Investigation of microwave irradiation as an energy source in polymerization reactions

Variable frequency microwaves were investigated as an energy source to cure unidirectional carbon fiber reinforced phenylethynyl-terminated polyimide composites, and to synthesize the poly( 3 -caprolactam), poly( 3 -caprolactone) and copoly(amide-ester) via ring opening polmerization. ^ The mechanism of the thermal and microwave cure reactions 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 (Mn ∼ 5000 g/mol) was studied by kinetics and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Both the model compound and PETI-5 exhibited much lower activation energies and higher rate constants by the microwave cure process than by the thermal cure process. Solid-state 13C-NMR studies revealed that the major cure reaction in both the model compound and PETI-5 resin is an ethynyl to ethynyl addition reaction, with a minor reaction to further form carbon-carbon single bonded structures.^ Microwave energy was successfully applied to fabricate carbon fiber reinforced phenylethynyl-terminated polyimide composites, PETI-5/IM7, with higher glass transition temperatures (by 11° to 16°C) and enhanced mechanical properties at both room temperature and 177°C and in one-half the time, compared to the standard thermal process. Equivalent physical and mechanical properties were obtained from microwave synthesized nylon-6 and poly( 3 -caprolactone) in reduced time relative to the commercially produced thermal products. Anionic copolymerization of 3 -caprolactam with 3 -caprolactone via microwave irradiation produced poly( 3 -caprolactam-co- 3 -caprolactone) with higher yield, higher amide content, and higher Tg\u27s relative to the thermally produced copolymer.