Synthesis and thermal properties of hybrid copolymers of syndiotactic polystyrene and polyhedral oligomeric silsesquioxane

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<ul><li><p>Synthesis and Thermal Properties of Hybrid Copolymers ofSyndiotactic Polystyrene and Polyhedral OligomericSilsesquioxane</p><p>LEI ZHENG, RAJESWARI M. KASI, RICHARD J. FARRIS, E. BRYAN COUGHLIN</p><p>Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003</p><p>Received 25 October 2001; accepted 14 January 2002Published online 00 Month 2002</p><p>ABSTRACT: Copolymerizations of styrene and the polyhedral oligomeric silsesquioxane(POSS)styryl macromonomer 1-(4-vinylphenyl)-3,5,7,9,11,13,15-heptacyclopentylpen-tacyclo [,9.15,15.17,13] octasiloxane have been performed with CpTiCl3 in con-junction with methylaluminoxane. Random copolymers of syndiotactic polystyrene(sPS) and POSS have been formed and fully characterized with 1H and 13C NMR, gelpermeation chromatography, differential scanning calorimetry, and thermogravimetricanalysis. NMR data reveal a moderately high syndiotacticity of the polystyrene back-bone consistent with this use of CpTiCl3 as a catalyst and POSS loadings as high as 24wt % and 3.2 mol %. Thermogravimetric analysis of the sPSPOSS copolymers underboth nitrogen and air shows improved thermal stability with higher degradationtemperatures and char yields, demonstrating that the inclusion of the inorganic POSSnanoparticles makes the organic polymer matrix more thermally robust. The polymer-ization activity and thermal stability are also compared with those of reported atacticpolystyrenePOSS copolymers. 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem40: 885891, 2002; DOI 10.1002/pola.10175Keywords: polyhedral oligomeric silsesquioxane (POSS); syndiotactic polystyrene;POSS copolymer; polyolefin nanocomposites; organicinorganic hybrid materials</p><p>INTRODUCTION</p><p>Recently, a great deal of attention has been fo-cused on nanocomposite materials, which typi-cally refer to discontinuous particulate inclusionswith one major dimension on a scale of 1100 nm.These materials have shown novel and often im-proved mechanical, thermal, electronic, magnetic,and optical properties.1 Although a two-phasecomposite may be organicorganic, inorganicinorganic, or organicinorganic, the last is moredesirable from the standpoint of combined advan-tageous performance relative to either of the non-</p><p>hybrid counterparts. Among the many syntheticapproaches to obtaining nanocomposites, the useof structurally well-defined inorganic nanopar-ticles or clusters is becoming an increasingly im-portant strategy.2 Despite the effort required toestablish methods to form covalently or ionicallybonded nanoparticles from suitable precursors,this approach has the advantage that it can con-trol and tune the beneficial phenomena associatedwith nanostructures over a range of length scales.Furthermore, the problem associated with mi-crophase separation in organicinorganic compos-ites can be alleviated either through the nanopar-ticles being covalently bound to a polymer backboneor through modification of their surface properties.</p><p>In particular, the use of polyhedral oligomericsilsesquioxane (POSS) nanoparticles has been</p><p>Correspondence to: E. Bryan Coughlin (E-mail: of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 885891 (2002) 2002 Wiley Periodicals, Inc.</p><p>885</p></li><li><p>demonstrated to be an efficient method for de-signing hybrid nanocomposite materials.3 A typi-cal POSS nanoparticle contains an inorganicSi8O12 nanostructured skeleton surrounded byeight organic groups labeled R (e.g., isobutyl, cy-clopentyl, or cycylohexyl) on the corners to pro-mote solubility in conventional solvents and com-patibility with a host polymer matrix. These aregenerally synthesized through kinetically con-trolled hydrolytic condensation reactions of RSiCl3in aqueous acetone. Two predominant stable struc-tures can be obtained: a fully condensed R8Si8O12cube and an incompletely condensed R7Si7O9(OH)3trisilanol.46 POSS macromonomers (Scheme 1)can be prepared from R7Si7O9(OH)3 trisilanol andRSiCl3 (R is a functional group) in a corner-cap-ping reaction.7 The resulting inorganic core is,therefore, surrounded by seven hydrocarbyl groupsand a unique group capable of undergoing poly-merization. A variety of functional groups, suchas styrene,7 methacrylate,8 epoxide,9 and nor-bornene,10 have been prepared. This has affordedthe opportunity to incorporate inorganic POSScages into organic matrices through copolymer-izations. This class of organicinorganic nano-composites, with nanosize silica covalentlybonded to the polymer matrix, is difficult to syn-thesize otherwise. A variety of POSS-containingcopolymers have been prepared with radical tech-niques, both conventional7,8 and atom transferprotocols,11,12 condensation,13 ring-opening met-athesis polymerization,14 and metallocene cataly-sis techniques.15,16 Materials prepared includepoly(methyl methacrylate),8,11,12 polystyrene,7</p><p>epoxy,9 polysiloxane,17 polynorbornene,10 poly-ethylene, and polypropylene copolymers.15</p><p>Our current research interests have focused onpolyolefinPOSS copolymers. Although these hy-brid materials are desirable because of the vastscale of production and wide range of applicationsfor polyolefins, the ability to copolymerize polyole-fins with POSS macromonomers with coordina-</p><p>tion polymerization (ZieglerNatta or single-sitecatalysis) is at an early stage of development.Efficient synthetic methods to produce polyethyl-enePOSS and polypropylenePOSS copolymershave not been reported until recently by ourgroup.15 In this study, we extend POSS-contain-ing copolymers to syndiotactic polystyrene (sPS).Since the initial report from Ishihara and cowork-ers,18,19 sPS has become an attractive engineer-ing thermoplastic because of its high melting tem-perature (Tm 270 C), fast crystallization kinet-ics, high modulus, and good resistance to waterand organic solvents. This work addresses amethod to synthesize nanocomposites of sPS andPOSS and their resulting thermal properties. Thecomposite matrix of sPS is a semicrystalline ma-terial, in contrast to the conventional amorphousatactic polystyrene synthesized by radical poly-merization.7 Different microstructures and,therefore, novel properties are expected becauseof this crystalline nature.20</p><p>EXPERIMENTAL</p><p>Materials</p><p>The cyclopentylPOSSstyryl macromonomer1-(4-vinylphenyl)-3,5,7,9,11,13,15-heptacyclopen-tylpentacyclo [,9.15,15.17,13] octasiloxane(1) was provided by the Air Force Research Lab-oratory of the Propulsion Directorate at EdwardsAir Force Base in California. CpTiCl3 was ob-tained from Strem. Styrene was obtained fromAldrich and distilled from calcium hydride beforeuse. Methylaluminoxane (MAO) was obtainedfrom Albemarle as a 30 wt % solution in toluene.Toluene was passed sequentially through col-umns of activated alumina (LaRoche A-2) andQ-5-supported copper redox catalyst (EngelhardCU-0226S) under a prepurified nitrogen atmo-sphere.21</p><p>Polymerization Procedures for sPSPOSSCopolymers</p><p>The styrenePOSS copolymerizations were car-ried out in a 100-mL, round-bottom flaskequipped with a magnetic stirring bar, 3.12 g(0.03 mol) of styrene, 1.7 mL of a 4.77 M MAOsolution (8 mmol), 0.30 g (0.3 mmol; styrene/POSS 100/1) of POSSstyryl monomer 1, andtoluene. The sealed flask was removed from the</p><p>Scheme 1. POSS macromonomer structure.</p><p>886 ZHENG ET AL.</p></li><li><p>glove box and heated to 50 C in an oil bath. Atoluene solution of 10 mol of CpTiCl3 was in-jected. The total volume of toluene was 20 mL.The polymer was observed to precipitate from thesolution during the course of polymerization. Af-ter a certain reaction time (see Table 1), polymer-ization was terminated by the addition of metha-nol. The copolymers were fully precipitated in 100mL of a 10% HCl/methanol solution. The copoly-mer was recovered by filtration, washed with acopious amount of hexanes for the removal ofresidual 1, and dried overnight in vacuo at 60 C.The polymerization protocol was repeated withvarious amounts of 1 (0.3, 0.9, 1.5, or 3.0 g) for thepreparation of a range of copolymers. The copol-ymers were then extracted with refluxing 2-bu-tanone for 24 h in a Soxhlet extractor for theremoval of atactic copolymers and possible resid-ual POSS macromonomer.</p><p>1H NMR: 7.0 (br, phenyl-para and ortho posi-tions), 6.5 (br, phenyl-meta position), 1.8 (br, CHof the backbone), 1.3 (br, CH2 of the backbone),1.61.5 (m, CH2 of cyclopentyl), 1.00.9 (m, CH ofcyclopentyl). 13C NMR (sPSPOSS copolymers):143.2 (ipso-C of phenyl), 125.9 (phenyl-meta posi-tion), 125.6 (phenyl-ortho position), 123.6 (phe-nyl-para position), 41.5 (CH of sPS backbone),38.4 (CH2 of backbone), 25.2, 25.1 (CH2 of cyclo-pentyl), 20.1 (CH of cyclopentyl).</p><p>Polymer Characterizations1H and 13C NMR spectra were obtained on aBruker AMX 500 FT NMR spectrometer with tet-</p><p>rachloroethane-d2 (CDCl2CDCl2) as the solvent.The residual proton resonance from the solvent at5.95 ppm was used as the internal reference for1H NMR, and hexamethyldisiloxane was used asthe reference at 0 ppm for 13C NMR. Quantitativespectra were obtained with a standard inverse-gated proton decoupling pulse sequence and arelaxation delay of 6 s. Gel permeation chroma-tography was performed with a Polymer Labora-tories PL-220 high-temperature gel permeationchromatograph equipped with a Wyatt min-iDAWN high-temperature light scattering detec-tor and a refractive-index detector at 135 C with1,2,4-trichlorobenzene as a solvent. The min-iDAWN detector contained a 690-nm diode laser.A dn/dc value of 0.047 mL/g for polystyrene wasused. Differential scanning calorimetry was per-formed on a TA DSC 2910 instrument equippedwith a liquid nitrogen cooling accessory unit un-der a continuous nitrogen purge (50 mL/min).Data reported were gathered for the second meltwith a heating and cooling scan of 10 C/min.Thermogravimetric analysis (TGA) was carriedout with a TA instrument TGA 2050 thermogravi-metric analyzer at a heating rate of 20 C/minfrom room temperature to 700 C under a contin-uous nitrogen (100 mL/min) or air (50 mL/min)purge.</p><p>RESULTS AND DISCUSSION</p><p>Synthesis of sPSPOSS Copolymers</p><p>Copolymers of sPS and POSS were synthesizedwith cyclopentadienyl titanium(IV) trichloride in</p><p>Table 1. Copolymerization of Styrene and POSSStyryl with CpTiCl3a</p><p>Sample[Styrene]/[POSS]</p><p>(Molar Ratio)ReactionTime (h)</p><p>Yield(g)</p><p>POSSb</p><p>Mw(103 g/mol)c PDIc</p><p>MEKInsolubility</p><p>(%)dwt % mol %</p><p>sPS 100/0 0.5 2.23 0 0 33 1.76 90sPSPOSS-1 100/1 1 1.57 3.3 0.36 37 1.45 89sPSPOSS-2 100/3 1 1.15 9.9 1.1 35 1.51 80sPSPOSS-3 100/5 1 0.90 16 1.9 18 1.50 60sPSPOSS-4 100/10 3 0.47 24 3.2 32 1.58 40</p><p>a Experimental conditions: 10 mol of CpTiCl3, 8 mmol of MAO (Al/Ti 800), 0.03 mol of styrene, and 20 mL of toluene at 50 C.b As determined by 13C NMR.c Weight-average molecular weight determined with a gel permeation chromatograph coupled to light scattering detectors in</p><p>1,2,4-trichlorobenzene at 135 C.d As calculated from the polymer insoluble in 2-butanone (MEK) after 24 h of Soxhlet extraction.</p><p>SYNDIOTACTIC POLYSTYRENE POSS COPOLYMERS 887</p></li><li><p>conjunction with MAO (Scheme 2).18 The reactionconditions, yields, POSS concentrations, and mo-lecular weight data are shown in Table 1. With anincreasing POSS concentration, the yields of thecopolymers dropped considerably. This observa-tion is in contrast to the trend observed byHaddad and Lichtenhan7 for the synthesis ofatactic poly(4-methyl styrene)POSS copolymersby a free-radical mechanism; they reported nosignificant influence on yield. The observed de-crease in the yield of the copolymers with theincrease in POSS content can be attributed to thecoordination polymerization mechanism. Thepolymers are formed by the coordination of thestyrene unit to the cationic titanium metal center,followed by 2,1-insertion of subsequent styreneunits in the propagation step.22 Therefore, in thecopolymers, because of the presence of very bulkyPOSSstyryl units and possible polar interactionbetween oxygen of the POSS cage and the tita-nium catalyst center, the propagation rate ismuch slower than that for styrene. The molecularweights of these copolymers are similar to those ofthe control homopolymer except for sPSPOSS-3,which decreased from 33 to 18 kg/mol. No signif-icant change, or trend, was observed for the poly-dispersity indices (PDIs). All were within a rangeof 1.51.8. These polymerizations were all run tomoderate levels of conversion for the preservationof random copolymer structures, thereby avoidingthe formation of sPS homopolymer. In general,POSSstyryl is less readily incorporated than sty-rene into the copolymers; an inspection of Table 1reveals a relative reactivity ratio of approxi-mately 1/300.</p><p>Ketones are commonly used to extract atacticpolystyrene from a mixture of atactic polystyreneand sPS as a semiquantitative means to measuretacticity. In our case, the Soxhlet extraction of thecopolymers was carried out for a day with reflux-ing 2-butanone as the extraction solvent; this wasfollowed by recovery of the insoluble material.The percentage of insoluble copolymer in reflux-ing 2-butanone decreased as the POSS contentincreased. It was first assumed that the increasedsolubility with an increase in POSS content wasdue to the decrease in the tacticity of the copoly-mers. However, 13C NMR of the copolymersproved otherwise (Fig. 1). The carbon peak at143.2 ppm of the ipso-carbon was used as a refer-ence for the tacticity determination in sPS. Forour syndiotactic styrenePOSS copolymers, a sin-gle peak at 143.2 ppm was observed for all thecopolymers both before and after extraction. Thisdemonstrates that the POSS incorporation doesnot decrease the tacticity of the copolymers; in-stead, the loss of crystallinity accounts for theincreased solubility (discussed later). 13C NMRalso provided information of the extent of thePOSS incorporation in the copolymers. As ex-pected, with increasing POSS content in the feed,the extent of the POSS concentration in the co-polymers also increased proportionally. The high-est concentration obtained in the copolymers was3.2 mol % (24 wt %).</p><p>Thermal Characterization of sPSPOSS Copolymers</p><p>The thermal properties of the sPSPOSS copoly-mers are shown in Table 2. Thermal analysis of</p><p>Scheme 2. Copolymerization of styrene and POSSstyryl.</p><p>888 ZHENG ET AL.</p></li><li><p>the sPSPOSS copolymers revealed that theglass-transition temperatures (Tgs) of the copol-ymers had a minor increase from 98 to 102 Ccorresponding to 03.2 mol % POSS. This is not</p><p>surprising because Haddad and Lichtenhan7 re-ported a dramatic increase in Tg when POSS wasincreased above 7.8 mol % in atactic poly(4-methyl styrene)POSS copolymers.7 We also ob-</p><p>Figure 1. 13C NMR spectra of sPSPOSS-3 (top) before and (bottom) after Soxhletextraction.</p><p>Table 2. Summary of the Thermal Characterization of sPSPOSS Copolymersa</p><p>SampleTg</p><p>(C)bTm</p><p>(C)bH</p><p>(J/g)bTdec under</p><p>N2 (C)c</p><p>Char Yieldunder N2 (%)</p><p>Tdec underAir (C)c</p><p>Char Yieldunder Air (%)</p><p>sPS 98 259 28.7 387 0 307 0sPSPOSS-1 99 255 25.1 398 1.1 307 2.1sPSPOSS-2 101 245 20.1 394 2.6 300 5.8sPSPOSS-3 100 237 15.1 397 4.3 321 8.9sPSPOSS-4 102 d d 387 17.2 34...</p></li></ul>