Properties of Epoxy Resins Modified with Liquid Crystalline Polyurethane

Anew kind of reactive toughening agent, defined as liquid crystalline polyurethane(LCPB), having both flexible chain and rigid biphenyl mesogenic groups wassynthesized by polyaddition of 2,4-toluenediisocyanate (TDI) with diethyleneglycol and 4,4 -dihydroxybiphenyl in DMF. The structure and morphology of the LCPBwere investigated systematically by Fourier transform infrared spectroscopy (FTIR),differential scanning calorimetry (DSC), wide angle X-ray diffractometry (WAXD) andpolarizing optical microscopy (POM), respectively. The POM observation confirmedthat the LCPB exhibited nematic texture and good mesophase stability. The LCPB wasused to modify the epoxy resin as toughening agents. The mechanical properties,dynamic mechanical behaviour, fracture surface morphology and apparent activationenergy (Ea) of the modified systems were systematically investigated. The experimentalresults revealed that the impact strength of the epoxy resin modified with LCPBreached the highest value of 47.21 kJ/m^2, and it is 1.8 times higher than that of theunmodified system when the content of LCPB loading reached 5 wt%. In addition, thetensile strength and the fracture strength also enhanced with increasing concentrationof LCPB. The DMA results showed that the storage modulus in rubbery state and glasstransition temperature (Tg) of the modified systems were higher than those of theunmodified system, indicating that the motion of epoxy matrix chains was stronglyrestricted by the rigidity of mesogenic units, while the Ea at glassy relaxation processof modified system was about 100-120 kJ/mol higher than that of the unmodifiedsystem. The effects of reinforcement on mechanisms of the damaging process of themodified systems were investigated by scanning electron microscopy.