Proton NMR study of molecular motion in polydimethylsiloxane

Proton spin–lattice relaxation times T1 were measured for two samples of polydimethylsiloxane (PDMS), one with weight‐average molecular weight Mw = 77,400 and the other with Mw = 609,000. Two T1 minima and a T1 discontinuity were observed for each compound. The high‐temperature T1 minima were attributed to a stretching and flexing motion of the PDMS chain. Quantitative comparison of the relaxation data with a theoretical model developed for this motion allowed the activation energy, 2.3 kcal/mole, and the maximum angular displacement of the methyl group symmetry axis to be determined. The latter was found to be 31°, independent of sample molecular weight. The low‐temperature minima were ascribed to methyl reorientation with an activation energy of 1.6 kcal/mole. The T1 discontinuities were attributed to melting and allowed the degree of crystallinity to be estimated.