High frequency ultrasonic studies of poly(dimethylsiloxane) networks

Abstract

Longitudinal ultrasonic measurements over a wide frequency range (5 to 650 MHz) are reported for poly(dimethylsiloxane)(PDMS) networks. Sound velocity data show no dependence on the crosslink density at these frequencies.

The sound absorption decreases slightly as the crosslink density increases in a linear manner; this effect is more apparent at the lower frequencies studied. Absorption measurements carried out at a frequency of 300 MHz and in the temperature range from 173 to 300 K show no shift of the α-relaxation due to the introduction of crosslinks when compared with PDMS gum.

The results are discussed in terms of activation energies derived from the transition map which is plotted together with data from the literature for liquids and networks, since the position of the α-relaxation appears to be crosslink density independent in the studied frequency ranges. The most widely used equations expressing the dependence of activation energy on temperature are applied and a new equation is proposed based on the experimental data.

A second peak in sound absorption at 224 K is rather broad and slightly shifted in comparison to previous observation. This peak is attributable mainly to crystallization and secondly to the helix transformation.