Synthesis Of Multi-Arm Liquid Crystals And Properties Of Composites

The relative molecular mass of multi-arm liquid crystal is between liquid crystal molecules' and liquid crystalline polymers',which leads to various performances that between liquid crystal molecules' and liquid crystalline polymers'.This takes high values in theoretical research and practical application.Liquid crystalline epoxy resin is a kind of advanced epoxy resin which is a widely used thermosetting resin,with appending many excellent features of liquid crystal into the traditional epoxy resin.Among of it,using liquid crystalline epoxy additive agent to improve the mechanical properties of epoxy resin is a toughening method with a vast application prospect.In this thesis,in the relative molecular mass range from 1000 to 2000,two multi-arm liquid crystals(QTPM and TTPM)with different types of core were synthetized to study the special performances of multi-arm liquid crystals and to further investigate the effects by cores on the performances.The coincidences between the structures of synthetic productions and the molecular designs were proved by Fourier transform infrared spectrums(FT-IR)and proton nuclear magnetic resonance spectrums(1H-NMR).Furthermore,the thermal behaviors and liquid crystalline properties of the multi-arm liquid crystals were systematic studied by the thermogravimetric analysis(TGA),differential scanning calorimetry analysis(DSC),polarizing microscopy analysis(POM)and x ray diffraction analysis(XRD).The studies found that QTPM performed similar thermal behaviors and liquid crystalline properties with liquid crystalline polymers:it had a low glass-transition temperature(121.0?)and a 119.0? wide mesophase temperature range.During the mesophase,QTPM performed as a nematic liquid crystal,what's more,a linear negative correlation between sample viscosity and temperature was observed under the POM.TTPM was different from QTPM and majority liquid crystals,whose unique thermal behaviors and liquid crystalline properties were found during the studies.TTPM had a low and weakly crystallinity melting point(127.9?),its 104.1? wide mesophase was segmented into two intervals:under the POM,a finely crushed and dense schlieren texture which belonged to nematic phase and a high fluidity drip shaped texture which also belonged to nematic phase were observed.It can be assumed that after the melting point,the arms of the ETTPM were able to move freely while the cores of multiple ETTPM aggregated towards each other to form clusters.Such clustering caused a liquid crystal texture which resulted from the arrangement of mesogenic units in a limited space and a high viscosity of the fluid of ETTPM,these performances were nearly to that of plastic mesophase.As the temperature rose up,with a significant endothermal phenomenon,the clusters disintegrated and the movement of molecules was free,and this caused the fluid viscosity decreased substantially.Under such temperature,the free-moving mesogenic units could form orientated structure in a larger range.Subsequently,TTPM was practically applied as the precursor of liquid crystalline epoxy additive agent and ETTPM was synthesized.The structure of ETTPM was also characterized by FT-IR and 1H-NMR.The thermal rechearch and liquid crystal rechearch showed that ETTPM inherited majority characteristics of TTPM,including the unique melting process,these characteristics ensured the advantaged actions as a liquid crystalline epoxy additive agent.The suitable curing agent(4,4'-diaminodiphenyl methane,DDM)and reasonable curing temperature(130?)were decided by DSC thermograms of dynamic curing reactions.The curing process was studied by DSC constant temperature curing reaction and POM,which proved that ETTPM met the requirements of liquid crystalline epoxy additive agent.In the end,to validate the actual toughening effect of ETTPM,the toughened epoxy resin composites with liquid crystal epoxy additive agent were prepared by the co-curing of ETTPM and bisphenol-A epoxy resin E51.The existence and distribution of aeolotropic domains in the composites were observed and researched by an original method that using the scattering on laser by aeolotropic domains.Mechanical tests showed that ETTPM was effective for the mechanical properties improvement of epoxy resin cured products,as demonstrated in the improvement of impact strength,tensile strength and tensile elasticity modulus.The impact fracture cross-sections and surface morphologies were studied further by POM and scanning electron microscopy(SEM).Crack deformation,crack split and notable plastic deformations were found in modified epoxy resin cured products,and these factors played important roles in improving the impact strength.