Elastomers having high functionality cross links viewed as bimodal networks. An alternative interpretation in terms of bimodal chain‐length distributions

Abstract

AbstractSeveral groups have now prepared poly(dimethylsiloxane) networks of high cross‐link functionality by end‐linking vinyl‐terminated chains by means of SiH groups in siloxane oligomers (CH3)3SiO[SiHCH3O]xSi(CH3)3. The elongation moduli of these networks were generally found to be considerably larger than the values predicted from the functionality and number density of the cross links (based on the stoichiometry of the end‐linking reaction). Not all the SiH groups in an oligomer are used in the end‐linking reaction, however, and the segments between cross‐links can themselves act as short network chains. The connectivity of these short chains to the long ones, in what is essentially a bimodal distribution, has been neglected in analyses to date. They are taken into account in the present analysis, giving much better agreement between experiment and theory. The stress‐strain behavior for such very short chains can be characterized by the use of Monte Carlo methods and the Fixman‐Alben non‐Gaussian distribution. This alternative analysis seems useful in reproducing the experimental observations, but further experimental and theoretical will be required to remove some remaining ambiguities. © 1995 John Wiley & Sons, Inc.