Optimization of foaming process using triblock polyimides with thermally labile blocks

Triblock polyamic acid was synthesized from the reaction of amine-terminated polystyrene with 4,4′-oxydianiline and pyromellitic dianhydride in N-methyl-2-pyrrolidone. IR and TGA were used to determine optimum curing conditions, in which imidization was completed and the thermally labile polystyrene block was intact. From the thermomechanical analysis (TMA), it was identified that the resulting triblock polyimide film showed contraction at several points over the temperature range of 50-400°C. The first contraction at 75°C and the second at 275°C were due to the residual stresses associated with the polystyrene and polyimide blocks, respectively. The third at 330°C was probably attributed to foam collapse. The structural instability in polyimide matrix induced by the residual stresses was believed to be the reason causing the foam collapse. To overcome such a problem, an annealing process was carried out. The annealed film showed no contraction and, moreover, no foam collapse was observed in the thermomechanical analysis.