Recovery of deformed polymers. II. Repeated cold drawing of polycarbonate, polyethylene, and polypropylene

Studies of repeated cold drawing with intermediate annealing have been carried out on three polymers: polycarbonate, polyethylene, and polypropylene. In the case of polycarbonate, samples recovered to their original lengths in the anneals. On testing, necking and yielding behavior were observed on all draws, with the modulus and yield stress dropping most significantly between the first and second draws, and by a smaller amount between the second and subsequent draws. The recovery kinetics are complex; activation energy spectra derived from these kinetics are centered about 22.5-24.5 kcal/mole, and do not vary significantly with the initial strain or number of the draw. In the case of polypropylene, yielding and necking were observed on all draws after annealing at 160°C; with annealing at lower temperatures, yielding was noted on only the first draw. For both polyethylene (PE) and polypropylene (PP) incomplete recovery was observed on the intermediate anneals; the elastic modulus and drawing stress were significantly smaller on the second draw than on the first draw, and were effectively unchanged between the second and subsequent draws. This softening effect decreased with increased extent of recovery in the intermediate anneal. Activation energy spectra derived from the recovery kinetics were centered about 14.5-15.5 kcal/mole for PP and 22-23 kcal/mole for PE; the occurrence of recovery processes having significantly lower activation energies is also indicated.