Partly imidized polyamic acid and its uniaxial stretched polyimide films

Partly imidized polyamic acid(PAA) has been used to prepare high performance polyimide films. The behaviors of two polyamic acids derived from pyromellitic dianhydride(PMDA)/4,4′-oxydianiline(ODA) and 3,3′,4,4′-biphenyltetracarboxylic diahhydride(BPDA)/paraphenylenediamine(PPD) containing dehydrating agents composed of acetic anhydride and a tertiary amine as the catalyst were investigated. The gel point was dependent on imidization degree in despite of temperature and the molar ratio of catalyst to acetic acid. Imdization content was about 35% for PMDA/ODA and about 22% for BPDA/PPD. The effect of catalyst on imidization possessed an order of triethylamine>3-methylpyridine>pyridine>isoquinoline>2-methylpyridine. The stretching of the films greatly reduced the coefficient of linear thermal expansion(CTE) either in the longitudinal direction or transversal direction. Compared to the film from polyamic acid, the partly imidized film had greater stretching ratio, so that the uniaxial stretched polyimide film from partly imidized PAA had higher tensile strength and tensile modulus, but lower elongation in the stretching direction.