Thicker Lamellae and Higher Crystallinity of Poly(lactic acid) via Applying Shear Flow and Pressure and Adding Poly(ethylene Glycol).

In this work, we explored the crystallization of poly(lactic acid) (PLA) blended with poly(ethylene glycol) (PEG) under two inevitable processing fields (i.e., flow and pressure) that coexist in almost all processing for the first time. Here, the PEG was incorporated into PLA as a molecular chain activity promoter to induce PLA crystallization. A homemade pressuring and shearing device was utilized to prepare samples and necessary characterization methods, such as differential scanning calorimetry, scanning electron microscopy, and synchrotron radiation, and were used to investigated the joint effects of PEG, pressure, and shear flow on the crystallization behaviors and morphologies of PLA/PEG samples. The results reveal that adding 3-5 wt % PEG into PLA can significantly increase the PLA crystallinity due to the efficient plasticization effect of PEG, while the PEG content reaches 10 wt %, the PLA crystallinity decreases drastically as the phase separation between PEG and PLA occurs. We also find that applying a higher pressure (∼100 MPa) can facilitate the formation of thicker lamellae with fewer defects as well as higher crystallinity under an equal degree of supercooling compared to normal pressure or a low pressure condition because the slip of molecular chains during crystallization makes the lamellae thicker under higher pressures. The PLA crystalline structure in the PLA/PEG sample is not influenced by the shear flow, yet the crystallinity is largely enhanced by applying a shear flow with an appropriate intensity (0-3.5 s-1 ). It is worth noting that pressure and shear flow show a synergetic effect to fabricate PLA/PEG samples with high crystallinity. These meaningful results could beyond doubt help comprehend the relationship between crystallization conditions and crystallization behaviors of PLA/PEG samples and thus provide guidance to obtain high-performance PLA/PEG products via controlling crystallization conditions.