Hydraulic permeation of liquids through swollen polymeric networks. I. Poly(vinyl alcohol)–water

Poly(vinyl alcohol) membranes were prepared by crosslinking with terephthalaldehyde. Hydraulic permeation of water through this network structure was measured as a function of pressure for temperatures ranging from 18° to 35.8°C. The data were analyzed via a previously developed solution–diffusion theory for hydraulic permeation to give mutual diffusion coefficients. The activation energy for diffusion was found to be 6.5 kcal/mole which compared to the value of 4.3 kcal/mole for viscous flow of water indicates an influence of polymer–liquid interaction on the energetics of the diffusion process.     

High gas permeability in open-structure membranes

For most polymeric membranes, the gas permeability coefficient (P) is often interpreted as the product of diffusivity (D) and solubility (S) of a penetrant gas in the polymer (P=D S). The basic assumption is that molecular diffusion is primarily responsible for mass transport in the membrane permeation process. However, for some open structure membranes, such as poly(1-trimethylsilyl-1-propyne) [PTMSP] or poly(dimethylsiloxane) [PDMS], the high permeabilities of some gases yield much higher diffusivities when calculated from the above relationship (P=D S) than when calculated by using the direct kinetic measurement of diffusivity. It is hypothesized that this discrepancy is due to the convective transport of gas molecules through such open structured polymers. In most cases, the convective contribution to mass transport through membranes is negligible. However, for polymer membranes with high free volume, such as PTMSP, whose free volume fraction is 20 to 25%, the convective term may dominate the permeation flux. In this study, a non-equilibrium thermodynamic formalism is employed to properly treat the diffusion term and convective term that constitute the Nernst-Planck equation. The current analysis indicates that the total permeation flux, which consists of a diffusion term and a convective term, agrees well with the experimental data for several permeation systems: pure components propane and n-butane/PTMSP, pure gas hydrogen/PTMSP, and mixed gas hydrogen/PTMSP. Also, the permeation systems of a nonporous rubbery membrane, PDMS, and eight organophosphorus compounds were included in the study. It is recommended that the proposed model be validated by using other polymers with high free volumes and high permeabilities of gases and vapors, such as poly(1-trimethylgermyl-1-propyne) [PTMGeP] and poly(4-methyl-2-pentyne) [PMP]. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Permeability of water vapor and carbon dioxide gas through poly(n-alkyl L-glutamate)

To elucidate the permeation mechanism in polypeptide membranes, permeation of water vapor and carbon dioxide gas through a series of poly(n-alkyl L -glutamate) (polymers of methyl, ethyl, propyl, butyl, amyl, hexyl, and octyl glutamate) were studied. It was confirmed that diffusion of small molecular substances in polypeptides takes place through the side chain region between helices. The diffusion coefficient of carbon dioxide gas increases with increase in side chain length. As for water vapor, the diffusion coefficient is highest with poly(n-butyl glutamate), and the clustering effect of water may contribute to the diffusion coefficient with increase in the hydrophobic nature of the polymer.     

Permeation of water and sodium chloride through cellulose acetate

The factors contributing to the selective permeation of water and sodium chloride through cellulose acetate membranes have been examined by the use of radioactive tracers. With decreasing acetyl content both the partition coefficient (solubility) and diffusion coefficient of water increased, the latter the more sharply. The effect was even more pronounced for salt, indicating that increasing selectivity with acetyl content stems mainly from increasingly preferential restrictions on salt mobility. Trends identical with those mentioned for decreasing acetyl content were found for increasing amounts of cellulose acetate solvents that had been extracted with water to yield more highly swollen membranes. A free-volume treatment for diffusion of small molecules below the glass transition temperature with the aid of subgroup motion in the polymer is used for both components. The water content of the membrane at (or near) saturation emerged as the predominant factor in the permeation behavior. In view of the similarity in the activation energies of water and salt diffusion the far steeper dependence of the salt diffusion coefficient on water content could not be accounted for by size differences between the diffusing species and has been attributed to confinement of salt ions to locations at which multiple water contacts are feasible.     

Gas permeation through glassy polymer membranes with relatively low glass-transition temperature

When the glass-transition temperature of the polymer is not so much higher than the experimental temperature, the pressure dependence of the mean permeability coefficient of the poly-mer membrane to a gas is apt to deviate from the prediction by the conventional dual-mode mobility model, and to obey a similar model with concentration-dependent diffusivities because of the plasticization action of sorbed gas in the polymer membrane. In this work, sorption and permeation for oxygen and carbon dioxide in a membrane of polystyrene whose glass-transition temperature is 95°C, were measured to discuss the mechanism of gas diffusion in glassy polymer membranes with relatively low glass-transition temperature at 30, 40 and 50°C respectively.     

The mechanism of liquid transport through swollen polymer membranes

The pressure-driven transport of liquids employed in reverse osmosis has been shown to occur by a solution-diffusion mechanism in highly swollen polymer membranes. A theory based on this mechanism was successfully used earlier to correlate permeation fluxes for such membranes. Positive confirmation of this theory is provided here by direct measurement of the proposed concentration gradient. A study of the temperature dependence of the liquid diffusion coefficient in the polymer membrane has provided additional evidence of a hydrodynamic regime of diffusion in highly swollen membranes. It is also shown that the proposed ceiling flux in reverse osmosis is equal to the pervaporation flux.     

Recovery of alcohols from water using polydimethylsiloxane–silica nanocomposite membranes: Characterization and pervaporation performance

Different polydimethylsiloxane (PDMS) nanocomposite membranes were synthesized by incorporating various contents of nanosized silica particles to improve the PDMS pervaporation (PV) performance. A uniform dispersion of silica nanoparticles in the PDMS membranes was obtained. The nanocomposite membranes were characterized morphologically by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that surface roughness increases by incorporating silica, and this decreases absorption of penetrants on the membrane. Swelling studies showed that the presence of silica nanoparticles into the PDMS membranes decreases degree of swelling, which can be attributed to rigidification of the PDMS matrix. Additionally, the results revealed that helium permeability decreases through the nanocomposite membranes, due to the more polymer chains packing. Effects of silica on recovery of isopropanol (IPA) from water mixtures were also investigated. Based on the results, incorporating silica nanoparticles promotes significantly the PDMS membrane selectivity because the polymer chains are rigidified and also the polymer free volume decreases. However, permeation flux decreases as diffusion of the penetrants reduces in the presence of silica nanoparticles within the PDMS membranes. As PV performance depends on operating conditions, effects of feed composition, and temperature were also studied. Moreover, recoveries of IPA, ethanol, and methanol from water mixtures were compared using the PDMS‐silica nanocomposite membranes. The results demonstrated that polarity and solubility of alcohols affect permeation flux and selectivity resulting in the higher permeation flux and selectivity for IPA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation of highly mesoporous carbon membranes via a sol-gel process using resorcinol and formaldehyde

This paper reports the preparation of highly mesoporous carbon membranes, which are obtained by the pyrolysis of sol-gel derived mesoporous polymer membranes using resorcinol and formaldehyde (RF). Two series of RF carbon membranes were prepared by changing the resorcinol to catalyst molar ratio. The nitrogen adsorption-desorption measurement shows that the RF carbon membranes possess a well-developed mesoporous structure with controlled pore diameters of 5.48 nm and 13.9 nm. The helium and nitrogen permeances of both RF carbon membranes were independent of the feed pressure, indicating that there was no contribution of viscous flow and the membranes are initially crack-free. The gas permeation result showed that the dominant mechanism of gas transport through both the RF carbon membranes is Knudsen diffusion. With regard to the permeation of condensable gases such as CH₄ and CO₂, it was observed that the surface flow also contributes to the total permeation.     

Effect of crosslinking on the physicochemical properties of proton conducting PVDF-g-PSSA membranes

Poly(vinylidene fluoride) (PVDF) membranes, radiation-grafted with styrene and sulfonated, were studied as a candidate material for polymer electrolyte fuel cell (PEFC). In particular the effect of the use of crosslinkers in the polymer structure was investigated using bis(vinyl phenyl)ethane (BVPE) and divinylbenzene (DVB) as reagents. Water uptake in the H+ form, proton conductivity and ion exchange capacity of the PVDF-g-PSSA membranes, as well as transport properties of oxygen and hydrogen were determined at room temperature. Crosslinking with DVB resulted in a more pronounced decrease in the properties; the use of BVPE had no significant influence. Even on the permeation of oxygen and hydrogen the BVPE had little effect: the diffusion coefficient and solubility remained at the same level as for the non-crosslinked membranes. Increasing the membrane thickness was found to be at least as effective in reducing the oxygen permeation rate as using crosslinkers.     

Effects of temperature,repeated exposure,and aging on polybutylene permeation by organic chemicals

The permeation parameters, partition coefficient and diffusion coefficient, were obtained for polybutylene (PB) with toluene using a gravimetric method. A PB sample was repeatedly exposed and desorbed, and the partition coefficient and diffusion coefficient were measured. Permeation parameters were also measured at different temperatures. The results were compared with those obtained from a 12-year-old PB pipe sample, which had experienced structural failure. Tests at an elevated temperature of 45°C failed to simulate the effect of polymer aging, whereas repeated sorption/desorption tests produced permeation parameters compatible with those of a 12-year-old PB pipe in service.     

Influence of annealing on the permeation properties of a thermoplastic elastomer

The use of plastic films with specific diffusion or permeation properties for industrial applications has grown at a considerable rate. Some useful applications are found in medical devices, bioreactors, and combustible fuel storage where polymer films function as separation membranes that allow permeation of different gases at different rates. In this work, the permeation and diffusion properties of a polyester‐based thermoplastic polyurethane (TPU) were investigated. TPU injected and extruded specimens were subjected to thermal treatment (annealing) at 100°C for 20 h. Injected samples were exposed to certain hygrothermal conditions and films were prepared to evaluate the influence of annealing on the permeation of gases. In order to achieve a complete analysis, tests such as differential scanning calorimetry, tensile tests, and Fourier transform infrared spectroscopy were conducted to examine the morphological changes. These were then correlated to the TPU permeation behavior after annealing. Water uptake by the polymer—measured as weight gain—likely indicates an increase in the free volume in the amorphous domains. Similarly, in permeation and water immersion tests, the diffusion rate of gases and H₂O through the TPU was higher for the annealed samples when compared to those without treatment, indicating that diffusion within the polymer is dependent on the postprocessing thermal treatment. POLYM. ENG. SCI., 59:1810–1817, 2019. © 2019 Society of Plastics Engineers     

Sorption and diffusion of solvent vapours in poly(vinylalcohol) membranes of different crystallinity degrees

Solvent sorption and diffusion are the key processes that control membrane performances in membrane processes. The sorption characteristic of water and ethanol vapours in poly(vinylalcohol) (PVA) membranes of different crystallinity degrees was measured by microgravimetry and the diffusion characteristic was calculated from the sorption kinetics at different water activities by curve fitting. The sorption isotherms for water vapour in membranes of 28, 37, 44 and 56% crystallinity degrees at 40°C obey the Flory equation based on the polymer lattice model. When the sorption extent was corrected by assuming that only the polymer amorphous phase is accessible to the penetrant, a unique Flory χ interaction parameter, 0.3, was obtained for all samples except for the 28% crystallinity sample. For the latter sample, the lower χ value (0.18) obtained can be explained by a change in the sorption behaviour of the original crystalline domains which may undergo partial destruction. The diffusion coefficient increases with the average water content in the membrane according to an exponential relationship characterized by a limit diffusion coefficient and a plasticization coefficient. The higher the crystallinity of the membrane, the lower the values of the limit diffusion coefficient and the plasticization coefficient. The ethanol sorption was also well described by the Flory–Huggins equation. The limit diffusion coefficient for water was two orders of magnitude larger than that for ethanol.     

热致相分离法制备聚(偏氟乙烯-六氟丙烯)/离子液体凝胶膜及 气体渗透性能

采用热致相分离（TIPS）法制备p(VDF-HFP)/[BMIM]PF6凝胶膜,对该膜的内部结构及机械性能进行表征,探讨了[BMIM]PF6添加量对凝胶膜的CO2/N2渗透性能影响。结果表明：随着[BMIM]PF6添加量的增加,p(VDF-HFP)/[BMIM]PF6体系的凝胶温度逐渐降低,凝胶膜的聚合物骨架结构由致密变成疏松的球晶结构;同时[BMIM]PF6的添加降低了凝胶膜的结晶度,改善了p(VDF-HFP)链段的柔韧性,因而膜的CO2和N2渗透系数显著增加,CO2/N2渗透选择性则先升高后降低;凝胶膜受扩散过程控制,当[BMIM]PF6添加量（质量分数）由0增加到60%时,凝胶膜的CO2渗透性系数从0.2 Barrer增加到94.3 Barrer。     

Polysulfone/poly(ether sulfone) blended membranes for CO2 separation

Polymer blending as a modification technique is a useful approach for augmenting the gas‐separation and permeation properties of polymeric membranes. Polysulfone (PSF)/poly(ether sulfone) (PES) blend membranes with different blend ratios were synthesized by conventional solution casting and solvent evaporation technique. The synthesized membranes were characterized for miscibility, morphology, thermal stability, and spectral properties by differential scanning calorimetry (DSC), field emission scanning electron microscopy, thermogravimetric analysis, and Fourier transform infrared (FTIR) spectroscopy, respectively. The permeation of pure CO₂ and CH₄ gases was recorded at a feed pressure of 2–10 bar. The polymer blends were miscible in all of the compositions, as shown by DSC analysis, and molecular interaction between the two polymers was observed by FTIR analysis. The thermal stability of the blend membranes was found to be an additive property and a function of the blend composition. The morphology of the blend membranes was dense and homogeneous with no phase separation. Gas‐permeability studies revealed that the ideal selectivity was improved by 65% with the addition of the PES polymer in the PSF matrix. The synthesized PSF/PES blend membranes provided an optimized performance with a good combination of permeability, selectivity and thermal stability. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42946.     

Permeation and diffusion of environmental pollutants through flexible polymers

Permeation and diffusion characteristics of high density polyethylene (HDPE), elasticized polyolefin, and polychloroprene membranes have been determined for various organic pollutants at 22 and 50°C. A standard ASTM permeation cell method was used to measure the permeation rate while an immersion/weight gain method was used to obtain the diffusion coefficient. Thickness-dependent permeation rates were determined for HDPE membranes. An equimolar mixture of various permeants was also used to see the possibility of synergistic effects by combination of permeants. The experimental results are discussed in relation to the suitability of HDPE membranes in severe environments.     

Production and characterization of membranes of recycled waste materials: Cellulose acetate,obtained from sugarcane bagasse with polystyrene from plastics cups

Membranes of cellulose acetate from sugarcane bagasse (CA), as well as blends of this cellulose acetate and polystyrene from plastic cups (CA/PS) were produced by casting utilizing dichloromethane as solvent at the concentration 12% w/w. The membranes were characterized regarding ion diffusion by dialysis and properties of pure water permeation rate, PEG rejection (utilizing an aqueous solution 1% w/v of polyethylene glycol (PEG), 45 and 80 kDa). Thermal characterization by thermogravimetric analysis and differential scanning calorimetry were also performed. The morphology of the membranes' cross‐sections was evaluated by scanning electron microscopy. The experiment of ion diffusion by dialysis showed that the ion diffusion coefficient of CA membrane is comparable to that found in the literature for membranes of commercial cellulose triacetate, 8.47 × 10^?8 cm² s^?1, while the ion diffusion coefficient of blends decreased as PS was added to the system. Regarding transport driven by pressure, CA membrane presented low rejection of PEG 80 kDa. These results showed that CA membrane could be used in a range of application comprehending the process of ultrafiltration or microfiltration. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Permeability of small molecules through vapor deposited polymer membranes

Free‐standing, thin films of poly(2‐hydroxyethyl methacrylate), p(HEMA), membranes deposited by initiated chemical vapor deposition (iCVD) technique are presented. Systematic studies of permeation of model dye molecules through these free‐standing thin films are performed and effects of process parameters on the permeability and selectivity of the membranes synthesized using an all‐dry process are demonstrated for the first time. Permeation studies of model dye molecules show that the diffusion of large dye molecules through the membrane is faster than the smaller dye molecules, due to loss of conformation entropy. Mesh sizes of the membranes are controlled by tuning the crosslink ratio of the polymer and diffusion coefficients of alizarin yellow model dye molecules are obtained as 0.44 × 10^?7 and 1.71 × 10^?7 cm²/s for high and low crosslinked membranes, respectively. Selectivity of the membranes is also demonstrated by the permeation studies of dyes with similar sizes but different polarities and the permeability coefficient of the hydrophilic dye is observed to be 30 times larger than that of the hydrophobic dye of similar size. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42453.     

Permeability of water and water vapor through cellulosic membranes

To elucidate the water transport mechanism through homogeneous membranes, water and water vapor permeation through crosslinked cellulose membranes, cellulose diacetate, and cellulose triacetate membranes are studied. It is found that the water flux increases with the degree of hydration; and as for cellulose membranes, the degree of hydration is an increasing function of the degree of crosslinking. Activation energy of hydraulic permeability (K_w) is not equal to that of purely viscous flow, and is smaller than that of the water vapor diffusion coefficient (D?) for all membranes. The free-volume concept relating the molar frictional coefficient to temperature and to degree of hydration explains reasonably the temperature dependence of hydraulic permeability and of water vapor diffusion coefficient and gives adequate values for the fractional free volume of the system. The critical volume V^*, appearing in the Cohen-Turnbull expression between friction coefficient and free volume fraction, may be considered as the size of the cluster of water molecules. The value of V^* in the case of hydraulic permeability is larger than that for water vapor diffusion by several times. Furthermore, the value V^* increases with increase of degree of hydration for water permeation and water vapor diffusion.     

Fabrication and characterization of PEI/PVP‐based carbon hollow fiber membranes for CO2/CH4 and CO2/N2 separation

Carbon hollow fiber membranes derived from polymer blend of polyetherimide and polyvinylpyrrolidone (PVP) were extensively prepared through stabilization under air atmosphere followed by carbonization under N₂ atmosphere. The effects of the PVP compositions on the thermal behavior, structure, and gas permeation properties were investigated thoroughly by means of differential scanning calorimetry, thermogravimetric analysis, X‐ray diffraction, and pure gas permeation apparatus. The experimental results indicate that the transport mechanism of small gas molecules of N₂, CO₂, and CH₄ is dominated by the molecular sieving effect. The gas permeation properties of the prepared carbon membranes have a strong dependency on PVP composition. The carbon membranes prepared from polymer blends with 6 wt % PVP demonstrated the highest CO₂/CH₄ and CO₂/N₂ selectivities of 55.33 and 41.50, respectively. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3167–3175, 2012     

Synthesis of graft copolymeric membranes of poly(vinyl alcohol) and polyacrylamide for the pervaporation separation of water/acetic acid mixtures

Graft copolymers of poly(vinyl alcohol) (PVA) with polyacrylamide were prepared and membranes were fabricated at 48 and 93% grafting of acrylamide onto PVA. These membranes were used in the pervaporation separation of water/acetic acid mixtures at 25, 35, and 45°C. The permeation flux, separation selectivity, diffusion coefficient, and permeate concentration were determined. The highest separation selectivity of 23 for neat PVA at 25°C and the lowest value of 2.2 for 93% acrylamide‐grafted PVA membranes were observed. A permeation flux of 1.94 kg m^?2 h^?1 was found for the 93% grafted membrane at 90 mass % of water in the feed mixture. The diffusion coefficients in a water/acetic acid mixture had an effect on the membrane permselectivity. The Arrhenius equation was used to calculate the activation parameters for permeation as well as for the diffusion of water and of acetic acid. The activation energy values for the permeation flux varied from 97 to 28 kJ/mol. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 244–258, 2002