Biaxial and uniaxial extension studies of polydimethylsiloxane networks prepared by end‐linking in solution

Abstract

Polydimethylsiloxane chains were tetrafunctionally end-linked with tetraethoxysilane in solution, with either a linear dimethylsiloxane oligomer or hexadecane as solvent. Stress-strain isotherms for the networks thus prepared were obtained in the extracted but unswollen state, both in elongation and in compression (biaxial extension, which was achieved by inflation of sheets of the materials). Measurements were carried out to the rupture points in order to provide as large a range of deformation as possible. Comparison of the experimental and theoretical stress-strain isotherms indicated that the experimental results were in good agreement with the constrained junction theory of rubberlike elasticity. The results showed that the constraint parameter k decreases with decrease in the volume fraction v2c of polymer present during cross-linking and incorporated into the network structure, which means that networks prepared at high dilutions have fewer chain-junction entanglements, as expected. However, the experimental values of the phantom modulus [f] are also a function of the weight fraction of soluble (uncross-linked) polymer which had been present. This confirms the expectation that networks prepared at high dilutions have imperfections such as loops and dangling ends. There is an excellent agreement, however, between experiment and theory with respect to the relationships among k, the Mooney-Rivlin 2C1 estimate of the phantom modulus, and v2c.