Engineering Styrenic Blends with Poly(lactic acid)

Polymer pollution from nonbiodegradable polymer blends is a source of increasing environmental concern. The obvious solution, the replacement of petroleum-based polymers with biodegradable polymers from renewable sources, is difficult, since these polymers are highly immiscible. Here, we show how the addition of only a few percent of a tertiary phase can enable compatibilization and enhancement of the mechanical properties. The tertiary phase can be selected for any binary polymer system using the spreading coefficient (λ_ij) and the work of adhesion (W_a) as guides. We demonstrate this method using poly(lactic acid) as the continuous matrix phase, styrenic polymers, polystyrene (PS) or high-impact polystyrene (HIPS), as the discontinuous minority phase, and styrene acrylic copolymers [styrene methyl methacrylate (SMMA)] as the tertiary phase. We show that a distinct maximum in the impact toughness, nearly 58 and 56% for PS and HIPS blends, respectively, occurs when only 1.2 and 1.5% SMMA copolymers are added.