Structures of side chain liquid crystalline poly(silylenemethylene)s

The structures of side chain liquid crystalline poly(silylenemethylene)s (-(SiCH₃R-CH₂)-: R = O(CH₂)_NO-Ph-Ph, Ph = phenyl) (PSM-N; N = 3, 6, 8, 11) have been studied by X-ray diffraction, differential scanning calorimetry (DSC), polarized optical microscopy, and transmission electron microscopy (TEM). DSC study showed that PSM-Ns have multiple transitions that are dependent on the side chain length. All PSM-Ns studied show a series of sharp equatorial reflections in the X-ray fiber pattern at room temperature, a characteristic of well-ordered smectic phases. Layered smectic structures of PSM-Ns persist up to the isotropic temperature during heating, although the specific kinds of smectic phases changed. In each, the layer thickness corresponds to the single side chain length and increases with slope of ∼1.3 Å per methylene spacer. This is a single-layer structure. For PSM-3 and PSM-6, the mesogens pack in a two-dimensional orthorhombic cell, a characteristic of a smectic E (S_E) phase, with dimensions a = 8.14 Å and b = 5.14 Å. The S_E phase of PSM-3 and PSM-6 converts directly to the isotropic phase. For PSM-8 and PSM-11, the mesogens pack in a hexagonal smectic B (S_B) cell at room temperature. This is manifest from the hexagonal six-point arrangement of reflections at d = 4.4 Å in the X-ray fiber pattern. During room temperature annealing, PSM-8 and PSM-11 crystallize into orthorhombic cells with parameters a = 8.80 Å, b = 5.08 Å, c = 50.8 Å for PSM-8 and a = 8.90 Å, b = 5.14 Å, c = 58.6 Å for PSM-11. Upon heating, the crystalline structures change to smectic A (S_A) before becoming isotropic. Micrographs of PSM-11 show a layered structure with thickness matching that observed in X-ray pattern.