Synthesis and processing of PMR‐15/LaRC‐TPI semi‐IPN systems

To improve the fracture toughness of PMR‐15 polyimide and to alleviate its high susceptibility to microcracking induced by thermal cycling, a thermoplastic polyimide, LARC‐TPI, was incorporated to form a sequential semi‐interpenetrating polymer network (semi‐2 IPN). The imidization kinetics of LARC‐TPI in the semi‐IPNs were studied using a thermal gravimetric analyzer. Both the solvent and the glass transition temperature of the semi‐IPN were found to have significant effects on the imidization kinetics. The kinetics could be modeled by a two‐step reaction: the first step being a second‐order reaction followed by a second step, which is a first‐order diffusion‐controlled reaction. Differential scanning calorimetry was chosen to investigate the curing of PMR‐15 and PMR‐15/LARC‐TPI semi‐IPNs. The curing process was well correlated by a first‐order reaction kinetics, which suggested that the reverse Diels‐Alder reaction of the Norbornene end group was the rate controlling step. The glass transition temperatures of these semi‐IPNs were again found to play important an important role in dictating the curing kinetics. A higher proportion of LARC‐TPI or a higher glass transition temperature of the semi‐IPN prepolymer tended to result in a slower curing reaction. The optimum molding cycle of PMR‐15 and PMR‐15/LARC‐TPI semi‐IPNs were then determined from the obtained kinetics.