SHORT COMMUNICATION

The preparation and properties of hybridized hydrogelsbased on cubic thiol-functionalized silsesquioxane covalentlylinked with poly(N-isopropylacrylamide)

Zhiqiang Xu & Caihua Ni & Bolong Yao & Lei Tao &

Changping Zhu & Qingbang Han & Jiaquan Mi

Received: 25 July 2011 /Revised: 2 September 2011 /Accepted: 3 September 2011 /Published online: 27 September 2011# Springer-Verlag 2011

Abstract Firstly, nano-sized polyhedral oligomeric silses-quioxane with functional mercapto groups (POSS-SH) wasprepared through hydrolytic condensation of 3-mercaptopropyl trimethoxysilane. Then N-isopropylacryla-mide (NIPAm) was allowed to polymerize at the presence ofPOSS-SH and N,N-methylene-bisacrylamide to yield hybrid-ized hydrogels. The hybridized hydrogels demonstratedthermosensitive behavior across the volume phase transitiontemperatures. The swelling and deswelling rates were greatlyaccelerated through the incorporating of POSS-SH into thegels, and the thermal properties of the hybridized hydrogelswere reinforced compared with the neat PNIPAm hydrogel.These results were ascribed to nano-effect created by thehydrophobic POSS-SH nanoparticles. The hybridized hydro-gels have potential applications in drug controlled release.

Keywords Hybridized hydrogel . Silsesquioxane .

N-Isopropylacrylamide

Introduction

Poly(N-isopropylacrylamide) (PNIPAm) is one of the mostattractive thermosensitive polymers and has been studiedextensively in both theory and application fields[1–5].However, conventional organic hydrogels display somedisadvantages of properties [6, 7] such as low mechanicalstrength, morphological inhomogeneity, low-equilibriumswelling ratio, and slow swelling and deswelling rates[8, 9].To overcome these problems, some strategies includingintroducing porosities, optimizing synthetic conditions, andimproving structural inhomogeneities have been attempted inthe past years. For examples, X.Z. Zhang et al. synthesizedmacroporous temperature-sensitive PNIPAm hydrogels usingpoly(ethylene glycol) as the pore-forming agent and ethylenetriethoxy silane as the cross-linking agent under acidicconditions[10]. They also obtained a hydrogel with expandednetwork structure through copolymerization of NIPAm andacrylic acid in alkaline solutions [11]. X.D. Xu et al. preparedpoly(N-isopropylacrylamide) hydrogels with pendent micellarstructure, and the hydrogel demonstrated improvedtemperature-sensitive properties, i.e., enlarged water-containing capability at room temperature, as well asimproved deswelling rate upon heating [12]. K. Haraguchiet al. prepared nanocomposite hydrogels composed of poly(N-isopropylacrylamide) with clay and investigated the effects ofcross-linker contents on various physical properties [13].

Polyhedral oligomeric silsesquioxane (POSS) is a promisingnanomaterial for making hybrids [14–18]. Usually, POSSderivatives possess a general formula of (RSiO1.5)n and awell-defined cage-like structure and appended eight organic

Electronic supplementary material The online version of this article(doi:10.1007/s00396-011-2510-0) contains supplementary material,which is available to authorized users.

Z. Xu : C. Ni :B. Yao : L. TaoSchool of Chemical and Material Engineering,Jiangnan University,Wuxi 214122, China

C. Ni (*) :C. ZhuState Key Laboratory of Hydrology-Water Resourcesand Hydraulic Engineering, Hohai University,Nanjing 210098, Chinae-mail: nicaihua2000@163.com

C. Zhu :Q. HanCollege of Computer and Information Engineering,Hohai University,Changzhou 213000, China

J. MiSchool of Foreign Language, Jiangnan University,Wuxi 214122, China

Colloid Polym Sci (2011) 289:1777–1782DOI 10.1007/s00396-011-2510-0

groups to the vertexes of the cage. These organic groups canbe further functionalized to yield POSS hybrids. It has beenproved that POSS is cytocompatible and hence suitable fordrug release and tissue engineering, and POSS is known as“The next generation material for biomedical applications.”There are plenty of reports on POSS-based inorganic/organichybrids which have been used in gene delivery [17], drugcontrolled release [18, 19], and cardiovascular device appli-cations [20]. There have been some reports on PNIPAmhydrogels modified by POSS macromers [21–24].

In this study, we have developed a new way for preparinginorganic/polymer hybridized hydrogels. Firstly, a caged octa(3-mercaptopropyl) octa-silsesquioxane (POSS-SH) with nano-sized structure was prepared. Secondly, N-isopropylacrylamidewas allowed to polymerize at the presence of POSS-SH as achain transfer agent. Hybridized hydrogels based on POSS-SHand PNIPAm were obtained when a cross-linker was added tothe polymerization system. It was found that the swelling,deswelling, and reswelling rates of the hybridized hydrogelswere increased obviously, and their thermal properties wereimproved compared to neat PNIPAm hydrogel.

Experimental

Materials

3-Mercaptopropyl trimethoxysilane(MPT), 2,2-azobisiso-butylnitrile(AIBN), N,N-methylene- bisacrylamide (BIS),and N,N-dimethylformamide (DMF) were purchased fromShanghai Reagent Co., and N-isopropylacrylamide(NIPAm) was purchased from Aldrich and was recrystal-lized from n-hexane before use, and other chemicals wereused as received.

Preparation of POSS-SH

Stoichiometric amounts of MPT (0.2 mmol), deionizedwater (5.4 ml), ethanol (40 ml), and hydrochloric acid(37%, 0.4 ml) were placed together in a flask (100 ml)equipped with a magnetic stirrer and a nitrogen catheter.The hydrolysis and condensation reactions were carried outat 60 °C for 36 h. A raw product was obtained afterremoval of the solvent at a reduced pressure. The rawproduct was dissolved in DMF and precipitated in distilledwater, and the procedure was repeated three times. Thesample was dried in a vacuum at the reduced pressure. Aviscous liquid product was obtained with a yield of 62%.

Characterization of POSS-SH

The infrared characteristic absorption peaks of the POSS-SH were determined by Fourier transform infrared spec-

troscopy (FTLA 2000, Boman, Canada). The thermalstability was determined by thermogravimetric analysis(TGA/SDTA851e Mettler Toledo, Switzerland), nitrogenatmosphere (flow rate, 200 mL/min), heating rate of 10 °C/min in a range of 20–500 °C. Nuclear magnetic resonance (1H-NMR and 29Si-NMR) spectra were recorded by Digital NMRSpectrometer (AVANCE III 400 MHz, Bruker Corporation).

Preparation of hybridized hydrogels of PNIPAmand POSS-SH

The monomer NIPAm (4.52 g), POSS-SH (0.226 g, 5 wt.%with respect to NIPAm), and BIS (0.136 g, 3 wt.% withrespect to NIPAm) were dissolved in 30 ml of DMFsolution. AIBN (1 wt.% with respect to NIPAm) was addedto the solution. Pure nitrogen was bubbled, passing throughthe solution in a reactor for 30 min, the reactor was sealed,and the polymerization was performed at 70 °C for 24 h.The hydrogel was allowed to swell in deionized water, andthe unreacted monomer and linear polymer were removedthrough washing with excess deionized water. A whitehybrid xerogel was attained after freeze-drying. Fivesamples marked as POSS-0, POSS-5, POSS-10, POSS-15,and POSS-20 were prepared when the weight percentage ofthe POSS-SH with respect to NIPAm was 0, 5, 10, 15, and20, respectively.

Measurements of the swelling ratios

Swelling ratios of hydrogels were gravimetrically mea-sured. A xerogel sample was placed in a non-woven fabricbag and immersed in deionized water with an optionaltemperature controlled for 1 day. The bag was removedfrom the water and weighted after the water on the surfaceof the bag was rapidly wiped off with moistened tissue. Theswelling ratio was calculated via Eq. (1):

Sr ¼ Ww �Wdð Þ=Wd � 100% ð1Þ

Where Sr stands for equilibrium swelling ratio and Ww

and Wd are sample weights in wet and dry state,respectively. The sample in water was then brought to anincreased temperature for 8 h to attain a new equilibrium,and a new swelling ratio was measured.

Measurements of deswelling kinetics

The hydrogels were allowed to swell in distilled water atroom temperature (25 °C) for 24 h prior to the measure-ments. Then the swollen hydrogels were transferred to hotwater with a temperature of 45 °C, and the weights weregravimetrically measured after wiping off water on the gelsurface with moistened tissue. The weight changes of

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hydrogels were recorded at regular time intervals. Thewater retention (Wr) is defined as follows:

Wr ¼ Wt �Wdð ÞÞ=Ww �WdÞ � 100% ð2Þwhere Wt is the weight of the hydrogel at regular timeintervals, and the other symbols are the same as definedabove.

Measurements of reswelling kinetics

A hydrogel was immerged in deionized water at 45 °C for24 h. Then it was transferred to water of the roomtemperature (25 °C). The reswelling ratio, defined as informula (1), was recorded after wiping off water on thesurface. The kinetics of reswelling of hydrogels wasgravimetrically measured at different intervals. The reswel-ling (Rs) was defined as a percentage of the reswelling ratiorelated to the initial equilibrium swelling ratio of thehydrogel.

Rs %ð Þ ¼ Sr=Se � 100% ð3ÞWhere Sr and Se are reswelling ratio and the initial

equilibrium swelling ratio of the hydrogel.

Results and discussion

Preparation and characterization of POSS-SH

The structure of POSS-SH is characterized by 1HNMR,FTIR, 29Si-NMR, and TGA, respectively. The 1HNMRspectrum (Fig. 1) shows that there are four strong resonancesignals (0.8, 1.7, 2.6, 1.4 ppm); they are attributed tochemical shifts of the protons respectively in a, b, c, and d

which are indicated in POSS-SH of the figure. Thehydrogen number ratio is 2:2:2:1 which is estimated byintegration area. The ratio is in agreement with the structureof –CH2–CH2–CH2–SH. The FTIR characteristic peaks ofPOSS-SH indicate that there is a sharp and strong peak at1,107 cm−1 which belongs to Si–O–Si stretching. The peakof 2,930 cm−1 is ascribed to C–H stretching of CH2 inmercaptopropyl groups. The characteristic peak of S–H canbe seen at 2,560 cm−1, and S–C symmetric stretching isshowed at 1,259 cm−1.

In the spectrum of 29Si NMR, there is a sharp peak with themaximum at −68 ppm which is ascribed to the mainstructures of the silsesquioxanes. The thermal stability ofPOSS-SH was determined by TGA. POSS-SH starts todecompose at 300 °C, and the decomposition stops at 650 °C.The residual is 40.8% as shown in the TGA curve. It isspeculated that the final product of the decomposition is SiO2.

FTIR spectra of the hybrid copolymers based on PNIPAmand POSS-SH

NIPAm is easily polymerized through the initiation ofAIBN in DMF solution. The mercapto groups at the eightvertexes of the caged POSS-SH act as a chain transfer agentand is connected to PNIPAm chains during the polymeri-zation. The hybridized hydrogels can be produced when theabove copolymerization is carried out at the presence of thecross-linking agent (BIS). Figure 2a shows FTIR spectra ofthe hybrid copolymers from POSS-0 to POSS-20. Thecurve POSS-0 is a typical FTIR of the neat PNIPAm. Thepeaks at 3,100–3,500 cm−1 show the existence of N–H ofamide groups, and 3,052 cm−1 is C–H stretch vibration ofCH. The peak 1,720 cm−1 is ascribed to C=O adsorption ofamide group. Meanwhile, a signal at 1,170 cm−1 appears,

Fig. 1 1HNMR of the POSS-SH

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and the peak signal is getting strong from curve POSS-5 toPOSS-20; this is attributed to Si–O–Si stretch vibration dueto the introduction of POSS-SH to the hydrogels.

Differential scanning calorimetry of the hybrid hydrogels

The effect of POSS-SH on the volume phase transitiontemperature (VPT) has been investigated using differentialscanning calorimeter (DSC 822e,Mettler Toledo, Switzerland).When the weight percentage of POSS-SH is 0, 5, 10, 15, and20 wt.%, respectively, with respect to NIPAm, the volumephase transition temperature of the hybrid hydrogels graduallydecreases from 32.5 °C of POSS-0 to 26.9 °C of POSS-20(Fig. 2b). It is explained that POSS-SH is hydrophobic, and itwill cause decrease in VPT of a hydrogel when incorporatedinto the gel. The more the amount of POSS-SH in a hybrid,the lower the VPT of the hydrogel. This observation suggeststhat a hybrid with a desirable VPT can be obtained byadjusting the amount of POSS-SH incorporated into thehybridized gel.

Swelling ratios of the hydrogels at different temperatures

The thermosensitive behavior of the hydrogels is furtherconfirmed through the measurements of swelling ratios. Thehybrid hydrogels are translucent and display various swellingratios respectively at the lower temperature (e.g., <32.5 °C).However, the hybridized hydrogels lose water and shrink uponbeing heated to the higher temperatures (e.g., >37 °C). Theequilibrium swelling ratio dependence on temperature is shownin Fig. 3a. As seen in the figure, all the hydrogel samplesdemonstrate thermosensitive swelling ratios. The neat PNI-PAm hydrogel (POSS-0) has a typical VPT at 32.5 °C.However, as POSS-SH content increases in the hybrid, theVPT shifts toward the lower temperature direction. This trendcorresponds with the result of DSC. The equilibrium swelling

ratio decreases with the increase in POSS-SH content. Thisphenomenon is also attributed to the hydrophobicity of POSS-SH in the hybrid.

Deswelling kinetics

Swelling or deswelling kinetics at a certain temperature isimportant in practical applications. Figure 3b demonstratesthe deswelling kinetics of the hybrid hydrogels after beingtransferred from an equilibrated swollen state at 25 °C to ahigh-temperature water medium at 45 °C. It is seen that allthe hydrogels shrink and lose water quickly when they areimmersed into hot water at 45 °C, and the hybrid hydrogelshave a faster deswelling rate than that of neat PNIPAmhydrogel. For example, the water retention of the neatPNIPAm hydrogel (POSS-0) decreased from 100% to 41%within 4 min, and 29.7% within 10 min. However, thewater retention of the hybrid hydrogels decreased from100% to 24.1%, 17.0%, 11.0%, and 7.2% within 10 min forsamples of POSS-5, POSS-10, POSS-15, and POSS-20,respectively. The time for neat PNIPAm hydrogel to reachhalf-to-the-equilibrium swelling is 4.5 min; however, thetime for hydrogels of POSS-5, POSS-10, POSS-15, andPOSS-20 to reach half-to-the-equilibrium swelling is 2.6,2.1, 1.7, and 1.5 (minutes), respectively.

This observation is in agreement with some reportedresearches. In this study, POSS-SH possesses hydrophobicproperty and has nano-sized structure that incorporates intothe PNIPAm hydrogel. The nano-sized hydrophobic micro-domains can behave as the nanoporogen and thus signifi-cantly increase the specific surface between PNIPAm chainand water molecules. Additionally, the increased quantity ofpores facilitates the shrinking process and water moleculediffusion inside the hydrogel network. For the reasonsabove, the hybrid hydrogels have a faster deswelling ratethan that of the neat PNIPAm hydrogel.

Fig. 2 a FTIR spectra of the copolymers of PNIPAm and POSS-SH, b DSC diagrams of the hybridized hydrogels

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Reswelling rates of the hybrid hydrogels

The reswelling behaviors show that the more the POSS-SH content in the hybrid hydrogels, the faster the gelsreswell. Within 20 min, the samples of POSS-0, POSS-5,POSS-10, POSS-15, and POSS-20 uptake water 27.0%,

40.2%, 44.5%, 54.7%, and 70.2% of their initialequilibrium swelling ratios, respectively. The reswellingrate increases as the POSS-SH component increased inthe hybrid hydrogels. The trend can also be attributed tothe enlarged average pore size and nano-sized hydropho-bic microdomains of these hybrid hydrogels, which has

Fig. 4 SEM images of thehybridized hydrogels: a POSS-5, b POSS-10, c POSS-15, dPOSS-20

Fig. 3 a Equilibrium swelling ratios of the hybridized hydrogels at varied temperatures, b deswelling behavior of the hybridized hydrogels at 45 °C

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been interpreted above. It is also observed that the hybridizedhydrogels maintain fixed shape with little deformationafter three cycles of swelling and deswelling. It isconcluded that the mechanic strength of the hybridizedhydrogels has been reinforced through incorporation ofthe nano-sized POSS-SH.

Observation by SEM of the hybrid hydrogels

To investigate morphology structures inside hybrid hydro-gels, scanning electronic microscope (SEM, JSMT300,Japan) tests for the four samples were conducted. We haveobserved that the hybridized hydrogels display homoge-nous macroporous network structure (Fig. 4). The pores aredistributed very evenly throughout the whole network. Thisresult is ascribed to the advantage of the synthetic strategy.The quantity of the pores gradually increases as the POSS-SH content is increased. This is explained that on one handnano-sized POSS-SH molecules are hydrophobic, and theyproduce numerous hydrophobic nanodomains inside thehybrid hydrogels. They play the role of porogen andincrease the interface area between the nanoparticles andPNIPAm segments, consequently creating more amount ofpores. On the other hand, the POSS-SH possesses multi-mercapto groups which act as cross-link agents to increasethe cross-linking degree. Therefore, the more condensedpores can be observed.

Conclusion

Nano-sized POSS-SH has been prepared through hydrolyticcondensation. The POSS-SH can be used as chain transferagent in the polymerization of NIPAm. The hybridizedhydrogels based on POSS-SH and PNIPAm can besynthesized in the presence of a cross-link agent. Theswelling and deswelling rates and thermal properties of thehybridized hydrogels were improved due to the incorporatingof nano-sized POSS-SH particles into the gels.

Acknowledgments We are grateful for the support of the State KeyLaboratory of Hydrology-Water Resources and Hydraulic Engineering,

Hohai University (no. 2010490911), National Natural Science Founda-tion of China (no. 10974044), and Fundamental Research Funds for theCentral Universities of Hohai University (20011B11014).

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