Molecular motions in crystalline and mesomorphic forms of poly(diethylsiloxane)

Molecular motions in low molar mass (46200 g mol^—1) poly(diethylsiloxane) (PDES) are studied by ²H NMR. From DSC measurements the existence of the crystalline phases α₁, β₁ and α₂, β₂, as well as a pronounced mesogenic phase α_m, is clearly established. Based upon calculations of averaged field gradient tensors based on cone models a detailed motional analysis in the different phases has been conducted. At low temperatures the only motion present besides methyl rotation are librations of the ethyl groups with amplitudes below 25°. The motion in the conformational disordered phase is characterized by conformational changes within the side chains, accompanied by occasional rotations through angles larger than 70° within the backbone. The flexibility of the ethyl groups, however, is limited by steric hindrance, prohibiting a full rotation. This situation is observed also in the meso phase where there is an additional rotational degree of freedom around the main chain axis.