Toughening Poly(lactide) with Diblock Copolymers

Abstract

With the rapid expansion of the plastic industry, plastic waste is generated at an alarming rate. Currently, only a small fraction of waste is recycled, leading to a buildup of plastic in landfills and the environment. Poly(lactide) (PLA), a bio-derived and industrially compostable polymer, provides an alternate approach to plastic disposal. Although PLA displays several advantageous mechanical properties, it is brittle and thus cannot be readily used in many applications. In this work, we blended PLA with a diblock copolymer poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) using industrially relevant preparation methods to produce tough blends. The incompatibility of the PBO block with PLA promotes phase separation, while the PEO block decreases the interfacial tension between the two phases, leading to well-dispersed PEO-PBO particles with uniform size in PLA. Melt mixing of 1.8 wt % PEO-PBO into amorphous PLA (PDLLA) led to a 20-fold increase in tensile toughness without affecting the modulus or transparency. The deformation mechanism was investigated by small angle X-ray scattering (SAXS), revealing that the particles cavitate and act as stress concentrators for craze deformation followed by necking, where the deformation mechanism transitions to shear yielding. As a result of the craze deformation mechanism, these blends remain tough after 114 days of aging, displaying a 10-fold increase in elongation at break compared to neat PDLLA. Semi-crystalline PLA (PLLA) was also blended with PEO-PBO, resulting in a unique combination of properties which can expand the applications of PLLA as a sustainable plastic. The blends were annealed at different temperatures, resulting in a range of crystallinities. Addition of 5 wt % PEO-PBO led to a minimum 5-fold reduction in the time required to achieve 50% of the final extent of crystallinity compared to neat PLLA. The blends exhibit a 7-15-fold increase in tensile toughness compared to neat PLLA, scaling inversely with crystallinity. The deformation mechanism was investigated by SAXS and wide-angle X-ray scattering (WAXS), indicating that the particles cavitate and induce craze deformation, while the crystal structure displayed minimal changes. These blends also remained ductile over time, and after 85 days of aging, the blends fail at > 50% strain while PLLA fails at 4% strain after 2 days of aging. The previous findings were applied to the final study, focusing on PLA and PEO-PBO/PLA films (both amorphous and semi-crystalline) with aligned chains, a common result of film processing techniques used to produce plastic packaging. Isotropic PDLLA and PEO-PBO/PDLLA samples were uniaxially stretched to various stretching ratios (λ). Both neat PDLLA and PEO-PBO/PDLLA films are tough when examined parallel to chain orientation (machine direction (MD)). At λ = 6, neat PDLLA and PEO-PBO/PDLLA display an 8- and 14-fold increase in toughness, respectively, compared to isotropic PDLLA. However only the PEO-PBO/PDLLA films are tough when examined perpendicular to chain orientation (transverse direction (TD)), exhibiting a minimum 10-fold increase in toughness. In the MD, the PDLLA films deform by shear yielding and the PEO-PBO/PDLLA blends deformation mechanism depends on λ. At λ ≤ 2 the films deform by crazing and at λ ≥ 4 the films deform by shear yielding. In the TD, the PEO-PBO blends deform by uniform crazing. When examined in the MD, both PDLLA and PEO-PBO/PDLLA films remain tough through 155 days of aging. The deformation mechanism of the PEO-PBO/PDLLA films changes with aging, and the propensity of the material to deform by crazing increases. Alternatively, the PEO-PBO/PDLLA films tested in the TD are more sensitive to aging, displaying a reduction in ductility with time. However, the films still exhibit a 5-fold improvement in toughness compared to neat PDLLA films in the TD after 155 days of aging. Film stretching of PLLA and PEO-PBO/PLLA films resulted in tough, transparent semi-crystalline blends. The PEO-PBO particles increase the crystallization kinetics and ductility in the TD. This work provides a framework to manufacture tough PLA blends by the addition of a diblock copolymer, addressing the main property limitation of PLA. The blends are processed through industrially relevant procedures and require low mass loadings of additive to achieve sustained toughness, independent of aging time. These results will greatly advance the applications of sustainable PLA, further supporting a more sustainable future.