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 phasesβ1 andβ2, a mesophaseαm, 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 phaseβ2 its anisotropy is reduced by 25% compared with the low temperature phaseβ1 due to the onset of oscillations around the chain axis and conformational transitions. In the mesophaseαm the polymer chain rotates about its long axis yielding an axially symmetric chemical shift tensor opposite in sign to that in theβ1,β2 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.