Mesomorphism, molecular structure and dynamics of polydiethylsiloxane

Abstract

The correlation between the phase behaviour of polydiethylsiloxane (PDES) and conformational and motional changes at the various disordering transitions has been investigated by nuclear magnetic resonance (n.m.r.), dielectric relaxation and shear experiments. Diffusive rotation of the chain segments around the long axis of the molecules is indicated by 29Si n.m.r. below the isotropization transition. The remarkably fluid character is explained in part by the coexistence of anisotropic and isotropic motional states of the -OSiEt 2- segments, which indicate microscopic domain formation. The molecular motion in the α 2- and β 2-phases is restricted but still fast with respect to the 29Si n.m.r. timescale. Transmission electron micrographs show, besides chain-folded lamellae, also extended-chain lamellae. These differences in the morphologies can explain why the interconversion of α 2-PDES into the thermodynamically more stable β 2-polymorph is slow, in spite of the pronounced mobility of the polymer segments. Long-range reorganization processes have to be considered to allow the morphological changes observed by electron microscopy.