Synthesis of PLLA-based block copolymers for improving melt strength and toughness of PLLA by in situ reactive blending

Super-tough polylactic acid (PLA)/poly(butylene succinate) (PBS) materials prepared through reactive blending with epoxy-functionalized PMMA-GMA copolymer

High-performance biosourced polylactic acid (PLA)/poly(butylene succinate) (PBS) blends with small amounts of compatibilizer, epoxy-functionalized methyl methacrylate-co-glycidyl methacrylate copolymer (PMMA-GMA), were fabricated by melt compounding. The properties of the modified PLA/PMMA-GMA, PBS/PMMA-GMA, and PLS(PLA/PBS)/PMMA-GMA blends were investigated systematically. DSC combined with X-ray diffraction revealed a low-order semi-crystalline structure for all samples. SEM and DMA showed that the compatibility between PLA and PBS was improved after addition of PMMA-GMA. Rheological behavior of blends showed that the addition of PMMA-GMA resulted in a significant improvement in the viscoelasticity. FT-IR spectra confirmed that the interfacial compatibilization between PLA and PBS phases was improved due to the reaction of epoxy groups with terminal groups of PLA and PBS. Finally, the toughness and notched impact strength of the PLA materials were increased significantly. The elongation at break and notched impact strength of PLS/PMMA-GMA was about 55.7 and 6.2 times than neat PLA after incorporation of 7 wt% PMMA-GMA, respectively.

The Role of the Interface of PLA with Thermoplastic Starch in the Nonisothermal Crystallization Behavior of PLA in PLA/Thermoplastic Starch/SiO2 Composites

Corn starch was plasticized by glycerol suspension in a twin-screw extruder, in which the glycerol suspension was the pre-dispersion mixture of glycerol with nano-SiO₂. Polylactide (PLA)/thermoplastic starch/SiO₂ composites were obtained through melt-blending of PLA with thermoplastic starch/SiO₂ in a twin-screw extruder. The nonisothermal crystallization behavior of PLA in the composites was investigated by differential scanning calorimetry. An interface of PLA with thermoplastic starch was proven to exist in the composites, and its interfacial bonding characteristics were analyzed. The interfacial binding energy stemming from PLA with thermoplastic starch exerts a significant influence on the segmental mobility of PLA at the interface. The segmental mobility of PLA is gradually improved by increasing interfacial binding energy, and consequently, the relative crystallinity on the interface exhibits progressive promotion. The Jeziorny model could well describe the primary crystallization of PLA in the composites. The extracted Avrami exponents based on the Jeziorny model indicate that the primary crystallization of PLA follows heterogeneous nucleation and three-dimensional growth. This study has revealed the intrinsic effect of the interfacial segmental mobility on the nonisothermal crystallization behavior of PLA in composites, which is of technological significance for its blow molding.

Polymeric Scaffolds Used in Dental Pulp Regeneration by Tissue Engineering Approach

Currently, the challenge in dentistry is to revitalize dental pulp by utilizing tissue engineering technology; thus, a biomaterial is needed to facilitate the process. One of the three essential elements in tissue engineering technology is a scaffold. A scaffold acts as a three-dimensional (3D) framework that provides structural and biological support and creates a good environment for cell activation, communication between cells, and inducing cell organization. Therefore, the selection of a scaffold represents a challenge in regenerative endodontics. A scaffold must be safe, biodegradable, and biocompatible, with low immunogenicity, and must be able to support cell growth. Moreover, it must be supported by adequate scaffold characteristics, which include the level of porosity, pore size, and interconnectivity; these factors ultimately play an essential role in cell behavior and tissue formation. The use of natural or synthetic polymer scaffolds with excellent mechanical properties, such as small pore size and a high surface-to-volume ratio, as a matrix in dental tissue engineering has recently received a lot of attention because it shows great potential with good biological characteristics for cell regeneration. This review describes the latest developments regarding the usage of natural or synthetic scaffold polymers that have the ideal biomaterial properties to facilitate tissue regeneration when combined with stem cells and growth factors in revitalizing dental pulp tissue. The utilization of polymer scaffolds in tissue engineering can help the pulp tissue regeneration process.

Crystallization, thermal and mechanical properties of stereocomplexed poly(lactide) with flexible PLLA/PCL multiblock copolymer

In this work, the synthesized PLLA/PCL multi-block copolymers with different compositions were introduced into a stereocomplexed poly(lactide) (sc-PLA) matrix to accelerate the stereocomplexation of PLA enantiomers and improve its inherent brittleness. The PLLA/PCL multi-block copolymers were in different compositions to adjust the molecular weight of the PLLA block. The structure, molecular weight, crystallization behavior, crystal structure and thermal stability of PLLA/PCL multi-block copolymers were investigated. The results indicated that PLLA/PCL multi-block copolymers with controllable structure and composition were successfully synthesized. On this basis, the blends of sc-PLA and PLLA/PCL multi-block copolymers were prepared by solution casting, and characterized. The results revealed that the introduction of PLLA/PCL multi-block copolymers promoted the stereocomplexation of the PLA enantiomers during the melting crystallization process to obtain a complete stereocomplexed material. But the presence of the PCL block leads to a decrease in the melting temperature of the stereocomplex and difficulty in homogeneous nucleation. Compared with sc-PLA, the elongation at break of the blends was significantly improved and their tensile strengths were only slightly reduced. And the thermal stability and mechanical properties of the blends could be adjusted by controlling the content and composition of PCL/PLLA multi-block copolymers. These results revealed that the degree of stereocomplexation and toughness of sc-PLA were improved, which may expand the application fields of PLA-based materials.

The PLLA/PCL multi-block copolymer was introduced into the stereocomplexed PLA matrix, and its effect on the crystallization, thermal and mechanical properties of the stereocomplexed PLA was discussed.

Poly-l-Lactic Acid (PLLA)-Based Biomaterials for Regenerative Medicine: A Review on Processing and Applications

Synthetic biopolymers are effective cues to replace damaged tissue in the tissue engineering (TE) field, both for in vitro and in vivo application. Among them, poly-l-lactic acid (PLLA) has been highlighted as a biomaterial with tunable mechanical properties and biodegradability that allows for the fabrication of porous scaffolds with different micro/nanostructures via various approaches. In this review, we discuss the structure of PLLA, its main properties, and the most recent advances in overcoming its hydrophobic, synthetic nature, which limits biological signaling and protein absorption. With this aim, PLLA-based scaffolds can be exposed to surface modification or combined with other biomaterials, such as natural or synthetic polymers and bioceramics. Further, various fabrication technologies, such as phase separation, electrospinning, and 3D printing, of PLLA-based scaffolds are scrutinized along with the in vitro and in vivo applications employed in various tissue repair strategies. Overall, this review focuses on the properties and applications of PLLA in the TE field, finally affording an insight into future directions and challenges to address an effective improvement of scaffold properties.

Development of Injection-Molded Polylactide Pieces with High Toughness by the Addition of Lactic Acid Oligomer and Characterization of Their Shape Memory Behavior

This work reports the effect of the addition of an oligomer of lactic acid (OLA), in the 5-20 wt% range, on the processing and properties of polylactide (PLA) pieces prepared by injection molding. The obtained results suggested that the here-tested OLA mainly performs as an impact modifier for PLA, showing a percentage increase in the impact strength of approximately 171% for the injection-molded pieces containing 15 wt% OLA. A slight plasticization was observed by the decrease of the glass transition temperature (Tg) of PLA of up to 12.5 °C. The OLA addition also promoted a reduction of the cold crystallization temperature (Tcc) of more than 10 °C due to an increased motion of the biopolymer chains and the potential nucleating effect of the short oligomer chains. Moreover, the shape memory behavior of the PLA samples was characterized by flexural tests with different deformation angles, that is, 15°, 30°, 60°, and 90°. The obtained results confirmed the extraordinary effect of OLA on the shape memory recovery (Rr) of PLA, which increased linearly as the OLA loading increased. In particular, the OLA-containing PLA samples were able to successfully recover over 95% of their original shape for low deformation angles, while they still reached nearly 70% of recovery for the highest angles. Therefore, the present OLA can be successfully used as a novel additive to improve the toughness and shape memory behavior of compostable packaging articles based on PLA in the new frame of the Circular Economy.

Effect of Poly(styrene-ran-methyl acrylate) Inclusion on the Compatibility of Polylactide/Polystyrene-b-Polybutadiene-b-Polystyrene Blends Characterized by Morphological, Thermal, Rheological, and Mechanical Measurements

A poly(styrene-ran-methyl acrylate) (S-MA) (75/25 mol/mol), synthesized by surfactant-free emulsion copolymerization, was used as a compatibilizer for polystyrene-b-polybutadiene-b-polystyrene (SBS)-toughened polylactide (PLA) blends. Upon compatibilization, the blends exhibited a refined dispersed-phase morphology, a decreased crystallinity with an increase in their amorphous interphase, improved thermal stability possibly from the thicker, stronger interfaces insusceptible to thermal energy, a convergence of the maximum decomposition-rate temperatures, enhanced magnitude of complex viscosity, dynamic storage and loss moduli, a reduced ramification degree in the high-frequency terminal region of the Han plot, and an increased semicircle radius in the Cole–Cole plot due to the prolonged chain segmental relaxation times from increases in the thickness and chain entanglement degree of the interphase. When increasing the S-MA content from 0 to 3.0 wt %, the tensile properties of the blends improved considerably until 1.0 wt %, above which they then increased insignificantly, whereas the impact strength was maximized at an optimum S-MA content of ~1.0 wt %, hypothetically due to balanced effects of the medium-size SBS particles on the stabilization of preexisting crazes and the initiation of new crazes in the PLA matrix. These observations confirm that S-MA, a random copolymer first synthesized in our laboratory, acted as an effective compatibilizer for the PLA/SBS blends.

Synergistic Mechanisms Underlie the Peroxide and Coagent Improvement of Natural-Rubber-Toughened Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Mechanical Performance

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a promising bio-based and biodegradable thermoplastic with restricted industrial applications due to its brittleness and poor processability. Natural rubber (NR) has been used as a toughening agent, but further physical improvements are desired. In this study, rubber toughening efficiency was significantly improved through the synergistic use of a trifunctional acrylic coagent and an organic peroxide during reactive extrusion of PHBV and NR. The rheological, crystallization, thermal, morphological, and mechanical properties of PHBV/NR blends with 15% rubber loading were characterized. The peroxide and coagent synergistically crosslinked the rubber phase and grafted PHBV onto rubber backbones, leading to enhanced rubber modulus and cohesive strength as well as improved PHBV⁻rubber compatibility and blend homogeneity. Simultaneously, the peroxide⁻coagent treatment decreased PHBV crystallinity and crystal size and depressed peroxy-radical-caused PHBV degradation. The new PHBV/NR blends had a broader processing window, 75% better toughness (based on the notched impact strength data), and 100% better ductility (based on the tensile elongation data) than pristine PHBV. This new rubber-toughened PHBV material has balanced mechanical performance comparable to that of conventional thermoplastics and is suitable for a wide range of plastic applications.