Effect of molecular weight on transverse microcracking in high-temperature LaRC-RP46T polyimide composites

This experimental study examines the role of molecular weight as well as crosslink density on the microcrack resistance in high-temperature LaRC-RP46T composites. LaRC-RP46T polyimides are prepared by in-situ polymerization of three monomer reactants: monoisopropyl ester of 5-norbornene-2,3-dicarboxylic acid (NE); 3,4′-oxydianiline (3,4′-ODA); and diisopropyl ester of 3,3′,4,4′-benzophenonetetracarboxylic acid (BTDE). Five formulations were prepared, in which formulated molecular weight between crosslink sites varies from 1,500 to 21,000 g/mole. By varying the molecular weight between the crosslink sites, a series of five resin systems with different crosslink densities was produced. Cross-ply composite laminates were compression molded from each formulation and IM-7 graphite fibers and were cut into a series of triangular test specimens. These test specimens were then subjected to 1,500 thermal cycles in order to investigate the damage associated with thermal cycling. The extent of the microcrack damage was determined by measuring the number of transverse microcracks and by noting changes in the density and moisture absorption of each specimen.