| Bismaleimide (BMI) is a high-performance thermosetting resin, which has good processing properties of EP as well as excellent thermal and mechanical properties of polyimide resin (PI), therefore, it meets the requirement of advanced polymer matrix composites (APC) in many respects, and it has become the dominant matrix of aerospace structural composites. However, the high crosslink density and poor toughness of unmodified BMI cured resin limit the application of BMI, thus it is necessary to improve the toughness of BMI in order to overcome its defects. BMI modified by allyl compound, its prepolymer is extremely stable, and its cured resin has excellent toughness and thermal properties.In this paper, liquid crystal structure and phthalide cardo structure were respectively introduced to the diallyl compounds to synthetize liquid crystalline diallyl compound (BAOBE) and chain-extended diallyl compound containing phthalide cardo structure (DAOPE). Then the BAOBE and DAOPE were respectively copolymerized with BDM and DABPA to modify BDM/DABPA resin system, with a view to improve toughness of BMI resins under the premise of maintaining the excellent thermal properties.Firstly, the novel diallyl compounds BAOBE and DAOPE were respectively synthesized through Williamson etherification, carboxylic acid acylation and esterification, FTIR and’H NMR tests were used to confirm the chemical structure of intermediates and target products. Then differential scanning calorimetry (DSC) and polarized microscope were used to detect the liquid crystal behavior of BAOBE, the results showed that liquid crystal color patterns were observed under polarizing microscope, and its phase transition temperature was consistent with the description of its DSC curves, which confirmed that BAOBE has liquid crystalline behavior, and it belong to nematic liquid crystal structure.Finally, the synthetic BAOBE and DAOPE were respectively used to modify BDM/DABPA resin system, at the molar ratio of maleimide to allyl groups of 1:0.87, a series of BAOBE/DABPA/BDM and DAOPE/DABPA/BDM modified resins were prepared. The curing behavior and curing kinetics of modified resin systems were studied by DSC. It demonstrated that the two systems had similar curing behavior and curing kinetics, and melting point of the prepolymer was lower generally. Besides, DMA, TGA, bending and impact test were used to study the influence of different molar ratio of novel allyl compound and DABPA on the thermal and mechanical properties of the modified system. The results showed that the char yield at 700℃ of each modified system was increased with the increase of BAOBE and DAOPE, the initial thermal decomposition temperature of BAOBE/DABPA/BDM tended to be increased at first and then decreased with the addition of BAOBE, while the initial thermal decomposition temperature of DAOPE/DABPA/BDM essentially remained unchanged with the increase of DAOPE, which indicated that each modified resin system maintained excellent thermal properties. Rubbery storage modulus (E’) and glass transition temperature (Tg) of each system tended to be decreased with the increasing content of BAOBE and DAOPE, the Tg of all of resins above 280℃. With the addition of BAOBE and DAOPE, the bending and impact strength of each system were increased at first and then decreased, while the flexural modulus was increased continuously at the same time. |
