Polyimide-modified epoxy system: time-temperature-transformation diagrams, mechanical and thermal properties

A hot-melt processable thermoset was prepared by blending tetraglycidyl-4,4′-diaminodiphenylmethane/4,4′-diaminodiphenylsulfone epoxy resin and a high- T g thermoplastic polyimide. The polyimide was synthesized from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 4,4′-(9 H-fluoren-9-ylidene)bisphenylamine to form a linear structure initially miscible with the neat epoxy resin. The thermal polymerization study, including determination of time-temperature-transformation cure diagrams, pointed out a lowering in crosslink density of epoxy network probably due to the polyimide viscosity effect. After cure, no phase separation could be observed by scanning electron microscopy and only one T g was detected by dynamic mechanical analysis, showing full miscibility between the blend components. The consequences of the thermoplastic incorporation (polyimide concentration = 10 wt%) were a slight increase in T g value ( ΔT g = +5°C) and rather limited improvements in stress at rupture (50%) and strain-energy release rate G Ic (27%) compared to the unmodified epoxy matrix.