Miscibility of ethylene–styrene copolymer blends

Binary blends of ethylene–styrene copolymers were studied over the full range of copolymer styrene content. A miscibility and cocrystallization map was determined from morphology as imaged with atomic force microscopy (AFM), glass transition behavior primarily from dynamic mechanical thermal analysis (DMTA), and melting behavior from differential scanning calorimetry (DSC). A difference in styrene content of about 9 wt% marked a transition from miscible to immiscible amorphous copolymer blends. The miscibility–immiscibility boundary encompassed a very small region, estimated as 9–10 wt% difference in styrene content, where partial miscibility was clearly evident. Blends in this region were characterized by an upper critical solution temperature (UCST). The phase behavior in the partial miscibility region was strongly dependent on temperature and molecular weight. The critical composition difference for blends of amorphous copolymers of about 9 wt% also applied to blends of semicrystalline copolymers. Cocrystallization of miscible, semicrystalline copolymers occurred if the styrene content difference was less than about 4 wt%. If the styrene content difference was between 4 and 9 wt%, partial cocrystallization was possible depending on the composition of the blend (wt/wt) and the thermal history of the blend.