Synthesis Of Liquid Crystalline Epoxy Oligomers And Their Modification Of Thermosetting Resins

Highly crosslinked thermosetting resins have been wildly used as matrix for advanced materials due to their high mechanical performance and good processing behaviors. However, their drawbacks of highly brittleness limited their further application, various modifiers, such as reactive rubbers, thermoplastics and block copolymers, are deeply studied to improve their toughness. Although liquid crystal (LC) -coil block copolymers with epoxy functional groups are expected to have excellent properties, their application on the modification of thermosetting resins have not been studied.A series of LC oligomers, including both rigid LC oligomer and rod-coil oligomers with different coil segments, were synthesized and characterized by infrared spectroscopy (IR), nuclear magnetic resonance (1H NMR), etc. The phase transition behaviors of these liquid crystals were also studied by differential scanning calorimeter (DSC) and polarizing optical microscope (POM).The toughness of DGEBA (DER331) with rigid LC oligomer (BMPE) was systematically studied, firstly. The effect of curing conditions and concentration of BMPE on the properties of toughened systems were investigated. In addition, the toughening mechanism was revealed by the combination of the results from DSC, POM and scanning electrical microscope (SEM), which suggested that with the increase of BMPE content, the mechanical properties and the glass transition temperature (Tg) of the system all increased attributed to the formation of microfibers. While the formation of microfibers could be prevented by the increased viscosity resulted from the introduction of polyethersulfone (PES), and thus the mechanical properties of the system was decreased.Furthermore, the modification of cyanate ester (L10) with BMPE was also studied. The structure and morphology change of the system with different BMPE/L10 content during curing process were monitored by IR and POM. SEM results showed phase separation in the BMPE/L10 (1:0.5 molar ratios) blend. In addition, the relation of liquid crystalline phase formation and reaction was also investigated. Based on this result, BMPE/L10/DER331 blends were also studied.Based on the above work, the structures of LC epoxy oligomers with different coil segments were investigated, and their toughness effects and mechanism on DER331 were revealed by thermal and mechanical properties as well as phase morphologies. The morphologies of LC epoxy oligomer LC400-P at crystalline state, liquid crystalline state and in epoxy resin solution were revealed by XRD, AFM and TEM etc. The tapping mode AFM images indicate that LC400-P self assembles into a nanoscale columnar structure with a length of about 10nm. In the liquid crystalline phase LC400-P showed organized discrete bundles of nanoscale, columnar structures about 30nm in length from the TM-AFM. A blend of LC400-P in epoxy precursor shows that the rod coil oligomer forms nanoscale columnar structures typical of its bulk state at room temperature. However, a significant difference in the structures of LC400-P while heating to higher temperature appeared as nanofiber structures. During the curing process, LC400-P self-assembled into fibril structures and was retained in the epoxy network.Further studies of different mesogenic structures and various coil lengths LC epoxy oligomer on mechanical, thermal properties and phase structure were investigated. Final properties of modified system were related to amounts of formed fibril structures. Influences of substitute on mesogenic unit on fibril formation were also revealed. Methyl group on the mesogenic unit benefited fibril formation and improvement of final properties. Longer coil length depressed the formation of fibril structure due to lower fraction of mesogenic unit. As to similar content of shorter coil length oligomer, its higher fraction of mesogenic unit benefit formation of more nanoscale columnar structure, but shorter coil leaded to aggregation of mesogenic unit and lower compatibility in epoxy resin.Influences of curing condition such as curing agents and temperatures on fibril formation and mechanical properties were also investigated. Fibril formation was hindered by very fast curing condition. Meanwhile, it was revealed that higher curing temperature can decrease the viscosity of blend and benefit nanoscale columnar structure formation.