Thermotropic homopolyesters: 5. Investigation of the smectic phase of polyesters based on p, p′-bibenzoic acid

Abstract

We report a study of the mesophases of the homologous series of polymers prepared from p, p′-bibenzoic acid and the tetra-, hexa-, octa-, and decamethylene glycols. These polymers are designated BB n, where n is the number of methylene units in the diol. The BB4 and BB6 polymers form a thermotropic smectic phase which was independently identified as S A by mutual miscibility studies with a known low molecular weight mesogen, TBBA, and by X-ray diffraction studies of oriented samples. The enthalpy change for the smectic-isotropic transition is only 1.4 Kcal mol −1 a value consistent with a smectic phase of low order. The smectic layer spacing is shorter than the fully extended length of the repeating unit for both BB4 and BB6. Low molecular weight samples of BB6 exhibit three crystal modifications. The two higher temperature forms have the same c-axis spacing as the smectic layer spacing, 18.7 Å, while the polymorph stable at room temperature has a fibre repeat of 32 Å, indicating a still smaller distance per repeating unit, 16 Å. All of these values are smaller than the 20 Å length of the fully extended repeating unit. Shearing the smectic hase of BB6 polymers having η inh < 0.45 dL/g produces a highly oriented sample in which the repeating units are perpendicular to the direction of flow. Shear of the smectic phases of polymers having higher intrinsic viscosities breaks up the smectic structure, and leads to less perfect orientation of the molecules parallel to the shear direction.