Thermomechanical and surface properties of novel poly(ether urethane)/polyhedral oligomeric silsesquioxane nanohybrid elastomers

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<ul><li><p>Thermomechanical and Surface Properties of NovelPoly(ether urethane)/Polyhedral OligomericSilsesquioxane Nanohybrid Elastomers</p><p>Juanjuan Tan,1,2 Zhiyuan Jia,1,2 Dekun Sheng,1,2 Xin Wen,1,2 Yuming Yang11 Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, China</p><p>2 Graduate School of the Chinese Academy of Sciences, Beijing 10080, China</p><p>A series of linear polyurethane (PU) elastomerscontaining various amounts (ca. 210 wt%) of a diol-functionalized polyhedral oligomeric silsesquioxane(diolPOSS) were synthesized from organic solventdispersions. Fourier transform infrared spectroscopywas used to characterize the chemical structure of thediolPOSS-reinforced PU. Transmission electron micros-copy (TEM) indicated that the diolPOSS aggregationwas formed in the polymer matrix at the scale of 2050nm. The introduction of diolPOSS into such system ledto high glass transition temperature, enhanced storagemodulus, and improved stability compared with thepristine PU according to the differential scanning calo-rimetry, dynamic mechanical analysis, and thermalgravimetric analysis, respectively. Moreover, contactangle measurements indicated a signicant enhance-ment in surface hydrophobicity as well as a reductionin surface free energy after introducing diolPOSSinto the synthesized PU. The improvement of surfaceproperties could be ascribed to the enrichment ofthe diolPOSS moiety on the surface of the hybrids.POLYM. ENG. SCI., 51:795803, 2011. 2011 Society ofPlastics Engineers</p><p>INTRODUCTION</p><p>In recent years, organicinorganic hybrid composites</p><p>have undergone a signicant evolution because of the</p><p>outstanding properties of these materials compared with</p><p>pristine polymers [1]. Polyhedral oligomeric silses-</p><p>quioxane (POSS)-based polymers are emerging as one</p><p>of the most promising high-performance materials for</p><p>fundamental studies as well as industrial applications.</p><p>POSS, rst reported in 1946 [2], is a cube-octameric</p><p>molecule with an inner inorganic SiOSi frameworkthat is externally covered with reactive or nonreactive</p><p>organic substituents, so it possesses organicinorganic</p><p>hybrid structure at molecular level. POSS moieties can</p><p>be incorporated into polymer systems through either</p><p>simple melt/solution blending [3, 4] or polymerization</p><p>via functional substituents at the vertexes of POSS</p><p>cages [5, 6]. The resulting POSSpolymer hybrid sys-</p><p>tems displayed various signicantly enhanced proper-</p><p>ties, including increased toughness, decreased amma-</p><p>bility, ultraviolet stability, and oxidation resistance [7,</p><p>8]. In most cases, the enhanced properties are achieved</p><p>by incorporation of only small amounts (\10 wt%) ofPOSS that are far below the levels needed for tradi-</p><p>tional llers.</p><p>Polyurethane (PU), a well-known versatile engineering</p><p>material, can be tailored to meet the highly diversied</p><p>demands of modern technologies, such as coatings, adhe-</p><p>sives, reaction injection molding, bers, foams, rubbers,</p><p>thermoplastics elastomers, and composites [9, 10]. With</p><p>the increasing demand in a wide range of application,</p><p>considerable academic and industrial efforts have been</p><p>devoted to preparing novel PU materials with improved</p><p>properties. During the past years, extensive work has been</p><p>reported in the literature on the modication of PUs via</p><p>POSS. The PU/POSS hybrids are normally obtained from</p><p>waterborne dispersions or nondispersions. In previous</p><p>studies, two typical kinds of POSS with different func-</p><p>tional groups have been synthesized into PU from homo-</p><p>geneous aqueous solution [1114]. The rst one is 2,3-</p><p>propanediol propoxy-heptaisobutyl-POSS (diol-POSS),</p><p>which signicantly reduced the surface free energy and</p><p>thus increased surface hydrophobicity of PUs [11, 12].</p><p>The second one is 3-(2-aminoethylamino) propyl-heptai-</p><p>sobutyl-POSS (diamino-POSS), which was incorporated</p><p>into aqueous PU dispersions [13, 14] to enhance the ther-</p><p>momechanical and surface properties of PU. As for the</p><p>Correspondence to: Yuming Yang; e-mail:</p><p>Contract grant sponsor: Chinese Academy of Sciences.</p><p>Contract grant sponsor: Commission of Science Technology and Industry</p><p>for National Defense.</p><p>DOI 10.1002/pen.21877</p><p>Published online in Wiley Online Library (</p><p>VVC 2011 Society of Plastics Engineers</p><p>POLYMER ENGINEERING AND SCIENCE-2011</p></li><li><p>PU/POSS hybrids obtained from nondispersions, Neu-</p><p>mann et al. [15] reported the synthesis of an octa(isocya-</p><p>nate)-functionalized POSS macromer, which can subse-</p><p>quently be used to prepare organicinorganic hybrid PUs.</p><p>Zheng and coworkers [16, 17] used octa-functional POSS</p><p>as cross-linking agents to react with PU prepolymer to</p><p>form octa-armed star-like PU prepolymer with silses-</p><p>quioxane core, and then the chain was extended into PU</p><p>networks. Fu et al. [18, 19] prepared PU elastomers con-</p><p>taining octasiloxanePOSS used for a chain extender</p><p>through one of its corner groups grafting onto the macro-</p><p>molecular backbone of PU chains. Janowski and Pieli-</p><p>chowski [20] investigated the thermo(oxidative) stability</p><p>of PU/POSS nanohybrid elastomers using thermal gravi-</p><p>metric analysis (TGA). Oaten and Choudhury [21] synthe-</p><p>sized transparent PU hybrid containing an open-cage</p><p>POSS for thin lm application.</p><p>In this work, a new type of diolPOSS was chosen</p><p>as precursor for PU synthesis. The whole synthetic pro-</p><p>cess was performed in two mixed organic solvents to</p><p>improve the dispersion of the monomers and facilitate</p><p>the polymerization reaction. The purpose of this article</p><p>is to prepare a novel poly(ether urethane)/POSS nano-</p><p>hybrid elastomer and investigate the effect of POSS on</p><p>the thermomechanical and surface properties of PU</p><p>elastomers in detail.</p><p>EXPERIMENT</p><p>Materials</p><p>4,40-Diphenylmethane diisocyanate (MDI), poly(pro-pylene glycol) (PPG, Mw 2000), and dibutyltin dilau-rate (DBTDL) catalyst were purchased from Aldrich.</p><p>MDI and DBTDL were used as received, and PPG was</p><p>dehydrated under vacuum at 1108C for 2 hr before use.Trans-CyclohexaneDiolIsobutyl POSS (C36H78O14Si8,diolPOSS), a white powder, was obtained from Hybrid</p><p>Plastics (USA). The molecular structure is given in</p><p>Scheme 1. 1,4-Butanediol (BDO), toluene, and n-hexanewere purchased from Shanghai Chemical Reagent Co.</p><p>(Shanghai, China). BDO was distilled under reduced pres-</p><p>sure. Toluene and n-hexane were dried over 4 A molecu-lar sieves and then distilled from sodium benzophenone</p><p>ketyl for further purication.</p><p>Preparation of Samples</p><p>In a dry 250-mL three-necked round-bottom ask</p><p>equipped with a mechanical stirrer, a nitrogen inlet, and a</p><p>condenser, MDI and diolPOSS were rst dissolved into</p><p>mixed solvents of toluene and n-hexane under ultrasonic</p><p>SCHEME 1. Synthesis of the PUdiolPOSS hybrids.</p><p>796 POLYMER ENGINEERING AND SCIENCE-2011 DOI 10.1002/pen</p></li><li><p>dispersion and then allowed to react with each other for 30</p><p>min with the existence of DBTDL at room temperature.</p><p>Later, calculated PPG (NCO Index 2:1, where [NCO]and [OH] are the concentration of NCO groups of MDI and</p><p>total OH groups of diolPOSS and PPG, respectively) was</p><p>charged into the ask while stirring, and the reaction ask</p><p>was immersed in an oil bath at 608C. The isocyanate(NCO) content was monitored during the reaction using the</p><p>standard dibutylamine back-titration method [22]. About 1</p><p>hr later, the NCO content reached the theoretical NCO</p><p>value (the content of left NCO was nearly 50% of that in</p><p>the original state). Then, the prepolymer was chain</p><p>extended with BDO, and the reaction continued for another</p><p>3 hr at 608C to complete polymerization. Finally, the mix-tures were precipitated with n-hexane after some additionaltoluene added and dried under vacuum at 608C for 1 week.The pristine PU was prepared in the same way without</p><p>addition of diolPOSS and n-hexane.The lms of the pristine PU and PU hybrids with dif-</p><p>ferent concentrations of diolPOSS were hot pressed at</p><p>1608C and 10 MPa with a hold time of 3 min, followedby quenching to room temperature between two thick</p><p>metal blocks.</p><p>Measurements</p><p>Fourier Transform Infrared Spectroscopy. Fouriertransform infrared spectroscopy (FTIR) measurements</p><p>were conducted on a FTIR-6100 spectrometer (JASCO,</p><p>Tokyo, Japan) equipped with an IMV-4000 multichannel</p><p>infrared microscope (JASCO, Tokyo, Japan) and a MCT</p><p>detector in the transmission mode at room temperature.</p><p>The sample lms were prepared by dissolving the poly-</p><p>mers with tetrahydrofuran (10 wt%) and then the solu-</p><p>tions casting onto KBr windows. The residual solvent was</p><p>removed in a vacuum oven at 608C for 2 hr. In all cases,the spectra was recorded 32 scans at a resolution of 2/cm</p><p>in the wavenumber range of 4000500 cm21.</p><p>Wide-Angle X-ray Diffraction. Wide-angle x-ray dif-fraction (WAXD) experiments were performed by the use of</p><p>Rigaku D/Max-II B X-ray diffractometer with a Cu anode</p><p>(CuKa1 1.5406 A). DiolPOSS powder and 1-mm thickPU hybrid lms were analyzed by WAXD at room condi-</p><p>tions. All of the measurements were operated at 40 kV and</p><p>200 mA from 4 to 408 at a 2y scan rate of 28/min.</p><p>Transmission Electron Microscopy. The morphologyof PU/POSS hybrids was observed using a transmission</p><p>electron microscope (TEM) (JEOL JEM-1011). The speci-</p><p>men for TEM observations was about 5060-nm in thick-</p><p>ness, which was prepared by ultramicrotoming under</p><p>cryogenic conditions using a leica ultramictome (Ultracut</p><p>R with FCS).</p><p>Differential Scanning Calorimetry. The calorimetricmeasurements were performed on a TA Instrument differ-</p><p>ential scanning calorimetry (DSC) Q20 with a Universal</p><p>Analysis 2000 under a continuous dry nitrogen purge (50</p><p>mL/min). Indium was used for temperature and enthalpy</p><p>calibration. All samples were sealed in aluminum pans</p><p>with mass in the range of 510 mg. All measurements</p><p>were conducted at a heating rate of 108C/min followingheatcoolheat procedure from 280 to 1208C. Glass tran-sition temperatures (Tg) were determined by the midpointof enthalpy change during the second heating.</p><p>Dynamic Mechanical Analysis. The dynamic mechani-cal tests were performed on DMA/SDTA861e (Mettler</p><p>Toledo, Switzerland) in the shear-sandwich mode. The</p><p>samples were cut on the 1-mm thick PU lms with a di-</p><p>ameter of 10 mm and measured under a nitrogen atmos-</p><p>phere from 2100 to 1308C at a heating rate of 38C/min,a frequency of 1 Hz, and a deformation 0.3%.</p><p>Thermal Gravimetric Analysis. A Mettler ToledoTGA STAR gravimetric analyzer was used to investigate</p><p>the thermal stability of all PU samples. The specimens</p><p>(about 10 mg) were heated under a nitrogen atmosphere</p><p>from ambient temperature up to 6008C at a heating rateof 208C/min in all cases. The temperature at which therate of mass loss (Tmax) is up to maximum was evaluatedfrom the differential thermogravimetry curves.</p><p>Contact Angle Measurements. The static contact anglemeasurements with ultrapure water and diiodomethane</p><p>were performed on a Kruss DSA100 (Germany) apparatus.</p><p>A droplet of water (or diiodomethane) was dropped onto</p><p>the surface of PU lms about 510 lm. The evolution ofthe droplet shape was recorded by a CCD video camera and</p><p>analyzed to determine the contact angle. About 10 inde-</p><p>pendent measurements were performed and the average</p><p>contact angle was reported. The specimens were dried in a</p><p>vacuum oven for 12 hr before measurement.</p><p>X-ray Photoelectron Spectroscopy. X-ray photoelec-tron spectroscopy (XPS) experiments of PU hybrid lms</p><p>were performed on a ThermoElectron ESCALAB 250</p><p>spectrometer at room temperature by using an Al and K</p><p>X-ray source (1486.6 eV). The spectra were recorded at</p><p>908 takeoff angle with 100 eV pass energy. The bindingenergies were calibrated by using the containment carbon</p><p>(C 1s 284.6 eV).</p><p>RESULTS AND DISCUSSION</p><p>Preparation of the InorganicOrganic Hybrids</p><p>The synthesis process of PUdiolPOSS hybrids is illus-</p><p>trated in Scheme 1. The contents of diolPOSS in the</p><p>hybrids were set at 2, 5, 8, and 10 wt%, and the corre-</p><p>sponding samples were marked as PU2, PU5, PU8, and</p><p>PU10, respectively. The compositions of the pristine PU</p><p>DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE-2011 797</p></li><li><p>and PUdiolPOSS hybrids are summarized in Table 1. To</p><p>compensate for the inclusion of diolPOSS monomers, the</p><p>content of PPG was reduced from 77 to 65 wt% of the</p><p>whole hybrids as the diolPOSS content varied from 0 to</p><p>10 wt%. All samples can dissolve completely into polar</p><p>solvents such as N,N-dimethylformamide and tetrahydro-furan, which proved that there was no covalent cross-link-</p><p>ing in their molecular structure.</p><p>The FTIR spectra of the diolPOSS, the pristine PU,</p><p>and PUdiolPOSS hybrids are shown in Fig. 1. For the</p><p>pristine PU, the stretching vibrations of the NH groupsoccurred at 3300 cm21, together with the carbonyl bands</p><p>at 1732 cm21 were indicative of the presence of urethane</p><p>moieties. For the diolPOSS, the broad band above 3000</p><p>cm21 was typical of the stretching vibrations of the</p><p>hydroxyl groups, and the band at 1106 cm21 was ascribed</p><p>to the stretching vibration of SiOSi groups in the sil-sesquioxane cages. For the PUdiolPOSS hybrids, this</p><p>band at 1106 cm21 unfortunately overlapped with that of</p><p>the aliphatic ether COC. It is worth noticing that thedisappearance of the bands at 22502275 cm21, which</p><p>are characteristic of isocyanate, indicated the completed</p><p>reaction between the hydroxyl and isocyanate groups.</p><p>WAXD</p><p>Figure 2 shows the WAXD patterns of the diolPOSS</p><p>monomer and all synthesized samples. The pure diolPOSS</p><p>monomer presented characteristic diffraction peaks at 2yof 8.18, 10.88, 12.08, and 18.98, corresponding to the crys-tallization of POSS, which are similar to the previous</p><p>reports [12, 13]. In particular, the most intense and sharp-</p><p>est diffraction peak at 2y of 8.18 is attributed to the 101reection of POSS rhombohedral unit cell [23]. In con-</p><p>trast, the pristine PU (PU0) showed only one broad amor-</p><p>phous halo centered at 2y of 19.58. With the incorporationof 2 wt% POSS, the pattern of PU2 resembled that of</p><p>PU0; with further increase of diolPOSS content, the dif-</p><p>fraction for PU0 still existed, and the peak shifted to a</p><p>slightly lower angle, indicating that the introduction of</p><p>diolPOSS expanded the intermolecular main chain spac-</p><p>ing. When the content of diolPOSS was[2 wt%, anotherbroad peak appeared at 2y of 9.18, which should beascribed to the aggregation of diolPOSS, indicating the</p><p>partial crystallization of diolPOSS in the hybrids [13, 24].</p><p>But the deviation from the diffraction of pure diolPOSS</p><p>indicated that the presence of diolPOSS in hybrids was</p><p>different from the 3D crystals of pure diolPOSS mono-</p><p>mer. It is apparent that the crystallization behavior and</p><p>crystal morphology of diolPOSS were inuenced by the</p><p>pendent state in the long polymer chains.</p><p>Morphology of the PUdiolPOSS Hybrids</p><p>For further conrming the diolPOSS distribution in the</p><p>hybrids, the morphology of the diolPOSS-containing PU</p><p>hybrids was observed by TEM. Figure 3 representatively</p><p>shows the TEM micrograph of the hybrids containing 5</p><p>wt% (PU5) and 10 wt% (PU10) of diolPOSS. According</p><p>to the difference in transmitted electronic density between</p><p>organic PU polymer and inorganic POSS component...</p></li></ul>