Biodegradable “Core–Shell” Rubber Nanoparticles and Their Toughening of Poly(lactides)

Nanoconfinement-Enhanced Fire Safety and Mechanical Properties of Polylactic Acid with Nanocerium Metal-Organic Frameworks

Ce-based metal-organic frameworks (Ce-MOFs) were successfully synthesized by coordinating binary acid and amino structures with cerium oxides. The quantum dot scale endows Ce-MOFs with enhanced modifiability. Additionally, the phosphorus-rich biomass phytic acid, with its numerous hydroxyl groups, strengthens the H-bond network within the system. Ce-MOFs-centered nanoconfinement can form through the multiple H-bond interactions between Ce-MOFs and polylactic acid (PLA) molecules, thereby improving the mechanical and flame-retardant properties of PLA. The PLA/CeCxOy-P-3 composite exhibited excellent fire retardancy and toxic gas suppression, with a 27.8% decrease in the peak heat release rate and a 50% reduction in the peak smoke production rate. Notably, PLA/CeCxOy-P-3 showed an 80% lower peak CO release compared with the pure PLA sample. Moreover, the incorporation of Ce-MOFs positively influenced the tensile properties of PLA, transforming it from brittle to tough. This work designed and synthesized Ce-MOFs on the quantum scale. The resulting Ce-MOFs with the anticipated structure offer a novel direction for preparing MOFs for flame retardant applications.

Melt stretching and quenching produce low-crystalline biodegradable poly(lactic acid) filled with β-form shish for highly improved mechanical toughness

High-toughness biodegradable poly(lactic acid) (PLA) has always been intensively pursued on the way of replacing traditional petroleum-based plastics. Regulating microstructures to achieve self-toughening holds great promise due to avoidance of incorporating other heterogeneous components. Herein, we propose a straightforward and effective way to tailor microstructures and properties of PLA through melt-stretching and quenching of slightly crosslinked samples. The melt stretching drives chains orientation and crystallization at high temperature, while the quenching followed can freeze the crystallization process to any stage. For the first time, we prepare a type of transparent and low-crystalline PLA filled with rod-like β-form shish, which displays an outstanding tensile toughness, almost 17 times that of the conventional technique-processed one. This mechanical superiority is enabled by an integration of high ductility due to oriented chain network, and high tensile stress endowed by nanofibrous filler's role of β-form shish. Furthermore, the mechanically toughened PLA is demonstrated to generate the richest micro-cracks and shear bands under loading, which can effectively dissipate the deformational energy and underlie the high toughness. This work opens a new prospect for the bottom-up design of high-performance bio-based PLA materials that are tough, ductile and transparent by precise microstructural regulation through scalable melt processing route.

Effects of Relative Positions of Defect to Inclusion on Nanocomposite Strength

It is generally accepted that material inhomogeneity causes stress concentrations at the interface and thus reduces the overall strength of a composite. To overcome this reduction in strength, some groups experimented on coating the nanoinclusions with a layer of rubbery material, aiming for higher energy absorption. However, representative volume element (RVE) nanocomposite models, established with randomly distributed core–shell nanoparticles and single nanoparticle cells, show that the enhancement in strength observed in some experiments remains elusive computationally. By including a pre-existing crack in the matrix of the RVE, the stress concentration at the crack tip is reduced for cases where the nanoparticle and precrack are aligned away from the loading direction. This suggests that stress concentrations around inherent defects in materials can sometimes be reduced by adding nanoparticles to improve material strength. The effect is reversed if the crack and nanoparticle are aligned towards the loading direction. Parametric studies were also carried out in terms of the relative stiffness of the nanoparticle to the matrix and crack length. Validation tests were performed on 3D RVEs with an elliptical crack as the initial defect, and the results match with the 2D findings.

Flexure-resistant and additive-free poly (L-lactic acid) hydrophobic membranes fabricated by slow phase separation

PLLA membranes with excellent ductility were successfully prepared by a simple solvent evaporation-induced precipitation method, without any additive. The excellent mechanical properties are mainly attributed to the interconnecting pore morphology and the plastic deformation of the pore wall during the stretching process. The interconnecting pore morphology is determined by delaying non-solvent diffusion and molecular chain pre-nucleation. It was found that the average pore size gradually decreased from 19.25 μm to 6.71 μm as the concentration of the polymer solution increased from 0.03 g/ml to 0.10 g/ml, and the elongation at break of the membrane can reach 130.8%. The crystallinity is between 33.4% and 44.5%, and the crystal form is a perfect α crystal. Membrane with interconnecting pore structure contributes to the formation of 91.2% porosity. Furthermore, the solvent evaporation-induced precipitation method can also form surfaces containing micro-nano structures which significantly improves surface hydrophobicity. The combination of high porosity and hydrophobicity makes the membrane potentially applicable to the field of oil-water separation.

Toughening Poly(l-lactide) Blends: Effectiveness of Sequence-Controlled Six-Arm Star-Branched Block Copolymers of Poly(l-lactide) and Poly(ε-caprolactone)

Well-defined six-arm star-branched bio-degradable block copolymers of l-lactide and ε-caprolactone were prepared using controlled ring-opening polymerization and a sequential monomer addition method using dipentaerythritol as the initiator core and organocatalysts at low temperatures in solution. Sequence of enchainment was changed by reversing the order of monomer addition giving, either, a crystalline PLA block or an amorphous PCL block as the outer segment. Well-defined six-arm poly(ε-caprolactone-b-l-lactide, 6s-PCL-b-PLA) block copolymers were obtained with a range of segment molecular weights. However, in the case of six-arm poly(l-lactide-b-ε-caprolactone, 6s-PLA-b-PCL), disruption of the block structure was observed on account of competing transesterification reactions accompanying a chain-growth reaction. Such sequence-controlled block copolymers showed interesting phase morphologies, as evidenced by differential scanning calorimetry (DSC) studies. 6s-PCL-b-PLA showed two glass-transition temperatures and two melting temperatures characteristic of the amorphous and crystalline blocks. 6s-PCL-b-PLA and 6s-PLA-b-PCL with different segment chain lengths were solution blended (10 wt %) with a commercially sourced PLA. All the blends were highly transparent. The structure and properties of the blend were examined by DSC, measurement of mechanical properties, and scanning electron microscopy. The results show that a phase-separated 6s-PCL-b-PLA copolymer results in two- to three-fold improvement in tensile toughness without the loss of modulus. A possible hypothesis for the mechanism of tensile toughness in the blend has been proposed.

Nanocrystallisation and self-assembly of biosourced ferulic acid derivative in polylactic acid elastomeric blends

HYPOTHESIS

The crystallisation of biosourced ferulic acid derivatives - Bis-O-feruloyl-1,4-butanediol (BDF) - in a polylactic acid (PLA) matrix produces thermoplastic elastomeric blends that are transparent and biodegradable. Elastomeric and transparency are controlled by the domain size. PLA-BDF blends up to a threshold BDF concentration providing elastomeric properties show no evidence of BDF crystallisation. Heat treatment weakens the PLA-BDF interaction, give BDF molecules mobility to interact with nearby BDF molecules, leading to BDF nano-crystallisation.

EXPERIMENTS

PLA-BDF blends were synthesised by hot-melt processing by mixing pure PLA with different concentrations of BDF (0-40 wt%) at 180 °C for 13 min. One set of blends was annealed at 50 °C for 24 h and compared with the unannealed set. The BDF crystallisation in the blends is studied by combining SAXS, SEM, XRD and Polarised Optical Microscopy. Monte-Carlo simulations were performed to validate SAXS data analysis.

FINDINGS

Unannealed PLA-BDF blends of up to the threshold of 20 wt% BDF are dominated by the semicrystalline behaviour of PLA, without any trace of BDF crystallisation. Surprisingly, the PLA-BDF 40 wt% blend shows BDF crystallisation in the form of large and nanoscale structures bonded together by weak interparticle interaction. At concentrations up to 20 wt%, the BDF molecules are homogenously dispersed and bonded with PLA. Increasing BDF to 40 wt% brings the BDF molecules close enough to crystallise at room temperature, as the BDF molecules are still bonded with the PLA network. Annealing of PLA-BDF blends led to BDF nanocrystallisation and self-assembling in the PLA network. Both BDF nanoparticle size and interparticle distance decrease as the BDF concentration increases. However, the number density of BDF nanocrystals increases. The formed BDF nanocrystals have size ranging between 100 and 380 Å with interparticle distance of 120-180 Å. The structure factor and potential mean force confirm the strong interparticle interaction at the higher BDF concentration. Heat treatment weakens the PLA -BDF interaction, which provides mobility to the BDF molecules to change conformation and interact with the nearby BDF molecules, leading to BDF crystallisation. This novel BDF crystallisation and self-assembly mechanism can be used to develop biodegradable shape memory PLA blends for biomedical, shape memory, packaging and energy applications.

Preparation of hetero-armed POSS-cored star-shaped PCL-PLA/PLA composites and effect of different diisocyanates as compatibilizer

Eight-armed A₄B₄-type hetero-arm star-shaped PCL-PLA polymers ((PCL)₄-POSS-(PLA)₄, SPLA30) with POSS core were successfully prepared via combination of the "arm-first" approach utilizing ring-opening polymerization (ROP) and click chemistry techniques. Firstly, alkyne-functional PLA and PCL polymers having arms with 30 repeating units were synthesized via ROP with utilizing propargyl alcohol as initiator and stannous octoate (Sn(Oct)₂) as catalyst. Then, the obtained hetero-armed star-shaped polymers were prepared by Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) click reaction between alkyne functional polymers (1:1 PCL:PLA) and azido functional polyhedral oligomeric silsesquoxane (POSS-(N₃)₈) molecules. Finally, these obtained star-shaped SPLA30 was blended with neat PLA at different PLA/SPLA30 ratios (95/5 and 90/10 wt%) via melt blending by utilizing micro-compounder (a lab-scale) to enhance thermal, morphological, and mechanical properties of neat PLA. Also, different diisocyanates (1,4-phenylene diisocyanate (PDI), isophorone diisocyanate (IPDI), methylene diphenyl diisocyanate (MDI), and toluene 2,4-diisocyanate (TDI)) at constant ratio (1 wt%) were used as a chain extender bonding terminal group of polymers. It was found that addition of SPLA30 and SPLA30+ diisocyanates provided improvements in mechanical properties (especially in elongation at break and impact strength) of neat PLA. When the thermal properties were examined, it was seen that the decomposition temperatures of the blends decreased significantly compared to neat PLA and that there was a significant increment in the T_g and T_m values. In addition, it has been found that especially the diisocyanates added to provide good interfacial adhesion with polymer blends and show a homogeneous distribution on the surface.

Toughening of Poly(l-Lactide) with Branched Polycaprolactone: Effect of Chain Length

In this work, a series of branched polycaprolactone (BPCL) samples with different ε-caprolactone (CL) chain lengths were synthesized and used to toughen poly (lactic acid) (PLA). The spherical structure increased the free volume, facilitating the free movement of the PLA chain segment and increasing the ductility. In addition, the hydrogen bonds between the multi-terminal hydroxyl group of BPCL x and PLA improved the interaction between them. The glass-transition temperatures (T g) and crystallization temperatures (T c) of the blends were significantly lower than those of PLA, and these temperatures increased with the chain length of polycaprolactone. BPCL x increased the crystallization rate of PLA through heterogeneous nucleation. A longer chain length of CL increased the mutual entanglement in the blends, reduced the hydrogen bonding between BPCL x and PLA, and increased the entanglement of BPCL x chains. When the chain length of CL was 6, the impact strength and elongation at break of the PLA/BPCL blends exhibited an increase of 151.72 and 465.8%, respectively, as compared with PLA.

Polyethylene oxide enhances the ductility and toughness of polylactic acid: the role of mesophase

A lack of understanding of the structure-property relationship of the polylactic acid (PLA)-based polymer composite system makes it a challenge to manufacture products with optimized mechanical performance by precisely regulating the microscopic structure and morphology. Herein, we chose the PLA/polyethylene oxide (PEO) blend as a model to investigate the structural reason for the enhanced ductility and toughness of this kind of material. We have demonstrated that a considerable amount of the PLA mesophases exist in the melt quenched films that display high ductility and toughness, in contrast to the PLA crystals in their counterparts of slowly cooled films that are dominated by brittle fracture. The mesophase formed by melt quenching is attributed to a moderate acceleration of PLA chain mobility due to the plasticizing effect of the flexible PEO. In situ experiments have revealed the further formation of oriented mesophases induced by tensile deformation, which presents a high consistency between the content increase and the tensile stress intensification. We illustrate that the mesophases directly develop into a microfibrillar morphology to transmit the external stress and prevent crack propagation under deformation. This work emphasizes the essential role of the PLA mesophase in acquiring the enhanced ductility and toughness of the PLA/PEO composite films, which may be generalized to other similar PLA-based polymer composite materials.

Supertough and Transparent Poly(lactic acid) Nanostructure Blends with Minimal Stiffness Loss

This contribution is an attempt to explore the effectiveness of a series of newly obtained thermoplastic elastomers (TPEs) as a toughening agent for modifying poly(lactic acid) (PLA). The TPEs, including ionically modified isotactic polypropylene-graft-PLA (iPP-g-PLA) copolymers with explicit graft length, graft density, and ionic group content, and an iPP-g-PLA copolymer with a very high molecular weight and explicit graft density, were elaborately designed and synthesized. The semicrystal or rubbery copolymer backbone originated from iPP was designed to improve the toughness and maintain a relatively high strength, while the grafted PLA side chain was to ensure a high level of compatibility with the PLA matrix. To obtain further enhancement in interfacial reinforcement, the imidazolium-based ionic group was also added during graft onto reaction. All of these graft copolymers were identified with randomly distributed PLA branches, bearing a very high molecular weight ((33-398) × 10⁴) and very high PLA content (57.3-89.3 wt %). Unprecedentedly, with a very small amount of newly designed TPE, the modified PLA blends exhibited a significantly increased elongation at break (up to about 190%) and simultaneously retained the very high stiffness and excellent transparency. The nanometer-scale phase-separated particles with good compatibility and refractive index matching to the PLA matrix were demonstrated to play a crucial role in the excellent performance. The findings suggested that the newly designed iPP-g-PLA copolymers are very economic, promising, and effective modifying agents for developing highly transparent and tough PLA-based sustainable materials.

Toughened Poly(lactic acid)/BEP Composites with Good Biodegradability and Cytocompatibility

Using novel biodegradable elastomer particles (BEP) prepared by the technologies of melt polycondensation, emulsification, and irradiation vulcanization, we successfully prepared advanced poly(lactic acid) (PLA)/BEP composites with higher toughness, higher biodegradability, and better cytocompatibility than neat PLA by means of the melt blending technology. The experimental results revealed that the elongation at break of the PLA/BEP composites containing 8 parts per hundred rubber (phr) by weight BEP increased dramatically from 2.9% of neat PLA to 67.1%, and the notched impact strength increased from 3.01 to 7.24 kJ/m2. Meanwhile, the biodegradation rate of the PLA/BEP composites increased dramatically in both soil environment and lipase solution, and the crystallization rate and crystallinity of the PLA/BEP composites increased significantly compared to those of neat PLA. The methyl thiazolyl tetrazolium (MTT) assay also showed that the viability of L929 cells in the presence of extracts of PLA/BEP composites was more than 75%, indicating that the PLA/BEP composites were not cytotoxic and had better cytocompatibility than neat PLA. Research on advanced PLA/BEP composites opens up new potential avenues for preparing advanced PLA products, especially for advanced biomedical materials.

Improving the Thermal and Mechanical Properties of Poly(l-lactide) by Forming Nanocomposites with an in Situ Ring-Opening Intermediate of Poly(l-lactide) and Polyhedral Oligomeric Silsesquioxane

In this study, a series of poly(l-lactide) and (3-amino)-propylheptaisobutyl cage silsesquioxane (PLLA-AMPOSS) intermediates were first fabricated using single-arm in situ solution polymerization of LLA monomers and AMPOSS nanoparticles with different contents, 0.02-1.00 mol%. Then, the PLLA-AMPOSS intermediate with 0.5 mol% AMPOSS was selected as a representative and investigated by nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). Afterwards, it was added into the pure PLLA with different mass fractions. Finally, the thermal behavior, crystallization kinetics, morphological characteristics, and mechanical properties of the obtained PLLA/PLLA-AMPOSS nanocomposites were carefully measured and investigated by differential scanning calorimetry (DSC), polarizing microscopy (POM), scanning electron microscopy (SEM), and tensile test. After comparing the PLLA-AMPOSS intermediate and PLLA/AMPOSS blend, the results show that the ring-open polymerization of PLLA-AMPOSS intermediate was successful. The results also show that the existence of PLLA-AMPOSS has a strong influence on the crystallization behavior of PLLA/PLLA-AMPOSS composites, which can be attributed to the heterogeneous nucleation effect of PLLA-AMPOSS. In addition, it was also found from the tensile test results that the addition of the PLLA-AMPOSS nanofiller improved the tensile strength and strain at break of PLLA/PLLA-AMPOSS nanocomposites. All of these results indicate the good nucleating effect of PLLA-AMPOSS and that the AMPOSS disperses well in the PLLA/PLLA-AMPOSS nanocomposites. A conclusion can be drawn that the selective nucleating agent and the combined method of in situ ring-opening polymerization and physical blending are feasible and effective.

Super Toughened Poly(lactic acid)-Based Ternary Blends via Enhancing Interfacial Compatibility

Novel super toughened bioplastics are developed through controlled reactive extrusion processing, using a very low content of modifier, truly a new discovery in the biodegradable plastics area. The super toughened polylactide (PLA) blend showing a notched impact strength of ∼1000 J/m with hinge break behavior is achieved at a designed blending ratio of PLA, poly(butylene succinate) (PBS), and poly(butylene adipate-co-terephthalate) (PBAT), using less than 0.5 phr peroxide modifier. The impact strength of the resulting blend is approximately 10 times that of the blend with the same composition without a modifier and ∼3000% more than that of pure PLA. Interfacial compatibilization among the three biodegradable plastics took place during the melt extrusion process in the presence of a controlled amount of initiator, which is confirmed by scanning electron microscopy and rheology analysis. The synergistic effect of strong interfacial adhesion among the three blending components, the decreased particle size of the most toughened component, PBAT, to ∼200 nm, and its uniform distribution in the blend morphology result in the super tough biobased material. One of the key fundamental findings through the in situ rheology study depicts that the radical reaction initiated by peroxide occurs mainly between PBS and PBAT and not with PLA. Thus, the cross-linking degree can be controlled by adjusting renewable sourced PLA contents in the ternary blend during reactive extrusion processing. The newly engineered super toughened PLA with high stiffness and high melt elasticity modulus could reasonably serve as a promising alternative to traditional petroleum plastics, where high biobased content and biodegradability are required in diverse sustainable packaging uses.

Toward Super-Tough Poly(l-lactide) via Constructing Pseudo-Cross-link Network in Toughening Phase Anchored by Stereocomplex Crystallites at the Interface

We demonstrated a novel strategy to toughen poly(l-lactide) (PLLA) by constructing pseudo-cross-link networks based on chain entanglements of long-chain branched structure in the toughening phase, which were anchored by stereocomplex (SC) crystallites at the interface. The formation of pseudo-cross-link network was achieved by simple blending of the copolymer of long-chain branched polycaprolactone and poly(d-lactide) (LB-PCL- b-DLA) with PLLA without introducing any chemical cross-linking structure or nonbiodegradable component. The microscopic morphology analysis suggests that the interface-formed SC crystallites not only enhanced the interfacial interaction between LB-PCL and PLLA but also obviously increased the matrix crystallization rate. Different from those blends without SC crystallites or long-chain branched structures, nano-microgels were observed in chloroform solution of the PLLA/LB-PCL- b-DLA blend, suggesting the formation of pseudo-cross-link network. The pseudo-cross-link network in LB-PCL toughening phase endows PLLA a significantly improved impact toughness (49.5 kJ/m²), which is almost 13 times than that of neat PLLA. Moreover, matrix crystallinity and spherulite size of the PLLA matrix also play significant roles in toughening. Only sufficiently crystallized PLLA with proper spherulite size can effectively trigger the matrix shear yielding, meanwhile, facilitate the energy dissipating.

Overcome the Conflict between Strength and Toughness in Poly(lactide) Nanocomposites through Tailoring Matrix-Filler Interface

Strength and toughness are the two most important prerequisites for structural applications. Unfortunately, these two properties are often in conflict in materials. Here, an effective and yet practical strategy is proposed to simultaneously strengthen and toughen poly(l-lactide) (PLLA) using a simple rigid-rubber "reinforcing element." This element consists of a rigid graphene oxide (GO) sheet covalently coupled with poly(caprolactone-co-lactide) (PCLLA) rubbery layers, which can be easily synthesized and incorporated into PLLA matrix to develop composites with well-tailored GO/PLLA interfaces. It is demonstrated that by adding the "reinforcing element," i.e., GO-graft-rubber-graft-polyd-lactide), PLLA exhibits higher strength and higher toughness, which could be attributed to the synergy of rigid GO and rubbery PCLLA working in tandem during deformation. It is further demonstrated that this strategy can also be applied to other filler systems, such as clay and particulate polyhedral oligomeric silsesquioxane, and other polymer systems, such as poly(methyl methacrylate). The strategy could be considered as a general design principle for reinforcing materials where both strength and toughness are the key concerns.

Preparation, Foaming and Characterization of Poly(l-lactic acid))/Poly(d-lactic acid)-Grafted Graphite Oxide Blends

Commercial poly(l-lactic acid) (PLLA) was blended with different contents of graphene oxide-graft-poly(d-lactic acid) (GO-g-PDLA), which was synthesized via ring-opening polymerization using modified GO as initiator. PLLA and PLLA/GO-g-PDLA blend foams were prepared in a batch process via varying-temperature mode using supercritical carbon dioxide as physical foaming agent. The results showed that the addition of GO-g-PDLA in PLLA leads to the formation of stereocomplex (sc)-crystallites. Increase in the GO-g-PDLA content enhances the IR absorption, diffraction peak and melting peak corresponding to the sc-crystallites. The addition of GO-g-PDLA to PLLA leads to the decrease of the cell diameter, increase of the cell density and to a little change in expansion ratio, which is attributed to the fact that the enhancement of PLLA crystallization restricts cell growth and GO-g-PDLA acts as nucleation point.

Fracture behavior of highly toughened poly(lactic acid)/ethylene-co-vinyl acetate blends

Poly(lactic acid) (PLA) is brittle which restricts the range of its applications. The toughness of PLA was effectively improved in this work by incorporation of rubber grade ethylene-co-vinyl acetate (EVM). For example, the elongation at break of PLA increased by about 50 times after the addition of the EVM (10–30 wt%), although the EVM was not miscible with the PLA matrix. Furthermore, the notched impact toughness of PLA/EVM blend (70/30 wt/wt) reached to 85 kJ/m2 even at a temperature as low as −10°C. The critical temperatures of brittle-to-ductile transition (BDT) for PLA/EVM blends are observed at −20~0°C depending on the composition, while no BTD transition appeared for neat PLA. The impact fracture surface morphology of PLA and PLA/EVM blends observed by SEM indicates that the toughening modification was achieved through obvious matrix yielding. Moreover, the toughening behavior of the PLA/EVM blends was also interpreted quantitatively by using a single-edge notched three-point bending model (SEN3PB). The SEN3PB experiments reveal that the fracture energy was consumed in an outer plastic zone away from the fracture surface rather than in the inner fracture process zone, which accounts for the high toughness of the PLA/EVM blends.

Fully biodegradable Poly(lactic acid)/Starch blends: A review of toughening strategies

Polylactic acid (PLA) and Starch are both bio-based biodegradable polymers that have properties that are complementary to each other. PLA/starch blend exploits the good mechanical property of PLA and the low cost of Starch. However, PLA/Starch blend is intrinsically brittle. This paper reviews the current state of arts in toughening of PLA/Starch blend, which are categorized as: Additive Plasticization, Mixture Softening, Elastomer Toughening and Interphase Compatibilization. These strategies are not mutually exclusive and can be applied jointly in a single blend, opening up a wide range of toughening techniques that can be employed in PLA/Starch blend. Even though significant progress has been made in this area, there is still much room for research, in order to achieve easy to process, fully bio-based and completely biodegradable PLA/Starch blends that have mechanical properties suitable for a wide range of applications.

Granular Nanostructure: A Facile Biomimetic Strategy for the Design of Supertough Polymeric Materials with High Ductility and Strength

The realization of high strength, large ductility, and great toughness for polymeric materials is a vital factor for practical applications in industry. Unfortunately, until now this remains a huge challenge due to the common opposing trends that exist when promoting improvements in these properties using materials design strategies. In the natural world, the cuticle of mussel byssus exhibits a breaking strain as high as 100%, which is revealed to arise from an architectural granular microphase-separated structure within the protein matrix. Herein, a facile biomimetic designed granular nanostructured polymer film is reported. Such biomimetic nanostructured polymer films show a world-record toughness of 122 (± 6.1) J g^-1 as compared with other polyvinyl alcohol films, with a breaking strain as high as 205% and a high tensile strength of 91.2 MPa, which is much superior to those of most engineering plastics. This portfolio of outstanding properties can be attributed to the unique nanoscale granular phase-separated structure of this material. These biomimetic designed polymer films are expected to find promising applications in tissue engineering and biomaterials fields, such as artificial skin and tendon, which opens up an innovative methodology for the design of robust polymer materials for a range of innovative future applications.

Recent advances in the development of biodegradable PHB-based toughening materials: Approaches, advantages and applications

Polyhydroxybutyrate (PHB) is a natural biodegradable polymer that is produced by many types of bacteria as an intracellular energy storage material. Due to its numerous advantages such as biodegradability, biocompatibility, availability and with physical properties comparable to petroleum-based thermoplastics, PHB is a potential substitute in biomedical and packaging fields. However, several physical drawbacks, such as high production cost, thermal instability, and poor mechanical properties, due to secondary crystallization and slow nucleation rate, limit its competition with traditional plastics in industrial and biomedical applications. Thereby, many attempts have been employed to improve the material performance of toughened PHB so as to achieve greater competitiveness and sustainability. In this review, the most recent developments of PHB-based toughening materials are discussed with respect to their approaches and strategies, which includes: drawing and thermal treatment, blending with materials from natural sources and synthetic polymers, as well as forming reinforced composites with natural fibers and inorganic fillers. The alternation of PHB chemical structure to form various types of functional copolymers with enhanced materials performance is also summarized. The expanded utilization of these newly developed sophisticated PHB materials as engineering materials and the biomedical significance in different domains are also addressed.

Grafting Modification of the Reactive Core-Shell Particles to Enhance the Toughening Ability of Polylactide

In order to overcome the brittleness of polylactide (PLA), reactive core-shell particles (RCS) with polybutadiene as core and methyl methacrylate-co-styrene-co-glycidyl methacrylate as shell were prepared to toughen PLA. Tert-dodecyl mercaptan (TDDM) was used as chain transfer agent to modify the grafting properties (such as grafting degree, shell thickness, internal and external grafting) of the core-shell particles. The introduction of TDDM decreased the grafting degree, shell thickness and the T_g of the core phase. When the content of TDDM was lower than 1.15%, the RCS particles dispersed in the PLA matrix uniformly-otherwise, agglomeration took place. The addition of RCS particles induced a higher cold crystallization temperature and a lower melting temperature of PLA which indicated the decreased crystallization ability of PLA. Dynamic mechanical analysis (DMA) results proved the good miscibility between PLA and the RCS particles and the increase of TDDM in RCS induced higher storage modulus of PLA/RCS blends. Suitable TDDM addition improved the toughening ability of RCS particles for PLA. In the present research, PLA/RCS-T4 (RCS-T4: the reactive core-shell particles with 0.76 wt % TDDM addition) blends displayed much better impact strength than other blends due to the easier cavitation/debonding ability and good dispersion morphology of the RCS-T4 particles. When the RCS-T4 content was 25 wt %, the impact strength of PLA/RCS-T4 blend reached 768 J/m, which was more than 25 times that of the pure PLA.

Poly(lactic acid) stereocomplexes: A decade of progress

Upon blending enantiomeric poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] and poly(d-lactide) (PDLA) [i.e., poly(d-lactic acid) (PDLA)] or synthesis of stereo block poly(lactide) [i.e., poly(lactic acid) (PLA)], a stereocomplex (SC) is formed. PLA SC has a higher melting temperature (or heat resistance), mechanical performance, and hydrolysis-resistance compared to those of neat PLLA and PDLA. Because of such effects, PLA SC has been extensively studied in terms of biomedical and pharmaceutical applications as well as commodity, industrial, and environmental applications. Stereocomplexation stabilizes and strengthens PLA-based hydrogel or nanoparticles for biomedical applications. Stereocomplexation increases the barrier property of PLA-based materials and thereby prolongs drug release from PLA based materials. In addition, PLA SC is attracting significant attention because it can act as a nucleating agent for the widely used biobased polymer PLLA and thereby the heat resistance of PLLA-based materials can be enhanced. Interestingly, a wide variety of SCs other than PLA SC are found to have been formed in the enantiomeric substituted PLA blends and stereo block substituted PLA polymers. In the present review article, a decade of progress in investigation of PLA SCs is summarized.

A Functionalized Cyclic Lactide Monomer for Synthesis of Water-Soluble Poly(Lactic Acid) and Amphiphilic Diblock Poly(Lactic Acid)

Biodegradable and bioabsorbable poly(lactic acid)s are one of the most important biomedical materials. However, it is difficult to introduce the functional groups into poly(lactic acid)s in order to improve their hydrophilicity and degradation rate. Here the authors describe the synthesis of functionalized cyclic lactide monomer 3,6-bis(benzyloxymethyl)-1,4-dioxane-2,5-dione (BnLA) using an advanced synthetic route. Water-soluble hydroxyl-functionalized homopoly(lactic acid) (P(OH)LA) is synthesized via ring-opening polymerization (ROP) of BnLA, followed by a hydrogenolytic deprotection reaction. Amphiphilic diblock poly(lactic acid) (P(OH)LA-PLA) is synthesized via ROP of DL-lactide using PBnLA as an initiator, followed by a hydrogenolytic deprotection reaction. P(OH)LA-PLA is able to form polymeric micelles with the diameter of sub-100 nm.