Crazing of Polymers at Low Temperatures in He, Ar, N2 and at Reduced Pressure

Abstract

IT is generally thought that having a polymer immersed in liquid nitrogen does not affect its stress-strain behaviour. We found that liquid and gaseous nitrogen and liquid argon affect the tensile stress-strain curve and the mode of fracture of amorphous polymers. Previous investigations at ∼78 K of the tensile behaviour of amorphous polymers have been conducted in liquid or gaseous nitrogen. Polymethyl methacrylate (PMMA), when stretched in liquid nitrogen, showed severe crazing1 with increased fracture toughness at 78 K. Beardmore and Johnston2 and Beardmore3 investigated PMMA tested in tension at 78° C; in the former paper no mention was made of test environment; in the latter paper, the specimen was surrounded with lead shot cooled with liquid nitrogen. Beardmore and Rabinowitz4 studied tensile crazing of PMMA, polycarbonate (PC) and polysulphone (PSF) at 78 K, but they did not specify the environment (S. Rabinowitz, private communication). Ekvall and Low5 studied PC from 78 to 300 K in nitrogen gas and found a ductile-to-brittle transition between 90 K and 143 K. Our work suggests that this transition was an environmental effect of the nitrogen gas.