Model networks of end-linked polydimethylsiloxane chains. IV. Elastomeric properties of the tetrafunctional networks prepared at different degrees of dilution

Abstract

Highly elastomeric ’’model’’ networks were prepared by end-linking polydimethylsiloxane chains of known molecular weight, using a tetrafunctional orthosilicate. The reactions were carried out in both the undiluted state and in solution, with n-dodecane, cyclic dimethylsiloxane oligomer, and linear dimethylsiloxane oligomer as diluents. The networks thus prepared were studied with regard to their stress–strain isotherms in the unswollen state, in elongation at 25° C, and with regard to their equilibrium swelling in benzene at room temperature. The results obtained did not depend significantly on the amount of diluent present during the preparation of the networks, which indicates that the end-linking reaction employed must be very nearly complete even in the undiluted state. Values of the modulus in the reaction employed must be very nearly complete even in the undiluted state. Values of the modulus in the limits of large and small elongation were obtained from the stress–strain isotherms and were found to be in satisfactory agreement with theoretical predictions for nonaffine and affine deformations, respectively. The results from the swelling equilibrium measurements were found to fall between these two extremes. They are in excellent agreement, however, with the results of a very recent theory of network swelling in which the degree of nonaffiness depends on both the network structure and the equilibrium degree of swelling. The relatively good agreement between experiment and theory suggests that interchain entanglements do not make a large contribution to the equilibrium elastomeric properties of a polymer network.