Abstract:
Thermotropic liquid crystal copolyesters (TLCPs) with high melting point were synthesized by acetylation and melt polycondensation using p-hydroxybenzoic acid (HBA), terephthalic acid (PTA), and hydroquinone (HQ) as monomers. HBA and HQ were converted into 4-acetyloxybenzoic acid (ABA) and 1,4-diacetoxybenzoic (AHQ) respectively by acetylation to improve the reactivity. Zinc acetate was selected as the catalyst, and the mole ratio of monomers was
n(ABA)∶
n(PTA)∶
n(AHQ)=60∶20∶20. The ternary copolyester shows excellent thermal stability and high melting point of 407 ℃. Furthermore, a series of new TLCPs (HBA/PTA/SuA/HQ) containing flexible segments were synthesized by incorporating the long-chain aliphatic 1,8-suberic acid (1,8-SuA), partially replacing PTA. The structure of the modified copolyesters were characterized by Fourier transform infrared spectrometer (FT-IR) and carbon-13 nuclear magnetic resonance (
13C-NMR). The thermal properties and the liquid crystal texture of the copolyesters were analyzed using differential scanning calorimetry (DSC), thermal gravimetric analyzer (TGA), polarized optical microscope (POM) and X-ray diffraction (XRD). The results show that the melting point of the HBA/PTA/SuA/HQ copolyester decreases from 407 ℃ to 214 ℃ with the increasing of the 1,8-SuA content, but the crystallinity increases from 25.2% to 32.2%. The synthesized TLCPs show good thermal stability with the maximum decomposition temperature over 428 ℃. The decomposition temperature of the copolyesters decreases with the 1,8-SuA increases accordingly, and the char yield is between 22.1% and 31.3%, slightly lower than the initial ternary copolyester. All the synthesized copolyesters exhibit the similar textures of nematic liquid crystals. The results indicate that the combination property of the new copolyesters is little influenced by the introduction of long-chain aliphatic 1,8-SuA but the processability can be significantly improved.