IN-PLANE ORIENTATION OF POLYIMIDE CHAINS INDUCED BY THERMAL IMIDIZATION

The degree of in-plane molecular orientation of the unstretched polyamic acid(PAA) films and the corresponding polyimide (PI) film cured thermally have been quantitatively estimated by measuring the dichroic ratio of a rigid-rod dye (perylenebisimide) dispersed molecularly in their films at an incidence angle. Thermal imidization of PAA(BPDA/PDA) cast on a substrate enhanced markedly the degree of in-plane orientation of the PI chains, meaning that spontaneous in-plane molecular orientation occurred without stretching of the film. However, no spontaneous in-plane orientation was observed for BPDA/ODA system as well as PMDA/ODA system. Thus, it was found that the spontaneous orientation behavior depends strongly on the PI chain rigidity or linearity in contrast to that the degree of in-plane orientation for various PAAs is essentially independent of the chemical structure. PI(BPDA/PDA) cured in free-standing showed the degree of the in-plane orientation much smaller than the PI film imidized on a substrate.