A water-soluble organic-inorganic hybrid material based on polyhedral oligomeric silsesquioxane and polyvinyl alcohol

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  • ORIGINAL PAPER

    A water-soluble organic-inorganic hybrid material basedon polyhedral oligomeric silsesquioxane and polyvinyl alcohol

    Xiaojing Zhang & Chunmian Yan & Shaoming Fang &Chenggui Zhang & Tiangang Jia & Yi Zhang

    Received: 22 June 2009 /Accepted: 22 September 2009 /Published online: 3 October 2009# Springer Science + Business Media B.V. 2009

    Abstract A novel series of organic-inorganic hybridmaterials involving cage-like octa(3-chloroammonium-propyl)silsesquioxane and Polyvinyl alcohol (OCAPS/PVA) were prepared via solution blending method. Theobtained hybrid films were optically transparent and solublein water. OCAPS/PVA hybrids were characterized by FT-IR,wide-angle X-ray diffraction (WAXD) scanning electronicmicroscopy (SEM), energy dispersive X-ray spectroscopy(EDS), differential scanning calorimetry (DSC), thermog-ravimetry analysis (TGA) and tensile test. The resultsshowed that the hydrogen bond interactions were formedbetween OCAPS and PVA. OCAPS could be dispersed wellin PVA matrix till its content was 10 wt%, while theaggregation and crystallization of OCAPS were observedwhen the content was up to 15 wt%. The glass transitiontemperature (Tg) of OCAPS/PVAwas found to increase from53 C to 60 C, and the melting temperature (Tm) decreasedfrom 180 C to 171 C with increasing OCAPS content from0 wt% to 15 wt%. The thermal stability of PVA main chainwas improved by the addition of OCAPS and the thermalresidue ratio also increased. The tensile strength of OCAPS/PVA decreased from 28 MPa to 19 MPa, while theelongation at break of hybrid films increased from 121% to175%.

    Keywords Polyhedral oligomeric silsesquioxane .

    Polyvinyl alcohol . Hybrid materials . Thermal properties .

    Mechanical properties

    Introduction

    Organic-inorganic hybrid materials have been paid on muchattention in the last decades due to the synergetic effect oforganic and inorganic components in nanoscales. Thecombination of organic and inorganic components isexpected to provide remarkable and complementary prop-erties, which cannot be obtained with a single material. Themost widely used synthetic approach is via the sol-geltechnique for preparing inorganic oxides at ambienttemperature by hydrolysis and condensation of alkoxysi-lanes and other metal alkoxides [1, 2]. Organic phases (suchas polymers) could be incorporated without any separationinto the silica matrix by using covalent bond or physicalinteractions between two phases or by controlling gelationof polymers and formation of silica gel through variousmolecular designs [3, 4]. Although the organic-inorganicpolymer hybrids prepared by the sol-gel reaction ofalkoxysilanes are known to have good properties such ashigh thermal stability and high strength, the obtained hybridmaterials could not be soluble again in any organic solventsbecause of the covalently crosslinking structure in thesilica-polymer matrix. The insolubility has limited thewidely use of organic-inorganic hybrid materials preparedby sol-gel method [5, 6].

    Apparently, water-soluble hybrid materials will breakthis limitation and start a new future, but research in thisfield has still been limited up to now. Wunder et al.synthesized an oligo(ethyleneoxide)-functionalized silses-quioxane, in which the hydrophilic oligo(ethyleneoxide)was linked to a cubic silsesquioxane core via hydro-silylation reaction [7]. Frey et al. and He et al. preparedmonosubstituted and multiarm poly(ethylene oxide)-cubicsilsesquioxane that were also water-soluble, whose solutionproperties and aggregation behavior were investigated

    X. Zhang (*) : C. Yan : S. Fang :C. Zhang : T. Jia :Y. ZhangCollege of Materials and Chemical Engineering,Henan Provincial Key Laboratory of Surface and InterfaceScience, Zhengzhou University of Light Industry,Zhengzhou 450002, Peoples Republic of Chinae-mail: zhangxj@iccas.ac.cn

    J Polym Res (2010) 17:631638DOI 10.1007/s10965-009-9351-2

  • [8, 9]. Recently, Mori et al. reported the synthesis andcharacterization of water-soluble silsesquioxane-based nano-particles that were prepared by hydrolytic condensation ofdifferent triethoxysilane precursors [10, 11].

    Silsesquioxanes is a special kind of siloxane compoundwith the emperical formula (RSiO1.5)n where R ishydrogen, or any alkyl, alkylene, aryl, arylene, ororgano-functional derivatives of the above groups. Thestructures of silsesquioxanes have been reported asrandom structure, ladder structure, cage structures, andpartial cage structure. Among them, polyhedral oligomericsilsesquioxane(POSS) has been paid on more attentionsrecently as a new class of nanofillers for preparation oforganic-inorganic hybrid materials. It generally consists ofthree-dimensional, rigid inorganic cores (Si-O cages) andflexible organic coronae, and can be considered to be thesmallest silica particles (13 nm) [1214]. POSS may beincorporated into a polymer matrix in two primary ways:chemically tethered to the polymer backbone or physicallyblended with the polymer matrix as untethered fillerparticles. The properties of hybrid polymers such asthermal stability usually have been enhanced when POSSwas covalently tethered to a polymer backbone [1518].Compared to the extensive reports about covalently POSS-polymer hybrid system, research about POSS/polymerhybrid materials prepared by physical blending has beenlimited [1921]. The physical blending method is simpleto use and fit to prepare materials on the large scale, whilewith the shortcoming of poor miscibility between POSS andpolymer matrix if no special interface interactions areinvolved in the blended materials. An enhanced compatibilityis expected when the substitute groups of POSS are similar orhave relative strong interactions with the polymer.

    Cage-like octa(3-chloroammoniumpropyl)silsesquioxane(OCAPS) with functional amino groups has good solubilityin water (Fig. 1), which is usually utilized as the buildingblocks of molecular assembly, drug control released

    materials, DNA delivery and so on [2225]. Till now, thereare a few reports about Polyvinyl alcohol (PVA)/silicahybrid materials by sol-gel methods [26, 27], yet water-soluble hybrid materials based on OCAPS and PVA as wellas their properties are seldom reported. In this report, aseries of hybrid materials involving OCAPS and PVA wereprepared via solution blending method, in which thehydrogen bonds were formed between amino group ofOCAPS and hydroxyl or carbonyl group of PVA. Thecompatibility and interactions between OCAPS and PVA aswell as the effects of OCAPS on the thermal andmechanical properties of PVA were investigated by FT-IR,wide-angle X-ray diffraction (WAXD), scanning electronicmicroscopy (SEM), energy dispersive X-ray spectroscopy(EDS), differential scanning calorimetry (DSC), thermog-ravimetry analysis (TGA) and tensile test.

    Experimental

    Materials

    3-Aminopropyltriethoxysilane was analytical pure gradeand purchased from Nanjing Shuguang Chemical Co. Ltd;PVA was purchased from KuRaRay Co. Ltd. (DP=1,700,degree of hydrolysis = 88%); All the other reagents wereanalytical pure grade and used as received. The synthesis ofOCAPS was according to literature 22 and 28. Thecharacterization data are as follows: FT-IR (KBr, cm1):3,224, 3,025, 1,604, 1,494, 1,122, 935 cm1; 1H-NMR(400 MHz, D2O): 4.68 (s, NH3, 24 H), 2.94(t, CH2NH3,16 H), 1.70 (m, SiCH2CH2, 16 H), 0.71 (t, SiCH2, 16 H);29Si-NMR (71.5 MHz, D2O): 66.4 (s).

    Preparation of OCAPS/PVA hybrid films

    The OCAPS solution was added quickly into 15 wt% (inmass, the same below) water solution of PVA afterultrasonic oscillation for 20 min. Then the mixed solutionwas stirred for 1 hr and treated by ultrasonic oscillation for30 min to make OCAPS disperse well in PVA matrix.Finally, OCAPS/PVA hybrid films were prepared insolution casting method and the contents of OCAPS were3 wt%, 5 wt%, 10 wt% and 15 wt% respectively.

    Characterization

    The FT-IR spectra of the neat polymer and the hybrid filmswere recorded in the range 4,000600 cm1 at a resolutionof 4.0 cm1 with a Bruker Tensor 27 FT-IR spectrometer(Germany); Dispersion of OCAPS in the polymer matrixwas observed through microscopic investigations using aJEOL JSM 6490LV SEM with an acceleration voltage of

    SiO Si

    O

    O

    O

    O

    Si SiOO

    O

    Si SiO

    Si SiO

    O

    O

    NH3+Cl-

    NH3+Cl-

    NH3+Cl-

    NH3+Cl--Cl+H3N

    -Cl+H3N

    -Cl+H3N

    -Cl+H3N

    Fig. 1 Scheme of the structure of OCAPS

    632 X. Zhang et al.

  • 10 kV. The samples were sputter coated with gold in orderto avoid the artifacts associated with sample charging. TheX-ray silicon mapping of the hybrid films was recorded inan Oxford EDS system, attached to the microscope. WAXDspectra were recorded on a Bruker D8 ADVANCEapparatus with a Ni-filtered Cu-K radiation, 40 kV,100 mA electric current, and a scanning rate of 2/min.The scanning range of 2 was from 360. Calorimetricmeasurements were performed on a Bruker Q100 DSC innitrogen atmosphere at a heating rate of 10 C/min. Allsamples (about 5 mg in weight) were heated from 30 C to210 C. Themogravimetric analysis of the hybrid materialswas performed by using a NETZSCH-499C TGA instru-ment from ambient temperature to 700 C with a heatingrate of 10 C/min in nitrogen atmosphere. A sampleweight of 10 mg was taken for all the measurements. Theweight loss against temperature was recorded. Tensilemechanical tests were carried out using a CMT-6104

    Electronic Universal Testing Machine (MTI Systems Co.Ltd., Shenzhen, China) according to GB/T1040-92 stan-dard. The samples are double-bell and measured at a strainrate of 30 mm/min. All the reported results are an averageof at least six successful measurements for the tensiledeterminations.

    Results and discussion

    Preparation of OCAPS/PVA hybrid films

    OCAPS was prepared in a yield of 30% by controlledhydrolytic condensation of 3-aminopropyltriethoxysilane instrongly acidic methanol. Like the reported synthesis ofother POSS compounds, the yield of OCAPS was not highand the synthetic process was time-consuming. For pureOCAPS, there was only one single resonance peak at

    Samples PVA(wt%) OCAPS(wt%) Appearance Solubility

    PVA 100 0 Transparent Water

    3POSS97PVA 97 3 Transparent Water

    5POSS95PVA 95 5 Transparent Water

    10POSS90PVA 90 10 Transparent Water

    15POSS85PVA 85 15 Transparent Water

    Table 1 Preparationof OCAPS/PVA hybridmaterials

    POSS

    POSS

    POSS

    OCAPS PVA Hydrogen bond

    POSS

    POSS

    Fig. 2 Scheme of the structureof OCAPS/PVA hybrid materialsbased on hydrogen bond

    A water-soluble organic-inorganic hybrid material 633

  • 66.4 ppm in 29Si-NMR, which matched the result reportedby literature [28]. The obtained OCAPS has a goodsolubility in water or dimethyl sulfoxide. A homogenousOCAPS/PVA solution can be conveniently formed bymixing the water solutions of OCAPS and PVA. Thecomposition and appearance of OCAPS/PVA hybrid filmswere summarized in Table 1. The solutions were treated byultrasonic oscillation before and after blending so thatOCAPS could be dispersed well, and transparent anduniform hybrid films could be obtained finally. TheOCAPS/PVA hybrid films could be dissolved in wateragain by stirring for a few minutes without heating. Thecage-like OCAPS molecules act as physical micro cross-linking points because of the formation of hydrogen bonds

    (a)

    (b)

    Fig. 3 FT-IR spectra of OCAPS, PVA (a) and OCAPS/PVA hybridmaterials (b)

    Fig. 4 SEM photograph of 10POSS90PVA hybrid material

    Fig. 5 X-ray silicon mapping of 10POSS90PVA hybrid material

    Fig. 6 WAXD patterns of PVA, OCAPS and OCAPS/PVA hybridmaterials

    634 X. Zhang et al.

  • between eight amino groups in OCAPS and hydroxyl orcarbonyl groups of PVA, so a physical cross-linkingnetwork structure can be formed in OCAPS/PVA hybridsystem, schemed in Fig. 2.

    FT-IR spectra analysis

    FT-IR spectra of OCAPS, PVA and OCAPS/PVA hybridmaterials were shown in Fig. 3. A Strong and sharp peakappeared at 1,123 cm1 can be attributed to Si-O-Siasymmetric stretching vibration in OCAPS, which is thetypical absorption of the cage structure of OCAPS. Thestrong peaks at 3,425 cm1 and 3,023 cm1 and the mediumpeaks at 1,604 cm1 and 1,494 cm1 are relativelystretching vibration and deformation vibration of N+-H inOCAPS. A broad absorption due to -OH stretchingvibration appeared at 3,303 cm1 in PVA. Also a sharppeak appeared at 1,742 cm1 can be observed, which can beattributed to C=O stretching vibration of the residual estergroup in PVA. The N+-H stretching vibration peaks over-lapped -OH stretching vibration in OCAPS/PVA hybridmaterials, while it can be seen that the absorption from3,000 cm1~3,700 cm1 was broadened in 15POSS85PVA,indicating that hydrogen bond interactions may be formedbetween OCAPS N+-H group and PVA hydroxyl group. Itshould be noticed that C=O stretching vibration has shiftedfrom 1,743 cm1 in PVA to lower wavenumbers in OCAPS/PVA hybrid materials. The red shift of C=O absorptionpeak suggests that the hydrogen bond also forms betweenOCAPS N+-H group and PVA carbonyl group beside thatbetween OCAPS N+-H group and PVA hydroxyl group.Possible hydrogen bond may occur between PVA hydroxylgroup and OCAPS siloxane group with a red shift ofOCAPS siloxane absorption peak, while no obvious shifts ofsiloxane absorption peaks were observed in this study. Theseresults confirm that the hydrogen bonds have been formedbetween N+-H groups in OCAPS and hydroxyl or carbonylgroups in PVA. Furthermore, a new shoulder peak near C=Oabsorption peak appeared around 1,600 cm1~1,700 cm1,whose intensity increased with increasing the OCAPS content.This absorption showed a red shift from 1,654 cm1 (5 wt%)to 1,639 cm1 (15 wt%). Similar result was also reported in

    Table 2 Thermal properties and Xcs of PVA, OCAPS and OCAPS/PVA hybrids materials

    Samples Tg(C) Tm(C) Xc(DSC)a Xc(WAXD)

    b Weightloss(C) Remaint(%)

    5% 50% 80%

    PVA 53 180 0.226 0.328 282 344 433 3.9

    3POSS37PVA 55 175 0.191 0.275 281 344 435 5.2

    5POSS95PVA 57 174 0.179 0.259 275 366 447 6.5

    10POSS90PVA 59 173 0.142 0.210 235 366 460 9.8

    15POSS85PVA 60 171 0.105 0.129 234 341 479 14.4

    OCAPS 305 >700 51.0

    a The degree of crystallinity, Xc(DSC), was determined by DSC as the ratio between the heat of fusion, Hm, of the samples and thethermodynamic enthalpy of melting of a 100% crystalline PVA, Hm

    0 (Hm0 =150 J/g)

    b The degree of crystallinity, Xc(WAXD), was determined by WAXD using the software MDI Jade 5.0

    (a)

    (b)

    Fig. 7 DSC curves of PVA and OCAPS/PVA hybrid materials: a Tg;b Tm

    A water-soluble organic-inorganic hybrid material 635

  • Xus study on poly(acetoxystyrene-co-octavinyl-POSS) nano-composites. It was considered that the shift should be assignedto the dipole-dipole interaction between the POSS siloxaneand the PVA carbonyl [18].

    Microscopic observations

    As shown in Table 1, OCAPS/PVA hybrid materials havegood film-forming properties and the obtained films areoptically transparent, which reveals that OCAPS has beenwell dispersed in a scale smaller than the wavelength ofvisible light. Microscopic morphology of the cross sectionof 10POSS90PVA hybrid film was further observed bySEM (Fig. 4). It can be seen that the cross section ofhybrid films was smoo...

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