TENSILE PROPERTIES OF CRYSTALLINE POLYMERS - RANDOM COPOLYMERS OF ETHYLENE

Force-elongation curves of a set of random ethylene-1-alkene copolymer s have been studied. The comonomers included 1-butene, 1-hexene, 1-oct ene, and 4-methyl-1-pentene. The copolymers all had the most probable molecular weight and narrow composition distributions. A set of hydrog enated poly(butadienes), random ethyl-branched copolymers, that have v ery narrow molecular weight and composition distributions were also st udied as reference. Only ductile type deformations were studied. Sever al important generalizations resulted from this work. An important fin ding was that the molecular weight, copolymer composition, and chemica l nature of the co-units have to be considered as independent variable s in analyzing the tensile behavior. The nominal stress-strain curves of even modest molecular weights are dominated by strain-hardening. Th is characteristic is similar to that of very high molecular weight lin ear polyethylenes. However,the effect is not as large for the ethylene -butenes as compared with the other copolymers. The ultimate propertie s of the ethylene-butene copolymers also differ from the others. An ex planation for these differences can be given by postulating that in th e melt structure, and thus in the residual liquid-like region, noncrys talline portions are affected by interaction of the side groups. In co ntrast, the yield stress and initial modulus do not depend on the chem ical nature of the co-unit but only on the crystallinity level. In the regions of overlap there is a substantial difference between the yiel d stress and initial modulus of Linear polyethylene and the copolymers . Different portions of the force-elongation curves are governed by di fferent 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 mol ecular understanding of the ductile deformation of random copolymers.