Reactive & Functional Polymers 73 (2013) 1646–1655

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Reactive & Functional Polymers

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Synthesis, characterization and self-assembly of hybrid pH-sensitiveblock copolymer containing polyhedral oligomeric silsesquioxane(POSS)

1381-5148/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.reactfunctpolym.2013.09.008

⇑ Corresponding authors. Tel.: +86 592 2186178; fax: +86 592 2183937.E-mail addresses: xyting@xmu.edu.cn (Y. Xu), lzdai@xmu.edu.cn (L. Dai).

Yiting Xu a,⇑, Min Chen a, Jianjie Xie a, Cong Li a, Cangjie Yang a, Yuanming Deng a, Conghui Yuan a,Feng-Chih Chang b, Lizong Dai a,⇑a Fujian Provincial Key Laboratory of Fire Retardant Materials, College of Materials, Xiamen University, Xiamen 361005, Chinab Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, Taiwan

a r t i c l e i n f o

Article history:Received 2 August 2013Received in revised form 26 September2013Accepted 29 September 2013Available online 9 October 2013

Keywords:Hybrid block copolymerPolyhedral oligomeric silsesquioxanepH sensitivitySelf-assembly behavior

a b s t r a c t

In this study, a series of novel hybrid pH-sensitive block copolymers containing POSS (HBCPs),poly(methacrylisobutyl-POSS)-b-poly(4-vinylpyridine) (PMAiBuPOSS-b-P4VP) and poly(methacrylisobu-tyl-POSS)-b-polystyrene-b-poly(4-vinylpyridine) (PMAiBuPOSS-b-PS-b-P4VP), were synthesized viareversible addition fragmentation chain-transfer (RAFT) polymerization. Their structures and molecularweight were characterized via 1H NMR, GPC and TEM. Their self-assembly behaviors, including pH-sen-sitive behaviors and self-assembly morphologies in aqueous solution, were investigated via DLS and TEM.It was found that the size of aggregates in aqueous solution would initially decrease and later increase asthe pH value increased. It is supposed that this behavior was caused by the pH sensitivity of the P4VPblock of the HBCPs. Our hybrid triblock copolymers were found to assemble nanowires and nanospheres.Unique dot-like phase separation was also observed in the aggregates of the HBCPs at pH 1. Furthermore,we investigated the effects of block length and structure on the self-assembly morphologies ofthe HBCPs.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

An amphiphilic polymer is a macromolecule with an affinity forboth phases, which generally means that the structure of the poly-mer molecules contains both hydrophilic and hydrophobic chains.Amphiphilic block copolymers are one of the most common typesof amphiphilic polymers. It is well known that in selective solvents,amphiphilic block copolymers can self-assemble into aggregates ofa variety of morphologies, which can be generally divided intostar-like and crew-cut types [1]; specific examples include spheres,rods, vesicles, cylinders and large compound micelles [2–4]. Be-cause of their prospective applications in microreactors, microelec-tronics, microcapsules, drug delivery, and metal-polymer catalysts[5–10], amphiphilic block copolymers have attracted considerableinterest from researchers in recent years [11–18]. Previous studieshave revealed that the aggregate morphology is generally con-trolled by the balance of various contributions to the free energy,including the interfacial energy between the core and the outsidesolution, the stretching of the core-forming blocks, and the repul-sive interactions among corona chains [2,3]. Thus, it is straightfor-

ward to conclude that the morphologies of block copolymeraggregates could be tuned by many factors, such as relative blocklength, ion content, and solvent properties, that directly influencethe balance of contributions to the free energy [19,20].

Stimuli-sensitive polymers are those polymers whose proper-ties or aggregate morphologies change in response to changes inthe environment [21]. These polymers have been of great interestin recent years because of their unique applications [22]. pH-sensi-tive copolymers often contain some ionizable groups or blocks thatcan accept or release protons, thereby granting them sensitivitiesto the pH value of the environment [23–25]. As a result, the amph-iphilicity of some block copolymers with ionizable groups can befinely tuned by controlling the pH value. Poly(4-vinylpyridine)(P4VP) is an important poly (base) with pH sensitivity. When thepH of the solution is greater than 5, P4VP is hydrophobic in itsdeprotonated state; however, when the pH is lower than 5, P4VPis protonated and soluble [26,27]. Thus, the aggregation behaviorof block copolymers with P4VP will change significantly withchanges in pH. Because of this property, P4VP-based block copoly-mers have received much attention [28–30], but most of this atten-tion has been paid to pure organic P4VP-containing copolymers.

At present, incorporating inorganic blocks into organic polymers toobtain organic–inorganic hybrid copolymers has become a popular

Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655 1647

area of study. Polyhedral oligomeric silsesquioxane (POSS) is a class ofinorganic components that can be used as building blocks for prepar-ing novel hybrid copolymers. The POSS molecule possesses a well-de-fined cage-like structure that is composed of an inorganic Si–Oframework (Si8O12) with an average diameter of 1.5 nm, and it is con-sidered to be the smallest silica particle. In addition, its inorganic coreis surrounded by eight organic corner groups, one or more of whichmight be reactive; because of this structure, it can be easily introducedinto polymer matrices or copolymerized with organic monomers toform hybrid polymers, thereby endowing it with superior mechanicaland thermal properties, oxidation resistance, and reduced flammabil-ity [31–35]. Most studies concerning POSS have focused on the prep-aration of nanocomposites with the combined properties of organicand inorganic components, but in recent years, studies regardingthe incorporation of hydrophobic POSS into amphiphilic polymersand their self-assembly behaviors have been reported by manyresearchers. It has been found that POSS molecules have a strongaggregating ability, and they can effectively control the motion ofthe chain and induce self-assembled molecular aggregates of a con-trolled nanometer-scale size [36]. Meanwhile, the introduction ofPOSS can effectively improve the thermal stability of the copolymer[37], which is a promising method of addressing the shortage ofassemblies stemming from pure soft matter and extending theirapplications in catalysis, separation and recycling at high tempera-tures. Recently, living/controlled polymerization techniques, includ-ing atom transfer radical polymerization (ATRP), living anionicpolymerization (LAP) and reversible addition fragmentation chaintransfer (RAFT), have been utilized to obtain well-defined POSS-con-taining copolymers [38–42], and their self-assembly behaviors insolution have also been studied extensively. Most recently, our grouphas synthesized amphiphilic block copolymers containing POSS,namely, PMAPOSS-b-PAA and PMAPOSS-b-P(AA-co-St), via RAFTpolymerization and selective hydrolysis and has obtained patternedcore-corona nanoparticles in solution [43]. Moreover, Chen et al. havereported a POSS-capped dipicolinic acid-functionalized poly(ethyleneglycol) amphiphile surfactant with metal-responsive properties [44].However, little attention has been paid to the synthesis and self-assembly of POSS-containing block copolymers with pH sensitivity.

Hence, in this work, we report a study concerning the synthesisand self-assembly behaviors of novel hybrid pH-sensitive blockcopolymers containing POSS (referred to as HBCP in this article).A novel series of poly(methacrylisobutyl-POSS)-b-poly(4-vinylpyr-idine) (PMAiBuPOSS-b-P4VP) diblock and poly(methacrylisobutyl-POSS)-b-polystyrene-b-poly(4-vinylpyridine) (PMAiBuPOSS-b-PS-b-P4VP) triblock copolymers of various chain lengths weresynthesized via RAFT polymerization. In these block copolymers,poly(methacrylisobutyl-POSS) (PMAiBuPOSS) and polystyrene(PS) functioned as hydrophobic blocks, and poly(4-vinylpyridine)(P4VP) functioned as pH-sensitive blocks, whose amphiphilicitycould be tuned by controlling the pH value. Because the PS blockis softer than POSS, the introduction of PS into the hydrophobicblock provides the possibility of forming many interesting mor-phologies in the aqueous phase. The synthesis route is illustratedin Scheme 1. The self-assembly behaviors, including pH-sensitivebehaviors and self-assembly morphologies of aggregates, were fur-ther investigated using dynamic light scattering (DLS) and trans-mission electron microscopy (TEM). The effects of block lengthand structure were also carefully considered in this investigation.

2. Experimental

2.1. Materials

2,2-Azobis-(isobutyronitrile) (AIBN, Aldrich, 99%) was used asreceived. Cumyl dithiobenzoate (CDB) was synthesized according

to the procedure described in the literature (purity > 95% accordingto 1H NMR analysis) [45]. 3-(3,5,7,9,11,13,15-Heptaisobutyl-penta-cyclo[9.5.1.13,9.15,15.17,13]-octasiloxane-1-yl)propyl methacry-late (MAiBuPOSS, Product No. MA0702) was purchased fromHybrid Plastics Company and used as received. 4-Vinylpyridine(4VP) and styrene (St) were purified by passing through a columnof basic aluminum oxide to remove inhibitors. Toluene (anhydrous,synthetic grade) and tetrahydrofuran (THF) were purchased fromAldrich and distilled prior to use. Hydrochloric acid (HCl, Aldrich,36.5–38%) was used as received. The reagents used were of analyt-ical grade unless otherwise noted.

2.2. Synthesis of PMAiBuPOSS homopolymer

In a typical experiment, MAiBuPOSS monomer (4.732 g,5 mmol),CDB (68.0 mg,0.25 mmol) and AIBN (8.2 mg,0.05 mmol) were dis-solved in 3.25 mL of toluene, placed in a Schlenk tube, and then thor-oughly deoxygenated by five consecutive freeze–pump-thaw cycles.The tube was subsequently placed in an oil bath at 65 �C for 48 h.The reaction was stopped by plunging the tube into liquid nitrogen.The product was drawn from the reaction mixture after 48 h and ana-lyzed using 1H NMR and GPC. The conversion was determined using1H NMR in CDCl3 from the relative integrations of the polymer estergroup methylene protons (CH2–O, 2nH, d = 3.85 ppm for the mono-mer, where n was the degree of polymerization) and the characteristicvinyl protons of the unreacted monomer (peaks at 5.58 and 6.15 ppm,2H). The PMAiBuPOSS theoretical molar mass (Mn,theo) was calculatedvia monomer conversion according to the following equation:Mn;theo ¼ ½MAiBuPOSS�0=½CDB�0 � conversion�MMAiBuPOSS þM0, inwhich MMAiBuPOSS and M0 represent the molecular weights of themonomer and the chain-transfer agent (CDB), respectively (MMAiBu-

POSS = 946.3 g mol�1 and M0 = 272.4 g mol�1). After purification viathree iterations of precipitation into a solvent mixture (methanol/ace-tic ether = 6/1,vol%) and drying in a 25 �C vacuum for 24 h, purePMAiBuPOSS pink polymer was obtained. The purified polymer wascharacterized using 1H NMR and GPC. The number-averaged degreesof polymerization (DPn;MAiBuPOSS) were calculated from the 1HNMR spectrum according to the following equation:DPn;MAiBuPOSS ¼ A3:85=A7:85, where A3.85 and A7.85 represent theintegration areas corresponding to the methylene protons of the poly-mer ester group (CH2–O, 2nH, d = 3.85 ppm) and the aromatic protonsof the dithiobenzoate group of CDB (Ar–H, 2H, d = 7.85 ppm), respec-tively. The molecular weight of PMAiBuPOSS (Mn;NMR) was calculatedfrom the 1H NMR spectrum according to the following equation:(Mn;NMR ¼ DPn;MAiBuPOSS�MMAiBuPOSS þM0, where MMAiBuPOSS

and M0 represent the molecular weights of the monomer and CDB,respectively (MMAiBuPOSS = 946.3 g mol�1 and M0 = 272.4 g mol�1).

1HNMR (CDCl3, ppm, TMS) d: 0.63 (–SiCH2), 0.96 (–CH2

CH(CH3)2), 1.88 (–CH2CH(CH3)2), 3.85 (–OCOCH2), 7.0–7.9 (–C6H5).

2.3. Synthesis of PMAiBuPOSS-b-P4VP

In a typical experiment, 4VP monomer (5 mL, 48.6 mmol),PMAiBuPOSS (1.8 g,0.1 mmol) and AIBN (2.5 mL of 0.005 M AIBNTHF solution) were dissolved in 5 mL of THF, placed in a Schlenktube, and then thoroughly deoxygenated by five consecutivefreeze–pump-thaw cycles. The tube was subsequently placedin an oil bath at 65 �C for 12 h. The reaction was stopped byplunging the tube into liquid nitrogen. The product wasdrawn from the reaction mixture after reaction and analyzedvia 1H NMR. The conversion of 4VP was determined using 1HNMR in CDCl3 from the relative integrations of the polymerpyridine group protons at the ortho-position of nitrogen (C–H,2nH, d = 8.34 ppm, where n is the degree of polymerization)and the pyridine group protons at the nitrogen ortho-positionof the unreacted monomer (C–H, 2H, d = 8.53 ppm). The

Scheme 1. The synthesis route for the production of PMAiBuPOSS-b-P4VP and PMAiBuPOSS-b-PS-b-P4VP via RAFT polymerization.

1648 Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655

PMAiBuPOSS-b-P4VP theoretical molar mass (Mn,theo) was calcu-lated via 4VP conversion according to the following equation:Mn;theo ¼ ½4VP�0=½PMAiBuPOSS�0 � conversion�M4VP þMPMAiBuPOSS,in which MPMAiBuPOSS and M4VP represent the molecular weights ofPMAiBuPOSS and the 4VP monomer, respectively (MPMAiBuPOSS wascalculated for the synthesized PMAiBuPOSS as discussed above,and M4VP = 105.14 g mol�1). The resulting polymer was obtainedafter precipitation from deionized water and drying in a 25 �C vac-uum for 24 h. The purified polymer was characterized using 1HNMR and TEM. The molecular composition of PMAiBuPOSSn-b-P4VPm was determined from the integration of the methylene pro-tons of the PMAiBuPOSS ester group (CH2–O, 2nH, d = 3.85 ppm)and the polymer pyridine group protons at the ortho-position ofnitrogen (C–H, 2 mH, d = 8.34 ppm). The number-averaged degreesof polymerization (DPn;4VP) were calculated from the 1H NMRspectrum according to the following equation: DPn;4VP ¼ A8:34

=A3:85 � DPn;MAiBuPOSS. The molecular weights of PMAiBuPOSS-b-P4VP (Mn;NMR) were determined from the equation Mn;NMR ¼DPn;4VP þMPMAiBuPOSS, where MPMAiBuPOSS and M4VP represent themolecular weights of PMAiBuPOSS and the 4VP monomer,respectively (MPMAiBuPOSS was calculated for the synthesizedPMAiBuPOSS as discussed above, and M4VP = 105.14 g mol�1).

1HNMR (CDCl3, ppm, TMS) d: 0.63 (–SiCH2), 0.96 (–CH2

CH(CH3)2), 1.44 (–CH2), 1.88 (–CH2CH(CH3)2), 3.85 (–OCOCH2),6.38, 8.34 (–C5H4N).

2.4. Synthesis of PMAiBuPOSS-b-PS-b-P4VP

To synthesize PMAiBuPOSS-b-PS-b-P4VP, the macro chain-transfer agent, PMAiBuPOSS-b-PS (MCTA), must be synthesizedfirst. In a typical experiment, styrene (1 mL, 10.7 mmol), PMAiBu-POSS (0.3 g, 1.41 � 10�2 mmol) and AIBN (0.15 mL of 0.004 M AIBNtoluene solution) were dissolved in 0.3 mL of toluene, placed in aSchlenk tube, and then thoroughly deoxygenated by five consecu-tive freeze–pump-thaw cycles. The tube was subsequently placedin an oil bath at 65 �C for 16 h. The reaction was stopped by plung-ing the tube into liquid nitrogen. After purification by precipitationinto methanol, the product was dried in a 25 �C vacuum for 24 h.The purified polymer was characterized using 1H NMR, GPC andTEM. The molecular composition of PMAiBuPOSSx-b-PSy was deter-mined from the integration of the methylene protons of thePMAiBuPOSS ester group (CH2–O, 2xH, d = 3.85 ppm) and the poly-mer phenyl group protons (Ar–H, 5yH, d = 6.49–7.4 ppm). After thisprocedure, 4VP (2 mL,19.44 mmol), PMAiBuPOSS22-b-PS169

(0.9 g,2.00 � 10�2 mmol) and AIBN (0.5 mL of 0.005 M AIBN THFsolution) were dissolved in 0.3 mL of THF, placed in a Schlenk tube,

and subjected to five consecutive freeze–pump-thaw cycles fordeoxygenation. The tube was subsequently placed in an oil bathat 65 �C for 20 h. The reaction was stopped by plunging the tubeinto liquid nitrogen. Aliquots were drawn from the reaction mix-ture after reaction and analyzed via 1H NMR. The resulting polymerwas obtained after precipitation from deionized water and dryingin a 25 �C vacuum for 24 h. The PMAiBuPOSS-b-PS-b-P4VP theoret-ical molar mass (Mn,theo) was calculated via 4VP conversion accord-ing to the following equation: Mn;theo ¼ ½4VP�0=½MCTA�0�conversion�M4VP þMMCTA, where MMCTA and M4VP representthe molecular weights of PMAiBuPOSS22-b-PS169 and the 4VPmonomer, respectively. The molecular composition ofPMAiBuPOSS22-b-PS169-b-P4VPz was determined as in the case ofPMAiBuPOSSn-b-P4VPm. The molecular weights of PMAiBuPOSS-b-PS-b-P4VP (Mn, NMR) were determined from the equation:Mn;NMR ¼ DPn;4VP �MMCTA.

1HNMR (CDCl3, ppm, TMS) d: 0.63 (–SiCH2), 0.96 (–CH2-

CH(CH3)2), 1.44 (–CH2), 1.88 (–CH2CH(CH3)2), 3.85 (–OCOCH2),6.38, 8.34 (–C5H4N), 7.0–7.9 (–C6H5).

2.5. Self-assembly of HBCPs in solutions

The preparation of copolymer aggregates was conducted fol-lowing the procedure adopted in our previous study with someslight modification [41]. The copolymers were first dissolved in asmall volume of THF, followed by the slow addition (2 mL/min)of a known volume of aqueous solution; aqueous solutions withvarious pH values were used for different samples In the typicalprocess, 4 mg of HBCPs (PMAiBuPOSS-b-P4VP or PMAiBuPOSS-b-PS-b-P4VP) was dissolved in 1 mL of THF, and then 5 mL of HClaqueous solution of a certain pH was added slowly. The solutionwas exposed to the air at room temperature until the THF hadcompletely evaporated; the duration of exposure was approxi-mately 48 h. The concentration of all aggregate solutions discussedbelow was 0.5 mg/mL. In each case, the concentration was muchhigher than the associated critical aggregate concentration (CAC),which was determined using fluorescence-emission spectrometry(as shown in Fig. S1 and S2).

2.6. Characterization methods

1H NMR measurements were performed on a BrukerAV400 MHz NMR spectrometer using CDCl3 as the solvent. Molec-ular weight determinations for the polymers were conducted usinggel permeation chromatography (GPC) analyses performed in tet-rahydrofuran using a series of Waters Styragel columns: HR2,

Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655 1649

HR4 and HR5. The eluent was THF at a flow rate of 1.0 mL/min. Aseries of low-polydispersity polystyrene standards were employedfor the GPC calibration. TEM measurements were performed usinga JEM2100 at an acceleration voltage of 200 kV. A 5 lL droplet ofself-assembled aggregate solution (0.5 mg/mL) was dropped di-rectly onto a copper grid (300 mesh) coated with a carbon film, fol-lowed by drying at room temperature.

The size of the aggregates was characterized via DLS performedon a Zetasizer NanoZS Instrument (Malvern Instruments, UK)equipped with a 400 mW argon-ion laser with wavelength of532 nm at 25 �C and a scattering angle of 90�. The CAC of the copoly-mers in aqueous solution was measured via fluorescence spectro-photometry on an Edinburgh FLS920 instrument using pyrene asthe fluorescence indicator (stimulation slit width: 1 nm, emissionslit width: 2 nm, and scanning speed: 120 nm/min). The absorbanceat 373 nm relative to that at 383 nm was plotted against the polymerconcentration, and the crossing point of the two extrapolatedstraight lines was defined as the CAC of the polymer.

3. Results and discussion

3.1. Synthesis and characterization of PMAiBuPOSS-b-P4VP andPMAiBuPOSS-b-PS-b-P4VP

In this paper, we investigated the synthesis of PMAiBuPOSS-b-P4VP diblock copolymers and PMAiBuPOSS-b-PS-b-P4VP triblockcopolymers with various chain lengths via RAFT polymerization.During the polymerization, AIBN was used as the initiator andCDB as the RAFT agent, because CDB is a convenient vehicle thatrequires mild experimental conditions and yields a narrow molec-ular-weight distribution [40]. The polymerization results of ourblock copolymers are listed in Table 1. From the polymerization re-sults, we find that the molecular weight of PMAiBuPOSS calculatedvia NMR is higher than that calculated from the GPC results. Thiscould be attributed to the compact structure of PMAiBuPOSS,which is different from that of GPC polystyrene standards [46].To synthesize a hybrid block copolymer containing POSS, PMAiBu-POSS homopolymer should be synthesized first because of its sterichindrance. PMAiBuPOSS18 (Mn,GPC = 5900, PDI = 1.06) and PMAiBu-POSS22 (Mn,GPC = 8200, PDI = 1.05) were utilized for chain extensionin our study. In contrast, if P4VP or PS is used as a macro transferagent, it will be difficult to further prepare hybrid block copoly-mers via the chain extension of P4VP or PS with MAiBuPOSS. Ourgroup, Deng et al. has attempted to prepare hybrid block copoly-mers via the chain extension of PMMA chains with MACyPOSS orMAiBuPOSS. However, this approach was severely limited becauseof a very low conversion and chain-transfer efficiency [40], andthat case is similar to the present one.

1H NMR was used for the characterization of our resulting poly-mers. The 1H NMR spectrum of PMAiBuPOSS18 is shown in Fig. 1a.The signals at 0.63 ppm are attributed to the methylene protons

Table 1The polymerization results of PMAiBuPOSS, PMAiBuPOSS-b-PS, PMAiBuPOSS-b-P4VP and

Samples Chemical formulaa 10�3Mb(Theo

P1 PMAiBuPOSS18 17.3P2 PMAiBuPOSS22 21.1MCTA PMAiBuPOSS22-b-PS169 44.5HBCP1 PMAiBuPOSS18-b-P4VP40 22.2HBCP2 PMAiBuPOSS18-b-P4VP126 35.2HBCP3 PMAiBuPOSS22-b-PS169-b-P4VP162 62.0HBCP4 PMAiBuPOSS22-b-PS169-b-P4VP215 67.6

a Chemical formula and Mn,NMR of different resulting polymers were determined fromb Mn,theo of different resulting polymers were calculated from the conversion of monoc Mn,GPC and PDI of different resulting polymers were determined from GPC analyses.

next to the silicon atom (Si), and the signals at 0.96 ppm are as-signed to the methyl protons of PMAiBuPOSS18. The signal of themethylene protons of the PMAiBuPOSS ester group, which playsan important role in the calculation of conversion and molecularcomposition, appears at 3.85 ppm. Fig. 1b presents the 1H NMRspectrum of PMAiBuPOSS18-b-P4VP126. In contrast with the previ-ous spectrum, we find that peaks at 6.38 ppm and 8.34 ppm ap-pear, which belong to the polymer pyridine group protons at themeta-position and ortho-position of nitrogen, respectively. Usingthese signals, the number-averaged degrees of polymerizationwere calculated, as mentioned above. In the 1H NMR spectrum ofPMAiBuPOSS18-b-P4VP126, we find no peaks corresponding to the4-vinylpyridine monomer, which means that our purification waseffective.

In the synthesis of the PMAiBuPOSS-b-PS-b-P4VP triblockcopolymer, MCTA (PMAiBuPOSS-b-PS) was further synthesizedon the basis of PMAiBuPOSS. The 1H NMR spectrum of PMAiBu-POSS22-b-PS169 is shown in Fig. 2b. By comparing this spectrumto the 1H NMR spectrum of PMAiBuPOSS22 (Fig. 2a), we can clearlysee that peaks of 6.49–7.4 ppm and 7.1–7.4 ppm appear, which areassigned to the aromatic protons of the PS block. GPC analysis alsodemonstrated the successful synthesis of PMAiBuPOSS-b-PS(Fig. 3). There is no shoulder or tail in the GPC curve of PMAIBU-POSS22, which indicates its narrow molecular-weight distribution.With further chain extension, the chromatograms of PMAiBu-POSS22-b-PS169 exhibit a shift toward lower elution volumes, indi-cating that PMAiBuPOSS-b-PS has been synthesized successfully.The small shoulder found in the GPC curve of PMAiBuPOSS22-b-PS169 is ascribed to a dead chain of PMAiBuPOSS22 and severalpolymers resulting from coupling termination. The 1H NMR spec-trum of PMAiBuPOSS22-b-PS169-b-P4VP162 is shown in Fig. 2c.The signals of the polymer pyridine group protons appear at6.38 ppm and 8.34 ppm, and the peaks at 6.38 ppm overlap withpeaks attributed to aromatic protons.

However, because the PMAiBuPOSS block is insoluble in DMFand the solubility of the P4VP block in THF is not very high, no idealeluent for the GPC measurement for our hybrid block copolymercould be found. Here, TEM was utilized for the characterizationof our resulting polymers. Fig. 4a and b presents TEM images ofcasting films of PMAiBuPOSS18-b-P4VP126. The stripes of phaseseparation in the TEM images are well in order. Because all TEMsamples are imaged by identifying the higher mass contrast ofthe POSS-containing block compared to the P4VP segments,PMAiBuPOSS domains appear as darker regions, while P4VP do-mains appear as brighter regions. Here, the densities of thePMAiBuPOSS and P4VP segments are imaged by referencing thoseof the MAiBuPOSS (1.114 g/ml) and 4VP (0.989 g/ml) monomersbecause of the difficulty in obtaining the exact values of the densityof the PMAiBuPOSS domains and P4VP domains. The area ratio be-tween the darker region and the brighter region is approximately1.26. Through calculation, it is found that the mass ratio of the

PMAiBuPOSS-b-PS-b-P4VP.

) 10�3Mna (NMR) 10�3Mn

c (GPC) PDIc

19.2 5.9 1.0622.0 8.2 1.0539.8 31.8 1.0821.6 – –32.2 – –51.5 – –66.8 – –

the integration of 1H NMR spectra.mers which have been mentioned above.

Fig. 1. 1H NMR spectra of PMAiBuPOSS18 and PMAiBuPOSS18-b-P4VP126.

Fig. 2. 1H NMR spectra of PMAiBuPOSS22, PMAiBuPOSS22-b-PS169 and PMAiBu-POSS22-b-PS169-b-P4VP162.

Fig. 3. GPC traces of PMAiBuPOSS22 and PMAiBuPOSS22-b-PS169.

1650 Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655

PMAiBuPOSS domains and the P4VP domains is close to the molec-ular-weight ratio of the PMAiBuPOSS block and the P4VP block.The observed well-ordered phase-separation morphology and thearea ratio served as proof of the successful synthesis of PMAiBu-POSS-b-P4VP.

TEM images of casting films of PMAiBuPOSS22-b-PS169 andPMAiBuPOSS22-b-PS169-b-P4VP162 are also shown in Fig. 4. Simi-lar to PMAiBuPOSS18-b-P4VP126, well-ordered stripes are alsofound in the casting films of PMAiBuPOSS22-b-PS169 (Fig. 4c)and PMAiBuPOSS22-b-PS169-b-P4VP162 (Fig. 4d). The PS domainsappear as brighter regions similar to the P4VP because the PSblock has a similar structure to P4VP. By comparing the images,it can be clearly seen that the stripes in the TEM images ofPMAiBuPOSS22-b-PS169-b-P4VP162 are bolder than those in theimages of PMAiBuPOSS22-b-PS169. We attribute this differenceto the successful chain extension of P4VP, which increased thearea of the brighter region. The 1H NMR, GPC and TEM results dis-cussed above demonstrate the successful preparation of PMAiBu-POSS-b-P4VP diblock copolymers and PMAiBuPOSS-b-PS-b-P4VPtriblock copolymers.

3.2. The effect of pH on the aggregate size of HBCPs

As we know, P4VP is an important poly(base) with pH sensitiv-ity. When the pH of the solution is greater than 5, P4VP is hydro-phobic in its deprotonated state; however, when the pH is lowerthan 5, P4VP is protonated and soluble [26,27]. Therefore, self-assembly behaviors of P4VP-containing block copolymers can betuned by controlling the pH value. In our work, a series of hybridblock copolymers containing POSS blocks and P4VP blocks weresynthesized and assumed to have pH sensitivity. For a study ofthe pH sensitivity of the HBCPs (PMAiBuPOSS-b-P4VP and PMAiBu-POSS-b-PS-b-P4VP), self-assembly aggregates of the HBCPs wereprepared using the methods described in the experimental section.DLS was utilized for the investigation of the aggregate sizes. Fig. 5shows the number-weighted size distribution of self-assembledaggregates of HBCP1, HBCP2, HBCP3 and HBCP4 (chemical formu-las are shown in Table 1) in aqueous solution at various pH values.It is shown that the HBCP aggregates are evenly distributed anduniform in size. The size of the aggregates is approximately10–200 nm. Furthermore, the change in the size of the HBCP aggre-gates with increasing pH value presents a certain regularity.Consider HBCP1 as an example (Fig. 5a): In pH-1 aqueous solution,the diameter of the aggregates is approximately 80 nm. When thepH value increases, the size of the aggregates begins to decrease,and the diameter of the HBCP1 aggregates is 15 nm at pH 3.However, with a further increase in pH, the aggregate size beginsto increase. Similar behavior is observed for HBCP2, HBCP3 andHBCP4.

To provide a better understanding of the trend of this change inaggregate size, the relation between the diameter of the HBCPaggregates and the pH value is shown in Fig. S3. It is found thatall aggregates of the HBCPs, including HBCP1, HBCP2, HBCP3 andHBCP4, exhibit a similar size change: the aggregates size initiallydecreases and then increases as the pH value increases. However,the minimum size of the HBCP1 and HBCP4 aggregates is achievedat pH 2, while the size minimum for the HBCP2 and HBCP3 aggre-gates appears at pH 3.

As we know, DLS results reflect the hydrodynamic size of theaggregates, which is closely related to their molecular chain statein a fluid. This relation can be simply explained: the P4VP blockis highly protonated when the pH value is low, so the molecularchain is stretched because of strong electrostatic repulsion, whichresults in a larger hydrodynamic size. As shown in Fig. 6, as the pHvalue increases, the degree of protonation of the P4VP block beginsto decrease, and the number of positive charges on the P4VP block

Fig. 4. TEM images of casting films of PMAiBuPOSS18-b-P4VP126 (a and b), PMAiBuPOSS22-b-PS169 (c) and PMAiBuPOSS22-b-PS169-b-P4VP162 (d).

Fig. 5. The number-weighted size distribution of self-assembled aggregates of HBCP1 (a), HBCP2 (b), HBCP3 (c) and HBCP4 (d) in aqueous solution with varying pH.

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decreases accordingly. The direct result of such a change in the pHvalue is a decrease of electrostatic repulsion, leading to morecurled molecular chains of P4VP, which is reflected in the smallersize of the HBCP aggregates in the aqueous phase. The self-assem-bly behaviors of the HBCP aggregates should be controlled by a bal-ance of three different interactions: the aggregating interactions ofthe hydrophobic blocks, the p–p stacking interaction of the pyri-dine group, and the electrostatic repulsion between protonatedP4VP blocks. When the pH value increases to a value close to thepKa of P4VP, the degree of protonation of the P4VP block is solow that some P4VP chains present hydrophobic properties. Conse-quently, a stronger p–p stacking interaction occurs, and more P4VPmolecular chains participate in the formation of aggregate cores.Therefore, larger HBCP aggregates with larger hydrophobic coresare formed in relatively high-pH conditions.

During the experiment, we observed that HBCPs would precip-itate from aqueous solution when pH > 5. HBCPs could no longerform aggregates in aqueous solution under these conditions be-cause of the disappearance of amphiphilicity. We were also sur-prised to find that the HBCPs that precipitated from aqueoussolution would form aggregates once again with the aid of mag-netic stirring when we reduced the aqueous pH to 4 or less.

3.3. The effect of pH on the self-assembly morphologies of HBCPs

Further investigation of the pH sensitivity of the HBCPs wasconducted via TEM. The TEM images in Fig. 7 illustrate the self-assembly morphologies of the HBCP aggregates at various pH val-ues. From these images, we could observe the existence of theHBCP aggregates directly, and we find that the change in size ofthe HBCP aggregates as a function of the pH value is consistentwith the DLS results. In the TEM images, the diameter of theHBCP1 aggregates is approximately 40 nm or 20 nm in aqueoussolution at pH 1 or pH 4, respectively. The diameter of the HBCP2aggregates is approximately 40 nm or 30 nm in aqueous solutionat pH 1 or pH 4, respectively. The morphology of the HBCP3aggregates is entirely different from those of HBCP1 and HBCP2.HBCP3 forms nanowires at pH 1 and forms nanospheres with a

Fig. 6. Schematic of the variation in a

diameter of 100–200 nm at pH 4. Meanwhile, the morphologyof the HBCP4 aggregates is nanospherical with a diameter of50 nm at pH 1 and 100–200 nm at pH 4. By comparing the TEMand DLS results, it is found that the diameters indicated by theDLS results are slightly higher than those observed in the TEMimages. The difference between the sizes measured via DLS andthose determined via TEM can be accounted for by consideringthat the sizes measured using DLS are based on the swollenaggregates in aqueous solution, while the sizes observed usingTEM are derived from the dried aggregates. It is worth noting thatthe diameter of the HBCP3 nanowires is approximately 25 nm,while the size deduced from the DLS results is approximately160 nm. Apparently, the staggered structure of the nanowirescauses their hydrodynamic size to appear larger.

3.4. The effects of block length and structure on the self-assemblymorphologies of HBCPs

It is well known that self-assembly morphologies are highlyinfluenced by the block length and structure of amphiphilic poly-mers. Here, the self-assembly morphologies of our HBCPs shouldbe influenced by the block lengths of PMAiBuPOSS, PS and P4VP.TEM and DLS were utilized for the investigation of these effects.Fig. 8 shows TEM images and diameter distributions of the self-assembled aggregates of HBCP1 and HBCP2 in aqueous solutionat pH 1. By comparing the two, we find that the diameter of theHBCP1 aggregates is similar to that of HBCP2 aggregates, althoughthe size of the HBCP1 aggregates is slightly larger. Compared toHBCP1, the chain length of the hydrophobic PMAiBuPOSS blockof HBCP2 is the same, but the chain length of its hydrophilic pro-tonated P4VP block is longer. Because of the longer hydrophilicchain, stronger electrostatic repulsion exists between the hydro-philic chains of HBCP2, and the growth of hydrophobic cores ismore strongly hindered by this repulsion, resulting in smalleraggregates.

We also investigated the PMAiBuPOSS-b-PS-b-P4VP hybrid tri-block copolymers. TEM images and diameter distributions of theself-assembled aggregates of HBCP3 and HBCP4 at pH 1 are shown

ggregate size as a function of pH.

Fig. 7. TEM images of aggregates formed by HBCP1 (a and e), HBCP2 (b and f), HBCP3 (c and g) and HBCP4 (d and h) in aqueous solution at pH 1(a–d) and pH 4 (e�h).

Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655 1653

in Fig. 9. From the TEM images, we observe that HBCP3 self-assem-bled into nanowires, but HBCP4 self-assembled into nanospheres,even though their diameter distributions are similar. The hydro-phobic chain of HBCP4 is the same as that of HBCP3, consistingof a PMAiBuPOSS block and a PS block, but the hydrophilic chainof HBCP3 is shorter. Furthermore, the hydrophobic chain of HBCP3is the longest of the four HBCPs in Table 1, and it also has the low-est CAC value (Fig. S2). Driven by strong aggregating interactions

Fig. 8. TEM images (a and b) and DLS results (c and d) for aggregates of HB

and weak electrostatic repulsion, the HBCP3 aggregates grew con-tinuously and tended to self-assemble into wire-like or rod-likeaggregates in aqueous solution. It is known that amphiphilic mol-ecules will self-assemble into rod micelles at high concentrations[47,48]. Because of the higher concentration of HBCP3 relative tothe CAC, nanowires formed in the HBCP3 dispersion.

Self-assembly morphologies were also compared between thehybrid diblock copolymers (HBCP1 and HBCP2) and the triblock

CP1 (a and c) and HBCP2 (b and d) formed in pH-1 aqueous solution.

Fig. 9. TEM images (a and b) and DLS results (c and d) for aggregates of HBCP3 (a and c) and HBCP4 (b and d) formed in pH-1 aqueous solution.

Fig. 10. Schematic of the phase separation formed in the HBCP aggregates.

1654 Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655

copolymers (HBCP3 and HBCP4). Larger aggregate sizes are foundfor HBCP3 and HBCP4. Apparently, the longer hydrophobic block,which contained a PMAiBuPOSS block and a PS block, resulted ina larger aggregate size. A unique self-assembly morphology is ob-served in the TEM images of the triblock copolymers (Fig. 9), which

is entirely different from the morphology of our diblock copoly-mers (Fig. 8). This morphological difference could be attributedto the difference in the block structures. The existence of the PShydrophobic block, which is softer than POSS, provided the possi-bility of forming many interesting morphologies in aqueous solu-tion. As a result, unique morphology is observed in the TEMimages of the triblock-copolymer aggregates.

In addition, as shown in Fig. 8b and b, dot-like darker regions onthe nanoscale appear among the aggregates, but these dot-like dar-ker regions are not found in the HBCP aggregates at pH 4. We attri-bute these curious dot-like regions to phase separation. It is shownin Fig. 10 that hydrolysis reactions are likely to occur on the estergroups of PMAiBuPOSS at lower pH values [49], and the cage-likePOSS structure would separate from the main chain of the HBCPsafter such a hydrolysis reaction. Because strong interactions existbetween the POSS structures that have separated from the HBCPs,they tend to aggregate into POSS-rich regions in hydrophobiccores, similar to the mechanism reported by Zhang et al. [34],and manifest as dark dots. At the same time, the poly(acrylic acid)groups are hydrophobic at pH 1.0, so they also tend to aggregate inhydrophobic cores. As a result, poly(acrylic acid) groups will notexert a great effect on the size of the nanostructures.

4. Conclusion

In summary, this article has described a simple strategy for pre-paring hybrid pH-sensitive block copolymers containing POSS andhas investigated their pH-sensitive behaviors and self-assemblymorphologies. PMAiBuPOSS-b-P4VP diblock copolymers andPMAiBuPOSS-b-PS-b-P4VP triblock copolymers were successfullysynthesized via RAFT polymerization. The aggregate size of the

Y. Xu et al. / Reactive & Functional Polymers 73 (2013) 1646–1655 1655

HBCPs was found to be sensitive to pH. As the pH value increased,the size of the aggregates in aqueous solution initially decreasedand later increased. The initial size decrease is attributed to theshrinking of the P4VP chain at higher pH values, but the subse-quent increase in the aggregate size was caused by the aggregatinginteraction and the p–p stacking interaction of the pyridine ring,which led to the participation of more P4VP molecular chains inthe formation of aggregate cores. Nanowires and nanosphereswere assembled by our hybrid triblock copolymers. Unique dot-like phase separation was also observed in the aggregates of theHBCPs at pH 1; these dots are considered to correspond to POSS-rich regions formed after the hydrolysis reactions of ester groupsof the HBCPs. The results of this study confirm, for our HBCPs,the effects of block length and structure on the self-assembly mor-phologies of block copolymers.

Acknowledgments

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

Appendix A. Supplementary material

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.reactfunctpolym.2013. 09.008.

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