Sc Crystallites Research Articles

Synthesis of core–shell ZnO nanoparticles and their effect on mechanical and antibacterial properties for PLLA/ZnO nanocomposites

AbstractTo overcome the inherent brittleness of renewable poly(l‐lactide) acid (PLLA) and enable it for use in a wide application, we successfully synthesized the toughening agent of ZnO nanoparticles grafted by poly(caprolactone‐co‐d‐lactide) (ZnO‐g‐PCLDLA) via sequential open‐loop polymerization. First, ZnO was silanized to produce hydroxyl groups on the surface of the nanoparticles. Initiated by hydroxyl groups on the ZnO core, the rubbery PCLDLA composed of caprolactone‐d‐lactide co‐polymeric segments and d‐lactide homo‐polymeric segments was successively polymerized, forming ZnO core–shell nanoparticles. PLLA/ZnO‐g‐PCLDLA nanocomposites were prepared by solution blending. The nanocomposites with 10 wt% ZnO‐g‐PCLDLA exhibited superior ductility, whose elongation at break was about 54 times higher than that of neat PLLA. This considerable increment of toughness was attributed to the rubbery shell of PCLDLA and the enhanced interfacial interaction from the interfacial stereocomplex crystallites, formed by co‐crystallization of the PLLA matrix and d‐lactide homo‐polymeric segments of the PCLDLA shell on ZnO. The PLLA/ZnO‐g‐ PCLDLA nanocomposite films possessed outstanding antimicrobial properties against Staphylococcus aureus and Escherichia coli. Positive ZnO produces an electrostatic force to interact with the negatively charged membrane of bacteria, thereby damaging the cell membrane and achieving significant antibacterial effects.Highlights ZnO‐g‐PCLDLA core–shell nanoparticles were constructed by sequential ROP. The ZnO core provided high rigidity and antimicrobial properties. The rubbery PCLDLA shell provided high elasticity and ductility. Interfacial interaction was greatly enhanced by the SC crystallites. The PLLA nanocomposite films exhibited outstanding antimicrobial activity.

Preferential formation of stereocomplex crystals in poly(L-lactic acid)/poly(D-lactic acid) blends by a fullerene nucleator

Selective formation of stereocomplex (sc) crystallization in enantiomeric poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends is considered as one of the most effective and promising way to improve the mechanical and thermal properties of polylactide (PLA) materials. However, homocrystallization (hc) prevails over sc crystallization in high-molecular-weight (HMW) PLLA/PDLA blends. Herein, we propose a simple and straightforward approach for fabricating sc crystallization and suppress hc crystallization for HMW PLLA/PDLA blends through the addition of C70 as a nucleator. Non-isothermal crystallization and wide-angel X-ray diffraction studies demonstrate that, the incorporation of 1 wt% C70 overwhelmingly leads to the formation of sc crystallites, while preventing the formation of hc crystallites. Isothermal crystallization experiments at 140 °C reveal a significant reduction in the half-crystallization period of the PLLA/PDLA blend upon the addition of C70. Fourier-transformed infrared spectroscopy suggests that, the improved intermolecular interactions between PLLA and PDLA chains, as well as the inhibition of molecular chain diffusion and mobility, contribute to the accelerated formation of sc facilitated by C70. The enhanced sc crystallization results in a 15.5 °C higher thermal stability in the as-prepared PLLA/PDLA blend with 1 wt% C70 compared to the neat counterpart.

Scalable Fabrication of Polymeric Composite Microspheres to Inhibit Oral Pathogens and Promote Osteogenic Differentiation of Periodontal Membrane Stem Cells.

Periodontitis is a worldwide bacterial infectious disease, resulting in the resorption of tooth-supporting structures. Biodegradable polymeric microspheres are emerging as an appealing local therapy candidate for periodontal defect regeneration but suffer from tedious procedures and low yields. Herein, we developed a facile yet scalable approach to prepare polylactide composite microspheres with outstanding drug-loading capability. It was realized by blending equimolar polylactide enantiomers at the temperature between the melting point of homocrystallites and stereocomplex (sc) crystallites, enabling the precipitation of sc crystallites in the form of microspheres. Meanwhile, epigallocatechin gallate (EGCG) and nano-hydroxyapatite were encapsulated in the microspheres in the designated amount. Such an assembly allowed the fast and sustained release of EGCG and Ca2+ ions. The resultant hybrid composite microspheres not only exhibited strong antimicrobial activity against typical oral pathogens (Porphyromonas gingivalis and Enterococcus faecalis), but also directly promoted osteogenic differentiation of periodontal ligament stem cells with good cytocompatibility. These dual-functional composite microspheres offer a desired drug delivery platform to address the practical needs for periodontitis treatment.

Enhanced polylactide stereocomplexes by aluminum oxide particles for reliable thermal conductivity at elevated temperature

Stereocomplex-type polylactide (sc-PLA) with high heat resistance provides great promise for renewable and environmentally friendly PLA as alternative engineering plastics. However, due to the weak melt memory of stereocomplex (sc) crystallites, that is, its poor ability to reform sc crystallites after melting, sc-PLA cannot be processed by conventional melting methods, and its lack of functional properties limits its practical applications. In this work, the interchain interactions between PLLA and PDLA chains are found to be well kept in equimolar PLLA/PDLA blends via the incorporation of Al2O3 particles for the first time, which effectively overcomes these drawbacks. The composites show good melt memory for sc crystallization, and exclusive sc crystallites form during cooling after eliminating the thermal history when the content of Al2O3 is higher than 3 vol%. Moreover, exclusive sc crystallites still can be formed in the composites with high Al2O3 contents of 40 vol%. The interchain interactions between PLLA and PDLA chains are well kept in the melt of the composites, endowing the composites with the capability to overcome the kinetic barrier and form sc crystallites rapidly during cooling. Benefiting from the high thermal conductivity of Al2O3, and enhanced sc crystallites with high heat resistance, the sc-PLA composite with 40 vol% Al2O3 shows a thermal conductivity of 1 Wm−1K−1, and the thermal conductivity can be maintained at elevated temperatures (25–180 °C). The synchronously enhanced sc formation and reliable thermal conductivity at elevated temperatures enabled by Al2O3 provide a new design approach for PLA as an alternative engineering plastic.

Substantially Enhanced Stereocomplex Crystallization of Poly(L-lactide)/Poly(D-lactide) Blends by the Formation of Multi-Arm Stereo-Block Copolymers

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.

A generalizable strategy toward highly tough and heat-resistant stereocomplex-type polylactide/elastomer blends with substantially enhanced melt processability

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.

Influence of miscible and immiscible sequences of poly(D-lactide) copolymers on the competition of stereocomplex- and homo-crystallization in poly(L-lactide) blends

A high molecular weight (HMw) PLLA has been blended with a low molecular weight (LMw) PDLA and its two types diblock copolymers (PEG-b-PDLA and PDMS-b-PDLA), respectively, to investigate the stereocomplex (sc) and homochiral (hc) crystallization behaviors. The miscible PEG and immiscible PDMS segments with comparable molecular weight were introduced into PDLA by bulk ring-opening polymerization method. Compared to PLLA/PDLA, the introduction of both sequences could inhibit the crystallization kinetics of sc and hc crystallites of the blends due to the dilution effect of LMw segments. The formation of sc crystallites is prior to its hc counterparts. The shorter induction period of hc crystallites of PLLA/PEG-b-PDLA blend indicates that the miscible PEG segments could promote the mobility of PLLA chains. Due to the involving of PEG chains, the spherulite exhibits coarsen and irregular texture and its lamellae show sparse string-like structure. The microphase separation between PLA and PDMS segments as evidenced by SAXS results in the increased sc nuclei and different growth mode of hc crystallites. The phase separated microdomains weakened the competition between sc and hc crystallites, and the former acts as a template for repeated assembly of the latter. X-ray analysis also reveal that sc crystallites possess much thicker lamellar thickness than hc crystallites in PLLA/PDLA and PLLA/PEG-b-PDLA blends.

Favorable formation of stereocomplex crystals in long-chain branched poly(L-lactic acid)/poly(D-lactic acid) blends: impacts of melt effect and molecular chain structure

A feasible approach is proposed to promote the formation of the stereocomplex crystal (Sc) in long-chain branched Poly(L-lactic acid)/Poly(D-lactic acid) (LCBPLA/PDLA) blends with hydrogen bond interactions. The synergistic effect of PDLA and long-chain branches significantly increases the crystalline ability of Sc from 20.1 to 30.4% due to the improved intermolecular crystal nucleation/growth. The frequency-independent loss tangent appeared at a PDLA concentration of 5.0 wt%, indicating a transition from the liquid-like to solid-like viscoelastic and the formation of a network composed of long-chain branches and the reserved Sc crystallites. Changing the processing temperatures from 190 to 230 °C seems to induce different melting behavior of Sc with diverse topological conformations so as to act as a template to attract PLLA molecule chains to perform the crystallization behavior. In situ wide-angle X-ray diffraction analysis reveals that a higher cooling temperature (less than 220 °C) significantly contributes to the Sc with more integrated structures and the Hc-to-Sc transition. The polarizing optical microscope results show that increasing the PDLA content and long-chain branched points significantly promote the spherulite growth and nuclei density of Sc, increasing the Sc size from 56.47 to 94.13 μm and nuclei density from 52.2 to 98.3%.

Stereocomplex- and homo-crystallization behavior, structure, morphology, and thermal properties of crystalline and amorphous stereo diblock copolymers, enantiomeric Poly(l-lactide)-b-Poly(dl-lactide) and Poly(d-lactide)-b-Poly(dl-lactide)

Enantiomeric stereo diblock poly (lactide) polymers of crystalline poly (l-lactide) or poly (d-lactide) with amorphous poly (dl-lactide) (PLLA-b-PDLLA and PDLA-b-PDLLA, respectively) were synthesized to have different molecular weights of crystalline block but similar overall molecular weights and the effects of fraction or molecular weight of crystalline block on the crystallization behavior, structure, morphology, and thermal properties of PLLA-b-PDLLA/PDLA-b-PDLLA blends were investigated for the first time. In the cases of isothermal crystallization from the melt, heating after melt-quenching, and solvent evaporation, PLLA-b-PDLLA/PDLA-b-PDLLA blends were crystallizable for crystalline block fraction down to 21.5%, whereas in the case of cooling from the melt, PLLA-b-PDLLA/PDLA-b-PDLLA blends were crystallizable only for crystalline block fraction down to 46.1%. Interestingly, the incorporated long amorphous PDLLA blocks facilitated exclusive SC crystallization in PLLA-b-PDLLA/PDLA-b-PDLLA blends with the crystalline block fractions of 46.1 and 21.5%. As with the results reported for unblended PLLA-b-PDLLA, the transition crystallization temperature from α-form to δ-form decreased with decreasing crystalline block fraction. The melting temperatures of SC crystallites and homo-crystallites, radial growth rate of spherulites, and total crystallization rate decreased with a decrease in crystalline block fraction. For Tc range wherein only SC crystallites were formed, only one regime was observed irrespective of crystalline block fraction and Avrami exponent was not significantly affected by the presence of amorphous PDLLA block and Tc. The present study revealed that the incorporation of PDLLA block into PLLA and PDLA can be effectively utilized for enhancement of SC crystallization during heating and isothermal crystallization from the melt.

Low-temperature sintering of stereocomplex-type polylactide nascent powder: The role of poly(methyl methacrylate) in tailoring the interfacial crystallization between powder particles

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.

Toward all stereocomplex-type polylactide with outstanding melt stability and crystallizability via solid-state transesterification between enantiomeric poly(l-lactide) and poly(d-lactide)

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.

Stereocomplexed Poly(lactide) Composites toward Engineering Plastics with Superior Toughness, Heat Resistance and Anti-hydrolysis

Poly(lactide), PLA, suffers from brittleness and low heat deflection temperature (HDT), which limits its application as an engineering plastic. In this work, poly(L-lactide)/poly(D-lactide)/ethylene-vinyl acetate-glycidyl methacrylate random copolymer (PLLA/PDLA/EVM-GMA = 1/1/x) composites were prepared by melt blending, and the in situ formed EVM-g-PLA copolymers improved the compatibility between PLA and EVM-GMA. Subsequently, the blends were subjected to a two-step annealing process during compression molding, i.e. first annealing at 120 °C to rapidly form a certain amount of stereocomplex (sc) crystallites as nucleation sites, and then annealing at 200 °C to guide the formation of new sc crystallites. Both differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) measurements confirmed the formation of highly stereocomplexed PLA products. Mechanical results showed that the PLLA/PDLA blend with 20 wt% of EVM-GMA had a notched impact strength up to 65 kJ/m2 and an elongation at break of 48%, while maintaining a tensile strength of 40 MPa. Meanwhile, dynamic mechanical analysis (DMA) and heat deflection tests showed that the PLA composite had an HDT up to 142 °C which is 90 °C higher than that of normal PLA products. Scanning electron microscopy (SEM) confirmed the fine dispersion of EVM-GMA particles, which facilitated to understand the toughening mechanism. Furthermore, the highly stereocomplexed PLA composites simultaneously exhibited excellent chemical and hydrolysis resistance. Therefore, these fascinating properties may extend the application range of sc-PLA material as an engineering bioplastic.

Electrospun membranes of low molecular weight di-stereoblock poly(lactic acid)s with high thermal stability and solvent resistance via low temperature sintering

Electrospinning is a highly efficient mild method to obtain stereocomplex poly(lactic acid) fibers from blends of high molecular weight (MW) poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). However, high MW PLAs are expensive and proved difficult to avoid homocrystallites which weaken the performances. Here, di-stereoblock poly(lactic acid)s (di-sb-PLAs) with low MW of 104 g mol−1 were synthesized and electrospun into membranes. The stereocomplex (sc) crystallites were induced by sintering the amorphous as-spun membranes at 200 °C, and further improved by applying 1 MPa pressure which promoted the segment migration across fiber intersections and welded the fibers by forming new sc crystallites. Due to more readily crystallize at molecular level than the blend of PLLA and PDLA, the sintered di-sb-PLA membranes exhibited higher thermal stability, better tensile strength, and were more stable during multiple cycles of selective separating solvent and water mixtures. Increasing MW of di-sb-PLA positively affected the membrane properties and inhibited dramatical embrittlement that occurred in sintered membrane of PLLA and PDLA blend.

Stereocomplex-type polylactide with remarkably enhanced melt-processability and electrical performance via incorporating multifunctional carbon black

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.

Green Method to Widen the Foaming Processing Window of PLA by Introducing Stereocomplex Crystallites

Polylactide (PLA) microcellular foams with well-defined cell morphology were successfully fabricated from poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) blends through a supercritical carbon dioxide (CO₂) foaming method. The gaps between the melting temperature of homocrystallites (Tₘ₍HC₎) and stereocomplex crystallites (Tₘ₍SC₎) were utilized to improve PLA’s melt foaming behaviors (i.e., cell nucleation and cell morphology) owing to the existence of exclusive SC crystallites when the foaming temperature fell in between Tₘ₍HC₎ and Tₘ₍SC₎. Thanks to the enhanced melt strength, which resulted from the introduction of SC crystallites, the cell morphology transformed from elliptical to nearly circular and the cell collapse and coalescence reduced when foamed at high temperatures and pressures. The wider foaming processing window is favorable for actual industrial manufacture such as the continuous extrusion foaming process. In addition, PLA foams possessing more SC crystallites exhibit higher heat resistance, which further broadens their applications in high-temperature conditions.

Enhancing the recrystallization ability of bio-based polylactide stereocomplex by in situ construction of multi-block branched conformation

To enhance the formation and regeneration ability and the toughness of stereocomplex polylactide (sc-PLA), a high molecular weight stereocomplex branched multi-block polylactide (sb-PCLDA) was synthesized from d-lactide, l-lactide and poly(e-caprolactone) diol (PCL). High molecular weight sc-PLA has attracted much attention in bioengineering because of its good mechanical and thermal properties, but its brittleness and the poor regeneration ability of the sc crystallites hindered the practical processability, that is, the mixed homochiral and sc crystallites would form in the sc-PLA products after the complete melting and recrystallization processing. The high molecular weight sc-PLA which is easy to form complete stereocomplex and suitable for melt processing needs further research. Herein, sb-PCLDA having PCL, PLLA and PDLA segments on one molecular chain has been synthesized by polymerization of l-lactied and d-lactide with PCL and in situ hydrolysis condensation with silane coupling agents. Differential scanning calorimetry and wide angle X-ray diffraction indicated that sb-PCLDA formed complete sc with high crystallinity and exhibited good sc reformulation ability after multiple melting, suggesting much improved processability. Meanwhile, the sb-PCLDA maintained good thermal properties and hydrolysis resistance. The polymer exhibited significant tensile toughness with an elongation at break of 185%. This facile method could simultaneously improve the mechanical property and recrystallization ability of sc-PLA.

Toward Supertough and Heat-Resistant Stereocomplex-Type Polylactide/Elastomer Blends with Impressive Melt Stability via in Situ Formation of Graft Copolymer during One-Pot Reactive Melt Blending

Stereocomplexation of enantiomeric poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) chains opens up a great opportunity toward sustainable PLA engineering plastic with exceptional heat resistance and durability. However, the processing and applications of stereocomplex-type PLA (SC-PLA) are significantly blocked by its inferior melt stability (i.e., the weak melt memory effect in triggering complete SC crystallization, which makes it hard to obtain exclusive formation of SC crystallites in melt-processed products) and inherent brittleness. In this contribution, we demonstrate an unprecedented strategy to address these obstacles by one-pot reactive melt blending of the equimolar PLLA/PDLA blend with reactive poly(ethylene–methyl acrylate–glycidyl methacrylate) (E-MA-GMA) in the presence of catalyst, where both the stereocomplexation and the grafting of some PLLA/PDLA chains onto E-MA-GMA backbones take place simultaneously and competitively. Intriguingly, the E-MA-graft-PLA copolymer in situ formed can substant...

Design of Supertoughened and Heat-Resistant PLLA/Elastomer Blends by Controlling the Distribution of Stereocomplex Crystallites and the Morphology

Supertoughened and heat-resistant poly(l-lactide) (PLLA)/elastomer blends were prepared by controlling the distribution of stereocomplex (sc) crystallites and the morphological change from sea–isla...

A promising strategy for fabricating high-performance stereocomplex-type polylactide products via carbon nanotubes-assisted low-temperature sintering

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.

Smart Design of Rapid Crystallizing and Nonleaching Antibacterial Poly(lactide) Nanocomposites by Sustainable Aminolysis Grafting and in Situ Interfacial Stereocomplexation

Sustainable antibacterial poly(lactide) (PLA) nanocomposites have attracted plenty of attention due to their potential application in biomedical and packaging fields. However, the preparation of nonleaching antibacterial PLA nanocomposites with high crystallinity is still a challenge due to their poor compatibility and the low crystallization rate of PLA. An effective and facile way by compounding poly(d-lactide) (PDLA) with poly(l-lactide) (PLLA)-grafted ZnO that was synthesized via in situ aminolysis reaction at the surface of ZnO was designed to simultaneously overcome these limitations. During solution mixing, the PDLA matrix and the grafted PLLA chains tend to orient side by side at the PDLA/ZnO interface and finally cocrystallize into stereocomplex (sc) crystallites. The interfacial stereocomplexation not only serves as a nucleator to increase the crystallization rate of the matrix but also effectively prevents ZnO from leaching out of the polymer matrix. Meanwhile, the nanocomposites have excellent...