Organic–inorganic hybrid hydrogels involving poly(N-isopropylacrylamide) and polyhedral oligomeric silsesquioxane: Preparation and rapid thermoresponsive properties

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<ul><li><p>OrganicInorganic Hybrid Hydrogels InvolvingPoly(N-isopropylacrylamide) and PolyhedralOligomeric Silsesquioxane: Preparation andRapid Thermoresponsive Properties</p><p>KE ZENG,1 YUAN FANG,1 SIXUN ZHENG1,2</p><p>1Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,Peoples Republic of China</p><p>2State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240,Peoples Republic of China</p><p>Received 21 August 2008; revised 26 November 2008; accepted 29 November 2008DOI: 10.1002/polb.21655Published online in Wiley InterScience (www.interscience.wiley.com).</p><p>ABSTRACT: 3-Acryloxypropylhepta(3,3,3-triuoropropyl) polyhedral oligomeric silses-quioxane (POSS) was synthesized and used as a modier to improve the thermalresponse rates of poly(N-isopropylacrylamide) (PNIPAM) hydrogel. The radicalcopolymerization among N-isopropylacrylamide (NIPAM), the POSS macromer andN,N0-methylenebisacrylamide was performed to prepare the POSS-containingPNIPAM cross-linked networks. Differential scanning calorimetry (DSC) and thermalgravimetric analysis (TGA) showed that the POSS-containing PNIPAM networks dis-played the enhanced glass transition temperatures (Tgs) and improved thermal sta-bility when compared with plain PNIPAM network. The POSS-containing PNIPAMhydrogels exhibited temperature-responsive behavior as the plain PNIPAM hydro-gels. It is noted that with the moderate contents of POSS, the POSS-containingPNIPAM hydrogels displayed much faster response rates in terms of swelling, desw-elling, and re-swelling experiments than plain PNIPAM hydrogel. The improvedthermoresponsive properties of hydrogels have been interpreted on the basis of theformation of the specic microphase-separated morphology in the hydrogels, that is,the POSS structural units in the hybrid hydrogels were self-assembled into thehighly hydrophobic nanodomains, which behave as the microporogens and promotethe contact of PNIPAM chains and water. VVC 2009 Wiley Periodicals, Inc. J Polym Sci PartB: Polym Phys 47: 504516, 2009</p><p>Keywords: amphiphiles; biological applications of polymers; copolymerization;hydrogels; networks; polyhedral oligomeric silsesquioxane; poly(N-isopropylacrylamide);self-assembly; stimuli-sensitive polymers; swelling</p><p>INTRODUCTION</p><p>Poly(N-isopropylacrylamide) (PNIPAM) is aninteresting water-soluble polymer and it possessesa lower critical solution temperature (LCST)around 32 C in aqueous solution.16 At theJournal of Polymer Science: Part B: Polymer Physics, Vol. 47, 504516 (2009)VVC 2009 Wiley Periodicals, Inc.</p><p>Correspondence to: S. Zheng (E-mail: szheng@sjtu.edu.cn)</p><p>504</p></li><li><p>lower temperatures PNIAPM chains are highlyhydrated and extend with a random coil confor-mation. When the temperature is increased to32 C, the dehydration occurs in water as the indi-vidual chains of the polymer are collapsed intoglobules. The LCST behavior is exhibited in theform of volume phase transition temperature(VPTT) behavior710 if PNIPAM is cross-linked,that is, the thermoresponsive PNIPAM hydrogelsare formed. The thermoresponse properties endowthe hydrogel systems with the promising poten-tials for application in biomedical and medicalareas.1118</p><p>The conventional hydrogels inherently possesssome drawbacks in morphology and propertiessuch as morphological inhomogeneity, low me-chanical strength, low swelling ratio at equilib-rium, and slow swelling and deswellingrates.16,1922 In the past years, considerable efforthas been performed to improve the properties ofhydrogels via a variety of approaches. It has beenproposed that fast swelling and deswelling can beachieved by introducing porosities,2327 and struc-tural inhomogeneity28,29 in hydrogels. However,care must be taken as the mechanical propertiesof porous hydrogels can be signicantly lowerthan non-porous hydrogels. The measures forimproving this quality of the porous hydrogelsmust be taken to balance with ratio of swellingand fast response rates. Recently, it is reportedthat the modication of hydrogels can be achievedthrough the formation of organicinorganichybrids.22,3034 Zhang et al. prepared an organicinorganic hybrid PNIPAM hydrogel by copolymer-izing NIPAM with vinyltriethoxysilane to incorpo-rate siloxane linkage into the hydrogels; thepurpose is to improve the response rates of thehydrogels.32 More recently, Haraguchi et al.33,34</p><p>proposed a new strategy to improve hydrogelproperties by preparing an organicinorganicnanocomposite PNIPAM hydrogel in which exfoli-ated clay layers acted as effective multifunctionalcross-linking agents.</p><p>Polyhedral oligomeric silsesquioxane (POSS)reagents, monomers, and polymers are emergingas new chemical feedstock for preparing organicinorganic nanocomposites; POSS technology isbecoming a main avenue to organicinorganicnanocomposites because of its simplicity in pro-cessing and the excellent comprehensive proper-ties of this class of hybrid materials.3538 A typicalPOSS molecule possesses the structure of cube-octameric frameworks represented by the formula(R8Si8O12) with an inorganic silica-like core</p><p>(Si8O12) surrounded by eight organic cornergroups, one or more of which is reactive or poly-merizable. As a class of new nanosized buildingblocks, POSS molecules have successfully beenincorporated into a variety of polymer systems viareactive blending, copolymerization, and macro-molecular reaction.3541 However, there are fewreports on POSS-modied polymer hydrogels.Schiraldi and coworkers.42 even used octametha-cryloxylpropyl POSS as one of cross-linkingagents of PNIPAM to promote the interactionsbetween the cross-linking agent and silicate llerin the clay-containing PNIPAM hydrogels. Theyproposed that the nature of cross-linking agents(organic and inorganic) in these systems had littleeffect on the overall water absorption of system.In the previous work, we reported the modica-tion via in situ cross-linking of PNIPAM using anoctafunctional POSS macromer. It is identiedthat the temperature response of the POSS-cross-linked PNIPAM hydrogels was improved whencompared with the plain hydrogel.43</p><p>In the present work, we report a modicationof PNIPAM hydrogels with a POSS macromer.The POSS macromer used in this work is a 3-acryloxypropylhepta(3,3,3-triuoropropyl) polyhe-dral oligomeric silsesquioxane, a new POSS mac-romer. Both hepta(3,3,3-triuoropropyl) groupsand cage-like silsesquioxane endow the POSSmacromer with the high hydrophobicity. It isexpected that the POSS blocks in the hydrogelscan self-assemble into the highly hydrophobicmicrodomains, which behave as the microporo-gens to promote the diffusion of water moleculesin the hydrogel and thus enhance the responserate of hydrogels. In this communication, werstly report the synthesis and characterizationof 3-acryloxypropylhepta(3,3,3-triuoropropyl)POSS and then the POSS macromer was intro-duced into PNIPAM hydrogels with the approachof copolymerization. The hydrogel behavior of thePOSS-containing PNIPAM hydrogels wasaddressed on the basis of swelling, deswelling,and re-swelling kinetics.</p><p>EXPERIMENTAL</p><p>Materials</p><p>3,3,3-Triuoropropyltrimethoxysilane was pur-chased from Zhejiang Chem-Technology, China. 3-Chloropropyltrichlorosilane was obtained fromShanghai Lingguang Chemical, China and wasused as received. Acryl chloride was purchased</p><p>POSS-MODIFIED PNIPAM HYDROGELS 505</p><p>Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb</p></li><li><p>from Alfa Regent, USA and used as received. 2,20-azobisisobutylnitrile (AIBN) and N,N0-methylene-bisacrylamide (BIS) were chemical pure gradeand purchased from Shanghai Reagent, Shang-hai, China. N-isopropylacrylamide (NIPAM) wasprepared in this lab by following the literaturemethod.44 Other reagents such as sodium, cal-cium hydride (CaH2), and sodium hydroxide(NaOH) were of chemically pure grade, purchasedfrom Shanghai Reagent, China. The solvents suchas tetrahydrofuran (THF), dichloromethane, pe-troleum ether (distillation range: 6090 C), andtriethylamine (TEA) were of chemically puregrade, also obtained from commercial resources.Before use, THF was reuxed above sodium andthen distilled and stored in the presence of themolecular sieve of 4 A. Triethylamine (TEA) wasreuxed over CaH2 and then was puried with p-toluenesulfonyl chloride, followed by distillatuion.</p><p>Synthesis of 3-Chloropropylhepta-(3,3,3-triuoropropyl) POSS</p><p>The preparation of hepta(3,3,3-triuoropropyl)tri-cycloheptasiloxane trisodium silanolate [Na3O12-Si7(C3H4F3)7] was synthesized by following themethod reported by Fukuda and coworkers.45 In atypical experiment, (3,3,3-triuoropropyl) trime-thoxysilane (50.0 g, 0.23 mol), THF (250 mL),deionized water (5.25 g, 0.29 mol), and sodium hy-droxide (3.95 g, 0.1 mol) were charged to a askequipped with a condenser and a magnetic stirrer.After reuxed for 5 h, the reactive system wascooled down to room temperature and held at thistemperature for 15 h with vigorous stirring. Allthe solvent and other volatile were removed by ro-tary evaporation and the white solids wereobtained. After dried at 40 C in vacuo for 12 h,37.3 g products were obtained with the yield of98%. The as-prepared 3-chloropropylhepta(3,3,3-triuoropropyl) polyhedral oligomeric silsesquiox-ane was prepared via the corner capping reactionbetween Na3O12Si7(C3H4F3)7 and 3-chloropropyltrichlorosilane. Typically, Na3O12Si7(CH2CH2CF3)7 (10.0 g, 8.8 mmol) and triethylamine(1.3 mL, 8.8 mmol) were charged to a askequipped with a magnetic stirrer, 200 mL anhy-drous THF were added with vigorous stirring.The ask was immersed into an ice-water bathand purged with highly pure nitrogen for 1 h.After that, 3-chloropropyltrichlorosilane (2.24 g,10.56 mmol) dissolved in 20 mL anhydrous THFwere slowly dropped within 30 min. The reactionwas performed at 0 C for 4 h and at room temper-</p><p>ature for 20 h. The sodium chloride was lteredout and the solvent together with other volatilewas removed via rotary evaporation and affordthe white solids. The solids were washed with50 mL methanol by three times and dried invacuo at 40 C for 24 h and 7.53 g product wasobtained with the yield of 73%.</p><p>FTIR (cm1, KBr window): 10901000 (SiAOASi), 29002850 (ACH2), 11201300 (ACF3).</p><p>1HNMR (ppm, acetone-d6): 3.62 (t, 2.0H, ACH2ACl),2.32 (m, 14.0H, SiCH2CH2CF3), 1.93 (m, 2.0H,ACH2ACH2ACl), 1.03 (m, 14.0H, SiCH2CH2CF3),0.93 (t, 2.0H, ACH2ACH2ACH2ACl).</p><p>29Si NMR(ppm, acetone-d6): 70.1, 69.2, 66.7 and 65.9.</p><p>Synthesis of 3-Hydroxypropylhepta-(3,3,3-triuoropropyl) POSS</p><p>3-Hydroxypropylhepta(3,3,3-triuoropropyl) poly-hedral oligomeric silsesquioxane [denoted 3-hydroxypropylhepta(3,3,3-triuoropropyl) POSS]was prepared via the hydrolysis of 3-chloropropyl-hepta(3,3,3-triuoropropyl) polyhedral oligomericsilsesquioxane [denoted 3-chloropropylhepta-(3,3,3-triuoropropyl) POSS] in the presence offresh silver oxide (Ag2O). Typically, 3-chloropro-pylhepta(3,3,3-triuoropropyl) POSS (10.0 g,9.38 mmol) was charged to a 250 mL ask equippedwith a magnetic stir bar. The mixture of 200 mLtetrahydrofuran with 200 mL ethanol, deionizedwater (5 mL), and fresh Ag2O (3.0 g) were added.The system was reuxed for 24 h and all the in-soluble solids were ltered out. The above proce-dure was repeated for three time to access thecomplete conversion of 3-chloropropylhepta(3,3,3-triuoropropyl) POSS into 3-hydroxypropyl-hepta(3,3,3-triuoropropyl) POSS. After that, allthe solvents were removed by rotary evaporation.After dried in vacuo at 60 C for 24 h, 8.36 g ofsolid product was obtained with the yield of 85%.</p><p>FTIR (cm1, KBr window): 3336 (AOH), 10901000 (SiAOASi), 29002850 (ACH2), 11201300(ACF3).</p><p>1H NMR (ppm, acetone-d6): 3.61 (t, 2.0H,ACH2AOH), 2.32 (m, 14.0H, SiCH2CH2CF3), 1.93(m, 2.0H, ACH2ACH2AOH), 1.03 (m, 14.0H,SiCH2CH2CF3), 0.93 (t, 2.0H, ACH2ACH2ACH2AOH).</p><p>29Si NMR (ppm, acetone-d6): 69.4,68.4, 67.0 and 66.1.</p><p>Synthesis of 3-Trimethylsilyetherpropylhepta-(3,3,3-triuoropropyl) POSS</p><p>To conrm the formation of 3-hydroxypropyl-hepta(3,3,3-triuoropropyl) POSS, the derivative</p><p>506 ZENG, FANG, AND ZHENG</p><p>Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb</p></li><li><p>of the aforementioned product was prepared andsubjected to 1H NMR analysis. The 3-hydroxypro-pylhepta(3,3,3-triuoropropyl) POSS (1.0 g,0.87 mmol), anhydrous THF (10 mL) and triethyl-amine (0.44 g, 4.35 mmol) were charged into aask. Then, the ask was immersed into an ice-water bath and chlorotrimethylsilane (0.47 g,4.35 mmol) was dropwise added into the askwith a syringe. The mixture was reacted at 0 Cfor 3 h and another 24 h at room temperaturewith vigorous stirring. After the insolubility wasremoved, the volatile components were removedvia rotary evaporation to obtain solid productswhich were further dried in a vacuum oven at40 C for 24 h with a yield of 90%.</p><p>1H NMR (ppm, acetone-d6): 3.62 (t, 2.0H,ACH2AOSi(CH3)3), 2.32 (m, 14.0H, SiCH2CH2CF3), 1.93 (m, 2.0H, ACH2ACH2AO Si(CH3)3),1.03 (m, 14.0H, SiCH2CH2CF3), 0.93 (t, 2.0H,ACH2ACH2ACH2AO Si(CH3)3), 0.09 (s, 9.0H,SiACH2CH2CH2AOASi(CH3)3). It should bepointed out that in the NMR measurement noTMS was used as the internal reference.</p><p>Synthesis of 3-Acryloxypropylhepta-(3,3,3-triuoropropyl) POSS</p><p>3-Hydroxypropylhepta(3,3,3-triuoropropyl) POSS(5.00 g 4.3 mmol) and TEA (1.2 mL, 8.6 mmol)were charged to a ask equipped with a magneticstirrer and 20 mL anhydrous THF was added.The ask was immersed into an ice-water bathand acryl chloride (0.7 mL, 8.6 mmol) dissolvedin 10 mL anhydrous THF were slowly droppedwithin 30 min. The reaction was performed at 0C for 1 h and then at room temperature for 24 h.The insoluble solids (salts) were ltered outand the solvents were eliminated by rotary evap-oration to obtain the crude solids. The solids wasdissolved in 50 mL chloroform and washed withdeionized water until the solution became neu-tral. The solvent was removed by rotary evapora-tion. After dried in vacuo at 25 C for 24 h, 4.26 gof product was obtained with the yield of 82%.</p><p>FTIR (cm1, KBr window): 10901000(SiAOASi), 29002850 (ACH2), 11201300 (ACF3),1734 (CO), 1628 (CC). 1H NMR (ppm, CDCl3):6.35, 5.78 (d, 2.0H, CH2CHA), 6.13 (t, 1.0H,CH2CHA), 4.16 (t, 2.0H, AOACH2ACH2A), 2.12(m, 14.0H, SiCH2CH2CF3), 1.83 (t, 2.0H,AOACH2ACH2A), 0.91 (m, 14.0H, SiCH2CH2CF3), 0.82 (t, 2.0H, ACH2ACH2ACH2AOA).</p><p>Preparation of PNIPAM-co-POSS Hybrids</p><p>Typically, the POSS-containing PNIPAM hybridcontaining 10 wt % POSS was prepared as fol-lows. 3-Acryloxypropylhepta(3,3,3-triuoropropyl)POSS (0.10 g), N-isopropylacrylamide (NIPAM)(0.9 g), and N,N0-methylenebisacrylamide (12.4mg) were charged to a glass tube and anhydrousTHF (2 mL) was added. The system was purgedwith highly pure nitrogen for 30 min and 2 mgAIBN was added. The radical polymerization wasinitiated and performed at 60 C for 24 h. As thepolymerization proceeds, the system was gradu-ally gelled. The gel was respectively extractedwith water and THF for 72 h to remove unreactedNIPAM and 3-acryloxypropylhepta(3,3,3-triuoro-propyl) POSS. The gel was dried in vacuo at 60 Cfor 48 h. In this work, the contents of the POSSwere controlled to be 5, 7.5, 10, a...</p></li></ul>

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