Molecular motions in poly(diethyl siloxane) studied by solid-state²⁹Si NMR

Abstract

Molecular motions in poly(diethyl siloxane) were studied by solid-state²⁹Si-NMR in the temperature range 180–350 K. In this temperature range two solid phasesβ ₁ andβ ₂, a mesophaseα _m, and an amorphous isotropic phase exist. The nature of the chain mobility in the different phases was deduced from the resulting changes in the NMR line-shape governed by anisotropic chemical shift. In the intermediate solid phaseβ ₂ its anisotropy is reduced by 25% compared with the low temperature phaseβ ₁ due to the onset of oscillations around the chain axis and conformational transitions. In the mesophaseα _m the polymer chain rotates about its long axis yielding an axially symmetric chemical shift tensor opposite in sign to that in theβ ₁,β ₂ phases. The broad transition of the mesophase into the isotropic phase is accompanied by an increase in a narrow Lorentzian line arising from the amorphous phase. The results are compared with previous¹H NMR, Raman-spectroscopy and x-ray measurements.