Molecular mobility of ultrasonically devulcanized silica-filled poly(dimethyl siloxane)

We used NMR relaxation and pulsed-gradient diffusion measurements at 70 degreesC in precipitated silica-filled poly(dimethylsiloxane) (PDMS; silicone rubber) after crosslinking, after subsequent devulcanization by intense ultrasound, and after subsequent revulcanization. As in unfilled PDMS, transverse relaxation displays three distinct rate components attributed to an entangled and crosslinked network (similar in T-2), light Sol plus dangling network fragments, and unreactive trace oligomers. Ultrasound produces copious amounts of extractable Sol. The T-2 relaxation times decreased modestly with increasing filler content, but they and the components' proportions correlated mainly with the Sol fraction, that is, the network degradation. In rupturing the network, devulcanization produces large diffusing fragments and dangling ends; revulcanization largely reverses these effects. The rates and amplitudes of the bimodal diffusivity distribution confirmed this conclusion. The weakness of the effects of filler suggests that ultrasound devulcanization is easily adaptable to the recycling of the preponderantly particulate-filled industrial rubbers. (C) 2003 Wiley Periodicals, Inc.