Structural, dielectric, and rheological characterization of a thermotropic polyester displaying smectic A, nematic, and isotropic phases

Abstract

We report structural, dielectric, and rheological properties of a thermotropic liquidcrystalline 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 (G') 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.

Correlation of the thermal properties of main-chain liquid crystalline polymers with the symmetry and anisometry of their mesogens

“…We selected the polymers with oligooxyethylene spacers and arylate long Synthesis, thermal properties and mesomorphic behaviour of polymers 1 a -e were studied in detail [1][2][3][4][5][6]22) . Kalika et al studied dynamomechanical properties of the polymer with diethylene glycol spacer 4) . The intrinsic viscosity of the sample was 0.12 dL/g.…”

“…LCP with a terephthaloyl bis(4-oxybenzoate) (TOB) mesogenic group and spacers of different length and chemical nature were under investigation by several research teams since the early 80th [1][2][3][4][5] . The most interesting feature of LCP carrying this and similar mesogenic groups is their increased smectogeneity which is displayed not by polymers only, but also by low molecular weight liquid crystals with the same mesogenic groups 6) .…”

Viscoelasticity of main chain liquid crystalline elastomers

“…Of course, phase shift evaluation was also affected. Anyway, we could not get acceptable data superposition in the whole temperature range, whatever the reference temperature chosen, so the data sets corresponding to each phase have been examined separately: the time-temperature superposition does not hold across the smectic-isotropic transition, as it was already reported in the case of SCLCPs, SCLCEs and a slightly crosslinked main-chain smectic polymer [5,9,12,19,29,32,33]. The phase shift increases from w48 to about 208 throughout the frequency range under examination.…”

Rheological properties in blends of poly(aryl ether ether ketone) and liquid crystalline poly(aryl ether ketone)

“…Provided that they are blended with the common polymers, there will certainly be a rapid reduction in mechanical properties of PAEKs. Compared with the conventional polymers, the thermotropic liquid crystalline polymers (TLCP) possess the unique structural properties, such as the rigid rod nature of TLCP molecules, molecule anisotropy, and low viscosity 6–9. By blending with TLCP, which can act as processing aids to reduce blend viscosity and reinforcing agents, both improvements of processing property and reinforcement of mechanical strength of poly(aryl ether ether ketone) (PEEK) can be achieved.…”