Behavior of temperature dependence of chi parameter in random copolymer blends showing an immiscibility window

Previously miscibility for the blends of poly(vinyl chloride-stat-vinyl acetate) (VCVAc90, containing 90 wt% of VC) with poly(isobutyl methacrylate-stat-n-butyl methacrylate) (iBMAnBMA) was found to be like a so-called immiscibility window. In this work, the temperature dependence of the Flory-Huggins chi parameter was calculated for the VCVAc90/iBMAnBMA blends using the Flory-type equation-of-state theory extended to random copolymer systems. The equation-of-state parameters necessary for calculation of chi was determined using the experimental results of osmotic pressures and heats of mixing for solutions of these polymers in cyclohexanone (CHN). The temperature dependence of chi was a monotonically increasing function over all the copolymer compositions of iBMAnBMA copolymers, and there existed a range of copolymer compositions in which chi for the copolymer blends VCVAc90/iBMAnBMA was positive though chi s for both homopolymer blends VCVAc90/PiBMA and VCVAc90/PnBMA (where VCVAc90 was regarded as a homopolymer) were negative. This corresponds qualitatively to miscibility behavior observed for the present copolymer blends. It was concluded that miscibility behavior in the present copolymer blends is one of two types of immiscibility windows classified previously by us.