Study On Some Chemical And Physical Issues In The Phase Separation Process Of Modified Bismaleimide Systems

In this work some chemistry and physics issues of the phase separation of modified bismaleimide systems were studied. In part I of this thesis, the reaction-induced phase separation of polyethersulfone (PES) modified bismaleimide (BMI) resin was investigated. The effect of PES content, PES molecular weight and curing temperature on the phase separation were discussed. In addition, the viscoelastic effects on the phase separation, the chemorheology and the critical gelation of the blends during isothermally curing were also studied. In part II of the thesis, the cure mechanism of the blends of bisphenol A dicyanate ester and 4,4'-bismaleimidodiphenylmethane (BMI) was studied.The phase structure of the PES modified BMI resin changed from dispersed phase, via co-continuous phase, to phase inversion with the increasing PES content. It was found that the addition of PES would obviously increase the tensile strength and elongation at break of pure BMI resin, while the module would decrease slightly. Comparing with unmodified BMI system, the curing reaction rate of the modified BMI systems decreased with the addition of PES because of the diluted effect.In addition, the effect of PES molecular weight on phase separation was studied. The lower the molecular weight of PES was, the higher the content of PES was needed to obtain the phase inversion in the modified systems. The tensile properties of the blends with low molecular weight PES were better than those of blends with high molecular weight PES at the same PES content. While the cure reaction activation energies of the modified BMI systems with high molecular weight PES is slightly higher than that of systems with low molecular weight PES.Furthermore, the effect of curing temperature on phase separation was also investigated. The higher the curing temperature was, the higher curing conversion and the smaller domain size exhibited. However, the phase structure remained same in spited of the curing temperature.According to the results from time-resolved light scattering (TRLS), the reaction-induced spinodal phase separation could take place in the modified blends with certain PES content. The evolution of scattering vector q_m followed Maxwell-type relaxation equation, and the value of relaxation time Ï„ at various temperature was fitted with Williams-Landel-Ferry equation. It was found that the experimental values fitted well with the equation and the reference temperature T_s , obtained from the fitting, was 28-40K higher above the T_g of the blends, which demonstrated viscoelastic effect experimentally.The chemorheological behaviors of the PES modified systems were examined, which indicated that phase morphology played an important role in the chemorheological behavior of PES modified BMI systems. Combined with the results of TRLS, the jumping-off point of the first rise of complex viscosity was identified as the occurrence of phase separation. At the beginning of phase separation, the complex viscosity abruptly dropped if PES dispersed phase structure formed, while the complex viscosity steadily increased if PES inversion structure exhibited. In addition, the complex viscosity of the blends with low PES content increased faster than the blends with high PES content owing to the diluted effect of PES. Moreover, the evolution of the complex viscosity of the blends with high PES content was relatively less sensitive to the curing temperature due to the formation of the continuous PES-rich phase. The PES molecular weight mainly influenced the onset of phase separation of modified BMI systems but was relatively less influential on the evolution of the complex viscosity in the succedent curing process.Particularly, two critical gel points were observed in the isothermal curing of the PES modified BMI systems and were identified as the fixing of the phase structure and the chemical gelation of the cross-link reaction of BMI, respectively. It was notable that the curing conversion of the first gelation was dependent on curing temperature, whereas the second gelation (chemical gelation) occurred almost at the constant conversion (about 0.54) at different curing temperature. The critical relaxation exponent n of the first gelation was about 0.66, which consisted with the prediction by the ZIMM model; while the critical relaxation exponent n of the secondgelation was about 0.33, which was much lower than that of the first gelation. In addition, the critical relaxation exponent n decreased with the increase of PES content and the decrease of curing temperature.In part II of the thesis, the cure mechanism of the blends of bisphenol A dicyanate ester and 4,4'-bismaleimidodiphenylmethane was studied. The results clarified that there were different kinds of cure mechanism in the blends of BMI and dicyanate ester. In non-catalyzed blends, the dicyanate ester and BMI cured independently and formed two kinds of network, which had two glass transition temperatures and had the absorption peak of triazine ring of polycyanurate in Fourier-Transform infrared spectroscopy (FTIR) spectra. Nonylphenol, a cyanate curing catalyst, could accelerate the cure of dicyanate ester but could not change the independent cure mechanism of two components. However, the presence of P-toluene sulphonic acid resulted in the co-reaction between two components and formed a homogeneous network, which had only one glass transition temperature and did not exhibit the absorption peak of triazine ring of polycyanurate in FTIR spectra. Thus, it could be concluded that the cure mechanism of the blends of BMI and dicyanate ester, which was far from being well understood, was strongly related to the catalyst presented in the blends.