Miscibility of Ethylene-Vinyl Alcohol Random Copolymer Blends

The incompatibility of polymers in blends presents a significant challenge to their recyclability. Ethylene-vinyl alcohol random copolymers (EVOH), widely used as an oxygen barrier in packaging materials, are of different compositions based on their applications. Though incompatible with both polyethylene and polyvinyl alcohol, the compatibility of these copolymers with each other is key to their behavior in mixed waste streams. Here, we study the melt miscibility and co-crystallization capability of commercial EVOH copolymers with varying ethylene content (48-27 mol%) in binary blends using calorimetric and diffraction techniques. We find that EVOH copolymers are miscible if the composition difference is less than 11 mol%, and are mechanically compatible over the entire composition range studied, but co-crystallization at the level of crystal stems is determined by crystallization temperature and cooling rate. X-Ray diffraction experiments suggest that co-crystallizing blends have a slightly larger unit cell than the individual components, implying imperfect crystallization in blends. In blends with same composition difference, co-crystallization is easier if both components crystallize in the same lattice structure, but difficult when individual components adopt different lattice structures (monoclinic vs. orthorhombic) under a given crystallization condition.