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Polyhedral oligosilsesquioxane (POSS)-reinforced thermosets based on octaglycidyl epoxy polyhedral oligosilsesquioxane cured with 4,4Ј-diaminodiphenyl sulfone (DDS) were prepared and studied for their cure, thermomechanical, and microstructural characteristics. Particular attention was paid to nanometer-scale deformation processes responsible for toughening, as revealed by transmission electron microscopy (TEM) in conjunction with the thermal properties. A cure analysis investigated with calorimetry and rheometry showed a significant dependence of the cure mechanism and kinetics on the DDS content, but all hybrid thermosets reacted completely below 300°C into rigid solids. A dynamic mechanical analysis of this hybrid resin system showed that increasing the DDS concentration used during cure increased the dynamic storage modulus in the glassy (temperature Ͻ glass-transition temperature) and rubbery (temperature Ͼ glass-transition temperature) states, simply through an increase in the crosslink density. The phase structures revealed by TEM with selective POSS staining were drastically affected by the DDS concentration and manifested as altered nanomechanical deformation structures. It was qualitatively found that the main toughening mechanism in the studied POSS-reinforced thermosets was void formation at the nanometer scale, possibly templated by limited POSS aggregation. As the crosslinking density increased with the DDS concentration, microshear yielding between voids prevailed, providing a balance of stiffness, strength, and toughness.

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ABSTRACT:The kinetic mechanism of the thermal cure of a phenylethynyl-terminated imide model compound, 3,4-bis[(4-phenylethynyl)phthalimido]diphenyl ether (PEPA-3,4-ODA) and a phenylethynyl-terminated imide oligomer PETI-5 (MW 5000 g/mol) was studied. FTIR was used to follow the cure of the model compound, while thermal analyses (DSC) was used to follow the cure of the PETI-5 oligomer. The changes in IR absorbance of phenylethynyl triple bonds at 2214 cm 01 of PEPA-3,4-ODA as a function of cure time were detected at 318, 336, 355, and 373ЊC, respectively. The changes in the glass transition temperature, T g , of PETI-5 as a function of time were measured at 350, 360, 370, 380, and 390ЊC, respectively. The DiBenedetto equation was applied to define the relative extent of cure, x, of the PETI-5 oligomer by T g . For the model compound, the reaction followed first order kinetics, yielding an activation energy of 40.7 kcal/mol as determined by infrared spectroscopy. For PETI-5, the reaction followed 1.5th order, yielding an activation energy of 33.8 kcal/mol for the whole cure reaction, as determined by T g using the DiBenedetto method. However, the cure process of PETI-5 just below 90% by this method followed firstorder kinetics yielding an activation energy of 37.2 kcal/mol. ᭧

The cure reactions of phenylethynyl end-capped polyimides were investigated using solid-state 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR). A 13C-labeled model compound (13C-PEPA-3,4‘-ODA) and an imide oligomer (13C-PETI-5) were synthesized and characterized. The thermal cure process for 13C-PEPA-3,4‘-ODA was followed over the temperature range 318−380 °C and for13C-PETI-5 over the temperature range from 350 to 400 °C. Our NMR results showed that, for the model compound, as curing proceeded, the percentage of polymeric structures containing double-bonded and single-bonded carbon increased while the percentage of triple-bonded carbon gradually decreased and finally disappeared at the elevated temperatures. The PETI-5 cure process was very similar to the PEPA-3,4‘-ODA cure process, and the percentage of double-bonded carbon structure of PETI-5 increased during the cure process as the percentage of triple-bonded carbon decreased. Moreover, for the PETI-5 resin system, a weak broad 13C signal due to a single-bonded structure was observed after cure. The carbonyl groups remained relatively constant during the curing reactions for both the model compound and PETI-5 resin. The appearance of single-bonded structures in the cure of the model compound and PETI-5 can be derived from polyene structures by a further intra- or intermolecular Diels−Alder reaction to form cycloolefinic ring or branched structures. On the basis of the chemical shift data of several low molecular weight compounds with aromatic ring structures and polyene structures, we cannot exclude the formation of substituted aromatic ring structures from PEPA-3,4‘-ODA or from PETI-5.

A novel process for synthesizing nylon‐6 and poly(ϵ‐caprolactone) by microwave irradiation of the respective monomers, ϵ‐caprolactam and ϵ‐caprolactone, is described. The ring opening of ϵ‐caprolactam to produce nylon‐6 was performed in a microwave oven by the forward power being controlled to about 90–135 W in the presence of an ω‐aminocaproic acid catalyst (10 mol %) and for periods of 1–3 h at temperatures varying from 250 to 280 °C. The ring opening of ϵ‐caprolactone to produce poly(ϵ‐caprolactone) was performed in a microwave oven by the forward power being controlled to about 70–100 W for a period of 2 h in the presence of stannous octoate with and without 1,4‐butanediol over a temperature range of 150–200 °C. The yields, conditions of the reactions, and properties of the products generated relative to the thermal processes are discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2264–2275, 2002

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