Sorption and transport studies of water in Kapton polymide. I

Sorption isotherms and diffusion coefficients of water in a 0.3-mil Kapton polyimide film at 30, 45, and 60°C are reported. The data are well described by the dual mode sorption and transport models at low activities. At high penetrant activities, clustering of water is suggested by a Zimm–Lundberg analysis of the sorption data and the fact that the diffusion coefficient for water decreases with increasing external vapor activity. The effect of temperature on the diffusion coefficients at infinite dilution and the dual mode sorption parameters k_D, b, and C are presented and discussed. The magnitude of the activation energy of the diffusion coefficient at infinite dilution, 5.4 kcal/mol, is smaller than the corresponding activation energy in more flexible chain polymers, perhaps suggesting that rather small backbone motions are associated with diffusion of water through the Kapton matrix. The predictions for the isosteric enthalpy of sorption from the dual mode model are presented and compared with the values determined from graphical analysis of the sorption isotherms performed independently without reference to the dual mode sorption model.     

Diffusion anomaly and blind pore character in carbon molecular sieve membrane

The structure of a carbon molecular sieve (CMS) membrane is characterized by the through pores and blind pores with non-linear sorption isotherms inside. Time-lag analysis was conducted for gas permeation in such a structure and mathematical formulations were derived for two cases. It is found that the pressure dependence of the time lag is dominated by the ratio of sorption affinities in the two types of pores. The experimental permeation data of pure component CO₂ and N₂ measured on a CMS membrane were used to validate the model. It is found that, for the adsorbing species (CO₂), the model is able to well describe the diffusion anomalies over a wide range of permeation pressure, while for the weakly adsorbing species (N₂), the model is inadequate to cope with the anomalies at the low end of permeation pressure.     

Effect of carbon black loading on fluoroelastomer–solvent interactions

An analysis of the molecular transport of organic liquids into fluoroelastomer membranes containing varying amounts of carbon black has been undertaken by the sorption–desorption gravimetric method. The variation in carbon black loading and temperature showed a significant effect on their transport characteristics. Diffusion coefficients were calculated from Fick's equation. Experimental sorption–desorption results were analyzed in terms of concentration profiles obtained from a solution of Fick's equation as well as by a numerical method based on the finite difference technique. Arrhenius activation parameters were estimated from the temperature-dependent diffusion and permeation data. The results of this study are discussed in terms of polymer–solvent interactions. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:815–825, 1998     

The transport properties of CO2 and CH4 for chemically modified polysulfones

The sorption and transport properties of pure CO₂ and CH₄ for a series of polysulfones were measured. The effects of molecular structure of polysulfones on transport properties were studied using chemically modified polysulfones, including TMSPSF (bisphenol‐A trimethylsilylated polysulfone), BPSF (bromobisphenol‐A polysulfone), and BTMSPSF (bromobisphenol‐A trimethylsilylated polysulfone). The effects of operating pressure on the sorption and permeation properties of polysulfones were examined. The permeation properties for a mixture of CO₂ and CH₄ were also measured and these results were compared with those obtained from the experiments of pure gases. The sorbed concentrations and permeability coefficients are well fitted to a conventional dual‐mode model. The permeability coefficients of each gas of a binary mixture are lower than those of pure gases, which shows the competition effect between each component. The permeability coefficients of polysulfones rank in the following order, TMSPSF > BTMSPSF > bisphenol‐A polysulfone (PSF) > BPSF. The effect of the substituents on chain packing was related to the gas‐permeation properties. Fractional free volume (FFV) calculations and X‐ray diffraction were used to judge chain packing. In comparison with PSF, the higher values of permeability coefficients for TMSPSF and BTMSPSF are due to higher FFV and d spacing. The lower permeability coefficients for BPSF is attributed to the strong induced dipole interchain interaction. Addition of bromo substituents to TMSPSF is also found to decrease the permeability coefficients for BTMSPSF, suggesting that the potential increase in FFV due to packing–disrupting bulky trimethylsilyl groups is overridden by the increase in cohesive energy density. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 391–400, 2000     

Mechanism of gas transport of NH3‐plasma‐treated poly(phenylene oxide) membrane

The effect of NH₃‐plasma treatment on glassy poly(phenylene oxide) (PPO) membranes on the diffusion process for carbon dioxide (CO₂), oxygen (O₂), and nitrogen (N₂) was investigated from the permeability measurements. The sorption equilibria and permeation behavior for O₂ and CO₂ in untreated PPO membranes were simulated well in terms of the dual‐mode sorption and mobility model. For O₂ transport, NH₃‐plasma treatment on PPO membrane had an influence on the diffusion process of Henry's law species, whereas for CO₂ transport, it promoted the transport of Langmuir mode, presumably through an increased Langmuir capacity constant for CO₂. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1845–1852, 2000     

Semi-empirical model of toluene transport in polyethylene membranes based on the data using a new type of apparatus for determining gas permeability, diffusivity and solubility

Solubility, diffusivity and permeability of toluene vapors in low-density polyethylene (LDPE) membranes of various thicknesses (approximately 48, 93, 138 and 187 μm) at different temperatures 30, 40 and 50 °C in the range of relative vapor pressure p/p⁰=(0.05; 0.95) were measured using new type of permeation apparatus. Moreover, special construction of the new cell enables determination of the permeant amount sorbed in the membrane in the steady state operation of vapor permeation.The simple semi-empirical model of toluene transport in a polyethylene membrane based on relation between experimentally obtained effective diffusion coefficients and concentration dependent diffusion coefficients evaluated from experiments on a new permeation apparatus was proposed. The model enables estimation of toluene fluxes, sorption in the steady state of vapor permeation and concentration profiles in a polyethylene membrane from equilibrium sorption isotherms and effective diffusion coefficients. Very good agreement between experimental and calculated values from the proposed model was obtained.     

Influence of casting and curing conditions on gas sorption and transport in polyimide films

Carbon dioxide and methane gas permeabilities, solubilities and diffusivities for poly [N, N′-(phenoxyphenyl)-pyromellitimide] films synthesized in our laboratory are contrasted with values measured for a chemically identical commercial polyimide film, Kapton H. The synthesized samples exhibited greater permeabilities, solubilities, and diffusivitles than those of the commercial sample. At the same time, the higher carbon dioxide permeability is associated with only a moderate reduction in permselectivity for the CO₂/CH₄ system. The increased sorption and transport coefficients for the synthesized sample are attributed to a lower degree of aggregation and orientation of this film as compared to the Kapton H sample.     

Interaction of CO2 with a series of detached organic films

Data obtained from gaseous sorption isotherms of CO₂ on eight organic films at three temperatures and three pressures were evaluated for evidences of chemisorption and physical adsorption. The rates of sorption, affected by both temperature and pressure, were analyzed by calculating the perameters of the Elovich equation and those of Barrer's solution of Fick's law for diffusion. Through a consideration of the Elovich parameters, the sorption coefficients, the limiting diffusion coefficients, and the activation energies for diffusion it appears that the rate of interaction of CO₂ with these organic films is primarily a diffusion-controlled process.     

A fundamental study on the removal of air pollutants (sulfur dioxide,nitrogen dioxide and carbon dioxide) by adsorption on activated carbon

Gravimetrically measured adsorption and desorption dynamics of sulfur dioxide, nitrogen dioxide and carbon dioxide on a commercial activated carbon are interpreted by a single-particle model based on three transport processes: macropore, micropore and sorbed-phase diffusion. Additional phenomena, concentration-dependent sorbed-phase diffusivity and sorbent non-isothermality, are incorporated to expand the applicability of the model. The dynamic sorption behaviour of all three gases is adequately described, without resorting to a different particle tortuosity factor for each sorbate. The value of the tortuosity factor (8) and the extracted diffusion coefficients are consistent with literature values. The affinity of the activated carbon for the adsorbates is, in increasing order, CO₂ < SO₂ < NO₂, while the extracted diffusion coefficients show the reverse trend, NO₂ < SO₂ < CO₂.     

Study of transport of pure and mixed CO2/N2 gases through polymeric membranes

The permeations of pure CO₂ and N₂ gases and a binary gas mixture of CO₂/N₂ (20/80) through poly(dimethylsiloxane) (PDMS) membrane were carried out by the new permeation apparatus. The permeation and separation behaviors were characterized in terms of transport parameters, namely, permeability, diffusion, and solubility coefficients which were precisely determined by the continuous‐flow technique. In the permeation of the pure gases, feed pressure and temperature affected the solubility coefficients of CO₂ and N₂ in opposite ways, respectively; increasing feed pressure positively affects CO₂ solubility coefficient and negatively affects N₂ solubility coefficient, whereas increasing temperature favors only N₂ sorption. In the permeation of the mixed gas, mass transport was observed to be affected mainly by the coupling in sorption, and the coupling was analyzed by a newly defined parameter permeation ratio. The coupling effects have been investigated on the permeation and separation behaviors in the permeation of the mixed gas varying temperature and feed pressure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 179–189, 2000     

Effects of uniaxial drawing and heat-treatment on gas sorption and transport in PVC

Sorption and transport measurements for various gases in rigid poly(vinyl chloride) were made following uniaxial drawing and heat treatment. The permeabilities of He, Ar, N₂, and CH₄ were found to be essentially independent of pressure in PVC while CO₂ showed a complex pressure dependence which varied with prior exposure and degassing history. Sorption isotherms were analyzed by the dual mode sorption model, and the parameters obtained were correlated with the Lennard–Jones potential-well depth of the gas. The Henry's law coefficient for CO₂ was found to be significantly larger than expected which is believed to be the result of a specific interaction with PVC. Uniaxial drawing of PVC above its glass transition caused significant reductions in gas permeabilities, of which roughly one-third is attributable to the accompanying heat treatment rather than molecular orientation per se. The physical state of the polymer was characterized by density, birefringence, and calorimetry. Changes in gas sorption and permeation behavior are discussed in terms of these results.     

Low pressure CO2 sorption in poly(vinyl cyclohexane-carboxylate)

A gravimetric method was applied to determine the solubility of CO₂ in poly(vinyl cyclohexanecarboxylate) below 1 atm. The temperatures were varied from 5 to 85°C, above and below the glass transition temperature. The sorption isotherms were concave to the pressure axis below T_g and they were tentatively analyzed by the dual-mode sorption model. Above T_g, the isotherms were linear and described by the Henry's law. Below 75°C, the diffusion coefficients of CO₂ were also obtained from the half-time of the sorption process.     

Competitive sorption of water vapor and CO2 in photocrosslinked PVCA film for a capacitive‐type humidity sensor

The sorption behavior of water vapor and CO₂ gas in photocrosslinked poly(vinyl cinnamate) (PVCA) film was examined at 30°C under atmospheric pressure. Both the water sorption isotherm and the CO₂ sorption isotherm obtained with quartz crystal microbalance (QCM) method obeyed the simple Langmuir's equation. Water vapor/CO₂ mixed‐gas sorption isotherms were also obtained. Total amount of sorbed mixed gases was clearly influenced by the partial pressure of water vapor (p_w) and CO₂ gas (p_c) in the atmosphere. A modified Langmuir's equation based on a dual‐site model was employed for predicting the competitive adsorption isotherm, and the isotherm was clearly described by the equation. The theoretically estimated amount of adsorbed water at the constant p_w decreased slightly with increasing p_c. The effect of this phenomenon on the sensitivity of the capacitive‐type relative humidity sensor was examined. As expected, the electrical capacitance of the sensor at the constant relative humidity decreased because of the coexistence of CO₂ gas. However, the influence was quite small in the CO₂ concentration range in the ordinary environment. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 401–407, 2002     

Characterization of physical aging of poly(methyl methacrylate) powders by a novel high pressure sorption technique

High pressure CO₂ sorption isotherms were measured at 35°C in previously dilated PMMA microspheres, which were subsequently aged in vacuum at 35°C. The dilation was induced by preswelling with high activity methanol vapor at 13°C, or alternatively with CO₂ at a pressure of 20 atm and a temperature of 5°C. The state of the samples during long-term aging was probed by rapid and intermittent determination of complete high pressure CO₂ isotherms. The dilation increased the sorption levels observed in the sorption experiments compared to those observed in untreated samples and, as the aging time increased, the sorption level progressively decreased. The isotherms under all conditions were well described by the dual mode sorption model. The observed time dependence of the isotherms was successfully described by confining all changes in sorption capacity to a systematic decay of the Langmuir capacity parameter. The equilibrium parameters of the model, however, were essentially unaffected by the preswelling and aging histories.     

Sorption and transport of linear and branched ketones in biaxially oriented polyethylene terephthalate

Equilibrium sorption and kinetics of acetone, methyl ethyl ketone (MEK), methyl n-propyl ketone (MnPK), and methyl i-propyl ketone (MiPK) uptake in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were determined at 35 °C and low penetrant activity. Sorption isotherms for all penetrants were well described by the dual-mode sorption model. Sorption and desorption kinetics were described either by Fickian diffusion or a two-stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients and equilibrium solubility at fixed relative pressure decreased in the following order: acetone>MEK>MnPK>MiPK. Diffusion coefficients for each penetrant increased with increasing penetrant concentration.     

Effects of Thermal Treatment on Gas Transport Through Porous Silica Membranes

Abstract

The effects of thermal treatment from 180°C to 1150°C on the gas transport properties of porous silica membranes were systematically studied for various gases. The permeance of all gases, except for CO₂, has a maximum at 800°C. The CO₂ permeance was constant from 180°C to 600°C and then decreased monotonically. Membranes thermally treated at 1150°C did not exhibit any gas permeation because of pore collapse. The gas transport behavior follows a combination of Knudsen diffusion and surface diffusion for all gases tested except for carbon dioxide. The permeation of carbon dioxide is strongly affected by capillary condensation. We propose a new transport model composed of two components; that is, the Knudsen diffusion factor, α, and the surface diffusion factor, β. A transition was observed for α and β at around 800–900°C, which is close to the strain point of the membrane. This transition treatment temperature can be correlated with the changes in gas permeance. The model allows qualitative evaluation of gas transport through porous membranes regardless of their actual microporous structures.     

Foam processing of poly(ethylene-co-vinyl acetate) rubber using supercritical carbon dioxide

Soft rubber foams like poly(ethylene-co-vinyl acetate) (EVA) are industrially applied in a broad range of products, including sports gear, insulation materials and drug delivery systems. In contrast to glassy polymers, few studies in literature concern the foaming of soft rubbers using supercritical carbon dioxide. In this study, open microporous matrices of EVA have been formed with CO₂. Prior to the foam expansion, sorption and swelling isotherms of CO₂ in EVA have been measured and the obtained isotherms have been correlated using the Sanchez-Lacombe equation of state. Additionally, a pressure-independent diffusion coefficient of CO₂ in EVA has been obtained from these experiments. The microporous foams have been formed by a pressure quench of the CO₂-swollen polymer matrix. Sorption pressure as well as temperature and decompression times appear to determine the pore size and bulk density of the foam. These parameters allow for a control of the foam structure of EVA rubbers.     

Measurement and prediction of diffusion coefficients of supercritical CO2 in molten polymers

The solubility and diffusivity of supercritical carbon dioxide (sc‐CO₂) in low‐density polyethylene (LDPE), high‐density polyethylene (HDPE), polypropylene (PP), ethylene‐ethylacrylate copolymer (EEA) and polystyrene (PS) were measured at temperatures from 150°C to 200°C and pressures up to 12 MPa by using the Magnetic Suspension Balance (MSB), a gravimetric technique for gas sorption measurements. The solubility of CO₂ in each polymer was expressed by Henry's constant. The interaction parameter between CO₂ and polymer could be obtained from the solubility data, and it was used to estimate the Pressure‐Volume‐Temperature relationship and the specific free volume of polymer/CO₂ mixtures. The diffusion coefficients were also measured by the MSB for each polymer. The resulting diffusion coefficients were correlated with the estimated free volume of polymer/CO₂ mixture. Combining Fujita's and Maeda and Paul's diffusion models, a model was newly developed in order to predict diffusion coefficients for the polymers studied. Polym. Eng. Sci. 44:1915–1924, 2004. © 2004 Society of Plastics Engineers.     

Sorption and diffusion of water vapor in poly(ethylene terephthalate) film

The sorption and diffusion of water vapor in poly(ethylene terephthalate) (PET) film were measured by applying a thermogravimetric analyzer (TG‐DTA), which customarily has been used to detect the weight loss of a sample with the increase of temperature under a given atmosphere. In this case, we detected the weight gain of PET film by sorption of water vapor under a given humidity at a constant temperature. Sorption‐rate curves were successfully obtained in spite of the low solubility of PET film and the presence of Fickian‐type curves. The solubility was better described according to the dual‐mode sorption model. The diffusion coefficients were determined in their initial slopes by the short‐time method. We found that the diffusion coefficient depended on vapor pressure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 67–74, 2000     

Sorption and diffusion of supercritical carbon dioxide into polysulfone

Sorption and diffusion of supercritical carbon dioxide (SCCO₂) into polysulfone (PSF) from 313 K and 20 MPa to 333 K and 40 MPa were investigated in this study. A simple gravimetric method was used to measure the mass gain of SCCO₂ in PSF, and the Fick's diffusion model was applied to describe the desorption process. The sorption amount, the sorption diffusivity under supercritical states, and the desorption diffusivity at ambient conditions are presented. Comparisons of the sorption amounts and diffusivities of CO₂ for polymers of polycarbonate and PSF are discussed according to the interactions between gas and polymers. The morphology change and plasticization effect attributed to gas sorption in PSF were studied. Effects of glass‐transition temperature and yielding stress for PSF and other polymers were used to describe the difference in their diffusivities for the sorption and desorption processes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 474–482, 2004