Stress behaviors and thermal properties of polyimide thin films depending on the different curing process

The effect of high boiling point solvent on the residual stress behaviors of semiflexible structure poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA) and pseudo-rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide was investigated. As a solvent, a mixed solution of 20 wt% cyclohexyl-2-pyrrolidone (CHP; bp = 307 °C) and 80 wt% n-methyl-2-pyrrolidone (NMP; bp = 202 °C) was used. The effects of solvent system and imidizing history on the morphological structure, as well as residual stress, were significantly high in the BPDA-PDA having high chain rigidity, but relatively low in the semiflexible PMDA-ODA with low chain rigidity. In addition, rapidly cured films prepared from PAA (NMP/CHP) showed higher residual stress and a lower degree of molecular anisotropy than slowly cured film imidized from PAA (NMP). This was induced by high chain mobility in polyimide thin films prepared from PAA (NMP/CHP) during the thermal cure process. Therefore, molecular anisotropy, depending on the solvent system and imidizing history, might be one of the important factors leading to low residual stress in polyimide thin films.