Crazing and Toughness in Diblock Copolymer-Modified Semicrystalline Poly(<scp>l</scp>-lactide)
Charles J. McCutcheon, Boran Zhao, Christopher J. Ellison, Frank S. Bates
Abstract
Sustainable semicrystalline poly(l-lactide) (PLLA) was melt mixed with 5 wt % poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) diblock copolymer, resulting in blends that display an exceptional combination of properties. The blends were annealed at various temperatures, leading to different degrees of crystallinity. The addition of 5 wt % PEO-PBO produced finely dispersed liquid particles that caused a significant reduction in the time for crystallization after quenching from the melt, where Tm = 166 °C. At 95 °C, the halftime for crystallization was t1/2(95 °C) = t1/2o/7, while at 135 °C, t1/2(135 °C) = t1/2o/5, where t1/2o is the time required to obtain 50% of the final extent of crystallization with pure PLLA. The block copolymer particles also enhanced the ductility of the blends by facilitating stress-induced cavitation and uniform crazing without impacting the modulus. Tensile toughness increased by 7–15 fold, scaling inversely with the degree of crystallinity. The deformation mechanism was investigated by small- and wide-angle X-ray scattering as a function of applied strain, revealing that the craze volume is dependent on crystallinity, while the crystal structure displayed minimal changes. Regardless of the extent of crystallinity, crazing was found to be the primary deformation mechanism, countering the ductile-to-brittle transition associated with the aging of PLLA. Adding 5 wt % PEO-PBO extends the strain at break from 4% for pure PLLA after 2 days to more than approximately 50% after 85 or more days of aging. These findings, along with the industrially relevant blend preparation method, reveal that PEO-PBO is a unique and potent additive that could expand the applications served by PLLA, promoting a more sustainable future.