Strength Enhancement From Heterogeneous Networks

At the typical crosslink densities of elastomers, the failure properties vary inversely with mechanical stiffness, so that compounding entails a compromise between stiffness and strength. Our approach to circumvent this conventional limitation is by forming networks of two polymers: (i) that are thermodynamically miscible, so that the chemical composition is uniform on the nm level; and (ii) have markedly different reactivities for network formation. The resulting elastomer consists of one highly crosslinked component and one that is lightly or uncrosslinked. This disparity in crosslinking causes their respective contributions to the network mechanical response to differ diametrically.

            Earlier results showed some success with this approach for blends of natural rubber and 1,2-polybutadiene and butyl rubber with polyisobutylene. We present new results on random copolymers of ethylene and propylene (EPR) with the corresponding unsaturated terpolymer (EPDM). The sulfur-vulcanized blends exhibit higher strength than the corresponding pure EPDM networks, when compared at equal modulus. Some preliminary work will also be described on the use of radiation crosslinking to achieve miscible blend networks with heterogeneous crosslinking.