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Stereocomplex (SC) crystallization can prominently improve the physico-chemical properties of poly(l -lactide)/poly(d -lactide) (PLLA/PDLA) blends, yielding a novel polylactide (PLA) material. However, the predominant formation of SC crystals in the melt-processing of high-molar-mass (high-MW, >100 kg/mol) enantiomeric PLA blends remains a huge challenge due to the competition between SC crystallization and homocrystallization. Herein, double-grafted copolymer having both PLLA and PDLA side chain has been designed and synthesized as an efficient crystallization promoter for the harvest of SC crystals in the high-MW PLLA/PDLA blends. The results show that, with the addition of such a copolymer, the blends can preferentially crystallize into SC crystals in both isothermal and non-isothermal conditions. Promisingly, the SC crystals can be exclusively formed by adding only small amounts (e.g., 0.5 wt%) of the copolymer, without the formation of any homocrystals. This interesting observation can be interpreted by the crucial role of the unique copolymer in suppressing the phase separation of the opposite PLA enantiomers upon melting as an efficient compatibilizer and then encouraging the generation of alternatingly arranged PLLA/PDLA chain clusters favored for SC nucleation and crystal growth. These findings provide new inspiration for the development of high-performance PLA with desirable SC crystallizability. [Display omitted] [ABSTRACT FROM AUTHOR]

Over the past two decades, biomass-derived and biodegradable polylactide (PLA) has sparked tremendous attention as a sustainable alternative to traditional petroleum-derived polymers for diverse applications. Unfortunately, the current applications of PLA have been mainly limited to biomedical and commodity fields, mostly because the poor heat resistance (resulting from low melting temperature) and hydrolysis stability make it hard to use as an engineering plastic. Stereocomplexation between enantiomeric poly( l-lactide) (PLLA) and poly( d-lactide) (PDLA) opens a new avenue toward PLA-based engineering plastics with improved properties. The formation, crystal structure, properties, and potential applications of stereocomplex-type PLA (SC-PLA) are summarized by some research groups. However, since it is challenging to achieve full stereocomplexation from high-molecular-weight PLLA/PDLA blends and to avoid serious thermal degradation of the PLAs after complete melting, the advances and progress in the processing of SC-PLA into useful products are quite rare in open publication. In this review, some important strategies for enhancing stereocomplex crystallization in practical processing operations are presented and recently developed processing technologies for SC-PLA are highlighted, such as low-temperature sintering. Furthermore, major challenges and future developments are briefly discussed. This review is expected to potentially open up new research activities in the processing of SC-PLA. [ABSTRACT FROM AUTHOR]

Sustainable stereocomplex-type polylactide (SC-PLA) has been recognized as a potential substitute for some traditional engineering plastics, however its widespread application still faces several hurdles primarily related to poor melt processability (i.e., the unfavorable melt memory effect in SC crystallization as well as the low melt strength) and insufficient fracture toughness. To overcome these hurdles, an equimolar poly(l -lactide)/poly(d -lactide) (PLLA/PDLA) blend toughened with poly(ethylene -co- vinyl acetate) (EVA) has been chosen as a model system and a facile strategy has been devised by incorporating trace amounts (i.e., 0.5 wt%) of epoxy-functionalized oligo(styrene-acrylic) (ESA) into the PLLA/PDLA/elastomer blends through direct melt-blending at 200 °C. Such a one-pot blending not only facilitates the stereocomplexation between PLLA and PDLA chains but also permits the highly efficient reaction between terminal hydroxyl groups of some enantiomeric PLAs and epoxy groups of ESA before their stereocomplexation. This results in the in situ generation of large amounts of long-chain-branched PLA- graft -ESA copolymer in the SC-PLA matrix. Very impressively, the generation of this PLA- graft -ESA copolymer gives rise to a substantial improvement in the melt memory effect of the SC-PLA matrix. This copolymer behaves as a compatibilizer to stabilize regular PLLA/PDLA chain clusters in blend melts and hence to stimulate the exclusive formation of SC crystallites in melt-processed blend products. In addition, long PLA branches endow the matrix with a considerably enhanced melt viscosity owing to the increased interchain entanglement density. Consequently, highly stereocomplexed SC-PLA/EVA products with exceptional impact toughness and heat resistance as well as robust mechanical strength have been prepared by injection molding. More notably, this strategy can be generalized to various SC-PLA-based blends, irrespective of whether the elastomers have reactive groups. Overall, these exciting findings illustrate a promising avenue toward the design and applications of high-performance SC-PLA-based materials with excellent melt-processability via the generation of unique graft copolymers having long PLLA and PDLA branches. [Display omitted] • SC-PLA/elastomer blends with markedly enhanced melt processability are designed. • Long-chain-branched copolymers are in-situ generated during melt-blending with ESA. • The chain branching induces drastically enhanced melt memory effect and melt strength. • The blends have been injection-molded into super-tough and heat-resistant products. • Our strategy can be generalized to various SC-PLA-based blends. [ABSTRACT FROM AUTHOR]

Abstract Polylactide (PLA), a promising biodegradable and renewable polymer, finds many applications, particularly, used as biodegradable fiber in textile industry. The key is to improve its heat resistance. In this work, small amount of poly (D-lactide) (PDLA) was added firstly into poly (L-lactide) (PLLA) matrix to form stereocomplex polylactide (SC-PLA), which has much better heat resistance than that of PLLA matrix. Then two melting temperatures were adopted for fiber spinning, the one (210 °C) is far below the melting temperature of SC-PLA (i.e., 220–230 °C), and the other (225 °C) is close to the melting temperature of SC-PLA. The boiling water shrinkage and tensile strength of PLLA/PDLA fibers were investigated as function of PDLA content. It was found, the boiling water shrinkage could be largely reduced to as low as 5.6% and a satisfactory tensile strength (628 MPa) is achieved for fibers spun at 225 °C, compared with fibers spun at 210 °C (7.5% and 500 MPa, respectively), as the PDLA content is 3 wt%. SEM observation suggests a formation of nanofibrillar like SC-PLA structure as spun at 225 °C, while only spherical SC-PLA particles as spun at 210 °C. DSC result indicates that adding PDLA could largely enhance the crystallization rate of PLLA, but almost a constant crystallinity and SC-PLA content were observed for both spun fibers, disregarding the spinning temperatures. Herman's orientation parameters of spun fiber were obtained from 2D-WAXD, which suggests that fiber spun at 225 °C has higher orientation than that of fiber spun at 210 °C. Thus it can be concluded that the formation of nanofibrillar like SC-PLA and enhanced orientation degree contribute mainly to the improved heat resistance and tensile strength. This work presents a feasible and effective way to fabricate all-PLA-based fibers with the outstanding heat resistance by adding a small amount of PDLA and using a suitable spinning temperature. Graphical abstract Image 1 Highlights • The influence of spinning temperature on the orientation and properties of PLLA/PDLA fibers was depicted in detail. • A critical PDLA content of increasing PLLA/PDLA fibers' heat resistant and mechanical property was revealed.. • All-PLA-based fiber with outstanding heat resistance was manufactured by melt spinning process. [ABSTRACT FROM AUTHOR]

Abstract As the most promising bio-derived and biodegradable polymers with tremendous application potential, the practical application of poly(l -lactide) (PLLA) has been severely restricted by its inherent brittleness. Unfortunately, blending with various rubbers can substantially enhance the toughness of PLLA, but usually causes a significant deterioration in mechanical strength and modulus. In this work, taking novel PLLA/core-shell rubber (CSR) blends as an example, we describe a facile and robust strategy to obtain good stiffness-toughness balance by promoting the networking of CSR particles within PLLA matrix with the aid of stereocomplex (SC) crystallites. To do this, small amounts (5–20 wt%) of poly(d -lactide) (PDLA) were incorporated into the blends through simple melt-blending. The results demonstrate that the CSR particles can be uniformly dispersed in PLLA matrix due to the good interfacial compatibility, but the stereocomplex (SC) crystallites formed between the incorporated PDLA and PLLA matrix chains induces the organization of the CSR particles into a network-like structure in the matrix. Compared with the well-dispersed structure, the network-like structure can endow the PLLA/CSR blends with dramatically enhanced impact toughness, without compromising their mechanical strength and modulus, indicating a marvelous balance between the stiffness and toughness. Furthermore, the elongation at break of the blends is also enhanced greatly. These interesting findings suggest that the formation of SC crystallites could provide a promising avenue for fabricating CSR particles toughened PLLA blends with balanced stiffness-toughness. Graphical abstract Image 1 Highlights • A promising strategy to induce the networking of CSR particles in PLLA/CSR blends has been reported. • The CSR particles can be organized into a network-like structure in PLLA matrix under the drive of SC crystallites. • The SC crystallites is formed during melt mixing of the blends with small amounts of PDLA. • The formation of the network structure can significantly enhance the toughening efficiency. • The obtained PLLA/CSR/PDLA blends exhibit a balanced stiffness and toughness. [ABSTRACT FROM AUTHOR]

Despite of its enormous application potential as a prominent engineering bioplastic, the processing of the stereocomplex-type polylactide (SC-PLA) formed from poly(l -lactide)/poly(d -lactide) (PLLA/PDLA) blends into useful products remains challenging. Recently, low-temperature sintering has emerged as a feasible and promising processing technology towards transparent highly-crystalline SC-PLA products. During the sintering of SC-PLA nascent powder, the interfaces between adjacent particles could be bound together through SC crystallization of interdiffused PLLA and PDLA chains across the interfaces. However, the massive generation of the interface-localized SC crystallites is significantly impaired by the insufficient chain interdiffusion resulting from the rapid SC crystallization. In this work, miscible poly(methyl methacrylate) (PMMA) is adopted to substantially promote the chain interdiffusion by fundamentally diminishing the SC crystallization rate. Intriguingly, we find that the crystallization of PLLA/PDLA chains is greatly decelerated with the incorporation of PMMA and thus more enantiomeric PLA chains could interdiffuse through the interfaces before their crystallization. Moreover, the PMMA is also favorable for the particle wetting of the densified nascent powders, which makes it easier for the enantiomeric chains to interdiffuse across the interfaces. The promoted chain interdiffusion leads to an evident increase in the number of the newly-generated SC crystallites and resulting interfacial weld strength. Consequently, the sintered SC-PLA products possessing superior thermomechanical properties and markedly enhanced hydrolytic resistance have been fabricated by incorporating 9 wt% PMMA, without compromising transparency. These findings could bode well for the industrial fabrication and applications of SC-PLA products. Image 1 • PMMA has been used to improve the low-temperature sintering of SC-PLA nascent powder. • The PMMA can promotes the interdiffusion of PLLA/PDLA chains across the particle interfaces. • Both the decelerated crystallization and enhanced particle wetting contribute to the promotion of the chain interdiffusion. • Sufficient amounts of interface-localized SC crystallites are generated to achieve mechanically robust interfaces. • The obtained SC-PLA products have superior thermomechanical properties, transparency, and hydrolytic stability. [ABSTRACT FROM AUTHOR]