Crazing Mechanism and Physical Aging of Poly(lactide) Toughened with Poly(ethylene oxide)-block-poly(butylene oxide) Diblock Copolymers

Sustainable polymers are important alternatives to plastics and elastomers derived from petroleum resources. Poly(lactide) (PLA), a commercially available sustainable plastic, is a well-known success story. However, PLA lacks ductility and toughness, limiting the number of potential uses. In this study, small amounts of a liquid poly(ethylene oxide)-block-poly(butylene oxide) (PEO-PBO) diblock copolymer additive were blended with PLA to enhance its toughness and ductility. The incorporated PEO-PBO diblock copolymers generated a macrophase-separated morphology with particle diameters of 0.2-0.9 mu m, and nearly matched refractive indices of PLA and PEO-PBO led to retention of optical transparency. Addition of just 1.8 wt % PEO-PBO into PLA led to a 20-fold increase in toughness, measured as the area under the stress-strain data in tension without affecting the bulk elastic modulus of the plastic. The micromechanical deformation process of the PEO-PBO/PLA blend was investigated via in situ small angle X-ray scattering during tensile testing. The total volume of the crazed material was proportional to the total surface area of the dispersed PEO-PBO particles, and both quantities increased with increasing PEO-PBO loading. Increasing the PEO-PBO loading also resulted in (A) an increase in particle size, causing a decrease in the craze initiation stress, and (B) an increase in fibril spacing, indicating a lower craze propagation stress. Furthermore, craze development was found to be independent of aging time. As a result, the PEO-PBO/PLA blend was able to remain ductile and tough for up to 114 days, exhibiting a 10-fold increase in elongation at break and toughness compared to neat PLA, which becomes brittle in less than 2 days. These results demonstrate that designing additives that promote deformation by crazing is an effective way to overcome the aging-induced embrittlement of glassy polymers.