Synthesis, morphology, and viscoelastic properties of cyanate ester/polyhedral oligomeric silsesquioxane nanocomposites

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<ul><li><p>Synthesis, Morphology, and Viscoelastic Properties ofCyanate Ester/Polyhedral Oligomeric SilsesquioxaneNanocomposites</p><p>KAIWEN LIANG,1 HOSSEIN TOGHIANI,1 GUIZHI LI,2 CHARLES U. PITTMAN JR.2</p><p>1Dave C. Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, Mississippi 39762</p><p>2Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762</p><p>Received 4 April 2005; accepted 5 April 2005DOI: 10.1002/pola.20861Published online in Wiley InterScience (</p><p>ABSTRACT: Cyanate ester (PT-15, Lonza Corp) composites containing the inorganicorganic hybrid polyhedral oligomeric silsesquioxane (POSS) octaaminophenyl(T8)-POSS [1; (C6H4NH2)8(SiO1.5)8] were synthesized. These PT-15/POSS-1 composites(99/1, 97/3, and 95/5 w/w) were characterized by X-ray diffraction (XRD), transmis-sion election microscopy (TEM), dynamic mechanical thermal analysis, solventextraction, and Fourier transform infrared. The glass-transition temperatures (Tgs)of the composite with 1 wt % 1 increased sharply versus the neat PT-15, but 3 and5 wt % 1 in these cyanate ester composites depressed Tg. All the PT-15/POSS compo-sites exhibited higher storage modulus (E0) values (temperature &gt; Tg) than theparent resin, but these values decreased from 1 to 5 wt % POSS. The loss factor peakintensities decreased and their widths broadened upon the incorporation of POSS.XRD, TEM, and IR data were all consistent with the molecular dispersion of 1 due tothe chemical bonding of the octaamino POSS-1 macromer into the continuous cyanateester network phase. The amino groups of 1 reacted with cyanate ester functions atlower temperatures than those at which cyanate ester curing by cyclotrimerizationoccurred. In contrast to 1, 3-cyanopropylheptacyclopentyl(T8)POSS [2; (C5H9)7(SiO1.5)8CH2CH2CH2CN] had low solubility in PT-15 and did not react with the resin below or atthe cure temperature. Thus, phase-separated aggregates of 2 were found in samples con-taining 110 wt % 2. Nevertheless, the Tg and E</p><p>0 values (temperature &gt; 285 8C) of thesecomposites increased regularly with an increase in 2. VVC 2005 Wiley Periodicals, Inc. J PolymSci Part A: Polym Chem 43: 38873898, 2005</p><p>Keywords: cyanate ester resins; FT-IR; nanocomposites; polyhedral oligomericsilsesquioxane (POSS); TEM</p><p>INTRODUCTION</p><p>Hybrid organic polymer/inorganic nanocompo-sites have generated intense recent interest.126</p><p>Inorganic nanophases include nanoclays,2732</p><p>carbon nanotubes,3336 vapor-grown carbonbers,3743 inorganic nanobers,44,45 and polyhe-dral oligomeric silsesquioxane (POSSs).1926,46</p><p>Silsesquioxanes are structures exhibiting theformula (RSiO1.5)n, where R is hydrogen or anyfunctionalized or unfunctionalized alkyl, alky-lene, aryl, or arylene group. Silsesquioxanes areknown to form ladder,4749 cage,47,5052 partial</p><p>Correspondence to: C. U. Pittman, Jr. (E-mail:</p><p>Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 38873898 (2005)VVC 2005 Wiley Periodicals, Inc.</p><p>3887</p></li><li><p>cage,53 and polymer structures.26,54 POSSs offera chance to prepare hybrid organicinorganicmaterials with molecularly dispersed inorganicstructural units in the nanocomposites. POSScompounds have cage structures with the empir-ical formulas (RSiO1.5)8,10,or12, which are calledT8, T10, and T12 cages, respectively. Each cagesilicon is bonded to three oxygens and to a sin-gle R substituent. Both organic (cyclohexyl, phe-nyl, etc.) and inorganic organic hybrid (e.g.,OSiMe2OPh) substituents exist. Partiallyclosed cage structures are also known. POSScompounds, with diameters of 13 nm, can beconsidered the smallest possible particles ofsilica, but unlike silica, silicones, or llers,POSS molecules contain either functionalized orunfunctionalized substituents at each of the cor-ner silicon atoms. These substituents can com-patibilize POSS molecules with polymers ormonomers.</p><p>Reactive POSS derivatives are available forpolymerization or grafting.1926,46,5558 Hence,POSS cages can be incorporated into commonplastics via copolymerization,57,5961 grafting,57,62</p><p>or blending.57,62,63 New hybrid inorganicorganicthermoset46,6470 and thermoplastic materi-als22,26,6063,71,72 can be prepared. These hybridscan exhibit dramatic improvements in polymerproperties such as higher use temperatures,73</p><p>oxidation resistance,62 surface hardening,62 andmechanical property modications.75,76 Further-more, reductions in ammability,77 heat evolu-tion,78 and processing viscosity79 have beenreported. Many thermoplastic and thermoset sys-tems, including methacrylates,80 styrenes,81 nor-bornenes,82 ethylenes,83 epoxies,84 and siloxanes,61</p><p>have been improved.In this study, octaaminophenyl(T8)POSS (1)</p><p>was incorporated into the PT-15 cyanate esterresin (Lonza Corp.) and cured thermally to formnanocomposites, in which 1 was distributed atthe molecular level. The morphology and visco-elastic properties of these composites were deter-mined by X-ray diffraction (XRD), transmissionelection microscopy (TEM), and dynamic mechani-cal thermal analysis (DMTA). Chemical incorpora-tion was studied by extraction and Fourier trans-form infrared (FTIR) analyses. Cyanate ester/3-cyanopropylheptacyclopentyl(T8)POSS (2) com-posites were also prepared to generate compo-sites in which aggregates and particles of 2 werepresent. The structures and properties of thesecomposites were also characterized.</p><p>EXPERIMENTAL</p><p>Materials</p><p>The phenolic-derived low-viscosity cyanate esterresin used in this work, PT-15, was supplied byLonza, Inc. PT-15 is a multifunctional, low-vis-cosity (35 cps at 80 8C) liquid cyanate esterresin. This cyanate ester blend is derived from abisphenol F mixture, which also contains somelarger oligomers, in which all the phenolichydroxyls have been converted to OCN func-tions by ClCN. PT-15 may be cured via a ther-mally driven cyclotrimerization to form triazinerings, each of which serves as a crosslinkingsite. This reaction can take place readily in theabsence of a catalyst at temperatures above165 8C, and this allows a large processing win-dow for blending the resin with other compo-nents at temperatures from 100 to 130 8C, atwhich the viscosity is very low.</p><p>The multifunctional monomer 1 [(C6H4NH2)8(SiO1.5)8], molecular weight 1153.63 g/mol]</p><p>3888 LIANG ET AL.</p></li><li><p>was prepared in our laboratory by H.-S. Choby the nitration of octaphenyl(T8)POSS tothe octanitro derivative, followed by reductionto POSS-1 in dry formic acid/triethyl amine.85</p><p>It is essential that all water be removed fromthe HCOOH/Et3N solution when Laines proce-dure is followed in this synthesis. The aminogroups positional distribution is approximately80% meta, 15% para, and 5% (maximum) ortho,and this is consistent with the electron-with-drawing nature of the Si8O12 cage substituent.The structure was conrmed by 1H, 13C, and28Si NMR spectroscopy.85 Monofunctional 2[(C5H9)7(SiO1.5)8CH2CH2CH2CN; molecular weight 968.66 g/mol] was purchased from HybridPlastics Co.</p><p>Preparation of the Composites</p><p>PT-15/1 composites (99/1, 97/3 and 95/5 w/w)were made by a solution blending process.POSS-1 was dissolved in tetrahydrofuran (THF),and this gave a transparent solution (0.5 g/mL).Then, the liquid cyanate ester components werealso dissolved in this THF solution. The result-ing solution was put in a vacuum oven (300350 mmHg) at 50 8C for 16 h to remove THF.The resulting cyanate ester/POSS-1 blends weretransparent in each case. These liquid blendswere cured in an oven. The cure protocol was (1)heating to 188 8C and holding for 120 min, (2)ramping the temperature to 250 8C at 5 8C/min,and (3) holding the samples at 250 8C for180 min. Periodic examination showed that thesamples remained transparent at all stages dur-ing the cure, and they remained transparentafter being cured and postcured.</p><p>PT-15/POSS-2 composites were prepared by adirect blending process followed by a thermalcure cycle. PT-15 (9.9, 9.7, 9.5, or 9.0 g) washeated to 120 8C (g, viscosity 8 cps) and heldthere for 10 min while being stirred magneti-cally. Then, 2 (0.1, 0.3, 0.5, or 1.0 g, respectively)was added as a ne powder to the liquid resin.These mixtures (total weight 10 g) were mag-netically stirred at 120 8C for 45 min, duringwhich time no viscosity increase occurred. Ineach case, 2 appeared not to completely dissolveinto the liquid cyanate ester resin. Some 2 wassuspended in the liquid resin. These suspensionswere translucent. Solution methods were alsotried. Dissolving 2 and PT-15 in THF, followedby evaporation of THF, resulted in similar sus-pensions of 2 in the resin. These mixtures were</p><p>placed into a mold without degassing, and eachsample was cured in an oven. The cure tempera-ture/time protocol was identical to that used forthe PT-15/1 systems: The solubility of 2 athigher temperatures in PT-15 is unknown, butthe systems remained translucent throughoutthe cure. Each sample was nally postcured at300 8C for 30 min. The PT-15/POSS-2 99/1 and97/3 composites were translucent, whereas the95/5 and 90/10 composites were opaque.</p><p>Measurements</p><p>XRD measurements were performed to examinethe potential POSS alteration of the solid-statepolymer microstructure in the POSS-1 compo-sites. XRD can probe ordered POSS aggregatesformed by phase separation. The samples wereexamined with a Philips XPERT X-ray diffrac-tometer. Philips analytical software and Cu Karadiation (40 kV, 45 mA) were employed. Scanswere taken over the 2h range of 1308 with astep size of 0.038 at 1 s per step.</p><p>TEM was used to characterize phase separa-tion in these cyanate ester resin/POSS compo-sites. A JEM-100 CXII transmission electronmicroscope (JEOL USA, Inc.) was used to char-acterize the phase morphology in the POSS-1nanocomposites. The samples were ultramicro-tomed to an approximately 7090-nm thicknessand mounted on carbon-coated Cu TEM grids.</p><p>The dynamic storage modulus (E0) and loss fac-tor (tan d) were determined in the bending modeversus the temperature with a Rheometrics Sci-entic model MK III DMTA instrument. A dual-level bending mode was employed. Small-ampli-tude bending oscillations (both 1 and 10 Hz) wereused at a gap setting of 8.00 mm. All measure-ments were carried out from 35 to 350 8C. Thetest samples were approximately 3.04.0 mmthick, 4.55.5 wide, and 38 mm long.</p><p>The composite densities were measured withan electronic densimeter (ED-120T) at 25 8C.</p><p>Specimens (1.0 g) of every composite wereimmersed into a large excess of THF at roomtemperature for 3 months to determine if anyPOSS could be extracted by THF. The extractionof 1 or 2 would have indicated that theextracted POSS had not chemically bonded tothe resin matrix. Concentrated residue/THF sol-utions were coated onto KBr plates, and THFwas removed. IR spectra were obtained on anFTIR instrument (MIDAC Corp.). Residues wereweighed after the removal of THF.</p><p>CYANATE ESTER/POSS NANOCOMPOSITES 3889</p></li><li><p>RESULTS AND DISCUSSION</p><p>The thermal curing of the PT-15 resin generatessolid, crosslinked resins via cyclotrimerization ofthe OCN functions. This process forms trisubsti-tuted triazine rings. The triazine rings serve asthermally stable crosslinking hubs throughoutthe resin. Amine groups can add across thecyanate ester function upon mild heating or inthe presence of base catalysts.86 This additionproduct is primarily of importance as a catalyticintermediate in triazine ring formation.87,88</p><p>Thus, the amino groups on 1 react easily byadding across the CN triple bond of the cyanateester groups of PT-1587 (Scheme 1). This causesmacromer 1 to react with the cyanate estermonomer at temperatures far below the temper-atures at which cyanate ester resins cure.Therefore, 1 dissolves into, and reacts with, theliquid PT-15. At 188250 8C, PT-15 cures, incor-porating 1 molecularly throughout the resin.The initial amino group addition to OCN func-tions generates RNHC(NH)OR groups. How-ever, the fate of these functions during the sub-</p><p>sequent 188250 8C cure or 300 8C postcure isunknown. The idealized nal resin structure isshown in Scheme 1. The structure shown for theresin is idealized because the fate of theRNHC(NH)OR functions at high temperaturesis not known.</p><p>Macromer 2 was selected in the hope that itssingle nitrile function would enter the cyanateester cyclotrimerizations at higher temperaturesso that this nitrile would become a portion ofthe triazine rings. This would generate a cross-linked cyanate ester resin with the POSS cagespendent along the network. Unlike 1, the POSS-2 cage would not constitute a crosslink center.However, separate phases were observed in eachPT-15/2 composite, so most of blend 2 was notpart of the network chemical structure.</p><p>Morphology of the Nanocomposites</p><p>POSS-1 was completely dissolved in THF, andthis gave a transparent solution. After theremoval of THF and curing, the PT-15/POSS-1composites remained transparent.</p><p>Scheme 1. Synthesis of the PT-15/POSS-1 composites.</p><p>3890 LIANG ET AL.</p></li><li><p>Figure 1 displays the wide-angle X-ray dif-fraction (WAXD) patterns for the cured PT-15and the PT-15/1 composites with compositions of99/1, 97/3, and 95/5 (w/w). For comparison, thediffraction pattern of solid 1 is also shown.POSS-1 is a complex isomer mixture withapproximately 80% meta-amino groups, approxi-mately 15% or more para-amino groups, andapproximately 5% other amino groups. There-fore, sharp narrow peaks were not observed forsolid 1. There is an amorphous peak at approxi-mately 7.88 in the diffraction pattern of 1. How-ever, this amorphous peak is absent from all thediffraction patterns of the PT-15/1 composites.The weak amorphous peak at 19.08 in the dif-fraction pattern of 1 is close to the amorphouspeak at 19.58 of the neat PT-15, but the inten-sity in this region does not change as the 1 con-tent increases. The amorphous peak at 19.58 inall the cyanate ester/1 composites is a contribu-tion from PT-15. This implies that 1 is dispersedinto the PT-15 network as unassociated andcompatible POSS units. Moreover, the extractiondata indicate that 1 is chemically bonded to thecyanate ester network.</p><p>TEM micrographs of both PT-15/POSS-1 99/1and 95/5 composites yield no clear-cut evidencefor the presence of POSS-1 aggregates. NoPOSS-1 particles were observed by TEM. This isconsistent with the interpretation that POSS-1has been molecularly dispersed within thematrix. The TEM observations agree with XRDstudies that show that the broad peak of POSS-1 at approximately 7.88 disappears in all the dif-fraction patterns of the PT-15/POSS-1 compo-sites (Fig. 1).</p><p>POSS-2 is not very soluble in PT-15 at 120 8C.When these two components are dissolved in aTHF solution, the removal of THF results in some</p><p>phase separation of 2 into small particles in theresin. Figure 2 displays the WAXD patterns forthe cured PT-15 and the PT-15/POSS-2 compo-sites with compositions of 99/1, 97/3, 95/5, and 90/10 (w/w). The crystalline features characteristicof pure POSS-2 and the amorphous features char-acteristic of PT-15 can be observed. A crystallinepeak at approximately 8.28 (equivalent to anin...</p></li></ul>