Semiflexible Thermotropic Liquid-Crystalline Polymers: The Characterization of a Polyester-Polyether

Abstract

ABSTRACT:The liquid crystal behavior of the thermotropic polyester derived from (4,4'-terephthaloyldioxy)dibenzoic acid and tetraethylene glycol was studied by differential scanning calorimetry, polarizing microscopy, and X-ray diffraction. The polymer sample exhibits the phase sequence isotropic-nematic-smectic A-smectic E-crystal, with decreasing temperature. Attention was devoted to the optical textures of the mesophases, as compared to those of low molar mass mesogens, and unusual high strength singularities were detected in the schlieren texture of the nematic phase. KEY WORDSLiquid-Crystalline Polymers/ Thermotropic Polyesters/ Liquid-Crystalline Polymorphism/ Smectic Mesophases / Schlieren Textures/ glycol ethers of different lengths:The thermotropic liquid-crystalline behavior of main chain polymers comprising alternating mesogenic units and flexible spacers is well established and attracts ever increasing interest. The great majority of such polymers presents only one mesophase, typically nematic ( or cholesteric ), but smectic phases have also been observed, including ordered smectic B 1 , E 2 , G 3 or H 4 phases. A few examples are known of mesophase polymorphism5, this feature being in marked contrast with what is most commonly observed in low molecular weight mesogens.In a previous paper 6 some of us reported on the synthesis and characterization of the thermotropic properties of a class of polyesters derived from 4,4' -(terephthaloyldioxy)dibenzoic acid (HTH) and ethylene (or propylene) Polymer samples having n = 2, 3, or 4 displayed two different enantiotropic liquidcrystalline phases, the lower temperature mesophase being a provisionally unidentified smectic phase and the higher temperature mesophase being recognized as nematic. 6 In the same context, unusual textural behaviors were observed in such polymers, especially at the mesophase-mesophase transition. All these features have stimulated further investigation and accordingly the polyester sample of that class with n = 4, here designated as HTH/TEG, was freshly prepared from HTH diacid chloride and tetraethylene glycol (TEG) and t To whom correspondence should be addressed.

Chiral liquid crystal polymers. 10. Thermotropic mesomorphism in chiral isomeric polyesters

“…The isotropization enthalpies (Af/j = 0.6-2.0 kJ/mol ru) and entropies (ASi = 1.2-4.2 J/(mol ru K)) are low and indicate that the high-temperature mesophase is nematic (or cholesteric, N*) in character. 22,26 The above transitions are reversibly observed on cooling from the isotropic melt with a limited degree of supercooling (5-15 °C), which is greater for the onset of the ordered smectic phases. Such transitions extend over rather broad ranges of temperature, which results in biphasic regions of 10-30 °C, The mesophase-crystal transition is very supercooled in sample 4 (Tc = 65 °C), but crystallization does not take place in the other polymers and the mesophase can be frozen-in in the solid state.…”

Practical considerations in the study of main-chain thermotropic liquid-crystalline polymers by vibrational microscopy

“…In order to illustrate several points, the thermotropic polyester poly(oxytetraethyleneoxide-oxycarbonyl-1,4-phenylene-oxyterephthaloyl-1,4-phenylcarbonyl), P4EGTOB, where the spacer unit, R (scheme 1) is -(CH 2 -CH 2 -O) 4 -, is considered because of its highly complex transition behaviour [43,56], as can be observed in the thermograms given in figure 2 in reference 43 for a series of heating and cooling cycles. This data combined with results from TOA and WAXD gives the approximate transitional diagram shown in scheme 2:…”