Lamellae Evolution of Stereocomplex-Type Poly(Lactic Acid)/Organically-Modified Layered Zinc Phenylphosphonate Nanocomposites Induced by Isothermal Crystallization

Toward exclusive stereocomplex crystallization of high-molecular-weight poly(L-lactic acid)/poly(D-lactic acid) blends with outstanding heat resistance via incorporating selective nucleating agents

In high molecular weight poly(L-lactic acid)/poly(D-lactic acid) (HMW PLLA/PDLA) blends, the construction of exclusive stereocomplex crystals (SC) with high crystallinity and strong melt memory remains a great challenge. In the present study, various norbornene dicarboxylate complexes (TMXNa, Mg, Al, or Ca) were employed as the stereo-selective nucleating agents (NAs), and their effect on the crystallization characteristics, rheological behavior, and heat resistance of PLLA/PDLA blends were thoroughly studied. Strikingly, TMX-Al facilitated the construction of exclusive SC with over 50 % crystallinity and excellent melt memory. The dense SC crystals network structure boosted the heat resistance of L/D-xAl blends with a VST as high as 145 °C. The strengthened intermolecular interaction fostered the generation of pre-ordered structure in the melt and enhanced chain interdiffusion, which contributed to intermolecular nucleation and SC crystallization in L/D-xAl blend. This study opens up a new avenue for melt processing and application development of SC-PLA materials.

Development of Stereocomplex Polylactide Nanocomposites as an Advanced Class of Biomaterials-A Review

This review paper analyzes the development of advanced class polylactide (PLA) materials through a combination of stereocomplexation and nanocomposites approaches. The similarities in these approaches provide the opportunity to generate an advanced stereocomplex PLA nanocomposite (stereo-nano PLA) material with various beneficial properties. As a potential "green" polymer with tunable characteristics (e.g., modifiable molecular structure and organic-inorganic miscibility), stereo-nano PLA could be used for various advanced applications. The molecular structure modification of PLA homopolymers and nanoparticles in stereo-nano PLA materials enables us to encounter stereocomplexation and nanocomposites constraints. The hydrogen bonding of D- and L-lactide fragments aids in the formation of stereococomplex crystallites, while the hetero-nucleation capabilities of nanofillers result in a synergism that improves the physical, thermal, and mechanical properties of materials, including stereocomplex memory (melt stability) and nanoparticle dispersion. The special properties of selected nanoparticles also allow the production of stereo-nano PLA materials with distinctive characteristics, such as electrical conductivity, anti-inflammatory, and anti-bacterial properties. The D- and L-lactide chains in PLA copolymers provide self-assembly capabilities to form stable nanocarrier micelles for encapsulating nanoparticles. This development of advanced stereo-nano PLA with biodegradability, biocompatibility, and tunability properties shows potential for use in wider and advanced applications as a high-performance material, in engineering field, electronic, medical device, biomedical, diagnosis, and therapeutic applications.

Enhanced Photodegradation Stability in Poly(butylene adipate-co-terephthalate) Composites Using Organically Modified Layered Zinc Phenylphosphonate

The enhancement of the ultraviolet (UV) photodegradation resistance of biodegradable polymers can improve their application efficacy in a natural environment. In this study, the hexadecylamine modified layered zinc phenylphosphonate (m-PPZn) was used as a UV protection additive for poly(butylene adipate-co-terephthalate) (PBAT) via solution mixing. The results from the Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction analysis of the m-PPZn indicated the occurrence of hexadecylamine intercalation. FTIR and gel permeation chromatography were used to characterize the evolution of the PBAT/m-PPZn composites after being artificially irradiated via a light source. The various functional groups produced via photodegradation were analyzed to illustrate the enhanced UV protection ability of m-PPZn in the composite materials. From the appearance, the yellowness index of the PBAT/m-PPZn composite materials was significantly lower than that of the pure PBAT matrix due to photodegradation. These results were confirmed by the molecular weight reduction in PBAT with increasing m-PPZn content, possibly due to the UV photon energy reflection by the m-PPZn. This study presents a novel approach of improving the UV photodegradation of a biodegradable polymer using an organically modified layered zinc phenylphosphonate composite.

Thermo-Mechanical Performance of Polylactide Composites Reinforced with Alkali-Treated Bamboo Fibers

In this study, polylactide acid (PLA) is filled with bamboo fibers (BFs) to fabricate a biodegradable natural composite for industrial applications. The influence of pre-treatment of BFs using 4 wt % sodium hydroxide (NaOH) solution at room temperature for 1 h on thermal and mechanical properties of resultant composites is systematically investigated. Differential scanning calorimetry and thermogravimetric analysis demonstrate that the incorporation of treated BFs promotes higher glass transition and crystallization temperatures of the resultant composites relative to untreated fiber composites, whereas alkali treatment results in superior thermal stability. Furthermore, the fracture surfaces are characterized by scanning electron microscopy. The changes in morphology reveal the possible dissolution of hemicellulose and lignin by alkalization with NaOH, indicative of an improved interfacial adhesion. An increment in the tensile strength of composites is achieved through the reinforcement with treated fibers. However, a lower tensile modulus is found for composites reinforced with chemically modified BFs, which might be due to the partial conversion of cellulose I into II. The results highlight that the use of BFs could be a feasible candidate as reinforcements for the development of biodegradable composites.

Non-isothermal Crystallization, Thermal Stability, and Mechanical Performance of Poly(L-lactic acid)/Barium Phenylphosphonate Systems

The introduction of a nucleating agent in semi-crystalline polymers is a frequently utilized way to improve the crystallization performance, and the use of a nucleating agent has a very great effect on the performance of the polymer in other areas including thermal stability and mechanical properties. In this investigation, barium phenylphosphonate (BaP) was prepared as a crystallization accelerator for Poly(L-lactic acid) (PLLA), and the non-isothermal crystallization behavior, thermal stability, and mechanical properties of PLLA modified by BaP were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electronic tensile testing. Non-isothermal crystallization analysis showed that the BaP could significantly accelerate the crystallization of PLLA, and the non-isothermal crystallization peak shifted to a higher temperature with increasing concentration of BaP, however, the corresponding crystallization peak became wider. XRD results after non-isothermal crystallization confirmed the non-isothermal crystallization DSC results. Additionally, the addition of BaP did not change the crystal form of PLLA. A comparative study on thermal stability indicated that BaP decreased the onset decomposition temperature of PLLA, resulting from the formation of more tiny and imperfect crystals. Whereas the influence of BaP on the thermal decomposition profile of PLLA was negligible. In terms of mechanical properties, the tensile strength and elastic modulus of PLLA/BaP increased compared to the virgin PLLA, unfortunately, the elongation at break decreased.