The relationship between mechanical properties and structure in rolled polypropylene

Abstract

AbstractThe relationship between mechanical properties and fine structure has been studied in polypropylene rolled both unidirectionally and biaxially (cross rolled). In unidirectionally rolled samples, a complex dependence with cold work is observed with a substantial change being observed at about 50 percent cold work. At 70 percent cold work, the yield strength and tensile strength increase substantially in the roll direction as compared with the starting billet but decrease only slightly in the transverse direction. Above 50 percent cold work, Young's modulus increases rapidly in the roll direction with a smaller increase in the transverse direction. The elongation to freak decreases in the roll direction but increases in the transverse direction. A striking feature is the large increase in ductility due to a small amount of cold work (ca., 10‐20 percent). Analogous property changes are observed for cross rolled samples although no significant variation with direction in the sheet was found. The complex property changes are accompanied by complex changes in the molecular orientation as observed by wide angle X‐ray pole figures and by changes in the morphology as observed with small angle x‐ray scattering. These changes are interpreted in terms of a model incorporating tilting both of lamellae and of chain stems within the lamellae at early stages of rolling followed by breakup of lamellae and molecular rearrangement at later stages.