Thermal Expansion and Viscoelasticity of Rubber in Relation to Crosslinking and Molecular Packing

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Abstract
Effects of crosslinking on specific volume, thermal expansion coefficient, glass transition temperature (Tg) and viscoelasticity of rubber have been studied. Materials were prepared by heating purified natural rubber with varying amounts of cumyl peroxide. This procedure formed networks by intermolecular carbon-to-carbon bonding, and an approximately sixty-fold range of crosslink density was obtained. Crosslink density could be estimated with reasonable confidence up to about 1020/g. At this level the effects observed were, approximately, a one per cent decrease in specific volume; 23 per cent and 6 per cent decreases in the thermal expansion coefficients respectively below and above Tg; 5° C increase in Tg; and a displacement of the viscoelastic response by +5° C. These changes are accounted for in terms of the reduction in free volume consequent on crosslinking. There was also a qualitative change in behavior, the thermal expansion and viscoelastic transitional regions widening as crosslinking increased. The thermal expansion behavior is explained in terms of a linear increase in the variance of monomeric free volume with crosslinking up to 1020/g. At higher densities crosslinks are so close that their packing effects interact and the nature of the phenomenon changes.

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