THERMODYNAMICS AND PHASE-SEPARATION IN MELT BLENDS OF POLYETHYLENE AND MODEL COPOLYMERS

Binary blends of linear polyethylene (PE) and hydrogenated polybutadiene (HPB) are employed to evaluate the interaction parameter-chi between chains containing different relative amounts of linear and ethyl-branched C4H8 repeat units. Conditions for liquid-liquid phase separation in the melt are determined unambiguously from the appearance of quench-crystallized blends which have been etched to define HPB-rich domains. For polyethylene and random poly(ethylene-co-butene-1) at 150-degrees-C, chi = 0.022 Y(br)2, where Y(br) is the fraction of branched C4H8 repeat units. Domain sizes range from 0.1 to 16-mu-m, increasing with chi/chi-c and the time permitted for phase separation. Sizes observed for blends near the critical point compare favorably to those predicted by the linearized theory for spinodal decomposition.