Orientation distribution of liquid‐crystalline polyester sheets studied by polarized infrared spectroscopy

Polarized infrared spectroscopy was applied to evaluate the orientation distribution in extrusion‐molded sheets of a liquid‐crystalline copolyester consisting of 73 mol % of 4‐hydroxy benzoic acid units and 27 mol % of 2‐hydroxy‐6‐naphthoic acid units. The surface orientation function was evaluated from the polarized attenuated total reflection/Fourier transform infrared spectra and the polarized specular reflection spectra. In the case of the specular reflection method, the absorption and refractive indices were obtained from the specular reflectance using the Kramers–Kronig relation and the Fresnel equation. On the other hand, the microscopic orientation functions inside the sheets were evaluated by Fourier‐transform infrared microspectroscopy. The polarized FTIR microspectra of microtomed section of sheets were measured in a microscopic domain as small as 40 μ at various positions from the center of the sheet. The surface orientation function was shown to be higher than the interior orientation function. Orientation functions obtained by the spectroscopic techniques are lower than crystal orientation functions determined by wide‐angle x‐ray diffraction, implying that the crystal orientation function is higher than the orientation function of noncrystalline molecular chains. The orientation distribution in the sheet is discussed in relation to the morphological structures studied by scanning electron microscopy.