Dependence of the Physical Properties and Molecular Dynamics of Thermotropic Liquid Crystalline Copolyesters on p-Hydroxybenzoic Acid Content.

Gi Tae Park,Jin-Hae Chang,Won Jun Lee,Ae Ran Lim

doi:10.3390/polym12010198

Gi Tae Park, Jin-Hae Chang + Show 2 more

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https://doi.org/10.3390/polym12010198

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Abstract

Two series of thermotropic liquid crystal copolymers (TLCPs) with different monomer structures and compositions were synthesized. The copolymers in the first series consisted of 2,5-diethoxyterephthalic acid (ETA), hydroquinone (HQ), and p-hydroxybenzoic acid (HBA), whereas those in the second series contained ETA, 2,7-dihydroxynaphthalene (DHN), and HBA. In both series, the molar ratio of HBA to the other monomers varied from 0 to 5. The thermal properties, degree of crystallinity, and stability of the liquid crystalline mesophase of the copolymers obtained at each HBA ratio were evaluated and compared. Overall, at each HBA content, the DHN-containing copolymer had better thermal properties, but the HQ-containing copolymer exhibited a higher degree of crystallinity and a more stable liquid crystalline mesophase. Furthermore, similar thermal stabilities were observed in both series. The dependence of the molecular dynamics of the TLCPs on the monomer structure was explained using 13C magic-angle spinning/cross-polarization nuclear magnetic resonance spectroscopy. An in-depth investigation of the relaxation time of each carbon revealed that the molecular motions of the TLCPs were greatly influenced by the structures of the monomers present in the main chain. The molecular dynamics of the HQ and DHN monomers in the two series were evaluated and compared.

Highlights

Thermotropic liquid crystal polymers (TLCPs), which contain a special monomer chemical structure, are already widely used as high-performance commercial engineering polymers owing to their good thermal properties, high strength, high modulus, low viscosity, and other excellent mechanical properties [1,2]
The results of the 2nd heating were used to obtain thermal properties (Tg, Tm, and Ti ) using differential scanning calorimetry (DSC), and the scanning temperature ranges were determined in advance using thermogravimetric analysis (TGA) to prevent thermal decomposition during scanning
The thermal properties of the two TLCP series are summarized in TLCP-I series, the copolymers were synthesized by varying the hydroxybenzoic acid (HBA) molar ratio between 0 and 5

Summary

Introduction

Thermotropic liquid crystal polymers (TLCPs), which contain a special monomer chemical structure, are already widely used as high-performance commercial engineering polymers owing to their good thermal properties, high strength, high modulus, low viscosity, and other excellent mechanical properties [1,2]. Interest in TLCPs and their composites has been increasing, to achieve enhanced physical properties, most studies have focused on rigid rod-type TLCPs with completely aromatic monomers in the main chain [8,9]. In almost all TLCPs, the thermo-mechanical properties have been enhanced by using straight, rigid rod-shaped monomer units, including terephthalic acid (TPA), hydroquinone (HQ), 4,40 -biphenol (BP), p-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), naphthalenediol. Rigid rod-type TLCPs exhibit excellent thermal and mechanical properties, their high melting points generally make processing difficult [6,14]. These TLCPs show low solubilities in common solvents. Many studies have investigated the introduction of substituents, flexible alkyl groups, or side-group TLCPs [15,16,17]

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