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-Tg thermoplastic polyimide. The polyimide was synthesized from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 4,4′-(9H-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 Tg 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 Tg value (ΔTg = +5°C) and rather limited improvements in stress at rupture (50%) and strain-energy release rate GIc (27%) compared to the unmodified epoxy matrix.