whether or not addition of base alone c m induce the collapse of the polyelectrolyte was aIso

investigated. Several solutions were prepared with different pH's and exposed to gamma

radiation. No other salt was added. None of the collected images showed any collapsed

structures. Based on AFM images, addition of bases/titration of the polyacrylic acid do not

induce collapse of the chains. However, it is interesting to note that in another preparation,

this time not exposing the solutions to gamma radiation and just directly observing them

with AFM, some unusual structures were observed. This is described in the necklace

configuration part.

Solutions with varying h were tried but based on M M images, it is not clear at what

particular h the poIymer collapses. The images obtained do not yet support concIusive

evidence to Say that at some A, spheres will be formed. More careful scrutiny is required.

Gamma radiation is known to cause chain crosslinking and chain scission in

polymers. The question of which dominates in a particular irradiation depends on the

polymer structure and the irradiation conditions. Various doses were tried to see the

dependence of the particle size to irradiation dose. Unfortunately, no conclusive evidence

was obtained using AFM imaging.

Polyacrylic acid with an average molecular weight of 90 000 was also studied.

However, no spheres were formed at al1 acidities and just polyrner film was seen in al1 AFM

images.

Necklace configuration Recent theoretical and computer modeling studies suggested a pearl necklace

configuration as an intermediate structure in the collapse process (24-26). Expe~mentally,

the pearl necklace structure was suggested as the cause of a peak in scattering experiments

(27). In some of the solutions, images such as the one shown in figure 5 indicate an

experimental verification of the pearl necklace configuration. A necklace configuration of

beadlike structure connected by a very thin string can be observed. In some areas, several

necklace structures can be seen to be arising fiom a large blob. Cross sections of the thin

string suggest that they maybe single polymer chains. The necklace structure was first

proposed for charged arnpholytes by Kantor and Kardar (28, 29). They postulated that it is

similar to the transition of the shape instability of charged drops. Dobrynin (24) extended

the idea to polyelectrolytes and used scaling theory to propose that the necklace structure

has a lower free energy than the cylindrical globule. The balance of the electrostatic

repulsion and surface tension determines the free energy. The polyelectrolyte chain in poor

solvents undergoes a cascade of transitions between necklace configurations with different

number of beads as a function of charge and solvent quality- Unfortunately, efforts to $ l,Mi),:?-3 CI+

reproduce this solution were unsuccessfùl. (The solution is a 0.5 mg/ml PAA prepared as A

described in the experimental part and with a pH = 8.50 and without anymore added salt).

Titration of a new polyacrylic acid soIution with NaOH and imaging the solution at various

acidities were tried but no similar structures were observed. The solution itself though,

displays the same necklace structure every tirne a drop is taken and imaged. This sarne

necWace structure was also observed two months after the solution was initially prepared.

Whether this condition is caused by the thermodynamics and kimetics of the drying is a good

question to explore. However, that question is not the focus of this thesis.

L mer Irradiation The discussion above focused on gamma radiation as the crosslinking agent. Laser

sources however can also be used as photochemical crosslinkers. Although this avenue was

not extensively studied, the same solutions were prepared. Sodium chloride was added to

polyacrylic acid, aerated, exposed to laser radiation and dialyzed before analysis. AFM

image of the results showed spheres with similar diameters. The control solution showed

polyrner film as the one before. Also, 15 minutes of laser irradiation with a frequency

doubled picosecond pulsed Nd-YAG laser at 266 nm is enough t o effect crosslinking. As is

with gamma irradiation, the starting pH of the solution does not really matter in coming up

with collapsed structures.

Aggregates seen using gamma irradiation were also observed with the laser as the

crosslinker. Figures 2e and 2f show such aggregates.

Metal colloids

Copper colloids

Addition of transition metal salts like copper sulfate to a solution of polyacrylic acid

induces the collapse of the polyelectrolyte. Subjecting this solution to gamma radiation

brhgs about crosslinking and reduction. A change in the color of the solution results and

this can be followed by visible spectroscopy.

As in the previous process, gamma radiation crosslinks the coliapse polyacrylic acid.

An AFM image is shown in figure 6 for this particular solution afier the crosslinking

procedure. Spheres typical of the collapsed polyectrolyte can be observed. However, a

more interesting result was the observation of the production of copper metal colloids as

evidenced by the change in the color of the solution and the observation of plasrnon peaks

for copper.

It is well known that gamma radiation is a very effkient and clean reducing agent.

Gamma radiation is believed to react with water producing reactive radicals like hydrated

electrons and hydroxy radicals. These radicals either react directly with the substrate

(copper ion) or with the solvent rnolecule producing further reactive radicals which

eventualIy reacts with the substrate.

Figure 7 shows a number of UV-Vis spectra of irradiated copper sulfate -

polyacrylic acid solution. The peak at 480 nm is very prominent while the peak at around

700 nm corresponding to the d-d transition for cu2' is hardly seen indicating that some

reduction have occurred. Similar studies done by Khatouri, et.al., (30) using low molecular

weight polyacrylic acid suggested that the 480 nm peak corresponds to the reduced form -

Cu' stabilized by the polyrner. This peak is produced at low pH due to its stabilization by

the protonated fom of polyacrylic acid and at low doses of radiation (0.4 Mrad). Increasing

the dose of radiation to 0.8 Mrad produces the surface plasrnon peak at 570 nm, due to the

increase in size of the aggregates. However, the results obtained in this experïrnent don't

seem to support the above studies. Varying different parameters, the 480-nm peak was

observed consistently with al1 the solutions.

The W-Vis spectra presented in figure 7 show the effects of various parameters.

Increasing the polymer concentration (increasing the value of h while keeping the copper

concentration constant) should increase the concentration of the reduced species since there

will be more room for the counterions to be attracted to. This is observed in figure 7(ii) as

expected. Obviously, addition of more copper sulfate (decreasing the value of h while

keeping the polymer concentration) will lead to an outright increase in the concentration of

the reduced species as evidenced in figure 7(i).

A look at the effect of the dose of radiation is shown in figure 7(iv). increasing the

dose of radiation while keeping other variables constant increases the intensity of the 480-

nm peak. However, the increase is only up to 1.5 Mrad and then a decrease in intensiv was

observed. Similar expenments(30) have s h o w that there is maximum dosage wherein after

that dosage there will be no more reduction that will take place. This observation was based

on the intensity of the plasmon peak. Since the plasmon peak is not the one being observed

here, it could be that some fùrther reaction/transformation have occurred in which the final

product does not absorbed in visible range.

Exposure of the solution to laser radiation first and then to gamma radiation produces

different effect. Laser irradiation crosslinks the polymer spheres as evidenced by AFM

imaging but c a ~ o t reduce the copper ions as monitored spectrophotometrically. In effect,

polymer spheres with copper ions as counterions are produced. The effect of this laser

irradiation to the production of copper reduced specie is shown in figure 7(iii). The intensity

of the solution exposed to laser irradiation is higher compared to the one not exposed to it

suggesting an increased concentration of the reduced specie. This could be due to more

copper ions encased and stabilized by the already crosslinked polyacrylic acid before

reduction. Whereas copper ions are attracted to the carboxylate charges on the polymer, it

could be that more carboxylate functionalities are stabilizing copper ions when the polymer

is already collapsed and pemanently crosslinked.

As noted before, the peak at 480 nm was observed on al1 solutions and this result is

somewhat contrary to what has been done before. Observation of the stability of this peak

with time is shown in figure 8. The peak is stable in some solutions but evolved to the

plasmon peak of copper in some solutions. There was a plain decrease in intensity,

complete disappearance of the peak and evolution to the plasmon peak of copper. This

signifies that fûrther processes are happening after the radiation has been stopped. Copper

clusters and coIloids are known to be very unstable systems. As is usually done, the coIloids

are stored in inert atmosphere because oxygen in the air has the potential of oxidizing the

species back to copper 2+ ions. It could be that some oxidation has occurred for solutions

exhibiting reduced intensities over time. The appearance of the plasmon peak indicating the

formation of CunO clusters is still unexplained.

In one experiment, the effect of starting pH on the solution was monitored. The 480

peak normally observed were not seen but the surface plasmon band was directly detected.

Figure 9 shows the spectra of solutions at three different initial pH's. The more basic the

starting solution, the higher the intensity of the resulting surface plasmon band. As the pH

increases, the polyelectrolyte is increasingly becoming more negative attracting more

positive copper ions. This leads to more copper ions that can be reduced at a given site or

more copper clusters aggregating and forming the colloid. Thus, the intensity of the

plasmon band increases.

Silver Co llo ids

ConceptuaIIy, almost al1 transition metal salts can be used as collapsing agent for the

polyacrylic acid. Due to extensive studies on silver colloids and their ease of obse~a t ion

(the plasmon band is in the visible range), silver salts were examined.

Using the same process, subjecting a silver nitrate - polyacrylic acid solution (h =

6.6) at pH = 8 to laser irradiation for 15 minutes and exposing it to gamma radiation (1.5

Mrad) aftenvards, no change of color was observed after laser irradiation. But the solution

became dark yellow orange after gamma radiation exposure. A typical UV-Vis spectmm

shows a peak at 400 nm (figure 10) corresponding to the well-known plasmon resonance

band of silver colloids (3 1). The silver colloids, unlike the copper colloids do not undergo

fùrther reaction, as changes in the surface plasmon band were not observed over tirne. This

is in addition to the fact that the solutions were not stored in inert atmosphere. Solutions

containing silver nitrate onIy showed no absorbance as the silver ions are completely in its

metaIlic reduced forrn.

Silver colloids are known to have surface enhanced Raman scattering property(32).

That is species adsorbed on the surface of the colloid will have an enhanced Raman

scattering signal. This property of silver is still not widely understood but prevailing

theories States that it is a combination of chemical effect and electromagnetic effect (33).

The existence of the silver colloids is further proven when the Raman spectrum of the

solution was acquired (figure 11). Clearly, there is an enhanced Raman scattering signal for

the polyacrylic acid compared to just pure polyacrylic acid.

An AFM image (figure 12) of the solution shows the same spheres as observed

before. Now the question of whether polyrner spheres, silver colloids or a composite of both

are being seen arises. The AFM scans the surface of the spheres and has no capabilities of

chemically differentiating the spheres or whether the sphere is a composite of two or more

things.

Visualizing this solution with TEM (figure 13) however reveals an inner core of

dark spot with a hazy coating. The TEM image displays the electron density of the sample

and to a certain extent differentiates them. In this case, the inner core is ascribed to be the

silver colloid while the outer coating is the encapsulating polymer. The image clearly shows

the formation of a novel poIyrner metal nanocomposite.

A solution (h = 3.5 at pH = 8.20) with a different preparation, this tirne adding 0.33

% isopropanol (radical scavenger) and omitting laser irradiation was made and exposed to

the same dose of gamma radiation. After the radiation, the solution exhibited the same UV-

Vis spectrum as the above solution indicating the formation of silver colloids. M M and

TEM images of this solution are shown in figure 14. Based on TEM measurements of 120

particles, the size of the inner core corresponding to the silver colloid is around 3.52 Ifr 0.53

nm while the whole nanocomposite is around 5.22 f 0.88 nm. The size of the encapsulating

matrix - the polyacrylic acid is 0.85 nm based on the difference on the two numbers above

for this kind of preparation.

The use of polyelectrolytes as stabilizing agents for colloidal solutions is already

well known (34). Polyelectrolytes act as both steric stabilizer and depletion stabilizer. In

the former, stabilization is through the macromolecules attached to the surface of the

particles by grafting or by physical adsorption while macromolecules that are free in

solution describes the latter. In some cases, both types of stabilization can be observed in

solution and is tenned as electrosteric stabilization. However, for the two cases above, the

polyelectrolyte is encapsulating the colloidal particle and not merely adsorbed on its surface.

This type of stabilization has never before been observed to our knowledge and presents

some interesting applications.

srti tic al distance = 3.8 nm

Figure 1: AFM image of the irradiated (a) and non-irradiated NaCl - Polyacrylic acid solution and their corresponding cross sections. The first image shows the permanently crosslinked polymer spheres while the second image shows polymer film corresponding to the control solution.

Figure 2: AFM images showing various aggregation patterns (a-e) observed for the collapsed polymer spheres. (f) is a close-up of one of the structures in (e).

1 Number 1 diameter (nm) 1

Figure 3: The effect of heat (increased temperature) on the polymer spheres. The first image (a) is an image at room temperature while the second image (b) is an image of the same area after heating the sarnple to 80 OC for one hour. Tabulation of the cross sections of the numbered particles showed a general decrease in the diameter of the particles. The large circular object on the upper right hand portion of the image is an unknown structure but serves as a marker to easily identify the particles.

Figure 4: The collapsed polymer spheres imaged under Ruid conditions showing the same features as the one obsewed under dried condition .S.

Figure 5: Necklace structures obsewed in the collapse of polyacrylic acid. (a), (b) and the inset in (c) are some examples of the images obtained while (c) is a close- up view of the beadlike string and the corresponding cross section.

Figure 6: AFM image of gamma irradiated copper sulfate - polyacrylic acid solution showing the polymer spheres formed.

(a) A = 3.4 (b) 1. = 5.2 (c) X = 10.4

(a) with laser irradiation (b) without laser irradition

(a) A = 5.2 (b) L = 2-6 (c) A = 0.5

(a) 1.5 Mrad (b) 2.0 Mrad (c) 1 -0 Mrad

Figure 7 : The effect of various pararneters on the copper sulfate-polyacrylic acid solution after gamma irradiation.

(i) effect of copper sulfate concentration - as the concentration of copper increases the intensity of the peak increases

(ii) effect of polyacrylic acid concentration - likewise as the concentration of the polyelectrolyte increases , the intensity of the peak increases

(iii) effect of laser irradiation - exposing the solution to laser radiation before irradiating with gamma increases the intensity of the peak

(iv) effect of dose of radiation - intensity of the peak increases as the dose increase to 1.5 Mrad but decreases afterwards

Figure 8: Examples of the evolution of the copper peaks 14 hours after irradiation. In the top two graphs, the 480-nm peak evolved to the well-known copper surface plasmon peak though the two peaks showed different shapes. In (c), the intensity of the peak decreased dramatically while the copper d-d (the broad peak at around 700 nm) transition peak increased. The intensity of the peak on (d) decreased but is still very visible and there is no noticeable peak at around 700 nm.

350 400 450 500 550 600 650 700 750 800

wavelength (nm)

Figure 9: The effect of ph on the surface plasrnon of copper colloidç: (a) pH = 8.53, (b) pH = 5.45 and (c) pH = 3.41. As the solution becomes more basic, the intensity of the surface plasrnon peak increases.

Figure I O : UV-Vis spectra of gamma - irradiated (a) silver nitrate - polyacrylic acid solution showing the surface plasmon of silver colloids and an almost Rat absorbance for just pure (6) silver nitrate.

O 50 O 1000 1500 2000 2500 3000

wavenum bers(cm-1)

Figure 1 The Raman spectra of (a) silver colloid - polyacrylic acid nanocomposite and (b) pure polyacrylic acid clearly showing the enhancement of the Raman signals for the former. The increased intensities are due to the SERS effect of the silver colloid on the polymer.

Figure 12: AFM image and cross section analysis of silver - polyacrylic acid nanocomposite.

vertical dkbncs (aj 8.9 nm ibj 3.8 nm

Figure 14: (a) AFM image of the silver nitrate - polyacrylic acid nanocomposite after gamma irradiation showing the sp. heres formed and the corresponding cross sections. (b) Highly magnified and cornputer enhanced TEM image of the silver colloid - polyacrylic acid nanocomposite utilizing a different preparation scheme. The image also shows the same dark spot at the middle interpreted to be the silver colloid and the lighter encapsulation which is taken to be polyacrylic acid.

Summary and Conclusions

The polyelectrolyte collapse transition process is a widely studied research area.

However, looking at it frorn a different perspective makes it rnuch more interesting to study.

In this regard, the use of gamma radiation opened up some room to further contribute to this

area.

Crosslinking of the collapsed state enabled the use of the Atomic Force Microscope

to help visualize and characterize the system. Various modes of the microscope can be

utilized to examine the system better. Basic properties such as size size distribution and

shape was obtained. Other properties such as charges, mechanical properties, aggregation

properties among others can be inferred directly or indirectly. The utility of the instrument

in this area is indeed very promising. The novel polyrner spheres formed also provide some

interesting applications to ponder.

Extension of the concept to synthesize metal colloids has been demonstrated through

the production of copper and silver colloids. In this case, novel polymer nanocomposites

were made. Initial characterization of the silver colIoid - polyacrylic acid nanocomposite

shows core-shell structure of silver colloid and polyacrylic acid, respectively. Formation of

the copper colloid through this method, however, is still unclear since the surface plasmon

band appeared hours after the radiation has stopped. It is worthwhile to consider also that

this process, taken in a different light, can be used as a way to control the size distribution of

the coIIoids formed.

While various aspects of the topic were explored, a lot of things can still be

envisioned. Thinking of the collapsed spheres as a confining space similar to micelles,

synthesis of size controlled semiconductor - polymer systems, bimetallics and magnetic

transition metai oxide - polymer systems is not a far-fetched idea. Of course, one should

not forget other usefùl rnetals in the penodic table, which might bring some other novelties.

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