Abstract
Stereocomplex-type polylactide (SC-PLA) created by alternate packing of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains in a crystalline state has emerged as a growingly popular engineering bioplastic that possesses excellent hydrolytic stability and thermomechanical properties. However, it is extremely difficult to acquire high-performance SC-PLA products via melt-processing of high-molecular-weight PLLA/PDLA blends because both SC crystallites and homocrystallites (HCs) are competitively formed in the melt-crystallization. Herein, a facile yet powerful way was employed to boost SC formation by introducing trace amounts of some epoxy-functionalized small-molecule modifiers into the enantiomeric blends during reactive melt-blending. The results show that the SC formation is considerably enhanced with the in situ generation of multi-arm stereo-block PLA copolymers, based on the reaction between epoxy groups of the modifiers and hydroxyl end groups of PLAs. More impressively, it is intriguing to find that the introduction of only 0.5 wt% modifiers can induce exclusive SC formation in the blends upon isothermal and non-isothermal melt-crystallizations. The outstanding SC crystallizability might be attributed to the suppressing effect of such unique copolymers on the separation of the alternately arranged PLLA/PDLA chain segments in molten state as a compatibilizer. Furthermore, the generation of these copolymers does not result in a significant increase in melt viscosity of the blends. These findings suggest new opportunities for the high-throughput processing of SC-PLA materials into useful products.
Highlights
In the past few decades, biodegradable polylactide (PLA) produced from bio-renewable feedstocks has aroused great repercussions because of its appealing physicochemical properties, such as favorable biocompatibility, excellent transparency, and high mechanical strength and stiffness [1–5]
We demonstrated that the introduction of trace amounts (e.g., 0.3–0.5 wt%) of epoxy-functionalized oligo(styrene-acrylic) (ESA) into the blends leads to the in situ generation of comb-like PLA-graft-ESA copolymers containing long PLLA/PDLA branches in the reactive melt-blending process, and these copolymers can behave as highly efficient compatibilizers to stimulate exclusive SC formation [57]
The SC crystallites can be exclusively formed with further increasing 4EG content up to 0.5 wt%. These results vividly demonstrate that the generation of sufficient amounts of multi-arm PLA copolymers can substantially promote the SC crystallization of racemic PLLA/PDLA blends
Summary
In the past few decades, biodegradable polylactide (PLA) produced from bio-renewable feedstocks has aroused great repercussions because of its appealing physicochemical properties, such as favorable biocompatibility, excellent transparency, and high mechanical strength and stiffness [1–5]. We demonstrated that the introduction of trace amounts (e.g., 0.3–0.5 wt%) of epoxy-functionalized oligo(styrene-acrylic) (ESA) into the blends leads to the in situ generation of comb-like PLA-graft-ESA copolymers containing long PLLA/PDLA branches in the reactive melt-blending process, and these copolymers can behave as highly efficient compatibilizers to stimulate exclusive SC formation [57]. This indicates that reactive blending is a relatively facile and low-cost strategy to remarkably enhance the SC crystallizability of the high-MW enantiomeric blends without sacrificing their valuable properties.
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