Nanocomposites through copolymerization of a polyhedral oligomeric silsesquioxane and methyl methacrylate

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<ul><li><p>Nanocomposites Through Copolymerizationof a Polyhedral Oligomeric Silsesquioxane andMethyl Methacrylate</p><p>NOA AMIR, ANASTASIA LEVINA, MICHAEL S. SILVERSTEIN</p><p>Department of Materials Engineering, Technion Israel Institute of Technology, Haifa 32000, Israel</p><p>Received 13 February 2007; accepted 13 April 2007DOI: 10.1002/pola.22168Published online in Wiley InterScience (www.interscience.wiley.com).</p><p>ABSTRACT: The mechanical properties and thermal stability of polymers can beenhanced through the formation of nanocomposites. Nanocomposites consisting ofhybrid copolymers of methacrylcyclohexyl polyhedral oligomeric silsesquioxane(POSS-1) and methyl methacrylate (MMA) with up to 92 wt % (51 mol %) POSS-1and with superior thermal properties were synthesized using solution polymerization.The POSS-1 contents of the copolymers were similar to or slightly higher than thosein the feeds, the polydispersity indices were relatively low, and the degree of polymer-ization decreased with increasing POSS-1 content. POSS-1 enhanced the thermal sta-bility, increasing the degradation temperature, reducing the mass loss, and prevent-ing PMMA-like degradation from propagating along the chain. The mass loss wasreduced in a high POSS-1 content copolymer since the polymerization of POSS-1with itself reduced sublimation. Exposure to 450 8C produced cyclohexyl-POSS-likeremnants in the POSS-1 monomer and in all the copolymers. The degradation ofthese remnants, for the copolymers and for the POSS-1 monomer, yielded 75% SiO2and an oxidized carbonaceous residue. VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A:Polym Chem 45: 42644275, 2007</p><p>Keywords: copolymerization; hybrid; nanocomposites; POSS; thermal properties</p><p>INTRODUCTION</p><p>Nanocomposites consisting of hybrid materialsthat combine organic and inorganic groups oftenexhibit synergistic properties (e.g., mechanical,thermal, optical, electrical).14 Silsesquioxanes(SSQ) are siliconoxygen frame-works with theempirical formula (RSiO1.5)n.</p><p>5 R can be hydrogenor an organic substituent, the simplest of whichis CH3. Oligomeric SSQ are obtained through thehydrolytic condensation of trifunctional silanessuch as RSiCl3 or RSi(OCH3)3. Polyhedral oligo-meric silsesquioxanes (POSS) are a relatively</p><p>new family of organicinorganic hybrids andhave been described in recent reviews.6,7 Themost common POSS unit is the cubic T8, com-posed of eight silicon atoms at each corner of acube and oxygen bridges between the siliconatoms.</p><p>POSS can be incorporated into polymer matri-ces through blending or through chemical reac-tion. POSS can be more easily mixed into a poly-mer matrix if the silicon atoms bear organicgroups that enhance compatibility with the poly-mer.8,9 POSS has been blended with poly(methylmethacrylate) (PMMA) to yield tougher materi-als.10 POSS becomes tethered to the polymerchain as a pendent group if one silicon atombears a group that either reacts with the poly-mer or copolymerizes with the monomer (theremaining seven silicon atoms bear nonreactive</p><p>Correspondence to: M. S. Silverstein (E-mail: michaels@tx.technion.ac.il)</p><p>Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 42644275 (2007)VVC 2007 Wiley Periodicals, Inc.</p><p>4264</p></li><li><p>groups).11 POSS can crosslink the polymer ifmore than one silicon atom bears groups that ei-ther react with the polymer or copolymerize withthe monomer. POSS bearing different reactivegroups have been copolymerized with organicmonomers using diverse polymerization proc-esses (radical, condensation, ring opening me-tathesis, and living).6,7,1216</p><p>Nanocomposites of PMMA containing silicananoparticles have exhibited enhanced thermalstability.17,18 Copolymers containing tetheredPOSS often exhibit enhanced thermal proper-ties.1923 The temperature at which degradationbegins increases on POSS incorporation. A silicalayer, formed on the surface during degradationin the presence of oxygen, serves as a barrier,preventing the degradation of the underlyingpolymer.21,24 The objectives of this research wereto synthesize nanocomposite copolymers of POSSand MMA and to describe the copolymers molec-ular structures and their behavior on exposure toelevated temperatures. Such a combination ofmolecular characterization and thermal analysishas proved useful in providing insight into vari-ous organicinorganic hybrid systems.2527</p><p>EXPERIMENTAL</p><p>Materials</p><p>POSS bearing seven nonreactive cyclohexylgroups and a reactive propylmethacrylate group((C6H11)7Si8O12C7H11O2, 1125.9 g/mol), hereafterreferred to as POSS-1, was supplied by HybridPlastics. The MMA was supplied by Merck Schu-chardt. The MMA was washed to remove the in-hibitor (once with an aqueous 5 wt % sodium hy-droxide (NaOH) solution and then three timeswith deionized water). Toluene, tetrahydrofuran(THF), benzoyl peroxide (BPO), NaOH, deuter-ated chloroform (CDCl3), polystyrene (PS) cali-bration standards for gel permeation chromatog-raphy (GPC), and potassium bromide (KBr) weresupplied by Aldrich and used as received.</p><p>Synthesis of POSS-1/MMA Copolymers</p><p>The POSS-1/MMA copolymers were prepared byfree radical solution polymerization, as illus-trated in Scheme 1. The polymerization solutionscontained a total of 10 g monomers. The propor-tions of POSS-1 and MMA in the monomer feedsare listed in Table 1. The mass of toluene used,</p><p>10 g, was equal to the mass of the monomer mix-ture. The moles of BPO used was 1% of the totalnumber of moles of POSS-1 and MMA. Themonomer mixture was added to toluene in around-bottomed three-necked ask under nitro-gen, and heated to 75 8C with constant stirring.The initiator was added after the solutionreached 75 8C and the solution was held at thattemperature for 5 h. The resulting copolymerwas precipitated in methanol, ltered, dried over-night under vacuum, and then ground using amortar and pestle.</p><p>Molecular Composition and Molecular Weight</p><p>Nuclear magnetic resonance (1H NMR) was usedto determine the amount of POSS-1 incorporatedinto the copolymers (Bruker 500-MHz NMR spec-trometer). The chemical structures were identi-ed and the comonomer compositions were calcu-lated. Measurements were made in solutions ofCDCl3 at room temperature. Chemical shifts (d)are reported in parts per million (ppm) withCDCl3 as an internal standard.</p><p>Fourier transform infrared (FTIR) measure-ments were made in transmission through KBrpellets containing 1% by weight of the sample(Brucker Equinox 55 spectrophotometer). Thepellets were made by mixing the sample withKBr, grinding until a homogenous powder res-ulted, and then pressing the powder in a mold.The spectra were taken from 500 to 4000 cm1 ata resolution of 2 cm1. The contributions of vari-ous groups to the FTIR spectra between 1000and 1300 cm1 were calculated using curve t-ting. The background was removed using theShirley method.28 The individual peaks weredescribed using a symmetric Gaussian/Lorent-zian product function whose variables are thepeak position, peak height, full width at halfmaximum, and Gaussian/Lorentzian fraction.28</p><p>The t of the sum of the peaks to the experimen-tal data was optimized and the area fraction ofeach peak, A, was calculated. X-ray photoelectronspectroscopy (XPS) was used to determine ele-mental compositions following pyrolysis (ThermoSigma Probe, VG Scientic).</p><p>The molecular weight, molecular weight distri-bution, and copolymer homogeneity were deter-mined using GPC (Waters Breeze GPC with aWaters Styragel HR 4E column). PS was used asa calibration standard, the mobile phase wasTHF, the ow rate was 1 mL/min, and the tem-perature was 30 8C.</p><p>NANOCOMPOSITE COPOLYMERS OF POSS AND MMA 4265</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>Glass-Transition Temperature and Exposure toElevated Temperatures</p><p>Differential scanning calorimetry (DSC) wasused to determine the glass-transition tempera-tures (Tg) and to characterize the behavior on ex-posure to elevated temperatures (Mettler DSC-821 calorimeter). The DSC samples were drypowders (810 mg) in an open pan, with an</p><p>empty aluminum pan as the reference, and meas-urements were made from 25 to 450 8C at 10 8C/min in owing nitrogen. Thermogravimetricanalysis (TGA) was used to characterize thebehavior on exposure to elevated temperatures(Thermogravimetric Analyzer, TA). The TGAsamples were dry powders (2025 mg) and meas-urements were made from 25 to 900 8C at 20 8C/min in owing nitrogen. The differential ther-</p><p>Table 1. Compositions, Molecular Weights, and Transition Temperatures</p><p>FeedPOSS-1(mol %)</p><p>CopolymerPOSS-1(mol %)</p><p>FeedPOSS-1(wt %)</p><p>CopolymerPOSS-1(wt %)</p><p>Mn(kg/mol) PDI N Tg (8C) Td (8C)</p><p>PMMA 0 0 0 0 316 1.21 3160 122 233C-11 8 11 50 58 93 1.87 437 113 280C-21 21 21 75 75 116 1.48 349 325C-51 44 51 90 92 189 1.31 310 370</p><p>Scheme 1. Copolymerization reaction.</p><p>4266 AMIR, LEVINA, AND SILVERSTEIN</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>mogravimetry (DTG) curves are derivatives ofthe TGA thermograms, calculated using the sup-plied software.</p><p>RESULTS AND DISCUSSION</p><p>Molecular Composition</p><p>The methoxy group in PMMA resonates at 3.58ppm and is marked M1 in Scheme 1.29,30 Thecyclohexyl hydrogens in POSS-1 resonate atthree different places because of different ringconformations (axial and equatorial) and differ-ent chemical environments. The single hydrogenon the carbon that is attached to silicon resonatesat 0.76 ppm and is marked P1 in Scheme 1.31 Thehydrogen marked P2 in Scheme 1 resonates at1.23 ppm. The 1H NMR spectra of the copolymersin Figure 1 were used to determine their compo-sitions. The relative areas of the P1 and P2 reso-nances (the areas normalized by the area of theM1 resonance, which is only found in the MMAunits) are listed in Table 2. The copolymer com-positions calculated from these results are listedin Table 1. The POSS-1 contents in the copoly-mers are similar to, or slightly higher than, thePOSS-1 contents in the feeds. The copolymerscontaining 11, 21, and 51 mol % POSS-1 will bereferred to as C-11, C-21, and C-51, respectively.</p><p>The FTIR spectra of PMMA and the copoly-mers, as-synthesized, and of POSS-1 are pre-sented in Figure 2. Selected bands from thePMMA and from POSS-1 are listed in Table 3.The heights of the bands associated with POSS-1(cyclohexyl CH2 (c-CH2) at 2929 cm</p><p>1, SiO at1110 cm1, and noncyclic CH2 (nc-CH2) and CH3at 1446 cm1), normalized by the height of theCOO band at around 1731 cm1, are presented asa function of the POSS-1 content in Figure 3(a).The normalized heights of these bands increaselinearly with increasing POSS-1 content, asexpected.</p><p>The overlapping bands between 1030 and1330 cm1 were described using curve tting.</p><p>Figure 1. 1H NMR spectra of the copolymers.</p><p>Table 2. 1H NMR Peak Positions andRelative Areas</p><p>1H Position (ppm) Relative Area</p><p>C-11 M1 3.56 1.00P1 0.76 0.31P2 1.23 1.43</p><p>C-21 M1 3.56 1.00P1 0.76 0.74P2 1.23 3.53</p><p>C-51 M1 3.56 1.00P1 0.76 2.35P2 1.23 10.8</p><p>NANOCOMPOSITE COPOLYMERS OF POSS AND MMA 4267</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>The labels F1F6 in Table 3 were assigned to thebands used for tting. POSS-1 exhibits a verysmall SiO network band at 1030 cm1 (F1), arelatively large SiO cage band at 1110 cm1(F2), and relatively small CO bands at 1135,1198, and 1272 cm1 (F3, F4, and F6, respec-tively). PMMA exhibits CO bands at 1152,1193, 1242, and 1271 cm1 (F3, F4, F5, and F6,respectively). The curve ts for PMMA, C-11, andPOSS-1 are shown in Figure 4. The area percen-tages of the bands, AFn, are summarized in Table 4.</p><p>The amount of SiO network structure (F1) isnegligible. The combined areas of F2 and F3increase with increasing POSS-1 content sinceF2 is found solely in POSS-1. The areas of F4,F5, and F6 decrease with increasing POSS-1 con-tent since they are not prominent in POSS-1.</p><p>Molecular Weight</p><p>The GPC results are summarized in Table 1. Thedegrees of polymerization, N, were calculated bydividing the number of average molecularweight, Mn, by the average monomer molecularweight calculated from the copolymer composi-tion as determined by NMR. All the chromato-grams were unimodal, indicating that only onepolymerization product was synthesized. Mndecreases from 316,000 g/mol for PMMA to93,000 g/mol for C-11. Mn increases with increas-ing POSS-1 content in the copolymers, but thisactually reects the relatively high molecularweight of the POSS-1, not the length of the poly-mer chain. The variation of N is instructive. Ndecreases from 3160 for PMMA to 437 for C-11and continues to decrease with increasing POSS-1 content. This decrease in N may be inuencedby the decreasing molar concentration of mono-mers with increasing POSS-1 content and theassociated increase in chain transfer to the sol-vent. The polydispersity indices (PDI) are in therange 1.211.87, typical for molecular weight dis-tributions from radical polymerization. PMMA</p><p>Figure 2. FTIR spectra of PMMA, the copolymers,the POSS-1 monomer at 25 8C, and of the POSS-1monomer following exposure to 450 8C.</p><p>Table 3. Selected FTIR Bands for PMMA and for the POSS-1 Monomer</p><p>Band (cm1) Group Comments</p><p>PMMA1731 CO Stretch1446 OCH3 Symmetric deformation1271, F6 CCO, CO Symmetric deformation1242, F5 CCO, CO Symmetric deformation1193, F4 COC Symmetric deformation1152, F3 COC Stretch</p><p>POSS-1 Monomer2929 CH2, Cyclohexane Asymmetric stretch1724 CO Stretch1446 OCH3 Symmetric deformation1446 CH2 Deformation1272, F6 CCO, CO Symmetric deformation1195, F4 COC Asymmetric stretch1135, F3 COC Stretch1110, F2 SiOSi cage Asymmetric stretch1030, F1 SiOSi network Asymmetric stretch</p><p>4268 AMIR, LEVINA, AND SILVERSTEIN</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>has the smallest PDI and C-11 has the largest.The PDI of the copolymers decreases withincreasing POSS-1 content and, therefore, withdecreasing N.</p><p>The number of MMA units in the chain,NMMA, and the number of POSS-1 units in thechain, NPOSS-1, were calculated from the Mn andfrom the copolymer composition. NMMA, NPOSS-1,and their sum, N, are plotted in Figure 5 as afunction of the POSS-1 content in the copolymer.NMMA decreases linearly, and NPOSS-1 increaseslinearly, with the amount of POSS-1 in the copol-ymer. The rate of decrease in NMMA is greaterthan the rate of increase in NPOSS-1. Therefore, Ndecreases with the amount of POSS-1 in the co-polymer.</p><p>Glass-Transition Temperature</p><p>DSC thermograms from 50 to 150 8C for the vari-ous polymers are shown in Figure 6(a). PMMAexhibits a glass-transition temperature (Tg) at</p><p>122 8C and C-11 exhibits a somewhat broader Tgat 113 8C (Table 1). Generally, the Tg has beenfound to increase with POSS content in otherPOSS-containing copolymers,21,22,32 althoughdecreases in Tg have also been observed.</p><p>15,16 NoTgs can be discerned for C-21 and C-51. The pres-ence of 21 mol % POSS-1 suppresses the segmen-tal motion associated with PMMA. A similar phe-nomenon has been observed in PMMA-containingsilica nanoparticles.17</p><p>Exposure to Elevated Temperatures</p><p>PMMA and POSS-1</p><p>DSC, TGA, and DTG thermograms for the poly-mers and for POSS-1 are shown in Figures 6(b),7, and 8...</p></li></ul>