Elasticity measurements on bimodal networks in elongation and compression: networks crosslinked in solution and studied unswollen, and networks crosslinked in the undiluted state and studied swollen

Abstract

Polydimethylsiloxane (PDMS) bimodal networks were prepared by tetrafunctionally end-linking very short and relatively long hydroxyl-terminated chains, both in solution and in the bulk state. A constant weight fraction of short chains, 30 wt%, was employed. The networks were studied with regard to their stress-strain isotherms, covering the entire range from high elongations to high compressions. In the first type of experiment, the bimodal networks crosslinked in solution were studied in the unswollen state. The dependence of the elastic properties of the networks on the volume fraction v 2s of polymer chains in the solution being crosslinked was determined. In particular, as v 2s increases, the modulus was found to go through a pronounced maximum with increase in deformation. In the second type, the bimodal networks crosslinked in the undiluted state were studied swollen. Decrease in the volume fraction v 2 of polymer network present during the stress-strain measurements was found to increase the modulus, which is the opposite of that invariably found for unimodal networks. This effect of swelling on the elastic properties of bimodal networks confirmed that the self-reinforcement they exhibit is due to the limited extensibility of short chains.