Rheokinetics of ring-opening metathesis polymerization of bio-based castor oil thermoset

摘要

Ring-opening metathesis polymerization (ROMP) of norbornenyl-functionalized castor oil has been evaluated using small-amplitude oscillatory shear flow experiments as a function of angular frequency, temperature, and curing time. At the onset temperature of the curing process, an abrupt increase in dynamic shear moduli, G and G″, and complex shear viscosity, ν~*, was observed during the dynamic temperature ramps (2 °C/min heating rate) of the sample over a wide range of angular frequencies. A dramatic increase in zero-shear viscosity, ν _0, was also observed at the gelation temperature, T _(gel). The value of T _(gel) obtained from the abrupt increase in ν _0 was found to be in good agreement with the value evaluated from the crossover point of G and G″. The real time curing kinetics was investigated under isothermal conditions over a wide range of angular frequencies at different constant curing temperatures (40, 45, 50, and 55 °C). The isothermal gelation kinetics was found to be strongly curing temperature dependent; i.e., the higher the curing temperature, the faster the gelation process. Both G and G″ showed a power law relationship with angular frequency at the gel point, with critical power law exponents at the gel point in good agreement with the value predicted using percolation theory. Furthermore, ν _0 and the equilibrium storage modulus, G _(eq), were found to be well described by power law scaling functions with the relative distance from the gel point. The molecular dynamics and thermal stability of the fully cured sample were also investigated by dynamic mechanical analysis and thermogravimetry, respectively.