Rubber‐modified epoxies: III. Influence of the rubber molecular weight on the phase separation process

The effect of varying the rubber molecular weight on the reaction induced phase separation taking place in rubber‐modified epoxies is reported. Three butadiene–acrylonitrile copolymers with number‐average molecular weights of 2030, 3600 and 6050 g mol−1 were used. The thermosetting polymer was an epoxy based on diglycidylether of bisphenol‐A cured with a cycloaliphatic diamine. Increasing the rubber molecular weight showed the following effects: (i) a decrease in the conversion at the cloud point; (ii) a corresponding decrease in the viscosity at the cloud point; (iii) a primary morphology of a low concentration of particles with a large size; (iv) a more complete segregation of the rubber from the matrix leading to a smaller decrease of the glass transition temperature of the system; (v) a decrease in the toughening. The Flory–Huggins lattice model may be used as a simple approximation to describe the phase separation process when varying the rubber molecular weight.