RUBBER-MODIFIED EPOXIES .3. INFLUENCE OF THE RUBBER MOLECULAR-WEIGHT ON THE PHASE-SEPARATION PROCESS

The effect of varying the rubber molecular weight on the reaction-indu ced phase separation taking place in rubber-modified epoxies is report ed. Three butadiene-acrylonitrile copolymers with number-average molec ular weights of 2030, 3600 and 6050 g mol-1 were used. The thermosetti ng 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 clo ud point; (iii) a primary morphology of a low concentration of particl es with a large size; (iv) a more complete segregation of the rubber f rom the matrix leading to a smaller decrease of the glass transition t emperature of the system; (v) a decrease in the toughening. The Flory- Huggins lattice model may be used as a simple approximation to describ e the phase separation process when varying the rubber molecular weigh t.