Synergy of supercritical CO2 and superheated H2O for enhanced processability of polyethersulfone towards open cell foams

Processing of a high glass transition (T_g) polymer such as polyethersulfone (PES; T_g = 225°C) poses a challenge as it requires high processing temperatures or sometimes toxic solvents. In this work, we report of a facile process using superheated water (shH₂O) and supercritical carbon dioxide (scCO₂) co‐media. PES solids were foamed in the scCO₂/shH₂O co‐media or scCO₂ alone in a batch process at different temperatures. The scCO₂/shH₂O produced a synergistic effect and achieved PES foams even at processing temperatures as low as 85°C below the nominal T_g; whereas, scCO₂ alone required higher processing temperatures. Moreover, the scCO₂/shH₂O co‐media produced highly porous PES foams that were at least 23% higher in porosity than what was obtained using scCO₂ alone. In addition, the scCO₂/shH₂O produced open cell foams at some processing conditions; whereas, scCO₂ produced closed cell morphologies. Since both CO₂ and H₂O are innocuous, this approach has potential for use in the preparation of ultrafiltration membranes, which currently require the use of toxic solvents for their fabrication by way of the phase inversion process. Moreover, the use of scCO₂/shH₂O is a cost‐effective approach for the processing of high T_g polymers at significantly lower temperatures. POLYM. ENG. SCI., 58:1108–1114, 2018. © 2017 Society of Plastics Engineers     

Density‐Dependent Formation of the Pure trans‐Isomer of the Unsaturated Alcohol by Selective Hydrogenation of Citral in Supercritical Carbon Dioxide

Selective hydrogenation of citral to unsaturated alcohol [geraniol (trans) + nerol (cis)] was carried out in supercritical carbon dioxide (scCO₂) using an MCM‐41 supported plantinum catalyst (∼1 wt% Pt). A remarkable rate of isomerization of the unsaturated alcohol [nerol (cis) to geraniol (trans)] during the hydrogenation of citral was achieved simply by tuning the density of CO₂. Optimum reaction conditions were developed to obtain only geraniol (trans) with a selectivity of 98.8% and citral conversion of 99.8%. A significant change in the cis:trans ratio of the product (1:82.3) from the substrate (1:1.3) was observed depending on the various reaction parameters like carbon dioxide and hydrogen pressure, reactant concentration, reaction time and, particularly, the total selectivity for unsaturated alcohol [geraniol (trans) +nerol (cis)]. It has been observed that the presence of hydrogen is necessary for isomerization. Our results were explained in terms of a density‐dependent, two‐step model. The kinetic behaviour shows that the rate of isomerization was higher in scCO₂ compared to other organic solvents and the pure form of geraniol (trans) was obtained exclusively. A probable reaction pathway was proposed in order to explain the isomerization during hydrogenation of citral in scCO₂ medium.     

Role of supercritical carbon dioxide for selective impregnation of decrosslinking reagent into isoprene rubber vulcanizate

The behavior of supercritical CO₂ (scCO₂)/low molar mass molecule/crosslinked rubber ternary system was investigated in relation to the impregnation of reagent into the isoprene rubber (IR) vulcanizates, which was the first step of new decrosslinking reaction. The diffusion coefficient of decrosslinking reagent, diphenyl disulfide (DD), into the IR network in scCO₂ was 3.2×10⁻¹¹ m²/s. The distribution coefficient (K_c) of DD between the solvent and IR matrix was also determined for scCO₂ and toluene. The K_c for scCO₂ was higher about four orders of magnitude than that for toluene. DD was uniformly dispersed in the crosslinked IR matrix under 10 MPa at 313 K in scCO₂. These phenomena are advantages of use of scCO₂ for the effective decrosslinking reaction of IR vulcanizate.     

Continuous‐Flow Asymmetric Hydrogenation of the β‐Keto Ester Methyl Propionylacetate in Ionic Liquid–Supercritical Carbon Dioxide Biphasic Systems

A continuous‐flow process for the asymmetric hydrogenation of methyl propionylacetate as a prototypical β‐keto ester in a biphasic system of ionic liquid and supercritical carbon dioxide (scCO₂) is presented. An established ruthenium/2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl (BINAP) catalyst was immobilised in an imidazolium‐based ionic liquid while scCO₂ was used as mobile phase transporting reactants in and products out of the reactor. The use of acidic additives led to significantly higher reaction rates and enhanced catalyst stability albeit at slightly reduced enantioselectivity. High single pass conversions (>90%) and good enantioselectivity (80–82% ee) were achieved in the first 80 h. The initial catalyst activity was retained to 91% after 100 h and to 69% after 150 h time‐on‐stream, whereas the enantioselectivity remained practically constant during the entire process. A total turnover number of ∼21,000 and an averaged space‐time yield (STY_av) of 149 g L⁻¹ h⁻¹ were reached in a long‐term experiment. No ruthenium and phosphorus contaminants could be detected via inductively coupled plasma optical emission spectrometry (ICP‐OES) in the product stream and almost quantitative retention by the analysis of the stationary phase was confirmed. A comparison between batch‐wise and continuous‐flow operation on the basis of these data is provided.     

Use of supercritical CO2 as a dispersing agent in the preparation of a polymer‐layered silicate nanocomposite

The usefulness of supercritical CO₂ (scCO₂) in the processing of thermoplastic olefin (TPO) nanocomposites was investigated using a simple extensional flow mixing device. An organoclay (Cloisite^® 20A), a maleated polypropylene (Polybond^® 3200), and a TPO polymer were selected for this study. The three components were combined in different manners to evaluate how scCO₂ influenced the microstructure of a final nanocomposite prepared by different mixing orders of the formulation components. The organoclay was examined in its as‐supplied state as well as after being conditioned under scCO₂. Analyses of the nanocomposites by X‐ray diffraction, transmission electron microscopy, and parallel plate rheology techniques showed that inclusion of scCO₂ during melt compounding significantly contributed to disrupting the crystalline order in the structure of the original organoclay. However, the order by which the formulation components are combined under scCO₂ was important to clay dispersion. It was shown that making use of the proper combination order led to an improvement in the modulus of the resulting nanocomposites as measured by rheology. POLYM. COMPOS., 38:987–995, 2017. © 2015 Society of Plastics Engineers     

Investigation of crystallization of PVCH‐PE‐PVCH triblock copolymer in supercritical carbon dioxide

Crystallization of glassy‐crystalline‐glassy poly(vinylcyclohexane)‐b‐polyethylene‐b‐poly(vinylcyclo hexane) (PVCH‐PE‐PVCH) triblock copolymer treated in supercritical Carbon Dioxide (scCO₂) was investigated by using differential scanning calorimetry (DSC) and atomic force microscope (AFM). It was found that the melting temperatures (T_m) and the crystallinity (X_c) of the PVCH‐PE‐PVCH samples treated in scCO₂ at different annealing temperatures (T) were all much higher than those of the untreated PVCH‐PE‐PVCH, indicating that the scCO₂ could effectively induce the samples to further crystallize. With increasing the T, the T_m of the samples linearly increased, even up to 108°C, close to the T_m (～ 110°C) of the PE homopolymer hydrogenated from polybutadiene which is equal to the PE block in the triblock copolymer. The results could be ascribed to the released PE chain ends linked to the PVCH block due to the lowered T_g of the PVCH block swollen by scCO₂. It suggested that the origin of the confined crystallization in PVCH‐PE‐PVCH was the fixed PE chain ends by the glassy PVCH. AFM images of the samples treated in scCO₂ showed that the PVCH lamella phase tended to connect each other and led to the aggregated structures. The result indicated that the PVCH block could be availably swollen by scCO₂. It supported the DSC experiment results of the samples treated in scCO₂. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2584–2589, 2006     

Hydrogenation of Nitrobenzene with Supported Transition Metal Catalysts in Supercritical Carbon Dioxide

Transition metal catalysts such as Pd, Pt, Ru, and Rh supported on carbon, silica and alumina have been examined for the hydrogenation of nitrobenzene (NB) in supercritical carbon dioxide (scCO₂) and in ethanol. The order of hydrogenation activity is Pt>Pd>Ru, Rh in scCO₂ and in ethanol. The effectiveness of the support is C>Al₂O₃, SiO₂ for either Pt or Pd in scCO₂. For all the catalysts, higher selectivity to aniline has been obtained in scCO₂ compared with ethanol. Hydrogenation of nitrobenzene catalyzed with Pd/C and Pt/C catalysts was successfully conducted in scCO₂ with a 100% yield to aniline at a lower reaction temperature of 35 °C. The product aniline (organic phase) can be easily separated from the side‐product water (aqueous phase), solvent (scCO₂), and catalyst (solid) by a simple phase separation process. The hydrogenation of NB is a structure‐sensitive reaction in ethanol as well as in scCO₂ except for a few Pt/C catalysts in which the degree of metal dispersion is small (<0.08).     

Rhodium complexes containing chiral P-donor ligands as catalysts for asymmetric hydrogenation in non conventional media

Abstract  

Rhodium-catalysed asymmetric hydrogenation using P-donor ligands, such as new fluorinated (R)-BINOL and azadioxaphosphabicyclo[3.3.0]octane derivatives was carried out in different reaction media such as organic solvent (CH₂Cl₂), ionic liquid ([BMI][PF₆]), supercritical carbon dioxide (scCO₂) and [BMI][PF₆]/scCO₂ mixture. The best enantioselectivities were obtained in neat [BMI][PF₆], allowing a recycling up to ten times without activity loss. However, the enantioselectivity was lost due to ligand leaching. The ionic liquid phase containing rhodium molecular species was supported on functionalized multi-walled carbon nanotubes in order to improve the recycling, but unfortunately the asymmetric induction was lost upon catalyst immobilization.     

Reaction Calorimetry in Supercritical Carbon Dioxide

Summary: Reaction calorimetry is an efficient tool used to obtain kinetic, thermodynamic and safety data. A reaction calorimeter, RC1e‐HP350, developed in collaboration with Mettler‐Toledo GmbH, allows investigating chemical reactions under supercritical conditions. The main technical difference, compared with a classical liquid system, is that the whole reactor volume is occupied by the media. Heat transfer analysis in supercritical carbon dioxide (scCO₂) by the Wilson plot method shows that the behavior of the internal heat transfer coefficient in scCO₂ is the opposite of the one observed for classical liquid. In scCO₂ the lower the temperature (above the critical point) the better the internal heat transfer coefficient. The evolution of scCO₂ thermodynamical and transport properties near the critical point are responsible for this behavior. The dispersion polymerization of methyl methacrylate in scCO₂, with the polydimethylsiloxane monomethacrylate as stabilizer, is used as a model reaction. A polymerization reaction enthalpy of ?56.9 ± 2.2 kJ · mol^?1 is determined, being in good agreement with previously reported data. The results presented illustrate the accuracy of the heat balance model used and emphasize the potential of reaction calorimetry for the promotion of supercritical fluids technologies.

Technical comparison between liquid and supercritical reaction calorimetry.     

Catalytic Oxidation of Oleic Acid in Supercritical Carbon Dioxide Media with Molecular Oxygen

Oxidation of oleic acid was performed over various ordered porous catalysts containing transition metal in supercritical carbon dioxide (scCO₂) media with molecular oxygen. Oleic acid was completely decomposed into mono- and dicarboxylic acids over porous catalysts, viz., mesoporous molecular sieves (CrMCM-41, MnMCM-41, CoMCM-41) and microporous molecular sieves (CrAPO-5, CoMFI, MnMFI) using scCO₂ at 353 K for 8 h. Among the different catalysts studied, microporous and mesoporous catalysts containing chromium, in presence of scCO₂ showed high distribution of azelaic and pelargonic acids as compared to their analogs containing cobalt or manganese. The presence of scCO₂ medium with the catalysts increased the distribution of azelaic and pelargonic acids. The effect of CO₂ pressure, reaction temperature and reaction time on oxidation of oleic acid over CrMCM-41 was also investigated. Additionally it is noticed that the catalyst can be recycled with negligible loss of catalytic activity.     

Multiple-step drawing innovative ultrahigh-molecular-weight polyethylene fibers modified with bacterial cellulose and scCO2-aid

Innovative supercritical carbon dioxide (scCO₂)-assisted ultrahigh-molecular-weight-polyethylene (UHMWPE)/modified bacterial cellulose (MBC) as-spun fibers were found to display substantially lower dynamic transition temperatures than those acquired for scCO₂-assisted UHMWPE or UHMWPE/MBC as-spun fibers prepared without scCO₂-assistance or incorporation of MBC nanofibers. Multiple-step drawing methods were first-time applied to these finely ''relaxed'' scCO₂-assisted UHMWPE/MBC fibers and considerably improved their achievable draw ratios (D_ras), orientation factor (f_os), and tensile tenacities (σ_tts). The best five-step drawn scCO₂UHMWPE/MBC fiber displayed a particularly high σ_tt of 135 g d⁻¹, which was ~35, ~3.75, and ~1.7 fold of σ_tts acquired for good steel fiber and the most appropriate single-step drawn scCO₂-assisted UHMWPE and UHMWPE/MBC fibers, respectively. The particularly high D_ras, f_o, and σ_tts acquired for the best multiple-step drawn scCO₂-assisted UHMWPE/MBC fibers is ascribed to their more ''relaxed'' UHMWPE structures, thinner lamellae, and successive increased drawing temperature in the multiple-step drawing processes.     

Synthesis of cross-linked poly(4-vinylpyridine) and its copolymer microgels using supercritical carbon dioxide: Application in the adsorption of copper(II)

Compared with conventional precipitation polymerization method, cross-linked poly(4-vinylpyridine) (P4VP) and its microgels copolymerized with α-methacrylic acid (MAA) were synthesized through a new route of stabilizer-free polymerization in supercritical fluids. The yellow, dry, fine powders were directly obtained from precipitation polymerization of 4-vinylpyridine in supercritical carbon dioxide (scCO₂) at pressures ranging from 70.0 to 230 bar, using N,N′-methylenebisacrylamide as cross-linker. The effects of the reaction pressure, cross-linker ratio, initiator concentration, and reaction time were investigated. The capacity of this microgel for adsorption of copper(II) was also studied. At higher cross-linker concentrations, a high yield of the cross-linked P4VP microgel was generated in scCO₂, and its particle size was less than 300 nm. Polymerization of cross-linked P4VP in scCO₂ was extremely sensitive to the density of the continuous phase. The adsorption followed the Langmuir isotherm. The adsorption capacities of cross-linked P(4VP-co-MAA) and cross-linked P4VP were 47.2 and 26.9 mg g⁻¹, respectively.     

In situ ATR-FT-IR study of the thermal decomposition of diethyl peroxydicarbonate in supercritical carbon dioxide

The thermal decomposition of the organic free-radical initiator, diethyl peroxydicarbonate (DEPDC), was monitored by in situ ATR-FT-IR in heptane, and in the green solvent supercritical carbon dioxide (scCO₂) both with and without supercritical ethylene. It was observed that the characteristic peaks of DEPDC at 1802-1803 and 1194-1203 cm⁻¹ decreased significantly upon heating corresponding to the decomposition of DEPDC, while two new intense peaks simultaneously appeared at 1747 and 1262 cm⁻¹ in heptane, and similarly at 1756 and 1250 cm⁻¹ in scCO₂. The changes in the absorbance intensity of the characteristic peaks of the initiator during the decomposition were used for the measurement of the decomposition rate constant (k_d) of DEPDC. It was found that the thermal decomposition of DEPDC at low concentration in either heptane under atmospheric N₂ or scCO₂ under high pressure was via the first-order kinetics of unimolecular decomposition. The activation energy of the thermal decomposition of DEPDC was found to be 115 kJ/mol in heptane from 40 to 74 °C and 118 kJ/mol in scCO₂ from 40 to 60 °C. These new peaks revealed the formation of carboxyl groups contained in the decomposed products, indicating incomplete decarboxylation. During removal of CO₂ after the reaction in scCO₂, the instable intermediate monoethyl carbonate was decarboxylated and converted into the major end product, ethanol.     

Utilization of supercritical CO2 as a processing aid for preparation of ultrahigh molecular weight polyethylene/functionalized activated nanocarbon fibers

This is the first investigation to report the processing and properties of ultrahigh molecular weight polyethylene (UHMWPE)/functionalized activated nanocarbon (FANC) gel solutions with the aid of supercritical carbon dioxide (scCO₂). The ultradrawing and ultimate tensile properties of scCO₂UHMWPE and scCO₂UHMWPE/FANC fibers were found to improve considerably compared to those of UHMWPE and UHMWPE/FANC fibers prepared in the conventional way. The maximum achievable draw ratio obtained for the optimal scCO₂UHMWPE/FANC fibers drawn at 95°C reached 445. The highest tensile tenacity (σ_f) of the fully drawn scCO₂UHMWPE/FANC fiber reached an extraordinary high value of 104 g/d, which is about 3.2 and 1.1 times of that of the optimal UHMWPE and UHMWPE/FANC fully drawn fibers, respectively. The σ_f obtained for the optimally fully drawn scCO₂UHMWPE/FANC fiber is about 25 times of those of steel fibers and is the highest tensile tenacity ever reported for single‐stage drawn polymeric fibers. Considerably lower dynamic transition temperatures and evaluated thinner crystal lamellae nucleated off of extended chains or FANC nucleants were found for as‐prepared scCO₂UHMWPE and scCO₂UHMWPE/FANC fibers compared with UHMWPE and UHMWPE/FANC fibers, respectively. Specific surface area, morphological, and Fourier transform infrared analyses of the activated nanocarbon (ANC), acid‐treated activated nanocarbon (ATANC) and FANC nanofillers and investigation of thermal, morphological, and orientation factor properties of the as‐prepared and drawn UHMWPE, UHMWPE/FANC, scCO₂UHMWPE, and scCO₂UHMWPE/FANC fibers were performed to understand the remarkable ultradrawing, dynamic transition, and ultimate tensile properties obtained for scCO₂UHMWPE and scCO₂UHMWPE/FANC fibers. POLYM. ENG. SCI., 59:1462–1471 2019. © 2019 Society of Plastics Engineers     

Tandem Staudinger-Aza-Wittig reaction in supercritical CO2: Synthesis of a pharmaceutical interest compound

The aim of this research was to investigate the capability of using supercritical CO₂ (scCO₂) as reagent and solvent in the synthesis of pharmaceutical cyclodextrinyl derivatives in smooth reaction conditions. In this way we have followed the kinetics of a synthesis which was previously realized in DMF, by using tandem Staudinger-Aza-Wittig (S.A.W.) reaction in scCO₂ at 200 bars pressure in a 100 mL reactor. The results show that the reaction in scCO₂ showed a second order kinetics with the yield of 92%.     

Phase appearance during polymerization of fluorinated polyimide monomers and deposition into the microscopic‐scale trenches in supercritical carbon dioxide

The phase appearance during the synthesis of fluorinated polyamic acid (PAA) from 2,2‐bis(3,4‐anhydrodicarboxyphenyl)‐hexafluoropropane (6FDA) and 2,2′‐bis(trifluoromethyl)?4,4′‐diaminobiphenyl (TFDB) in supercritical carbon dioxide (scCO₂) was investigated to obtain fundamental data for the deposition of fluorinate polyimides (PI) using scCO₂. All polymerizations were carried out at 30 MPa for 60 min. The experimental temperatures ranged from 50 to 70 °C, and each of the monomer concentrations ranged from 0.67 × 10^?5 to 3.3 × 10^?5 mol cm^?3. The holding time of the transparent phases, which was the time from the beginning of the polymerization to the appearance of a turbid phase, was increased with either a decrease in the polymerization temperature or a decrease in the initial monomer concentration. The holding time of the fluorinated PAA was longer than that of the monomers of Kapton‐type PAA. The deposition of PI into the microscopic‐scale trenches that had formed on the silicon wafer was successful in scCO₂. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43334.     

Predicting the effect of dissolved carbon dioxide on the glass transition temperature of poly(acrylic acid)

The morphology and size of poly(acrylic acid) (PAA) particles produced by precipitation polymerization in supercritical CO₂ (scCO₂) depends on the glass transition temperature (T_g) of the polymer at reaction conditions. In this study, the use of the Sanchez–Lacombe equation of state (SL‐EOS), in conjunction with Chow's equation, to predict the effect of CO₂ pressure on the T_g of PAA was evaluated. Characteristic parameters for PAA were determined by fitting density data. Characteristic parameters for CO₂ were determined by fitting density data in the supercritical region. When the SL‐EOS was used in a purely predictive mode, with a binary interaction parameter (ψ) of 1, the solubility of CO₂ in PAA was underestimated and T_g was overestimated, although the trend of T_g with CO₂ pressure was captured. When was determined by fitting the SL‐EOS to the measured sorption of scCO₂ in PAA, the calculated T_g's agreed very well with measured values. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Enhanced electrical properties of polycarbonate/carbon nanotube nanocomposites prepared by a supercritical carbon dioxide aided melt blending method

Polycarbonate/carbon nanotube (CNT) nanocomposites were generated using a supercritical carbon dioxide (scCO₂) aided melt blending method, yielding nanocomposites with enhanced electrical properties and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Baytubes^® C 150 P CNTs were benignly deagglomerated with scCO₂ resulting in 5 fold (5X), 10X and 15X decreases in bulk density from the as-received CNTs. This was followed by melt compounding with polycarbonate to generate the CNT nanocomposites. Electrical percolation thresholds were realized at CNT loading levels as low as 0.83 wt% for composites prepared with 15X CNT using the scCO₂ aided melt blending method. By comparison, a concentration of 1.5 wt% was required without scCO₂ processing. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO₂ processing compared to direct melt blending, but was significantly worse than that of twin screw melt compounded nanocomposites reported in the literature. A rheologically percolated network was observed near the electrical percolation of the nanocomposites. The importance of maintaining longer carbon nanotubes during nanocomposite processing rather than focusing on dispersion alone is highlighted in the current efforts.     

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.     

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.