Polycarbonate/Copolyester Blends Having Optical-Grade Clarity and Their Upper Critical Solution Temperature Type Phase Behavior

The miscibility of bisphenol A polycarbonate (PC) and poly(ethylene terephthalate-co-1,4-dimethyl cyclohexane terephthalate) copolyesters (PECT) containing various amount of 1,4-dimethyl cyclohexane were explored to produce a blend having optical-grade clarity. It is known that PC does not form miscible blends with poly(ethylene-terephthalate) (PET), while it forms a miscible blend with poly(1,4-dimethyl cyclohexane terephthalate) (PCT). PC blends with PECT copolyesters prepared by solvent casting were not miscible regardless of the copolyester composition. However, PC and PECT copolyesters containing 1,4-dimethyl cyclohexane from 32 to 80 mol % formed homogeneous mixtures upon heating. Since immiscibility of solvent casting samples can be driven by solvent effects even though PC/PECT binary blends are miscible, upper critical solution temperature (UCST) type phase behavior was confirmed by exploring phase reversibility. The phase homogenizing temperature of PC/PECT blends exhibiting UCST-type phase behavior was increased by decreasing the 1,4-cyclohexane dimethanol (CHDM) content in copolyester. The metastable blends obtained by quenching from above the phase-homogenization temperature exhibited a single glass transition temperature and had optical-grade clarity.