Mechanical properties, crystallization characteristics, and foaming behavior of polytetrafluoroethylene-reinforced poly(lactic acid) composites

A Double-Expanded Polytetrafluoroethylene Fabrication Method for Increased Mechanical Compliance in Tubular Vascular Graft Applications

A novel manufacturing technique has been developed to enhance the compliance of expanded polytetrafluoroethylene (ePTFE) for vascular graft applications. This new method involves modifying the existing processing procedures by introducing an additional expansion step while using a lower temperature during the first expansion stage. The new process results in the production of highly compliant ePTFE grafts without the need for supplementary additives or inherent material alterations. Tensile testing in both the longitudinal and circumferential directions as well as cyclical tensile testing were conducted to characterize the mechanical properties of double-expanded ePTFE grafts prepared using varying expansion ratios. The double-expanded ePTFE grafts consistently outperformed the prevailing, single-expanded counterparts in both tensile stress tests and cyclical assessments of its elastic compliance. Notably, the double-expanded ePTFE samples exhibited the desirable, biomimetic "toe-region" and an elastic strain capacity of up to 50%, comparable to native vascular materials. Scanning electron microscopy (SEM) imaging was used to examine the morphological characteristics of the wavy fibers within the double-expanded PTFE samples, which contributed to the enhanced compliance that is needed for vascular graft applications.

Green synthesis of bio-based flame retardant/natural rubber inorganic-organic hybrid and its flame retarding and toughening effect for polylactic acid

Polylactic acid (PLA) has garnered significant interest as a bio-based polymer due to its favorable thermal and processing characteristics, as well as its notable economic and environmental benefits. However, the drawbacks such as flammability and poor toughness of PLA severely constrained its applications in more fields. Here, based on the outstanding flame-retardant properties of core-shell flame retardant (CSFR) and the toughening potential of natural rubber (NR), we synthesized inorganic-organic hybrid of CSFR-NR using an aqueous synthesis to synchronous optimization of the comprehensive performance of PLA. The as-prepared CSFR-NR with "hard core and soft shell" possess the ability to promote char formation and facilitate uniform dispersion in the PLA matrix. Consequently, the PLA/CSFR-NR showed an excellent flame retardancy with the limiting oxygen index (LOI) value of 31.5 % and UL-94 V-0 rating and synergistic toughening effect with absolutely improvement in elongation at break and notched izod impact strength, achieving a balance between the fire safety and mechanical performance. Moreover, the degradation rate of PLA has also been substantially promoted by CSFR-NR in simulated seawater. Hence, this study offers a straightforward, efficient, and environmentally friendly strategy for creating high-performance flame retardant and toughened bioplastics.

Hydrophobic and oleophilic open-cell foams from in-situ microfibrillation blends of poly(lactic acid) and polytetrafluoroethylene: Selective oil-adsorption behaviors

Confronted with severe water contamination induced by the spillage of oils, seeking oil-selective adsorbent to recover oil from oily wastewater is extremely urgent. In particular, the functionalized polymer foams with open-cell structures are highly promising oil-selective adsorbent. Herein, a simple, effective and green method was presented to prepare open-cell poly(lactic acid) (PLA)/polytetrafluoroethylene (PTFE) foams with selective oil-adsorption behaviors via melt blending and supercritical CO₂ batch foaming technique. The introduction of PTFE had a distinct positive influence on the melt viscoelasticity and crystallization performances of various PLA specimens. The resulted PLA/PTFE4 foam with a volume expansion ratio of 10.17 ± 0.93 and a cell density of 1.58 × 10⁸ cells/cm³ possessed the highest open-cell content of 90.81 ± 0.78 %. Meanwhile, PLA/PTFE4 foam revealed oil/water selective adsorption capacity of 1.2-6.1 g/g for various organic solvents and oils. The adsorption capacity of PLA/PTFE4 foam for CCl₄ exhibited no significant decrement during ten adsorption-desorption cycles. This research offered a guideline for the manufacture of green environmental open-cell polymer foams for oil-selective adsorption.

The synergistic effect of polytetrafluoroethylene in-situ fibrillation and dibenzoyl sebacate hydrazide on the crystallization and foaming behavior of poly (lactic acid)

The fully degradable poly (lactic acid) foam with green environmental protection characteristics can alleviate the shortage of petroleum resources caused by the application of plastics. However, due to the inherent low melt strength and slow crystallization rate of linear PLA. It is difficult to obtain PLA microcellular foam with good morphology. In order to obtain PLA microcellular foam with ultra-high expansion ratio and small cell size, PTFE (polytetrafluoroethylene) nanofibers with excellent CO₂ adsorption rate were introduced. Self-assembled nucleator TMC-300(dibenzoyl sebacate hydrazide) was also introduced to blend with PLA to obtain small-sized cells. The results show that the PTFE entanglement network as a self-assembled template can effectively improve the early crystallization nucleation efficiency and increase the crystallinity of branched PLA (CBPLA)/TMC by 7 %. The microcellular foam with PTFE content of 0.5 wt% (CBPLA/TMC/PTFE 0.5) was successfully prepared by physical foaming agent, which had the lowest cell size (8.7 μm) And high expansion ratio (1200 %).

Microcellular injection molded lightweight and tough poly (L-lactic acid)/in-situ polytetrafluoroethylene nanocomposite foams with enhanced surface quality and thermally-insulating performance

High-performance microcellular polymer foams have been widely used all over the world, while the excessive usage of petroleum-based polymers caused serious environmental problems. As the eco-friendly awareness is increasing significantly, poly (L-lactic acid) (PLLA), as a typical biomass polymer, has gradually attracted widespread attention. However, the slow crystallization and poor melt strength of PLLA lead to low foaming ability and thus limiting its industrial applications. Herein, a novel and scalable strategy by coupling in-situ fibrillation and mold-opening microcellular injection molding (MOMIM) was developed to fabricate lightweight and tough PLLA/polytetrafluoroethylene (PTFE) foams. Thanks to the reticulated in-situ PTFE nanofibrils with a diameter of 100-200 nm, the crystallization and viscoelasticity of PLLA were dramatically promoted, and further contributing to its foaming ability. The expansion ratio of the MOMIM PLLA/PTFE foam was increased by 86 % compared with the regular microcellular injection molded (RMIM) PLLA foam. Moreover, the lower foam density and the toughening effect of PTFE nanofibrils resulted in the outstanding ductility of the PLLA/PTFE foams, whose tensile elongation, flexural strength, and impact strength were maximally increased by 52 %, 28 %, and 48 %, compared with PLLA foams. More importantly, the thermally-insulating performance and surface quality of PLLA/PTFE foams were also greatly improved.

ScCO2-assisted fabrication and compressive property of poly (lactic acid) foam reinforced by in-situ polytetrafluoroethylene fibrils

As an effective alternative for petrochemical-based polymers, bio-based poly (lactic acid) (PLA) foam has been anticipated to alleviate enormous environmental pollution caused by microplastics. However, some difficulties involved in PLA foaming process due to the inherently poor melt strength and crystallization properties. In this context, a small amount of polytetrafluoroethylene (PTFE) was incorporated into PLA matrix to solve the aforementioned issues. Scanning electron microscopy measurement exhibited that PTFE fibrils and their physical networks were formed in molten PLA after blending. Due to these PTFE networks, approximately 2 orders of magnitudes increment in the storage modulus and more than 20% improvement in crystallinity of PLA were obtained. Diverse PLA samples were successfully foamed by a cost-effective, green and supercritical CO₂-assisted foaming method. The PLA/PTFE foam with the PTFE content of 5 wt% (PLA/PTFE5) possessed the smallest pore size (9.51 μm) and the highest pore density (2.60 × 10⁸ pores/cm³). In addition, the average specific compressive strength of PLA/PTFE5 foam was enhanced 30% in comparison with that of pure PLA foam. Overall, this study could provide a prospective strategy for developing bioderived and biodegradable polymer foams with controllable pore structures and high compression property.

A facile strategy for preparation of strong tough poly(lactic acid) foam with a unique microfibrillated bimodal micro/nano cellular structure

This work reports the design and fabrication of strong tough poly(lactic acid) (PLA) foam by combining pressure-induced-flow (PIF) processing with supercritical CO₂ foaming. PIF processing widened the foaming window of PLA to 40-120 °C, while supercritical CO₂ foaming released the undesired internal stress of PLA samples with PIF processing (P-PLA). The prepared PLA foams displayed a unique microfibrillated bimodal micro/nano cellular structure which is strongly affected by saturation temperature (T_s). Both micron and nano cells showed decreasing cells size and increasing cell density as T_s elevated. The orientation factor as well as internal stress of PLA foams decreased with increased T_s. Compared with P-PLA samples, PLA foam prepared at T_s of 40 °C showed negligible reduction of orientation from 0.45 to 0.41 and release of internal stress characterized by the rightward shift of Raman peak (stretching vibration of CO bond from 1763 to 1766 cm^-1). Furthermore, PLA foam prepared at T_s of 40 °C presented excellent impact strength (32.3 kJ/m²), tensile strength (42.0 MPa), and ductility (14.2%). The combination of PIF processing and supercritical CO₂ foaming provides a facile and effective method to prepare strong tough PLA foam that has immense potential in biomedical, aerospace, automotive, and other structural applications.

The effect of polytetrafluoroethylene particle size on the properties of biodegradable poly(butylene succinate)-based composites

Poly(butylene succinate) (PBS)/polytetrafluoroethylene (PTFE) composites, including three types of PTFE powders, were prepared by melt blending using a HAAKE torque rheometer. Microcellular foams were successfully fabricated by batch foaming with supercritical fluids (scCO₂). The effects of PTFE powder type on crystallization, rheological properties and foaming behavior were studied. PTFE L-5 and PTFE JH-220 powders showed good dispersion in the PBS matrix, and PTFE FA-500 powder underwent fibrillation during the melt blending process. All three PTFE powders gradually increased the crystallization temperature of PBS from 78.2 to 91.8 ℃ and the crystallinity from 45.6 to 61.7% without apparent changes in the crystal structure. Rheological results revealed that PBS/PTFE composites had a higher storage modulus, loss modulus, and complex viscosity than those of pure PBS. In particular, the complex viscosity of the PBS/P500 composite increased by an order of magnitude in the low-frequency region. The foamed structure of PBS was obviously improved by adding PTFE powder, and the effect of fibrillated PTFE FA-500 was the most remarkable, with a pore mean diameter of 5.46 μm and a pore density of 1.86 × 10⁹ cells/cm³ (neat PBS foam: 32.49 μm and 1.95 × 10⁷ cells/cm³). Moreover, PBS/P500 foam always guarantees hydrophobicity.

Foaming behavior of poly(lactic acid) with different D-isomer content based on supercritical CO2-induced crystallization

Using supercritical carbon dioxide (sc-CO2) as a physical foaming agent, the effect of sc-CO2 on the formation of crystalline domains and subsequently on the foaming behaviors of the two grades of PLA with different D-isomer content were investigated in a wide foaming temperature range. The PLA’s final crystallinity is significantly increased with decreasing annealing temperature and by reducing the D-isomer content. Cellular structure results show that not only the crystallinity but also the crystalline morphology play an important role in cellular structure. A novel spherulite morphology including ringless and ring-banded morphology in the same spherulite was formed at lower foaming temperature, as a result, some entities were nonuniformly distributed in the PLA foams. Uniform and closed cellular structure were obtained when only the ring-banded spherulites were formed. An opened and interconnected cellular structure is tended to be formed because of the synergistic effect of high temperature and plasticization of CO2. Based on the crystallinity and morphology, a suitable foaming window as a function of temperature is proposed. It is found that PLA with 4.1% D-isomer content had much broader suitable foaming window range to produce homogeneous cellular structure.

Foaming of PLA Composites by Supercritical Fluid-Assisted Processes: A Review

Polylactic acid (PLA) is a well-known and commercially available biopolymer that can be produced from different sources. Its different characteristics generated a great deal of interest in various industrial fields. Besides, its use as a polymer matrix for foam production has increased in recent years. With the rise of technologies that seek to reduce the negative environmental impact of processes, chemical foaming agents are being substituted by physical agents, primarily supercritical fluids (SCFs). Currently, the mass production of low-density PLA foams with a uniform cell morphology using SCFs as blowing agents is a challenge. This is mainly due to the low melt strength of PLA and its slow crystallization kinetics. Among the different options to improve the PLA characteristics, compounding it with different types of fillers has great potential. This strategy does not only have foaming advantages, but can also improve the performances of the final composites, regardless of the implemented foaming process, i.e., batch, injection molding, and extrusion. In addition, the operating conditions and the characteristics of the fillers, such as their size, shape factor, and surface chemistry, play an important role in the final foam morphology. This article proposes a critical review on the different SCF-assisted processes and effects of operating conditions and fillers on foaming of PLA composites.

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.

Highly Durable Superhydrophobic Polymer Foams Fabricated by Extrusion and Supercritical CO2 Foaming for Selective Oil Absorption

The severe water contamination caused by oil leakage is calling for low-cost and high-performance absorbent materials for selective oil removal. In this study, a scalable green method was proposed to produce polypropylene (PP)/poly(tetrafluoroethylene) (PTFE) composite foams via conventional processing techniques including twin-screw extrusion and supercritical carbon dioxide foaming. To produce the superhydrophobic foam, micro- and nanosized PTFE particles were melt blended with PP and subsequently foamed. Ascribed to the nanofibrillation of microsized PTFE during processing, the fabricated foam exhibited a special highly porous structure with PTFE nanofibrils and nanoparticles uniformly distributed on the pore surfaces within the PP matrix, which resulted in a remarkably high water contact angle of 156.8° and a low contact angle hysteresis of 1.9°. Unlike traditional surface-modified superhydrophobic absorbers, the foams prepared are entirely superhydrophobic, which means that they remain superhydrophobic when being fractured or cut. Moreover, they are highly durable and maintained the superhydrophobicity when subjected to ultrasonication and mechanical sanding. When used in selective oil absorption, the durable foams exhibited excellent absorption efficiency and high stability in repetitive and long-term use. These advantages make the PP/PTFE foam a promising superabsorbent material for water remediation.

Chemical Modification and Foam Processing of Polylactide (PLA)

Polylactide (PLA) is known as one of the most promising biopolymers as it is derived from renewable feedstock and can be biodegraded. During the last two decades, it moved more and more into the focus of scientific research and industrial use. It is even considered as a suitable replacement for standard petroleum-based polymers, such as polystyrene (PS), which can be found in a wide range of applications-amongst others in foams for packaging and insulation applications-but cause strong environmental issues. PLA has comparable mechanical properties to PS. However, the lack of melt strength is often referred to as a drawback for most foaming processes. One way to overcome this issue is the incorporation of chemical modifiers which can induce chain extension, branching, or cross-linking. As such, a wide variety of substances were studied in the literature. This work should give an overview of the most commonly used chemical modifiers and their effects on rheological, thermal, and foaming behavior. Therefore, this review article summarizes the research conducted on neat and chemically modified PLA foamed with the conventional foaming methods (i.e., batch foaming, foam extrusion, foam injection molding, and bead foaming).

Multiple actions of poly(ethylene octene) grafted with glycidyl methacrylate on the performance of poly(lactic acid)

Poly(ethylene octene) grafted with glycidyl methacrylate (POE-g-GMA) was employed to improve the rheological and thermal properties, toughness, and foaming behaviors of poly(lactic acid) (PLA) through a chain extension effect.

Fabrication of poly(ε-caprolactone) tissue engineering scaffolds with fibrillated and interconnected pores utilizing microcellular injection molding and polymer leaching

Three-dimensional fibrillated interconnected porous poly(ε-caprolactone) scaffolds were prepared by microcellular injection molding and polymer leaching.