Biaxial Stretching Inducing Formation of Spherical Mesophase of Poly(lactic Acid) with Highly Improved Strength and Toughness.

Highly toughened poly-(lactic acid) films with balanced stiffness and strength were successfully prepared through biaxial stretching, inducing the formation of sea-island morphology. Attributing to biaxial stretching, the spherical mesophase was formed at various temperatures from 65 to 80 °C. The content and average diameter size of spherical mesophase first increased and then decreased with increasing stretching temperature. The resulting sea-island morphology functions as rigid nodes bridging the compliant amorphous domains, thereby reinforcing the film structure. Notably, the incorporation of small-sized spherical mesophases (e.g., T65) stimulates the generation of abundant microfibrillated crazes within the PLA matrix, which serves as a primary mechanism for tensile energy dissipation. The exceptional toughness and mechanical balance stem from two key factors: (i) the mesophase acts as a reinforcement to the matrix, mitigating potential loss in tensile strength; and (ii) the spherical geometry of these domains promotes craze initiation while simultaneously suppressing the transition from crazes to macroscopic cracks. Consequently, this study offers a straightforward and effective pathway to achieve mechanically balanced PLA films, providing a potential toughening paradigm for similar brittle thermoplastics.