The Effects of Network Imperfections on the Small-Strain Moduli of Polydimethylsiloxane Elastomers Having High Functionality Cross Links

Abstract

Poly(dimethylsiloxane) networks of high cross-link functionality have been prepared by end linking vinyl-terminated chains with multifunctional poly(methylhydrosiloxane) chains. They covered a wide range in the extent of reaction, P_vi , of the vinyl end groups. At small strains, these networks had elongation moduli that significantly exceeded the values predicted by the Flory-Erman theory. Neglected in such standard analyses, however, is the fact that the segments between cross links along the junction precursor molecule can themselves act as short network chains, contributing to the modulus and giving a strongly bimodal distribution of both network chain lengths and cross-link functionalities. As would be expected, an unmistakable transition is observed in values of the shear modulus G toward the phantom limit of deformation as the crosslink density increases. Calculations based on recognition of such short chains give results in much better agreement with experiment. The results so obtained showed strong dependence of the elastomeric properties on the extents of reaction and the inherent network imperfections. Such imperfections have a pronounced effect on the equilibrium modulus, more specifically on the empirical constant 2C ₂. The dependence of 2C ₂ on the volume fraction of the elastically “effective” chains is thus established. Moreover, the results unambiguously demonstrate that the empirical constant 2C ₂ is essentially a topological contribution and contains no contributions from the chemical network.