Synthesisi,Preparation And Properties Of Two Biphenyl Liquid Crystal Epoxy Resins

Epoxy resin has excellent comprehensive properties and is indispensable in modern industry,but high crosslinking density brings huge internal stress and poor toughness to the material,so epoxy resin toughening is a long-term hot issue.However,toughening usually leads to a significant decrease in strength and rigidity,and the two contradict each other,which seriously restricts the application of high-performance epoxy resins in cutting-edge engineering and technical fields such as aerospace,electronics and electrical appliances.Therefore,the reinforcement and toughening of epoxy resins is necessary.Different from other toughening methods,the unique one-dimensional or two-dimensional ordered structure of liquid crystal polymers can significantly improve the toughness of the resin on the basis of improving the mechanical properties,modulus and thermal properties of the resin.Using the most stable biphenyl structure as the mesogen to synthesize liquid crystal epoxy resin with better operability with large aspect ratio and wide mesogenic temperature range is the current focus of academic research.In addition,biomass materials are renewable and are an inexhaustible source for the preparation of new materials.The combination of bio-based materials and liquid crystal thermosetting resins is also in line with the current concept of sustainable development.In this paper,starting from the requirements of high crystallinity and large aspect ratio of liquid crystal epoxy resin,a"one-step precipitation method"is proposed,and a biphenyl type liquid crystal epoxy resin with high crystallinity is prepared.Using the structural characteristics of hydroxyl and double bonds in bio-based eugenol,a novel eugenol-biphenyl liquid crystal epoxy resin with large aspect ratio and wide liquid crystal temperature range was designed and prepared.On this basis,the mesogenic properties,thermal curing behavior,toughening modification and curing kinetics of the above two liquid crystal epoxy resins were studied.The main contributions of the paper are as follows:（1）In order to prepare liquid crystal epoxy resin with high crystallinity and high epoxy value,based on the"one-step method",a"one-step precipitation method"was developed to prepare 4,4-diphenol diglycidyl ether（DGEBP）.The optimum synthesis conditions were obtained.Its structure was determined by FTIR,¹H NMR,¹³C NMR and other means;its mesogenic properties were characterized by POM,DSC and other means,and its crystallinity and thermal properties were investigated by TGA and XRD.The results show that DGEBP prepared by"one-step precipitation method"has higher crystallinity（91%）and epoxy value（0.635）than the data reported in the literature,and has good thermal properties.It is a non-bulk liquid crystal epoxy resin.resin.Then,the thermal curing behavior of DGEBP and six curing agents was discussed,and the thermal properties of the cured products were characterized by means of DSC and TGA.The results show that DGEBP with high crystallinity has better heat resistance compared with the data reported in the literature.Finally,DGEBP was used as a strengthening and toughening modifier,added to the binary blend system of（bisphenol A diglycidyl ether）DGEBA and polyphenol oxide resin,and a composite modification system was prepared by using dicyandiamide as a curing agent.The mechanical and impact properties of the cured product show that the modified impact strength and tensile fracture energy are increased by 20%and 26%respectively,the toughness is significantly improved,and the strength,glass transition temperature（T_g）,modulus and thermal properties are all at the same time.have been improved to varying degrees.（2）With the depletion of fossil energy and the increasingly serious environmental problems,bio-based materials have also received great attention.In order to prepare liquid crystal epoxy resin with large aspect ratio and wide liquid crystal temperature range,a diene intermediate（BPEU）was prepared from 4,4-biphenyldicarboxylic acid（BP）and bio-based material eugenol（EU）.Then,BPEU was double-bonded epoxidation to obtain a bio-based biphenyl liquid crystal epoxy resin（BPEU-ER）.Infrared,nuclear magnetic and mass spectrometry analysis showed that its molecules were targeted products,and their dielectric properties were characterized by POM,DSC and other means,and TGA and XRD explored their crystallinity and thermal properties.The above data show that BPEU-ER is a nematic double-transition liquid crystal with a large aspect ratio and a wide liquid crystal temperature range.The liquid crystal phase exists in the temperature range of 154 ℃～207 ℃ and 107 ℃～204 ℃ The thermal curing behavior of BPEU-ER was studied by means of DSC and TGA,and the thermal properties of the cured product were obtained.Using BPEU-ER to modify DGEBA and polyphenol oxide resin binary blend matrix resin.The test results of the modified system show that BPEU-ER has a good toughening effect,and the impact strength and tensile fracture energy are increased by 19%and 48%,respectively.At the same time,the strength,glass transition temperature（T_g）,modulus and thermal properties There are slight increases.Finally,the curing kinetics of the two curing systems DGEBP/DICY and BPEU-ER/DICY were studied.The average reaction activation energies of the two systems were142.39 KJ/mol and 87.42 KJ/mol,respectively,and the fitting conditions of the constructed curing reaction rate equations good.