High performance polymer films 4. Mechanical behavior

Tensile properties of the polyimide and copolyimide films based on two dianhydrides, pyromellitic dianhydride (PMDA) and 3,3^′,4,4^′-benzophenonetetracarboxylic dianhydride (BTDA) and two diamines, 4,4^′-oxydianiline (ODA), and a proprietary aromatic diamine (PD) have been described. The tensile strength of the films containing higher proportions of BTDA or PMDA and PD is much higher (except the fully rigid film based on PMDA-PD which is brittle in nature) than the films containing higher proportion of ODA moiety. The films containing PD as the diamine moiety exhibit high initial moduli than the films containing exclusively or mainly ODA as the diamine moiety. The films having higher concentration of the -O- linkage originated from diamine ODA are found to exhibit higher elongation values. There is found to be no direct correlation between η_inh of the precursor casting solutions and mechanical properties of structurally different polyimide/copolyimide films. For a particular polyimide or copolyimide film, the tensile strength value is found to be less sensitive than the elongation to the variation of η_inh value of the precursor poly(amic acid) or copoly(amic acid). Tensile strength and elongation of the film, basically rigid in nature, may be improved by post-curing at 360°C/370°C. While Kapton H film retains 78% and 63.5% of its tensile strength and % elongation at break (% E_b) respectively after hot-wet mechanical test, the film based on BTDA 80, PMDA 20 and PD shows an increase of about 27% and 22% in its tensile strength and % E_b respectively.