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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 Recently, powder metallurgy inspired low-temperature (below the melting temperature) sintering has been proposed as an innovative technology for the processing of eco-friendly stereocomplex-type polylactide (SC-PLA) from its nascent powder, without triggering noticeable thermal degradation. The physicochemical performances of the obtained SC-PLA products are governed by the newly formed SC crystallites with a remarkable ability to weld the interfaces between adjacent powder particles, however, the insufficient interdiffusion of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains across the interfaces makes it challenging to form sufficient amounts of new SC crystallites. In the present work, we describe a facile and robust strategy to address this challenge by coating a trace amount (e.g., 0.001 wt%) of disentangled carbon nanotubes (CNTs) on the SC-PLA particle surfaces. During the subsequent sintering, these interface-localized CNTs can function as efficient nucleating agent to substantially promote the SC crystallization of PLLA/PDLA chains on their surfaces and finally countless SC crystallites are formed to strongly weld the interfaces as welded joints. Accordingly, SC-PLA/CNTs products possessing superb heat/solvent resistances and mechanical strength have been fabricated. Impressively, with the introduction of 0.001 wt% CNTs, the tensile strength is significantly enhanced from 45.8 MPa to 67.2 MPa. Most importantly, there is a linear relationship between the tensile strength and the content of the CNTs induced SC crystallites. Overall, this wok could provide a practical guidance for the design and fabrication of high-performance SC-PLA products through low-temperature sintering assisted by nanofillers like CNTs. Graphical abstract Image 1 Highlights • SC-PLA products with superb performances has been fabricated by CNTs assisted low-temperature sintering. • The disentangled CNTs were coated on the surface of SC-PLA powder particles before sintering. • These CNTs can substantially promote SC crystallization across particle interfaces as nucleating agent. • Countless new SC crystallites are induced to strongly weld the interfaces and thus enhance the product performances. • The linear relationship between tensile strength and CNTs induced SC crystallites has been established. [ABSTRACT FROM AUTHOR]

Selective localization of nanoparticles at the interface of immiscible polymer blends has been witnessed as an efficient method to improve blend properties and even provide some added functionalities. Nevertheless, it is still a great challenge to achieve thermodynamically stable interface-localization of the nanoparticles mainly due to their low interfacial stabilities as well as high transfer speeds between the blend phases, especially for those with high aspect ratios. In this work, taking poly( l -lactide)/poly( d -lactide) grafted ethylene-acrylic ester copolymer/multi-walled carbon nanotubes (PLLA/EGD/MWCNTs) ternary composite as an example, we describe a new and facile strategy to address this challenge via anchoring high-aspect-ratio MWCNTs at the interface of PLLA/EGD blends using interface-localized stereocomplex (SC) crystallites as anchoring agents. During melt mixing of EGD with PLLA/MWCNTs mixture, such SC crystallites can be rapidly formed at the blend interface to prevent MWCNTs transferring from the thermodynamically unfavorable PLLA matrix into favorable EGD phase as physical barriers, and meanwhile some of these transferring MWCNTs could serve as nucleating agents to induce SC crystallization on their surfaces. As a result, many MWCNTs are firmly anchored at the blend interface by these SC crystallites. The anchored MWCNTs can not only function as effective interfacial enhancers to remarkably enhance interfacial strength and resulting impact toughness of PLLA/EGD blends but also enable the formation of electrical conductive pathway in the network-like PLLA/EGD blends at a much lower percolation threshold. This wok could offer a promising opportunity for preparing high-performance and multifunctional PLLA-based composites through controlling particle localization at the interface of immiscible PLLA blends with the aid of SC crystallization. [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]

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]

Stereocomplex-type polylactide (SC-PLA) has recently emerged as an up-and-coming engineering Bio-plastic because the SC crystallization between enantiomeric poly(l -lactide) (PLLA) and poly(d -lactide) (PDLA) can substantially enhance the heat resistance and hydrolytic stability. Unfortunately, it is exceedingly challenging for high-molecular-weight PLLA/PDLA blends to preferentially crystallize into SC crystals (SCs) during melt processing and the concomitant formation of homocrystals (HCs) causes a significant deterioration in physio-chemical properties of final products. Herein, a dibenzoylhydrazide compound has been utilized as an efficient nucleating agent (NA) for the SC crystallization of PLLA/PDLA blends. The results indicate that, in both non-isothermal and isothermal crystallizations, the SC crystallization is markedly promoted with the incorporation of 0.2–1.5 wt% NA. Specially, the enantiomeric blends with relatively high amounts (≥0.5 wt%) of NA are found to exclusively crystallize into SCs without any HCs formation. Meanwhile, the NA can accelerate the SC crystallization and reduce the crystal size. The preferential SC crystallization should be associated with the hydrogen-bonding interactions between PLAs and NA molecules, which could lower barriers for the nucleation and crystal growth of SCs. Our work provides a facile yet powerful strategy for the preparation of high-performance SC-PLA materials with exceptional SC crystallization ability. [Display omitted] • TMC can act as an efficient NA to promote the SC crystallization of PLLA/PDLA blends. • Exclusive formation of SCs is achieved after adding 0.5 wt% TMC into the blends. • The crystallization is greatly accelerated due to the increase in nucleation density. • The hydrogen bonding between TMC and PLA is responsible for the enhanced SC formation. • A facile strategy has been proved to develop SC-PLA with exceptional crystallizability. [ABSTRACT FROM AUTHOR]

Recently, stereocomplex-type polylactide (SC-PLA) has generated growing interest because the unique SC crystals can provide drastic improvement in the heat/chemical resistances and durability of bio-derived and biodegradable PLA, exhibiting great potential to compete with some petroleum-derived engineering plastics in diverse applications. However, SC-PLA suffers from poor melt memory effect as well as significant thermal degradation after being completely melted at temperatures above 250 °C, which make it challenging to converse SC-PLA into useful products using versatile melt-processing technologies. With these challenges in mind, in this work, we propose a facile and practical strategy to fabricate SC-PLA products with exceptional thermomechanical properties through low-temperature melt-processing of SC-PLA powder with a bimodal melting temperature distribution. The results show that the melting temperature (T m) bimodality makes SC-PLA powder able to be easily injection molded at a relatively low temperature of 210 °C (between the two T m s) due to the selective melting of the low- T m SC-PLA component (SC- l PLA, ca. 20–40 wt%), indicating a good melt-processability. Meanwhile, the high- T m SC-PLA (SC- h PLA) component can directly deliver its desirable properties to the injection molded SC-PLA products. More importantly, during the injection molding process, the unmelted SC- h PLA particles can also induce the exclusive SC crystallization of the SC- l PLA melt as nucleating agent and greatly amplify the shear stress imposed on the melt as "rigid particles", finally generating numerous oriented SC structure in the SC- l PLA. Consequently, the injection molded SC- h PLA/SC- l PLA products exhibit impressively high tensile strength (74.3 MPa) and Vicat softening temperature (211.6 °C). Overall, this work presents an effective guidance for achieving desirable melt-processability of SC-PLA without sacrificing its thermomechanical properties and provides an industrial processing route towards high-performance SC-PLA products. Image 106841 • SC-PLA products were fabricated by low-temperature melt-processing of bimodal SC-PLA powder. • The melting temperature bimodality plays a key role in achieving balanced melt-processability and product performance. • The SC- l PLA component can provide the powder with good low-temperature melt-processability. • The SC- h PLA component can endow the melt-processed products with exceptional thermomechanical performance. • SC- h PLA induced exclusive SC formation and crystal orientation can also contribute to the enhanced performance. [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]

Recently, we proposed a novel strategy for fabricating high-performance stereocomplex-type polylactide (SC-PLA) products through low-temperature (180–210 °C, lower than the melting temperature of SC crystallites) sintering from its nascent powder, without serious thermal degradation involved in the conventional melt processing at higher temperatures. During the sintering, some poly( l -lactide) (PLLA) and poly( d -lactide) (PDLA) chains from adjacent powdery particles can interdiffuse across the interfaces and subsequently co-crystallize into new SC crystallites capable of welding the interfaces. The interfacial strength is dominated by the content of the newly-formed SC crystallites, however, the fast cocrystallization of PLLA/PDLA chains could hinder their sufficient interdiffusion at the interfaces and thus only limited interface-localized SC crystallites could be formed. In this work, we attempt to substantially promote the chain interdiffusion via depressing the cocrystallization rate. To do this, SC-PLA nascent powders with different optical purities of the lactate units were prepared, and the role of optical purity in directing the chain interdiffusion and cocrystallization has been investigated. Very interestingly, we demonstrate that decreasing optical purity (from 99.5 to 96%) is favorable for the formation of numerous SC crystallites at the interfaces because the lowering of cocrystallization rate enables more PLLA/PDLA chains to interdiffuse sufficiently before their cocrystallization. As a result, SC-PLA products with superior heat resistance have been fabricated by the sintering of low-optical-purity SC-PLA powder. These fascinating findings could not only provide new understanding on the low-temperature sintering mechanism of SC-PLA powders but also greatly expand the possibilities for the fabrication of SC-PLA products with superb properties. [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]

Stereoblock copolymerization is regarded an appealing strategy to substantially enhance the melt stability (i.e., the survival ability of the intermolecular collaboration between poly(l -lactide) (PLLA) and poly(d -lactide) (PDLA) upon melting) of high-molecular-weight (high-MW) stereocomplex-type polylactide (SC-PLA), however it usually suffers from complicated synthetic procedures and even impaired sustainability. Herein, solid-state transesterification (SST) has been devised as a facile and robust route to prepare unique multi-stereoblock PLA copolymers with outstanding melt stability and unexpectedly strong crystallizability from commercial available linear high-MW PLLA/PDLA (50/50) blends. The SST was performed through low-temperature (i.e., 180 °C) melt-blending of PLLA and PDLA in the presence of trace amounts (0.03–0.035 wt%) of catalysts, where the enantiomeric PLA chains could rapidly co-crystallize into solid-state SC crystallites upon melting and subsequently the hetero-chain transesterification reaction occurs exclusively in the mobile amorphous phase of the pre-formed SC crystallites. As a result, multiblock-like PLLA- b -PDLLA- b -PDLA copolymers with long crystallizable PLLA and PDLA blocks are in-situ generated. The results show that high-content SC crystallites can be exclusively formed in the melt-crystallization of the copolymers even at high cooling rates (e.g., 40 °C/min) and low temperatures (e.g., 140 °C). More interestingly, the presence of the non-crystallizable PDLLA blocks not only does not disturb the SC crystallizability of the enantiomeric PLA blocks, as evidenced by the same high crystallinity and melting temperature as those of starting PLLA/PDLA blends, but also increase the crystallization rate by stabilizing the clusters of ordered PLLA/PDLA segments in the copolymer melt. Overall, these findings could open a new pathway to develop melt-processible all SC-PLA for engineering applications. Image 1 • Unique stereoblock PLA copolymers are prepared via solid-state transesterification (SST). • The SST was performed during low-temperature melt-blending of high-MW PLLA and PDLA. • SC crystallites rapidly formed are subjected to hetero-chain reaction in the amorphous phase. • Multiblock-like PLLA- b -PDLLA- b -PDLA copolymers with long crystallizable blocks are in-situ generated. • The obtained copolymers possess outstanding melt stability and strong crystallizability. [ABSTRACT FROM AUTHOR]

As a popular "green" engineering plastic, stereocomplex-type polylactide (SC-PLA) exhibits great application potential in various fields owing to its outstanding physicochemical performance and durability. However, the applications of SC-PLA still face formidable challenges mostly associated with its inferior melt-processability (i.e., the weak melt memory effect to motivate exclusive SC crystallization and extremely low melt viscosity) and the lack of necessary functional features (e.g., electrical conductivity) in some cases. Herein, we devise a facile and robust strategy to overcome these obstacles by incorporating carbon black (CB) into equimolar poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blend. It is interesting to find that the CB particles can adsorb many PLLA/PDLA chain segments on their surface and such strongly adsorbed PLA segments could interact with PLA chains outside the surface to form physical junctions capable of stabilizing the PLLA/PDLA chain assemblies in the melt, finally inducing the exclusive SC formation during subsequent crystallization. Meanwhile, the CB particles can substantially enhance the melt viscosity of the blend (from 3.9 Pa s to 844.1 Pa s when measuring at 250 °C and 50 Hz). Because of the greatly improved melt-processability, the PLLA/PDLA/CB composites have been successfully processed into highly crystalline products with exclusive SC crystallites and excellent thermomechanical performance by injection molding. Additionally, the CB particles can endow the composite products with fascinating electrical conductivity (19.0 S/m) and electromagnetic interference shielding effectiveness (26.6 dB). This work could open up a promising avenue towards high-performance and multifunctional PLA engineering Bioplastic. Image 1 • CB particles are used to improve the melt-processability and electrical performance of SC-PLA. • The CB particles exhibit strong adsorbability for PLLA/PDLA chain segments. • The adsorbed PLA segments contribute to the greatly enhanced melt memory effect and melt viscosity. • The PLLA/PDLA/CB composites can be injection molded into products with exclusive SC crystallites. • The obtained products possess superb thermomechanical and electrical properties. [ABSTRACT FROM AUTHOR]