Rheological behavior of thermoset/thermoplastic blends during isothermal curing: Experiments and modeling

The evolution of the viscoelastic properties of a molten thermoplastic/thermoset system during the course of the isothermal polymerization of the thermoset precursors has been investigated and modeled. Such systems are initially homogenous and phase separate upon polymerization of the monomers. In the present study, atactic polystyrene (85 and 60 wt%) is blended to a stoichiometric mixture diglycidyl ether of bisphenol A with 4,4′-methylenebis(2,6-diethylaniline). During the polymerization, polystyrene becomes the thermoplastic-rich matrix and an epoxy-rich dispersed phase appears. Both phases experience changes in their composition and viscoelastic properties. A rheokinetic model is proposed to take into account four contributions to the viscoelastic behavior: progressive deplastification of the polystyrene matrix involving a modification of the glass transition and thus of free volume, dilution of the network of entanglements of the matrix by the non yet converted low molar weight molecules, emulsion behavior after the separation of the epoxy-rich phase and finally interparticular interactions being assimilated to a mechanical percolation. Provided that the glass transition temperature of the matrix and the dynamic moduli of the neat components are known, the changes in the viscoelastic behavior of the system with time can be predicted with no ad hoc parameter and model calculations are in good agreement with the experimental data.