Gas foaming of segmented poly(ester amide) films

Biodegradable segmented poly(ester amide)s, based on dimethyl adipate, 1,4-butanediol and N,N′-1,2-ethanediyl-bis[6-hydroxy-hexanamide], with two distinct melting transitions were gas foamed using carbon dioxide (CO₂). Polymer films were saturated with CO₂ at 50 bar for 6 h after which the pressure was released. The samples were immersed in octane at the desired temperature after which foaming started immediately. Just above the lower melt transition the polymers retain adequate mechanical properties and dimensional stability, while the chain mobility increased sufficiently to nucleate and expand gas cells during the foaming process. In this way semi-crystalline poly(ester amide)s can be gas foamed below the flow temperature.Two poly(ester amide)s with 25 mol% (PEA2,5-25) and 50 mol% (PEA2,5-50) of bisamide segment content were foamed at 70 and 105 °C, respectively. The storage modulus (G′) of both pure polymers at the onset foaming temperature is 50-60 MPa. Closed-cell foams were obtained with a maximum porosity of ∼90%. The average pore size of PEA2,5-25 ranges from 77 to 99 μm. In contrast, the average pore size of PEA2,5-50 is in between 2 and 4 μm and can be increased to 100 μm by lowering the CO₂ saturation pressure to 20 bar. The porosity of PEA2,5-50 foams using this saturation pressure decreased to 70%.     

Batch foaming of SAN/clay nanocomposites with scCO2: A very tunable way of controlling the cellular morphology

This paper aims at elucidating some important parameters affecting the cellular morphology of poly(styrene-co-acrylonitrile) (SAN)/clay nanocomposite foams prepared with the supercritical CO₂ technology. Prior to foaming experiments, the SAN/CO₂ system has first been studied. The effect of nanoclay on CO₂ sorption/desorption rate into/from SAN is assessed with a gravimetric method. Ideal saturation conditions are then deduced in view of the foaming process. Nanocomposites foaming has first been performed with the one-step foaming process, also called depressurization foaming. Foams with different cellular morphology have been obtained depending on nanoclay dispersion level and foaming conditions. While foaming at low temperature (40 °C) leads to foams with the highest cell density (∼10¹²-10¹⁴ cells/cm³), the foam expansion is restricted (d∼0.7-0.8 g/cm³). This drawback has been overcome with the use of the two-step foaming process, also called solid-state foaming, where foam expansion occurs during sample dipping in a hot oil bath (d∼0.1-0.5 g/cm³). Different foaming parameters have been varied, and some schemes have been drawn to summarize the characteristics of the foams prepared - cell size, cell density, foam density - depending on both the foaming conditions and nanoclay addition. This result thus illustrates the huge flexibility of the supercritical CO₂ batch foaming process for tuning the foam cellular morphology.     

Direct synthesis of poly(l-lactic acid) in supercritical carbon dioxide with dicyclohexyldimethylcarbodiimide and 4-dimethylaminopyridine

The direct synthesis of poly(l-lactic acid) (PLLA) from an l-lactic acid oligomer has been performed in supercritical carbon dioxide (scCO₂) using an esterification promoting agent, dicyclohexyldimethylcarbodiimide (DCC), and 4-dimethylaminopyridine (DMAP) as a catalyst. PLLA within M_n of 13,500 g/mol was synthesised in 90% yield at 3500 psi and 80 °C after 24 h. The molecular weight distribution of the products was narrower than PLLA prepared with melt-solid phase polymerisation under conventional conditions. Both DCC and DMAP showed high solubility in scCO₂ (DCC: 7.6 wt% (1.63×10⁻² mol/mol CO₂) at 80 °C, 3385 psi, DMAP: 4.5 wt% (1.62×10⁻²mol/mol CO₂) at 80 °C, 3386 psi) and supercritical fluid extraction was found to be effective at removing excess DMAP and DCC after the polymerisation was complete. We show that DCC and DMAP are effective esterification promoting reagents with further applications for condensation polymerisations in scCO₂.     

Tailoring the pore structure of PCL scaffolds for tissue engineering prepared via gas foaming of multi-phase blends

The aim of this study was the design of poly(ε-caprolactone) (PCL) scaffolds characterized by well controlled pore structures obtained by gas foaming of multi-phase blends of PCL and thermoplastic gelatin (TG). Co-continuous blends made of PCL and TG were prepared by melt mixing and, subsequently gas foamed in an autoclave to induce the formation of the porous network. A mixture of N₂ and CO₂ was used as blowing agent and the foaming process performed at temperature higher than PCL melting, in the range 70–110 °C. The foams were finally soaked in water at 37 °C to selectively extract the TG and achieve the final pore structure. The results of this study demonstrated that the proposed approach allowed to tailor the micro-structural properties of PCL scaffolds for tissue engineering.     

Two-step micro cellular foaming of amorphous polymers in supercritical CO2

Microcellular foaming of amorphous rigid polymers, polymethylmethacrylate (PMMA) and polystyrene (PS) was studied in supercritical CO₂ (ScCO₂) in the presence of several types of additives, such as triblock (styrene-co-butadiene-co-methylmethacrylate, SBM and methylmethacrylate-co-butylacrylate-co-methylmethacrylate, MAM) terpolymers. This work is focused in the two-step foaming process, in which the sample is previously saturated under ScCO₂ being expanded in a second step out of the CO₂ vessel (e.g. in a hot oil bath) where foaming is initiated by the change of temperature near or above the glass transition temperature of the glass/polymer glassy system. Samples were saturated under high pressures of CO₂ (300 bar), at room temperature, for 16 h, followed by a quenching at a high depressurization rate (150 bar/min). In the last step, foaming was carried out at different temperatures (from 80 °C to 140 °C) and different foaming times (from 10 s to 120 s). It was found that cellular structures were controlled selecting either the additive type or the foaming conditions. Cell sizes are ranging from 0.3 μm to 300 μm, and densities from 0.50 g/cm³ to 1 g/cm³ depending on the polymers considered.     

Preparation and characterization of foams from sheet glass and fly ash using carbonates as foaming agents

Glass foams were produced using sheet glass cullet and fly ashes from thermal power plant with added carbonates (commercial dolomite- and calcite-based sludges) as foaming agents. The influence of type and amount of carbonates as well as of the sintering temperature on the apparent density, compressive strength, microstructure and crystalline phases was evaluated. The experimental results showed that homogenous microstructures of large pores could be obtained by adding just 1–2 wt.% carbonates and using low sintering temperature (850 °C), leading to foams presenting apparent density and compressive strength values of about 0.36–0.41 g/cm³ and 2.40–2.80 MPa, respectively. Good correlations between compressive strength, apparent density and microstructure (pore size, struts’ thickness and internal porosity) were observed.     

Integrated process of supercritical CO2-assisted melt polycondensation modification and foaming of poly(ethylene terephthalate)

An integrated process of melt polycondensation modification and foaming of poly(ethylene terephthalate) (PET) was performed in a high pressure vessel assisted by supercritical carbon dioxide (scCO₂). ScCO₂ was firstly employed to sweep PET melt, i.e., high pressure CO₂ continuously flows through the vessel at a fixed flow rate to remove small molecules for higher molecular weight PET, then this modified PET melt was directly foamed through a rapid depressurization process using scCO₂ as blowing agent. In this integrated process, PET with high melt strength after polycondensation modification could be foamed directly in molten state. Therefore, re-molten process of solid modified PET pellets was canceled to avoid its degradation and CO₂ saturation time could be saved in foaming process, thus processing time could be shortened and energy efficiency could be improved. The influences of scCO₂ sweeping treatment time, pressure and flow rate on properties of the modified PETs and cell morphologies of the foamed PETs were investigated respectively. The results showed that CO₂ sweeping treatment could effectively enhance PET melt polycondensation modification process, which was superior to that of N₂ treatment. PET foams with average cell diameter of 32–62 μm and cell density of 1 × 10⁷ to 4 × 10⁷ cells/cm³ have been obtained in the integrated process. Compared with the process of only foaming modified PET by scCO₂ or performing scCO₂ assisted modified PET further melt polycondensation process and scCO₂ foaming process separately, this integrated process produced better cell morphology.     

A dynamic supercritical carbon dioxide foaming method for fabricating wrinkled surface to enhance triboelectric nanogenerator performance

Triboelectric nanogenerator (TENG) is a promising energy harvester to overcome the energy depletion issue. The surface structure has been considered as an effective way to enhance the triboelectric performance. Herein, a dynamic supercritical carbon dioxide (scCO₂) foaming method, which introduced a scCO₂ flow field during scCO₂ saturation, was proposed to fabricate thermoplastic polyurethane (TPU) foams with surface wrinkly structures. The size of the surface wrinkles could be regulated in the range of 1.8–10 μm by varying the foaming pressure. The surface wrinkled TPU film with wrinkle wave length of 2.4 μm demonstrated an excellent enhancement in output voltage (130%), current (180%), and maximum transfer charge (130%) when paired with surface structured polydimethylsiloxane film in a TENG. Due to the excellent durability and flexibility of the composing materials, the developed TENG showed outstanding stability in long-term continuous operation. With a high power density of 0.5 W/m² achieved on a 10⁷ Ω external load, the flexible TENG could be used to charge capacitors, power light-emitting-diodes, and served as a self-powered sensor to detect various human movement behaviors. This work provides a new path for the fabrication of surface wrinkled films for the sustainable development of high performance TENGs.     

Fabrication of bimodal porous PLGA scaffolds by supercritical CO2 foaming/particle leaching technique

Specific pore structure is a vital essential for scaffolds applied in tissue engineering. In this article, poly(lactide‐co‐glycolide) (PLGA) scaffolds with a bimodal pore structure including macropores and micropores to facilitate nutrient transfer and cell adhesion were fabricated by combining supercritical CO₂ (scCO₂) foaming with particle leaching technique. Three kinds of NaCl particles with different scales (i.e., 100–250, <75, <10 μm) were used as porogens, respectively. In particular, heterogeneous nucleation occurred to modify scCO₂ foaming/particle leaching process when NaCl submicron particles (<10 μm) were used as porogens. The observation of PLGA scaffolds gave a formation of micropores (pore size <10 μm) in the cellular walls of macropores (pore size around 100–300 μm) to present a bimodal pore structure. With different mass fractions of NaCl introduced, the porosity of PLGA scaffolds ranged from 68.4 ± 1.4 to 88.7 ± 0.4% for three NaCl porogens. The results of SEM, EDS, and in vitro cytotoxicity test of PLGA scaffolds showed that they had uniform structures and were compatible for cell proliferation with no toxicity. This novel scCO₂ foaming/particle leaching method was promising in tissue engineering due to its ability to fabricate scaffolds with precise pore structure and high porosity. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43644.     

Porosity effect on microstructure,mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting

In this study, Ti₂AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k₁) and non‐Darcian (k₂) permeability coefficients displayed values in the range 0.30‐93.44 × 10^?11 m² and 0.39‐345.54 × 10^?7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti₂AlC foams.     

Supercritical CO2-assisted extrusion foaming: A suitable process to produce very lightweight acrylic polymer micro foams

A strategy of CO₂-assisted extrusion foaming of PMMA-based materials was established to minimize both foam density and porosities dimension. First a highly CO₂-philic block copolymer (MAM: PMMA-PBA-PMMA) was added in PMMA in order to improve CO₂ saturation before foaming. Then the extruding conditions were optimized to maximize CO₂ uptake and prevent coalescence. The extruding temperature reduction led to an increase of pressure in the barrel, favorable to cell size reduction. With the combination of material formulation and extruding strategy, very lightweight homogeneous foams with small porosities have been produced. Lightest PMMA micro foams (ρ = 0.06 g cm⁻³) are demonstrated with 7 wt% CO₂ at 130°C and lightest blend micro foams (ρ = 0.04 g cm⁻³) are obtained at lower temperature (110°C, 7.7 wt% CO₂). If MAM allows a reduction of T^foaming, it also allows a much better cell homogeneity, an increase in cell density (e.g., from 3.6 10⁷ cells cm⁻³ to 2 to 6 10⁸ cells cm⁻³) and an overall decrease in cell size (from 100 to 40 μm). These acrylic foams produced through scCO₂-assisted extrusion has a much lower density than those ever produced in batch (ρ ≥ 0.2 g cm⁻³).     

Low‐temperature clean preparation of poly(lactic acid) foams by combining ethyl lactate and supercritical CO2: correlation between processing and foam pore structure

This study reports the low‐temperature and clean fabrication of porous poly(lactic acid) (PLA) through solid‐state foaming using various mixtures of ethyl lactate (EL) and supercritical CO₂ (scCO₂) as the blowing agent. Results showed that adding a small amount of EL (up to 0.2% molar fraction) to scCO₂ enhanced the plasticizing effect of the blowing agent mixture. As a direct consequence, at an operating temperature of 35 °C, PLA foams could be manufactured with homogeneous morphology, density as low as 0.09 ± 0.01 g cm^?3, mean pore sizes up to 519.0 ± 205.0 µm and pore densities in the range 2.0 × 10⁵ to 3.4 × 10⁸ pores cm^?3. Conversely, at a temperature of 40 °C, an increase of plasticizer concentration in the blowing agent mixture up to 0.2% promoted the crystallization of the polymer during sorption stage and, consequently, foaming was slightly reduced. © 2013 Society of Chemical Industry     

Ceramic shell foams produced by direct foaming and gelcasting of proteins: Permeability and microstructural characterization by X-ray microtomography

In this work, ceramic shell foams produced by direct foaming and gelcasting of proteins containing 35 vol.% of solids were efficiently tailored through the optimization of suspension parameters, foaming and also according to the gelling temperature. These were key parameters employed for controlling the foamability and foam stability, and hence the foam porosity and pore characteristics after the sintering process. The potential of using microtomography in characterizing the morphometric parameters of foams was explored. Airflow permeability measurements (∼24 °C to 727 °C) were performed in order to investigate the applicability of such materials in the fluid flow field. The samples exhibited a wide range of pore sizes (60 ± 5–1700 ± 90 μm), porosity values higher than 80 %, Darcian (k₁) and non-Darcian (k₂) permeability coefficients values in the ranges of 1.32–1.83 x 10⁻⁹ m² and 8.34–22.46 x 10^-5 m, respectively.     

Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part I. Miscibility and foaming of poly(l-lactic acid) in carbon dioxide + acetone binary fluid mixtures

Miscibility and foaming of poly(l-lactic acid) (PLLA) in carbon dioxide + acetone mixtures have been explored over the temperature and pressure ranges from 60 to 180 °C and 14 to 61 MPa. Liquid-liquid phase boundaries were determined in a variable-volume view-cell for polymer concentrations up to 25 wt% PLLA and fluid mixtures containing 67-93 wt% CO₂ over a temperature range from 60 to 180 °C. Even though not soluble in carbon dioxide at pressures tested, the polymer could be completely solubilized in mixtures of carbon dioxide and acetone at modest pressures.Foaming experiments were carried out in different modes. Free-expansions were carried out by exposure and swelling in pure carbon dioxide in a view-cell followed by depressurization. Foaming experiments were also carried out within the confinement of specially designed molds with porous metal surfaces as boundaries to direct the fluid escape path and to generate foams with controlled overall shape and dimensions. These experiments were conducted in pure carbon dioxide and also in carbon dioxide + acetone fluid mixtures over a wide range of temperatures and pressures. Foaming in carbon dioxide + acetone mixtures was limited to 1 and 4 wt% acetone cases. Microstructures were examined using an environmental scanning electron microscope (ESEM). Depending upon the conditions employed, pore diameters ranging from 5 to 400 μm were generated. At a given temperature, smaller pores were promoted when foaming was carried out by depressurization from higher pressures. At a given pressure, smaller pores were generated from expansions at lower temperatures. Foams with larger pores were produced in mixtures of carbon dioxide with acetone.     

Controlling the foam morphology of supercritical CO2-processed poly(methyl methacrylate) with CO2-philic hybrid nanoparticles

Highly CO₂-philic nanoparticles, octatrimethylsiloxy polyhedral oligomeric silsesquioxanes (POSS) are used to increase the affinity of poly(methyl methacrylate) (PMMA) to CO₂ in supercritical carbon dioxide (scCO₂) foaming, thus to improve its foaming performance and the foam morphology. PMMA and PMMA-POSS composite foams were produced based on the two-factorial design, at the upper and lower experimental conditions of pressure, temperature, processing time, and venting rate. The foams of PMMA-5% POSS composites exhibited smaller average pore sizes and higher pore densities than neat PMMA and PMMA-0.5% POSS composites. The smallest average pore diameter (0.3 μm) and the highest pore density (6.33 × 10¹² cm⁻³) were obtained with this composite processed at 35°C, 32 MPa, for 24 h and depressurized with fast-venting rate (0.4 MPa/s). ScCO₂ processing decreased the density of the polymer by more than 50%.     

Polymeric nanoparticles from macroscopic crystalline monomers by facile solid-state polymerization in supercritical CO2

When macroscopic crystalline monomers were polymerized by a free-radical solid-state reaction in the presence of supercritical CO₂ (scCO₂), the resultant products were found to be composed of unexpected nanoparticle morphologies. In particular, the solid-state polymerization (SSP) of amino acid based monomers, acryloyl-β-alanine (ABA) and methacryloyl-β-alanine (MBA), initiated by azobisisobutyronitrile in scCO₂ (at 65 °C and 34.5 MPa), produced corresponding polymers having aggregated spherical architectures. The average diameters of the PABA and PMBA particles were measured to be 94 and 102 nm, respectively. In addition, high molecular weight polymers (PMBA, M_w = 3.8 × 10⁵ g/mol) with a high yield (∼96%) were obtained. The microscopic investigation revealed that a unique particle formation mechanism was involved in the SSP in which large sized crystalline monomers were chipped into small pieces during the initial stage of polymerization and subsequently converted into nanoscale objects after 24 h.     

Synthesis and foaming of water expandable polystyrene-activated carbon (WEPSAC)

In this study, water acts as a co-blowing agent to support carbon dioxide (CO₂) in the extrusion foaming process of polystyrene (PS) to produce foams with very low density for thermal insulation applications. Herein, we report a simple suspension polymerization method to prepare water expandable polystyrene (WEPS) based on a PS/water containing activated carbon (AC) composite. AC pre-saturated with water was introduced into the styrene monomer to form a water-in-oil inverse emulsion without emulsifiers. Via suspension polymerization, water expandable PS/AC (WEPSAC) beads could be subsequently obtained. Low density PS foams (∼0.03 g/cc) were successfully produced in the CO₂ extrusion foaming process using WEPSAC. Because of lower foam density and better IR absorption due to the presence of water containing AC, WEPSAC foams provided a lower thermal conductivity than conventional talc reinforced PS foams.     

Microstructure and permeability of porous zirconia ceramic foams prepared via direct foaming with mixed surfactants

The permeability performance of porous ceramics from particle-stabilized foams is closely associated with the connectivity between bubble-evolved pores. In order to regulate the connectivity of pore structure, the zirconia ceramic foams were fabricated by direct foaming with mixed surfactants of cetyltrimethyl ammonium bromide (CTAB) and sodium N-lauroyl sarcosinate (SLS). Different solid loadings and CTAB:SLS mixing ratios were used in this study. The results indicate that the pore structures of zirconia ceramic foams were interconnected by open windows on the cell walls, and the porosity and average size of cell and cell windows could be tailored by adjusting the solid loading and CTAB:SLS mixing ratio. The decrease in solid loading and CTAB:SLS mixing ratio caused larger porosity and size of cells and cell windows, and thus resulted in the obvious augment of Darcian (k₁) and non-Darcian (k₂) permeability constants. The ranges of k₁ and k₂ of the as-fabricated zirconia ceramic foams are 6.92 × 10^?13-4.05 × 10^?10 m² and 2.09 × 10^?5-3.19 × 10^?9 m respectively.     

Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part II. Foaming of poly(?-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone fluid mixtures and formation of tubular foams via solution extrusion

This paper reports on the foaming of poly(?-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone mixtures. Experiments were carried out in specially designed molds with porous metal surfaces and fluid circulation features to generate foams with uniform dimensions at 60, 70 and 80 °C at pressures in the range 7-28 MPa. Depending upon the conditions, foams with pores in the range from 5 to 200 μm were generated. Adding acetone to carbon dioxide improved the uniformity of the pores compared to foams formed by carbon dioxide alone. In addition, a unique high-pressure solution extrusion system was designed and used to form porous tubular constructs by piston-extrusion of a solution from a high-pressure dissolution chamber through an annular die into a second chamber maintained at controlled pressure/temperature and fluid conditions. Long uniform porous tubular constructs with 6 mm ID and 1 mm wall thickness were generated with glassy polymers like poly(methyl methacrylate) by extruding solutions composed of 50 wt% polymer + 50 wt% acetone, or 25 wt% polymer + 10% acetone + 65% carbon dioxide at 70 °C and 28 MPa. Pores were in the 50 μm range. The feasibility of forming similar tubular constructs were demonstrated with poly(?-caprolactone-co-lactide) as well. Tubular foams of the copolymer with interconnected pores with pore sizes in the 50 μm range were generated by extrusion of the copolymer solution composed of 25 wt% polymer + 10 wt% acetone + 65 wt% carbon dioxide at 70 °C and 28 MPa. Reducing the acetone content in the solution led to a reduction of pore sizes. Comparisons with the foaming behavior of the homopolymer poly(?-caprolactone) that were carried out in the molds with porous metal plates show that the foaming behavior of the copolymer is more akin to the foaming behavior of the caprolactone homopolymer component.     

A food grade approach for the isolation of major alkylresorcinols (ARs) from rye bran applying tailored supercritical carbon dioxide (scCO2) extraction combined with HPLC

Tailored supercritical carbon dioxide (scCO₂) extraction of alkylresorcinols (ARs) from rye bran resulting in pre-purification of ARs already during extraction process. The pre-purification was achieved by splitting the extraction process in 2 steps._. In both steps, the CO₂ pressure was set at 25 MPa with a flow rate of 10 g/min. The first step was carried out at 70 °C, 0.06% of ethanolic co-solvent for 2 h, followed by the second step at 45 °C using 10% of ethanolic co-solvent for 4 h, resulting in a pre-purified ARs extract. From the pre-purified scCO₂ extract, chromatographed on C8 column, were obtained pure ARs homologues of C17, C19 and C21 (68% of total) eluting separately in a linear gradient of ethanol. Additionally, a rapid method for the isolation of high purified pools of ARs homologues using a disposable solid phase extraction SPE-C18 column and a step gradient of ethanol was developed.