Deformation of Elastomeric Ethylene−Octene Copolymers

The elastomeric behavior of low-crystallinity ethylene−octene copolymers prepared by Dow's INSITE constrained geometry catalyst technology is described. Deformation in uniaxial tension was examined as a function of comonomer content and molecular weight. Within the melting range of copolymers, temperature was used as an experimental variable to reveal the relationship between crystallinity and stress response. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions provided the structural basis for analysis of the elastic behavior. The rubber modulus scaled with crystallinity. Furthermore, the dimension of the fringed micellar junction obtained from the modulus correlated well with the average crystallizable sequence length of the copolymer. Because classical rubber theory could not account for the large strain dependence of the modulus, a theory which incorporates the contribution of entanglements to the network response was considered. Slip-link theory described the entire stress−strain curve. The slip-link density correlated with crystallinity; the cross-link density did not depend on crystallinity and appeared to represent a permanent network. The latter was further revealed by the effect of molecular weight on the stress−strain behavior. It is proposed that lateral attachment and detachment of crystallizable chain segments at the crystal edges provide the sliding topological constraint attributed to slip-links, and entanglements that tighten into rigid knots upon stretching function as permanent network junctions.