Moisture-induced stress relaxation of polyimide thin films

Four different polyimides: rodlike poly(p-phenylene pyromellitimide) (PMDA-PDA), pseudo-rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semiflexible poly(4,4-oxydiphenylene pyromellitimide) (PMDA-ODA) and poly(4,4-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA) were prepared from two different precursors, poly(amic diethyl ester) (PAE) and poly(amic acid) (PAA). For polyimides prepared from the PAE precursors, stress relaxation coefficient at 25°C, 100% RH was 14.76×10−10 cm2/s for PMDA-PDA, 3.29×10−10 cm2/s for BPDA-PDA, 24.91×10−10 cm2/s for PMDA-ODA and 9.34×10−10 cm2/s for BPDA-ODA. For polyimides prepared from the PAA precursors, stress relaxation coefficient at 25°C, 100% RH was 10.50×10−10 cm2/s for PMDA-PDA, 1.91×10−10 cm2/s for BPDA-PDA, 28.84×10−10 cm2/s for PMDA-ODA and 9.10×10−10 cm2/s for BPDA-ODA. The effect of bulky ethyl ester group on the stress relaxation behavior is significantly high in rodlike PMDA-PDA or pseudo-rodlike BPDA-PDA having high chain rigidity and intermolecular packing order. The decreased birefringence, crystallinity and the looser packing in amorphous region of rigid polyimide derived from PAE may induce fast stress relaxation compared to flexible polyimide. Specially, the different chain mobility of polyimide also induced significant effect on the stress relaxation.