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EFFECTS OF DILUTION DURING CROSSLINKING ON STRAIN-INDUCED CRYSTALLIZATION IN CIS -1,4-POLYISOPRENE NETWORKS. I. EXPERIMENTAL RESULTS

Preparing an elastomer by crosslinking polymer chains in solution and then drying the resulting network should have at least two major effects. Specifically, there should be fewer entanglements, and the drying part of the process should compress the chains into a “supercontracted” state. The former effect would be expected to increase crystallization and the latter to reduce it. The present study was conducted to investigate these opposing effects on strain-induced crystallization in cis-1,4-polyisoprene chains that had been crosslinked by peroxide thermolysis in decalin solutions, and subsequently dried. The initiation of crystallization was taken to be the elongation at which an upturn in the modulus became discernible. Higher elongations were required for the networks prepared at higher dilutions, suggesting that with regard to strain-induced crystallization, the compressed states of the chains is more important then their reduced entangling. It is likely that quantitative comparisons are complicated by non-Gaussian effects. In addition to the qualitative insights obtained, the results should be useful for evaluations of the theories of rubberlike elasticity and strain-induced crystallization, as is illustrated in the following paper of this two-part series.