SOME DYNAMIC MECHANICAL-PROPERTIES OF UNIMODAL AND BIMODAL NETWORKS OF POLY(DIMETHYLSILOXANE)

Elastomeric networks of polydimethylsiloxane prepared by end-linking chains having molecular weights in the range 18,500 to 220 g mol-1 were studied from -128 to 50-degrees-C using a Rheovibron DDV III Viscoelastometer. In the case of the unimodal networks, the glass transition temperature T(g) was generally insensitive to degree of cross-linking. The intensity of the tan delta relaxation, however, increased by over an order of magnitude over the range of cross-link densities investigated. Bimodal networks prepared from mixtures of relatively long and very short PDMS chains also have values of T(g) which were insensitive to degree of cross-linking. Finally, as expected, the intensities of the tan delta peak for the biomodal networks could not be explained on the basis of simple additivity of contributions from the relatively long and the very short network chains.