Structure and Phase Transitions of Poly(diethylsiloxane)

Phase transitions and morphology of poly(diethylsiloxane) have been studied with help of differential scanning calorimetry and transmission electron microscopy on carbon−platinum replicas of freeze-fractured samples. The TEM experiments showed distinct lamellae in the case of the β and γ crystalline polymorphs, whose thicknesses correlated well with the calculated chain lengths, demonstrating an extended chain conformation for both polymorphs. A low heating rate dependence of the β → μ transition temperature in DSC and a linear relationship between isotropization temperature and reciprocal molecular weight indicated the existence of extended chain lamellae for the mesophase as well. The α crystalline polymorph displayed a greater softness than β- and γ-PDES, and no lamellar structures could be observed. DSC experiments on bimodal mixtures of PDES samples with strongly different molecular weights demonstrated molecular weight dependent phase separation in the case of β-PDES and the occurrence of mixed lamellae for the mesophase obtained from α-PDES. For the latter, a less ordered packing with chain ends incorporated into the lamellae was proposed. Upon heating γ-PDES no mesophase was formed, which was explained by surface energy effects. The critical molecular weight for mesophase formation was found to be M n = 28 000 g/mol.