Embed Size (px)
Citation preview
Soft Matter
PAPER
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article OnlineView Journal | View Issue
Comparison of L
State Key Laboratory of Molecular Engineeri
Center of Polymers and Polymer Composite M
Science and Laboratory for Advanced Mater
China. E-mail: [email protected]
† Electronic supplementary information (the synthesis of the polymers; molecular ch(Table S1); 1H NMR spectra of PNIPAM, d(Fig. S1), DLS curves of PNIPAM, mixtuPVCL in H2O upon heating at the concehydrodynamic diameter (Dh) distributionscopolymers in H2O during the heatinmeasurements (Fig. S3). See DOI: 10.1039
Cite this: Soft Matter, 2015, 11, 2771
Received 6th January 2015Accepted 9th February 2015
DOI: 10.1039/c5sm00026b
www.rsc.org/softmatter
This journal is © The Royal Society of C
CST-transitions of homopolymermixture, diblock and statistical copolymers ofNIPAM and VCL in water†
Lei Hou and Peiyi Wu*
The LCST-transitions of linear, well-defined polymers of N-isopropylacrylamide (NIPAM) and N-
vinylcaprolactam (VCL), including a homopolymer mixture, diblock and statistical copolymers, in water
are explored and compared by applying turbidity and FTIR measurements in combination with two-
dimensional correlation spectroscopy (2Dcos). Only one transition is observed in all polymer systems,
suggesting a dependent aggregation of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-
vinylcaprolactam) (PVCL) parts in the phase transition processes. With the help of 2Dcos analysis, it is
discovered that the hydrophobic interaction among C–H groups is the driving force for simultaneous
collapse of the two distinct thermo-responsive segments. Additionally, the delicate differences within the
LCST-transitions thereof have been emphasized, where the phase separation temperatures of the
homopolymer mixture and the diblock copolymer are close while that of the statistical copolymer is
relatively higher. Moreover, both diblock and statistical copolymers exhibit rather sharp phase transitions
while the homopolymer mixture demonstrates a moderately continuous one.
1. Introduction
Over the past decades, considerable efforts have been devotedto the exploration of thermal-responsive polymers because oftheir great potential in a wide range of applications in tissueengineering scaffolds, drug delivery, biological separation andsensing.1–3 It is generally known that the thermal-responsivepolymers possess either a lower critical solution temperature(LCST) or an upper critical solution temperature (UCST) in thesolutions during their phase transition processes. Particularly, alarge amount of attention has been focused on the polymersthat exhibit the LCST-type transitions in aqueous solutions.4
Among them, poly(N-isopropylacrylamide) (PNIPAM) is themost famous example, which undergoes a rapid coil-to-globuletransition in the aqueous solution on heating above its LCST(�32 �C).5–7 Another frequently investigated temperature-sensi-tive polymer is poly(N-vinylcaprolactam) (PVCL).8–10 PVCL is not
ng of Polymers, Collaborative Innovation
aterials, Department of Macromolecular
ials, Fudan University, Shanghai 200433,
ESI) available: Detailed information onaracterizations of the polymer samplesiblock, statistical copolymer and PVCLre, diblock, statistical copolymer andntration of 0.2 mg ml�1 (Fig. S2) andof the mixture, diblock and statisticalg process obtained from the DLS/c5sm00026b
hemistry 2015
only thermally responsive but also biocompatible. The hydro-philic amide group linked to the hydrophobic polymer back-bone will not be able to produce low molecular-weight amines.Additionally, N-vinylcaprolactam (VCL) has also been reportedto be less cytotoxic than N-isopropylacrylamide (NIPAM).11,12
Nowadays, polymers containing more than one thermo-responsive component have also aroused much attention, dueto their complex self-assembly behavior and their potential inthe development of reversible encapsulation and deliveryareas.13 Some of the researches are dedicated to the diblockcopolymers which consist of two distinct temperature-sensitivesegments. Sato et al.14 have reported the investigation of dehy-dration and self-association behavior of a novel diblockcopolymer composed of poly(2-isopropyl-2-oxazoline) and PNI-PAM (PIPOZ-b-PNIPAM) upon heating in water and water–methanol mixtures. It is found out that the phase transitions oftwo segments that occur depend on each other; even thedifference in the transition temperatures of PIPOZ and PNIPAMhomopolymers is enhanced by increasing methanol fraction,which may be attributed to the interference of the con-onsolvency effect of the PNIPAM block and the cosolvency effectof the PIPOZ block. Thermal properties of diblock copolymersPVCL and poly(2-dimethylaminoethyl methacrylate) (PVCL-b-PDMAEMA) in aqueous buffer solutions have been studied byKaresoja et al.15 Those block copolymers displayed one or twoseparate LCSTs depending on the pH. With increase intemperature, the PVCL block collapsed rst and the PDMAEMAchains turned out from the collapsed PVCL globule toward theaqueous phase to provide colloidal stability to the aggregate
Soft Matter, 2015, 11, 2771–2781 | 2771
Soft Matter Paper
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
particles. Some of the researches are concentrated on therandom copolymers composed of different thermal-responsiveunits. Cloud points of aqueous solutions of statistic copolymersof NIPAM and N-isopropylmethacrylamide (NIPMAM) havebeen determined turbidimetrically by Djokpe and Vogt, where alinear dependence of the cloud point on the copolymer contentwas discovered.16 Lately, Crespy et al.17 have described thesynthesis of VCL and di(ethylene glycol methyl ether methac-rylate) copolymers in solution and miniemulsion. It was shownthat the cloud point temperature of the random copolymerscould be ne-tuned in the temperature range between 26 and35 �C. Meanwhile, investigations on the phase transition ofphysically mixed aqueous solutions of two thermosensitivehomopolymers have been reported as well. Spevacek et al.18,19
studied the heat-induced phase separations in aqueous solu-tions of poly(N-isopropylmethacrylamide) (PNIPMAM)/poly-(vinyl methyl ether) (PVME) and PNIPAM/PNIPMAM mixturesby applying 1H NMR spectroscopy. The authors discovered that,in both systems, the phase transition of the component withlower LCST is not affected by the presence of the higher tran-sition component, while the phase transition of the componentwith higher LCST is affected by the phase separation of thelower transition component in the mixture. More recently, theyhave investigated the phase transition behavior of the PNIP-MAM/PVCL mixture in water and found that both transitions ofpolymeric components were shied �2 K toward highertemperatures in comparison with neat polymers, indicatingthat the existence of the second component hindered the phasetransition of the given one either by direct interactions betweentwo segments or indirectly by changing the order and structuresof water molecules surrounding the polymer chains.20 In thework of Chen et al.,21 the aggregation behavior of poly (vinylmethyl ether) (PVME)/poly (2-ethyl-2-oxazoline) (PEOZ) chainsin water was probed by elastic light scattering (ELS) spectros-copy, and two onset temperatures of phase transition duringheating were identied. However, systematic studies on thecomparison of the phase transition behavior of copolymers(both diblock and random copolymers) and mixtures with thehomopolymers are rather seldom. Richtering et al.22 havereported on the thermosensitive properties of a diblock copoly-mer poly(N,N-diethylacrylamide)-b-poly(N-isopropylacrylamide)(PDEAAM-b-PNIPAM) and compared it to the homopolymers,mixtures thereof, and to the statistical copolymer with the samemolecular weight. It was revealed that the macroscopic andmicroscopic properties of the diblock copolymer resembledmore the averaged properties of the homopolymers due to thecompetition between the block length dependency onthe respective cloud point and the effect of connectivity. Incontrast, the mixture and the random copolymer exhibitednonideal behavior leading to a synergistic depression of thecloud points.
Despite the similarities in the phase transition behavior ofPNIPAM and PVCL, differences for both polymers are obvious.For instance, the LCST transition is rather sharp and rapid forPNIPAM, whereas PVCL exhibits a relatively broad and contin-uous change.23 The cloud point of PVCL could range from 30 to50 �C, strongly dependent on the molecular weight and
2772 | Soft Matter, 2015, 11, 2771–2781
concentration,24 while that of PNIPAM is somewhat stable.Moreover, PNIPAM displays cononsolvency (insolubility in amixture of good solvents),25 which is hardly detectable forPVCL.8 Previously, we had explored the self-aggregationbehavior of the diblock copolymer PNIPAM-b-PVCL during theheat-induced phase transition in water, in which only onetransition point was observed, indicating that the two distincttemperature-sensitive segments behaved cooperatively.26 It isgenerally considered that the monomer pattern along thepolymer chain plays a vital part in its nal chemical and phys-ical properties. Under such a circumstance, we are interested onthe inuence of NIPAM and the VCL pattern along the polymerchain on the eventual LCST-behavior. In addition, the mixing oftwo thermally responsive polymers demonstrates a facilemethod to develop novel “smart”materials. As a result, how onegiven component is affected by the appearance of the other oneis of great signicance in practical applications. Moreover, bycomparing the phase transitions of the homopolymer mixture,diblock and statistical copolymers of NIPAM and VCL in water,we will be able to gure out what role each thermoresponsivecomponent possesses in different polymer structures and betterunderstand the inherent nature of thermosensitive behavior.
Thus, in this work, we utilize turbidity measurements toexamine the transition points and features of homopolymers ofPNIPAM and PVCL, the mixture thereof, diblock and statisticalcopolymers of NIPAM and VCL rstly, which may provide amacroscopic vision of the phase transition processes. Then,Fourier transform infrared spectroscopy (FTIR), proved to be aquite useful tool to probe variations in the hydration states orhydrogen bonds of individual chemical groups,27–29 is employedto describe the microscopic changes with regard to the LCST-transitions of different investigated systems and uncover theunderlying diverse molecular interactions thereinto. Further-more, the literature lacks integrated explorations of the phasetransition behavior of the homopolymer mixture and thestatistical copolymer of NIPAM and VCL, which will beemphasized in our work. Therefore, two-dimensional correla-tion spectroscopy (2Dcos) analysis is applied hereby to illustratethe dynamic dehydration behavior of the homopolymer mixtureand the statistical copolymer during the heating process. In thisway, additional information on the specic order taking placerelated to chain conformation changes during the transitioncould be extracted, which is very helpful for better under-standing of the evolution mechanisms.
2. Experimental2.1 Materials
N-Isopropylacrylamide (NIPAM) was obtained from Aladdinreagent Co. and recrystallized from cyclohexane. N-Vinyl-caprolactam (VCL) was purchased from Alfa Aesar Co. andpuried by passing through a short alumina column prior touse. 2,20-Azobis(isobutyronitrile) (AIBN, Aladdin reagent Co.)was recrystallized from ethanol. Methanol was distilled undervacuum aer drying with calcium hydride for 12 h. D2O waspurchased from Cambridge Isotope Laboratories Inc. (D-99.9%). The reversible addition–fragmentation transfer (RAFT)
This journal is © The Royal Society of Chemistry 2015
Paper Soft Matter
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
agent, O-ethyl-S-(1-phenylethyl)dithiocarbonate (CTA), wasprepared according to the literature procedure.30–32
2.2 Polymer synthesis
All the polymers studied in this work were synthesized via RAFTpolymerization according to ref. 33. The chemical structuresand reactions are presented in Fig. 1.
A typical procedure for the preparation of the statisticalcopolymer (PNIPAM-st-PVCL) was as follows. NIPAM (0.56 g, 5mmol), VCL (1.13 g, 10 mmol), AIBN (0.0032 g, 0.02 mmol) andCTA (0.0236 g, 0.1 mmol) were placed in a 25 ml ask anddissolved in 5 g of methanol. Aer three freeze–evacuate–thawcycles, the reaction mixture was sealed under vacuum and thepolymerization was conducted at 60 �C for 24 h. The reactionwas then quenched by rapid cooling in liquid nitrogen andexposure to air. Aer purifying by three times' precipitation withdiethyl ether, the product was nally vacuum-dried for 24 h. Thedetailed synthesis procedures of other polymers are shown inthe ESI.†
2.3 Instruments and measurements
The number average molecular weight (Mn) and polydispersityindex (PDI ¼ Mw/Mn) of PNIPAM, PVCL, diblock and statisticalcopolymers of NIPAM and VCL were determined by GPCmeasurements with monodisperse poly(methylmethacrylate)(PMMA) as a standard and DMF as an eluent phase at 35 �C. 1HNMR spectra of the investigated polymers were recorded on aVarianMercury plus (400MHz) spectrometer using CDCl3 as thesolvent. Turbidity measurements were performed on a Lamda35 UV-vis spectrometer at 500 nm with deionized water as a
Fig. 1 Chemical structures and synthesis of the polymers.
This journal is © The Royal Society of Chemistry 2015
reference (100% transmittance). Temperatures were manuallyregulated with a water-jacketed cell holder at the rate of ca.0.4 �C min with an increment of 1 �C. Each temperature pointwas held for 2 min before measurements to ensure the thermalequilibrium of the sample cell. The samples for turbiditymeasurements were highly diluted in water (0.2 mg ml�1) toprevent aggregation.
In the FTIR measurements of polymers solutions, theconcentration of different samples was xed at 10 wt%. Partic-ularly, in the homopolymer mixture, the weight ratio of PNI-PAM : PVCL was 1 : 1. Hereby, the solutions were sealedbetween two pieces of CaF2 tablets, which had no absorbance inthe middle infrared bands. All the temperature-dependent FTIRspectra were recorded by applying a Nicolet Nexus 6700 FTIRspectrometer equipped with a DTGS detector. 32 scans with aresolution of 4 cm�1 were accumulated to achieve an acceptablesignal-to-noise ratio. Temperatures were under programmedcontrol with an electronic cell holder at a heating rate of ca.1 �Cper 3 min with an increment of 0.5 �C (accuracy: 0.1 �C). Theneat lms of the studied polymers for FTIR tests were preparedby drop casting on the CaF2 tablets from their CH2Cl2 solutions.The baseline correction was performed by the soware ofOMINIC 6.1a.
2.4 Investigation method
2.4.1 Two-dimensional correlation spectroscopy (2Dcos).The temperature-resolved FTIR spectra recorded at an incre-ment of 0.5 �C in specic wavenumber ranges were selected toconduct 2Dcos analysis. 2Dcos was carried out by utilizing thesoware of 2D Shige ver. 1.3 (Shigeaki Morita, Kwansei GakuinUniversity, Japan, 2004–2005) and then plotted into contourmaps with Origin Program ver. 8.0. In the contour maps, redcolors are dened as positive intensities, while the green colorsare dened as negative ones.
3. Results and discussion3.1 Polymer preparation and characterization
The diblock and statistical copolymers of NIPAM and VCL, aswell as their homopolymers are synthesized via RAFT poly-merization. And the corresponding molecular characterizationsare summarized in Table S1.† Hereby, all the studied polymersare of almost the same molecular weight, but differentsegmental arrangements. Moreover, to better control the vari-ations, the diblock and statistical copolymers, together with thehomopolymer mixture are prepared with similar compositions(mole ratio of NIPAM to VCL is around 1/0.8), which can bedetermined from the 1H NMR spectra (Fig. S1†). The chemicalcompositions can be also conrmed from the FTIR spectra ofthose polymers, where C–H stretching bands of the diblock andstatistical copolymers and the homopolymer mixture are nearlyin the same shape (Fig. 2).
3.2 Turbidity measurements
To illustrate the thermoresponsive properties of the homopoly-mer mixture, diblock and statistical copolymers, turbidity
Soft Matter, 2015, 11, 2771–2781 | 2773
Fig. 2 FTIR spectra of neat films of PNIPAM (black), mixture (red),diblock (blue), statistical copolymer (green) and PVCL (pink).
Soft Matter Paper
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
measurements are rst employed to provide a direct view of thesolubility changes of their dilute aqueous solutions uponheating. For the sake of comparison, aqueous solutions of purePNIPAM and PVCL, prepared by RAFT polymerization, are alsoanalyzed under the same conditions. The resulting cloud pointcurves of those studied polymers at the concentration of 0.2 mgml�1 in the heating process are displayed in Fig. 3. Herein, thecloud point is taken as the initial turn point in the trans-mittance versus temperature curves. Notably, it is shown thatthe cloud point temperature (Tcp) of PVCL is remarkably higherthan that of PNIPAM, which could be attributed to the effect ofdilution since the transition point of PVCL is highly concen-tration dependent.22 To some extent, the enhanced difference ofTcps of PNIPAM and PVCL homopolymer aqueous solutionswith dilution will help us to better understand the phase tran-sition behavior of the homopolymer mixture, diblock andstatistical copolymers. Under such conditions, several
Fig. 3 Turbidity curves of PNIPAM (black), mixture (red), diblockcopolymer (blue), statistical copolymer (green) and PVCL (pink) in H2Oupon heating at the concentration of 0.2 mg ml�1.
2774 | Soft Matter, 2015, 11, 2771–2781
observations can be made by comparing the turbidity behaviorof the samples. It is clearly demonstrated that there exists onlyone LCST-transition in all the cloud point curves, indicatingthat the dehydration of PNIPAM and PVCL segments does notoccur independently in the investigated systems. Moreover, theTcps of the homopolymer mixture and the diblock copolymerare close, and they are both slightly higher than that of purePNIPAM, which might be attributed to the increased hydro-philicity of the polymer component with the incorporation ofthe PVCL part. Diblock copolymers are usually expected to formthe micelle structures in selective solvent. However, such aphenomenon has not been observed here in the PNIPAM-b-PVCL system, which could be explained by the hydrophobicinteractions of PNIPAM and PVCL segments.26 To be specic,aer heating the solution above the cloud point of PNIPAM butbelow that of PVCL, PVCL segments preferentially move towardsthe collapsed PNIPAM segments, attracted by the hydrophobicinteractions, rather than dissolve and spread into water. In thisway, the PNIPAM and PVCL segments in the diblock copolymerexhibit cooperative aggregation during the phase transitionprocess. Unlike the sharp decrease in transmittance of thediblock copolymer system at the cloud point, the homopolymermixture exhibits a more continuous transition, behaving in asimilar way to pure PVCL. This reveals that the dehydrationprocess of polymers occurs differently in the homopolymermixture and the diblock copolymer systems, which will befurther discussed later. As for the statistical copolymer, itdisplays a relatively abrupt LCST-transition with the cloud pointlying between that of pure PNIPAM and PVCL. Besides, the Tcpof the statistical copolymer is obviously higher than that of thehomopolymer mixture and the diblock copolymer, which couldbe ascribedto that copolymerization of the VCL unit with PNI-PAM would not only interrupt the interactions between NIPAMunits but also increase the hydrophilicity of the whole polymerchains. Similar results can be as well conrmed by the dynamiclight scattering (DLS) curves (Fig. S2†), where the hydrodynamicdiameter (Dh) displays an abrupt increase at the Tcp whichlocates at almost the same temperature as that found in theturbidity curves for each polymer system. In addition, it is foundout that both unimers and associates exist in the mixture,diblock and statistical copolymer solutions below the cloudpoints (Fig. S3†), suggesting the possibility of the formation ofmicelle structures in those thermal-responsive systems, whichmight be derived from the slight interactions (hydrogenbonding or hydrophobic interaction) among PNIPAM or PVCLunits. Nevertheless, it is should be noted that the scatteringintensity of large particles is far stronger than that of small onesin DLS measurements. Thus, it is presumed that mainlyunimers exist in the solutions below the cloud points, and theamount of associates could be neglected.
3.3 Conventional IR analysis
To further elucidate and compare the phase transition behaviorof those polymer systems from the molecular level, tempera-ture-dependent FTIR spectra of the homopolymer mixture,diblock and statistical copolymers in D2O (10 wt%) were
This journal is © The Royal Society of Chemistry 2015
Paper Soft Matter
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
obtained, as shown in Fig. 4. It should be noticed that D2O,rather than H2O, is selected as the solvent here in order toeliminate the overlap of the d(OH) relating to H2O around 1640cm�1 with the amide I region of PNIPAM and PVCL, as well asreduce the inuence of broad n(OH) of H2O on v(CH). As havebeen reported, the transition temperatures of both PNIPAM andPVCL are only slightly affected by the isotopic substitutions.9,34
Thus, it is suitable to use D2O instead of H2O for this study.Herein, we will mainly focus on the C–H stretching vibra-
tion region (3020–2835 cm�1) and the C]O stretching vibra-tion region (1680–1560 cm�1), so that we are able to tracenearly all the group motions in both PNIPAM and PVCL parts.By carefully inspecting the temperature-resolved FTIR spectra,two main changes can be observed upon heating in these threesystems: all the bands related to C–H stretching vibrationexhibit an evident red shi while the Amide I regions presentan apparent bidirectional spectral intensity variation. It isconsidered that water clathrates, where water molecules havea well-ordered structure, exist around the hydrophobic moie-ties (such as C–H groups) of water-soluble polymers.35 Thehigher the number of water molecules surrounding C–Hgroups is, the higher the vibrational wavenumber is.36 Fromthis point of view, the changes of the v(CH) bands could beexplained by the dehydration of C–H groups with increasedtemperature. Referring to the v(C]O) bands, they can all beroughly divided into two parts: the higher wavenumber moietyand the lower one. It is clearly demonstrated in the spectrathat the intensity of the higher wavenumber moiety increaseswhile the lower one decreases in the heating process, indi-cating that there exists a transformation among differentstates of carbonyl groups in the phase transition of thoseLCST-type polymers. Additionally, it is easy to nd that thehomopolymer mixture and the diblock copolymer share arelatively similar peak shape and variation of the v(C]O)region, which differs largely from that of the statisticalcopolymer. This phenomenon, to some extent, parallels theresults of Tcps in the turbidity curves.
For better understanding of those discoveries within theC]O groups, the second-derivative analysis of the amide Iregion is carried out, as presented in Fig. 5. It is believed thatthe sharpened minima in the second-derivative curves can berelated to themaxima in the original absorption spectra. For the
Fig. 4 Temperature-dependent FTIR spectra of mixture, diblock and sta
This journal is © The Royal Society of Chemistry 2015
homopolymer mixture and diblock copolymer systems, thesecond-derivative curves look the same and four peaks, locatedat around 1650, 1625, 1610 and 1590 cm�1 can be identied.On the basis of previous reports,7,23,26 those four peaks couldbe distributed to hydrogen bonds of C]O/D–N (PNIPAM),C]O/D–O–D (PNIPAM), C]O/D–O–D (PVCL) andC]O/2D–O–D (PVCL), respectively. With regard to the statis-tical copolymer, the situation is somewhat different. As shownin the gure, only three distinct bands, namely 1650, 1625 and1590 cm�1, are recognized. The band, 1610 cm�1, whichappears in the homopolymer mixture and diblock copolymersystems, is missing here. Through deeper examination, weshould nd that the missed band in the statistical copolymersystem is compensated by the increased intensity of band at1590 cm�1. Meanwhile, it is also the origin of the aboveobservation where the temperature-dependent FTIR spectra ofthe statistical copolymer in the v(C]O) region express differ-ently. By taking the corresponding assignments of those twobands into consideration, it can be deduced that the C]Ogroups of PVCL segments in the statistical copolymer aqueoussolution system are more hydrated than that in the homopol-ymer mixture and diblock copolymer systems. In spite of thedissimilarities among those three systems, the responses of theC]O groups during the phase transition upon heating allfollow the analogous change: interactions between carbonylgroups (of both PNIPAM and PVCL parts) and water moleculesgradually break with the formation of intra-/inter-molecularhydrogen bond C]O/D–N (PNIPAM part) and free or relativelyfree C]O (PVCL part). Apart from above observations, we couldas well gure out that the direct hydrogen bonding interactionof C]O in PVCL parts and D–N in PNIPAM parts is not thatpreferential in the three systems since no obvious bands relatedto C]O (PVCL)/D–N (PNIPAM) have been detected in thesecond-derivative spectra of the amide I region. Such a situationis quite different from that observed in the PNIPAM/PDEAAMsystems,22,37 where intra-/inter-molecular hydrogen bondingbetween the hydrogen-bond donor (PNIPAM) and the hydrogen-bond acceptor (PDEAAM) holds priority in the homopolymermixture and the statistical copolymer. In some sense, thisinspection might explain the difference of the LCSTs of thehomopolymer mixture and the statistical copolymer betweenPNIPAM/PVCL and PNIPAM/PDEAAM systems.
tistical copolymers in D2O (10 wt%) on heating between 25 and 45 �C.
Soft Matter, 2015, 11, 2771–2781 | 2775
Fig. 5 Second-derivative spectra in the C]O stretching region of 10 wt% mixture, diblock and statistical copolymers in D2O during the heatingprocess.
Soft Matter Paper
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
As has been discussed in our previous report,26 we are able tomonitor the dehydration behavior of PNIPAM and PVCL partsseparately by simply tracking the frequency variations ofvas(CH3) and vs(CH2), respectively. Therefore, quantitativeanalysis of the temperature-dependent frequency shis of theseC–H stretching bands is carried out, which are plotted in Fig. 6.For the convenience of comparison, the heat-induced changesof vas(CH3) in pure PNIPAM and vs(CH2) in pure PVCL are alsodisplayed hereby. It is observed that both C–H bands exhibitdecrease in wavenumber during the phase transition process inall investigated systems, revealing the decreased hydrophobichydration of C–H groups upon heating. However, we can alsond that different systems demonstrate different changebehavior. For example, the dehydration behaviour of C–Hgroups in the LCST-transition of pure PNIPAM is rather sharpwhile that in pure PVCL is relatively continuous, which is inconsistence with the previous literature,7,23 indicating thatPNIPAM presents a sharp transition while PVCL presents acontinuous one. Under such a circumstance, we may wonder
Fig. 6 Temperature-dependent frequency shifts of vas(CH3) and vs(CH2) ccopolymer (green) and PVCL (pink) in D2O during the heating process.
2776 | Soft Matter, 2015, 11, 2771–2781
how the transition behavior of PNIPAM and PVCL parts will bein the homopolymer mixture, diblock or statistical copolymers.In the phase transition of the homopolymer mixture, thesudden frequency decrease of vas(CH3) and vs(CH2) begins atalmost the same temperature, which, to some extent, conrmsthe results of turbidity curves and suggests that there shouldexist some kind of interaction between PNIPAM and PVCLduring the LCST-transition of the homopolymer mixturesystem. Then, by examining the variation behavior of the C–Hgroups in the homopolymer mixture more carefully, we candiscover that the changes with regard to PNIPAM and PVCLparts show obvious inconformity, where PNIPAM exhibits anabrupt change while PVCL exhibits a gradual one around theLCST. In a sense, this detection clearly implies that PNIPAM andPVCL parts resemble the properties of their own homopolymersin the phase transition of the homopolymer mixture systemapart from those interactions. In contrast, the changes of PNI-PAM and PVCL parts are identical, namely, both sharp, in theLCST-transition of the diblock copolymer aqueous solution
orresponding to PNIPAM (black), mixture (red), diblock (blue), statistical
This journal is © The Royal Society of Chemistry 2015
Paper Soft Matter
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
upon heating. Then, we are able to conclude that the chemicalbond between PNIPAM and PVCL within the diblock copolymerchain plays an important role in the cooperative dehydration ofboth segments. In this way, the difference in the phase transi-tion between the homopolymer mixture and the diblockcopolymer can be emphasized, though their LCSTs stay close.Moreover, this microscopic difference should be also respon-sible for the dissimilarity of the optical change in the LCST-transition of the homopolymer mixture and diblock copolymersystems, where the transmittance loss of the former is markedlyslower than that of the latter during the heating process (Fig. 3).Concerning the statistical copolymer, it is noted that both thetransition temperature and the transition behavior of the PNI-PAM and PVCL segments act in unison, hinting that PNIPAMand PVCL segments herein work collaboratively during thephase separation process. Furthermore, the dehydrationprocedure of the statistical copolymer chains consists of twosteps: a somewhat steep dehydration near the LCST and arelatively continuous change thereaer, which approximates tothe situation of the diblock copolymer and could be originatedfrom the combined LCST-transition features of pure PNIPAMand PVCL.26 Even though, when focusing on the details, we canapparently nd that the statistical copolymer exhibits a ratherbroader LCST-transition than the diblock copolymer. Consid-ering that the sharp phase transition of pure PNIPAM is rootedfrom the formation of inter-/intra-molecular hydrogen bondsC]O/H–N, we might attribute this nding to the fact that theincorporation of VCL units into PNIPAM chains would interruptthe construction of hydrogen bonding structure C]O/H–Namong PNIPAM segments, further making the LCST-transitionless precipitous. What's more, it is clearly demonstrated in thecurves that the initial wavenumber of vas(CH3) is lower andvs(CH2) is higher in the homopolymer mixture, diblock andstatistical copolymer systems than that in the correspondingpure PNIPAM and PVCL ones, respectively, indicating that watermolecules have been redistributed along the polymer chainswhen PNIPAM and PVCL segments are brought into one system.To be more specic, water molecules bind preferentially ontoPVCL sites, while that around PNIPAM ones are reduced in thecomposite system. This could be explained by the fact that VCL
Table 1 Comparison of phase transition temperatures and behavior
Turbidity
Transition temperaturea (�
FTIR
vas(CH3) (PNIPAM part)
PNIPAM 34 29PVCL 47 —Homopolymer mixture 35 30.5Diblock copolymer 35 31.5Statistical copolymer 38 31.5
a The transition temperatures herein are determined as the initial turn pvas(CH3) and vs(CH2).
This journal is © The Royal Society of Chemistry 2015
units are more hydrophilic than NIPAM ones. Additionally, thesituation is especially remarkable in the statistical copolymersystem, which can be also veried in the above second-deriva-tive analysis of the amide I region. And we might ascribe it tothe enhanced competition of hydration between PNIPAM andPVCL segments caused by the monomer pattern along thestatistical copolymer chains.
The phase transition behavior of the homopolymer mixture,diblock and statistical copolymers in their aqueous solutions,as well as the inherent distinction among them, has beenbasically pictured until now. For clarity, the comparison of thephase transition temperatures and behavior of the PNIPAM,PVCL, PNIPAM/PVCL mixture, diblock and statistical copoly-mers are summarized in Table 1. While, a more concretedescription with regard to the group motions from the molec-ular level still remains unclear. Thereupon, 2Dcos analysis ofFTIR spectra is performed. Herein, we will mainly focus on thehomopolymer mixture and statistical copolymer systems, due tothe lack of exploration.
3.4 Two-dimensional correlation spectroscopy (2Dcos)
2Dcos is a mathematical method, and its basic principles wererstly proposed by Noda.38,39 Until now, it has been intensivelyutilized to interpret spectroscopic uctuations of variouschemical species under different types of external perturbations(e.g., temperature, time, pressure, concentration, electromag-netic, etc.).40–42 By spreading the original peaks along a seconddimension, those complex or overlapped features, not readilyvisible in the conventional analysis, can be sorted out and thus,an improved spectral resolution is obtained. Furthermore,additional information on molecular motions or conforma-tional changes can be achieved in 2Dcos for different responsesof different species to external variables, which will help tounderstand the perturbation-induced change from an intui-tionistic view.
3.4.1 2Dcos analysis of the homopolymer mixture system.To acquire the 2Dcos maps of the homopolymer mixturesystem, all the FTIR spectra during heating from 25 to 45 �Cwith an increment of 0.5 �C have been employed and the cor-responding synchronous and asynchronous contour maps are
C) Transition behavior
PNIPAM part PVCL partvs(CH2) (PVCL part)
— Sharp —33.5 — Gentle31 Sharp Gentle31.5 Sharp Sharp32.5 Relatively sharp Relatively sharp
oint in turbidity curves and temperature-dependent frequency shis of
Soft Matter, 2015, 11, 2771–2781 | 2777
Fig. 7 2D synchronous and asynchronous spectra of 10 wt% PNIPAM/PVCL mixture in D2O obtained from all spectra between 25 and 45 �Cduring heating (red colors are defined as positive intensities, while the green colors are defined as negative ones).
Soft Matter Paper
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
shown in Fig. 7. A 2D synchronous map can provide informa-tion of simultaneous variations between two given wave-numbers. For instance, the bands at 2991 and 1587 cm�1 holdpositive cross-peaks in the synchronous map, reecting asimilar response of spectral intensities to the increasingtemperature of those two bands. Aer reading the raw data forreference, we will nd that the intensities of the two bands bothdecrease upon heating. Meanwhile, the 2D asynchronous mapcan signicantly enhance the resolution of the original spectra.In the C–H stretching region, hydrated and dehydrated states ofC–H groups, which have been superimposed in the originalspectra, are recognized. Moreover, several subtle bands relatedto C]O groups appear in the 2D asynchronous map, revealingthe diversity in the hydrogen bonding structure with regard tothose C]O groups. For the convenience of discussion, all thebands detected in the 2D asynchronous map and their corre-sponding assignments are listed in Table 2.
Table 2 Tentative band assignments of the PNIPAM/PVCL mixture inD2O according to 2Dcos results7,23,26
Wavenumber(cm�1) Tentative assignments
2991 vas(hydrated CH3) (PNIPAM segments)2972 vas(dehydrated CH3) (PNIPAM segments)2953 vas(hydrated CH2) (PNIPAM and PVCL segments)2929 vas(dehydrating CH2) (PNIPAM and PVCL segments)2914 vas(dehydrated CH2) (PNIPAM and PVCL segments)2856 vs(CH2) (PVCL segments)1660 v(free C]O) (PNIPAM segments)1647 v(C]O/D–N) (PNIPAM segments)1628 v(C]O/D–O–D) (PNIPAM segments)1606 v(C]O/D–O–D) (PVCL segments)1587 v(C]O/2D–O–D) (PVCL segments)
2778 | Soft Matter, 2015, 11, 2771–2781
In addition to enhancing spectral resolution, 2Dcos can bealso used to discern the specic sequence occurring alongexternal perturbations. A cross-peak in the 2D asynchronousmap can develop only if the intensities of two spectral featureschange out of phase with each other (i.e., accelerated or delayedwhen the external variable is time). The determining rule can beoutlined as Noda's rule43—namely, if the cross-peaks (v1, v2,assuming v1 > v2) in the synchronous and asynchronous maphave the same sign, the change at v1 may take place prior to orearlier than that of v2, and vice versa. A simplied method forthe determination of the sequence order has been reportedpreviously.44 And herein, the nal sequence order for the heat-induced phase transition of the homopolymer mixture in D2O isdescribed as follows: 2953/ 1628/ 2856/ 2914/ 2929/
2991 / 2972 / 1647 / 1660 / 1587 / 1606 cm�1
or vas(CH2) (hydrated) (PNIPAM and PVCL segments) /
v(C]O/D–O–D) (PNIPAM segments) / vs(CH2) (PVCLsegments) / vas(CH2) (dehydrated) (PNIPAM and PVCLsegments) / vas(CH2) (dehydrating) (PNIPAM and PVCLsegments) / vas(CH3) (hydrated) (PNIPAM segments) /
vas(CH3) (dehydrated) (PNIPAM segments) / v(C]O/D–N)(PNIPAM segments) / v(free C]O) (PNIPAM segments) /
v(C]O/2D–O–D) (PVCL segments)/ v(C]O/D–O–D) (PVCLsegments). Deduced from the overall sequence, it is noticeablydemonstrated that almost all the hydrophobic C–H groups holdan earlier response to temperature rising than the C]O groups,indicating that the hydrophobic interaction among C–H groupsshould be the driving force for the LCST-transition of thehomopolymer mixture. Accordingly, we may ascribe such aninteraction to be the root of simultaneous dehydration of PNI-PAM and PVCL parts. Considering the bands of C]O groupsseparately, we can discover that the changes of PNIPAM partstake place prior to that of PVCL ones. It is generally known that
This journal is © The Royal Society of Chemistry 2015
Table 3 Tentative band assignments of the statistical copolymer inD2O according to 2Dcos results7,23,26
Wavenumber(cm�1) Tentative assignments
2979 vas(hydrated CH3) (PNIPAM segments)2970 vas(dehydrated CH3) (PNIPAM segments)2939 vas(hydrated CH2) (PNIPAM and PVCL segments)2918 vas(dehydrated CH2) (PNIPAM and PVCL segments)2860 vs(hydrated CH2) (PVCL segments)2852 vs(dehydrated CH2) (PVCL segments)1661 v(free C]O) (PNIPAM segments)1649 v(C]O/D–N) (PNIPAM segments)1628 v(C]O/D–O–D) (PNIPAM segments)1599 v(C]O/D–O–D) (PVCL segments)1589 v(C]O/2D–O–D) (PVCL segments)
Paper Soft Matter
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
the sharp LCST-transition of pure PNIPAM comes from thetransformation of the hydrogen bonding structure C]O/H–O–Hto C]O/H–N. In this case, the earlier response of C]O groupsin PNIPAM herein is also responsible for its abrupt change inthe homopolymer mixture system, as have been observed inFig. 6. Thereupon, in combination with above conventionalFTIR analysis, the chain collapse mechanism of PNIPAM andPVCL in the homopolymer mixture in D2O during the heatingprocess could be proposed. At temperatures lower than LCST,both PNIPAM and PVCL chains are hydrophilic and well solublein water, with C–H and C]O groups associating with watermolecules. As temperature increases, connections betweenpolymer chains and water molecules gradually break, leading tothe strengthened hydrophobic interactions among both poly-mer chains. As soon as the LCST is reached upon heating, thosehydrophobic C–H groups in both PNIPAM and PVCL parts startto aggregate abruptly, further resulting in the parallel collapseof both kinds of polymer chains. Due to the lack of chemicalbonds between PNIPAM and PVCL chains, the cooperativedehydration among PNIPAM and PVCL chains is not thatsignicant. Hence, PNIPAM chains exhibit a sharp andexhaustive collapse while PVCL chains display a rather contin-uous and slow one.
3.4.2 2Dcos analysis of the statistical copolymer system. Tofurther gain the details of group motions in the LCST-transitionof the statistical copolymer aqueous solution, 2Dcos analysis isalso applied with all the FTIR spectra on heating between 25and 45 �C with an interval of 0.5 �C, and the obtainedsynchronous and asynchronous maps are presented in Fig. 8.Thanks to the enhanced spectra resolution of 2Dcos, manysubtle bands with regard to various states of C–H and C]Ogroups have been identied. Herein, it should be noted thatthough the original peak shape of C]O stretching region
Fig. 8 2D synchronous and asynchronous spectra of 10 wt% statistical c25 and 45 �C during heating (red colors are defined as positive intensitie
This journal is © The Royal Society of Chemistry 2015
differs much in the homopolymer mixture and diblock copoly-mer systems, the differentiated bands related to C]O groupsachieved by the 2D asynchronous map remain almost the same,inferring that the species of C]O groups do not vary betweenthe two systems. This phenomenon, in some sense, conrmsour above discussion that the distinctive shape of the C]Ostretching band of the statistical copolymer system is notderived from the formation of new hydrogen bonding structuresbut the transformation among different ones. For clarity, Table3 presents all the bands recognized from 2D asynchronousmaps and their tentative assignments.
Similarly, the specic change sequence of different chem-ical groups taking place under heating of the statisticalcopolymer in D2O can be discerned. According to Noda's rule,the nal sequence is deduced: 2979 / 2939 / 2860 / 1628/ 2970 / 1649 / 2918 / 2852 / 1661 / 1589 / 1599
opolymer of NIPAM and VCL in D2O obtained from all spectra betweens, while the green colors are defined as negative ones).
Soft Matter, 2015, 11, 2771–2781 | 2779
Fig. 9 Schematic illustration and comparison of the LCST-transitionprocesses of the homopolymer mixture, diblock and statisticalcopolymers of NIPAM and VCL in water.
Soft Matter Paper
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
cm�1 or vas(CH3) (hydrated) (PNIPAM segments) / vas(CH2)(hydrated) (PNIPAM and PVCL segments) / vs(CH2)(hydrated) (PVCL segments) / v(C]O/D–O–D) (PNIPAMsegments) / vas(CH3) (dehydrated) (PNIPAM segments) /
v(C]O/D–N) (PNIPAM segments) / vas(CH2) (dehydrated)(PNIPAM and PVCL segments) / vs(CH2) (dehydrated)(PVCL segments) / v(free C]O) (PNIPAM segments) /
v(C]O/2D–O–D) (PVCL segments) / v(C]O/D–O–D)(PVCL segments). Without considering the differences instretching modes, we have C–H/ C]O, which behaves in thesame way as the homopolymer mixture demonstrates, indi-cating that the LCST-transition of statistical copolymer is alsodriven by hydrophobic interactions among those C–H groups.Nevertheless, unlike the obviously predominant rule of thedehydration of C–H groups in the homopolymer mixturesystem, the changes of C]O and C–H take place alternatelyherein, revealing that C]O and C–H groups work coopera-tively in the phase transition process of the statistical copol-ymer aqueous solution. Through careful examination of thesequence order, it was gured out that both C–H and C]Ogroups in PNIPAM segments, respectively, exhibit an earlierresponse than those in PVCL segments. As has been discussedbefore, due to the close connection of PNIPAM and PVCLsegments along the statistical copolymer chains, the compe-tition of hydration between them is enhanced. Since PVCLsegments are more hydrophilic, water molecules willdistribute preferentially around PVCL segments. Under thiscircumstance, hydrophobic interactions in PNIPAM segmentsare intensied, which would further bring about their quickerresponse in the LCST-transition. On the basis of abovediscussion, the comprehensive self-aggregation behavior ofthe statistical copolymer of NIPAM and VCL in water can beobtained. The copolymer chains are well soluble in waterthrough hydrophobic hydration of C–H groups and hydrogenbonding of hydrophilic C]O groups with water molecules atlower temperatures. Due to the competitive hydration betweenPNIPAM and PVCL segments, there exists a heterogeneousdistribution of water molecules, where PVCL segments aresurrounded by more water molecules. Aer heating the solu-tion above LCST, the whole polymer chains begin to collapse,driven by enhanced hydrophobic interactions among C–Hgroups with increase in temperature. Particularly, PNIPAMsegments present a relatively earlier response to heating byvirtue of their less hydrated states. With the help of thecooperative dehydration of C–H and C]O groups in bothsegments and the formation of inter-/intra-molecularhydrogen bonds C]O/H–N among PNIPAM segments, thestatistical copolymer exhibits an abrupt LCST-transition inwater upon heating. Moreover, on account of the continuousdehydration of PVCL segments, further collapse of the statis-tical copolymer chain is observed at the later stage of the LCST-transition. For limpidity of demonstration, the illustrationand comparison of the LCST-transition processes of thehomopolymer mixture, diblock and statistical copolymers ofNIPAM and VCL in water upon heating are displayed in Fig. 9.(The description of the LCST-transition of diblock copolymerin water is according to ref. 26.)
2780 | Soft Matter, 2015, 11, 2771–2781
4. Conclusion
In this work, we mainly employed turbidity and FTIRmeasurements, as well as 2D correlation spectroscopy (2Dcos)analysis to explore the LCST-transitions of the homopolymermixture of PNIPAM and PVCL, diblock and statistical copoly-mers of NIPAM and VCL aqueous solutions in comparisonwith pure PNIPAM and PVCL. Only one transition point isdetected for all of the three investigated polymer systems,revealing that PNIPAM and PVCL segments do not behaveindependently in the LCST-transitions. The cloud pointtemperatures (Tcps) and macroscopic features of the transi-tions are determined from the turbidity curves, where thehomopolymer mixture and the diblock copolymer displayclose Tcps while the statistical copolymer presents a relativelyhigher one. Besides, both diblock and statistical copolymersexhibit rather sharp phase transitions while the homopolymermixture demonstrates a moderately continuous one. Suchobservations can be then conrmed and explained by thequantitative analysis of temperature-dependent FTIR spectraon the frequency shis of vas(CH3) and vs(CH2), which reectsthe dehydration behavior of PNIPAM and PVCL segments fromthe molecular level upon heating, respectively. By comparingwith pure homopolymers, it is discovered that water distri-butions are slightly changed along the polymer chains in thecomposite polymer systems, in which PNIPAM segmentsbecome less hydrated while PVCL segments become morehydrated. This situation is especially obvious in the statisticalcopolymer system, and it could be derived from the enhancedcompetition of hydration between PNIPAM and PVCLsegments under close connection. In addition, it is displayedin both 2Dcos analyses of the homopolymer mixture andstatistical copolymer systems that C–H groups, as the hydro-phobic part, exhibit an earlier response than C]O groups totemperature rising, which is also in accordance with ourprevious nding in the diblock copolymer system, indicatingthat the hydrophobic interaction is the driving force andpredominates the LCST-transitions thereof.
This journal is © The Royal Society of Chemistry 2015
Paper Soft Matter
Publ
ishe
d on
10
Febr
uary
201
5. D
ownl
oade
d by
Fud
an U
nive
rsity
on
27/0
3/20
15 0
7:56
:21.
View Article Online
Acknowledgements
We are very grateful for the nancial support of the NationalNatural Science Foundation of China (NSFC) (no. 21274030).
Notes and references
1 S. Dai, P. Ravi and K. C. Tam, So Matter, 2009, 5, 2513.2 D. Roy, W. L. A. Brooks and B. S. Sumerlin, Chem. Soc. Rev.,2013, 42, 7214.
3 M. I. Gibson and R. K. O'Reilly, Chem. Soc. Rev., 2013, 42,7204.
4 E. S. Gil and S. M. Hudson, Prog. Polym. Sci., 2004, 29, 1173.5 H. G. Schild, Prog. Polym. Sci., 1992, 17, 163.6 H. Wei, S. X. Cheng, X. Z. Zhang and R. X. Zhuo, Prog. Polym.Sci., 2009, 34, 893.
7 B. Sun, Y. Lin, P. Wu and H. W. Siesler, Macromolecules,2008, 41, 1512.
8 Y. Maeda, T. Nakamura and I. Ikeda, Macromolecules, 2002,35, 217.
9 A. Laukkanen, L. Valtola, F. M. Winnik and H. Tenhu,Macromolecules, 2004, 37, 2268.
10 W. Li and P. Wu, Polym. Chem., 2014, 5, 761.11 H. Vihola, A. Laukkanen, L. Valtola, H. Tenhu and
J. Hirvonen, Biomaterials, 2005, 26, 3055.12 A. C. W. Lau and C. Wu, Macromolecules, 1999, 32, 581.13 I. Dimitrov, B. Trzebicka, A. H. E. Muller, A. Dworak and
C. B. Tsvetanov, Prog. Polym. Sci., 2007, 32, 1275.14 R. Takahashi, X. P. Qiu, N. Xue, T. Sato, K. Terao and
F. M. Winnik, Macromolecules, 2014, 47, 6900.15 M. Karesoja, E. Karjalainen, S. Hietala and H. Tenhu, J. Phys.
Chem. B, 2014, 118, 10776.16 E. Djokpe and W. Vogt, Macromol. Chem. Phys., 2001, 202,
750.17 S. Zuber, K. Landfester, D. Crespy and A. M. Popa, J. Polym.
Sci., Part A: Polym. Chem., 2013, 51, 3308.18 L. Starovoytova, J. Spevacek, L. Hanykova and M. Ilavsky,
Polymer, 2004, 45, 5905.19 L. Starovoytova, J. Spevacek and M. Ilavsky, Polymer, 2005,
46, 677.20 H. Kourilova, J. Spevacek and L. Hanykova, Polym. Bull.,
2013, 70, 221.
This journal is © The Royal Society of Chemistry 2015
21 W. Z. Zhang, X. D. Chen and M. Q. Zhang, Polym. Bull., 2014,71, 243.
22 F. A. Plamper, A. A. Steinschulte, C. H. Hofmann, N. Drude,O. Mergel, C. Herbert, M. Erberich, B. Schulte, R. Winter andW. Richtering, Macromolecules, 2012, 45, 8021.
23 S. Sun and P. Wu, J. Phys. Chem. B, 2011, 115, 11609.24 F. Meeussen, E. Nies, H. Berghmans, S. Verbrugghe,
E. Goethals and F. Du Prez, Polymer, 2000, 41, 8597.25 F. M. Winnik, H. Ringsdorf and J. Venzmer, Macromolecules,
1990, 23, 2415.26 L. Hou and P. Y. Wu, So Matter, 2014, 10, 3578.27 Y. Maeda, T. Nakamura and I. Ikeda, Macromolecules, 2001,
34, 8246.28 Y. Lu, K. Zhou, Y. Ding, G. Zhang and C. Wu, Phys. Chem.
Chem. Phys., 2010, 12, 3188.29 B. Sun, H. Lai and P. Wu, J. Phys. Chem. B, 2011, 115, 1335.30 C. Leddet, A. Fischer, A. Brembilla and P. Lochon, Polym.
Bull., 2001, 46, 75.31 N. Gaillard, A. Guyot and J. Claverie, J. Polym. Sci., Part A:
Polym. Chem., 2003, 41, 684.32 H. Mori, H. Ookuma, S. Nakano and T. Endo, Macromol.
Chem. Phys., 2006, 207, 1005.33 D. C. Wan, Q. Zhou, H. T. Pu and G. J. Yang, J. Polym. Sci.,
Part A: Polym. Chem., 2008, 46, 3756.34 H. Shirota, N. Kuwabara, K. Ohkawa and K. Horie, J. Phys.
Chem. B, 1999, 103, 10400.35 E. C. Cho, J. Lee and K. Cho,Macromolecules, 2003, 36, 9929.36 P. Schmidt, J. Dybal andM. Trchova, Vib. Spectrosc., 2006, 42,
278.37 M. Keerl, V. Smirnovas, R. Winter and W. Richtering, Angew.
Chem., Int. Ed., 2008, 47, 338.38 I. Noda, Appl. Spectrosc., 1993, 47, 1329.39 I. Noda, A. E. Dowrey, C. Marcott, G. M. Story and Y. Ozaki,
Appl. Spectrosc., 2000, 54, 236A.40 I. Noda, J. Mol. Struct., 2008, 883, 2.41 I. Noda, J. Mol. Struct., 2014, 1069, 3.42 I. Noda, J. Mol. Struct., 2014, 1069, 23.43 I. Noda, Appl. Spectrosc., 2000, 54, 994.44 S. Sun, H. Tang, P. Wu and X. Wan, Phys. Chem. Chem. Phys.,
2009, 11, 9861.
Soft Matter, 2015, 11, 2771–2781 | 2781
