Synthesis And Characterization Of Aryl Ethynyl Terminated Liquid Crystal Aromatic Polyester

Liquid crystalline polyarylates are widely used in aerospace,national defense and military industries and other cutting-edge materials fields due to their excellent mechanical properties and chemical resistance.Most existing Liquid crystalline polyarylates are thermoplastic polymers,with high molecular weight and fully aromatic backbone structure,endowing them with high melting point and melt viscosity.Thermosetting liquid crystal polyarylate(LCT)is synthesized by introducing reactive end-capping groups into the backbone of liquid crystalline polyarylates,which effectively improves the pocessibility and remains the orientation after curing.Therefore,LCT shows great potential in high-performance composite material and exhibits better performance than other polymer materials.However,the existing end groups can be used for capping LCT are limited by high curing temperature and high requirements for processing equipment,Although LCT can be cured in multi-step at low temperature,the long cure time and poor mechanical properties of resultant products are still problematic.Therefore,developing phenylalkynyl end-capper with suitable curing temperature through molecular structure design,and synthesizing LCT with excellent processability and thermal-mechanical properties for high-performance composites is an important research topic.In this paper,a series of LCT with low curing temperature and excellent comprehensive properties were obtained.Meanwhile,the effects of phenylethynyl end-capper structure on the properties of LCT were systematically investigated as well.The commercial Xydar^?consisting of p-hydroxybenzoic acid,biphenol and terephthalic acid(HBA/BP/TA)was copolymerized with 2,6-naphthalenedicarboxylic acid(NDA)with a kinked structure to obtain a new liquid crystalline polyarylate(LC).By altering the positions of the benzene ring and the ethynyl group and changing the active cross-linking point,four different active phenylalkynyl end-capping groups,EA-COOH,EA-NH₂,PE-COOH and PE-OAc,were synthesized.Four phenylethynyl end-groups with different reactivity capp HBA/NDA/BP/TA to prepare a series of nematic LCTs,including PEOAc-LC-EACOOH,EANH₂-LC-EACOOH,EANH₂-LC-PECOOH and PEOAc-LC-PECOOH,via a simple one-pot melt polycondensation.The introduction of phenylethynyl end-cappers reduce the degree of crystallinity of reference polymer HBA/NDA/BP/TA and exhibit stable liquid crystalline phase behavior simultaneously.The kinetics of curing reaction of LCTs were studied using rheometer,suggesting that EANH₂-LC-EACOOH is the most suitable composition for low temperature curing.EANH₂-LC-EACOOH exhibits a stable and low viscosity plateau,indicating its excellent processability.After curing at different temperatures,all the cured films show outstanding thermomechanical properties in terms of high storage modulus(2-3 GPa)at 40?.It is worth noting that the thermomechanical properties of EANH₂-LC-EACOOH and PEOAc-LC-EACOOH cured films are not influenced by curing temperature.For instance,they can be cured at a much lower temperature(320?)than that(370?)required for the existing LCT within the same time(45 min).In order to further improve the end-capping efficiency of phenylethynyl end-cappers and reduce the polymerization temperature,3-(acetylamino)phenylacetylene(EPA)was prepared by acetylation of m-aminophenylacetylene(EA-NH₂)with acetic anhydride.By adjusting the polymerization temperature(T_p),solid polymerization temperature(T_s)and other process conditions,the resultant LCT(EPA-LC-EACOOH-275)exhibit good processability with a minimum melt viscosity of 2×10~4 Pa·s at 275?.EPA-LC-EACOOH-275 film with good mechanical properties was cured at 320?/45 min.The glass transition temperature(T_g)of EPA-LC-EACOOH-275increased from 206?(EANH₂-LC-EACOOH)to 215?,and its rubbery storage modulus remained stable at 3×10~7 Pa(350?).These results demonstrate that the new LCT(EPA-LC-EACOOH)has controllable melt viscosity and excellent thermomechanical properties,which is expected to provide high-quality matrix for high-performance composites.