EFFECT OF CHAIN-LENGTH DISTRIBUTION ON ELASTOMERIC PROPERTIES .1. COMPARISONS BETWEEN RANDOM AND HIGHLY NON-RANDOM NETWORKS

Two dimethylsiloxane polymers were prepared, both with 0.57 mol % methylvinylsiloxane units. In the first polymer, the potentially reactive vinyl groups were randomly distributed, while in the second they were segregated at the two ends of the chains. Networks were prepared from both polymers, and from various mixtures thereof, using γ-radiation and benzoyl peroxide (which are not selective for vinyl groups), and a silicon hydride cross-linking agent (which is highly selective). They were characterized in terms of their stress-strain isotherms in elongation and their degree of equilibrium swelling in benzene. The networks prepared by selectively cross-linking through the segregated vinyl sites were found to have relatively large values of the molecular weight Mc between cross-links, thus indicating that very short chains have significantly reduced contributions to the elastomeric properties of a network. In effect, two or more closely spaced cross-links act as a single cross-link of higher functionality. The ratio Mc(f)/Mciv2m) of the values of Mc from the stress-strain isotherm and degree of equilibrium swelling was generally found to be close to the expected value of unity. The values of the ratio for the very nonrandom networks suggest, however, that the very short chains contribute even less to the swelling response than they do to the modulus in elongation. © 1979, American Chemical Society. All rights reserved.