Synthesis, Morphology, and Viscoelastic Properties ofCyanate Ester/Polyhedral Oligomeric SilsesquioxaneNanocomposites

KAIWEN LIANG,1 HOSSEIN TOGHIANI,1 GUIZHI LI,2 CHARLES U. PITTMAN JR.2

1Dave C. Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, Mississippi 39762

2Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762

Received 4 April 2005; accepted 5 April 2005DOI: 10.1002/pola.20861Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Cyanate ester (PT-15, Lonza Corp) composites containing the inorganic–organic 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 (Tg’s)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 > 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 1–10 wt % 2. Nevertheless, the Tg and E0 values (temperature > 285 8C) of thesecomposites increased regularly with an increase in 2. VVC 2005 Wiley Periodicals, Inc. J Polym

Sci Part A: Polym Chem 43: 3887–3898, 2005

Keywords: cyanate ester resins; FT-IR; nanocomposites; polyhedral oligomericsilsesquioxane (POSS); TEM

INTRODUCTION

Hybrid organic polymer/inorganic nanocompo-sites have generated intense recent interest.1–26

Inorganic nanophases include nanoclays,27–32

carbon nanotubes,33–36 vapor-grown carbonfibers,37–43 inorganic nanofibers,44,45 and polyhe-dral oligomeric silsesquioxane (POSSs).19–26,46

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,47–49 cage,47,50–52 partial

Correspondence to: C. U. Pittman, Jr. (E-mail: cpittman@ra.msstate.edu)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 3887–3898 (2005)VVC 2005 Wiley Periodicals, Inc.

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cage,53 and polymer structures.26,54 POSSs offera chance to prepare hybrid organic–inorganicmaterials 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 1–3 nm, can beconsidered the smallest possible particles ofsilica, but unlike silica, silicones, or fillers,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.

Reactive POSS derivatives are available forpolymerization or grafting.19–26,46,55–58 Hence,POSS cages can be incorporated into commonplastics via copolymerization,57,59–61 grafting,57,62

or blending.57,62,63 New hybrid inorganic–organicthermoset46,64–70 and thermoplastic materi-als22,26,60–63,71,72 can be prepared. These hybridscan exhibit dramatic improvements in polymerproperties such as higher use temperatures,73

oxidation resistance,62 surface hardening,62 andmechanical property modifications.75,76 Further-more, reductions in flammability,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

have been improved.In this study, octaaminophenyl(T8)POSS (1)

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.

EXPERIMENTAL

Materials

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.

The multifunctional monomer 1 [(C6H4NH2)8(SiO1.5)8], molecular weight ¼ 1153.63 g/mol]

3888 LIANG ET AL.

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

It is essential that all water be removed fromthe HCOOH/Et3N solution when Laine’s proce-dure is followed in this synthesis. The aminogroup’s 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 confirmed by 1H, 13C, and28Si NMR spectroscopy.85 Monofunctional 2[(C5H9)7(SiO1.5)8CH2CH2CH2CN; molecular weight¼ 968.66 g/mol] was purchased from HybridPlastics Co.

Preparation of the Composites

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 (300–350 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.

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 ¼ 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 fine powder to the liquid resin.These mixtures (total weight ¼ 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

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 finally 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.

Measurements

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 1–308 with astep size of 0.038 at 1 s per step.

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 70–90-nm thicknessand mounted on carbon-coated Cu TEM grids.

The dynamic storage modulus (E0) and loss fac-tor (tan d) were determined in the bending modeversus the temperature with a Rheometrics Sci-entific 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 3.0–4.0 mmthick, 4.5–5.5 wide, and 38 mm long.

The composite densities were measured withan electronic densimeter (ED-120T) at 25 8C.

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.

CYANATE ESTER/POSS NANOCOMPOSITES 3889

RESULTS AND DISCUSSION

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

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 188–250 8C, PT-15 cures, incor-porating 1 molecularly throughout the resin.The initial amino group addition to ��OCN func-tions generates RNHC(¼¼NH)OR groups. How-ever, the fate of these functions during the sub-

sequent 188–250 8C cure or 300 8C postcure isunknown. The idealized final resin structure isshown in Scheme 1. The structure shown for theresin is idealized because the fate of theRNHC(¼¼NH)OR functions at high temperaturesis not known.

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.

Morphology of the Nanocomposites

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.

Scheme 1. Synthesis of the PT-15/POSS-1 composites.

3890 LIANG ET AL.

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.

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).

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

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 aninterplanar spacing of 1.08 nm) of 2 is present ineach composite. Furthermore, peaks at 11.0(0.8 nm) and 19.18 (0.47 nm) are evident in thecomposites with 3, 5, and 10 wt % 2. The intensityof these crystalline peaks increases as the loadingof 2 increases.

Confocal micrographs are shown in Figure 3, inwhich particles of 2 are dispersed in the resinmatrix. Some particles are about 100 lm in diam-eter. TEM micrographs of the 1 and 5 wt % 2 com-posites reveal the presence of both spherical andirregularly shaped particles. Particles from 25 nmto the micrometer range in diameter can be seenin both samples. The 1 wt % 2 sample has numer-ous particles 25–135 nm in diameter (Fig. 4),whereas the average size of these small particlesincreases for the 5 wt % sample. These averagesizes continue to increase in the samples with10 wt % 2 (Fig. 5). A close examination of the par-ticles in the 200–400-nm-diameter range showscomplex patterns of different contrast, whichsuggest some resin incorporation into aggregatesof smaller particles to form these features.

Viscoelastic Properties of the Cyanate EsterResin/1 Nanocomposites

The curves of bending E0 versus the tempera-ture, obtained at 1 Hz, are displayed in Figure 6

Figure 1. XRD patterns of the neat cured PT-15resin, its PT-15/POSS-1 composites, and the as-received POSS-1 monomer.

Figure 2. XRD patterns of the neat cured PT-15resin, its PT-15/POSS-2 composites, and the as-received POSS-2 monomer.

CYANATE ESTER/POSS NANOCOMPOSITES 3891

for the neat PT-15 resin and the 1, 3, and 5 wt %1 nanocomposites. The E0 values of the 99/1 and97/3 composites are higher than those of the neat

PT-15 over the entire temperature range. In con-trast, the 5 wt % 1 composite displays lower E0

values at 40 8C than that of neat PT-15 [E0 ¼ 1.50

Figure 3. Confocal optical micrographs of the PT-15/POSS-2 99/1 composite.

Figure 4. TEM micrographs of the PT-15/POSS-2 99/1 composite.

3892 LIANG ET AL.

(PT-15), 1.61 (1 wt % 1), 2.12 (3 wt % 1), and1.41 GPa (5 wt % 1) at 40 8C]. The E0 values of5 wt % POSS-1 are higher than that of neat PT-15 at temperatures above 320 8C. The E0 valuesof neat PT-15 and the 99/1, 97/3, and 95/5 nano-composites at 350 8C [greater than the glass-transition temperature (Tg)] are 86.8, 197.5,144.8, and 118.4 GPa, respectively.

The bending tan d (1 Hz) peak intensitiesdrops and the peaks broaden in the first heatingcycle as the POSS-1 loading increases (Fig. 7).The Tg values (tan d peak temperatures) are305, 336, 300, and 258 8C, for the neat PT-15resin and the 1, 3 and 5 wt % 1 composites,respectively. Tg decreases as the 1 content israised above 1 wt %.

The effect of the thermal history on the vis-coelastic response of the 5 wt % POSS-1 compo-site is shown in Figure 8. The E0 values increasemore upon its second heating cycle than uponits third heating cycle across the entire 35–370 8Crange. The Tg values of the 5 wt % POSS-1 com-posite change little (258, 263, and 261 8C in thefirst, second, and third cycles). The bending tan dpeaks broaden and their peak intensities weakenduring the second and third heatings.

There are eight amino functional substituentson POSS-1, and they can react with the cyanateester functions of the liquid PT-15 resin. Thismight generate higher network crosslink den-sities around POSS-1 moieties, raising the E0

and Tg values of the 99/1 composite above thoseof the neat PT-15 resin. The reaction of theamino groups of 1 with the cyanate ester func-tions is fast. PT-15/POSS-1 mixtures, obtainedafter the removal of THF from PT-15/1 solu-tions, precure at room temperature and becomesolids. This precuring reaction might cause some

changes in the formation of the PT-15 networkduring subsequent high-temperature curing.The two network structures cannot be the same.As the stoichiometry of 1 increases from 1 to5 wt % (�0.28–1.42 mol %), the amine to cyan-ate ester functional group ratio increases fromapproximately 0.0090 to 0.0461. This willchange the overall distribution of curing reac-tions, lowering both the stoichiometric and vol-ume fraction of the triazine rings formed. Thesechanges will influence both E0 and Tg valuesobtained with a given curing protocol. The addi-tion of the aryl amine function of 1 acrossthe cyanate ester generates the Ar0��NH��C(OAR)¼¼NH functional group. The reactionpathways of this new function (and its kineticswith respect to the cyanate ester function) dur-ing subsequent high-temperature curing areunknown.

Figure 6. Bending E0 versus the temperature at1 Hz (from DMTA) for the cured PT-15 and PT-15/POSS-1 composites (first heating cycle).

Figure 5. TEM micrographs of the PT-15/POSS-2 90/10 composite.

CYANATE ESTER/POSS NANOCOMPOSITES 3893

The tan d peaks broaden during the secondand third heatings of the pure cyanate ester(Fig. 9), but Tg remains almost the same. Thus,some reorganization and repacking of PT-15 seg-ments occur as the curing continues to advance.

Viscoelastic Properties of the CyanateEster Resin/2 Composites

The E0 values of all the PT-15/2 composites arehigher than those of the neat cyanate ester resinat all temperatures, with the exception of thePT-15/2 95/5 and 90/10 composites in the firstheating cycle at temperatures between 197 and260 8C (Fig. 10). In the glassy region (tempera-ture < Tg), the E0 values of these composites areabove those of the neat PT-15 resin. For exam-ple, the E0 values of the neat PT-15 resin andthe composites with 1, 3, 5, and 10 wt % 2(Table 1) are 1.50, 1.96, 2.01, 1.85, and 1.66

GPa, respectively, at 40 8C. The E0 valuesincrease regularly with an increase in theweight percentage of 2 at temperatures greaterthan 285 8C. For example, at 350 8C, E0 is 86.8,139.2, 142.9, 184.4, and 209.4 MPa for the purecyanate ester and the samples containing 1, 3,5, and 10 wt % 2, respectively. The E0 value ofthe 90/10 composite is about 2.5 times greaterthan that of the neat PT-15 resin. The incorpo-ration of 2 improves the mechanical propertiesabove their Tg’s, raising the heat distortion tem-perature. These improvements increase as thecontent of 2 increases.

The Tg values, defined as the temperature atthe peak maximum of the high-temperature apeak in the bending tan d curves, increase withan increase in the amount of 2. These Tg valuesare 305, 323, 320, 331, and 333 8C for the neatPT-15 resin and the 1, 3, 5, and 10 wt % 2 compo-sites, respectively. The tan d peak intensitiesdecrease as the weight percentage of 2 increases.

The effect of the thermal history on the vis-coelastic properties of the 5 wt % 2 compositewas studied through three heating cycles (Fig.11). The sharp decrease of E0 at about 230 8C inthe glass plateau, present in the first heatingcycle, disappears in the second cycle. The E0 val-ues increase over the entire 35–370 8C range inthe second and third cycles. Furthermore, thetan d peaks broaden and their intensitiesdecrease in the second and third cycles.

Densities, Solvent Extraction, and IR Data

The densities of the POSS-1 nanocompositesdecrease below those of the PT-15 resin as theloading of 1 goes up (Table 1). The POSS-1 com-posites might contain more free volume than

Figure 7. Bending tan d versus the temperature at1 Hz (from DMTA) for the cured PT-15 and PT-15/POSS-1 composites (first heating cycle).

Figure 8. Bending E0 and tan d versus the tempera-ture at 1 Hz (from DMTA) for the PT-15/POSS-1 95/5composite for three successive heating cycles.

Figure 9. Bending E0 and tan d versus the tempera-ture at 1 Hz (from DMTA) for the neat PT-15 resin inthree different heating cycles.

3894 LIANG ET AL.

neat PT-15 because of changes in the extent ofcure or because of resin packing around theunits of 1 in the matrix. More segmental motionis allowed in PT-15/1 than in the pure PT-15sample after curing in the first DMTA heatingcycle.

All samples were immersed in THF to dis-solve and extract oligomeric and linear poly-meric components over a period of 3 months.None of the samples swelled. The extracted resi-dues were isolated by solvent removal andweighed. Only very small amounts of PT-15cyanate ester oligomers (0.29–0.35 wt %) wereextracted (Table 1).

No 1 was detected in these residues or in resi-dues isolated by the extraction of powdered sam-ples of each of the PT-15/1 composites withrefluxing THF. The FTIR spectrum of 1 has

strong SiO cage absorption at 1080–1105 cm�1

and a ��CBN stretch at 2338 cm�1. These bandswere not present in the extracted residues. Allthe 1 present was chemically bonded to the PT-15 network. This agrees with the high reactivityof the amino groups of 1 with R��OCBN func-tions and the complete dispersion of 1 suggestedby XRD and TEM studies.

The densities of the PT-15/POSS-2 (1, 3, 5, and10 wt %) composites are all very similar andslightly higher than that of the neat PT-15 resin(Table 1), in contrast to the densities of the PT-15/1 nanocomposites. In the first DMTA heatingcycle, PT-15/2 systems exhibit less segmentalmotion than the PT-15/1 composites after curing.

Small amounts of THF-extracted residueswere isolated from the PT-15/2 composites bysolvent removal and weighed (Table 1).

FTIR spectra were obtained for all THF-extracted crosslinked samples (THF-insoluble)and for the residues that had been extracted(THF-soluble). The cured neat PT-15 exhibitedits strongest absorption around 1365 cm�1 and astrong 1565-cm�1 absorption due to the trioxy-gen-substituted triazine rings.87 The cyanogroup stretching frequency was at 2360 cm�1 forpure 2. The strongest ��Si��O�� absorption forpure 2 appeared at 1088 cm�1. This band wasshifted to 1107 cm�1 in the PT-15/2 composites.The weight percentages of the materialsextracted are given in Table 1 for each sample.IR spectra of the soluble residues confirmed thatpure 2 was extracted from the PT-15/2 compo-sites. In each case, 54–65% of 2, originallyblended, was removed. The remaining unex-tracted POSS may have been modified duringthe 300 8C postcure.

Table 1. Bending E0 Values, Tg Values, Densities, and Extraction Percentages of Neat PT-15 Resin andPT-15/POSS-1 and PT-15/POSS-2 Composites

Sample (w/w) Tg (8C)aE0 at 40 8C

(GPa)aE0 at 350 8C

(MPa)aDensity(g/cm3)

ExtractionPercentage (wt %)

PT-15 305 (303) 1.50 (2.12) 86.8 (121.9) 1.279 0.30PT-15/POSS-1 (99/1) 336 (342) 1.61 (2.43) 197.5 (229.6) 1.279 0.29PT-15/POSS-1 (97/3) 300 (294) 2.12 (2.69) 144.8 (171.3) 1.275 0.35PT-15/POSS-1 (95/5) 258 (261) 1.41 (2.25) 118.4 (143.6) 1.267 0.32PT-15/POSS-2 (99/1) 323 (328) 1.96 (2.63) 139.2 (185.3) 1.283 0.65b

PT-15/POSS-2 (97/3) 320 (329) 2.01 (2.68) 142.9 (203.5) 1.282 1.86b

PT-15/POSS-2 (95/5) 331 (349) 1.85 (2.62) 184.4 (254.6) 1.283 3.01b

PT-15/POSS-2 (90/10) 333 (352) 1.66 (2.51) 209.4 (269.5) 1.282 5.38b

a The values from the first heating cycle and third heating cycle (in parentheses) are given.b The IR spectrum of the extracted material was identical to that of 2, confirming that unreacted 2 was extracted. In every

case, 54–65% of 2 originally added to the composite was extracted.

Figure 10. Bending E0 versus the temperature at1 Hz (from DMTA) for the PT-15/POSS-2 composites(first heating cycle).

CYANATE ESTER/POSS NANOCOMPOSITES 3895

The intensity ratio of the 1107-cm�1 (Si��O)absorption to that at 1365 cm�1 (cyanurate) canbe use to follow the incorporation of 2 into thecured composites. These ratios, obtained fromthe THF-insoluble portion, increased with anincrease in the content of 2 (Table 1): 0.49 (1 wt% 2), 0.58 (3 wt % 2), 0.80 (5 wt % 2), and 0.86(10 wt % 2). However, these ratios were lowerthan those of the POSS-2 composites (beforeTHF extraction), which were 0.62 (1 wt % 2),0.72 (3 wt % 2), 0.85 (5 wt % 2), and 1.18 (10 wt% 2). For the 10 wt % sample, the intensityratios of 1107 cm�1 to 1365 cm�1 dropped from1.18 to 0.86 after THF extraction (almost equalto the number of that for the 5 wt % samplebefore THF extraction). This suggests almost 50wt % of 2 present was extracted by THF fromthe 10 wt % sample. This agrees with the 5.38wt % loss on extraction.

CONCLUSIONS

Multifunctional 1 was incorporated into the PT-15 cyanate ester resin network by thermal(250 8C) curing. The amino functions of 1reacted rapidly with cyanate ester functionsbefore high-temperature curing. The tan d peakintensities of the PT-15/1 resins (first heatingcycle) were lowered and their widths werebroadened by the incorporation of 1. The Tg val-ues of the PT-15/1 composites first increasedwith 1 wt % 1 and then decreased as the contentof 1 increased. The E0 values continued toincrease during the second and third heatingcycles for the 5 wt % 1 nanocomposites. Also,the bending tan d peaks broadened and their

peak intensities weakened in the glass-transitionregion during the second and third heatings.

XRD, TEM, extraction, and IR data all indi-cated that 1 was totally dispersed and chemi-cally bonded to the continuous matrix phase ona molecular scale in all samples.

In contrast to 1, the monofunctional nitrilePOSS, 2, did not chemically react with the cyan-ate ester at lower temperatures. Thus, substan-tial phase separation into both nanometer- andmicrometer-sized particles occurred during cur-ing. Only a small fraction of 2 was molecularlydispersed. At 250 and 300 8C, some reaction of 2with the resin may have occurred, allowing sur-face molecules of 2 to bind to the resin, andsome other self-reactions of the cages of 2 mayhave occurred during the 300 8C postcure, mak-ing it impossible to extract all the 2 originallyblended.

Increasing the weight percentage of 2 in thecomposites progressively raised the Tg values andthe E0 values of 2. Thus, both types of POSS addi-tives can improve the high-temperature mechani-cal properties of cyanate ester resins despite theirvery different degrees of dispersion.

This work was supported by the Air Force Office of Sci-entific Research (grant no. F49620-02-1-026-0) and theNational Science Foundation (grant no. EPS0132618).

REFERENCES AND NOTES

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