| A homologous series of main-chain thermotropic polyesters (PBTn, n=2,4,6) containing biphenyl and triad aromatic ester mesogenic units interconnected by n-methylene spacers in the main chain were prepared from terephthaloyl bis(4-oxybenzoyl chloride) (TOBC) and 4,4’-bis(ω-hydroxyalkyloxy)biphenyls (nBP, n=2,4,6) by solution polycondensation. The chemical structures of the monomers and PBTn were characterized by spectroscopic techniques and elemental analysis. The solubility, crystallinity, thermal and liquid crystalline properties of PBTn were investigated by differential scanning calorimetry (DSC), polarizing microscopy (POM), small-angle X-ray scattering (SAXS), thermogravimetric analyses (TG) and wide-angle X-ray diffraction (WAXD), and they were found to be closely dependent on the length of the methylene spacers. The increasing methylene units led to improved solubility, broader liquid crystalline range, reduced mesophase-isotropic transition point, higher thermal stability and decreasing crystallinity. The decomposition temperatures (5% mass loss) of the polymers were above 380℃in nitrogen atmosphere and only PBT4 and PBT6 exhibited smectic mesophases as well as high viscosity in the mesophase.A new homologous series of epoxy resin curing accelerators (LCECAn, n=2,4, 6) containing 4,4’-biphenyl and n-methylene units were also successfully synthesized. The curing behaviors of a commercial diglycidyl ether of bisphenol-A epoxy (E-51) and 4,4’-bis(2,3-epoxypropoxy)biphenyl (LCE) by using LCECAn as the curing agents have been investigated by DSC, respectively. The Ozawa equation was applied to the curing kinetics based upon the dynamic DSC data, and the isothermal DSC data was fitted using an autocatalytic curing model. The glass transition temperatures (Tg) of the cured epoxy systems were determined by DSC upon the second heating, and the thermal decomposition temperatures (Td) were obtained by TG. The results show that the number of methylene units in LCECAn has little influence on the curing peak temperatures of E-51/LCECAn and LCE/LCECAn systems. In addition, the activation energies obtained by the dynamic method proved to be larger than those by the isothermal method. Furthermore, both the Tg and Td of the cured E-51/LCECAn systems and LCE/LCECAn systems decreased with the increase in the number of methylene units in LCECAn.Finally, the promoting effects of LCECAn on the cure of LCE/DDM and LCE/Dicy systems and their dynamic curing kinetics were studied by DSC. The textures of LCE/DDM/LCECAn and LCE/Dicy/LCECAn curing systems were observed by POM. The dynamic mechanical properties and thermal stabilities of their thermosets were also investigated by dynamic mechanical analysis (DMA) and TG, respectively. It was found that the add of LCECAn (10%) to LCE/DDM and LCE/Dicy systems decreased their curing peak temperatures by 15 to 20℃. When observed under POM, the cured products of LCE/DDM/LCECAn and LCE/Dicy/LCECAn showed obvious briefregence of liquid crystal. Moreover, aα’ relaxation caused by liquid cyrstal was found on their DMA curves, suggesting an improved toughness. As the number of the methylene units in LCECAn increased, the thermal stability of LCE/DDM/LCECAn improved while the thermal stability of LCE/Dicy/LCECAn dropped. |
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