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

Organic–Inorganic Hybrid Hydrogels InvolvingPoly(N-isopropylacrylamide) and PolyhedralOligomeric Silsesquioxane: Preparation andRapid Thermoresponsive Properties

KE ZENG,1 YUAN FANG,1 SIXUN ZHENG1,2

1Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,People’s Republic of China

2State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240,People’s Republic of China

Received 21 August 2008; revised 26 November 2008; accepted 29 November 2008DOI: 10.1002/polb.21655Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: 3-Acryloxypropylhepta(3,3,3-trifluoropropyl) polyhedral oligomeric silses-quioxane (POSS) was synthesized and used as a modifier 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 (Tg’s) 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 specific 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 Part

B: Polym Phys 47: 504–516, 2009

Keywords: amphiphiles; biological applications of polymers; copolymerization;hydrogels; networks; polyhedral oligomeric silsesquioxane; poly(N-isopropylacrylamide);self-assembly; stimuli-sensitive polymers; swelling

INTRODUCTION

Poly(N-isopropylacrylamide) (PNIPAM) is aninteresting water-soluble polymer and it possessesa lower critical solution temperature (LCST)around �32 �C in aqueous solution.1–6 At the

Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, 504–516 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: S. Zheng (E-mail: [email protected])

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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) behavior7–10 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.11–18

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.16,19–22 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,23–27 and struc-tural inhomogeneity28,29 in hydrogels. However,care must be taken as the mechanical propertiesof porous hydrogels can be significantly 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 modification of hydrogels can be achievedthrough the formation of organic–inorganichybrids.22,30–34 Zhang et al. prepared an organic–inorganic 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

proposed a new strategy to improve hydrogelproperties by preparing an organic–inorganicnanocomposite PNIPAM hydrogel in which exfoli-ated clay layers acted as effective multifunctionalcross-linking agents.

Polyhedral oligomeric silsesquioxane (POSS)reagents, monomers, and polymers are emergingas new chemical feedstock for preparing organic–inorganic nanocomposites; POSS technology isbecoming a main avenue to organic–inorganicnanocomposites because of its simplicity in pro-cessing and the excellent comprehensive proper-ties of this class of hybrid materials.35–38 A typicalPOSS molecule possesses the structure of cube-octameric frameworks represented by the formula(R8Si8O12) with an inorganic silica-like core

(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.35–41 However, there are fewreports on POSS-modified 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 fillerin 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 modifica-tion via in situ cross-linking of PNIPAM using anoctafunctional POSS macromer. It is identifiedthat the temperature response of the POSS-cross-linked PNIPAM hydrogels was improved whencompared with the plain hydrogel.43

In the present work, we report a modificationof PNIPAM hydrogels with a POSS macromer.The POSS macromer used in this work is a 3-acryloxypropylhepta(3,3,3-trifluoropropyl) polyhe-dral oligomeric silsesquioxane, a new POSS mac-romer. Both hepta(3,3,3-trifluoropropyl) 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, wefirstly report the synthesis and characterizationof 3-acryloxypropylhepta(3,3,3-trifluoropropyl)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.

EXPERIMENTAL

Materials

3,3,3-Trifluoropropyltrimethoxysilane was pur-chased from Zhejiang Chem-Technology, China. 3-Chloropropyltrichlorosilane was obtained fromShanghai Lingguang Chemical, China and wasused as received. Acryl chloride was purchased

POSS-MODIFIED PNIPAM HYDROGELS 505

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

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: 60–90 �C), andtriethylamine (TEA) were of chemically puregrade, also obtained from commercial resources.Before use, THF was refluxed above sodium andthen distilled and stored in the presence of themolecular sieve of 4 A. Triethylamine (TEA) wasrefluxed over CaH2 and then was purified with p-toluenesulfonyl chloride, followed by distillatuion.

Synthesis of 3-Chloropropylhepta-(3,3,3-trifluoropropyl) POSS

The preparation of hepta(3,3,3-trifluoropropyl)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-trifluoropropyl) 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 flaskequipped with a condenser and a magnetic stirrer.After refluxed 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-trifluoropropyl) polyhedral oligomeric silsesquiox-ane was prepared via the corner capping reactionbetween Na3O12Si7(C3H4F3)7 and 3-chloropropyltrichlorosilane. Typically, Na3O12Si7(CH2CH2

CF3)7 (10.0 g, 8.8 mmol) and triethylamine(1.3 mL, 8.8 mmol) were charged to a flaskequipped with a magnetic stirrer, 200 mL anhy-drous THF were added with vigorous stirring.The flask 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-

ature for 20 h. The sodium chloride was filteredout 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%.

FTIR (cm�1, KBr window): 1090–1000 (SiAOASi), 2900–2850 (ACH2), 1120–1300 (ACF3).

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). 29Si NMR(ppm, acetone-d6): �70.1, �69.2, �66.7 and �65.9.

Synthesis of 3-Hydroxypropylhepta-(3,3,3-trifluoropropyl) POSS

3-Hydroxypropylhepta(3,3,3-trifluoropropyl) poly-hedral oligomeric silsesquioxane [denoted 3-hydroxypropylhepta(3,3,3-trifluoropropyl) POSS]was prepared via the hydrolysis of 3-chloropropyl-hepta(3,3,3-trifluoropropyl) polyhedral oligomericsilsesquioxane [denoted 3-chloropropylhepta-(3,3,3-trifluoropropyl) POSS] in the presence offresh silver oxide (Ag2O). Typically, 3-chloropro-pylhepta(3,3,3-trifluoropropyl) POSS (10.0 g,9.38 mmol) was charged to a 250 mL flask 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 refluxed for 24 h and all the in-soluble solids were filtered out. The above proce-dure was repeated for three time to access thecomplete conversion of 3-chloropropylhepta(3,3,3-trifluoropropyl) POSS into 3-hydroxypropyl-hepta(3,3,3-trifluoropropyl) 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%.

FTIR (cm�1, KBr window): 3336 (AOH), 1090–1000 (SiAOASi), 2900–2850 (ACH2), 1120–1300(ACF3).

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). 29Si NMR (ppm, acetone-d6): �69.4,�68.4, �67.0 and �66.1.

Synthesis of 3-Trimethylsilyetherpropylhepta-(3,3,3-trifluoropropyl) POSS

To confirm the formation of 3-hydroxypropyl-hepta(3,3,3-trifluoropropyl) POSS, the derivative

506 ZENG, FANG, AND ZHENG


of the aforementioned product was prepared andsubjected to 1H NMR analysis. The 3-hydroxypro-pylhepta(3,3,3-trifluoropropyl) POSS (1.0 g,0.87 mmol), anhydrous THF (10 mL) and triethyl-amine (0.44 g, 4.35 mmol) were charged into aflask. Then, the flask was immersed into an ice-water bath and chlorotrimethylsilane (0.47 g,4.35 mmol) was dropwise added into the flaskwith 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%.

1H NMR (ppm, acetone-d6): 3.62 (t, 2.0H,ACH2AOSi(CH3)3), 2.32 (m, 14.0H, SiCH2CH2

CF3), 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.

Synthesis of 3-Acryloxypropylhepta-(3,3,3-trifluoropropyl) POSS

3-Hydroxypropylhepta(3,3,3-trifluoropropyl) POSS(5.00 g 4.3 mmol) and TEA (1.2 mL, 8.6 mmol)were charged to a flask equipped with a magneticstirrer and 20 mL anhydrous THF was added.The flask 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 0�C for 1 h and then at room temperature for 24 h.The insoluble solids (salts) were filtered 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%.

FTIR (cm�1, KBr window): 1090–1000(SiAOASi), 2900–2850 (ACH2), 1120–1300 (ACF3),1734 (C¼¼O), 1628 (C¼¼C). 1H NMR (ppm, CDCl3):6.35, 5.78 (d, 2.0H, CH2¼¼CHA), 6.13 (t, 1.0H,CH2¼¼CHA), 4.16 (t, 2.0H, AOACH2ACH2A), 2.12(m, 14.0H, SiCH2CH2CF3), 1.83 (t, 2.0H,AOACH2ACH2A), 0.91 (m, 14.0H, SiCH2CH2

CF3), 0.82 (t, 2.0H, ACH2ACH2ACH2AOA).

Preparation of PNIPAM-co-POSS Hybrids

Typically, the POSS-containing PNIPAM hybridcontaining 10 wt % POSS was prepared as fol-lows. 3-Acryloxypropylhepta(3,3,3-trifluoropropyl)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-trifluoro-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, and 12.5 wt %.

Nuclear Magnetic Resonance Spectroscopy

The 1H NMR measurements were performed on aVarian Mercury Plus 400 MHz NMR spectrome-ter. The samples were dissolved with deuteratedacetone (acetone-d6) or chloroform (CDCl3) andthe solutions were measured with tetramethylsi-lane (TMS) as the internal reference.

Fourier Transform Infrared Spectroscopy

The fourier transform infrared spectroscopy(FTIR) measurements were conducted on a Per-kin–Elmer Paragon 1000 Fourier transform spec-trometer at room temperature (25 �C). The sam-ple films were prepared by dissolving the poly-mers with THF (5 wt %) and the solutions werecast onto KBr windows. The residual solvent wasremoved in a vacuum oven at 60 �C for 2 h. Thecrosslinked samples were mixed with the powderof KBr and then pressed into the small flakes. Allthe specimens were sufficiently thin to be withina range where the Beer-Lambert law is obeyed. Inall cases, 64 scans at a resolution of 2 cm�1 wereused to record the spectra.

Photon Correlation Spectroscopy

The linear poly[NIPAM-co-3-acryloxypropylhepta-(3,3,3-trifluoropropyl) POSS] copolymer (contain-ing 10 wt % POSS) (0.01 g) was dissolved in 0.5mL THF and then 50 mL ultra-pure water wasadded dropwise through a dropping funnel withvigorous stirring. After additional stirring for 30min, a transparent emulsion was formed with the



concentration of the copolymer being 0.02 wt %.The photon correlation spectroscopy (PCS) mea-surement was performed on a Zetasizer 3000HSA(Malvem Instrument, Malvem, Worcestershire,UK) with a He-Ne laser operating at the wave-length of 633 nm. The scattering angle for sizeanalysis was fixed at 90 degrees. All measure-ments were performed at 20 �C and the solventrefractive index was set to 1.330 and the solventviscosity to 1.00 for analysis.

Atomic Force Microscopy

The atomic force microscopy (AFM) experimentswere performed with a Nanoscope IIIa scanningprobe microscope (Digital Instruments, SantaBarbara, CA). Contacting mode was employed inair using a tip fabricated from silicon (125 lm inlength with approximately 500 kHz resonant fre-quency). Typical scan speeds during recordingwere 0.3–1 lines � s�1 using scan heads with amaximum range of 15 lm � 15 lm.

Differential Scanning Calorimetry

The differential scanning calorimetry (DSC) mea-surement was performed on a Perkin–ElmerPyris-1 thermal analysis apparatus in a dry nitro-gen atmosphere. The instrument was calibratedwith standard indium. All samples (about 10 mgin weight) were heated from 20 to 250 �C and thethermograms were recorded using the heatingrate of 20 �C/min. The glass transition tempera-tures were taken as the midpoint of the capacitychange.

Thermogravimetric Analysis

A Perkin–Elmer TGA-7 thermal gravimetric ana-lyzer was used to investigate the thermal stabilityof the nanocomposites. All the thermal analysiswas conducted in nitrogen atmosphere from ambi-ent temperature to 800 �C at the heating rate of20 �C/min. The thermal degradation temperaturewas taken as the onset temperature at which 5 wt% of weight loss occurs.

Swelling, Deswelling, and Reswelling Kinetics ofHydrogels

The specimens for swelling, deswelling andreswelling experiments were cut from the driedcylindered samples with the identical diameterand the heights of specimens were controlled tobe about 5 mm. Swelling ratios of the hydrogels

were gravimetrically measured after wiping offthe water on the surface with moistened filter pa-per in the temperature range from 25 to 48 �C. Allof the gel samples were immersed in distilledwater for 24 h at every particular temperature.Swelling ratio (SR) is defined as follows:

SR ¼ Ws=Wd (1)

where Ws is the weight of the water in the swol-len gel at a particular temperature and Wd is theweight of the PNIPAM (or POSS-containing) net-works.

The deswelling kinetics of hydrogels was gravi-metrically measured at 48 �C after wiping offwater on the surface with moistened filter paper.Before the measurement, the gel samples wereequilibrated in distilled water at room tempera-ture (25 �C) for 24 h. The weight changes of gelswere recorded with regular time intervals. Waterretention (WR) is defined as follows:

WR ¼ ðWt �WdÞ=Ws� 100% (2)

where Wt is the weight of the gel at regular timeintervals and the other symbols are the same asdefined above.

The kinetics of reswelling of hydrogels wasgravimetrically measured at 25 �C. Because theutilization of gel devices in practical applicationsis impossibly in a dried state before reabsorbingwater, the reswelling kinetics of gels was meas-ured at the equilibrium in distilled water at 48 �Cfor 24 h. After wiping off water on the surface, theweight changes of gels were recorded at regulartime intervals. Water uptake (WU) is defined asfollows:

WU ¼ ðWt �WdÞ=Ws � 100% (3)

where Wt is the weight of the gels at regular timeintervals and the other symbols are the same asdefined above.

RESULTS AND DISCUSSION

Synthesis of 3-Acryloxypropylhepta-(3,3,3-trifluoropropyl) POSS

The synthetic route of 3-acryloxypropylhepta-(3,3,3-trifluoropropyl) POSS is shown in Scheme1. In the first step, hepta(3,3,3-trifluoropropyl)tri-cycloheptasiloxane trisodium silanolate [Na3O12-

Si7(CH2CH2CF3)7] was prepared by following themethod reported by Fukuda and coworkers.45 Af-ter that, the so-called ‘‘corner-capping’’ reaction



between Na3O12Si7(CH2CH2CF3)7 and 3-chloro-propyltrichlorosiloxane was performed to afford 3-chloropropylhepta(3,3,3-trifluoropropyl) POSS.The 1H, 13C, and 29Si NMR spectroscopy indicatesthat 3-chloropropylhepta(3,3,3-trifluoropropyl)POSS was successfully obtained. The 3-chloropro-pylhepta(3,3,3-trifluoropropyl) POSS was thenconverted into 3-hydroxypropylhepta(3,3,3-tri-fluoropropyl) POSS via the substitution reactionof chlorine atom with hydroxyl group in the pres-ence of water. To promote the reaction to comple-tion, freshly-made Ag2O was added to the sys-tem.46,47 The reaction between 3-hydroxypropyl-hepta(3,3,3-trifluoropropyl) POSS and acrylchloride was employed to obtain 3-acryloxypropyl-hepta(3,3,3-trifluoropropyl) POSS. To demon-strate the completion of the substitution reaction,we prepared the derivative of 3-hydroxypropyl-hepta(3,3,3-trifluropropyl) POSS via its reactionwith trimethylchlorosilane [ClSi(CH3)3]. Shown inFigure 1 are the 1H NMR spectra of these POSSmacromers together with the assignment of thespectra. It should be pointed out that for 3-trime-thylsilyetherpropylhepta(3,3,3-trifluoropropyl)POSS tetramethylsilane (TMS) was not used asthe internal reference to obtain the 1H NMRspectrum. In terms of the ratios of integration in-tensity of 3,3,3-trifuluorpropy protons to that ofthe proton of the ligands (i.e., 3-chloropropyl, 3-hydroxypropyl, 3-trimethylsilyetherpropyl and 3-acryloxypropyl groups), it is judged that these

POSS macromers possess the cage-like structureof octameric silsesquioxane. The FTIR and NMRspectroscopy indicates that the 3-acryloxypropyl-hepta(3,3,3-trifluoropropyl) POSS macromerswas successfully obtained.

Scheme 1. Synthesis of 3-acryloxypropylhepta(3,3,3-trifluoropropyl) polyhedraloligomeric silsesquioxane.

Figure 1. 1H NMR spectra of POSS macromers.



Preparation of POSS-Containing PNIPAMCross-Linked Copolymers

The POSS-containing PNIPAM cross-linkedcopolymers were prepared via radical polymeriza-tion among NIPAM, 3-acryloxypropylhepta(3,3,3-trifluoropropyl) POSS and N,N0-methylenebisa-crylamide. The contents of POSS were controlledto be 5, 7.5, 10, and 12.5 wt %, respectively.Within the polymerization proceeding, all the re-active mixtures were gelled in success, indicatingthat the cross-linked networks were formed. Allthe cross-linked networks were extracted withdeionized water and tetrahydrofuran for 72 h, toremove any unreacted NIPAM and 3-acryloxypro-pylhepta(3,3,3-trifluoropropyl) POSS, respec-tively. The contents of POSS in the resultingcross-linked networks were determined in viewsof the ceramic yield in the thermal gravimetricanalysis (TGA) experiments. The TGA curves ofplain PNIPAM and its copolymers with 3-acryloxy-propylhepta(3,3,3-trifluoropropyl) POSS wereshown in Figure 2. All the samples displayed twosteps of thermal degradation at 110 and 400 �C.The former is ascribed to the loss of a trace ofwater in the water-soluble polymer during theheating scans. It is seen that the temperatures ofthermal degradation for the POSS-containingPNIPAM copolymers are slightly lower than thatof the plain PNIPAM network possibly due to thepresence of the long aliphatic linkages betweenPNIPAM main chains and POSS cages. However,

it is noted that the rates of degradation for thePOSS-containing copolymers are lower than thatof the plain PNIPAM network. As expected, nochar yield was detected for the plain PNIPAMnetwork at 800 �C owing to the complete decom-position. It is proposed that the POSS componentwill be transformed into ceramics (viz. SiO2) dueto thermal decomposition and oxidation. Thusthe ceramic yields can be exploited to estimatethe content of POSS in the copolymers. In thiswork, the ceramic (SiO2) yields were measured tobe 1.36, 2.4, 2.9, and 3.77 wt % for the copoly-mers containing 5, 7.5, 10, and 12.5 wt % POSS,respectively. The values of ceramic yield arequite close to those of calculated values, accord-ing to the feed ratios of POSS. Therefore, it isjudged that the POSS macromer completely tookpart in the formation of the copolymers. ThePOSS-containing PNIPAM copolymers were sub-jected to differential scanning calorimetry (DSC).The DSC curves are presented in Figure 3. Theplain PNIPAM network displayed a glass transi-tion at 144 �C. Compared with the plain PNIPAMnetwork, the POSS-containing copolymers dis-played the enhanced Tg’s, which increased withincreasing the content of POSS. The increasedTg’s could be attributed to the nanoreinforcementof POSS cages on polymer matrices. The disper-sion of POSS cages at the nanometer level couldsignificantly restrict the motions of macromolec-ular chains as shown in other POSS-reinforcedpolymers.48

Figure 2. TGA curves of the plain PNIPAM and thePOSS-containing PNIPAM copolymers.

Figure 3. DSC curves of the plain PNIPAM and thePOSS-containing PNIPAM copolymers.



All the POSS-containing PNIPAM cross-linkedcopolymers can be swollen in water. At the lowtemperature (e.g., \30 �C) the POSS-containinghydrogels were homogeneous and translucent.When the temperature was increased (e.g.,[40 �C),the hydrogels became shrank and cloudy, imply-ing the occurrence of volume-phase transition.when re-cooled to the low temperatures, thehydrogels became translucent again, that is, thisprocess is reversible. This observation indicatesthat all the POSS-containing hydrogels possesstemperature-sensitive behavior as the plain PNI-PAM hydrogel. It is of interest to investigate theswelling, deswelling, and reswelling kinetics ofthe POSS-containing hydrogels.

Swelling Kinetics of POSS-Containing PNIPAMHydrogels

Swelling Behavior

All the hydrogels were subjected to the measure-ments of swelling and deswelling kinetics. Theplots of stable swelling ratios as functions of tem-perature are shown in Figure 4. It is seen thateach hydrogel displayed a sigmoid curve of swel-ling ratio versus temperature, which is the char-acteristic of volume-phase transition behavior oftemperature-sensitive hydrogels. The plain PNI-PAM hydrogel possessed the VPTT of about 32 �Cand all the POSS-containing PNIPAM hydrogelssimilarly exhibited the volume phase transition at

this temperature. It is noted that the hybridhydrogels had lower water content than the plainPNIPAM hydrogel before the volume-phase tran-sition. With increasing the content of POSS thewater contents of the hybrid hydrogels decreased.The decrease in water contents is responsible forthe incorporation of POSS, the hydrophobic com-ponent, which gave rise to the decreased affinityof the PNIPAM networks with water. With theoccurrence of volume phase transition, the equi-librium swelling ratio was reduced from 8.21 to1.56 for the plain hydrogel, that is, 63.7 wt %absorbed water was released with the occurrenceof volume phase transition. All the POSS-contain-ing hydrogels exhibited much lower swellingratios than the plain PNIPAM hydrogels with thevolume phase transition. By comparing the swel-ling ratios of the hybrid hydrogels after and beforevolume phase transition, it is noted that with theoccurrence of volume phase transition, 92.8, 83.8,87.7, and 94.0 wt % absorbed water was releasedfor the hybrid hydrogels containing 5, 7.5, 10, and12.5 wt % of POSS, respectively. The swellingexperiments suggests that with the occurrence ofvolume phase transition, the release of theabsorbed water for the POSS-containing hydro-gels is much fast than the plain PNIPAM hydro-gel under the identical condition.

Deswelling Behavior

The deswelling properties were evaluated bymeasuring the swelling ratios of the hydrogels at48 �C for variable internals of time. The deswel-ling curves of the hydrogels were shown in Figure5. All the hydrogels shrank and lost the absorbedwater upon increase in temperatures above theVPTT. However, it is worth noticing that upondeswelling the POSS-containing PNIPAM hydro-gels displayed much lower stable water retentionwithin the same time than the plain PNIPAMhydrogel. This observation suggests that thedeswelling of the POSS-containing hydrogelsattained to its stable water retention much fasterthan the plain PNIPAM hydrogels. For example,it takes about 10 min to attain the stablewater retention of about 27% for the plainPNIPAM hydrogel. For the POSS-containinghydrogels, all the stable water retentions arelower than 16.4 wt % within the same time. Forthe hybrid hydrogel containing 7.5 wt % POSS,the stable water retention is even as low as10.7 wt %. This result suggests that within theidentical time, the POSS-containing hydrogels are

Figure 4. Swelling curves of the plain and POSS-containing PNIPAM hydrogels.



capable of releasing much more absorbed waterthan the plain hydrogels. To compare the reswel-ling rates of the different hydrogels, we havedefined a half-reswelling time (t1/2), at which 50%water uptake occurred.25

Reswelling Behavior

Figure 6 shows the plots of water uptake as func-tions of time for the plain hydrogel and the hybridPNIPAM hydrogels. In the experiments firstly allthe hydrogels were deswelled to reach the equili-briums at 48 �C in distilled water, the reswellingratio of the gels was measured gravimetrically at25 �C, which is quite lower than the VPTTs of thehydrogels. It is noted that the POSS-containinghydrogels gave the higher water uptake than theplain hydrogels after the 5-minite induction pe-riod elapsed. For the plain PNIPAM hydrogel, ittakes 120 min to attain the water uptake of 37.4wt %. With the same time, the swelling ratios ofall the hybrid hydrogels containing POSS are sig-nificantly higher than this value. The wateruptake of the hybrid hydrogel containing 5-wt %POSS is more than 69 wt %. This observationindicates that the reswelling of the POSS-contain-ing hybrid hydrogels is also much faster than theplain PNIPAM hydrogels. By fitting the data inthe Figure 6, the values of t1/2 for the hydrogelscontaining different amount of POSS wereobtained. The plot of t1/2 as a function of POSS

content for these hydrogels was also incorporatedin Figure 6. It is seen that the t1/2 decreased withincreasing the POSS contents in the hybridhydrogels, suggesting that the reswelling rates ofthe hybrid hydrogels are significantly increased.

Interpretation of Hydrogel Behavior

The POSS macromer used in this work is 3-acryl-oxypropylhepta(3,3,3-trifluoropropyl) POSS. Itconsists of seven 3,3,3-trifluoropropyl groups andone reactive 3-acryloxypropyl group. The 3,3,3-tri-fluoropropyl groups and cage-like silsesquioxanecore endows the POSS macromer with the highhydrophobilicity and the polymerizable 3-acrylox-ypropyl group facilitates the introduction of thePOSS blocks into PNIPAM with the formation ofchemical bonds. In this work, the highly hydro-phobic POSS blocks were introduced into cross-linked PNIPAM networks via the radical copoly-merization among NIPAM, 3-acryloxypropyl-hepta(3,3,3-trifluoropropyl) POSS, and BIS to pre-pare the POSS-containing PNIPAM hydrogels.

Figure 6. Reswelling curves of the plain and POSS-containing PNIPAM hydrogels. The top and left panelis the plot of t1/2 as function of POSS content for thereswelling experiment. The t1/2 is the half-reswellingtime (t1/2), at which 50% water uptake occurred. Thetimes for t1/2 were determined just after the tempera-ture was jumped from 25 to 48 �C.

Figure 5. Deswelling curves of the plain and POSS-containing PNIPAM hydrogels.



Morphology of POSS-ContainingPNIPAM Hydrogels

It is identified that the POSS-containing PNIPAMhydrogels significantly exhibited the improvedresponse rates when compared with the plainPNIPAM hydrogel in terms of the evaluations ofswelling, deswelling and deswelling kinetics. Theimproved properties of hydrogles could beascribed to the formation of the specific morpho-logical structures in the hydrogls. Because of the

immiscibility between organic chains (viz. PNI-PAM) and the pendant POSS blocks, all thePOSS-containing PNIPAM copolymers will bemicrophase-separated, that is, the POSS microdo-mains were dispersed into the continuous PNI-PAM matrices. In this work, the morphology ofthe POSS-containing PNIPAM copolymers wasinvestigated by means of atomic force microscopy(AFM). Towards this end, the PNIPAM copoly-mers were prepared via the radical polymeriza-tion between NIPAM and 3-acryloxypropyl-hepta(3,3,3-trifluoropropyl) POSS. The film speci-mens were prepared via spin coating of the THFsolution of the copolymers onto glass slides andsubjected to morphological observation by meanof AFM in contact mode. Representatively shownin Figure 7 is the micrograph of AFM for the co-polymer containing 10 wt % POSS. In terms ofthe volume fraction of POSS and the difference inviscoelastic properties between PNIPAM andPOSS phases, the light continuous regions couldbe ascribed to the PNIPAM matrix whereas thespherical dark regions are assignable to POSSdomains. It is seen that the POSS microphase atthe size of 100–1000 nm (in diameter) were dis-persed in the continuous PNIPAM. At the inter-face between air and the surface of the specimenthe observed spherical POSS domains (with deepcolor) are larger than the POSS domains in bulk(with shallow color). It is plausible to propose that

Figure 7. AFM micrograph of poly(NIPAM-co-3-acryloxypropylhepta(3,3,3-trifluoropropyl) POSS) con-taining 10 wt % POSS.

Scheme 2. Formation of microphase-separated structures in the POSS-containinghydrogels.



the spherical POSS domains have the tendencyto coarsen to the larger size owing to the charac-teristics of low surface energy of the fluorine andsilicon-containing blocks.46,49–56 Because of themore confinement of PNIPAM matrix, the POSSmicrodomains embedded in bulks of materials(with shallow color) would possess the smallersize than those at the surface of the specimens.The similar result has been found in epoxy ther-mosets containing hepta(3,3,3-trifluoropropyl)POSS-capped.46 It is proposed that the micro-phase-separated morphology will be reserved inthe hydrogels due to the hydrophobicity of POSSblocks, which can be confirmed via photon correla-tion spectroscopy (PCS). The linear poly[NIPAM-co-3-acryloxypropylhepta(3,3,3-trifluoropropyl)POSS] copolymers were used as the modelcopolymers of the POSS-containing hydrogels toinvestigate the self-assembly behavior of materi-als in water. Because of the high hydrophobicityof POSS block and hydrophilicity of PNIPAMchains, the polymers would be self-assembledinto the structures of micelles. The size of the mi-celle particles is readily measured by means ofphoton correlation spectroscopy (PCS). Represen-tatively shown in Figure 8 is the plot of size ofthe micelle particles in the aqueous PNIPAM co-polymer containing 10 wt % POSS. It is noted

that in the aqueous solution of the copolymer, thesize of the micelle particles is about 353 nm andpossesses the quite narrow distribution of size.This result is an indicative of the presence ofPOSS domains. Based on the above results, themorphology of POSS-containing PNIPAM net-works is proposed as Scheme 2. The dried POSS-containing PNIPAM networks are microphase-separated due to the immiscibility of PNIPAMwith the POSS component, and the microphase-separated morphology can be reserved when thenetworks were subjected to the swelling experi-ments with water.

Effect of Hydrophobic POSS Microdomains onHydrogel Behavior

The presence of POSS microdomains in the hydro-gels could exert a profound impact on the swel-ling, deswelling, and reswelling behavior. Whenthe POSS-containing PNIPAM cross-linkedcopolymers were subjected to the swelling experi-ments, the hydrophilic component (viz. PNIPAMchains) will form the stable hydration intera-ctions with aqueous medium via hydrogen bond-ing interactions between the amide moiety(ACONHA) and water molecules at the tempera-ture below the LCST. In addition to the hydro-philic part, there are a great number of hydropho-bic POSS microdomains in the hydrogels. Thehydrophobic microdomains could behave as themicroporogens and thus the area of the contactinterfaces between PNIPAM chain and water mol-ecules are significantly increased. The increasedarea of contact interfaces between PNIPAM chainand water molecules will facilitate the diffusionof water molecules in the cross-linked net-works.22,23,56 It should be pointed that the aggre-gated POSS microdomains could additionally actas the physical cross-linking sites of the hydrogelsand result in the additional cross-linking density.The higher the POSS content in the hybridhydrogels the higher the additional cross-linkingdensity. The increased cross-linking densityowing to the physical cross-linking could giverise to the additional decrease in swelling ratio ofhydrogels (See Fig. 4). In addition, it is seen thatwith the inclusion of the hydrophobic block (i.e.,POSS), the volume-phase transition temperatureof the hydrogels shifted to the lower tempera-tures. This observation is attributed to thereduced affinity of the hydrogels with waterbecause of the inclusion of the hydrophobic com-ponent (i.e., POSS).

Figure 8. Plot of distribution of micelle particle sizeas a function of diameter of particle for poly(NIPAM-co-3-acryloxypropylhepta(3,3,3-trifluoropropyl) POSS)containing 10 wt % POSS.



CONCLUSIONS

A novel inorganic macromer, 3-acryloxypropyl-hepta(3,3,3-trifluoropropyl) polyhedral oligomericsilsesquioxane (POSS) was synthesized and char-acterized. The POSS macromer was used toimprove the rate of temperature response ofpoly(N-isopropylacrylamide) (PNIPAM) hydrogels.The radical copolymerization among N-isopropy-lacrylamide (NIPAM), N,N0-methylenebisacryla-mide (BIS) and the POSS macromer was per-formed to prepare the organic–inorganic net-works. Differential scanning calorimetry (DSC)and thermal gravimetric analysis (TGA) showedthat the POSS-containing PNIPAM copolymersdisplayed the enhanced glass transition tempera-tures (Tg’s) and improved thermal stability. ThePOSS-containing PNIPAM hydrogels exhibitedtemperature-responsive behavior as the organic-only hydrogels. It is identified that with the mod-erate contents of POSS, the POSS-containingPNIPAM hydrogels displayed much faster res-ponse rates in terms of swelling, deswelling, andre-swelling experiments than the plain PNIPAMhydrogel. The improved hydrogel properties havebeen interpreted on the basis of the formation ofthe specific microphase-separated morphology inthe hydrogels, that is, the highly hydrophobicPOSS microdomains in the hybrid hydrogelsmight act as the microporogens and promote thecontact of PNIPAM chains and water.

The financial supports from Natural Science Founda-tion of China (Nos. 20474038 and 50873059) andNational Basic Research Program of China (No.2009CB930400) are acknowledged. The authors thankShanghai Leading Academic Discipline Project (ProjectNumber: B202) for the partial support.

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