Characterization of water vapor transport in glassy polyacrylonitrile by combined permeation and sorption techniques

Diffusion time lags, steady state permeabilities and sorption/desorption kinetics are reported for water vapor in biaxially oriented, solvent cast polyacrylonitrile (PAN) films. A wide range of vapor activities was studied at 15°C, 30°C, and 45°C. The transient and steady state permeation behavior at low and intermediate upstream vapor activities suggests that Fickian transport occurs under most of the conditions studied. Specifically, time lags predicted by Fick's law using the concentration-dependent diffusion coefficient derived from steady state permeation measurements agree reasonably well with experimentally measured values in most cases. p]Integral sorption/desorption kinetics at low and intermediate vapor activities also appear to be Fickian with a concentration-dependent diffusion coefficient. The form of the concentration dependency, evaluated from the “long time” solution of the diffusion equation for sorption experiments, is consistent with the form established for the diffusion coefficient from the steady-state permeation data. The diffusion coefficient exhibits a maximum near the concentration at which clustering is initiated. Presumably, the effective diffusion coefficient of water increases initially due to plasticizing or dual mode sorption effects associated with gap filling in the glassy matrix. As clustering becomes significant, the effective mobility of water is substantially reduced; therefore, the diffusion coefficient decreases at higher activities as clustering becomes the dominant mode of sorption. p]A tendency of the “early time” sorption/desorption kinetic data to exhibit concavity to the square root time axis at high activities suggests that time-dependent reductions in the diffusion coefficient may be occurring. Such reductions could be related to the kinetics of cluster formation at the higher vapor activities during sorption and to slow polymer consolidation during desorption. Any such non-Fickian effects, related to chain segment relaxations occurring over time scales similar to those of a diffusional jump, appear to be of importance only at short times. The short time nature of any such processes is suggested by the fact that diffusion coefficients evaluated from the “long time” solution to the diffusion equation for sorption are consistent with coefficients evaluated from steady state permeation data, in which case all time-dependent relaxation phenomena should be absent.     

Kinetics and equilibria of sulfur dioxide sorption in kapton polyimide

The kinetics and equilibria of SO₂ sorption in Kapton polyimide film have been measured at 25°C for pressures up to 58 cm Hg using a quartz spring balance. The observed equilibria are described well by the dual-mode model for sorption of penetrants in glassy polymers. Observed hysteresis in sorption-desorption cycling indicates that the diffusivity of SO₂ increases markedly with increasing local penetrant concentration in the polymer. The concentration dependence of the effective average diffusion coefficient is described well by an expression from dual-mode theory. The Langmuir component of the sorption population has an extremely low mobility compared to that of the Henry's law component; in fact, the so-called “total immobilization” limiting case, which assigns a zero diffusion coefficient to this component, appears to be satisfactory under the conditions studied. These results, coupled with published results for other penetrants, suggest that the degree of immobilization of the Langmuir population depends to a great extent on the condensability and/or the molecular size of the penetrant. Predictions of the SO₂ diffusion time lag and permeability in Kapton as functions of the upstream SO₂ pressure are presented and discussed in terms of the dual-mode theory.     

“Second component” effects in sorption and permeation of gases in glassy polymers

Data for CO² permeability through Kapton polyimide at 60°C are reported for upstream pressures up to 240 psia (16.33 atm) in the presence and absence of water vapor in the feed. The carbon dioxide flux was depressed by the presence of the water vapor. This phenomenon is analyzed in terms of the dual mode sorption and transport models. Together with other recent sorption and permeation data, this study suggests that competition of mixed penetrants for sorption sites and transport pathways associated with unrelaxed volume in glassy polymers is a general feature of gas/glassy polymer systems. The permselectivity of a membrane to a mixture of penetrants is strongly related to its ability to maintain a size and shape differentiating matrix, that is, to remain essentially unplasticized under operating conditions. Under such conditions, competition among penetrants for excess volume will be a generally important consideration for modeling gas permeation in permselective membranes.     

Kinetics of water sorption in flax and PET fibers

Composite materials are usually reinforced by synthetic fibers as carbon, glass etc…. Because of their good mechanical properties and low density, natural fibers are being considered more preferentially as reinforcement. The application of natural fibers as reinforcements in composite materials requires a strong adhesion between the fiber and the matrix. The poor resistance towards water absorption is one of the drawbacks of natural fibers which makes it more important to understand the dynamic properties of penetration of water molecules through these heterogeneous materials. Water vapour sorption kinetics in natural flax fibers have been performed at 25 °C by using an electronic microbalance (IGA, Hiden). By using the Fickian model for a complete cylinder water diffusion coefficients have been determined and calculated at short times (first half-sorption) and long times (second half-sorption) of kinetic curve and for different water activities. The values obtained for D₁ and D₂ are rather similar on the all range of water activity. Generally, water diffusivity increases and then decreases with water activity. The increase of diffusivity at low water activities may be explained on the basis of the dual mode sorption (Langmuir and Henry sorption’s combination) whereas the decrease for the higher activities can be attributed to the immobilization of sorbed water molecules due to the water clustering.     

Natural gas permeation in polyimide membranes

Polyimide membranes derived from 6FDA-DAM:DABA and 6FDA-6FpDA:DABA copolymers have been used to separate 50/50 CO₂/CH₄ mixtures and multicomponent synthetic natural gas mixtures at 35 °C and feed pressures up to 55 atm. For 6FDA-DAM:DABA 2:1 membranes the effects of thermal annealing and covalent crosslinking are decoupled with respect to effects on permeabilities and selectivity. Crosslinking at 295 °C with 1,4-butylene glycol and 1,4-cyclohexanedimethanol increases CO₂ permeabilities by factors of 4.1 and 2.4, respectively, at 20 atm feed pressure, without a loss in selectivity, relative to crosslinking at 220 °C. Thermal annealing and crosslinking also reduce CO₂ plasticization effects. Crosslinking of DABA-containing copolymers, therefore, can produce membranes with tunable transport properties that offer significantly higher performance with better plasticization-resistance than that reported in the literature for the commercial polymers Matrimid^® and cellulose acetate for CO₂ removal from natural gas mixtures. Separation of complex mixtures containing CO₂, CH₄, C₂H₆, C₃H₈, and C₄H₁₀ or toluene results in a significant decrease of the CO₂ permeability, but only a moderate decrease in the CO₂/CH₄ selectivity.     

Kinetics of saturated water vapor sorption on soot particles

In order to study the condensation activity of carbon-containing aerosols, the kinetics of sorption of saturated water vapors was investigated on model soot samples modified by the deposition of organic and inorganic substances corresponding to different classes of compounds contained in solid state products of natural fuel combustion. The values of limiting water sorption were determined and rate constants were calculated. It was established that the limiting sorption on graphitized thermal soot (GTS) modified by acids and salts (e.g., phenol, benzoic, phthalic, trimellitic, and sulfuric acids; sodium benzoate; and ammonium sulfate) per modifier molecule rose by two orders of magnitude, relative to the initial carbon matrix, and ranged from ≈2 to 200 molecules. It was found that hydrophilic centers covalently bound to the surface of furnace black (FB) and its oxidized form are shielded to a considerable degree as a result of modification by hydrophobic substances (hexadecane, octacosane, anthracene, and stearic acid). We conclude that hexadecane modification of GTS containing hydrophilic centers not bound to its surface (sulfuric acid and ammonium sulfate molecules) have no effect on the sorption of saturated water vapors.     

Evaluation of gas sorption parameters and prediction of sorption isotherms in glassy polymers

A model of continuous‐site distribution for gas sorption in glassy polymers is examined with sorption data of CO₂ and Ar in polycarbonate. A procedure is presented for determining from a measured isotherm the number of sorption sites in a polymer, an important parameter that previously had to be assumed. With this parameter value and solubility data obtained at zero pressure, the model can reasonably predict sorption isotherms of CO₂ in glassy polycarbonate for a wide temperature range. The number of sorption sites and the average site volume evaluated from CO₂ sorption isotherms are employed for the prediction of Ar sorption isotherms with zero‐pressure solubility data and the independently measured partial molar volume of Ar. A reasonable fit to the measured isotherms of Ar is achieved. With the proposed procedure, the continuous‐site model shows several advantages over the conventional dual‐mode sorption model. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 883–888, 2000     

Water vapor sorption and diffusion in glassy polymers

By establishing relationships between polymer structure and gas permeation behavior, significant advances have been made in designing materials for membrane separation of gas mixtures. However, the situation is not so well understood when water vapor is one of the components since water molecules may interact with the polymer (plasticization) or each other (clustering) in ways that complicate the structure-property relationships. In addition, accurate measurement of water sorption, diffusion, and permeation is more complicated than for gases because of the unique hydrogen bonding capability of water, e.g., its tendency to strongly adsorb on high-energy surfaces and high heat of vaporization. A progress report on a broad program to understand water sorption and diffusion in glassy polymers that may be of interest for membrane applications is outlined; specific strategies include studies of structurally related polymers and miscible blends of hydrophobic/hydrophilic polymer pairs.     

Water sorption and activation energy in polyimide thin films

The water sorption behavior and the activation energy were investigated for various chemical structure polyimide thin films; BPDA‐PDA, BPDA‐ODA, PMDA‐ODA, and 6FDA‐ODA. The activation energy for the water diffusion varied in the range of 5.53 to 9.27kcal/mol, and was in the increasing order: BPDA‐PDA < BPDA‐ODA < PMDA‐ODA < 6FDA‐ODA. BPDA‐PDA and BPDA‐ODA polyimide films showed relatively well‐ordered morphological structure, which results in relatively low diffusion coefficient and high activation energy. It was found that the diffusion coefficient and the activation energy are significantly related to the in‐plane orientation, crystallinity, and packing order in the polyimide thin films. The morphological structure was predominant factors for the water diffusion coefficient and activation energy in the polyimide thin films. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2714–2720, 2000     

Dual-mode sorption kinetics of gases in glassy polymers

The transport of gases in many glassy polymers can be described satisfactorily by means of a “dual-mode sorption” model. The transport behavior observed with a given gas/polymer system can be characterized by the model parameters, which are obtained from solubility measurements in conjunction with absorption/desorption or permeability measurements. The present study discusses the inverse problem, namely, the prediction of the absorption/desorption behavior of a gas in a glassy polymer from a specified set of dual-mode sorption parameters. Satisfactory agreement is obtained between reported absorption rates of sulfur dioxide in glassy polycarbonate and of water vapor in Kapton® ?

1 ?Trademark of E. I. du Pont de Nemours & Co.

and the rates predicted by the dual-mode sorption model. This study also confirms the consistency of the model.     

Dual-mode sorption and transport of sulfur dioxide in kapton polyimide

The kinetics and equilibria of SO₂ sorption in Kapton polyimide film have been studied at temperatures from 25 to 55°C and equilibrium sorption pressures up to 0.76 atm. The data are described well by the dual-mode model of sorption and transport in glassy polymers. The assumption of “partial immobilization” is required to correlate the transport data: the mobility of the Langmuir component of the sorbed population relative to the Henry's-law component is close to zero at 25°C, and increases to roughly 5% of the Henry's-law component mobility at 55°C. The heat of sorption is anomalously low, suggesting the presence of residual solvent in the film, a suggestion confirmed by annealing studies. However, the study also shows that partial removal of the residual has a relatively minor effect on the measured SO₂ sorption level and transport rates at 55°C.     

Water vapor sorption in naphthalenic sulfonated polyimide membranes

The water vapor uptake of sulfonated polyimides (SP) was investigated by electronic microbalance (IGA, Hiden) from 15 to 55°C. The sigmoïdal isotherms obtained (BET II type) are considered as dual sorption (concave part) plus clustering (convex part) and are fitted with good agreement by Park’s equation. Zimm–Lundberg’s method is used to study the clustering process of water molecules: limit clustering activity, a^*, and the number of molecules per cluster are estimated.

To obtain a better understanding of polymer structure and isotherm analysis, H⁺ (counter-ions of sulfonic groups) were replaced by ions with a smaller hydration shell (Cs⁺ and EDAH⁺). Comparison of the three isotherms shows no significant difference in the water affinity of the cations. This is attributed to a partial control of the sorption by microcavities existing in the membrane.     

Kinetics of radioeuropium sorption on Gorleben sand from aqueous solutions and ground water

Kinetics of Eu sorption on sandy sediment from Gorleben, Germany, containing humic substances, was studied by radiotracer method in batch experiments at very low europium concentration (3.4^.10^-8 mol/l), with the aim to find kinetic parameters suitable for modeling Eu migration in bed of the sediment and to elucidate the mechanism of the sorption. Experiments were evaluated using a new simplified method taking into account simultaneous sorption/desorption of Eu on the walls of sorption vessel. Five kinetic functions were tested, of which that describing diffusion in inert surface layer of sorbent proved most suitable. The effects of pH (3.0-8.8) by addition of Aldrich humic acid (10 mg/l), addition of hydrogencarbonates (5^.10^-3 mol/l) and preequilibration of the sediment with solution or of Eu with solution were examined. From the results it has been concluded that the kinetics and mechanism of the sorption strongly depend on pH. At pH 4.8 Eu is sorbed mainly as humate complex from the solution of humic acid. The addition of humic acid accelerates the sorption. Carbonate complexes of Eu are the probable forms sorbed at pH 8.8. The presence of humic substances in the slightly alkaline solutions suppresses the rate of the sorption due to slow dissociation of Eu-humate complexes.     

Gas permeation and sorption studies on stereoregular polynorbornene

Advances in the field of membrane gas separation over the past decade have encouraged the search for even better polymers and membranes for gas separation. They also have motivated studies on the relationship between the structure and the gas transport properties of different classes of polymers. Interest has recently been demonstrated in the literature in norbornene polymers with different side groups. These polymers can easily be prepared via ring-opening metathesis polymerization (ROMP) of norbornene (bicyclo[2.2.1]hept-2-ene) derivatives. So far, information on the microstructure of the polymers is scarce, so little is known about the influence of stereochemistry and tacticity of the polymer on gas separation properties. In this work, we present gas permeability and selectivity data for stereoregular unsubstituted polynorbornene. It will be shown that a high order of tacticity has a positive effect on both permeation and separation results. © 1997 John Wiley & Sons, Inc.     

Model for sorption of mixed gases in glassy polymers

A model is presented for analysis of the sorption of mixed gases in glassy polymers at concentrations below which significant plasticization occurs. The well-known dual-mode sorption model comprised of a Henry's law term and a Langmuir isotherm term, which has been used extensively for interpretation of single-component gas sorption data, forms the basis for the analysis of binary mixtures discussed here. Measurements using pure gases provide dual mode parameters which can then be used to predict the resultant sorption isotherms for binary mixtures of any of the pure gases. The proposed analysis is based upon recognition that the Langmuir component of the overall sorption concentration should be governed by competition between the two penetrants for the fixed unrelaxed volume in the polymer, which is believed to be the locus of the Langmuir capacity. This effect may result in a significant depression of the measured sorption of similar penetrants competing for the limited Langmuir capacity. A numerical example is considered which illustrates the range of behavior expected for CO₂ and CH₄ in polycarbonate. Deviations from the theoretical predictions of the simple dual-mode model for binary systems are discussed in terms of plasticizing effects on the Henry's law constant and the Langmuir affinity constant. The analyses proposed here are of direct and critical interest to the applied problems of migration of trace contaminants in glassy polymers and analysis of barrier packaging for foods since all of these applied problems involve mixed-penetrant sorption. Specifically, it is predicted that the presence of residual monomers or solvents in glassy polymers can produce both anomolously low Langmuir sorption affinity constants and sorption enthalpies compared with the residual-free case.     

Kinetics of the sorption of metal ions from sea water on clinoptilolite

Based on a study of the sorption kinetics of metal ions from sea water onto granular clinoptilolite and powdered clinoptilolite incorporated into a grain of a highly permeable inert support it has been shown that the rate of the sorption processes is limited principally by diffusion in the microgranules of zeolite. For the Tedzami clinoptilolite studied the characteristic dimensions of the microgranules are 15–20 m and are commensurate with the dimensions of microcrystals determined independently.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 263–267, February, 1990.We thank E. V. Venetsianov, L. S. Kamenetskii and M. V. Veksler for assistance in the investigation.     

Kinetics of imidization and crosslinking in PMR polyimide resin

Infrared spectroscopy and differential scanning calorimetry were used to study the imidization and crosslinking kinetics of norbornenyl-capped addition type of polyimide resins (designated PMR for polymerization of monomer reactants). The spectral and thermal analyses were performed on resin specimens that had been isothermally aged at temperatures appropriate for imidization (120–204°C) and crosslinking (275–325°C). Imidization occurs rapidly (about 10^?2 min^?1) at short times, but, at times longer than about 0.5 hr, the rate decreases significantly (about 10^?4 min^?1). The crosslinking reaction exhibits first-order kinetics during the initial part of the reaction, and its rate appears to be limited by the reversion of the norbornenyl Diels-Alder adduct. Both the first-order dependence and the activation energy (about 44 kcal) are consistent with this interpretation. The total heat evolved per mole of end cap during crosslinking shows an inverse dependence on the molecular weight of the imido prepolymers. This reflects the effect of end-cap dilution and decreased mobility of the larger oligomers.     

Permeation and sorption for oxygen and nitrogen into polyimide membranes

The permeation rate curves and sorption rate curves of oxygen and nitrogen below 1.3 atm were measured for seven polyimides of which chemical structures were systematically changed. These rate curves were applied Fickian model curves. The Fick's law was found to hold from the pressure independencies of diffusion coefficients for both the experiments. The solubility was better described according to dual-mode sorption model rather than Henry-s law from the sorption experiments. The diffusivities of both the gases were correlated with packing density (reciprocal of specific free volume) of the polymer, except two polyimides. The packing density of these two polymers could not be successfully calculated from Bondi's method. However, all the diffusion coefficients decreased linearly with an increase in the cohesive energy density (CED), which was calculated by the group contribution method of van Krevelen. These results suggest that the gas diffusions in polyimides better correlate with CED than with the packing density. Therefore, the cohesive energy density may be considered as a more reliable indication of the efficient gas separation. © 1995 John Wiley & Sons, Inc.     

Kinetics of water vapor sorption on porous glass as a method of porometry

Equilibrium low temperature nitrogen adsorption and a new method based on the kinetics of water vapor sorption at room temperature are used for a comparative study of pore morphology of high-silica porous glass. The values of the total pore volume, the specific surfaces, the effective mesopore diameters, and the fraction of each mode in the total mesopore volume are established using both techniques. It is found that the rate of water vapor sorption at constant pressure grows in inverse ratio to the diameter of cylindrical menisci in pores. The time dependence of the volume of adsorbed water is linear, while the characteristic time of filling for each pore mode varies directly with the square of the effective pore mode diameter. The proposed new kinetic porometry technique based on analyzing water vapor adsorption kinetics at room temperature is quite simple as it requires no complicated vacuum equipment and allows the simultaneous examination of large numbers of porous material samples.