Plastic yielding of partially crystalline polymers

Measurements are described of characteristic stress-strain properties in tension for several representative partially crystalline polymers prepared by cooling from the melt. Striking differences were found in the natural draw ratio, ranging from 2x to 11x. They are interpreted in terms of a simple molecular model of the yielding process in which polymer chains, folded to different degrees in the crystallites, are pulled taut. Values of yield and draw stress were found to be of the same order as the free energy U_m of melting, suggesting that yielding and drawing take place by stress-induced disruption of crystallites, analogous to melting. The mechanical work of drawing was similar to, but generally larger than, that required for melting, ranging from U_m to about 4U_m. The difference is attributed to strain energy of drawn material and possible recrystallization during drawing.