Cell Morphology and Improved Heat Resistance of Microcellular Poly(l-lactide) Foam via Introducing Stereocomplex Crystallites of PLA

Bio-based poly(lactic acid) foams with enhanced mechanical and heat-resistant properties obtained by facilitating stereocomplex crystallization with addition of D-sorbitol

The preparation of bio-based poly(lactic acid) (PLA) foams with high mechanical properties and heat resistance is of great significance for environmental protection and green sustainable development. In this paper, D-sorbitol (DS) containing six hydroxyl groups was introduced into poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blends for first time to promote the formation of stereocomplex (SC) crystals, which could improve the foaming behavior and enhance mechanical properties and heat resistance of PLA foams. The results showed that DS could improve the formation efficiency and crystallinity of SC crystals by enhancing the hydrogen bonding between the enantiomeric molecular chains. Furthermore, the compression modulus and interactions Vicat softening temperature of the PLLA/PDLA/DS blend foam increased about 854% and 16% compared to the pure PLLA foam, respectively. Besides, when the annealing process was introduced, the compression and heat resistance of the PLA foams increased further. This study provided a feasible strategy for the preparation of bio-based and biodegradable PLA foams with outstanding compressive and heat resistance properties.

Syntheses and properties of tri- and multi-block copolymers consisting of polybutadiene and polylactide segments

Biomaterials have drawn considerable attention in recent years because of environmental concerns. In this paper, several different poly(lactide)-b-poly(butadiene)-b-poly(lactide) (PLA-b-PB-b-PLA) triblock copolymers were synthesized by the bulk ring-opening polymerization of lactide initiated by flexible macro-initiator hydroxyl-terminated polybutadiene (HTPB) by adjusting the ratio of HTPB to lactide and the optical isomer of lactide. Afterwards, a chain-extension reaction with hexamethylene diisocyanate (HDI) was carried out to prepare (PLA-b-PB-b-PLA)_n multi-block copolymers with enhanced molecular weight. The structures and properties of these block copolymers were then characterized by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), atomic force microscope (AFM) and Fourier-transform infrared (FTIR). Toughening effect of the (PLA-b-PB-b-PLA)_n multiblock copolymers on biodegradable poly(l-lactide) (PLLA) film was investigated and the blended film with higher (poly(d-lactide)-b-poly(butadiene)-b-poly(d-lactide))_n (PDLA-b-PB-b-PDLA)_n loading (15 wt%) exhibited better toughness nearly without loss of the tensile strength. The mechanical properties of the (PLA-b-PB-b-PLA)_n/PLLA blended film were proved to be influenced by the different isomers of PLA and rubbery PB chains.

Toughen effect of the multiblock copolymers (PLA-b-PB-b-PLA)_n on PLLA was investigated and the mechanical properties of the blended films were proved to be influenced by the optical isomers of PLA, stereocomplexation and the rubbery PB chains.

Promoted formation of stereocomplex in enantiomeric poly(lactic acid)s induced by cellulose nanofibers

Stereocomplex (SC) crystallization between enantiomeric poly(_L-lactic acid) (PLLA) and poly(_D-lactic acid) (PDLA) is believed to yield poly(lactic acid) (PLA) with superior physiochemical properties. However, homocrystallization (HC) crystallites are inevitably generated in the PLLA/PDLA blends. Herein, we report a simple approach to fabricate PLLA/PDLA racemic blends with high contents of SC crystallites by introducing cellulose nanofibers (CNFs). The isothermal crystallization results revealed that the half-crystallization time of the PLLA/PDLA blend was significantly decreased by adding CNFs. Additionally, with the incorporation of 3 wt% modified CNFs, the PLLA/PDLA blend was overwhelmingly crystallized into SC crystallites with no HC crystallite formation. Based on Fourier transform infrared spectroscopy findings, it was speculated that the preferred SC crystallization of PLLA/PDLA/CNF was caused by enhanced interchain molecular interactions between CNFs and PLA. This work presents a feasible and efficient method to fabricate PLA with exclusively SC crystallites, which possesses great potential for producing high-performance PLA materials.

Hierarchical crystallization strategy adaptive to 3-dimentional printing of polylactide matrix for complete stereo-complexation

A novel strategy adaptive to 3D printing of PLA matrix for complete stereo-complexation was designed. Stereo-complexation has been demonstrated for its effectiveness in simultaneously improving aqueous stability and heat resistance of PLA. However, current techniques could not be directly incorporated into 3D printing of stereo-complexed PLA since stereo-complexed crystallites are easily formed before printing. High printing temperatures are thus required but decompose PLA materials at the same time. The hypothesis for this research is that controllable hierarchical crystallization in three thermal processes, the filament preparation, 3D printing and post annealing, could ensure feasibility of the strategy and a 100% stereo-complexation level in PLA matrices. Effects of extrusion, ambient and annealing temperatures on material structures were analyzed via WAXD, DSC and DMA. Resistance to hydrolysis and heat of the 3D printed PLA matrix was evaluated under practical conditions. It was showed that homo-crystallites anchored molecular chains of PLA during the post-annealing process for a high retention of tensile properties, while stereo-complexed crystallites provided stronger intermolecular interactions for improved hydrolytic and thermal resistance. This novel strategy via incorporating controlled hierarchical crystallization into 3D printing would enrich the fabrication and exploration of high-performance 3D printed PLA materials.

A green fabrication method of poly (lactic acid) perforated membrane via tuned crystallization and gas diffusion process

Poly (lactic acid) (PLA) perforated membrane is typically obtained through the solvent-volatilization-induced or non-solvent-induced phase separation (NIPS) method. However, the residual organic solvents would unavoidably limit the application of PLA perforated membrane in biomedical and high-end water purification fields. Herein, an innovative solution-free method was proposed for preparing the PLA perforated membrane via a simple and environmentally friendly way. We have successfully fabricated the PLA perforated membrane using a physical foaming technique with CO₂ as the blowing agent. By tuning the primary film thickness, saturation pressure, and foaming temperature, PLA perforated membrane's cell morphology could be accordingly adjusted. The PLA perforated membrane with a highly-ordered straight pore channel and high open cell content (OCC) approximately 72% was obtained under a mild condition. The formation mechanism of the PLA perforated membrane was discussed via the interaction of crystallization behavior and gas diffusion process. This green and solvent-free PLA perforated membrane possesses great potential for use in areas like the tissue engineering and high-end water purification.

The crystallization morphology and process of stereocomplex crystallites of polylactide under CO2: the effect of H-bonding and chain diffusion

The crystallization of PLA SC under CO2 was in situ investigated for the first time.

Influence of Polylactide (PLA) Stereocomplexation on the Microstructure of PLA/PBS Blends and the Cell Morphology of Their Microcellular Foams

Polylactide foaming materials with promising biocompatibility balance the lightweight and mechanical properties well, and thus they can be desirable candidates for biological scaffolds used in tissue engineering. However, the cells are likely to coalesce and collapse during the foaming process of polylactide (PLA) due to its intrinsic low melt strength. This work introduces a unique PLA stereocomplexation into the microcellular foaming of poly (l-lactide)/poly (butylene succinate) (PLLA/PBS) based on supercritical carbon dioxide. The rheological properties of PLA/PBS with 5 wt% or 10 wt% poly (d-lactide) (PDLA) present enhanced melt strength owing to the formation of PLA stereocomplex crystals (sc-PLA), which act as physical pseudo-cross-link points in the molten blends by virtue of the strong intermolecular interaction between PLLA and the added PDLA. Notably, the introduction of either PBS or PDLA into the PLLA matrix could enhance its crystallization, while introducing both in the blend triggers a decreasing trend in the PLA crystallinity, which it is believed occurs due to the constrained molecular chain mobility by formed sc-PLA. Nevertheless, the enhanced melt strength and decreased crystallinity of PLA/PBS/PDLA blends are favorable for the microcellular foaming behavior, which enhanced the cell stability and provided amorphous regions for gas adsorption and homogeneous nucleation of PLLA cells, respectively. Furthermore, although the microstructure of PLA/PBS presents immiscible sea-island morphology, the miscibility was improved while the PBS domains were also refined by the introduction of PDLA. Overall, with the addition of PDLA into PLA/10PBS blends, the microcellular average cell size decreased from 3.21 to 0.66 μm with highest cell density of 2.23 × 10¹⁰ cells cm⁻³ achieved, confirming a stable growth of cells was achieved and more cell nucleation sites were initiated on the heterogeneous interface.

Stereocomplex formation in mixed polymers filled with two-dimensional nanofillers

Nanosheets promote the formation of stereocomplex crystallites due to the heterogeneous nucleation of mixed polymer chains on filler surfaces.

Monte Carlo simulations of stereocomplex formation in multiblock copolymers

Local miscibility and relative size of block length and crystal thickness codetermine stereocomplex formation in multiblock copolymers.

Mechanical properties and heat resistance of stereocomplex polylactide/copolyester blend films prepared by in situ melt blending followed with compression molding

This work focuses on the process to obtain high heat-resistant stereocomplex polylactide (scPLA)/copolyester blend films by in situ melt blending of high molecular-weight poly(L-lactide) (PLLA), low molecular-weight poly(D-lactide) (PDLA) and copolyester followed with compression molding. A copolyester of poly(ε-caprolactone-co-L-lactide) was used as a film former. Stereocomplexation, mechanical properties and heat resistance of the scPLA/copolyester blend films were investigated by differential scanning calorimetry (DSC), tensile testing and dynamic mechanical analysis (DMA), respectively. The PDLA fractions enhanced stereocomplexation and heat resistance of the blend films while the copolyester fraction reduced film brittleness. Dimensional stability to heat of blend films was also determined and was accorded to their DMA results. It was concluded that the high heat-resistant and less brittle scPLA films could be prepared using 70/30 (w/w) PLLA/PDLA with 20 wt% copolyester through melt blending before compression molding. This film showed similar stress at break and heat resistance to those of polypropylene film but with lower strain at break.

Poly (lactic acid) blends: Processing, properties and applications

Poly (lactic acid) or polylactide (PLA) is a commercial biobased, biodegradable, biocompatible, compostable and non-toxic polymer that has competitive material and processing costs and desirable mechanical properties. Thereby, it can be considered favorably for biomedical applications and as the most promising substitute for petroleum-based polymers in a wide range of commodity and engineering applications. However, PLA has some significant shortcomings such as low melt strength, slow crystallization rate, poor processability, high brittleness, low toughness, and low service temperature, which limit its applications. To overcome these limitations, blending PLA with other polymers is an inexpensive approach that could also tailor the final properties of PLA-based products. During the last two decades, researchers investigated the synthesis, processing, properties, and development of various PLA-based blend systems including miscible blends of poly l-lactide (PLLA) and poly d-lactide (PDLA), which generate stereocomplex crystals, binary immiscible/miscible blends of PLA with other thermoplastics, multifunctional ternary blends using a third polymer or fillers such as nanoparticles, as well as PLA-based blend foam systems. This article reviews all these investigations and compares the syntheses/processing-morphology-properties interrelationships in PLA-based blends developed so far for various applications.

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.

Introduction of stereocomplex crystallites of PLA for the solid and microcellular poly(lactide)/poly(butylene adipate-co-terephthalate) blends

Solid and microcellular poly(l-lactide)/poly(butylene adipate-co-terephthalate) (PLLA/PBAT) blends with or without poly(d-lactide) (PDLA) were prepared via melt blending and batch foaming process with supercritical carbon dioxide, respectively. The introduction of PDLA on the rheological properties, crystallization behavior and dynamic mechanical properties of the PLLA matrix were investigated. The formed PLA stereocomplex between PLLA and PDLA enhanced the storage modulus and complex viscosity of PLLA/PBAT blends efficiently. Interestingly, the addition of 5 wt% or 10 wt% PDLA in the PLLA/PBAT blends was unfavorable for the PLLA crystallization behavior. The potential reason can be sc-PLA crystallites acting as the physical crosslinking points, which constrained the molecular mobility of the PLLA matrix and even blocked the nucleating effect of PBAT domains. Both the enhanced melt strength and decreased crystallinity of the PLLA matrix are favorable for the cell nucleation and growth and the gas adsorption, respectively. The designed partially foaming of PLLA/PBAT with or without PDLA was carried out to investigate the foaming mechanism. The final cell morphology of PLLA/PBAT foams exhibited typical open-cell structure mainly attributed to the soft immiscible PBAT phase as separated domains. With further addition of PDLA in the PLLA/PBAT blends, the microcellular morphology exhibited decreased average cell size and increased cell density. The sc-PLA crystallites networks in the PLLA matrix acted as cell nucleating agents, which meanwhile resisted the force of cell growth and then prevented the cell collapse.

The introduced PLA stereocomplex could enhance the melting strength of PLLA/PBAT blends efficiently. The microcellular morphology of PLLA/PBAT foams with PDLA exhibited decreased average cell size and increased cell density.

Pressure-controlled crystallization of stereocomplex crystals in enantiomeric polylactides with remarkably enhanced hydrolytic degradation

Enantiomeric biopolymers, with improved combinatorial heat resistance, hydrolytic degradation and hydrophilicity, were fabricated by pressure-controlled crystallization of stereocomplex crystals.

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.

Improved expansion ratio and heat resistance of microcellular poly(L-lactide) foam via in-situ formation of stereocomplex crystallites

It is critical to broaden the applications of poly(L-lactic acid) foams by improving heat resistance properties. The stereocomplex crystallites that are formed by melt blending of poly(L-lactic acid)/polylactide possess high melting point of about 220℃ and thus exhibit high heat resistance; therefore, the introduction of stereocomplex crystallites tends to improve the thermal stability of poly(lactic acid) foam. Unfortunately, using the solid-state foaming method, it was found that the expansion ratio of the obtained poly(lactic acid) foams was compromised with the value of 1.7 times once the stereocomplex crystallites were formed during the sample saturation stage. In this study, by applying a high compression molding temperature of 230℃, the as-prepared poly(L-lactic acid) and poly(L-lactic acid)/polylactide blends were amorphous. After being CO2 saturated at a mild condition, the specimens were foamed at 90–160℃. The wide-angle X-ray diffraction profiles presented that the stereocomplex crystallites and PLA homocrystals were in-situ generated during the foaming process. It is observed that the in-situ formed stereocomplex crystallites could act as the physical cross-linking agent to stabilize the nucleated bubbles and suppress cell coalescence, resulting in the increased expansion ratio (with value of about 23.6–25.6 times) and cell density, especially at high foaming temperatures and extended foaming time. Furthermore, the in-situ formed stereocomplex crystallites during the foaming increased the heat resistance performance of poly(L-lactic acid) foams. This novel crystallization control method helps us to find a balance point in preparing poly(L-lactic acid) foam with high expansion ratio, well-defined cell structure and high heat resistance performance.

WITHDRAWN: PLA Stereocomplexes: A Decade of Progress

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Multiporous open-cell poly(vinyl formal) foams for sound absorption

A series of multiporous open-cell poly(vinyl formal) (PVF) foams were obtained by crosslinking poly(vinyl alcohol) (PVA) with different contents of formaldehyde in aqueous solution.

Poly(lactic acid)-Based in Situ Microfibrillar Composites with Enhanced Crystallization Kinetics, Mechanical Properties, Rheological Behavior, and Foaming Ability

Melt blending is one of the most promising techniques for eliminating poly(lactic acid)'s (PLA) numerous drawbacks. However, success in a typical melt blending process is usually achieved through the inclusion of high concentrations of a second polymeric phase which can compromise PLA's green nature. In a pioneering study, we introduce the production of in situ microfibrillar PLA/polyamide-6 (PA6) blends as a cost-effective and efficient technique for improving PLA's properties while minimizing the required PA6 content. Predominantly biobased products, with only 3 wt % of in situ generated PA6 microfibrils (diameter ≈200 nm), were shown to have dramatically improved crystallization kinetics, mechanical properties, melt elasticity and strength, and foaming-ability compared with PLA. Crucially, the microfibrillar blends were produced using an environmentally friendly and cost-effective process. Both of these qualities are essential in guarantying the viability of the proposed technique for overcoming the obstacles associated with the vast commercialization of PLA.