Structural, Dielectric, and Rheological Characterization of a Thermotropic Polyester Displaying Smectic A, Nematic, and Isotropic Phases

We report structural, dielectric, and rheological properties of a thermotropic liquid-crystalline polyester, which exhibits a crystal-crystal transition at 137 °C and crystal-to-smectic A, smectic A-to-nematic, and nematic-to-isotropic transitions at 164, 180, and 220 °C, respectively; initial phase identifications were made by using optical microscopy techniques and confirmed by X-ray diffraction measurements. The dielectric results are highly sensitive to phase state, particularly at the lower frequencies investigated, where interfacial polarization effects dominate due to the broad biphasic regions present in our samples. Higher frequency measurements indicate a sizable increase in dipolar (rotational) mobility at the crystal-crystal transition, no change in the dielectric constant at the crystal-to-smectic A transition, and a decrease in e’ at the onset of the nematic phase. In contrast to the dielectric results, dynamic rheological measurements are insensitive to the crystal-crystal transition, but large, nearly stepwise decreases in complex viscosity are observed across both the crystal-to-smectic A and smectic A-to-nematic transitions, with a slight increase encountered at the onset of isotropy. Constant-temperature frequency sweeps recorded for the smectic A phase indicate a persistent shear thinning behavior, with the storage modulus (C) greater than the loss modulus (G”) over the investigated frequency range. Steady shear viscosity measurements obtained during cooling are consistent with the dynamic results and indicate that the layering of the smectic A phase is apparently not disrupted by steady shearing at low rates. A lack of reversibility is observed for the rheological measurements carried out from 230 °C relative to those initiated at 190 °C; this may be the result of transesterification reactions occurring at the higher temperature.