21360 | Phys. Chem. Chem. Phys., 2016, 18, 21360--21370 This journal is© the Owner Societies 2016

Cite this:Phys.Chem.Chem.Phys.,

2016, 18, 21360

Exploring the influence of the poly(4-vinylpyridine) segment on the solution propertiesand thermal phase behaviours of oligo(ethyleneglycol) methacrylate-based block copolymers:the different aggregation processes with variousmorphologies†

Yalan Dai and Peiyi Wu*

The assembly properties, thermal phase behavior and microdynamics of well-defined P(MEO2MA-co-

OEGMA)-b-P4VP, (poly(2-(2-methoxyethoxy)ethylmethacrylate)-co-poly(oligo(ethylene glycol) methacrylate))-

b-poly(4-vinyl pyridine), in aqueous solution during heating are investigated in detail by dynamic light

scattering (DLS), turbidity measurements, temperature-variable 1H NMR and FTIR spectroscopy in

combination with two-dimensional correlation spectroscopy (2Dcos) and the perturbation correlation

moving window (PCMW) technique. It is observed that the chain length of the relatively hydrophobic

P4VP segment strongly affects the temperature-induced phase transition behavior of the block

copolymers: the copolymers with shorter P4VP7/10 segments exhibit an abrupt phase transition process,

while the copolymer with longer P4VP19 blocks presents a relatively gradual transition behavior.

Moreover, the two systems with different P4VP segment lengths have different morphologies in aqueous

solution: a single-chain globule for shorter P4VP7/10 systems and a core–shell micelle consisting of a

relatively hydrophobic P4VP core and a hydrophilic POEGMA-based shell for the longer P4VP19 system.

Analysis of spectral results clearly illustrates that the dehydration of the CQO groups at the linkages

between backbones and pendant chains predominates the sharp phase transition of P(MEO2MA-co-

OEGMA)-b-P4VP10, while the dehydration of hydrophobic C–H groups on the side chains in P(MEO2MA-

co-OEGMA)-b-P4VP19 leads to the continuous increase of the hydrodynamic diameter (Dh) upon heating.

1. Introduction

Over the past few decades, temperature-responsive polymershave been the focus of considerable interest from both fundamentaland applied perspectives, with applications in controlled drugdelivery, bio-separation and sensing, protein purification, tissueengineering scaffolds, etc.1–4 As the most extensively exploitedthermosensitive polymer, poly(N-isopropylacrylamide) (PNIPAM)is regarded as a classic polymer exhibiting a lower criticalsolution temperature (LCST) in water with a thermally reversible

coil-to-globule phase transition.5 However, it is found that PNIPAMhas several inherent defects such as obvious hysteresis, releasingtoxic amines during hydrolysis and strong adhesion to proteinsby collaborative hydrogen bonding interactions.6 Recently, anemerging class of thermoresponsive polymers that may competewith or even surpass PNIPAM are polymers bearing oligo ethyleneglycol (OEG) side chains, such as poly(oligo(ethylene glycol)metha-crylate) (POEGMA), which combine the biocompatibility of poly-ethylene glycol (PEG) with a controllable and versatile LCSTbehavior.7,8 The LCST value of OEGMA-based polymers can befine-tuned in the range of physiological temperature by simplecontrollable random copolymerization of OEGMA monomerswith different initial comonomer ratios and oligo(ethylene glycol)side chain lengths.9–12 Owing to its excellent features includingtunable LCST, good biocompatibility, antifouling propertiesbelow LCST, and nearly complete reversibility of thermal transition,POEGMA has been widely used in the design of functional polymerswith complex topology structure, hydrogels, polymer brushes,nano-hybrid materials, bioactive surfaces, etc.13–17

The State Key Laboratory of Molecular Engineering of Polymers, Collaborative

Innovation Center of Polymers and Polymer Composite Materials, Department of

Macromolecular Science, and Laboratory of Advanced Materials, Fudan University,

Shanghai 200433, China. E-mail: peiyiwu@fudan.edu.cn

† Electronic supplementary information (ESI) available: GPC measurements and1H NMR spectra (in CDCl3) of P(MEO2MA-co-OEGMA), P(MEO2MA-co-OEGMA)-b-P4VP7, P(MEO2MA-co-OEGMA)-b-P4VP10, and P(MEO2MA-co-OEGMA)-b-P4VP19

and PCMW synchronous and asynchronous spectra of P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19. See DOI: 10.1039/c6cp04286d

Received 19th June 2016,Accepted 7th July 2016

DOI: 10.1039/c6cp04286d

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Recently, the synthesis and self-assembly of well-definedstimuli-responsive block copolymers, especially double hydro-philic block copolymers (DHBCs), consisting of two differenthydrophilic blocks, have attracted considerable interest becauseof their potential applications in the fields of drug delivery,nanotechnology, biological vectors, etc.18–20 As is well known,upon subtle adjustment of the environmental conditions suchas pH, light intensity, temperature or ionic strength, these blockcopolymers can self-assemble into a variety of nanostructures,such as micelles.21 Moreover, DHBCs will exhibit a specific‘schizophrenic’ character when both blocks are environment-sensitive, i.e., the AB block copolymer can form different micelleswith the A-core or the B-core in response to external stimuli.22

Several systems, such as poly(4-vinylbenzoic acid)-block-poly((2-diethylamino)ethylmethacrylate) (PVBA-b-PDEA),23 poly(methacrylicacid)-block-poly((2-diethylamino)ethyl methacrylate) (PMAA-b-PDEA),24 poly(N-isopropylacrylamide)-block-poly(3-[N-(3-methacryl-amidopropyl)-N,N-dimethyl]ammoniopropane sulfonate) (PNI-PAM-b-PSPP),25 and poly(N-isopropylacrylamide)-block-poly(4-vinylpyridine) (PNIPAM-b-P4VP),26 have been investigated. As one of thecommonly used polymers in biomedical applications, such as drugrelease systems, gene therapy studies and antimicrobial polymericsystems,27–30 poly(4-vinyl pyridine) (P4VP) is soluble in water whenthe pH is below 4.7 and becomes insoluble at higher pH withadjustable hydrophilicity.31 In particular, the hydrophilicity andsolution behavior of poly(vinyl pyridine) can be easily modulatedby changing the structure of the quaternizing agent.32,33 Zhu et al.used the atom transfer radical polymerization (ATRP) method toprepare the PNIPAM-b-P4VP block copolymer which couldassemble into micelles with the P4VP core when the temperatureis below its LCST and the pH is above 4.7, and a reverse micellecould be formed under modulating external conditions.26 The pH-and thermo-induced micellization behavior of the PNIPAM59-b-P2VP102 block copolymer using reversible addition-fragmentationchain transfer polymerization (RAFT) in dilute aqueous solutionwas investigated by Shi et al.34 Similarly, a reversible transitionbetween P2VP-core and PNIPAM-core micelles was observedthrough an intermediate unimer state in aqueous solution. Fromthe viewpoint of application, Dincer and his coworkers studied thedi-block copolymers of 4VP and OEGMA as potential drug deliverysystems and demonstrated that the copolymer chains formedmicelles at pH values higher than 5, whereas unimeric polymerswere observed at pH below 5 due to the repulsion of positivelycharged P4VP blocks.35 However, to the best of our knowledge,most of the research studies have only focused on the formation ofvarious micelle morphologies in a DHBC system upon distinctchanges in external stimuli, and the variation of the micro-structure and the thermal microdynamics of thermoresponsiveblocks, such as POEGMA, influenced by the introduction of theP4VP segment have been rarely studied.

Herein, a series of well-defined P(MEO2MA-co-OEGMA)-b-P4VP block copolymers, especially with a relatively higher molarfraction of hydrophilic OEGMA-based polymer blocks, aresynthesized via the RAFT technique. Different from previousresearch studies that focused only on light scattering techniquesand fluorescence measurements, temperature-variable 1H NMR

and temperature-resolved IR spectroscopy in combination withtwo-dimensional correlation spectroscopy (2Dcos) and the per-turbation correlation moving window (PCMW) technique areapplied here to illustrate the microdynamical phase separationmechanism of P(MEO2MA-co-OEGMA)-b-P4VP with different4VP block lengths in aqueous solutions upon thermal perturbation.In particular, the influence of the relatively hydrophobic P4VPsegment on the solution properties of P(MEO2MA-co-OEGMA)-b-P4VP will be discussed. Detailed information on the sequentialorder corresponding to chain conformation changes duringphase transition could be extracted.

2. Experimental section2.1 Synthesis of P(MEO2MA-co-OEGMA)-b-P4VP blockcopolymers

P(MEO2MA-co-OEGMA)-b-P4VP block copolymers were synthe-sized according to previous report.35 Materials and the typicalsynthesis process are provided in the ESI.† The molecularstructures of copolymer samples used in the investigation areshown in Scheme 1 and Table 1. Details of NMR spectra andGPC traces of all the copolymers are provided in Fig. S1 (ESI†).35

2.2 Instruments and measurements1H NMR spectra were recorded on a Varian Mercury plus 400 Mspectrometer with CDCl3 as solvent. A gel permeation chromato-graph (GPC) instrument with a G1362A refractive index detectorwas used to identify the weight-average molecular weights (Mw),number-average molecular weights (Mn), and the polydispersityindex (PDI, Mw/Mn). N,N-Dimethylformamide (DMF) was used as

Scheme 1 Chemical structure of P(MEO2MA-co-OEGMA)-b-P4VP,wherein m E 3 and n E 28.

Table 1 Characterization results and phase transition temperature (Tp) ofthe copolymers

CopolymerMn

a

(g mol�1)Mn

b

(g mol�1) Mw/Mnb

Tpc

(1C)

P(MEO2MA28-co-OEGMA3) 6975 7500 1.4 39P(MEO2MA28-co-OEGMA3)-b-P4VP7 7711 12 300 1.35 33P(MEO2MA28-co-OEGMA3)-b-P4VP10 8026 12 500 1.30 32P(MEO2MA28-co-OEGMA3)-b-P4VP19 8973 15 000 1.22 22

a Calculated using 1H NMR data. b Measured by GPC in DMF (PMMAas calibration). c Determined by turbidity measurements at 0.1 wt%copolymer concentration.

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the eluent at a flow rate of 1.0 mL min�1 and monodispersedpoly(methyl methacrylate) (PMMA) was used as the standard forthe calibration curve. The temperature-variable average Dh ofP(MEO2MA-co-OEGMA)-b-P4VP in H2O (0.1 wt%) was measuredon a dynamic light scattering (DLS)-Zetasizer nanosystem (Malvern)(scattering angle: 901) with an increment of 1 1C. Transmittancemeasurements were performed at 550 nm using a Lambda 35UV-vis spectrometer with a water-jacketed cell holder at the rateof 1 1C min�1. It was held for 2 min at each temperature pointbefore measurement to ensure thermal equilibrium of the samplecell. Temperature-variable 1H NMR spectra of copolymers in D2Osolutions (10 wt%) were recorded on a Bruker AV (500 MHz)spectrometer.

The samples of P(MEO2MA-co-OEGMA)-b-P4VP solutions(10 wt%) in D2O for FT-IR measurements were prepared, andsealed between two ZnS tablets. All the temperature-dependentFTIR spectra were recorded on a Nicolet Nexus 6700 spectro-meter equipped with a deuterated triglycine sulfate (DTGS)detector. And it was collected by signal-averaging 32 scans ata 4 cm�1 resolution to obtain an appropriate signal-to-noiseratio. Temperature was controlled with an electronic cell holderat a rate of ca. 0.3 1C min�1 with a programmed increment of 1 1C.Baseline correction was carried out using OMINIC 8.0 software.

Perturbation correlation moving window (PCMW) and two-dimensional correlation analysis (2Dcos) analysis were performedwith the collected FTIR spectra with an interval of 1 1C duringheating. Primary data processing was carried out and furthercorrelation was calculated using software 2D Shige, ver. 1.3(Shigeaki Morita, Kwansei-Gakuin University, Japan, 2004–2005)with an appropriate window size (2m + 1 = 11). Moreover, it wasplotted using Origin program ver. 8.0 for the final contour maps,with warm colors (red and yellow) defined as positive intensitiesand cool color (blue) regions as negative ones.

3. Results and discussion3.1 Turbidity measurements

Herein, UV/vis transmittance measurements of 0.1 wt% copolymeraqueous solution as a function of temperature were performed,as shown in Fig. 1. The phase transition temperature (Tp) istaken as the initial temperature point where the transmittance

drops below 50%. It can be found that the P(MEO2MA-co-OEGMA)-b-P4VP copolymers, with the introduction of P4VP segments,show lower phase transition temperatures compared with theP(MEO2MA-co-OEGMA) precursor, and the LCST of the copolymeraqueous solution shifts to a lower temperature upon increase ofthe P4VP block length. This phenomenon indicates that thepoly(4-vinyl pyridine) segment in aqueous solution is regardedas a hydrophobic segment, and the transition temperaturedecreases with the higher degree of chain collapse and a lowerdegree of hydration of the OEGMA-based segments due to theassembled structures of these double hydrophilic block copolymersupon heating. Moreover, obviously, the transition temperaturerange of P(MEO2MA-co-OEGMA)-b-P4VP19 (ca. 14 1C) is muchbroader than P(MEO2MA-co-OEGMA)-b-P4VP7 and P(MEO2MA-co-OEGMA)-b-P4VP10 (ca. 6 1C), illustrating that the phasetransition of the copolymer with longer P4VP blocks occursmore slowly than the shorter P4VP block copolymers. It may beattributed to the different hydrophilic–hydrophobic propertiesby introducing P4VP blocks of different lengths and the variousmicelle morphologies existing in aqueous solution. The differencewill be discussed in detail later.

3.2 DLS measurements

To further ascertain the results shown in the turbidity curvesand investigate the LCST-type transition of P(MEO2MA-co-OEGMA)-b-P4VP from a micro aspect, DLS measurements werecarried out with 0.1 wt% copolymer aqueous solutions, aspresented in Fig. 2. The Dh value of P(MEO2MA-co-OEGMA)-b-P4VP chain aggregates can be determined and the aggregationbehavior of the copolymers can be monitored under variationof solution temperature. As expected, upon heating, the Dh

sizes of all the copolymers undergo three stages: the relativeequilibrium stage, then, the increase, and the final slightdecrease. However, the Dh size exhibits an abrupt increase atthe cloud points, 33 and 32 1C, respectively, indicating theaggregation of copolymer chains for P(MEO2MA-co-OEGMA)-b-P4VP7 and P(MEO2MA-co-OEGMA)-b-P4VP10. In contrast, a con-tinuous increase occurs for the longer P4VP block system,P(MEO2MA-co-OEGMA)-b-P4VP19, which parallels well withthe turbidity curves. It indicates that the increased length of

Fig. 1 Transmittance vs. temperature for P(MEO2MA-co-OEGMA)-b-P4VP aqueous solution (0.1 wt%) at the wavelength of 550 nm and aheating rate of 1 1C min�1.

Fig. 2 Dh of 0.1 wt% P(MEO2MA-co-OEGMA)-b-P4VP7 (red), P(MEO2MA-co-OEGMA)-b-P4VP10 (blue) and P(MEO2MA-co-OEGMA)-b-P4VP19

(green) aqueous solutions from 5 to 55 1C with an interval of 1.0 1C.

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hydrophobic P4VP blocks broadens the transition temperatureregion and results in a lower LCST of P(MEO2MA-co-OEGMA)-b-P4VP19. Since there might exist hydrogen bond associated withpyridine and water molecules as well as weak pyridine–pyridineinteraction,36–40 and the hydration of the copolymers in aqueoussolution may be destabilized, it is inferred that the specificinteraction of P4VP and water may impact the initial interactionpattern between the POEGMA chain segment and water molecules,altering the thermal phase behavior consequently. In addition,P(MEO2MA-co-OEGMA)-b-P4VP7 and P(MEO2MA-co-OEGMA)-b-P4VP10 show a hydrodynamic diameter of 5–10 nm in water at 5 1C,and a higher Dh (B600 nm) at 55 1C. While, a relatively narrow Dh

(B200 nm) at initial temperature and a final Dh (B500 nm) at hightemperature belong to P(MEO2MA-co-OEGMA)-b-P4VP19.

For clarity, the Dh number-average size distributions of theseblock copolymers are portrayed under the same conditions inFig. 3. Herein, P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19 are chosen as representatives to study thethermal transition behavior of POEGMA-based copolymers withshorter and longer P4VP block lengths. At low temperature, it isinteresting to note that unimers (Dh B 5–10 nm) are observed inP(MEO2MA-co-OEGMA)-b-P4VP10, demonstrating that the systemhas a single-chain structure, probably a loose single-chainglobule with P4VP segments located at the inside.13,41–44 WhileP(MEO2MA-co-OEGMA)-b-P4VP19 shows a homogeneous Dh sizearound 200 nm at the starting temperature since P4VP is apH-sensitive polymer which is soluble in water (when the pHvalue is below 4.7) and becomes insoluble when the pH ishigh.31 At low temperature, a core–shell micelle is expected tobe formed in aqueous solution with the relatively hydrophobiccore of P4VP blocks and the hydrophilic shell of POEGMA-based blocks.26 With the Dh size increasing with temperature,the thermoresponsive POEGMA-based shell probably under-goes assembly by the hydrophobic interaction with the watermolecule, forming some small aggregates. When it is above thetransition temperature, the Dh shows a slight decrease withthe temperature increasing persistently, which may be due tothe further collapse of the aggregates by the removal of morewater molecules and the formation of more compact andregular structures. The same phenomenon is also observed inPNIPAM-b-poly(ionic liquid).45 Notably, at high temperature, theaggregates of copolymers with shorter P4VP7/10 blocks have a largerstructure (Dh sizes are larger) as a result of a higher aggregationnumber and weaker hydrophobic interaction, in contrast,the aggregates with long P4VP19 blocks are formed with a

smaller size and aggregation number, which can be ascribedto the stronger hydrophobic interactions derived from thelonger P4VP19 segment chains.

3.3 Temperature-variable 1H NMR analysis

To quantitatively describe the phase transition degree ofP(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19, temperature-variable 1H NMR measurements wereperformed, as shown in Fig. 4(a) and (b), respectively. Almost allthe proton types in P(MEO2MA-co-OEGMA)-b-P4VP copolymers canbe observed in 1H NMR spectra in D2O with correspondingassignments in Fig. 4(a). Compared with the sharp and intenseproton signals in CDCL3 in Fig. S1(b) (ESI†), it is noted that in D2O,the intensities of the signals from the pyridine ring (Hl, Hk), thebackbone (Ha, Hb), and protons close to the backbone (Hc)are considerably weakened and broadened, revealing that thehydration of the copolymers in D2O is not uniform with morewater molecules around the hydrophilic side chains and lesswater molecules surrounding the hydrophobic pyridine ringsand the backbones.44

Obviously, in Fig. 4, almost all the protons shift toward alower field along with a drastic intensity decrease upon heating,similar to the case of PNIPAM and POEGMA.6,46 Furthermore,after the phase transition, the signals of protons correspondingto pyridine rings (Hl, Hk), the backbone of P(MEO2MA-co-OEGMA)-b-P4VP(Ha, Hb,) and protons in the proximity of thebackbone (Hc) are hardly detectable, and only the protonsignals from the hydrophilic side chains in the surface ofaggregates can be detected. It is helpful in understanding themicrodynamic responses of P4VP segments and different partsof POEGMA-based chains during the phase transition processby quantitatively analyzing the varying pattern of specific peaks.Hence, the integral area changes of Hl,k in pyridine andHa,b,c,d,e,f,g,h were quantitatively analyzed to figure out thedifferent phase behaviors between P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19. Note that the integralareas are calculated with the HDO peak as the reference.

As presented in Fig. 5, it is obvious that the integral areas ofall the protons decrease upon heating, indicating that theprotons are wrapped after phase transition. Notably, the integralarea shifts of all the peaks in P(MEO2MA-co-OEGMA)-b-P4VP10

solution are more drastic than those in P(MEO2MA-co-OEGMA)-b-P4VP19, which demonstrates that the changes in the hydrationdegree in P(MEO2MA-co-OEGMA)-b-P4VP10 is larger than thatin P(MEO2MA-co-OEGMA)-b-P4VP19 solution. Compared withP(MEO2MA-co-OEGMA)-b-P4VP10, it is inferred that the weakhydration degree in P(MEO2MA-co-OEGMA)-b-P4VP19 solution atinitial temperature is due to the more dehydrated environmentcaused by the core–shell micelle structures formed in aqueoussolution. Additionally, Hl, Hk, Hc, Hd,e,f,g, Hh, Ha, and Hb inP(MEO2MA-co-OEGMA)-b-P4VP10 as well as Hl and Hk in P(MEO2MA-co-OEGMA)-b-P4VP19 exhibit ‘‘anti-S’’ shaped variations, while theintegral area shifts of Hc, Hd,e,f,g, and Hh in P(MEO2MA-co-OEGMA)-b-P4VP19 are inconspicuous and display approximately linearchanges. From the macrocosmic aspect, it is consistent with thedrastic phase transition with P(MEO2MA-co-OEGMA)-b-P4VP10

Fig. 3 Size distributions of (a) P(MEO2MA-co-OEGMA)-b-P4VP10 and (b)P(MEO2MA-co-OEGMA)-b-P4VP19 (0.1 wt%) with an interval of 1.0 1Cduring heating.

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and the gradual dehydration change in P(MEO2MA-co-OEGMA)-b-P4VP19 solution.

3.4 Conventional IR analysis

Temperature-resolved FT-IR measurements of P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19 in D2O(10 wt%) are measured upon heating to elucidate their dynamicmechanism of the thermoresponsive behavior, as shown inFig. 6. Taking account of the slight effect on the thermoresponsivephase behavior of the OEGMA-based polymer by the deuteriumisotope effect,6 H2O was replaced by D2O to eliminate the overlapof the d(O–H) band of H2O around 1640 cm�1 with n(CQO) ofPOEGMA-based blocks as well as the broad n(O–H) band of H2Oaround 3300 cm�1 with the C–H stretching region.47,48 Herein, theC–H stretching region (3050–2810 cm�1), the CQO stretchingregion (1755–1680 cm�1), the CQC stretching region in pyridine(1618–1589 cm�1) and the C–O–C stretching region (1118–1053 cm�1) are specifically focused on.

In Fig. 6, we can find that all the C–H stretching bands of thetwo copolymers shift to lower wavenumbers during the heatingprocess. Since water clathrates exist around the hydrophobicmoieties of water-soluble polymers in a well-ordered structure,a higher number of water molecules surrounding C–H groupswould result in higher vibrational frequency.49,50 The red shiftof the C–H band confirms that the aliphatic groups in thecopolymers suffer from dehydration process upon heating.Besides, obviously, the C–H groups of P(MEO2MA-co-OEGMA)-b-P4VP19 have an relatively abrupt decrement of absorbancecompared with the gradual decrease of P(MEO2MA-co-OEGMA)-b-P4VP10, suggesting that the dehydration of C–H groups inP(MEO2MA-co-OEGMA)-b-P4VP19 probably play a vital role inthe whole dehydration process during heating. As for CQO andC–O–C groups, both the copolymers exhibit a blue shift,demonstrating the dehydration of hydrophobic ester linkagesand the breakage of hydrogen bonds between ether oxygengroups and water.6 However, it should be mentioned that the

Fig. 4 (a) Normalized temperature-variable 1H NMR spectra of P(MEO2MA-co-OEGMA)-b-P4VP10 in D2O (10 wt%) (20–50 1C) with an increment of2 1C. (b) Normalized temperature-variable 1H NMR spectra of P(MEO2MA-co-OEGMA)-b-P4VP19 in D2O (10 wt%) (10–40 1C) with an increment of 2 1C.

Fig. 5 Temperature-dependent integral area shifts of different protons (a) Hl and Hk, (b) Hc, (c) Hd,e,f,g, (d) Hh, and (e) Ha and Hb in P(MEO2MA-co-OEGMA)-b-P4VP10 (black) and P(MEO2MA-co-OEGMA)-b-P4VP19 (red).

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blue shifts of P(MEO2MA-co-OEGMA)-b-P4VP10 are much morenotable than P(MEO2MA-co-OEGMA)-b-P4VP19, which can beascribed to the different dehydration degrees of CQO andC–O–C groups, and we will analyze them quantitatively in detaillater. Similar to the C–H group, a relatively hydrophobic moietyof water soluble copolymers, the red shift changes of CQCgroups in the pyridine ring of these two copolymers areobserved during heating in the present study. Accordingly, itis inferred that the relatively hydrophobic pyridine rings in theinner of the loose single chain globules or micelles formed inaqueous solution experience a gradual dehydration processupon heating.36,50 It can be carefully found that the shifts ofP(MEO2MA-co-OEGMA)-b-P4VP10 in CQC band regions areslighter compared with P(MEO2MA-co-OEGMA)-b-P4VP19, whichmight be attributed to the difference between the morphologystructures of these two copolymers and the different interactiondegrees caused by various lengths of P4VP blocks.

To straightforwardly elucidate the effect of hydration onthe frequency of the above four characteristic bands, spectralcomparisons of P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19 before and after their phase transition andneat film samples are plotted in Fig. 7. It is well known that thechemical groups cannot form hydrophobic hydration or hydrogen

bonds with water in the neat film. Compared with the neat film,the C–H, CQO, CQC (in pyridine) and C–O–C groups in bothcopolymers are all partially hydrated or hydrogen bonded after thephase transitions, indicating that those groups in the copolymerchains undergo only partial dehydration with temperatureincrease. The phenomena may result from the formation ofthe single-chain globules or micelle structure, leading to the lessflexible movement of dehydrated groups. In a sense, by compar-ison of the shifts of C–H and CQO groups below and abovephase transition temperatures, it seems that the dehydrationdegrees of CQO in P(MEO2MA-co-OEGMA)-b-P4VP10 and theC–H group in P(MEO2MA-co-OEGMA)-b-P4VP19 copolymer aremore obvious than the others, which will be deeply investigatedin the following FTIR study.

Additionally, we quantitatively examined the temperature-dependent frequency shifts of the three bands around 2832,1720, and 1600 cm�1, corresponding to ns(CH2)(–OCH2CH2O–),n(CQO) and n(CQC) in pyridine, to obtain more detailedinformation on the influence on the hydration states of differentchemical groups during the phase transition, as presented inFig. 8. Obviously, the frequency shifting tendency of the threebands all exhibits an asymmetric sigmoid curve with a sharpshift below LCST but a gradual shift above LCST, demonstratinga gradual dehydration process upon LCST. However, the C–Hregions of P(MEO2MA-co-OEGMA)-b-P4VP19 show a dramatic changeat phase transition temperature compared with P(MEO2MA-co-OEGMA)-b-P4VP10, suggesting that the dehydration of C–H groupsmight dominate the self-aggregation process of P(MEO2MA-co-OEGMA)-b-P4VP19. To quantitatively analyze the hydration statevariations during the phase transition process, we calculated theshifts of CQO groups in both copolymers with different P4VPblock lengths. The shifts of P(MEO2MA-co-OEGMA)-b-P4VP10

are 6.5 cm�1, which are larger than those in P(MEO2MA-co-OEGMA)-b-P4VP19 (4.0 cm�1), concluding that P(MEO2MA-co-OEGMA)-b-P4VP10 experiences larger hydration changes thanP(MEO2MA-co-OEGMA)-b-P4VP19. This phenomenon can beexplained by the fact that water molecules are expelled deeply

Fig. 6 Temperature-dependent FTIR spectra of P(MEO2MA-co-OEGMA)-b-P4VP10 (20–50 1C) and P(MEO2MA-co-OEGMA)-b-P4VP19 (10–40 1C) inD2O (10 wt%) during heating with an interval of 1 1C in the regions 3050–2810, 1755–1680, 1618–1589, and 1118–1053 cm�1.

Fig. 7 Spectral comparison of P(MEO2MA-co-OEGMA)-b-P4VP10 andP(MEO2MA-co-OEGMA)-b-P4VP19 in D2O (10 wt%) at low and hightemperatures and neat film samples.

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in the inner of the relatively loose P(MEO2MA-co-OEGMA)-b-P4VP10 single-chain globule, accompanied by the adjustment ofconformation and the formation of aggregates.

3.5 Perturbation correlation moving window (PCMW)

In consideration of the asymmetric sigmoid curves displayed inFig. 8, it is difficult for us to accurately determine the specifictransition points of the corresponding groups due to thegradual changes during heating. Therefore, PCMW techniqueis employed to ascertain the transition temperatures as well astransition temperature regions regarding various chemical groupsin the two copolymers with different P4VP segment lengths.51

Further introduction of PCMW technique is covered in the ESI.†Herein, the synchronous and asynchronous spectral maps of

the PCMW are drawn for deeper investigation of the phasetransitions of P(MEO2MA-co-OEGMA)-b-P4VP10 (20–50 1C) andP(MEO2MA-co-OEGMA)-b-P4VP19 (10–40 1C) in D2O duringheating, as shown in Fig. S2 (ESI†). For clarity, all the pointsread from PCMW synchronous and asynchronous spectra areplotted in Fig. 9. In P(MEO2MA-co-OEGMA)-b-P4VP10, the phasetransition temperature of CQO groups is determined to be30 1C, which is lower than those of C–H and CQC groups(around 33 1C), indicating that CQO groups have an earlierresponse than the other three groups. Additionally, the transitiontemperature range of C–H, CQC in pyridine and C–O–C groupsin P(MEO2MA-co-OEGMA)-b-P4VP10 is determined to be between29 and 35 1C, while CQO groups experience the transitionprocess between 27 and 33 1C. The relatively narrow transitiontemperature regions suggest that the C–H, CQC in pyridineand C–O–C groups of P(MEO2MA-co-OEGMA)-b-P4VP10 havecooperative responses to the increasing temperature under

the driving of CQO groups. As for P(MEO2MA-co-OEGMA)-b-P4VP19, there are relatively wide transition temperature rangesalmost from 17 to 26 1C of different groups, presenting a slowdehydration process. These observations accord well with turbidityand DLS results that P(MEO2MA-co-OEGMA)-b-P4VP10 presents asharp phase transition behavior while P(MEO2MA-co-OEGMA)-b-P4VP19 reveals a relatively gradual one. In fact, incorporating P4VPmoieties into linear POEGMA leads to the formation of micellesand more interaction between water and the copolymer, and alonger P4VP length may impact the hydrophilic–hydrophobicbalance and the hydrophobic aggregation of POEGMA. Furthermore,it is noteworthy that in P(MEO2MA-co-OEGMA)-b-P4VP19, the C–Hgroups respond earlier than CQO and the other groups, whichdiffer from P(MEO2MA-co-OEGMA)-b-P4VP10. It is inferred thatthe higher P4VP moieties in the block copolymer significantlyimpact the microscopic dynamic process of dehydration and theresponse of C–H groups may play a crucial role in the continuousphase transition behavior of the longer P4VP19 block system.

3.6 Two-dimensional correlation spectroscopy (2Dcos)

2Dcos analysis of P(MEO2MA-co-OEGMA)-b-P4VP10 (20–50 1C)and P(MEO2MA-co-OEGMA)-b-P4VP19 (10–40 1C) spectra duringheating with an internal temperature of 1 1C is employed toelucidate the contraction of the POEGMA-based block and themicroscopic phase transition mechanism influenced by therelatively hydrophobic P4VP blocks of different lengths, aspresented in Fig. 10 and 11. The generated synchronous spectrawhich reflect simultaneous changes between two wavenumbersand asynchronous spectra greatly enhancing the spectral resolutionof the two systems are shown. All the peaks observed inasynchronous spectra as well as their tentative assignmentsare listed in Table 2. Many additionally observed bands relatedto subtle group conformations are obtained, and they couldprovide more detailed information and significantly assist infiguring out the mechanism of the complex phase transitionprocess. Besides, 2Dcos can also discern the specific ordertaking place under external perturbation. According to Noda’srule (in the ESI†), the resultant sequence orders of all groups inthe two copolymers can be inferred directly and more detailedinformation can be figured out in the following discussion.

Analysis of 2Dcos for P(MEO2MA-co-OEGMA)-b-P4VP10 withshorter P4VP blocks. According to the 2Dcos spectra, using asimplified method for the determination of sequence orderdescribed before,55 the final sequence order of different

Fig. 8 Temperature-dependent frequency shifts of ns(CH2)(–OCH2CH2O–), n(CQO) and n(CQC in pyridine) groups in P(MEO2MA-co-OEGMA)-b-P4VP10 (blue) and P(MEO2MA-co-OEGMA)-b-P4VP19 (green) during heating.

Fig. 9 Transition temperatures and transition temperature regions uponheating, which are read from PCMW synchronous and asynchronousspectra (starting point of the transition process, blue; transition temperature,green; end-point of the transition process, red).

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chemical groups in the P(MEO2MA-co-OEGMA)-b-P4VP10 sampleduring heating is displayed as follows (-means prior to or earlierthan): 1727 - 1099 - 1079 - 1710 - 1598 - 2838 - 3002 -

2952 - 2927 - 2939 - 2987 - 2894 - 2879 - 1608 -

2825 cm�1 or n(CQO) (dehydrated) - n(C–O–C) (dehydrated) -n(C–O–C) (hydrated) - n(CQO) (hydrating) - n(CQC inpyridine) (dehydrated) - ns(CH2)(–CH2CH2O–) (hydrated) -

nas(CH3)(–OCH3) (hydrated) - nas(CH2)(–OCH2CH2O–)(hydrated) - nas(CH2) (backbone) - nas(CH2)(–OCH2CH2O–)

(dehydrated) - nas(CH3)(–OCH3) (dehydrated) - ns(CH3)-(–OCH3) (hydrated) - ns(CH3)(–OCH3) (dehydrated) - n(CQCin pyridine) (hydrated) - ns(CH2)(–OCH2CH2O–) (dehydrated).

Without considering the differences in stretching modes ofthe chemical groups in the copolymer with shorter P4VP blocks, thespecific order can be summarized as follows: CQO - C–O–C -

CQC in pyridine - –OCH2CH2O– - –OCH3 - CH2 (backbone).The dehydration of hydrophobic ester groups responds to thetemperature perturbation at the earliest during the heating

Fig. 10 2D synchronous and asynchronous spectra of P(MEO2MA-co-OEGMA)-b-P4VP10 in D2O (10 wt%) during heating between 20 and 50 1C. Warmcolors (red) are defined as positive intensities, while cool colors (blue) as negative ones.

Fig. 11 2D synchronous and asynchronous spectra of P(MEO2MA-co-OEGMA)-b-P4VP19 in D2O (10 wt%) during heating between 10 and 40 1C. Warmcolors (red) are defined as positive intensities, while cool colors (blue) as negative ones.

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process in the block system, which is in good agreement withthe results obtained from PCMW. Obviously, different from thePOEGMA based linear polymer system in which the dehydrationof C–H groups on the side chains has an early response duringthe transition process,6,54 the sensitivity of C–H groups totemperature seems to be considerably weakened. In other words,the driving force for phase transition of the P(MEO2MA-co-OEGMA)-b-P4VP10 sample is the damage of hydrogen bondswith CQO groups and water. Among the four stretching regions, itis reasonable to believe that CQO groups at the linkages betweenbackbones and pendant chains play a vital part in the self-aggregation transition of the single chain globules formed inaqueous solution. Unexpectedly, the dehydrated CQO and C–O–C respond prior to the hydrated ones in the sequence. Thisunusual phenomenon indicates that CQO and C–O–C groupswould not start to dehydrate until their conformational changeswere completed in P(MEO2MA-co-OEGMA)-b-P4VP10 solution. Asthe conformational changes occur before the dehydration process,the C–H groups have a higher degree of hydration before the LCSTphase transition. In fact, the temperature sensitivity weakeningphenomenon of C–H groups has also been found in the POEGMAbased hydrogel system and the PIL based system.53,56

Analysis of 2Dcos for P(MEO2MA-co-OEGMA)-b-P4VP19 withlonger P4VP blocks. With the same method utilized above, thespecific order in the P(MEO2MA-co-OEGMA)-b-P4VP19 sample isdescribed as 2954 - 3000 - 2994 - 1598 - 2923 - 2890 -

2836 - 2877 - 2827 - 1729 - 1606 - 1099 - 1716 - 1079 -

1704 cm�1 or nas(CH2)(–OCH2CH2O–) (hydrated) - nas(CH3)(–OCH3)(hydrated) - nas(CH3)(–OCH3) (dehydrated) - n(CQC in pyridine)(dehydrated) - nas(CH2) (backbone) - ns(CH3)(–OCH3)(hydrated) - ns(CH2)(–OCH2CH2O–) (hydrated) - ns(CH3)(–OCH3)(dehydrated) - ns(CH2)(–OCH2CH2O–) (dehydrated) - n(CQO)(dehydrated) - n(CQC in pyridine) (hydrated) - n(–C–O–C–)(dehydrated) - n(CQO) (hydrating) - n(–C–O–C–) (hydrated) -n(CQO) (hydrated).

Ignoring the differences in the stretching modes, the specialorder could be expressed as follows –OCH2CH2O– - –OCH3 -

CQC in pyridine - CQO - C–O–C. This sequence reveals that,

the dehydration of C–H groups takes place before the dehydrationof the pyridine rings and carbonyl groups during heating, which isconsistent with PCMW analysis. Thus, the phase transition of thecopolymer with longer P4VP19 blocks is driven by the dehydrationof the hydrophobic C–H groups located on the side oligo(ethyleneglycol) chains, followed by the dehydration of the pyridine ringsinside of the micelles. It is believed that the increase of thehydrophobic P4VP length may result in a more compact micellestructure, and a more hydrophobic environment near the core.While under the highly confined circumstance in the core, themolecular motion is limited. Therefore, as the relatively hydro-philic shell of the micelles formed in aqueous solution, thePOEGMA blocks squeeze water out of the molecular chains inthe first place under the driving of hydrophobic interaction ofthe aliphatic group on the side chain, and the inner core startsto expel water molecules subsequently. From the overallsequence order, we can additionally find that the changes ofCQO and C–O–C groups take place alternately at the end of theorder, indicating that in the transition process CQO and C–O–Cgroups work cooperatively to make the polymer chains collapseunder the driving of C–H groups and gradually expel watermolecules out of the system.

Moreover, upon a careful observation of CQO regions, com-pared to P(MEO2MA-co-OEGMA)-b-P4VP10 above in which onlytwo bands are observed, 1710 and 1727 cm�1 corresponding tothe (relatively) hydrated CQO and the (relatively) dehydratedCQO, there are three subtle bands, 1704 cm�1, 1716 cm�1,1729 cm�1, emerging in the asynchronous spectra of the carbonylgroups in the longer P4VP19 block system. It indicates theexistence of diverse dehydrating states of CQO during thephase transition. And it is inferred that the closer to the P4VPcore, the more dehydrated state belongs to CQO groups.Actually, the various dehydrating states of CQO groups havealso been observed in the POEGMA-based hydrogel system.52

Thus, it reveals that the water molecules are gradually squeezedout around CQO groups, resulting in the existence of a distributiongradient of water molecules in P(MEO2MA-co-OEGMA)-b-P4VP19,which parallels well with the continuous transition of CQOgroups in Fig. 8.

Based on the above analysis, we can clearly understand thephase transition dynamic mechanisms of P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19 aqueous solutionsupon heating. For a more intuitive demonstration of the dynamicmechanisms, a schematic illustration is depicted in Scheme 2.

In general, for the P(MEO2MA-co-OEGMA)-b-P4VP10 solution,below LCST, a relatively loose single-chain globule with relativelyhydrophobic P4VP blocks on the inside is formed, and sufficientlyhydrated in the aqueous solvent. When the temperature increasesabove LCST, compared with the hydrophobic alkyl chains that areconnected to the side chains of POEGMA blocks, the hydrophobicester carbonyls at the linkage between the backbone and thependant chains mainly provide the hydrophobic interactions, whichare the driving force for the LCST-type phase transition. At the sametime, the conformational change happens and the single-chainglobules start to merge together and further collapse under theincreasing hydrophobic interaction upon heating. As a result,

Table 2 Tentative band assignments of P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19 in D2O according to 2Dcosresults6,52–54

Wavenumber (cm�1) Tentative assignments

3002/3000 nas(CH3)(–OCH3) (hydrated)2994/2987 nas(CH3)(–OCH3) (dehydrated)2954/2952 nas(CH2)(–OCH2CH2O–) (hydrated)2939/2937 nas(CH2)(–OCH2CH2O–) (dehydrated)2927/2923 nas(CH2) (backbone)2894/2890 ns(CH3)(–OCH3) (hydrated)2879/2877 ns(CH3)(–OCH3) (dehydrated)2838/2836 ns(CH2)(–OCH2CH2O–) (hydrated)2827/2825 ns(CH2)(–OCH2CH2O–) (dehydrated)1727/1729 n(CQO) (dehydrated)1716/1710 n(CQO) (hydrating)1704 n(CQO) (hydrated)1606 n(CQC in pyridine) (hydrated)1598 n(CQC in pyridine) (dehydrated)1106/1099 n(–C–O–C–) (dehydrated)1079/1064 n(–C–O–C–) (hydrated)

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a core–shell assembly structure with relatively hydrophobicP4VP segments wrapped in the inside is formed subsequently.With continuous heating, the pendant ether segments start todehydrate and collapse toward the backbone simultaneously,and the D2O molecules around the pyridine ring inside theaggregates should be squeezed out, then the hydration ofhydrophilic side chains on the POEGMA-based shell is damagedcontinuously until the whole phase transition assembly processis finished.

In the case of P(MEO2MA-co-OEGMA)-b-P4VP19 solution, attemperatures below LCST, it exhibits a more compact micelle inthe solution due to the increased length of P4VP blocks, leadingto enhanced hydrophobic interactions near the core. When thetemperature increases above the LCST, the dehydration of thehydrophobic side chain of POEGMA-based blocks is carriedout, which plays a dominant role during the whole process ofdehydration. And accompanying the aggregation of side chainand collapse of the micelle shell, the much less water aroundthe pyridine rings in the core of the micelles is expelled outsubsequently. Then with the help of cooperative dehydrationamong the CQO and C–O–C groups and the resulting adjustmentof the polymer chains in the system, water molecules are ultimatelysqueezed out of the surrounding environment step by step.

4. Conclusion

In this work, P(MEO2MA-co-OEGMA)-b-P4VP block copolymersare synthesized via the RAFT method. Turbidity, DLS, 1H NMRas well as FTIR measurements in combination with 2Dcosmethods are used to investigate the thermal phase transitionbehavior and the assembly properties of P(MEO2MA-co-OEGMA)-b-P4VP solutions with different P4VP segment lengths at the molecularlevel. As shown by turbidity measurements, the transitiontemperature of P(MEO2MA-co-OEGMA)-b-P4VP decreases with

the increase of the P4VP block length. Specifically, the copolymerswith shorter P4VP7/10 blocks exhibit a relatively narrow phasetransition temperature range and a higher LCST, while thecopolymer with longer P4VP19 segments displays a broad transitionregion and a lower LCST during heating. Additionally, the DLSmeasurements suggest the noteworthy distinctions of hydro-dynamic diameters and morphologies between the shorter andlonger P4VP block systems. The copolymers with shorter P4VP7/

10 blocks possess a single-chain structure (single-chain globule)in solution while the copolymer with longer P4VP19 blocks isregarded as a micelle consisting of a relatively hydrophobic P4VPcore and a hydrophilic shell of the POEGMA block in aqueoussolution. The results are further confirmed by temperature-variable1H NMR analysis. And the temperature-dependent frequency shiftsof C–H, CQO, and CQC in pyridine and C–O–C groups in conven-tional FT-IR analysis indicate that the gradient distribution of watermolecules exists in the copolymer solutions ranging from relativelyhydrophobic pyridine rings to hydrophilic POEGMA chains. On thebasis of the 2D correlation analysis, we attribute the gradualdehydration of CQO groups at the linkages between backbonesand pendant chains to the dominant effect on the sharp phasetransition behavior of P(MEO2MA-co-OEGMA)-b-P4VP10. While thehydrophobic C–H groups on the side chains of POEGMA-basedblocks, due to the relatively more dehydrated environment resultingfrom the increased P4VP length and the formation of micelles with ahydrophilic POEGMA-based shell, are the driving forces for thecontinuous phase transition and play a significant role during thewhole dehydration process of P(MEO2MA-co-OEGMA)-b-P4VP19.

Acknowledgements

We gratefully acknowledge the financial support from theNational Science Foundation of China (NSFC) (No. 21274030and 51473038).

Scheme 2 Schematic illustration of the dynamic phase transition mechanism of P(MEO2MA-co-OEGMA)-b-P4VP10 and P(MEO2MA-co-OEGMA)-b-P4VP19 solutions during heating.

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