Polymerization of acrylonitrile in supercritical carbon dioxide

A series of fluorinated diblock copolymers, consisting of styrene (St)-acrylonitrile (AN) copolymer [poly(St-co-AN)] and poly-2-[(perfluorononenyl)oxy]ethyl methacrylate, with various compositions as well as with different molecular weights were synthesized by atom transfer radical polymerization and characterized. Dispersion polymerization of acrylonitrile in supercritical carbon dioxide (scCO₂) at 30 MPa and at 65 °C with this kind of amphiphilic block copolymer as a stabilizer and 2,2′-azobisisobutyronitrile as an initiator was investigated. The experimental results indicated that, in the presence of a small amount of poly(St-co-AN) (5 wt% to AN), spherical particles of polyacrylonitrile (PAN) were prepared with small diameter and narrow polydispersity (d_n = 153 nm, d_w/d_n = 1.12), resulting from the high stabilizing efficiency of this fluorinated block copolymer. Furthermore, the polymerization of AN in scCO₂ under different initial pressures especially under low pressure (<14 MPa) was studied. When the polymerization was carried out around the critical pressure of CO₂ (7.7-7.8 MPa), the PANs with high molecular weight (M_ν ≈ 130,000-194,000) were synthesized at high monomer conversion (>90%) no matter whether the stabilizer was added, compared to those synthesized by dispersion polymerization at 30 MPa. It was also found that the crystallinity of PAN synthesized at 7.7-7.8 MPa was somewhat higher than that synthesized at 30 MPa, while its crystallite size did not change.     

Purification of hydrocarbons from aromatic sulfur compounds by supercritical carbon dioxide extraction

Supercritical carbon dioxide extraction to purify samples of model hydrocarbons (tetralin, decalin, and tetradecane) containing various aromatic sulfur compounds (benzothiophene, dibenzothiophene, and 4,6-dimethydibenzothiophene) was studied. The influence of extraction temperature and pressure was investigated for the extraction from a tetralin-dibenzothiophene system in the range of 293-353 K and 8-15 MPa, and it was found that the amount of tetralin extracted increased with an increase in carbon dioxide density, while the separation factor decreased with an increase in carbon dioxide density. High recovery and high separation factor values for the tetralin-dibenzothiophene system were obtained under 10 MPa at 313 K. Higher separation factor was obtained for tetralin than decalin and tetradecane, containing 4,6-dimethyldibenzothiophene than that containing dibenzothiophene.     

Kinetics of N-isopropylacrylamide polymerizations in supercritical carbon dioxide fluids

Free-radical polymerization kinetics of N-isopropylacrylamide (NIPAAm) using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator in supercritical carbon dioxide (scCO₂) was investigated. A high-pressure differential scanning calorimeter revealed that the melting temperatures of NIPAAm and AIBN were both decreased with increasing CO₂ pressure in a linear fashion and the polymerization could occur in CO₂ at 55 °C. The polymerization kinetics of NIPAAm in scCO₂ was compared with that in methanol (MeOH). At 55 °C, the induction periods of polymerizations in scCO₂ of 27.6 MPa were much longer (up to 6 h) than those in MeOH (about 30 min) for similar feed concentrations. The monomer conversions reached and molecular weights produced were much higher in scCO₂ than in MeOH. The reaction orders for initial monomer and initiator concentrations, [NIPAAm]₀ and [AIBN]₀, in initial stage of polymerizations were respectively 1.48 and 0.79 in scCO₂ and 1.27 and 0.51 in MeOH.     

Acetylation of plant cellulose fiber in supercritical carbon dioxide

Natural cellulosic ramie fiber was acetylated using supercritical carbon dioxide (sc-CO₂) as a reaction medium. The structure and properties of the acetylated fibers were investigated using infrared spectroscopy, scanning electron microscopy, X-ray diffraction (including synchrotron microbeam X-ray diffraction), nano-Raman scattering, and a tensile test. The acetylation reaction proceeded without using an organic solvent, and it reached to the core part of the fiber within a short period while maintaining the fiber morphology. The crystallites of cellulose triacetate II and cellulose coexist in the fiber. The acetylated fiber with an average degree of substitution of 1.9 showed high modulus (34.5 GPa) and high strength (763 MPa), which are the highest values for cellulose diacetate so far reported to date.     

Solubility of chlorpheniramine maleate in supercritical carbon dioxide

Solubility of chlorpheniramine maleate in supercritical carbon dioxide at different temperatures (308–338 K) and pressures (160–400 bar) is measured using static method coupled with gravimetric method. The measured solubility data demonstrated that the solubility of chlorpheniramine maleate was changed between 1.54 × 10⁻⁵ and 4.26 × 10⁻⁴ based on the mole fraction as the temperature and pressure are changed. The general trend of measured solubility data shows a direct effect of pressure and temperature on the solubility of chlorpheniramine maleate. Finally, the obtained solubilities correlated using four semi-empirical density-based correlations including Mendez Santiago–Teja (MST), Kumar and Johnston (KJ), Bartle et al., and Chrastil models. Although the results of modeling showed that the KJ model leads to the average absolute relative deviation percent (AARD %) of 8.1% which is the lowest AARD %, deviation of other utilized correlations are rather the same.     

Cross-linking polymerization of acrylic acid in supercritical carbon dioxide

Cross-linking polymerization of acrylic acid in supercritical carbon dioxide (scCO₂) was studied in a batch reactor at 50 °C and 207 bar with either triallyl pentaerythritol ether or tetraallyl pentaerythritol ether as the cross-linker and with 2,2′-azobis(2,4-dimethyl-valeronitrile) as the free radical initiator. All polymers were white, dry, fine powders. Scanning electron microscopy showed that the morphology of the polymer particles was not affected by cross-linking. As the cross-linker concentration was increased, the polymer glass transition temperature first decreased, then increased. Water-soluble and water-insoluble polymers were synthesized by adjusting the cross-linker concentration. Viscosity measurements showed that the polymer thickening effect strongly depended on the degree of cross-linking. Finally, cross-linking polymerization of acrylic acid in scCO₂ was carried out in a continuous stirred tank reactor. The use of cross-linker decreased the monomer conversion in this system.     

Effect of supercritical carbon dioxide on polystyrene extrusion

A study on the extrusion of polystyrene was carried out using supercritical carbon dioxide (scCO₂) as foaming agent. scCO₂ modifies the rheological properties of the material in the barrel of the extruder and acts as a blowing agent during the relaxation at the passage through the die. For experiments, a single-screw extruder was modified to be able to inject scCO₂ within the extruded material. The effect of operating parameters on material porosity was studied. Samples were characterized by using water-pycnometry, mercury-porosimetry and scanning electron microscopy. Polystyrene with expansion rate about 15–25% was manufactured. A rapid cooling just downstream the die is important to solidify the structure. The die temperature allows the control of the porosity structure. CO₂ concentration shows no significant influence.     

Surface modification of polymeric nanocomposite thin films using supercritical carbon dioxide

We report on an efficient and environmentally friendly means to modify surface properties of polymer films supported for nanoparticles. Ultrathin polystyrene (PS) films (<300 Å), in which inorganic nanoparticles were embedded, were exposed to supercritical carbon dioxide (scCO₂). The swollen structure was then preserved by quickly evaporating CO₂. X-ray reflectivity (XR) results showed that this procedure produced polymeric nanocomposite films with a low-density region of about 150Å at the polymer/air interface. The formation of the low-density layer was independent of the nature of the particles, indicating that the surface modification through exposure to scCO₂ may be a universal phenomenon regardless of a choice of nanoparticles.     

Deposition of Pd into mesoporous silica SBA-15 using supercritical carbon dioxide

Pd was deposited into mesoporous silica SBA-15 using supercritical CO₂ (scCO₂). Palladium hexafluoroacetylacetonate [Pd(hfac)₂] was dissolved in scCO₂ and impregnated into the support at very mild conditions, 40 °C and 85 bar. Then the organometallic precursor was reduced with H₂ in the CO₂ mixture or, after depressurization, in pure H₂. Materials were characterized by TGA, XRD, TEM, SEM, EDX, ICP-OES and N₂-adsorption experiments. Pd nanoparticles evenly distributed into the support (1-3 mol% Pd by ICP-OES) are only obtained when the reduction is performed in pure H₂. Cluster size is limited in two dimensions by the pore size of the support but clusters grow larger with increasing impregnation time and turn into small nanowires. The catalytic activity of the Pd/SiO₂ composite material was confirmed following the reduction of 4-nitrophenol to 4-aminophenol in water by UV-vis spectroscopy.     

Drying of agar gels using supercritical carbon dioxide

The use of supercritical carbon dioxide (scCO₂) for the removal of water from agar gels has been investigated and compared to air and freeze drying. Experiments were conducted to evaluate how gel formulation (with and without sucrose) and drying conditions (with and without ethanol as a co-solvent, flow rate and depressurisation rate) affected the microstructure of the gels dried using scCO₂. X-ray micro-computed tomography (X-ray micro-CT) was used to determine the voidage (% open pore space) of the dried structures, which can be used to indicate the extent of drying-induced structural collapse (in general, the lower the voidage, the greater the collapse). For formulations containing sucrose, which displayed the best structural retention, voidage was found to increase in the order: air drying (4% voidage) < supercritical drying with pure CO₂ (48%) < supercritical drying with ethanol-modified CO₂ (68%) < freeze drying (76%). The relatively high voidage of samples dried in the presence of ethanol, was due in part to foaming of the gels, hypothesised to result from an interaction between the agar and ethanol, rather than an effect of the supercritical fluid. CO₂ flow rate (1 vs. 3 l/min) during supercritical drying and depressurisation rate (0.4 vs. 1.6 MPa/min) had no effect on the dried microstructure.     

Bright nickel film deposited by supercritical carbon dioxide emulsion using additive-free Watts bath

This paper is aimed on studying film smoothening effect of supercritical carbon dioxide emulsion (Sc-CO₂-E) on nickel film electroplated using an additive-free Watts bath. Morphology of nickel film electroplated with Sc-CO₂-E was found to be similar to that of nickel film prepared from electroless plating. Surface roughness (R_a) of nickel film electroplated with Sc-CO₂-E was lower than that of nickel film electroplated through conventional method. A minimum R_a was found for nickel film electroplated through conventional method and Sc-CO₂-E when increasing current density from 0.010 to 0.150 A/cm². The minimum R_a was 69.8 nm at 0.020 A/cm² and 14.0 nm at 0.030 A/cm², respectively for conventional and Sc-CO₂-E case. After the minimum point, increasing rate of R_a increased was lower for Sc-CO₂-E case; this was because of higher hydrogen solubility in Sc-CO₂. Grain size of nickel film electroplated with Sc-CO₂-E was found to be finer than that of conventional case.     

Solubility of cyproheptadine in supercritical carbon dioxide; experimental and modeling approaches

Solubility of solute in supercritical fluids at different pressures and temperatures is one of the most important parameters necessary for design of any supercritical fluid-based processes. Among different supercritical fluids, carbon dioxide is one of the most widely used solvents due to its useful and green characteristics. In this work, with the assist of supercritical carbon dioxide as the solvent, solubility of cyproheptadine in different temperatures (308–338 K) and pressures (160–400 bar) are measured using static method. The obtained results demonstrated that solubility of cyproheptadine ranged between 3.35 × 10⁻⁵ and 3.09 × 10⁻³ based on mole fraction. A closer examination of measured solubility data show that not only solubility of cyproheptadine increases by increasing pressure but also experiences a cross over pressure about 200 bar. At last, the measured solubility data are correlated using four widely used density based correlations namely Mendez Santiago–Teja (MST), Kumar and Johnston (KJ), Bartle et al., and Chrastil models. The obtained results demonstrated that the best correlative capability was observed for KJ model leads to the average absolute relative deviation percent (AARD %) of 6.3%.     

Reactive extraction of succinic acid using supercritical carbon dioxide

Reactive extraction using supercritical carbon dioxide (scCO₂) and tri-n-octylamine (TOA) was evaluated as a separation method of succinic acid from an aqueous solution. The reactive extraction of succinic acid was performed at varying initial acid concentrations in aqueous solution (0.07–0.45 mol?dm^?3), temperature (35–65°C) and pressure (8–16 MPa). The succinic acid separation was conducted in both batch mode and semi-continuous mode. The highest reactive extraction efficiency of approx. 62% was obtained for the process conducted in semi-continuous mode at 35°C and 16 MPa for the initial acid concentrations in aqueous phase of 0.39 mol?dm^?3.     

Catalyst retention in continuous flow with supercritical carbon dioxide

This review discusses the retention of organometallic catalysts in continuous flow processes utilizing supercritical carbon dioxide. Due to its innovative properties, supercritical carbon dioxide offers interesting possibilities for process intensification. As a result of safety and cost considerations, processes that use supercritical carbon dioxide are preferably done in continuous flow, as they require a pressure upwards of 74 bar. Many of the reactions that benefit from the application of supercritical carbon dioxide also involve the use of a homogeneous catalyst however, requiring efforts to recycle the catalyst when these are applied in continuous flow. Alternatively, the catalyst may be retained in the reactor by modifying the process or catalyst, such as by catalyst immobilization, membrane separation, or biphasic processing exploiting the properties of supercritical carbon dioxide. Each of these methods is discussed, including their advantages and drawbacks. Also discussed are milli- and micro-flow processes and their possibilities for integrated catalyst retention and handling supercritical carbon dioxide.     

Experimental investigation and modeling of the solubility of carvedilol in supercritical carbon dioxide

This study was aimed to measure the solubility of carvedilol in the temperature and pressure ranges of 308⿿338 K and 160 bar to 400 bar, respectively. In this direction, a homemade high pressure visual equilibrium cell was used to measure the solubility of carvedilol using a static method coupled with gravimetric technique. The results revealed that the carvedilol solubility was ranged between 1.12 ÿ 10^⿿5 and 5.01 ÿ 10^⿿3 based on the mole fraction (mole of carvedilol/mole of carvedilol + mole of CO₂) in this study as the temperature and pressure was changed. Finally, the results were correlated using four density-based semi-empirical correlations including Chrastil, Mendez⿿Santiago⿿Teja (MST), Bartle et al., and Kumar and Johnston (K-J) models. Results revealed that although the K-J model leads to the lowest average absolute relative deviation percent (AARD %) of 6.27%, but it could not be considered as the most accurate correlation since all the used four correlations introduces AARD % of about 6⿿10% which may be in the same range as the experimental error.     

Tailoring thermoresponsive microbeads in supercritical carbon dioxide for biomedical applications

The preparation of smart polymeric particles in supercritical carbon dioxide (scCO₂) presents many advantages for biomedical applications over conventional processes due to the easy elimination of trace contaminants rendering highly pure particles. Herein we report the successful optimization of poly(N-isopropylacrylamide) (PNIPAAm) synthesis strategy to obtain cell-sized hydrogel microbeads with defined and systematically varied mechanical properties. The effect of using different hydrophilic cross-linkers such as N,N-methylenebisacrylamide (MBAm), di(ethylene) glycol dimethacrylate (DEGDMA) and glycerol dimethacrylate (GDMA), on beads morphological, physico-chemical and mechanical properties was investigated. In agreement with a larger water uptake ability beads cross-linked with DEGDMA are more compliant than those containing MBAm or GDMA, having lower stiffness as accessed through oscillatory measurements on a rotational rheometer. Cytotoxicity assays showed that the obtained cross-linked PNIPAAm microbeads do not present any toxic effect on fibroblast cell cultures. Microbeads biocompatibility and adequate mechanical compliance enable their potential application on biomedical settings.     

Solubilities of chlormezanone,metaxalone and methocarbamol in supercritical carbon dioxide

The solubilities of three active pharmaceutical ingredients (APIs) in supercritical carbon dioxide were measured in this study using a semi-flow apparatus. These APIs are chlormezanone (C₁₁H₁₂ClNO₃S), metaxalone (C₁₂H₁₅NO₃) and methocarbamol (C₁₁H₁₅NO₅) that are all used as skeletal muscle relaxants. The solubility data are reported for three isotherms at 308.2, 318.2 and 328.2 K, with the pressure range from 12 to 24 MPa. Most solubility data are within the range of 10⁻⁶ to 10⁻⁴ mole fraction for each API. The crossover phenomena were observed from the experimental results for all three systems. These solubility data satisfied the thermodynamic consistency tests. They were then correlated using three semi-empirical models. With the optimally fitted binary interaction parameters, satisfactory correlation agreement is presented for each binary mixture.     

pH Sensitive polypropylene porous membrane prepared by grafting acrylic acid in supercritical carbon dioxide

pH sensitive membrane was prepared by grafting acrylic acid (AA) on the porous polypropylene (PP) membrane using supercritical (SC) CO₂ as a solvent. The monomer (AA) and the initiator (benzyl peroxide, BPO) were impregnated into the PP substrate with the aid of SC CO₂, and were grafted onto the microporous PP substrate. The grafted membranes were characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and the water permeability of the virgin and grafted membranes were determined at different pH values. It was demonstrated that the grafting degree (Dg) could be easily controlled by varying operating conditions, such as the monomer concentration, pressure, and temperature during the soaking process. The water permeation of the virgin membrane is nearly independent of pH. However, the water permeation of grafted membranes decreases dramatically with pH as the pH varies from 3 to 6 because the conformation of the PAA changes significantly with the pH of the contacting solution.     

Enhanced crystallization of bisphenol-A polycarbonate by nano-scale clays in the presence of supercritical carbon dioxide

The crystallization behavior of bisphenol-A polycarbonate (PC) and PC/clay nanocomposites were studied in the presence of supercritical carbon dioxide (SCCO₂) using DSC, WAXD and AFM. In the absence of SCCO₂, nano-scale clay itself does not change the crystallization behavior of PC under our experimental conditions. In the presence of SCCO₂, clay appears to be an efficient nucleating agent and enhances the crystallization of PC. The addition of clay reduces the induction time of crystallization and increases the crystallization rate. The increase in crystallinity with clay depends on the crystallization time. When the crystallization time is sufficient, PC and PC/clay composites tend to have similar crystallinity in the range of 26%. Two melting temperatures are observed during the DSC heating scan, and are mainly associated with the melting of both secondary and primary crystals. Results show that the clay influences the primary crystallization process more than the secondary crystallization process.     

Group A particle fluidization in supercritical carbon dioxide: Effect of operating conditions on fluidization efficiency

An experimental study was carried out to determine the fluidization behavior of group A glass bead beds under supercritical conditions. The fluidization state was followed experimentally by pressure drop measurements. Particle fluidization was obtained with carbon dioxide conditions far from the critical point, whereas near this point gas channeling should occur. Particle aggregation can be attributed to increased interparticle forces under these conditions.