ORIGINAL CONTRIBUTION

Temperature and pH dual-responsive behavior of polyhedraloligomeric silsesquioxane-based star-block copolymerwith poly(acrylic acid-block-N-isopropylacrylamide) as arms

Yu Bai & Jia Wei & Liping Yang & Chaobin He &

Xuehong Lu

Received: 14 April 2011 /Revised: 27 September 2011 /Accepted: 23 November 2011 /Published online: 20 December 2011# Springer-Verlag 2011

Abstract Poly(acrylic acid-block-N-isopropylacrylamide)(PAAc-b-PNIPAm)-tethered octafunctional polyhedral olig-omeric silsesquioxane (POSS) were synthesized via POSS-initialized atom transfer radical polymerization. When veryshort PAAc blocks are placed between the POSS core andPNIPAm short chains, the critical temperature (Tc) of POSS-PAAc-b-PNIPAm determined by cloud point measurementsvaries in a wide temperature range with pH, and its Tc iseven lower than that of POSS-PNIPAm with similar chainlength at pH05.0, indicating a synergic effect betweenhigh local chain density and intramolecular interactionbetween PAAc and PNIPAm. When the PAAc blocksare relatively long, the pH response of POSS-PAAc-b-PNIPAm diminishes owing to the reduced local PNIPAmchain density.

Keywords Poly(N-isopropylacrylamide) (PNIPAm) .

Polyhedral oligomeric silsesquioxane (POSS) . Temperatureresponsive . pH responsive . Critical temperature (Tc)

Introduction

Thermo-responsive polymers are of great scientific andtechnical interests in recent years owing to their risingpotentials in various fields such as controlled drug delivery,gene delivery, and membrane separation [1–4]. Poly(N-isopropylacrylamide) (PNIPAm) is the most widely studiedthermo-responsive polymer which is soluble in aqueousmedia at room temperature, whereas the solution shows adramatic increase in turbidity at around 34 °C termed as thecloud point or critical temperature (Tc) [5, 6]. At the Tc,PNIPAm undergoes a hydration–dehydration transition,expels water, and collapses to a compact state.

Many research groups have studied the effect of chainstructures, such as molecular weight [7–9], end groups [10,11], and molecular architecture [12, 13], on thermal responsebehaviors of PNIPAm. For linear PNIPAm, the scaling of theTc with chain length is expected in high molecular weightrange [8]. However, when molecular weights are low, linearPNIPAm chains would exhibit rod-like behavior, preventingintra-chain hydrophobic interactions. Asher et al. estimatedthat the persistent length of PNIPAm in water is about 10 m,implying that the oligomers with DP <10 are unable to exhibitthe volume phase transition in dilute solutions [14]. Withhydrophobic end groups, the Tc of linear PNIPAm reducesas the aggregation of the hydrophobic chain ends promotes theformation of micelles [12, 15, 16]. Sumerlin et al. synthesizedhyperbranched PNIPAm via reversible addition–fragmentationchain transfer (RAFT) polymerization [13]. The branchedPNIPAm exhibits reduced Tc as compared with linear PNI-PAm, which is attributed primarily to an increased contributionof hydrophobic end groups. Whittaker et al. synthesized starPNIPAm with four arms also via RAFT polymerization andfound that the Tc value of the star PNIPAm with relatively lowmolecular weight was significantly lower than those of their

Electronic supplementary material The online version of this article(doi:10.1007/s00396-011-2562-1) contains supplementary material,which is available to authorized users.

Y. Bai : J. Wei : L. Yang :X. Lu (*)School of Materials Science and Engineering,Nanyang Technological University,50 Nanyang Avenue,Singapore 639798, Singaporee-mail: asxhlu@ntu.edu.sg

C. He (*)Institute of Materials Research and Engineering,3 Research Link,Singapore 117602, Singaporee-mail: cb-he@imre.a-star.edu.sg

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linear counterparts, which is attributed to the presence of ahydrophobic core and hydrophobic end groups in the starPNIPAm molecules [12]. More recently, Liu et al. synthesizedwell-defined 7-arm and 21-arm star-like PNIPAm with β-cyclodextrin cores via click chemistry [11]. The high chaindensity around the β-CD core is believed to be the key factorfor the lower Tc of the star-like PNIPAm.

In addition to manipulating temperature response behav-ior of PNIPAm, pH-sensitive monomers, such as acrylicacid (AAc) and its derivatives, have been copolymerizedwith N-isopropylacrylamide (NIPAm), and the Tc of thecopolymers can be tuned via both temperature and pHstimuli [17–19]. Hoffman et al. reported that the Tc ofNIPAm-AAc random copolymers are always higher than thatof PNIPAm homopolymer, whereas when PNIPAm chains aregrafted on poly(acrylic acid) (PAAc), the Tc drops sharply aspH approaches the pKa of PAAc owing to the intramolecularhydrogen bonding between the PNIPAm chains and non-ionized PAAc segments on backbone [17]. PNIPAm-PAAcblock copolymers with low polydispersity have also beenprepared by Müller et al. via RAFT polymerization [18]. Theblock copolymers form micelles in aqueous solutions independence of pH and temperature, and they showsimilar thermal response behavior to that of the graftcopolymers when pH is slightly above PAAc's pKa.

Recently, much attention has been given to the use ofoctafunctional polyhedral oligomeric silsesquioxane (POSS)to construct star-shaped hybrid polymers [20–25]. The attrac-tiveness of POSS for synthesis of star polymers lies in thatthey are well-defined nanometer-sized cages with eight reac-tion sites and can be easily functionalized with groups that caninitialize living polymerizations to achieve well-controlledarm length [26–28]. POSS has been used to prepare thermo-responsive PNIPAm networks [22] and tadpole-shaped POSS-PNIPAm hybrids [29–31]. Recently, Yao et al. reported POSS-based star-block copolymers containing poly(ε-caprolactone)and PNIPAm that form nano micelles in aqueous media andexhibit temperature-dependent drug release behavior [32]. Inthis article, we report the synthesis of POSS-cored star-blockPAAc-PNIPAm (POSS-PAAc-b-PNIPAm) via two steps ofcore-initialized atom transfer radical polymerization (ATRP).The motivation of the research is to tune the critical tempera-ture of PNIPAm via star approach coupled with the incorpo-ration of pH-responsive blocks and investigate if there issynergy between the two approaches. By attaching eight PNI-PAm arms to POSS, a very high local chain density is createdin the region near the core, while the introduction of shortPAAc blocks next to the core would facilitate the hydrogenbonding between the arms. These star-block copolymers andreference POSS-PNIPAm star polymers allowed us to system-atically study the effects of star architecture and block lengthon temperature and pH responsive behaviors of the polymersin aqueous solutions.

Experimental section

Materials

Octa-aminophenyl-silsesquioxane (OAPS) was purchasedfrom Hybrid Plastics™ and used without further purifica-tion. NIPAm (Sigma-Aldrich, 97%) was recrystallized froma mixture of hexane and toluene (v/v04/1) and dried invacuum at 40 °C prior to use. Tert-butyl acrylate (t-BA,Sigma-Aldrich) was dried and distilled from CaH2 beforeuse. Tris[2-(dimethylamino)ethyl]amine (Me6TREN) wassynthesized from tris(2-aminoethyl)amine (Sigma-Aldrich,96%) [33]. Acetone was purchased from Fisher and used asreceived. 2-Bromoisobutyryl bromide (99%), ethyl 2-bromo-isobutyrate, copper(I) bromide (CuBr), 2-propanol, hydro-fluoric acid (HF), N,N,N',N',N"-pentamethyldiethylenetri-amine (PMDETA), trifluoroacetic acid (TFA), dimethylformamide (DMF), lithium bromide (LiBr), basic aluminumoxide for chromatography, and pH buffer solutions with pH05.0 (citric acid and sodium hydroxide), 6.0 (citric acid andsodium hydroxide), 7.4 (sodium phosphate dibasic and potas-sium phosphate monobasic) and 10.0 (boric acid, potassiumchloride and sodium hydroxide) were purchased from Sigma-Aldrich and used without further purification. Regeneratedcellulose dialysis membrane Spectra/Por® MWCO 1000 waspurchased from Spectrum Laboratories, Inc. and was washedwith deionized water before use.

Methods

1H and 13C NMR spectra were recorded on a 400 MHzBruker Avance-DRX-400 NMR spectrometer. Weight-averagemolecular weights (Mw) and polydispersities weremeasured viaabsolute size-exclusion chromatography (ASEC) using aWaters 2690 separation module with Hitachi GL-S300MDT-5column coupled with multi-angle light scattering detector fromWyatt technology and differential refractive index detectorWaters 410. DMF and THF (v/v01/1) containing 0.1% LiBrwas used as the eluent and the data were collected and pro-cessed by ASTRA software.

Cloud point measurements

Critical temperatures were determined using the turbid-ity method using a Shimazu 2501PC UV–vis spectrom-eter. Aqueous solutions were prepared at the polymerconcentration of 2 mg/mL. The transmittance of visiblelight at the wavelength of 500 nm was recorded as afunction of temperature. Samples were equilibrated ateach temperature for sufficient time to ensure that thetransmittance have a stable value. Tc was defined as thetemperature corresponding to 50% of total transmittancechange.

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Synthesis

POSS-PtBA-b-PNIPAm PtBA-PNIPAm block copolymer-tethered POSS (POSS-PtBA-b-PNIPAm) was synthesizedvia sequential ATRP of t-BA and NIPAm using octafunction-alized POSS as initiator. The POSS-based ATRP initiator wassynthesized by reaction of OAPS with 2-bromoisobutyl bro-mide according to the method reported by Hussain [20]. Thestructure of the initiator was verified by 1H NMR and 13CNMR in CDCl3, and elemental analysis (cf. “Electronic sup-plementary material”—Figs. S1 and S2 and Table S1). In atypical polymerization, the POSS initiator, t-BA, andPMDETA were dissolved in 2-propanol in a Schlenk tubeand degassed three times via freeze–pump–thaw process.Then CuBr was added into the tube when the solution was atfrozen state, followed by two more times of freeze–pump–thaw. The typical molar ratio of (initiator group):(t-BA):(PMDETA):(CuBr) was 1:10:2.2:2 and the molecular weightwas controlled by varying the monomer to initiator ratio andthe reaction time. The solution was then stirred under Argon atroom temperature. The polymerization was terminated byfreezing in liquid nitrogen and adding a few drops of THFduring exposure to air. Subsequently the polymer was purifiedby passing through a basic aluminium oxide column to re-move the copper catalyst. After dialysis against THF, thesolvent was removed by rotary evaporation followed by vac-uum drying to obtain yellowishwax-like product POSS-PtBA.Using POSS-PtBA as macroinitiator, a similar ATRP proce-dure, as well as purification method, was carried out to obtainPOSS-PtBA-b-PNIPAm as white or yellowish powder exceptthat Me6TREN was used as the ligand to replace PMDETA.1H NMR (acetone-d6), δ (in parts per million): 6.8–7.8 (br, N–H and aromatic H), 4.02 (s, –CH–(CH3)2), 2.31 (br, >CH–CH2–), 1.64 (br, >CH–CH2–), 1.50 (s, –C(CH3)3), 1.30(s, >C–(CH3)2), 1.16 (s, –CH–(CH3)2) (1H NMR: cf.“Electronic supplementary material”—Fig. S3).

POSS-PAAc-b-PNIPAm POSS-PAAc-b-PNIPAm was obtainedby hydrolysis of POSS-PtBA-b-PNIPAm. Typically, POSS-PtBA-b-PNIPAm was added in a round bottom flaskequipped with rubber stopper in ice bath while excess ofTFA was injected into the flask using a syringe under theprotection of Ar. After 1 h the ice bath was removed andthe solution was stirred at room temperature overnight.The solvent was then removed by rotary evaporation andthe product was dissolved in acetone and purified bydialysis against water. 1H NMR (acetone-d6) δ (in parts permillion): 6.8–7.8 (br, N–H and aromatic H), 4.02 (s, –CH-(CH3)2), 2.22 (br, >CH–CH2–), 1.65 (br, >CH–CH2–),1.30 (s, >C–(CH3)2), 1.16 (s, –CH–(CH3)2).

POSS-PNIPAm and linear PNIPAm POSS-PNIPAm wassynthesized via previously described ATRP process using

POSS initiator, NIPAm, Me6TREN, and CuBr at typicalmolar ratio of (initiator group):(NIPAm):(Me6TREN):(CuBr) at 1:20:2.2:2. After dialysis against water, freezedrying was used to obtain the white powder product. 1HNMR (400 MHz, D2O), δ(in parts per million): 6.6–8.0 (br,aromatic H), 3.80 (s, –CH–(CH3)2), 1.92 (br, >CH–CH2–),1.49 (br, >CH–CH2–), 1.17 (s, >C–(CH3)2), 1.05 (s, –CH–(CH3)2) (

1H NMR: cf. “Electronic supplementary material”—Fig. S4). Linear PNIPAmwas synthesized in analogy to POSS-PNIPAm using ethyl 2-bromo-isobutyrate as the initiator (1HNMR: cf. “Electronic supplementary material”—Fig. S5).

Hydrolysis To estimate the average number of arms on eachPOSS cage, typical POSS-PtBA-b-PNIPAm and POSS-PNIPAm samples were cleaved by the treatment with HF inacetone and THF (v/v01/1) at room temperature overnight. Thesolution was then diluted with an excess amount of acetone androtary evaporated to obtain yellowish powder product.

Results and discussion

Synthesis and structure verification

POSS-PtBA-b-PNIPAm was obtained by sequential ATRPof t-BA and NIPAm using octafunctionalized POSS asinitiator. Because of the different reactivities of t-BA [34]and NIPAm [35] in ATRP, PMDETA and Me6TREN wereused as the ligand for their polymerization, respectively, asshown in Fig. 1. The livingness of the polymerization wasproved by successful two-step reaction and the molecularweights were controlled by varying the monomer feed ratioand reaction time. From the 1H NMR spectra in Fig. 2, thelengths of the PtBA and PNIPAm blocks and number aver-age molecular weights of the star-block copolymers can beestimated from the integrations of the characteristic peaks ofPtBA, PNIPAm, and POSS-based initiator based on theassumption that there are eight arms on each POSS cage.The detailed calculation method is provided in supportinginformation. The estimated number average molecularweights are listed in Table 1 with estimated DP of PtBAand PNIPAm blocks indicated in the sample names as sub-scripts. The molecular weight distributions of the star-blockcopolymers are significantly broader than that of POSS-PNIPAm as the two-step synthesis route would inevitablybroaden the molecular weight distribution. In addition, thepolymerization of t-BA with POSS initiator was terminatedat a very short time and low conversion to obtain short PtBAblock. This made the distribution even broader becausepolydispersity decreases with increasing molecular weightin typical ATRP reactions [9]. POSS-PAAc-b-PNIPAm wassynthesized by hydrolysis of POSS-PtBA-b-PNIPAm in thepresence of TFA. This reaction allows effective conversion

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Fig. 1 Synthesis routes forPOSS-PNIPAm andPOSS-PAAc-b-PNIPAmcopolymers

Fig. 2 1H NMR of aPOSS-PtBA8-b-PNIPAm11 andb POSS-PAAc8-b-PNIPAm11

in acetone-d6

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of t-BA group to AAc group without breaking the Si–O–Sibonds of the POSS cage [36]. The 1H NMR peak of tert-butyl at δ01.50 (Fig. 2a) disappeared after the hydrolysis(Fig. 2b), indicating the complete conversion of PtBA toPAAc. Star POSS-PNIPAm and linear PNIPAm with similararm/chain lengths were also synthesized via ATRP as refer-ence samples. Their molecular weights are listed in Table 1.

Octafunctional POSS has been frequently used to con-struct star polymers owing to its precise cubic structure [26,27]. However, as the initiator contains multiple isomers [20,21, 28], the POSS-PNIPAm and POSS-PAAc-b-PNIPAmpolymers synthesized inevitably have varied arm numbers.Furthermore, the molecular weights of star-block polymersare very low, and hence there is a chance that the number ofthe arms may be less than eight on each cage. In order toclarify this issue, we used HF to hydrolyze the POSS cagesof a typical POSS-PtBA-b-PNIPAm star-block copolymer.The molecular weight changes brought by the HF treatmentwere traced by ASEC (Fig. 3). After hydrolysis it is evidentthat there is only a single peak, implying that POSS cagesare completely hydrolyzed, leading to the formation oflinear arms. The average number of arms estimated via

dividing Mn, ASEC of POSS-PtBA-PNIPAm by that of itshydrolyzed product is 7.9 that is fairly close to the averagenumber of active sites in each POSS initiator molecule andour assumption, confirming the star architecture of thecopolymers synthesized. The corresponding POSS-PNIPAmreference sample was hydrolyzed via the same procedureand its star structure is confirmed (cf. “Electronicsupplementary material”—Fig. S6).

Dual responsive behavior

To study the effects of the high chain density near the POSScore on the dual-responsive behaviors of the star-blockcopolymers, we inserted short segments of PAAc betweenPOSS and PNIPAm chains via two steps of ATRP thatoffered good control over polymer chain length and linear-ity. By varying the length of the PAAc blocks and pH value,

Table 1 Number average molecular weights (Mn) and polydispersities(PDI) of linear PNIPAm, POSS-PNIPAm and POSS-PAAc-b-PNIPAmdetermined by ASEC and NMR

Polymera Mn, NMR Mn, ASEC PDI by ASEC

Linear PNIPAm11 1,427 3,122 1.09

POSS-PNIPAm13 14,385 24,950 1.15

POSS-PtBA2-b-PNIPAm11 13,270 18,170 1.62

POSS-PtBA8-b-PNIPAm11 18,820 24,410 1.51

a DP indicated in sample names were estimated from 1H NMR inte-gration (details in “Electronic supplementary material”)

Fig. 3 ASEC curves of POSS-PtBA-b-PNIPAm and the correspondinghydrolysis product with their Mn, ASEC indicated

Fig. 4 a The effect of pH on temperature response behaviors of POSS-PAAc2-b-PNIPAm11. b The temperature response behavior of linearPNIPAm11 and star POSS-PNIPAm13

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we can manipulate the hydrophilicity of the molecules andintramolecular interactions.

In POSS-PtBA-b-PNIPAm copolymers, the PtBA seg-ments are hydrophobic. As a result, POSS-PtBA2-b-PNI-PAm11 forms a translucent micellar solution. Different fromPtBA, PAAc is a pH-responsive polymer that is hydrophilicat high pH values owing to ionization and becomeshydrophobic as pH reduces owing to protonation of thecarboxylic groups. Figure 4a shows the thermal responsebehavior of POSS-PAAc2-b-PNIPAm11 at different pH,which are all above the pKa value of PAAc (4.3). Thecritical temperature increases drastically from ca. 26.2°Cto 48.7°C when pH increases from 5.0 to 10.0. The temper-ature variation range is wide yet fairly different from thedual-responsive behaviors of PNIPAm-PAAc block copoly-mers [18] and grafting copolymers [37].

To reveal the mechanism for the dual-responsive behav-ior of POSS-PAAc2-b-PNIPAm11, we studied the effect ofthe star molecular architecture on thermal response behaviorof PNIPAm in low molecular weight range. The thermalresponse behavior of the linear PNIPAm11 and POSS-PNIPAm13 are shown in Fig. 4b. The Tc of linear PNIPAmdecreases with increasing molecular weight [38]. When theDP is 11 that is slightly larger than the persistent lengthpredicted by Asher [14], the Tc is as high as 56.0 °C,showing that short linear PNIPAm chains with length closeto persistent length do have rod-like behavior that sup-presses intramolecular hydrophobic interactions. In contraryto linear PNIPAm11, the Tc of POSS-PNIPAm13 is onlyabout 28.5 °C, which is lower than the well-known valueof around 34 °C. This can be attributed to the high localchain density in the near core region that promotes

Fig. 5 a Illustration of thepossible hydrogen bondsbetween adjacent arms ofPOSS-PAAc2-b-PNIPAm11 andb A molecular model ofadjacent arms of POSS-PAAc2-b-PNIPAm11 in the near coreregion showing a hydrogen bondbetween AAc and NIPAm

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intramolecular interaction between the adjacent PNIPAmarms. It is also worth noting that comparing with the starPNIPAm ended with chain transfer agents from RAFT [12,16] or long alkyl chains [15], the end groups of POSS-PNIPAm synthesized by ATRP is bromine, which has lim-ited hydrophobic effect. In addition, although POSS is ahighly hydrophobic inorganic core, it is surrounded by rigidphenyl and amide groups, as well as short AAc segments,which prevent the interactions of the POSS core with theshort PNIPAm segments.

Based on the above analysis, it is easy to deduce that athigh pH, the carboxylic groups are more ionized and sol-vated that push the arms away from each other owing to theelectrostatic repulsion between the charged carboxylicgroups and hence the short PNIPAm segments in POSS-PAAc2-b-PNIPAm11 behave like isolated rods, leading tomuch higher Tc. Indeed, POSS-PAAc2-b-PNIPAm11 exhibitscloud point at 48.7 °C in pH 10 solution, which is closer tothe behavior of linear PNIPAm13 rather than POSS-PNIPAm11. With similar PAAc short blocks but much lon-ger PNIPAm blocks, the Tc of the star-block copolymer atpH010 is much lower (cf. “Electronic supplementarymaterial”—Fig. S7), which verifies that the high Tc ofPOSS-PAAc2-b-PNIPAm11 is indeed due to the rod-likebehavior of the isolated short chains.

In contrast, at low pH, the PAAc blocks near the coreare more hydrophobic and hence can contribute to intra-molecular interactions. Thus, the thermal response behaviorof POSS-PAAc2-b-PNIPAm11 resembles more that ofPOSS-PNIPAm13. At pH05.0, POSS-PAAc2-b-PNIPAm11

shows even lower Tc (26.2 °C) than that of POSS-PNIPAm13 (28.5 °C). As PAAc units are able to formhydrogen bonds with PNIPAm under acidic conditions[18, 37], the lower Tc of POSS-PAAc2-b-PNIPAm11 atpH05.0 implies that there is a synergistic effect betweenhigh local chain density near the core and inter-chainhydrogen bonding. The PAAc blocks may have a hightendency to form hydrogen bonds with PNIPAm blocksin the region near the core, facilitating the intramolecularhydrophobic interactions between the short PNIPAmblocks. The molecular model in Fig. 5 illustrates that themolecular geometry of POSS-PAAc2-b-PNIPAm11 allowssuch inter-arm hydrogen bonds.

It is striking to see that different from POSS-PAAc2-b-PNIPAm11, POSS-PAAc8-b-PNIPAm11 exhibits muchbroader but similar Tc at about 42 °C under both acidicand basic conditions (Fig. 6). The likely reason is that thepH response of the star-block copolymers is dominated bythe interactions between PAAc and PNIPAm segments. Theanchoring of longer PAAc blocks next to the core reducesthe probability of formation of inter-chain hydrogen bondsbetween the PAAc and PNIPAm blocks on neighbor armsowing to the increased distance between them, while the

chain rigidity brought by the low molecular weight alsosuppresses the interactions between the PAAc and PNIPAmblocks on the same arm. Thus, the Tc of POSS-PAAc8-b-PNIPAm11 is not very sensitive to the pH change. Further-more, the intramolecular hydrophobic interaction betweenPNIPAm blocks is also suppressed owing to the lower localPNIPAm chain density. As a result, the Tc of PAAc8-b-PNIPAm11 in the whole pH range studied is higher than thatof POSS-PNIPAm13, but still lower than that of thecorresponding linear short PNIPAm chains (56 °C). Anotherimplication is that although the inorganic POSS core mayhave some hydrophobic effect to reduce Tc of the copoly-mer, POSS-PAAc8-b-PNIPAm11 still exhibits fairly high Tcat low pH with an even larger hydrophobic core comprisingPOSS and protonated PAAc segments, proving that thehydrophobicity of the core is not the major factor to affectTc in this case.

Fig. 6 a The effect of pH on temperature response behavior of POSS-PAAc8-b-PNIPAm11. b The Tc of POSS-PAAc2-b-PNIPAm11 andPOSS-PAAc8-b-PNIPAm11 determined at 50% of total transmittancechange as a function of pH

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Conclusions

Star-block copolymers containing POSS as the core andshort PAAc-b-PNIPAm chains as arms have been success-fully synthesized via two-step core-first ATRP of t-BA andNIPAm followed by hydrolysis. The star architecture isconfirmed by ASEC analysis of a POSS-PtBA-b-PNIPAmcopolymer and its cleaved arms. Corresponding low molec-ular weight linear PNIPAm and POSS-PNIPAm with shortarms were also synthesized via ATRP. The Tc of POSS-PNIPAm is much lower than that of linear PNIPAm withsimilar chain length owing to the high local chain densitynear the POSS core that significantly increases intramolec-ular interactions between the PNIPAm arms, while suchinteraction is largely inhibited for linear PNIPAm chainswhen their lengths approach the persistent length. Whenvery short PAAc blocks are placed between the POSS coreand PNIPAm short chains, POSS-PAAc2-b-PNIPAm11

varies with pH in a wide temperature range, and its Tc iseven lower than that of POSS-PNIPAm at pH05.0, imply-ing a synergic effect between high local chain density andintramolecular hydrogen bonding. When the PAAc blocksare longer, the Tc of POSS-PAAc8-b-PNIPAm11 is broad andhas almost no response to pH as intramolecular hydrogenbonding between PAAc and PNIPAm blocks may be hin-dered by the increased distance between them. The resultssuggest that with the star-block structure, the high localchain density near the core may lead to enhanced intramo-lecular interactions between different types of units andhence significantly alter the critical phase transition behav-ior. The findings may be used to design new thermo- andpH-responsive polymeric and hybrid systems, such as den-drimer and surface-grafted nanoparticles.

Acknowledgment Yu Bai thanks Nanyang Technological University,Singapore for providing his Ph.D. scholarship in the course of this work.

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