Effects of Mesogenic Cycloaliphatic Units on the Microstructure and Properties of Thermotropic Liquid Crystalline Polyesters

Abstract

We report the synthesis and structure‐property relationship of new thermotropic liquid crystalline polyesters (TLCPs) with different mesogenic cycloaliphatic unit content in the mainchain. For this purpose, a series of TLCPs composed of three different repeating units based on 4‐hydroxybenzoic acid (HBA, 75.0 mol%), 2‐hydroxy‐6‐naphthoic acid (HNA, 25.0–15.0 mol%), and terephthalic acid/1,4‐cyclohexanedimethanol (TPA/CHDM, 0.0–10.0 mol%) were synthesized via two‐step bulk polymerization. The structure, thermal and mechanical properties of the TLCPs were investigated by considering the effects of size and content of TPA/CHDM‐derived repeating units, in comparison to HNA‐based repeating units. Infrared spectroscopic data showed that TPA/CHDM‐derived repeating units were successfully incorporated into HBA/HNA‐based TLCPs. The polarized optical and electron microscopic images revealed the existence of thermotropic liquid crystalline features and associated fibrillar structures for all TLCPs. The melting temperatures of the TLCPs decreased with increasing the TPA/CHDM‐based repeating units, whereas the glass transition temperature increased, which is due to the trade‐off effect of the segmental mobility restriction in the amorphous phase and the destruction of the thermotropic liquid crystalline phase by the introduction of TPA/CHDM‐derived repeating units in the TLCP mainchain. In addition, the maximum elastic storage modulus was attained for the TLCP with 2.5 mol% TPA/CHDM‐derived repeating units.

Effects of Para‐ or Meta‐Aromatic Amide Units on the Microstructure, Thermal, Mechanical, Rheological, and Dielectric Properties of Thermotropic Liquid Crystalline Poly(ester amide)s

“…After the discovery of high‐performance lyotropic liquid crystalline polymers through poly( p ‐phenylene terephthalamide) (Kevlar) and poly( p ‐benzamide), thermotropic liquid aromatic polyesters (TLCPs) have also attracted great attention from industry and academia because of their high thermal stability, high strength/modulus, good chemical resistance, dimensional stability, and easy melt‐processibility. [ 1–10 ] TLCPs form partially ordered and anisotropic liquid crystals even above their melting transition temperatures due to rigid rod‐shaped structural units called mesogenic units. [ 6,11–16 ] Accordingly, TLCPs are applied to super‐engineering thermoplastics, technical fibers, composite materials for automobile parts, etc.…”