Tailoring Crystalline Morphology via Entropy-Driven Miscibility: Toward Ultratough, Biodegradable, and Durable Polyhydroxybutyrate

The excessive use and disposal of plastic products have become a severe threat to the environment, animal welfare, and human health. Naturally synthesized, marine-degradable polyhydroxybutyrate (PHB) represents a viable green substitute for conventional plastics. However, the inherent brittleness of PHB remains a major challenge due to undesirable large spherulites and secondary crystallization. Herein, we report PHB-based (up to 70 wt %) ductile and flexible materials by facile physical blending with edible poly(vinyl acetate) (PVAc). Theoretical and experimental analyses show that entropy rather than enthalpy drives the high miscibility between two polymers. Entropic mixing turns fragile PHB spherulitic crystals (>70 μm) into myriads of ultrafine domains (<2 μm). Interfacial entanglements between PVAc and PHB further prevent secondary crystal formation of the rigid amorphous phase. The resultant biopolymer blends demonstrate mechanical properties similar to commercial polyethylene plastics, such as high ductility (elongation >500%), toughness (∼62 MJ m^-3), flexibility, and shape recovery under repeated bending (180°) or twisting (360°). Under controlled composting conditions, the food-safe bioblends exhibit ∼2.4 times weight loss of virgin PHB. The proposed strategy proves applicable to other crystalline/amorphous polymeric mixtures. This discovery sheds new light on the rational design of green plastics for future sustainable electronics, agriculture, and biomedicine.