The effect of swelling on the stress-strain isotherms and ultimate properties of polydimethylsiloxane networks in elongation

Abstract

Gamma radiation was used to prepare three cross-linked, unfilled samples of highly elastomeric polydimethylsiloxane. Portions of each sample were studied in elongation to their rupture points, at 30 °C, in both the unswollen state and swollen with low molecular weight dimethylsiloxane fluid. Values of the volume fraction ν2 of polymer in the networks ranged from 1.00 to 0.40. None of the stress-strain isotherms obtained showed any upturn in the reduced force or modulus [f*] at high elongation, an observation in agreement with the conclusion that such increases in [f*], when observed, are due to strain-induced crystallization. All of the isotherms were well represented by the semi-empirical equation [f*]=2C1+2C2α−1, whereα is the elongation, and 2C1 and 2C2 are the Mooney-Rivlin constants. Values of 2C2, which serves as a measure of the extent to which [f*] varies with elongation, showed a decrease with decrease in ν2, and generally also with decrease in degree of cross-linking. The ultimate properties reported were the valuesλr and [f*], of the total elongation and the reduced force, respectively, at the rupture point. Decrease in degree of cross-linking causes a significant increase inλr and a significant decrease in [f*]r; decrease in ν2, however, has only a relatively small effect onλr and [f*]r.