Studies On 1,3,4-Oxadiazole-Containing Asymmetric Bismaleimides And Their Modified Resin Systems

Bismaleimides (BMI) is one of the high-performance thermosetting polymers with good processibility of epoxy resins and the excellent thermal stability, mechanical properties of polyimides. It is widely employed form aerospace and weaponry industry to multilayer printed circuit boards and electrical insulators. However, the pure BMI monomers have poor solubility in common solvents, high melting and molding temperature, poor processability due to the rigid benzene and imide rings. The high crosslink density results in inherent brittleness and poor impact resistance of the cured resins. We must solve the mentioned defects of BMI resins for their further application in high-technology fields. Hence, in this paper, we modified BMIs from three aspects in order to improve their solubility and processability without sacrificing the thermal stability and mechanical properties of cured resins.1) From the perspective of molecular design, we incorporated 1,3,4-oxadiazole ring and flexible ether linkages into the BMI skeletons, and designed asymmetric molecular structure, which made the terminal double bonds of BMIs be located in different chemical surroundings, and displayed different reaction activation. Hence, the cure reaction of BMI could be initiated at different temperature, and the reaction heat colud be released steadily for reducing thermal stress of material.2) In order to improve the combination performances of the resins, the novel BMI monomers were modified through the copolymerization with other resins, such as 4,4'-bismaleimidodiphenylmethane (BMDM) and o.o'-diallyl bisphenol A (DABPA).3) We prapred the prepolymers using 1,3,4-oxadiazole-containg asymmetric diamines (MDA) and BMDM, and investigated their toughening effects on the BMDM/DABPA systems.Firstly, We designed and synthesized two novel bismaleimide monomers containing 1,3,4-oxadiazole and asymmetric structure, e.g.,p-Mioxd and m-Mioxd. The chemical structures of intermediates and target compounds were confirmed by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and elemental analysis. The two monomers diaplayed good solubility, and m-Mioxd exhibits better solubility in low boiling organic solvents, such as dichloromethane, acetone due to meta-substitution. We investigated the curing behavior and kinetics of the BMI monomers by dynamic differential scanning calorimeter (DSC), the results indicated that the Mioxd monomers had broad curing exothermic peaks, and p-Mioxd has higher meting temperature due to the symmetry molecular structure and crystallinity. The polarized optical microscope and X-ray scattering diffraction (XRD) indicated thermal behavior below melting temperature for p-Mioxd is attributed to change of crystal morphology. The thermal stability of the BMI monomers were characterized using thermogravimetric analysis (TGA), and they displayed high decomposition temperature and residual mass for high temperature. We investigated the curing behavior, thermal stability of Mioxd/BMDM blends and thermal mechanical properties of their composites using DSC, TGA and dynamic mechanical analysis (DMA). The resultes indicated that the blends have excellect thermostability and high glass transition temperature (Tg).The Mioxd/DABPA copolymes with different molar ratio were prepared. The curing behavior, kinetics and curing process were investigated by DSC. The reaction mechanism during isothermal cure at 150? was investigated using FTIR. The TGA results showed that the thermostability of modified BMI resins was improved with the increasing Mioxd content. We investigated the effect of molar ratio in the m-Mioxd/DABPA blends on DMA and mechanical properties of the composites, and the composites gained best combination property with a little much m-Mioxd. We prepared Mioxd/BMDM/DABPA copolymer resins, and investigated the effects of mole concentration of Mioxd on their curing process, mechanical properties, fracture toughness and heat resistance by DSC, DMA, TGA. The results indicated that with inceasing addition of Mioxd, the thermostability and absorption performance of modified resins were improved, the flexural strength and impact strength increased firstly and then decreased, but the flexural modulus kept increasing. Increasing concentration of Mioxd make the viscosity increased, and the bubbles can not be completely discharged, so there may be some defects in material and the strength of cured resins were decreasd. m-Mioxd could improve the impact toughness more effectively.The MDA/BMDM prepolymers (MB) were prepared based on 1,3,4-oxadiazole-containg asymmetric diamines (MDA) and BMDM. We studied the influence of molar ratio on curing behavior, reactivity, mechanism and thermal stability of prepolymers by DSC, FTIR and TGA. The results showed that molar ratio of 1:2 is the most benefical to the addition reaction between-NH2 and BMI, and the obtained prepolymers have lower melting point between 120-150?. The Ea is calculated to be 118.05 kJ mol-1, and n is 1.21, which confirmed there are selfpolymerization of BMI and Michael addition reaction between -NH2,-NH-and double bond of BMI in curing process. TGA analysis indicated that the prepolymer consist of oligomer with variety of different molecular weight, and the thermal decomposition process is completed step-by-step with complex mechanism. The BMDM/DABPA was toughen through copolymerzaiton with MB to prepare MB/BMDM/DABPA systems. The influence of MB content on curing behavior, heat resistance, mechanical properties and moisture absorption properties was studied by DSC, DMA and TGA. The results showed that the little remaining-NH2 and-NH can catalyze the polymerization of BMI, and the peak temperature for cure reaction (Tp) was decreased. If the content of MB is increased, the viscosity is higher, and Tp would be increased. The heat resistance and storage modulus (E") in glassy region increased firstly and then decreased with increasing MB content, the E' in rubber region state and Tg were decreased. The flexural strength and impact strength increased firstly and then decreased, but the flexural modulus kept increasing. The crosslinking density plays a main effect on the water absorption performance, hence moisture resistance wered decreased with the addition of MB.