Synthesis,Characterization And Cure Of Aromatic Ester Mesogenic Liquid-Crystalline Epoxy Resins

As a novel kind of toughening agent, liquid crystalline polymer (LCP) possesses excellent properties of high strength, high modulus, high temperature resistance property and low thermal expand coefficient. LCP can not only improve the toughness and strength of epoxy resin but also modify its heat resistance, mechanical propertie, optical properties and dielectric properties. LCP has attracted considerable research interest in the aspect of modifying epoxy resin.In this study, four liquid crystalline epoxy resin (LCER) based on triaromatic ester mesogenic groups were synthesized to investigate the effect of the structure on the liquid crystalline (LC) property. The structure and phase behavior were characterized by FTIR, EA, 1H-NMR, DSC and hot stage POM. All the 4 kind LCERs with different structures showed nematic liquid crystalline phase between T_m and T_i in the heating scan. Introduction of either methyl substituent or flexible ethoxyl spacer can lower T_m and T_i of LCER. By incorporation of both methyl substituent on the mesogenic core and flexible ethoxyl spacer into the chain, Tm of MPEPEB was lowered to 78.7℃and T_i was lowed to 133.9℃.The curing kinetics of LCER/diamine mixtures were studied by DSC during isothermal and dynamic processes. Although there is little difference between the activation energies obtained for the four mixtures (MPEPB/DDM, PEPEB/DDM, MPEPEB/DDM and MPEPEB/DAE), marked different curing behavior was found in isothermal and dynamic investigation. During the dynamic curing, two exothermal peaks appeared in the DSC curves of MPEPEB/DAE and MPEPEB/DDM. The curing reactions of the four systems roughly obey n-th kinetic model in the isothermal investigation. The comparison between isothemal and dynamic data shows that the curing rate is not a unique function of curing degree and temperature.Nematic liquid crystalline phase maintained on curing at appropriate temperature observed in POM for all the cured samples. The schlieren texture of MPEPEB transform to nematic droplets while cured with DDM conformed by POM observation. The texture of MPEPEB/DAE disappeared at 85℃and reappeared in cooling. While no change was found in the DSC heatflow.The glass-transition temperatures (T_g) of the cured network were obtained by DSC. T_gs of MPEPB/DDM vary in 140~160℃, PEPEB/DDM in 90~100℃, MPEPEB/DDM in 40~50℃and MPEPEB/DAE in 32~36℃.