NMR relaxation and pulsed gradient NMR diffusion measurements of ultrasonically devulcanized poly(dimethylsiloxane)

We report ¹H NMR transverse relaxation and pulsed-gradient spin-echo self-diffusion measurements at 70 °C in poly(dimethylsiloxane) (PDMS) rubber before and after cross-linking, and after subsequent devulcanization by intense ultrasound. The effort Serves the purpose of characterizing at a molecular level this novel method of rubber recycling. Relaxation spectra extracted from the transverse magnetization decays display three distinct components, which are further refined by a direct fit. The components are attributed to entangled and cross-linked networks; light sol and dangling network fragments; and oligomers including 4% of an unreactive trace. Ultrasound produces extractable sol strongly dependent on feed rate and transducer amplitude. Our results vary monotonically with the amount of sol: all three mobilities (T₂) and the amounts of the two most mobile fractions increase with sol content, but the diffusion rates decrease slightly, due to the production of higher molecular weight sol. Data from our earlier NMR studies of ultrasound devulcanization of styrene-butadiene and natural rubber shows strong similarities with the present results. The main difference appears to be a greater extent of loosely attached network fragments in PDMS.