STRUCTURE AND MECHANICAL PROPERTIES OF BIMODAL PDMS NETWORKS IN THE SWOLLEN STATE

Abstract

We have investigated mechanical properties of bimodal networks, which have a two-peak distribution of the length of the network chains between crosslinks, in equilibrium swollen state. The bimodal networks were prepared by end-linking mixtures of short and long poly (dimethylsiloxane) (PDMS) chains with tetra-functional crosslinker. The ratio of molecular mass of the short and long PDMS chains was ca. 11. The stress-strain relationships have been investigated as a function of molar fraction of short chains. The networks with more than 98mol% short chains are brittle and they have high elastic moduli. When the molar fractions of short chains fall below 95mol%, the networks become markedly extensible. The stress-strain relations of unimodal networks and bimodal network with small fraction of short chains obey the prediction of the classical theory of rubber elasticity, while those of other bimodal networks deviate from the theoretical prediction. The dependence of network structure on the composition of precursor chains has been estimated from the analysis of the stress-strain behavior. The structure model proposed in small angle X-ray scattering (SAXS) study on bimodal networks has been re-examined on the basis of the results of mechanical experiments in this study.