synthesis, characterization, and temperature-responsive behaviors of novel hybrid amphiphilic block...

Synthesis, characterization, and temperature-responsive behaviors of novel hybridamphiphilic block copolymers containingpolyhedral oligomeric silsesquioxaneYiting Xua*, Jianjie Xiea, Lingnan Chena, Hui Gaoa, Conghui Yuana, Cong Lia,Weiang Luoa, Birong Zenga and Lizong Daia*

Organic/inorganic hybrid amphiphilic block copolymer poly(methacrylate isobutyl POSS)-b-poly(N-isopropylacrylamide-co-oligo(ethylene glycol) methyl ether methacrylate) (PMAPOSS-b-P(NIPAM-co-OEGMA)) was synthesized via reversibleaddition–fragmentation chain transfer polymerization. The self-assembly behavior of this block copolymer in aqueoussolution was investigated by dynamic light scattering (DLS) and transmission electron microscopy. The results indicatethat the novel block copolymer can self-assemble into spherical micelles with PMAPOSS segment as the hydrophobic partand P(NIPAM-co-OEGMA) segment as the hydrophilic part. The temperature-responsive characteristics of the assemblieswere tested by UV–Vis spectra and DLS. Some factors such as the concentration, molecular weight, and copolymer gener-ation that may affect the cloud point were studied systematically. The results reveal that this copolymer exhibits a sharpand intensive lower critical solution temperature (LCST). The essentially predetermined LCST can be conveniently achievedby adjusting the content of NIPAMorOEGMAdomain. In addition, these novel hybridmicelles can undergo an association/disassociation cycle with the heating and cooling of solution and the degree of reversibility displaying a tremendous con-centration dependence, as a novel organic/inorganic hybrid material with distinctive virtues can be potentially used in bi-ological andmedical fields, especially in drug nanocarriers for targeted therapy. Copyright © 2014 JohnWiley & Sons, Ltd.Additional supporting information may be found in the online version of this article at the publisher web-site.

Keywords: hybrid organic/inorganic amphiphilic block copolymer; reversible addition–fragmentation chain transfer (RAFT);POSS; self-assemble; lower critical solution temperature (LCST)

INTRODUCTION

Thermo-responsive polymers have been attracting significantattention from various research fields including bioconjugatechemistry, biomedicine, and molecular actuators.[1–5] Forinstance, water-soluble polymers exhibiting a lower criticalsolution temperature (LCST) in water can be exploited to induceself-assembly (micellization or vesiculation) or promote theuptake and release of particular guest molecules.[6–9] The prom-inent representative is poly(N-isopropylacrylamide) (PNIPAM),which displays an LCST in water around 32°C and has been themost studied thermo-sensitive polymer in bioapplications. Thereason for its biomedical popularity is attributed to the fact thatthe LCST of PNIPAM is not only close to body temperature butalso is relatively insensitive to other environmental conditions,such as slight variations of pH, concentration, or chemicalenvironment.[3,10,11] Poly(ethylene glycol) (PEG) as an uncharged,water-soluble, nontoxic, biocompatible polymer has alsobecome an extremely popular material for biotechnology appli-cations. Various (meth)acrylic PEG analogs can exhibit athermo-responsive property. For example, poly(oligo(ethyleneglycol) methyl ether methacrylate (Mmonomer ∼ 475 g/mol)) canexhibit an LCST at 60°C in water. Certainly they are not the onlyknown macromolecules that can exhibit thermo-responsiveproperties; other cases with LCSTs include N,N-diethylacrylamide(LCST, H2O∼, 32°C), N-vinylcaprolactam (LCST, H2O∼, 32.5°C), and

2-(dimethylamino)ethyl methacrylate (LCST, H2O∼, 50°C).[10,12–25]

They are potentially useful for several biomedical applicationssuch as smart bioactive surfaces, selective bioseparation, phaseseparation, or drug delivery.[26]

Incorporating inorganic or organometallic blocks into organicpolymers to obtain organic–inorganic hybrid with new propertieshas attracted considerable interest.[11] Polyhedral oligomericsilsesquioxane (POSS), a hybrid molecule with nanoscale dimen-sion, is emerging as a new chemical feedstock for preparingorganic–inorganic nanocomposites.[27] A typical POSS moleculepossesses the structure of cube-octameric frameworksrepresented by the formula R8Si8O12 with an inorganic silica-likecore (Si8O12) surrounded by eight organic corner groups, one ormore of which are reactive or polymerizable.[28–31] Based on this,it can be easily introduced into polymer matrices to form hybridpolymers and endow them with superior mechanical and thermalproperties, oxidation resistance, and reduced flammability.Hybrid polymer containing POSS can be easily prepared by

* Correspondence to: Yiting Xu and Lizong Dai, Fujian Provincial Key Laboratoryof Fire Retardant Materials, College of Materials, Xiamen University, Xiamen,Fujian, 361005, China.E-mail: [email protected]

a Y. Xu, J. Xie, L. Chen, H. Gao, C. Yuan, C. Li, W. Luo, B. Zeng, L. DaiFujian Provincial Key Laboratory of Fire Retardant Materials, College ofMaterials, Xiamen University, Xiamen, Fujian, 361005, China

Research article

Received: 1 April 2013, Revised: 18 December 2013, Accepted: 19 December 2013, Published online in Wiley Online Library: 7 February 2014

(wileyonlinelibrary.com) DOI: 10.1002/pat.3258

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polymerization of POSS-functionalized monomers. In particular,POSS-containing block copolymers with well-defined structurehas been synthesized, thanks to the promising living/controlledpolymerization techniques such as atom transfer radical polymeri-zation, living anionic polymerization, and reversible addition–frag-mentation chain transfer (RAFT).[12,32–39] Much attention has beenfocused on the preparation methods and the properties of POSS-containing hybrid polymers. However, there are still many othernovel aspects worthy to be studied, especially the self-assemblybehavior of these hybrid polymers with novel architectures.[40]

The self-assembly behavior of POSS-containing block copolymeris quite dependent on the strong hydrophobic association ofpoly(POSS) segments.[27] By controlling the type of the organicblocks connected to the poly(POSS), one can construct various as-semblies and endow the hybrids with novel properties.

In this contribution, we extend the study of the self-assemblybehaviors of POSS-containing hybrid polymers by the synthesisof a novel thermo-responsive, hybrid amphiphilic block copolymercontaining POSS, poly(methacrylate isobutyl POSS)-b-poly(N-isopropylacrylamide-co-oligo(ethylene glycol) methyl ether meth-acrylate) (PMAPOSS-b-P(NIPAM-co-OEGMA)), via RAFT polymeriza-tion (the synthesis route is described in Scheme 1). Theintroduction of PMAPOSS domains make it possible for thesehybrid copolymers to self-assemble into aggregations owing tothe strongly hydrophobic nature of the silicon-containing blocks.More importantly, the P(NIPAM–OEGMA) domains are tempera-ture-responsive and are capable of displaying a phase transitionin aqueous solution upon heating. The self-assembly behavior ofthese block copolymers in aqueous solution was investigated bymeans of dynamic light scattering (DLS) and transmission electronmicroscopy (TEM). The temperature-responsive characteristics ofthe assemblies were tested by UV–Vis transmittance and DLS.

EXPERIMENTAL

Materials

Methacrylate isobutyl POSS without further purification prior touse was obtained from Hybrid Plastic Co. (Hattiesburg, Mississippi,

USA) N-Isopropylacrylamide (NIPAM) (98%, Aladdin Co., China) waspurified by recrystallization from hexane prior to use. Oligo(ethyleneglycol) methyl ether methacrylate (OEGMA, Mmonomer =475g/mol )(97%, Aladdin Co.) was purified by basic alumina columns prior touse. Cumyl dithiobenzoate (CDB) was synthesized according to aliterature method.[41] The synthesis scheme and 1H NMR character-ization are shown in the Supporting Information (SI) Figures S1 andS2, respectively. 2,2′-Azoisobutyronitrile (AIBN) was recrystallizedfrom ethanol. Tetrahydrofuran (THF) was dried by refluxing oversodium and distilled prior to use. Toluene (anhydrous, syntheticgrade) was purchased from Aldrich, (Aldrich: Milwaukee, Wisconsin,USA) and used as received. Other chemicals such as ethyl acetate,hexane, and methanol were used as received.

Synthesis of PMAPOSS (macro-CTA) by RAFT polymerization

The RAFT polymerization of MAPOSS was conducted using aSchlenk tube. In a typical synthesis, MAPOSS (2.04 g, 2.16mmol),CDB (49mg, 0.18mmol) and AIBN (1.47mg, 0.03mmol) werecharged into a Schlenk tube filled with 1.0ml of toluene. Themixture was deoxygenated through five freeze–pump–thawcycles. The Schlenk tube was then filled with argon and kept in athermostated oil bath at 65°C. After 48h, the Schlenk tube wasquenched into liquid nitrogen to stop the polymerization. Thecrude solution was precipitated into excess solution of methanol/acetic ether (8/1, vol%), and this precipitation procedure wasrepeated three times. The slightly pink powder obtained was driedovernight under vacuum at room temperature, yield 78.5wt%. Theconversion was determined by 1H NMR in CDCl3 from the relativeintegrations of the characteristic vinyl protons of the reacted andunreacted monomer (peaks between 5.50 and 6.50ppm) usingthe peak at 0.68 ppm (assigned to the methylene protons next tosilicon) as internal standard. The actual degree of polymerizationof the PMAPOSS homopolymer was determined to be 9 by1H NMR. The molecular weight and molecular weight distributionof PMAPOSS homopolymer were determined by gel permeationchromatography (GPC).

1H NMR (CDCl3, ppm, TMS) δ: 7.40–7.90 (m, CH of phenylgroup next to disulfide ester group in CDB); 3.85 (s, CH2 of estergroup); 1.85 (m, CH of isobutyl group ); 1.69 (m, CH2 next toCOOCH2 group ); 1.49 (s, CH in main chain); 0.96 (m, CH3 inMAPOSS units); and 0.68 (m, CH2 next to silicon).

Synthesis of PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymer

The RAFT polymerization of PMAPOSS9-b-P(NIPAM180-co-OEGMA20)was performed using PMAPOSS as macro-CTA and proceededin a Schlenk tube. PMAPOSS (0.3478 g, 0.02mmol), NIPAM(0.4068 g, 3.6mmol), OEGMA (0.176ml, 0.4mmol), and AIBN(0.7mg, 0.004mmol) were charged into the Schlenk tubecontaining THF (2.0ml). The mixture was deoxygenated throughfive freeze–pump–thaw cycles. The Schlenk tube was then filledwith argon and kept in an oil bath at 65°C. After 48 h, the Schlenktube was quenched into liquid nitrogen to stop the polymeriza-tion. The crude product was precipitated into excess hexane,and this precipitation procedure was repeated three times. Theresulting white powder was dried overnight under vacuum atroom temperature, yield 42.68wt%. The conversion was alsodetermined by 1H NMR from the relative integrations of the char-acteristic vinyl protons of the unreacted and reacted monomers(peaks between 5.50 and 6.50 ppm) using the peak at 0.60 ppm(assigned to the methylene protons next to silicon) as internal

Scheme 1. Synthetic route of PMAPOSS-b-P(NIPAM-co-OEGMA) blockcopolymer via RAFT polymerization. This scheme is available in colouronline at wileyonlinelibrary.com/journal/pat

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standard. The molecular weight and molecular weight distribu-tion of PMAPOSS9-b-P(NIPAM180-co-OEGMA20) polymer weredetermined by GPC. Similarly, we also synthesized two amphi-philic diblock copolymers, namely PMAPOSS9-b-P(NIPAM180-co-OEGMA15) and PMAPOSS9-b-P(NIPAM240-co-OEGMA15).

1H NMR (CDCl3, ppm, TMS) δ: 4.05 (m, CH of isopropyl inNIPAM units); 3.75–3.85 (s, CH2 of ester group in MAPOSSand OEGMA units); 3.58 ppm (m, CH2 in the oligo(ethyleneglycol) side chain); 3.25 ppm (s, OH in OEGMA units); 1.20–2.35(m, CH2 and CH in the main chain, CH of isobutyl group andCH2 next to COOCH2 group in MAPOSS units); 0.84 (m, CH3 inMAPOSS units and NIPAM units); and 0.60 (m, CH2 next to silicon).

1H NMR (D2O, ppm, TMS) δ: 4.05 (m, CH of isopropyl in NIPAMunits); 3.75 ppm (s, CH2 of ester group in OEGMA units); 3.58 ppm(m, CH2 in the oligo(ethylene glycol) side chain); 3.25 ppm (s, OHin OEGMA units); 1.23–2.16 (m, CH2 and CH in the main chain);and 1.01 ppm (s, CH3 of isopropyl in NIPAM units).

Preparation of PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymermicelles in aqueous solution

PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymers were firstdissolved in THF, which is a good solvent for the hybrid copoly-mers. Then the solution was diluted slowly by a certain volumeof deionized water at a speed of 1ml/min. A typical 5mg/ml mi-celle solution was prepared as follows: 10mg PMAPOSS-b-P(NIPAM-co-OEGMA) was dissolved in 2ml THF, followed by theaddition of 2ml deionized water at a speed of 1ml/min. Thesolution was stirred for 24 h to completely remove THF at roomtemperature. Other micelle solutions with different concentra-tions were achieved by the same process.

Polymer characterization1H NMR characterization1H NMR spectra were recorded on a Bruker (Germany), AV300MHz NMR spectrometer using CDCl3 and D2O as the solventsat the room temperature.

Molecular weight measurement

Molecular weight determinations for polymerswere performed usingGPC analyses using a series of Waters Styragel HR2, HR4, and HR5,(Waters: Milford, MA 01757,USA). The eluent was THF at a flow rateof 1.0ml/min. A series of low polydispersity polystyrene standardswere used for the GPC calibration.

LCST behavior observation

Optical transmittance measurement

The optical transmittance of PMAPOSS-b-P(NIPAM-co-OEGMA) poly-mer micelle solutions at various temperatures was measured at500nm using a UV-2550 UV–Vis spectrometer (Shimadzu, Japan).Sample cells were thermostated in a water bath at differenttemperatures for 3min prior to the measurements, and the heatingrate was set at 1°C/min. The LCST is defined as a sharp decrease inoptical transmittance when the temperature rises to a certain point.

Size distribution measurement

DLS measurements were performed on a Zetasizer NanoZS Instru-ment (Malvern Instruments, Worcestershire, UK) at a scatteringangle of 90° and analyzed by Malvern Zetasizer Software version6.20. The heating rate of measurements at different temperatureswas set at 1°C/min. The LCST is defined as a sharp increase indiameter when the temperature rises to a certain point.

TEM observation

TEManalysiswasperformedona JEM2100 transmissionelectronmicro-scope (JEOL, Japan) with an accelerating voltage of 200 kV. Thesample was directly dropped onto a copper grid (300 mesh) coatedwith a carbon film. Then the sample was dried at room temperature.

RESULTS AND DISCUSSION

Synthesis of PMAPOSS (macro-CTA) by RAFT polymerization1H NMR was employed to characterize the structure of polymersof MAPOSS. Figure 1 shows the characteristic peaks of PMAPOSS.

Figure 1. 1H NMR spectra of the PMAPOSS homopolymer (CDCl3, 25°C). This figure is available in colour online at wileyonlinelibrary.com/journal/pat

HYBRID COPOLYMER: SELF-ASSEMBLY, TEMPERATURE-RESPONSIVE BEHAVIORS

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By comparing the peaks of methylene (2H) protons of ester groupsat 3.85 ppm with the aromatic (2H) protons of terminal benzenering at 7.85 ppm, the actual degree of polymerization of thePMAPOSS homopolymer is determined as DPn, MAPOSS= I3.85/I7.85. GPC was utilized to determine the molecular weight and mo-lecular weight distribution. As depicted in Table 1, theMn and poly-dispersity (Mw/Mn) of PMAPOSS are 8560 and 1.07, respectively.

Synthesis of PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymer

PMAPOSS-b-P(NIPAM-co-OEGMA) was synthesized via the RAFTblock copolymerization by using PMAPOSS as the macro-CTA.Table 1 summarizes the results obtained for block copolymerswith different monomer unit ratios of NIPAM to OEGMA. Thekinetics of the chain extension of NIPAM/OEGMA comonomerwas investigated with [NIPAM] : [OEGMA] : [Macro-RAFT] : [AIBN] =180:15:1:0.2. The samples at reaction times 16, 24, 36, and 48 hwere chosen for study. Figure 2 shows the evolution of GPCchromatograms with reaction time. With the increasing reactiontime, the chromatograms shift to lower elution volume.The relationship of monomer conversion with polymerizationtime is given in Fig. 3. The result demonstrates that themonomer conversion has a linear relationship with polymeriza-tion time. Meanwhile, a good linear relationship is observedbetween Mn of PMAPOSS-b-P(NIPAM-co-OEGMA) and the con-version of NIPAM and OEGMA from Fig. 4. The molecular weightdistributions of all samples are narrow (PDI = 1.12–1.27), which isconsistent with the living nature of the RAFT polymerization.

1H NMR was also employed to characterize the structureof PMAPOSS-b-P(NIPAM-co-OEGMA). Figure 5 shows the characteris-tic peaks of PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymer in

Figure 2. GPC traces of chain extension of NIPAM/OEGMA after differentreaction times with [NIPAM] : [OEGMA] : [PMAPOSS9] : [AIBN]=180:15:1:0.2.

Figure 4. Dependence of the molecular weight (GPC) and PDI onNIPAM/OEGMA comonomers conversion with [NIPAM] : [OEGMA] :[PMAPOSS9] : [AIBN] = 180:15:1:0.2. This figure is available in colour onlineat wileyonlinelibrary.com/journal/pat

Table 1. Experimental conditions and results of the copolymerization of macro-CTA and PMAPOSS-b-P(NIPAM-co-OEGMA) viaRAFT at 65°C

Sample name [MAPOSS] : [CDB] : [AIBN] or [NIPAM] :[OEGMA] : [PMAPOSS] : [AIBN]a

Time (h) Yieldb (%) Mnc (GPC) PDI

PMAPOSS9d 12:1:0.05 48 78.50 8,560 1.07

PMAPOSS9-b-P(NIPAM180-b-OEGMA20) 180:20:1:0.2 48 42.68 19,861 1.14PMAPOSS9-b-P(NIPAM180-b-OEGMA15) 180:15:1:0.2 48 40.82 19,027 1.16PMAPOSS9-b-P(NIPAM240-b-OEGMA15) 240:15:1:0.2 48 36.28 19,974 1.12PMAPOSS9-b-P(NIPAM180-b-OEGMA15) 180:15:1:0.2 16 20.18 9,681 1.27PMAPOSS9-b-P(NIPAM180-b-OEGMA15) 180:15:1:0.2 24 25.83 11,280 1.18PMAPOSS9-b-P(NIPAM180-b-OEGMA15) 180:15:1:0.2 36 34.78 16,205 1.16aThe feed molar ratio of monomers, macro-CTA and initiator.bThe conversion determined from 1H NMR spectra.cMeasured by GPC against poly(styrene) standards.dThe final composition (subscript numbers) determined from 1H NMR spectra.

Figure 3. Kinetics of RAFT polymerization of NIPAM/OEGMA comonomerwith [NIPAM] : [OEGMA] : [PMAPOSS9] : [AIBN] = 180:15:1:0.2. This figure isavailable in colour online at wileyonlinelibrary.com/journal/pat

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CDCl3 and D2O. More specifically in CDCl3, which is a good solvent forboth PMAPOSS and the P(NIPAM-co-OEGMA) segments, the signalsof MAPOSS and P(NIPAM-co-OEGMA) segments can be observed(Fig. 5A). When it comes to D2O, the PMAPOSS segment is insolublein aqueous solution, and it is assumed that it forms the core of a mi-celle with P(NIPAM-co-OEGMA) segment forming the shell (Fig. 5B).As a result, only the signals of NIPAM and OEGMA are revealed.

Micellization of PMAPOSS-b-P(NIPAM-co-OEGMA) blockcopolymer in aqueous solution

The morphology of micelles was investigated by TEM. The typicalTEM images of the micelles formed from PMAPOSS9-b-P

(NIPAM180-co-OEGMA15) (5mg/ml, 25°C) in water solution arepresented in Fig. 6. The micelles display well-defined sphericalshapes (as illustrated in Fig. 6A) with a narrow size distributionranging from 18 to 30 nm. The DLS data (the inset of Fig. 6B)measured by a Zetasizer indicates that the micelles show anaverage diameter of 93.9 nm with a narrow size distribution.The difference between the sizes measured by DLS and deter-mined by TEM can be attributed to the fact that the DLSmeasures swollen micelles in aqueous solutions, and the sizesobserved by TEM are derived from the dried micelles. Moreimportantly, Fig. 6 shows that these spherical micelles tend todeposit into a single-layer film with a uniform distribution. Thebehavior of micellar rearrangement may be induced by the

Figure 5. 1H NMR spectra of the PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymer (A) CDCl3, 25°C; (B) D2O, 25°C. This figure is available in colouronline at wileyonlinelibrary.com/journal/pat

Figure 6. TEM images of the micelles of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) in aqueous solutions: (A) lower magnification; (B) higher magnifica-tion. The inset of (A) represents model of core-shell micelles, and the inset of (B) represents DLS data of micelles in aqueous solutions. This figure isavailable in colour online at wileyonlinelibrary.com/journal/pat



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volatilization of solvent. When preparing TEM samples, thestrong associative intermicellar force, which stems from thehydrogen bonds formed between P(NIPAM180-co-OEGMA15)segments, makes the micelles pack into a dense film.

Nanoparticle size is well known to play a significant role inseveral applications such as drug delivery. One emphasis of thiswork was to study the factors that may affect the micellar size.The results in Fig. 7 reveal that the size of micelles is significantlyaffected by the original concentration of the copolymer solution.As shown in Fig. 7, the micellar size can shift from 73.6 to117.9 nm with the concentration of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) solution increasing from 0.5 to 10mg/ml. To somedegree, these results suggest that the size of the hybrid micellecan be finely tuned by adjusting the polymer concentration.Further emphasis was placed on studying the influence of

copolymer composition on micellar size at the same originalconcentration (5mg/ml) and the same temperature (25°C). FromFig. 8, it is obvious to see that the size increases with the increaseof OEGMA monomers and decreases with the increase of NIPAMmonomers. When the unit fraction of OEGMA is raised from 15 to20, the micellar size can shift to 151.2 nm from 93.9 nm. Themicelle size was reduced to 80.8 nm when the unit fraction ofNIPAM was increased from 180 to 240. These results also suggestthat the micellar size is more sensitive to OEGMA units thanNIPAM units because of the difference in hydrophilicity betweenOEGMA and NIPAM. OEGMA is a perfectly water-soluble mono-mer with a strong hydrophilic side chain oligo(ethylene glycol).Increasing the content of OEGMA can significantly strengthenthe hydrophilicity of the copolymer chain in water; thus thepolymer chains can self-assemble into larger micelles, which isconsistent with the report by Zhang et al.[40] that increasingthe hydrophilicity of copolymer can lead to the formation oflarger micelles. On the contrary, increasing the content of NIPAMunits reduces the overall hydrophilicity of the copolymer chainsowing to the weak hydrophilicity of NIPAM segments. As a result,only the smaller assemblies are generated.

Thermo-responsive properties of PMAPOSS-b-P(NIPAM-co-OEGMA) micelles

Temperature is an essential factor in the morphology behavior ofamphiphilic copolymer PMAPOSS-b-P(NIPAM-co-OEGMA)micelles.The solubility of PMAPOSS-b-P(NIPAM-co-OEGMA) in water at var-iable temperatures was observed by UV–Vis and DLS. Figure 9(A)shows the transmittance and diameter evolution of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) in water with a concentration of 5mg/mlduring heating. Below 41°C, the transmittance remained relativelyconstant at around 80%. However, it shows a sharp decrease intransmittance upon heating to over 41°C, which is in good agree-ment with the size increase plot. The diameter is sharply increased

Figure 7. Size of micelles prepared from PMAPOSS9-b-P(NIPAM180-co-OEGMA15) solution under different concentrations in H2O at 25°C. This figure isavailable in colour online at wileyonlinelibrary.com/journal/pat

Figure 8. Size of micelles prepared at various generations of PMAPOSS-b-P(NIPAM-co-OEGMA) block copolymer in H2O at 25°C. This figure isavailable in colour online at wileyonlinelibrary.com/journal/pat

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to a value of more than 2500nm from 107nm at 41°C. When thetemperature is further increased, the transmittance drops to nearly0%, and the diameter can reach and remain stable at nearly4100nm. Obviously, this phenomenon reflects a sharp transitionpoint consistent with an LCST behavior. The macroscopic changecan be observed from the pictures in Fig. 9(A), which were takenat 25°C and 42°C, respectively. The transformation of the copolymersolution from transparent to turbid is apparent when the tempera-ture rises from room temperature to above the cloud point. TEMwas used to further certify the morphology transition behavior.TEM images illustrated in Fig. 10 demonstrate that the novel hybridmicelles have a tendency to aggregate at elevated temperatures.This transition is related to the dehydration of the polymer

that occurs with a rearrangement of water molecules. NIPAMcan hydrogen bond with H2O via amide group below the criticaltemperature. Once the solution is heated to the LCST, hydrogenbonds between H2O and amide group are disrupted. As a result,NIPAM units change from hydrophilic to hydrophobic. The

process is also applied to OEGMA units. OEGMA exhibits an LCSTbehavior owing to the existence of oligo(ethylene glycol) units,which possess a temperature-sensitive hydrogen bonding abilitywith the water. The phase transition can also be explained fromthe theory of Flory. For an LCST transition, the disruption ofhydrogen bonds between P(NIPAM-co-OEGMA) segment andwater molecules results in the increase in the associated interac-tion parameter χ with increasing temperature. Thus the polymerbecomes hydrophobic above the critical temperature.[10,42–44]

However, it does not mean that one can just attribute the dif-ference in LCST to a simple manifestation of the aggregate size.As shown in Fig. 9(B), only a slight decrease in micellar size isobserved with the increase of temperature below the LCST, andthe largest difference is only 7 nm. It demonstrates that the mi-celles are nearly immune to temperature changes under the cloudpoint. When the temperature is elevated to the critical tempera-ture (41°C), the average size has an abrupt increase from 86nmat 40°C to 107nm. Interestingly, the single peak separates into

Figure 9. (A) Variation of optical transmittance at 500nm (red line) and diameter (blue line) with temperature for PMAPOSS9-b-P(NIPAM180-co-OEGMA15).(B) Variation of size distribution with temperature from 39°C to 44°C for PMAPOSS9-b-P(NIPAM180-co-OEGMA15). The polymer concentration was 5mg/ml.This figure is available in colour online at wileyonlinelibrary.com/journal/pat



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two parts at 40°C, and their proportions are 65.4% and 34.6%,respectively. The size of small size zone at 41°C is much smallerthan that below 41°C. This phenomenon proves the coexistenceof the shrunken micelles and micellar clusters, which are eithersmaller or larger above and below the cloud point, respectively.The process is generally described by four stages: (i) as the temper-ature elevated, hydrogen bonds betweenwater molecules and theP(NIPAM-co-OEGMA) segments are disrupted, resulting in thedecrease in the hydrophilicity of micellar shell; (ii) the formationof the shrunken micelles stemming from the reduction in thehydrophilicity of micellar shell; (iii) the formation of smaller micellar

clusters through the aggregation of the shrunken micelles in orderto reduce the surface energy via hydrophobic interaction and theformation of hydrogen bonds between shrunken micelles; and(iv) the formation of large micellar clusters from further entangle-ment of small micellar clusters. Stages 1 and 2 can take place atthe same time. The macroscopic phase transition of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) solution is explicated by the picturesshown in Fig. 11. When the solution is heated, the randommicellescondense to the shrunken micelles. These small micelles canquickly move and collide to aggregate into micellar clusters. In thisway, the optical transmittance of the copolymer solution quicklydecreases, and the micellar solution switches from transparentto turbid.

The effect of copolymer composition on the cloud point

In this work, three copolymer systems were prepared with thesame hydrophobic blocks but different hydrophilic blocks. Theeffect of hydrophilic blocks on the aqueous phase behaviors ofthese polymers was examined thoroughly. To identify the pointat which the LCST transformation occurs in these block copoly-mers, turbidity measurements at various temperatures werecarried out. The results in Fig. 12 presents both the UV–Vis andDLS data. The solution of PMAPOSS9-b-P(NIPAM180-co-OEGMA15)copolymer undergoes a phase transition upon heatingwith amea-sured cloud point of 41°C (solid blue line). When the content ofOEGMA units is raised to 20, a higher LCST with a value of 49°C(solid red line) is observed. However, when raising the unit fractionof NIPAM from 180 to 240, the cloud point decreases from 41°C to40°C (solid green line). The aforementioned results illustrate thatthe cloud point can be conveniently adjusted by varying the con-tent of OEGMA or NIPAM units. Moreover, the OEGMA units have amuch larger influence than NIPAM units. The reason for the

Figure 10. TEM images of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) micelles at 45°C with different scale bars.

Figure 11. Schematic representation of the phase changes for PMAPOSS-b-P(NIPAM-co-OEGMA) micelle with the solution heating and the macro-scopic phase transition of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) solutionwith the temperature ranging from 40°C to 43°C. This figure is available incolour online at wileyonlinelibrary.com/journal/pat

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phenomenon can also be ascribed to the significant difference insolubility between NIPAM and OEGMA as it is explained in the partof the micellization of PMAPOSS-b-P(NIPAM-co-OEGMA) block

copolymer in aqueous solution. The introduction of OEGMA units,which are more hydrophilic than NIPAM, improves the overallsolubility of the copolymer chain. The phase transition is thenshifted to a much higher temperature. On the contrary, theinclusion of NIPAM units, in some extent, decreases the overallsolubility; thus the cloud point shifts to a lower value.

The result also indicates that the synthetic route described inthis report offers a simple and convenient approach to thermo-responsive copolymers with essentially predetermined LCSTbehavior. Meanwhile the sharp and distinct LCST transitions sug-gest that the novel copolymer has great potential in the field ofdrug carrier for targeted therapy, and it can greatly improve theefficiency of drug action and positioning accuracy (the promisingapplication of PMAPOSS-b-P(NIPAM-co-OEGMA) micelle in thedrug carrier for targeted therapy is depicted in SI Figure S3).

The effect of copolymer concentration on the cloud point

The polymer concentration exhibits a significant impact on thephase transition temperature as well as on the micellar size.The effect of copolymer concentration on the cloud point ofPMAPOSS9-b-P(NIPAM180-co-OEGMA15) was observed by DLS,and the results are shown in Fig. 13. At the lowest concentration,0.5mg/ml, this amphiphilic copolymer exhibits a measuredcloud point of 43°C. When the concentration is increased to5mg/ml, a decrease of the cloud point of nearly 2°C to 41°C isobserved. Above this concentration, there appears to be littleconcentration dependence on the LCST with a stable phase tran-sition temperature 41°C.

Similar trends are found in the solutions of PMAPOSS9-b-P(NIPAM180-co-OEGMA20) and PMAPOSS9-b-P(NIPAM240-co-OEGMA15),but the critical stable concentration is 2mg/ml rather than 5mg/ml ofPMAPOSS9-b-P(NIPAM180-co-OEGMA15). As shown in Fig. 14, whendecreasing the concentration from 2 to 0.5mg/ml, the cloud pointof PMAPOSS9-b-P(NIPAM240-co-OEGMA15) increases 2°C from 40°Cto 42°C. In the case of PMAPOSS9-b-P(NIPAM180-co-OEGMA20)solution, at a concentration of 1mg/ml, the cloud point jumps toan additional 3–52°C from 49°C at 2mg/ml. However, when the con-centration is further diluted to 0.5mg/ml, no phase transition occursin the PMAPOSS9-b-P(NIPAM180-co-OEGMA20) solution. The polymeris completely soluble in water over the entire observed temperaturerange below this concentration. This copolymer possesses the largesthydrophilicity among the three systems.

Figure 12. Effect of composition on temperature dependence of optical transmittance changes at 500 nm (A) and on temperature dependence ofdiameter changes (B) for 5mg/ml solution of PMAPOSS9-b-P(NIPAM180-co-OEGMA20) (red line), PMAPOSS9-b-P(NIPAM180-co-OEGMA15) (blue line),and PMAPOSS9-b-P(NIPAM240-co-OEGMA15) (green line). This figure is available in colour online at wileyonlinelibrary.com/journal/pat

Figure 13. Effect of concentration on temperature dependence of di-ameter changes for PMAPOSS9-b-P(NIPAM180-co-OEGMA15) solution. Thisfigure is available in colour online at wileyonlinelibrary.com/journal/pat

Figure 14. Influence of copolymer concentration on the cloud point forPMAPOSS9-b-P(NIPAM180-co-OEGMA20) (blue curves), PMAPOSS9-b-P(NIPAM180-co-OEGMA15) (red curves), and PMAPOSS9-b-P(NIPAM240-co-OEGMA15) (greencurves). Thisfigure is available in colour online atwileyonlinelibrary.com/journal/pat



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The effect of chain length on the cloud point

A series of copolymers PMAPOSS9-b-P(NIPAM180-co-OEGMA15) wereprepared with the same composition but different molecularweights by adjusting the polymerization time. UV–Vis was appliedto examine the influence of molecular weight on the thermalbehavior of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) solutions. Theresult in Fig. 15 exhibits that the solution of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) has a remarkable dependence of its LCSTon the polymer molecular weight. When the molecular weightjumps from 9681g/mol (16h) to 19,020g/mol (48h), the LCSTundergoes a decrease from 45°C to 41°C. The differences in LCSTcan be as large as 4°C between short-chain and long-chain samples.

Heat-induced reversible phase transition of PMAPOSS-b-P(NIPAM-co-OEGMA) micelle

There was a great interest in studying the reversibility of theheat-induced phase transition. The result shown in Fig. 16demonstrates that the PMAPOSS-b-P(NIPAM-co-OEGMA) micellesolutions undergo an association/disassociation cycle with theheating and cooling of solution, and the degree of reversibilityhas a strong concentration dependence. The inset of Fig. 16 morespecifically illustrates this concentration dependence, and thedegree of reversibility of PMAPOSS9-b-P(NIPAM180-co-OEGMA15)solution can vary from 0.69% to 75.90% when the concentrationis decreased from 10 to 0.5mg/ml. Here the degree of reversibilityis calculated from the ratio of the optical transmittance (25°C)after cooling to that before heating. This phenomenon can beattributed to the difference in aggregation degree of the shrunkenmicelles at different solution concentrations. In the high concen-tration solution, the distance of adjacent micelles is much shorter;thus they can aggregate into largemicellar clusters via the impetusderiving from the hydrophobic interaction and the formation ofintermicellar hydrogen bonds. This can be confirmed by the stabletransmittance of different concentration solutions. The stabletransmittance can be elevated from 0.106% to 30.25% with the so-lution diluted from 10 to 0.5mg/ml. However, when the high con-centration solution is cooled, it needs much more energy toreform the hydrogen bonds between copolymer chains andwater molecules. Micellar clusters can only either disassociate

into small ones or fail to disassociate in the high concentrationsolutions. Conversely, for the low concentration case, the micellarclusters can disassociate into smaller ones or even intounshrunken micelles.

CONCLUSIONS

We have demonstrated the controlled synthesis of a thermo-responsive, hybrid amphiphilic block copolymer containingPOSS, PMAPOSS-b-P(NIPAM-co-OEGMA) by RAFT. TEM and 1HNMR results show that the PMAPOSS-b-P(NIPAM-co-OEGMA)amphiphilic block copolymer can self-assemble into sphericalmicelles composed of a hydrophobic PMAPOSS core and hydro-philic P(NIPAM-co-OEGMA) shell in water. Date measured fromDLS indicates that the micellar size can be finely tuned byadjusting the polymer concentration or copolymer composition.Results from UV and DLS reveal that this hybrid micellar solutionundergoes a sharp temperature-sensitive phase transitionprocess upon heating. The transition from soluble to insolubleis dependent on concentration and molecular weight, whichstrongly influence the phase separation (LCST) for the same co-polymer generation. The hydrophilic block P(NIPAM-co-OEGMA)plays an important role in phase transition process. An essentiallypredetermined LCST can be achieved just by varying the contentof NIPAM or OEGMA units. Furthermore, these novel hybridmicelles can undergo an association/disassociation cycle withthe heating and cooling of copolymer solution, and the degreeof reversibility is concentration dependent. Hence, as a novelorganic–inorganic hybrid material with the distinctive virtues,these novel thermo-responsive PMAPOSS-b-P(NIPAM-co-OEGMA)copolymers are relevant for many applications in biotechnologyand medical fields, especially in drug carrier for targeted therapy.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (51273164, U1205113), the NaturalScience Foundation of Fujian Province of China (2012J01233),the Fundamental Research Funds for the Central Universities(2012121031), and NCET.

Figure 15. Effect of molecular weight on temperature dependence ofoptical transmittance changes at 500 nm for PMAPOSS9-b-P(NIPAM180-co-OEGMA15) with the concentration of 5mg/ml. This figure is availablein colour online at wileyonlinelibrary.com/journal/pat

Figure 16. Heat-induced reversible phase transition of PMAPOSS9-b-P(NIPAM180-co-OEGMA15) in response to temperature changes between25°C and 50°C. Here the green and blue lines represent the heatingand cooling processes, respectively. This figure is available in colouronline at wileyonlinelibrary.com/journal/pat

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