Synthesis And Kinetics Of A Liquid Crystalline Epoxy Resin Curing Agent With Aromatic Ester Mesogen

As a novel kind of toughening agent, liquid crystalline polymer（LCP）possessesexcellent properties, such as high strength, high modulus, high temperature resistanceproperty and low thermal expand coefficient. LCP can not only obviously improve thetoughness and strength of epoxy resin but also modify its heat resistance, mechanicalproperties, optical properties and dielectric properties. LCP has attracted considerableresearch interest in the aspect of modifying epoxy resin.In order to minimize the disadvantage of brittleness and flammability of epoxyresin, a novel liquid crystalline epoxy resin curing agent （LCCA） with aromatic estermesogenic groups was synthesized by4,4’-oxybis （benzoic acid）（OBBA） andhydroquinone based on the recent literatures. The chemical structure was characterizedwith FTIR and1H-NMR. The mesogenic phase transition of the curing agent wasstudied by differential scanning calorimetry （DSC） and hot stage polarizingmicroscopy （POM）. It was found that LCCA was a pure mesogenic hardeningcompound with a large mesophase temperature range （218oC～241oC）.The thermal properties of o-cresol novolac epoxy resin （CNE） cured withdifferent curing agents were investigated by DSC and thermogravimetric analysis（TGA）, and the results showed that compared with the controls the CNE/LCCAsystem had much higher glass transition temperature （Tg=231.6oC）, better thermalstability （temperature at5%weight loss was376oC）, and higher char yields（temperature at600oC and800oC were37.1%and32.2%respectively）. Theintroduction of long rigid rod mesogen and high content of aromatic group wereconsidered to be responsible for the combination of these good thermal properties.The curing kinetics of CNE/LCCA was explored by non-isothermal differentialscanning calorimetry （DSC） technique. The basic cure process was determinedthrough extrapolation. It showed that the procuring temperature, curing temperatureand the post cure temperature were109.1℃,132℃and153.6℃, respectively. It isvery convenient to product and beneficial to industrial efficiency. Three methodswhich are Kissinger, Ozawa and Flynn-Wall-Ozawa method were used to calculate theactivation energy of the CNE/LCCA system, and the results showed that the Eascalculated from the three methods were very close. Flynn-Wall-Ozawa method wasemployed to investigate the relationship between the Eaand the curing degree. It is displayed that the activation energy values tended to decrease slightly with the degreeof conversion, this may have been caused by two major reasons. First, the Eavaluescan be affected by mass-transfer processes. As the reaction of curing went on, CNEbecame melt with the temperature increase, and the viscosity was reduced. Second, itis because that the hydroxyl group resulting from the ring opening of CNE could act asa curing agent for the epoxy ring. The autocatalytic kinetic model could well reflectthe investigated curing reaction as a result of Friedman method. What’s more, thepredicted curves from the kinetic model fit well with the nonisothermal DSCthermograms.Finally, the kinetics of the thermal degradation of CNE/LCCA in nitrogen wasinvestigated by TGA. Flynn-Wall-Ozawa method and Coats–Redfern method wereused to determine the kinetic parameters （activation energy Eaand decompositionorder n） of degradation. The results showed that the reaction order was1. Theactivation energy of thermal degradation of CNE/LCCA was184kJ/mol.