Fabrication of rigid poly(lactic acid) foams via thermally induced phase separation

Tailoring the cell morphology and characteristics of poly(lactic acid) foams via a stereocomplex crystallization-assisted phase separation strategy

Stereocomplex (SC) crystallization between poly(l-lactide) (PLLA) and its enantiomer poly(d-lactide) (PDLA) provides an effective approach to enhancing the physicochemical properties of poly(lactic acid) (PLA). In this study, we systematically investigate the effects of SC crystallization on the thermal-induced phase separation (TIPS) behavior of PLA enantiomers/1, 4-dioxane solutions, as well as the microstructure, pore morphology, and properties of the resulting PLA foams. The results show that increasing PDLA content leads to a higher relative fraction of SC crystallites and alters the cell morphology, transitioning from fishbone-like tubular pores to irregular small pores formed by densely packed submicron particles. In-situ Raman and X-ray analyses reveal that this phenomenon is closely related to the TIPS mechanism of PLA enantiomers in 1, 4-dioxane, where SC crystallites induce a shift from solid-liquid phase separation to crystallization-assisted liquid-liquid phase separation. Compared to α-crystal PLA foams, SC-type foams exhibit a larger specific surface area and greater thermal resistance. Additionally, the unique pore morphology of SC-type foams results in a more hydrophobic surface, demonstrating effective oil-water separation properties. Overall, this study elucidates the interaction mechanism between SC crystallization and phase separation during the TIPS process and proposes a novel strategy for fabricating PLA porous materials.

Preparation Method and Application of Porous Poly(lactic acid) Membranes: A Review

Porous membrane technology has garnered significant attention in the fields of separation and biology due to its remarkable contributions to green chemistry and sustainable development. The porous membranes fabricated from polylactic acid (PLA) possess numerous advantages, including a low relative density, a high specific surface area, biodegradability, and excellent biocompatibility. As a result, they exhibit promising prospects for various applications, such as oil-water separation, tissue engineering, and drug release. This paper provides an overview of recent research advancements in the fabrication of PLA membranes using electrospinning, the breath-figure method, and the phase separation method. Firstly, the principles of each method are elucidated from the perspective of pore formation. The correlation between the relevant parameters and pore structure is discussed and summarized, subsequently followed by a comparative analysis of the advantages and limitations of each method. Subsequently, this article presents the diverse applications of porous PLA membranes in tissue engineering, oil-water separation, and other fields. The current challenges faced by these membranes, however, encompass inadequate mechanical strength, limited production efficiency, and the complexity of pore structure control. Suggestions for enhancement, as well as future prospects, are provided accordingly.

Tuning pore size and density of rigid polylactic acid foams through thermally induced phase separation and optimization using response surface methodology

Rigid polylactic acid (PLA) foams fabricated via thermally induced phase separation (TIPS) utilizing a ternary solution of PLA, Tetrahydrofuran (THF), and water. The PLA gels were stabilized mechanically by the substituting of the THF/water solvent mixture with ethanol as non-solvent and subsequently vacuum dried. A comprehensive characterization of PLA foams was achieved by Scanning Electron Microscopy (SEM), X-ray Diffractometry (XRD) and Brunauer-Emmett-Teller (BET) analyses. The BET area obtained in the PLA foam is up to 18.76 m2/g. The Response Surface Methodology (RSM) was utilized to assess the impacts of four independent variables (polymer concentration, solvent composition, quench temperature, and aging time) on the pore size and density of PLA foam. The experimental findings demonstrated that the fabrication parameters could be fine-tuned to govern the morphology of the pores, comprising their size and density. The optimal values of parameters for cell size were identified by RSM to be 8.96 (wt%), 91.60 (w/w), 5.50 °C, and 3.86 h for the optimum cell size of 37.96 µm (37.78 by Genetic Algorithm). Optimum density by RSM 88.88 mgr/cm3 (88.38 mgr/cm3 by Genetic Algorithm) was obtained at 5.00 (wt%), 89.33 (w/w), 14.40 °C and 2.65 h.

Passive Daytime Radiative Cooling by Thermoplastic Polyurethane Wrapping Films with Controlled Hierarchical Porous Structures

Current research has focused on effective solutions to mitigate global warming and the accelerating greenhouse gas emissions. Compared to most cooling methods requiring energy and resources, passive daytime radiative cooling (PDRC) technology offers excellent energy savings as it requires no energy consumption. However, existing PDRC materials encounter unprecedented problems such as complex structures, low flexibility, and performance degradation after stretching. Thus, this study reports a porous structured thermoplastic polyurethane (TPU) film with bimodal pores to produce high-efficiency PDRC with efficient solar scattering using a simple process. The TPU film exhibited an adequately high solar reflectivity of 0.93 and an emissivity of 0.90 in the atmospheric window to achieve an ambient cooling of 5.6 °C at midday under a solar intensity of 800 W m^-2 . Thus, the highly elastic and flexible TPU film was extremely suitable for application on objects with complex shapes. The radiative cooling performance of 3D-printed models covered with these TPU films demonstrated their superior indoor cooling efficiency compared to commercial white paint (8.76 °C). Thus, the proposed design of high-efficiency PDRC materials is applicable in various urban infrastructural objects such as buildings and vehicles.

Morphology and properties of foamed high crystallinity PEEK prepared by high temperature thermally induced phase separation

Polyetheretherketone (PEEK) is a high‐performance semi‐crystalline thermoplastic polymer with outstanding mechanical properties, high thermal stability, resistance to most common solvents, and good biocompatibility. A high temperature thermally induced phase separation technique was used to produce PEEK foams with controlled foam density from PEEK in 4‐phenylphenol (4PPH) solutions. Physical and mechanical properties, foam and bulk density, surface area, and pore morphology of foamed PEEK were characterized and the role of PEEK concentration and cooling rate was investigated. Porous PEEK with densities ranging from 110 to 360 kg/m³ with elastic moduli and crush strength ranging from 13 to 125 MPa and 0.8 to 7 MPa, respectively, was produced.

Three Component Composite Scaffolds Based on PCL, Hydroxyapatite, and L-Lysine Obtained in TIPS-SL: Bioactive Material for Bone Tissue Engineering

In this research, we describe the properties of three-component composite foam scaffolds based on poly(ε-caprolactone) (PCL) as a matrix and hydroxyapatite whiskers (HAP) and L-Lysine as fillers (PCL/HAP/Lys with wt% ratio 50/48/2). The scaffolds were prepared using a thermally induced phase separation technique supported by salt leaching (TIPS-SL). All materials were precisely characterized: porosity, density, water uptake, wettability, DSC, and TGA measurements and compression tests were carried out. The microstructure of the obtained scaffolds was analyzed via SEM. It was found that the PCL/HAP/Lys scaffold has a 45% higher Young’s modulus and better wettability compared to the PCL/HAP system. At the same time, the porosity of the system was ~90%. The osteoblast hFOB 1.19 cell response was also investigated in osteogenic conditions (39 °C) and the cytokine release profile of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α was determined. Modification of PCL scaffolds with HAP and L-Lysine significantly improved the proliferation of pre-osteoblasts cultured on such materials.

Crystallization-Induced Polymer Scaffold Formation in the Polymer/Drug Delivery System Poly(l-lactic acid)/Ethyl Butylacetylaminopropionate (PLLA/IR3535)

Polymer/mosquito-repellent scaffolds exhibit increasing importance in long-lasting human skin protection to be used as wearable devices and allowing for controlled release of repellents. In this study, ethyl butylacetylaminopropionate (IR3535) was used as a human and environmental friendly active mosquito-repellent serving as a solvent to form functional poly(l-lactic acid) (PLLA) scaffolds by crystallization-based solid-liquid thermally induced phase separation. Crystallization of PLLA in the presence of IR3535 is faster than melt-crystallization of neat PLLA, and in the investigated concentration range from 5 to 50 mass % PLLA, its maximum crystallization rate increases with the PLLA content, by both, increases of the maximum crystal growth rate and of the nuclei density. By adjusting the polymer concentration and the crystallization temperature, microporous scaffolds of different fine structures are obtained, hosting the mosquito-repellent in intra- and interspherulitic pores for its intended later evaporation.

The Effect of Pore Size Distribution and l-Lysine Modified Apatite Whiskers (HAP) on Osteoblasts Response in PLLA/HAP Foam Scaffolds Obtained in the Thermally Induced Phase Separation Process

In this research, we prepared foam scaffolds based on poly(l-lactide) (PLLA) and apatite whiskers (HAP) using thermally induced phase separation technique supported by the salt leaching process (TIPS-SL). Using sodium chloride having a size of (a) 150-315 μm, (b) 315-400 μm, and (c) 500-600 μm, three types of foams with different pore sizes have been obtained. Internal structure of the obtained materials has been investigated using SEM as well as μCT. The materials have been studied by means of porosity, density, and compression tests. As the most promising, the composite prepared with salt size of 500-600 μm was prepared also with the l-lysine modified apatite. The osteoblast hFOB 1.19 cell response for the scaffolds was also investigated by means of cell viability, proliferation, adhesion/penetration, and biomineralization. Direct contact cytotoxicity assay showed the cytocompatibility of the scaffolds. All types of foam scaffolds containing HAP whiskers, regardless the pore size or l-lysine modification induced significant stimulatory effect on the cal-cium deposits formation in osteoblasts. The PLLA/HAP scaffolds modified with l-lysine stimulated hFOB 1.19 osteoblasts proliferation. Compared to the scaffolds with smaller pores (150-315 µm and 315-400 µm), the PLLA/HAP foams with large pores (500-600 µm) promoted more effective ad-hesion of osteoblasts to the surface of the biomaterial.

Recent Progress on Biodegradable Tissue Engineering Scaffolds Prepared by Thermally-Induced Phase Separation (TIPS)

Porous biodegradable scaffolds provide a physical substrate for cells allowing them to attach, proliferate and guide the formation of new tissues. A variety of techniques have been developed to fabricate tissue engineering (TE) scaffolds, among them the most relevant is the thermally-induced phase separation (TIPS). This technique has been widely used in recent years to fabricate three-dimensional (3D) TE scaffolds. Low production cost, simple experimental procedure and easy processability together with the capability to produce highly porous scaffolds with controllable architecture justify the popularity of TIPS. This paper provides a general overview of the TIPS methodology applied for the preparation of 3D porous TE scaffolds. The recent advances in the fabrication of porous scaffolds through this technique, in terms of technology and material selection, have been reviewed. In addition, how properties can be effectively modified to serve as ideal substrates for specific target cells has been specifically addressed. Additionally, examples are offered with respect to changes of TIPS procedure parameters, the combination of TIPS with other techniques and innovations in polymer or filler selection.

Morphology Control and Metallization of Porous Polymers Synthesized by Michael Addition Reactions of a Multi-Functional Acrylamide with a Diamine

Porous polymers have been synthesized by an aza-Michael addition reaction of a multi-functional acrylamide, N,N',N″,N‴-tetraacryloyltriethylenetetramine (AM4), and hexamethylene diamine (HDA) in H2O without catalyst. Reaction conditions, such as monomer concentration and reaction temperature, affected the morphology of the resulting porous structures. Connected spheres, co-continuous monolithic structures and/or isolated holes were observed on the surface of the porous polymers. These structures were formed by polymerization-induced phase separation via spinodal decomposition or highly internal phase separation. The obtained porous polymers were soft and flexible and not breakable by compression. The porous polymers adsorbed various solvents. An AM4-HDA porous polymer could be plated by Ni using an electroless plating process via catalyzation by palladium (II) acetylacetonate following reduction of Ni ions in a plating solution. The intermediate Pd-catalyzed porous polymer promoted the Suzuki-Miyaura cross coupling reaction of 4-bromoanisole and phenylboronic acid.

Phase behavior of solvent-rich compositions of the polymer/drug system poly(butylene succinate) and N,N-diethyl-3-methylbenzamide (DEET)

Poly(butylene succinate) (PBS) is used to produce micro-/nanoporous biodegradable scaffolds, suitable for the release of the mosquito repellent N,N-diethyl-3-methylbenzamide (DEET), based on thermally induced phase separation. For solvent-rich compositions up to 30 m% PBS, it was found that PBS dissolves in DEET at elevated temperatures. During cooling, spherulitic crystallization of PBS occurs, with the crystallization temperature decreasing with the content of DEET and the cooling rate, as determined by cloud-point measurements, differential scanning calorimetry, and polarized-light optical microscopy. Scaffold morphologies of quenched solutions were analyzed by scanning electron microscopy as a function of the polymer concentration and the quenching temperature. These two parameters control the nucleus density/spherulite size, the degree of intermeshing of spherulites, and the intra- and interspherulitic pore size, with the latter typically being of the order of magnitude of few micrometers.

Fire-extinguishing characteristics and flame retardant mechanism of polylactide foams: Influence of tricresyl phosphate combined with natural flame retardant

Extrusion and compression molding techniques were used to process polylactide (PLA) foams using a mixture blowing agent. Tricresyl phosphate (TCP) and natural flame retardants (NFR) from pumpkin (PK) and soybean (SB) were added to rigid PLA foams and the flame retardant properties of the foams were investigated. The effects of TCP content, types and amounts of NFR, and the ratio of TCP to NFR were determined on the physical, thermal and morphological characteristics of the foams. The fire-extinguishing characteristics and flame retardant mechanism of PLA foams with TCP were studied by thermogravimetric analysis-Fourier transform infrared (TGA-FTIR), scanning electron microscope (SEM), limiting oxygen index (LOI), and UL-94 techniques. The results revealed the efficiency of TCP as a flame retardant for PLA foams. Increased TCP levels contributed to a significant enhancement in fire-extinguishing characteristics. Phosphoric acid from thermal decomposition of TCP was a key factor in the proposed mechanism of flame retardation. Flame inhibition and retarded ignition of the PLA foams were achieved at all compositions of TCP and NFR. Due to the presence of compounds such as cellulose, phosphate, and silica, both PK and SB could be used as effective NFRs for PLA foams. All these characteristics promise extended applications for PLA foam in bio, circular, and green economies.

Preparation and characterization of PLA foam chain extended through grafting octa(epoxycyclohexyl) POSS onto carbon nanotubes

Improving foamability of poly (lactic acid) (PLA) resin is a key issue for its critical foaming applications with high-performance and ultralow density. However, owing to the rheological nature of linear PLA chain structure with relatively low molecular weight, the overall foamability of PLA resin cannot meet the processing requirements of foaming purpose. Here, we describe a simple and versatile technique to prepare high foamability PLA resin by inducing chain extender through grafting octa(epoxycyclohexyl) polyhedral oligomeric silsesquioxanes (POSS) on carbon nanotubes (CNT). After the orderly assemble of the two nanoparticles, an obvious increase in melt elasticity of PLA is observed. The enhanced melt elasticity of PLA had a significant effect on controlling subsequent foaming behavior. Thus, a homogeneous and finer cellular morphology of PLA rigid foam was obtained with a proper content of CNT-POSS. Eventually, the expansion ratio of chain-extended PLA foam was 13 times higher than that of unmodified PLA foam. The proposed design methodology will potentially pave a way for designing and preparing high-performance PLA rigid foam products.

Integration of an ultra-strong poly(lactic-co-glycolic acid) (PLGA) knitted mesh into a thermally induced phase separation (TIPS) PLGA porous structure to yield a thin biphasic scaffold suitable for dermal tissue engineering

We aimed to capture the outstanding mechanical properties of meshes, manufactured using textile technologies, in thin biodegradable biphasic tissue-engineered scaffolds through encapsulation of meshes into porous structures formed from the same polymer. Our novel manufacturing process used thermally induced phase separation (TIPS), with ethylene carbonate (EC) as the solvent, to encapsulate a poly(lactic-co-glycolic acid) (PLGA) mesh into a porous PLGA network. Biphasic scaffolds (1 cm × 4 cm × 300 μm) were manufactured by immersing strips of PLGA mesh in 40 °C solutions containing 5% PLGA in EC, supercooling at 4 °C for 4 min, triggering TIPS by manually agitating the supercooled solution, and lastly eluting EC into 4 °C Milli-Q water. EC processing was rapid and did not compromise mesh tensile properties. Biphasic scaffolds exhibited a tensile strength of 40.7 ± 2.2 MPa, porosity of 94%, pore size of 16.85 ± 3.78 μm, supported HaCaT cell proliferation, and degraded in vitro linearly over the first ∼3 weeks followed by rapid degradation over the following three weeks. The successful integration of textile-type meshes yielded scaffolds with exceptional mechanical properties. This thin, porous, high-strength scaffold is potentially suitable for use in dermal wound repair or repair of tubular organs.

Durable and Multifunctional Superhydrophobic Coatings with Excellent Joule Heating and Electromagnetic Interference Shielding Performance for Flexible Sensing Electronics

Superhydrophobic coatings have wide applications in many fields. However, superhydrophobic and smart coatings with multifunctionality and their applications in flexible sensing electronics are seldom reported. In this work, durable, superhydrophobic, and anticorrosive coatings with excellent Joule heating and electromagnetic interference (EMI) shielding performance are prepared on the basis of Ag precursor reduction and synchronous nonsolvent induced phase separation. Silver nanoparticles (AgNPs) coated with the copolymer (polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene: SEBS) are uniformly distributed on the target substrate, forming mechanically durable conductive network. SEBS could not only endow the surface coating with superhydrophobicity but also improve the interaction among individual Ag nanoparticles and the interfacial adhesion between AgNPs and the substrate. The multifunctional coating possesses excellent anticorrosive, self-cleaning, and deicing properties. The high conductivity endows the coatings with excellent Joule heating and EMI shielding performance. The multifunctional coating can be applied to a variety of different substrates with outstanding surface stability and reliability. The conductivity for the smart coating can reach as high as 107 S/cm with the EMI shielding effectiveness up to 37.8 dB. At a low applied voltage of 1 V, the conductive fabric can be heated up to over 80 °C in 60 s and displays good recyclability during dozens of heating and cooling cycles. The Joule heating-induced temperature increase could be used for efficient surface deicing. When used for the flexible and wearable strain sensors, the multifunctional coating has a very low strain detection limit of 0.5% and large sensitivity (with the gauge factor as high as 1075) and excellent repeatability. In addition, it can be used for precisely monitoring different body motions, including both large and subtle joint movement.

Recent Trends of Foaming in Polymer Processing: A Review

Polymer foams have low density, good heat insulation, good sound insulation effects, high specific strength, and high corrosion resistance, and are widely used in civil and industrial applications. In this paper, the classification of polymer foams, principles of the foaming process, types of blowing agents, and raw materials of polymer foams are reviewed. The research progress of various foaming methods and the current problems and possible solutions are discussed in detail.

Unique Ivy-Like Morphology Composed of Poly(lactic acid) and Bacterial Cellulose Cryogel

This study examines the unique morphology and properties of enhanced poly(l-lactic acid) (PLLA) monoliths having a bacterial cellulose (BC) framework. Open-porous BC/PLLA monoliths were successfully prepared using thermally induced phase separation (TIPS) and a freeze-drying technique. The BC/PLLA monoliths exhibited a unique ivy-like structure composed of leaf-like PLLA units and a BC fiber network. We demonstrated for the first time that the interpenetrating BC fiber gives PLLA monoliths four times higher compressive strength than the pristine PLLA. Scanning electron microscopy observation and the N₂ adsorption test revealed that the size of PLLA units and the surface area of the monoliths can be manipulated by varying the starting PLLA concentration during the TIPS process. Moreover, the hydrophilicity of the PLLA monoliths was easily controlled by incorporating BC; the neat PLLA monoliths showed a high static water contact angle of as high as 128.8 ± 1.1°, whereas the BC/PLLA monoliths exhibited a much lower contact angle (102.1 ± 1.7°) and greater absorbability to water.

Renewable metal–organic-frameworks-coated 3D printing film for removal of malachite green

A renewable metal–organic-frameworks (MOFs)-polylactic acid (PLA) film was designed and prepared in order to remove malachite green (MG) in waste water.

Formation mechanism of thermally expandable microspheres of PMMA encapsulating NaHCO3 and ethanol via thermally induced phase separation

The formation mechanism of TEMs via TIPS was investigated. It was supported by calculation, the effect of quench rate and stirring speed.

Unique leafy morphology of poly(lactic acid) monoliths controlled via novel phase separation technology

Poly(lactic acid) (PLA) monoliths with unique leaf-like morphology and physical properties, designed by a newly developed thermally induced phase separation.