X-ray diffraction study of the smectic A phases of some side-chain polysiloxanes

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Abstract
A series of four mesomorphic side chain polysiloxanes has been studied by X-ray diffraction. They all exhibit smectic A phases around 100–150°C and smectic B or crystalline phases at room temperature. Oriented diffraction patterns in the smectic A phase display up to five orders of reflection on the smectic layers. The projection of the electron density profile along the normal to the layers has been derived by measuring the intensities of the different orders. This profile agrees well with the molecular features as measured from Dreiding stereomodels. The backbones are confined in a layer of width 6 ± 2 Å along the director. In some cases, the backbones are sufficiently confined to present an electron density maximum comparable to that of the mesogenic cores. Then, a period d/2 appears in the electron density profile, and therefore the second order reflection becomes stronger than the first.