TENSILE PROPERTIES OF CRYSTALLINE POLYMERS - RANDOM COPOLYMERS OF ETHYLENE

Force-elongation curves of a set of random ethylene-1-alkene copolymers have been studied. The comonomers included 1-butene, 1-hexene, 1-octene, and 4-methyl-1-pentene. The copolymers all had the most probable molecular weight and narrow composition distributions. A set of hydrogenated poly(butadienes), random ethyl-branched copolymers, that have very narrow molecular weight and composition distributions were also studied as reference. Only ductile type deformations were studied. Several important generalizations resulted from this work. An important finding was that the molecular weight, copolymer composition, and chemical nature of the co-units have to be considered as independent variables in analyzing the tensile behavior. The nominal stress-strain curves of even modest molecular weights are dominated by strain-hardening. This characteristic is similar to that of very high molecular weight linear polyethylenes. However,the effect is not as large for the ethylene-butenes as compared with the other copolymers. The ultimate properties of the ethylene-butene copolymers also differ from the others. An explanation for these differences can be given by postulating that in the melt structure, and thus in the residual liquid-like region, noncrystalline portions are affected by interaction of the side groups. In contrast, the yield stress and initial modulus do not depend on the chemical nature of the co-unit but only on the crystallinity level. In the regions of overlap there is a substantial difference between the yield stress and initial modulus of Linear polyethylene and the copolymers. Different portions of the force-elongation curves are governed by different structural and molecular features indicating the complexity of the problem. The wide range of experimental data that are presented, for thoroughly characterized samples, can serve as a basis for the molecular understanding of the ductile deformation of random copolymers.