DEFORMATION IN POLYMER THIN-FILMS

In thin amorphous polymer films, the entanglement density, v(c), stron gly influences the draw ratio during micronecking and the competition between cavitational and non-cavitational processes, often referred to as crazing and shear deformation respectively. TEM investigations sug gest this also to be true of polymers capable of crystallizing althoug h modes of deformation involving chain slip may be suppressed owing to strain induced crystallization. In either case, v(c) is determined no t only by the intrinsic properties of the polymer in question (chain s tiffness and packing density), but also by the sample history. In melt crystallized semicrystalline films, for example, if crystallization t akes place. relatively slowly compared with the rate of relaxation of chain conformations in the melt, v(c) may be substantially reduced owi ng to a decrease in chain interpenetration. Evidence for this comes fr om observations of the draw ratios in micronecks in isotactic polyprop ylene films isothermally crystallized at different temperatures, which increase sharply in regimes where entanglement loss is predicted to o ccur during crystallization.