Stereocomplex formation of poly(l-lactide)-poly(ε-caprolactone) multiblock copolymers with Poly(d-lactide)

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.

Super-Tough and Biodegradable Poly(lactide-co-glycolide) (PLGA) Transparent Thin Films Toughened by Star-Shaped PCL-b-PDLA Plasticizers

To obtain fully degradable and super-tough poly(lactide-co-glycolide) (PLGA) blends, biodegradable star-shaped PCL-b-PDLA plasticizers were synthesized using natural originated xylitol as initiator. These plasticizers were blended with PLGA to prepare transparent thin films. Effects of added star-shaped PCL-b-PDLA plasticizers on mechanical, morphological, and thermodynamic properties of PLGA/star-shaped PCL-b-PDLA blends were investigated. The stereocomplexation strong cross-linked network between PLLA segment and PDLA segment effectively enhanced interfacial adhesion between star-shaped PCL-b-PDLA plasticizers and PLGA matrix. With only 0.5 wt% addition of star-shaped PCL-b-PDLA (Mn = 5000 g/mol), elongation at break of the PLGA blend reached approximately 248%, without any considerable sacrifice over excellent mechanical strength and modulus of PLGA.

Polymorphic Crystallization Behavior of a Poly(butylene adipate) Midblock within a Poly(L-lactide-butylene adipate-L-lactide) Triblock Copolymer

New biodegradable aliphatic PLLA-PBA-PLLA copolymers with soft poly(butylene adipate) (PBA) and hard poly(l-lactide) (PLLA) building blocks were synthesized via ring-opening polymerization (ROP). Proton nuclear magnetic resonance (¹HNMR) was utilized to confirm the volume fraction of PBA (f_PBA) within PLLA-PBA-PLLA. It was found that a PBA midblock (PBA-mid) within PLLA-PBA-PLLA-s (PLLA-PBA-PLLA triblock copolymer with a short PLLA block length) might display lamellar domain structure. However, PBA-mid within PLLA-PBA-PLLA-l (PLLA-PBA-PLLA triblock copolymer with a long PLLA block length) might locate itself as a nanoscale cylindrical domain surrounded by a PLLA continuous phase. Polymorphic crystals of PBA-mid within the PLLA-PBA-PLLA copolymers were formed after melt crystallization at the given temperatures, which were studied by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) analysis. According to the WAXD and DSC analyses, it was interesting to find that the α-type crystal of PBA-mid was favorable to develop in the lower temperature region regardless of the state (crystallization or amorphous) of the PLLA component. Additionally, when the PLLA component was held in its amorphous state, it was easier for PBA-mid within the PLLA-PBA-PLLA copolymers to transform from the metastable β-form crystal to the stable α-form crystal. Furthermore, polarized optical microscopy (POM) photos provided direct evidence of the polymorphic crystals of PBA-mid within PLLA-PBA-PLLAs.

Synergistic effects of PEG middle-blocks and talcum on crystallizability and thermomechanical properties of flexible PLLA-b-PEG-b-PLLA bioplastic

In this study, talcum was melt-blended with a flexible poly(l-lactide)-b-polyethylene glycol-b-poly(l-lactide) triblock copolymer (PLLA-PEG-PLLA) with 1, 2, 4, and 8 wt% talcum, for improvement of the crystallization and thermomechanical properties of PLLA-PEG-PLLA compared with PLLA. The crystallizability of PLLA-PEG-PLLA/talcum composites was better than that of PLLA/talcum composites as determined from differential scanning calorimetry. X-ray diffractometry showed that the PLLA-PEG-PLLA/talcum films had a higher degree of crystallinity than the PLLA/talcum films. PEG middle-blocks and talcum showed a synergistic effect for crystallization of PLLA end-blocks. The PLLA-PEG-PLLA/talcum films showed better thermomechanical properties than those of the PLLA/talcum films as determined from dynamic mechanical analysis. This was confirmed from the results of dimensional stability to heat. In summary, the PLLA-PEG-PLLA/talcum composites have potential for use as flexible bioplastics with good dimensional stability to heat.

Synthesis and Antiplatelet Adhesion Behavior of a Poly(L-lactide-co-glycolide)-Poly(1,5-dioxepan-2-one) Multiblock Copolymer

Platelet adhesion and denaturation on artificial medical implants induce thrombus formation. In this study, bioabsorbable copolymers composed of poly(l-lactide-co-glycolide) (PLGA) and poly(1,5-dioxepan-2-one) (PDXO) were synthesized and evaluated for their antiplatelet adhesive properties. The PLGA-PXO multiblock copolymer (PLGA-PDXO MBC) and its random copolymer (PLGA-PDXO RC) showed effective antiplatelet adhesive properties, and the number of adhered platelets was similar to those adhered on poly(2-methoxyethylacrylate), a known antiplatelet adhesive polymer, although a large number of denatured platelets were observed on a PLGA-poly(ε-caprolactone) multiblock copolymer (PLGA-PCL MBC). Using monoclonal antifibrinogen IgG antibodies, we also found that both αC and γ-chains, the binding sites of fibrinogen for platelets, were less exposed on the PLGA-PDXO MBC surface compared to PLGA-PCL MBC. Furthermore, free-standing films of PLGA-PDXO MBC were prepared by casting the polymer solution on glass plates and showed good tensile properties and slow hydrolytic degradation in phosphate-buffered saline (pH = 7.4). We expect that the unique properties of PLGA-PDXO MBC, i.e., antiplatelet adhesive behavior, good tensile strength, and hydrolytic degradation, will pave the way for the development of new bioabsorbable implanting materials suitable for application at blood-contacting sites.

Synthetic approaches for multiblock copolymers

Multiblock copolymers (MBCs) are an emerging class of synthetic polymers that exhibit different macromolecular architectures and behaviours to those of homopolymers or di/triblock copolymers.

Controlling Actuation Performance in Physically Cross-Linked Polylactone Blends Using Polylactide Stereocomplexation

Within the field of shape-changing materials, synthetic chemical modification has been widely used to introduce key structural units and subsequently expand the mechanical functionality of actuator devices. The introduction of architectural elements that facilitate in situ control over mechanical properties and complete geometric reconfiguration of a device is highly desirable to increase the morphological diversity of polymeric actuator materials. The subject of the present study is a multiblock copolymer with semicrystalline poly(l-lactide) and poly(ε-caprolactone) (PLLA-PCL) segments. By harnessing the stereocomplexation of copolymer chains with a poly(d-lactide) oligomer (PDLA), we provide anchoring points for physical network formation and demonstrate how a blending process can be used to efficiently vary the mechanical properties of a shape-memory actuator. We investigate the effect of molecular structure on the actuation performance of the material in cyclic thermomechanical tests, with a maximum reversible shape change ε_rev' = 13.4 ± 1.5% measured at 3.1 wt % of polylactide stereocomplex content in the multiblock copolymer matrix. The thermophysical properties, crystalline structure, and phase morphology were analyzed by DSC, WAXS and AFM respectively, elucidating the structure-to-function relationship in physically cross-linked blended materials. The work demonstrates a one-step technique for manufacturing a polymeric actuator and tuning its performance in situ. This approach should greatly improve the efficiency of physically cross-linked actuator fabrication, allowing composition and physical behavior to be precisely and easily controlled.

Synthesis and evaluation of injectable thermosensitive penta-block copolymer hydrogel (PNIPAAm-PCL-PEG-PCL-PNIPAAm) and star-shaped poly(CL─CO─LA)-b-PEG for wound healing applications

BACKGROUND

Loss of skin integrity due to injury, burning, or illness makes the development of new treatment options necessary. Skin tissue engineering provides some solutions for these problems.

OBJECTIVE

The potential of a biodegradable star-shaped copolymer [Poly(CL─CO─LA)-b-PEG] and penta-block copolymer hydrogel (PNIPAAm-PCL-PEG-PCL-PNIPAAm) was assessed for skin tissue engineering applications.

METHODS

Two copolymers were synthesized for cellular culture scaffolds and their mechanical properties were compared. The resulting star-shaped copolymer and thermosensitive penta-block copolymer were characterized using Fourier transform infrared and nuclear magnetic resonance spectroscopy. The crystallizability of the two copolymers was analyzed using X-ray diffraction. The resulting thermosensitive penta-block copolymer was evaluated by differential thermal analysis, differential scanning calorimetry and thermogravimetric analysis. Scanning electron microscopy and in vitro degradation of the polymer network in phosphate buffer solutions (pH 7.4) at 37°C were also examined. The pore size of the gels was calculated with Image Analyzer software. Finally, the cytotoxic, morphological, and gene expression effects of copolymers on the skin fibroblast were evaluated.

RESULTS

The experiments showed that the PNIPAAm-PCL-PEG-PCL-PNIPAAm polymer with the right composition and the expected molecular weight was achieved. The hydrogel had less crystallizability compared with its precursors. The resulting thermosensitive hydrogel had a three-dimensional structure with interconnected pores that mimicked the extracellular matrix. The control of the degradability rate can be possible by weight percent changes. The pore size correlated with the polymer concentration in aqueous solution and the pore sizes of the 20 wt% hydrogel were better for fibroblast cultivation than those of the 10 wt% hydrogel. Cell proliferation on the 20% gel was more than that of the 10% gel. The hydrogel not only preserved the viability and phenotypical morphology of the entrapped cells but also stimulated the initial cell-cell interactions and proliferation of fibroblasts. The hydrogel did not influence cell conformation and this property of the polymer underlined its safety. Cells seeded on this copolymer showed a normal and spear shape and formed a focal adhesion with the hydrogel surface. Notably, the hydrogel increased collagen I α1 and collagen III mRNAs expression.

CONCLUSION

Due to the low molecular weight and poor mechanical strength of the star-shaped copolymer, it was not considered for fabrication of the scaffolds for wound healing. The biodegradable, biocompatible, injectable and thermosensitive PNIPAAm-PCL-PEG-PCL-PNIPAAm hydrogel in 20 wt% demonstrated a desirable potential for future application as a cell scaffold in skin tissue engineering and wound healing.

In situ formation of PLA-grafted alkoxysilanes for toughening a biodegradable PLA stereocomplex thin film

Current work provides a synergistic approach to prepare super tough PLA without any significant loss of its excellent intrinsic mechanical properties.

A versatile strategy for the synthesis and mechanical property manipulation of networked biodegradable polymeric materials composed of well-defined alternating hard and soft domains

Networked materials composed of well-defined alternating domains of two types of biodegradable polymers, hard poly(l-lactide) and soft poly(ε-caprolactone), were successfully synthesized.