RUBBER-MODIFIED CYANATE ESTERS - THERMODYNAMIC ANALYSIS OF PHASE-SEPARATION

Cloud-point curves were determined for blends of a rubber (butadiene-a crylonitrile random copolymer terminated in non-functional groups, NFB N) and a cyanate ester (4,4'-dicyanate-1,1'-diphenylethane, CE). Measu rements were made in the initial blend and in partially polycondensed blends (thermal polycyclotrimerization of the CE monomer). Cloud-point conversions and temperatures were determined for different rubber fra ctions in the initial formulation. A thermodynamic analysis based on a Flory-Huggins equation, taking polydispersity of both components into account, led to the following conclusions. (1) Phase separation was t he result of the change in two different contributions to the free ene rgy of mixing: (a) a decrease in the entropic contribution due to the increase in the oligomer size; (b) a decrease in the enthalpic contrib ution as a result of the decrease in the cohesive energy density of th e CE-oligomer in the course of polycondensation, (2) Spinodal demixing was excluded as a possible mechanism of phase separation during polyc ondensation in solutions containing less than 12% rubber by volume.