Thermally and anionically initiated cure of bismaleimide monomers

International audienceA kinetic and thermodynamic study of the thermally or anionically induced cure of aromatic bismaleimide monomers (namely 2,4-bismaleimidotoluene, 4,4'-bismaleimidodiphenylmethane and their eutectic mixture) is presented. Calorimetric investigations allowed the determination of heats of polymerization and activation energies for both types of initiations, and the values found are compared with data in the literature. Isothermal cures were also carried out with and without anionic initiator (typically imidazole). SEC measurements were used to monitor the conversion of the monomers as a function of time. The thermally induced polymerization seems to proceed in a rather heterogeneous way, owing to an initiation that is too slow: gelation appears at first only as the formation of microgels, with no immediate increase in the viscosity; macrogelation occurs some time after. On the contrary, the anionically induced polymerization is more homogeneous. A complete study of a particular reacting system revealed no interference between radical and anionic polymerization over a wide range of temperatures, and in this case gelation results in an immediate increase in viscosity. However, the conversion at the gel point is unexpectedly high, which can be interpreted as the result of a very fast initiation as well as of transfer reactions. Glass transition temperatures were also measured as a function of time and curing temperature in order to study the influence of vitrification on the kinetics and to check for the existence of a unique relation between Tg and conversion. This actually proved to be correct as long as diffusion does not slow down the reaction; in this case Gillham's treatment of Tg versus Int was successfully applied and led to the correct value of the activation energy. All the measurements allowed us to establish the time-temperature-transformation cure diagram for a specific reacting system