The correlation between free volume and gas separation properties in high molecular weight poly(methyl methacrylate) membranes

Poly(methyl methacrylate) membranes of different fractional free volume (FFV) were prepared by dry casting from different solvents. Free volume data were determined by means of Bondi method and positron annihilation lifetime spectroscopy (PALS). It was found that both the boiling point and the solubility parameter of casting solvent affect the membrane’s free volume. It was believed that the difference in free volume was arisen from the difference in polymer packing.The gas permeability is higher when membranes are cast from higher molecular weight PMMA. But the plasticizing effect of CO₂ is less serious compared with the low molecular weight one. The high molecular weight PMMA membrane also has an extremely high O₂/N₂ selectivity, indicating its high structure uniformity. These results indicate that membranes made from polymer of higher molecular weight have the advantages of high permeability, gas selectivity and are less sensitive to CO₂ plasticization. The intrinsic gas transport properties such as the permeability, solubility and diffusivity of O₂, N₂, and CO₂ are measured or calculated. The effects of fractional free volume on membrane gas separation properties were investigated. It was found that the fractional free volume had no definite effects on gas solubility, but the gas permeability and diffusivity increased accordingly to the measured free volume.     

Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: Polybenzodioxane PIM-1

A detailed study of gas permeation, thermodynamic properties and free volume was performed for a novel polymer of intrinsic microporosity (PIM-1). Gas permeability was measured using both gas chromatographic and barometric methods. Sorption of vapors was studied by means of inverse gas chromatography (IGC). In addition, positron annihilation lifetime spectroscopy (PALS) was employed for investigation of free volume in this polymer. An unusual property of PIM-1 is a very strong sensitivity of gas permeability and free volume to the film casting protocol. Contact with water in the process of film preparation resulted in relatively low gas permeability (P(O₂) = 120 Barrer), while soaking with methanol led to a strong increase in gas permeability (P(O₂) = 1600 Barrer) with virtually no evidence of fast aging (decrease in permeability) that is typical for highly permeable polymers. For various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂) the data points on the Robeson diagrams are located above the upper bound lines. Hence, a very attractive combination of permeability and selectivity is observed. IGC indicated that this polymer is distinguished by the largest solubility coefficients among all the polymers so far studied. Free volume of PIM-1 includes relatively large microcavities (R = 5 Å), and the results of the PALS and IGC methods are in reasonable agreement.     

Biological Xenogenic Vessel Prostheses: Effect of H<Subscript>2</Subscript>O/CO<Subscript>2</Subscript> and EtOH Media on the Structure and Mechanical Properties of the Bovine Jugular Veins

The effect of the H₂O/CO₂ and EtOH/CO₂ media at pressures and temperatures corresponding to the formation of supercritical (sc) carbon dioxide on the structure and mechanical properties of bovine jugular veins was studied for the first time. Treatment of bovine jugular veins in an sc-CO₂ + ethanol or sc-CO₂ + water medium considerably devitalized the tissue and increased the tissue elasticity, which allows considering the treated veins as promising vascular grafts with expected high capacity to suppress calcification and good hemodynamic properties.     

High sensitivity of positron annihilation lifetime to time and pressure effects in gas-exposed polymers

Positron annihilation lifetime (PAL) experiments are performed in polycarbonate (PC) exposed to CO₂ and He gases as a function of time and pressure. In PC/CO₂ systems, hole size and fraction reduced from PAL data increase as a function of CO₂ pressure and exposure time. Significant hysteresis in positron lifetime data is observed during CO₂ abasorption/desorption. In PC/He systems, no variation is observed. Hole size distribution in the CO₂-exposed polymers is found to be significantly broader than in unexposed samples. The high sensitivity of PAL results to CO₂ exposure of PC is thought to relate to the microstructural changes in the polymer matrix, such as penetrant plasticization, gas hydrostatic pressure effect, and molecular filling and creation of holes.     

Polyimide-silica sol–gel membranes from a novel alkoxysilane functionalized polyimide: preparation,characterization and gas separation properties

Novel polyimide-organosilicate hybrid films were prepared by sol–gel process from a novel functionalized polyimide with alkoxysilanes as pendant groups that increase the affinity between inorganic and organic phases. The synthesis of this functionalized polyimide was carried out by an esterification reaction of a copolyimide containing carboxylic acid groups with allyl alcohol and subsequent hydrosililation. Tetraethoxysilane was used as precursor of silica in different amounts to obtain hybrid membranes with a silica content of 5, 10 and 20 %. The polymers and hybrid membranes were structural, mechanical and thermally characterized. The ²⁹Si-NMR solid state spectroscopy confirmed that silica was covalently bonded to the polyimide. SEM pictures showed a good dispersion of the silica particles and an amorphous morphology was observed by WAXS. DSC analyses revealed an increase in rigidity with the increase in silica content. The mechanical strength of the hybrid membranes decreased with the silica amount, exhibiting a brittle behavior. The evaluation of the gas permeation properties revealed that the film with the lowest silica content showed the highest permeability coefficients for O₂, N₂, CH₄, and CO₂ with 34, 8, 6, and 128 barrers respectively, while all hybrid membranes showed similar permselectivities around 4 and 22 for O₂/N₂ and CO₂/CH₄ respectively. The fractional free volume of hybrid membranes determined by positron annihilation lifetime spectroscopy followed the same trend that permeability coefficients, confirming that the gas transport properties are mainly governed by the free volume elements.     

Preparation and characterisation of polyaniline based membranes for gas separation

Transport rates (permeability) and ideal separation factors for several gas pairs through dense polyaniline membranes are reported. The ideal separation factors for all gas pairs tested were found to be independent of the polyaniline membrane thickness whereas the permeability of the single gases showed significant variations. Both dedoped and redoped films (film thickness between 9 and 67 μm) were studied. The highest selectivities α_(A/B) found were 7.6 for the gas pair H₂/CO₂ in the case of the dedoped membrane and 10 for the gas pair H₂/CO₂, 6 for O₂/N₂ and 200 for H₂/N₂ in the case of the redoped membrane. Statistical analysis of a large number of membranes allowed the critical comparison with results obtained by other groups.Comparison with other membrane materials shows that an approximately sixfold enhancement of the respective separation factors is possible for gas pairs containing hydrogen. Similar separation factors are observed for the gas pairs CO₂/O₂, CO₂/N₂ and N₂/O₂.Membranes for which Knudsen diffusion was observed exhibited regularly distributed micropores (400 nm diameter).     

Effects of cross-linkers with different molecular weights in cross-linked Matrimid 5218 and test temperature on gas transport properties

The poly(ethylene oxide) (PEO) was introduced by the cross-linking method in the commercial Matrimid 5218. The two kinds of membranes were prepared from the Matrimid 5218 and the cross-linkers poly(propylene glycol) block poly(ethylene glycol) block poly(propylene glycol) diamine (PPG/PEG/PPGDA) with different molecular weights. The cross-linking reaction process was monitored by FTIR. The cross-linked Matrimid 5218 membranes display excellent CO₂ permeability and CO₂/light gas selectivity. The effects of cross-linkers with different molecular weights on gel content, thermal properties and H₂, CO₂, N₂ and CH₄ gas transport properties were reported. The effect of temperature on gas transport properties was also reported, and the permeabilities of these materials as a function of temperature were compared with other gas membrane materials.     

Prediction of Carbon Dioxide Adsorption via Deep Learning

Porous carbons with different textural properties exhibit great differences in CO₂ adsorption capacity. It is generally known that narrow micropores contribute to higher CO₂ adsorption capacity. However, it is still unclear what role each variable in the textural properties plays in CO₂ adsorption. Herein, a deep neural network is trained as a generative model to direct the relationship between CO₂ adsorption of porous carbons and corresponding textural properties. The trained neural network is further employed as an implicit model to estimate its ability to predict the CO₂ adsorption capacity of unknown porous carbons. Interestingly, the practical CO₂ adsorption amounts are in good agreement with predicted values using surface area, micropore and mesopore volumes as the input values simultaneously. This unprecedented deep learning neural network (DNN) approach, a type of machine learning algorithm, exhibits great potential to predict gas adsorption and guide the development of next‐generation carbons.     

Effect of tacticity on permeation properties of poly(methyl methacrylate)

The effect of stereoregularity on gas permeation properties of poly(methyl methacrylate) (PMMA) was investigated. The gas permeability coefficients for He, H₂, O₂, N₂, Ar, CH₄, and CO₂ at 35°C near atmospheric pressure have been measured for three different PMMAs. Apparent diffusion and solubility coefficients were obtained from time lag data, and these were compared with data for a commercial PMMA previously reported. The permeability, solubility, and diffusion coefficients increase as the content of syndiotactic sequences increases. These observations are consistent with more dense packing of the isotactic form in the glassy state that stems in part from its lower glass transition temperature. The transport behavior for a 50:50 isotactic/syndiotactic blend was also studied. These so-called stereocomplexes exhibit permeation behavior comparable to other weakly interacting miscible blend systems.     

Comparison of three models for permeation of CO2/CH4 mixtures in poly(phenylene oxide)

Two models for the permeability of pure gases have been extended to include binary gas mixtures. The first is an extension of a pure gas permeability model, proposed by Petropoulos, which is based on gradients of chemical potential. This model predicts the permeability of components in a gas mixture solely on the basis of competition for sorption sites within the polymer matrix. The second mixed gas model follows an earlier analysis by Barrer for pure gases which includes the effects of saturation of Langmuir sites on the diffusion as well as the sorption processes responsible for permeation. This generalized “competitive sorption/diffusion” model includes the effect of each gas component on the sorption and diffusion of the other component in the mixture. The flux equations from these two models have been solved numerically to predict the permeability of gas mixtures on the basis of pure gas sorption and transport parameters. Both the mixed gas Petropoulos and competitive sorption/diffusion model predictions are compared with predictions from the earlier simple competitive sorption model based on gradients of concentration. An analysis of all three models is presented for the case of CO₂/CH₄ permeability in poly(phenylene oxide) (PPO). As expected, the competitive sorption/diffusion model predicts lower permeability than either of the models which consider only competitive sorption effects. The permeability depression of both CO₂ and CH₄ predicted by the competitive sorption/diffusion model is roughly twice that predicted by the competitive sorption model, whereas the mixed gas Petropoulos model predictions for both gases lie between the other two model predictions. For the PPO/CO₂/CH₄ system, the methane permeability data lie above the predictions of all three models, whereas CO₂ data lie below the predictions of all models. Consequently, the competitive sorption/diffusion model gives the most accurate prediction for CO₂, while the simple competitive sorption model is best for methane. The effects of mixed gas sorption, fugacity, and CO₂-induced dilation were considered and do not explain the inaccuracies of any of the models. The relatively small errors in mixed gas permeability predictions using either of the three models are likely to be related to “transport plasticization” of PPO owing to high levels of CO₂ sorption and its effect on polymer segmental motions and gas diffusivity.     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

Extraction of brominated flame retardants from polymeric waste material using different solvents and supercritical carbon dioxide

Different procedures were examined to extract pure and high concentrations of a series of brominated flame retardants from various polymer materials. These procedures include supercritical carbon dioxide (sc-CO₂), modified sc-CO₂, solvent and soxhlet extraction. Extraction with sc-CO₂ gave low extraction efficiencies (between 6 and 20%) probably due to the low pressure of sc-CO₂ used. The use of toluene, acetonitrile and THF as modifier in sc-CO₂ raised the extraction efficiencies for many flame retardants. High extraction efficiencies were achieved for tetrabromobisphenol A (TBBPA), TBBPA-bis-(2,3-dibromopropylether) (TBBPA-dbp), TBBPA-carbonatoligomer (TBBPA-co) and decabromodiphenylether (DECA) (between 93 and 100%) by using 1-propanol as solvent during soxhlet extraction. Toluene instead of 1-propanol was used where insufficient extraction of the flame retardant occurred. The materials (before and after extraction) were analysed with energy dispersive X-ray fluorescence analysis (EDXRF), high performance liquid chromatography with ultraviolet detection (HPLC/UV), gas chromatography/mass spectrometry (GC/MS) and infrared spectroscopy (IR) techniques. The properties of the extracted flame retardants such as TBBPA, TBBPA-dbp and 1,2-bis(tribromophenoxy)-ethane (TBPE) are in good agreement with those of standard reference materials.     

Preparation of CO_2 selective composite membranes using Pebax/CuBTC/PEG-ran-PPG ternary system

Three phase Pebax~? MH 1657/PEG-ran-PPG/CuBTC(polymer/liquid/solid) was successfully deposited as a selective layer on a porous Polysulfone(PSF) support. In fact, the beneficial properties of PEG(high selectivity) with those of PPG(high permeability, amorphous) have been combined with superior properties of mixed matrix membrane(MMMs). The membranes were characterized by DSC, TGA and SEM, while CuBTC was characterized by CO_2 and CH_4adsorption test. Statistically based experimental design(central composite design, CCD) was applied to analyze and optimize the effect of PEG-ran-PPG(10–50 wt%) and CuBTC(0–20 wt%) mass contents on the CO_2 permeance and CO_2/CH_4 ideal selectivity. Based on the regression coefficients of the obtained models, the CO_2 permeance was notably influenced by PEG-ran-PPG,while CuBTC has the most significant effect on the CO_2/CH_4 ideal selectivity. Under the optimum conditions(PEG-ran-PPG: 32.76 wt% and CuBTC: 20 wt%), nearly 620% increase in the CO_2 permeance and43% enhancement in the CO_2/CH_4 ideal selectivity was observed compared to the neat Pebax. The effect of pressure(3, 9 and 15 bar) on the pure and mixed gas separation performance of the composite membranes was also investigated. The high solubility of CO_2 in the membranes resulted in the enhancement of CO_2 permeability with increase in gas pressure.     

Development and characterization of homogeneous membranes de from high molecular weight sulfonated polyphenylene oxide

The high molecular weight polyphenylene oxide (PPO) was sulfonated to different ion exchange capacity (IEC) values using chlorosulfonic acid. The physico-chemical properties along with the gas transport properties of the membranes prepared from sulfonated PPO (SPPO) were evaluated. Sulfonation of PPO results in a linear increase of density with the IEC value while the average d-spacing in polymer remains constant. Sulfonic groups attached to the aromatic ring in the PPO backbone are not thermally stable. On the other hand, when tested with CO₂ at room temperature, the SPPO membranes maintained a constant permeability over the period of 60 days. An increase in IEC value of SPPO results in an increase in O₂/N₂ and CO₂/CH₄ ideal selectivities and a decrease in O₂ and CO₂ permeabilities. The combination of permeability and ideal selectivity for both gas pairs places the SPPO membranes below the respective upper-bound lines for polymeric membranes. However, an increase in the IEC value brings the permeability versus ideal selectivity relationship closer to the upper-bound line, especially for the O₂/N₂ gas pair.     

Positronium annihilation data and actual free-volume distribution in polymers

Determination of the size distribution of free-volume holes in solids, in particular, polymers, is an important physicochemical problem. The positron annihilation technique has been proposed for this purpose. The central point in this technique is the quantitative interpretation of data, especially, for substances with a high specific surface area. A developed free-volume system in open-pore membrane materials, such as poly(trimethylsilylpropyne) PTMSP and the spirocyclically bound benzodioxane polymer PIM-1, and polymeric sorbents (hypercrosslinked polystyrenes) makes it possible for the first time to compare the sorption characteristics and positron annihilation data on the character of size distribution of nanopores in these polymers. In combination with the results of mathematical simulation of the structure and radiothermoluminescence measurements, the array of data indicate the structural inhomogeneity of the test amorphous materials. It was shown that this inhomogeneity in relation to the positron annihilation technique is expressed in the insufficiency of the representation of the orthopositronium decay curve by one component that takes into account the Gaussian lifetime distribution (symmetrical pore size distribution) and in the necessity of use of several decay components. The feasibility of revealing a nonrandom character of pore size distribution gives the positron annihilation technique an advantage over other approaches (inverse gas chromatography, ¹²⁹Xe NMR) to investigation of nanopores in polymers.     

Gas Permeation Properties of Organic-Inorganic Hybrid Membranes Prepared from Hydroxyl-Terminated Polyether and 3-isocyanatopropyltriethoxysilane

Organic-inorganic hybrid materials were prepared by reacting 3-isocyanatopropyltriethoxysilane (IPTS) with hydroxyl terminated poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG) and poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) (PEPG), followed by hydrolysis and condensation with acid catalysis. Composite membranes have been obtained by casting hybrid sol on the microporous polysulfone substrate. The membranes were characterized by Fourier transform infrared (FT-IR), ¹³C NMR and ²⁹Si NMR. The permeability coefficients of N₂, O₂, CH₄ and CO₂ were measured by variable volume method. The gas permeability coefficients increase with increasing molecular weight of the polyethers. For the membranes containing PEG and PEPG, the higher values of CO₂ permeability coefficients and CO₂/N₂ separation factors are due to the presence of ethylene oxide segments. In case of PEPG membranes, molecular weight has more influence on CO₂ permeability than the effect of facilitation by ethylene oxide. The addition of TEOS into hybrid sol results in the decrease of all the gas permeability and does not affect the gas selectivity. PEG2000 membrane display the most performance among the hybrid membranes investigated here. The best values observed are CO₂ permeability of 94.2 Barrer with selectivity of 38.3 for CO₂/N₂ and 15.6 for CO₂/CH₄.     

Subnanometer hole properties of CO2-exposed polysulfone studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime (PAL) spectroscopy has been employed to study subnanometer hole properties in polysulfone (PSF). In this study, hole properties of size, fraction, and distribution of PSF exposed to CO₂ are reported. In the PSF/CO₂ system, the hole size and fraction significantly increase and the free-volume distribution broadens as a function of CO₂ pressure in the range of 0–1000 psi. Hysteresis in hole properties is observed during CO₂ sorption/desorption cycle. The high sensitivity of PAL results due to CO₂ exposure in PSF is explained in terms of the microstructural changes in the polymer matrix, i.e., filling penetrant and plasticization, gas hydrostatic pressure effect, and creation of free volumes and holes. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3049–3056, 1998     

The effects of CO2 exposure on pure and mixed gas permeation behavior: comparison of glassy polycarbonate and silicone rubber

Pure and binary mixture permeabilities have been investigated for the C0₂/CH₄ system in polycarbonate and silicone rubber. Upstream pressure conditions ranging from zero up to the critical point of CO₂ were investigated. No permeability hysteresis was observed for the silicone rubber sample with pure or binary feeds of CO₂ and CH₄. On the other hand, perturbation treatments with CO₂ resulted in long-lived increases in the permeability of the conditioned polycarbonate films compared to untreated films. Increases in CO₂ permeability of 50% persisted for the polycarbonate sample over a period spanning more than two months. CO₂/CH₄ mixed gas measurements on the conditioned polycarbonate film also reflect changes in its permselection properties. The CO₂ permeability in the mixed gas system is based on a fugacity driving force to provide a rigorous comparison with the pure gas system. The conditioning treatment caused reductions in permselectivity of 30% relative to the as-received film for mixed gas feed streams of CO₂/CH₄ under the conditions studied. The permselectivity effects also appear to be semi-permanent and continue if the conditioning medium is not totally removed after the conditioning treatment.     

Gas permeability of cardo-poly(ether-ether-ketone) and poly(phenylene sulfide)

The gas permeability and sorption of CO₂ and N₂O was measured on cardo-poly(ether-ether-ketone) (C-PEEK) and poly(phenylene sulfide) (PPS) at 298 K. The results are discussed on the basis of the dual-mode model. Results obtained from measurements at 308 K are compared with literature data of poly(phenylene oxide) (PPO), poly(sulfone) (PSU) and poly(carbonate) (PC). While C-PEEK shows similar transport properties as PC and PSU, the values of PPS are distinctly lower. The solubility of CO₂ in the various polymers as well as the correlation of the permeability coefficients of the same polymers for He, Ar, CO₂, N₂, and CH₄ with the kinetic molecular diameter of the gases indicate a simple relation of the transport properties with the polymer density.     

Effects of tertiary amines and quaternary ammonium halides in polysulfone on membrane gas separation properties

Polymers containing CO₂‐philic groups are of great interest for CO₂/light gas separation membranes because the affinity toward CO₂ can effectively increase CO₂ solubility and thus permeability. In this study, polysulfones (PSUs) modified with different degrees of benzyldimethylamine (DMA), benzyltrimethylammonium fluoride (TMAF), and benzyltrimethylammonium iodide (TMAI) were synthesized using sequential post‐functionalization reactions and investigated for CO₂/N₂ and CO₂/CH₄ separation. The physical properties of these polymers were studied, including density, fractional free volume, and glass transition temperature. In contrast to the conventional wisdom that tertiary amines exhibit an affinity toward CO₂, this study convincingly shows that the DMA substituent has a minimal impact on CO₂ solubility and CO₂/light gas solubility selectivity in PSUs under dry condition. On the other hand, incorporating TMAF and TMAI in PSU significantly increases CO₂ solubility. Particularly, introducing TMAI with a molar ratio of 1.07 relative to PSU repeating units increases CO₂/CH₄ solubility from 4.4 to 5.2, CO₂/CH₄ permeability selectivity from 21 to 45, and CO₂/N₂ permeability selectivity from 24 to 33 at 35 °C, while the CO₂ permeability decreases from 5.6 to 1.7 Barrers. The effect of these functional groups in PSUs on gas diffusivity and diffusivity selectivity can be satisfactorily described by the free volume model. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1239–1250