Curing effect on the relaxation modulus and thermal expansion coefficient of rodlike polyimide films

For the films of PMDA-PDA (pyromellitic dianhydride-p-phenylenediamine) cured on silicon (Si) and gallium arsenide (GaAs) substrates at 400 degree C, the corresponding thermal stresses, thermal expansion coefficients, and relaxation moduli increase with the increase of film thickness. The relaxation modulus increases with the increase of observation temperature. This may be attributed to the inherent characteristics of cross-linking and in-plane orientation of the films. With the increase of curing temperature, the variation of thermal stress with respect to temperature lessens and the thermal expansion coefficient decreases. For curing temperature changing from 200 to 250 degree C, the decreases in the variation of thermal stress and thermal expansion coefficient and the increase in the relaxation modulus may mainly be attributed to the effect of a further imidization. Above 250 'C, the decreases in the variation of thermal stress, thermal expansion coefficient, and relaxation modulus with respect to curing temperature may presumably be attributed to the conversion of some intermolecular links to imides.