Infrared spectroscopic studies of CO2 sorbed in glassy and rubbery polymeric membranes

Infrared spectra of CO₂ sorbed in rubbery and glassy polymers were measured to examine the relationships between the spectroscopic data and physical properties of the polymeric membranes. The “V-shape” tendency in the plot of W₁ [i.e., half-width of CO₂ peak sorbed in the membranes] vs glass-transition temperature (T_g) is observed, and has exactly the same tendency that is widely known from the plot of log D (diffusion coefficient) vs T_g. It is suggested that the membranes having a wider W₁ give a faster diffusion coefficient, since W₁ is inversely related to the moment of inertia of CO₂ in the membranes. Two distinct peaks of CO₂ were not observed in the infrared spectra of CO₂ sorbed in the glassy polymers. This suggests that the states of CO₂ in the Henry mode and Langmuir mode in the glassy polymers are similar in the spectroscopic measurements. © 1994 John Wiley & Sons, Inc.     

Hydrocarbon separations by glassy polymer membranes

Polymers are unarguably the most broadly used membrane materials for molecular separations and beyond. Motivated by the commercial success of membrane-based desalination and permanent gas separations, glassy polymer membranes are increasingly being studied for hydrocarbon separations. They represent a class of challenging yet economically impactful bulk separations extensively practiced in the refining and petrochemical industry. This review discusses recent developments in membrane-based hydrocarbon separations using glassy polymer membranes relying on the sorption-diffusion mechanism. Hydrocarbon separations by both diffusion-selective and sorption-selective glassy polymer membranes are considered. Opinions on the likelihoods of large-scale implementation are provided for selected hydrocarbon pairs. Finally, a discussion of the challenges and outlook of glassy polymer membrane-based hydrocarbon separations is presented.     

Dissolution of rubbery and glassy polymers

A mathematical model is formulated for solvent dissolution of rubbery and glassy polymers. An exact solution to the problem is derived for the constant diffusivity case, and a weighted residual solution is developed for the case of a concentration-dependent diffusion coefficient. The solution is used to calculate concentration profiles, dissolution curves, dissolution half-times, and pseudointerface positions at various times. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2607–2614, 1998     

Permeation of binary gas mixture through glassy polymer membranes with concentration-dependent diffusivities

An analytical solution has been obtained for the modified dual-mode mobility model for a single gas proposed by Zhou and Stern and extended to a binary gas mixture to describe the pressure dependence of mean permeability coefficients for CO₂ and CH₄ mixtures in homogeneous cellulose triacetate membranes. The permeabilities calculated from the model fitted the corresponding experimental results quite well. Permeation experiments for equimolar CO₂ and CH₄ mixture in a homogeneous membrane of methyl methacrylate and n-sbutyl acrylate copolymer were performed along with sorption experiments for pure CO₂ and CH₄ to test the applicability of the model. The experimental permeabilities were close to those calculated from the model.     

Gas permeability in rubbery polyphosphazene membranes

The synthesis, characterization, and gas permeability of 10 new polyphosphazenes has been studied. Additionally, the first gas permeation data has been collected on hydrolytically unstable poly[bis-(chloro)phosphazene]. Gases used in this study include CO₂, CH₄, O₂, N₂, H₂, and Ar. CO₂ was the most permeable gas through any of the phosphazenes and a direct correlation between the T_g of the polymer and CO₂ transport was noted with permeability increasing with decreasing polymer T_g. To a lesser degree, permeability of all the other gases studied also yielded increases with decreasing polymer T_g. The trend observed for these new polymers was further supported by published data for other phosphazenes. Furthermore, permeability data for all gases were found to correlate to the gas condensability and the gas critical pressures, except for hydrogen, suggesting that the nature of the gas is also a significant factor for permeation through rubbery phosphazene membranes. Ideal separation factors () for the CO₂/H₂ and CO₂/CH₄ gas pairs were calculated. For CO₂/CH₄, no increase in was observed with decreasing T_g, however increases in were noted for the CO₂/H₂ pair.     

Extension and compression behavior of rubbery polymer networks

The elastic behavior of different types of rubbery polymers (natural rubber, polybutadiene, silicone, and polyisoprene) networks at various degrees of vulcanization and swelling was examined in extension and compression. The data are represented by the Mooney-Rivlin equation. In compression, although C₂ is zero, C₁ decreases strongly with increasing swelling to a limiting value which, in some cases, may be correlated with the value of C₁ found in extension and hence related to the theoretical modulus for highly swollen networks. A possible explanation is presented in terms of supramolecular order in the amorphous materials.     

Permeation of gases through metallized polymer membranes

The permeation of He, H₂, CO₂, Ar, N₂ and Kr at 50°C through polyethyleneterephthalate, PET, membranes metallized with Pd, Ni and Cu was studied. It was found that metallizing a PET membrane changed its permeability for the gases studied, and that the permeability for H₂ varied slightly with differing H₂ pressure. In the range of 0-50°C the temperature dependence of the permeability for He and H₂ was determined. The results obtained were interpreted by assuming that the permeation of all gases, including H₂, through the metal layers of the membranes takes place by diffusion through fine defects which exist in their structure and, moreover, that H₂ also permeates through the Pd and Ni layers themselves. An important point is that by this method an increase of up to an order of magnitude of the membrane selectivity for H₂ was obtained     

Diffusion in glassy polymers

Two versions of the free‐volume theory of diffusion are compared by considering differences in the predictions for the activation energy for the diffusion process. A number of data‐theory comparisons for free‐volume theory are discussed and evaluated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 785–788, 2003     

Thermodynamic interactions in rubbery polymer/gas systems

The Flory–Huggins interaction parameters χ for 23 gases (He, Ne, Ar, Kr, Xe, H₂, N₂, O₂, N₂O, CO₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, 1,3-C₄H₆, four C₄H₈'s, n-C₄H₁₀, iso-C₄H₁₀, and n-C₅H₁₂) in five rubbery polymers (1,2-polybutadiene (PB), poly(ethylene-co-vinyl acetate)) (EVAc), polyethylene (PE), polypropylene (PP), and poly(dimethyl siloxane) (PDMS) were determined from either literature data on Henry's law coefficient and partial molar volume or those on sorptive dilation for each polymer/gas system. Values of χ for the gases increased in the order of PDMS < PP ≡ PB < EVAc ≡ PE. Among the gases except He and H₂ whose χ values are not reliable, Ne and Xe have respectively the highest and the lowest values of χ for the polyolefins. The χ values of the hydrocarbons were compared together with previously reported χ values of n-alkanes C₃-C₁₀. The dependencies of χ upon concentration and temperature were discussed on the basis of the literature data. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1049–1053, 1997     

Viscoelasticity of low molecular weight polystyrene. Separation of rubbery and glassy components

The complex shear modulus was measured for four low molecular‐weight polystyrenes (M_w = 10,500, 5970, 2630, and 1050) near and above the glass transition temperature. For the lowest molecular weight sample, the method of reduced variables, the time–temperature superposition principle, was applicable, while it was not applicable for the higher M samples. For these higher M samples, it was assumed that the complex modulus is composed of two components (R and G components). The R component was estimated by subtracting the G component, which was assumed to be the same as the modulus of the lowest molecular weight sample. The time–temperature superposition principle was applicable to each of the R and G components, and the shift factors were different from each other. The contribution of the R component to the total complex modulus decreased with decreasing M. Anomalous temperature dependence of the steady‐state compliance for low M polymers as Plazek reported could be attributed to difference in temperature dependence of the two components. The estimated complex modulus for the R component was in accord with that calculated by spring‐bead model theory. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 389–397, 1999     

Diffusion of water vapor through a hydrophilic polymer film

Multifilm techniques have been used to measure the diffusion coefficient of water vapor in cellophane, and the data have been compared with the integral diffusion coefficient obtained in previous work with single films. The multifilm techniques lead to a much sharper resolution of the effect of concentration on diffusion, and the maximum integral diffusion coefficient. The diffusion coefficient for water vapor in cellophane peaks at a moisture content corresponding to about 70% R. H., which is presumably the “critical concentration” discussed in previous work on the thermodynamics of water sorption by cellophane.     

Diffusion of radioactively tagged 1,1-diphenylethane and n-hexadecane through rubbery polymers: Dependence on temperature and dilution with solvent

The diffusion of 1,1-diphenylethane in trace amounts through eight rubbery polymers has been studied by radioactive tagging of this penetrant with ¹⁴C. For several polymers, the dependence on temperature and on dilution (swelling) by untagged diphenylethane was investigated. In the diluted systems, tagged n-hexadecane was also used as a trace penetrant. The temperature and concentration dependences were interpreted rather successfully in terms of the free volume. In comparing different polymers, with a 4000-fold range of diffusion coefficients, the translatory friction coefficient of 1,1-diphenylethane was found to be proportional to that of n-hexadecane to the power 1.06. This is interpreted qualitatively by the free volume concept to indicate a slightly less efficient mobility mechanism for the diphenylethane.     

Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes

This study examines the effect of treatment of defective glassy polymer membranes with a variety of vapors and liquids which have varying solvency power for the polymer. The pure-gas oxygen/nitrogen selectivities of defective, asymmetric membranes are shown to be permanently increased, in special cases, by treatment with certain solvents which have adequate solvency power to cause a critical level of swelling in the membrane skin layer. Three distinct types of membranes have been treated; asymmetric polysulfone membranes formed by dry/wet phase inversion, spin-coated poly(phenylene oxide)-ceramic composite membranes and solution-deposited polyimide-ceramic composite membranes. While the detailed fundamental processes controlling the elimination of surface defects are complex, our results suggest that plasticization of the selective skin layer, coupled with surface-tension driven cohesive forces are likely to be the key factors at play.     

Steady-state permeation of carbon dioxide through a miscible,glassy polymer blend

Steady-state permeation measurements are reported for carbon dioxide (CO₂) through quenched, amorphous films of a miscible blend of poly(butylene terephthalate) (PBT) and a random copolyester of bisphenol-A and iso/terephthalate acids (PAr). Permeabilities were determined at 35°C on blends with up to 60 wt % PBT and for CO₂ pressures up to 300 psi (2.06 MPa). At a fixed blend composition, the permeability, P̄, decays with driving pressure, p, as described by dual-mode models for gas transport in glassy polymers. From regression fits of the data to dual-mode model predictions for P̄(p), high-and low-pressure limiting permeabilities are determined. These decrease with PBT content in a manner indicating strong, favorable energetic interactions between the PBT and PAr components in the blend. © 1996 John Wiley & Sons, Inc.     

Mixed-matrix membranes comprising graphene-oxide nanosheets for CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation: A comparison between glassy and rubbery polymer matrices

The effect of graphene oxide (GO) nanosheets on the CO₂/CH₄ separation performance of a rubbery (poly(dimethylsiloxane), PDMS) as well as a glassy (polyetherimide, PEI) polymer is studied. Interfacial interactions between the nanosheets and both polymers are revealed by FTIR and SEM. The results of gas permeation through the membranes demonstrate that GO nanosheets enhance CO₂/CH₄ diffusivityselectivity of PEI and CO₂/CH₄ solubility-selectivities of the PEI and PDMS polymers, while diminish CO₂/CH₄ diffusivity-selectivity of PDMS. Furthermore, the possibility of overcoming the common tradeoff between CO₂ permeability and CO₂/CH₄ selectivity of rubbery and glassy polymers by incorporating very low amounts of graphene oxide nanosheets is addressed. In other words, at 0.25 wt % GO loading, the PEI membrane shows simultaneous enhancement of CO₂ permeability (16%) and CO₂/CH₄ selectivity (59%). Also, for the PDMS membrane simultaneous enhancement of CO₂ permeability (29%) and CO₂/CH₄ selectivity (112%) is occurred at 0.5 wt % GO loading. Finally, the capability of the well known Nielsen model to predict the gas permeability behavior of the nanocomposites is investigated.     

Diffusion in glassy polymers. III

The diffusion studies of several solvents in epoxy polymer reported by Kewi and Zupko in Part I of this series are explained with the solution obtained from the generalized diffusion equation which includes the internal stress contribution. The rate of permeation of a penetrant through a polymer film and the time lag needed to reach steady state are also given for the generalized diffusion equation.