M. Métayer

water vapor sorption in naphthalenic sulfonated polyimide membranes

Abstract The water vapor uptake of sulfonated polyimides (SP) was investigated by electronic microbalance (IGA, Hiden) from 15 to 55°C. The sigmoidal 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.

Sorption and diffusion behaviors of water in Nation 117 membranes with different counter ions

Abstract The sorption kinetic and equilibrium data obtained at different water activities allowed us to determine the water sorption isotherms and mean diffusion coefficient in the Nafion® membrane. The water sorption isotherm is reminiscent of systems in which there is water clustering at high water activities, with a marked Langmuir sorption at low water activity for the acid form of the membrane. The variation of the diffusion coefficient with the water activity showed a maximum value whose coordinates depend on the counter-cation nature. The smaller the counter-cation, i.e. the larger the counter-cation hydration energy, the higher the diffusion coefficient and the lower the water activity at the maximum point.

Correlation between transport parameters of ion-exchange membranes

Abstract A deduction of a relation between transport coefficients of ion-exchange membranes is considered by comparison the Kedem–Katchalsky and Onsager forms of transport equations in the framework of irreversible thermodynamics. This relation is analysed by using the experimental data of Narebska et al., Berezina et al., and the results of other authors. Transport equations generalising the Nernst–Planck equation with the coefficients determined directly from the practical transport characteristics are obtained either taking into account or disregarding the above mentioned relation. The Nernst–Einstein relation and its generalizations are discussed.

Diffusion of water through various polymer films: a new high performance method of characterization

Abstract Diffusion of water in polymer films has been characterized by using a new high performance and reproducible method based on permeation measurements. With this method, which uses a fast response water-specific sensor based on dew point measurement, it is possible to determine, from the transient step, the time lag water diffusion coefficient D L and, when the steady state is reached, a mean water permeability coefficient P . In this work we have emphasized the water concentration dependence of diffusion coefficient D . After calibration, the technique yields accurate values of P , consistent with literature data. By testing various polymers, different behaviours with respect to water have been observed, particularly with low density polyethylene (LDPE) which shows significant hydrophobic properties.

Molecular recognition of carbohydrates by a resorcinarene. Selective transport of alditols through a supported liquid membrane.

A supported liquid membrane (SLM) containing a resorcinarene carrier has been used for the selective transport of erythritol, threitol, ribitol and xylitol from concentrated (1.0-0.01 M) aqueous solutions. The membrane is made of a microporous polytetrafluoroethylene film impregnated with a 0.01 M solution of the carrier in CCl4. The permeabilities of the SLM for all alditols were calculated. On the basis of the flux dependence on the initial concentrations of carrier and alditol, the rate-determining step in the transport mechanism is shown to be the migration of the 1:1 carrier-carbohydrate complex in the immobilized organic phase. The flux of sugar is related to the initial concentration of alditol in the feed phase by a saturation law, which allowed the determination of the apparent diffusion coefficients and the stability constants of the resorcinarene complexes of alditols formed in the liquid membrane.

Kinetics of water vapor sorption in sulfonated polyimide membranes

Abstract Water vapour sorption kinetics in a H+ form naphtalenic sulfonated polyimide membrane at 25°C has been investigated by using an electronic microbalance (IGA, Hiden). An effective diffusion coefficient Dexp has been calculated from kinetic data by using the Fickian model for the final part of the sorption. The variation of Dexp with the equilibrium water activity (0

Study of transport of small molecules through ethylene-co-vinyl acetate copolymers films. Part A: Water molecules

Abstract The transport of liquid water through poly(ethylene-co-vinyl acetate) (EVA) films with 4.5%, 19%, 33%, 50% and 70% w/w VA contents, is studied by permeation measurements. A low density polyethylene LDPE, used as reference, is also studied. The diffusion of water is determined from transient permeation. For a better understanding of the role played by the structure and the surface of these copolymers whose crystallinity varies with the presence of VA groups, Differential Scanning Calorimetry and contact angle (surface free energy) measurements are performed. It is found for LDPE and 4.5% w/w VA membranes that the water diffusion coefficient decreases with increasing water concentration, while for 19% VA a constant diffusion coefficient is obtained. For membranes with 33, 50 and 70% w/w VA a diffusion coefficient which increases exponentially with the local water concentration is obtained. These results are explained by the formation at low VA content of small water clusters, clusters evidenced by means of IR spectrometry. On the other hand, the plasticization effect (concentration-dependent diffusion coefficient) is observed for VA content greater than 19% wt. VA is explained by the increase of the amorphous phase ratio, which allows an increase of the number of interactions between polar groups (carboxyl units in the statistical copolymer) and water molecules

Study of transport of small molecules through ethylene-co-vinyl acetate copolymers films. Part B: CO2 and O2 gases

Abstract The transport of gases (pure oxygen and carbon dioxide) through poly(ethylene-co-vinyl acetate) (EVA) of different VA contents, was studied by permeation measurements. Permeabilities, diffusivities, and solubilities were determined for EVA films containing 19, 33, 50 and 70% (w/w) VA. A low density polyethylene LDPE (used as reference with 0% VA) was also studied. The diffusion of oxygen was determined from transient permeation fluxes and the diffusion of carbon dioxide from time-lag permeation. In the case of gas permeation, whatever the VA content of the polymer used, the experimental transient fluxes are characterized by a constant diffusion coefficient for both O2 and CO2. In terms of gas permeability, compared with water permeability (Part A), these copolymers are characterized by very low coefficients, especially for oxygen. Assuming that D is constant, the O2 and CO2 solubility coefficient was then deduced and its value is shown to increase proportionally with the VA content in these rubbery copolymers (whose quantity of crystalline phases decreases and become nil for 70% VA). The H2O/CO2 and H2O/O2 ideal selectivity α increases with the VA content, in agreement with the increase in average polarity of the copolymer.

Diffusion and permeation of water through unsaturated polyester resins—influence of resin curing

Abstract The diffusion and permeation properties of liquid water through an isophthalic-maleic acid-propanediol-based copolyester resin were studied with a highly sensitive permeameter. The value of the parameters of the exponential dependence of the diffusion coefficient on the water concentration decreases with the curing extent, except that of the plasticization coefficient, which decreases after a post-curing at 120°C. Infrared-spectrophotometric and calorimetric data show that the curing involves mainly a polymerization of styrene during the curing at 25°C, and a crosslinking by the opening of the double bonds on the polyester chains in the subsequent curing step at 80°C. The water sorption by the resin results in a 20°C-decrease in the glass transition temperature of the water saturated resin, leading to the diffusivity enhancement by the plasticization effect.

Surface modification by low-pressure glow discharge plasma of an unsaturated polyester resin: effect on water diffusivity and permeability

Abstract Unsaturated polyester resin (UPR) films have been modified by a tetrafluoromethane microwave plasma. The treated surface morphologies investigated by contact angle, atomic force microscopy and permeation measurements show that the CF 4 plasma treatment decreases drastically the surface energy by increasing the hydrophobic character. The surface modification due to plasma fluorination decreases the water content diffusing through the UPR film and the time lag diffusion coefficient and thus, the sorption kinetic. From these results, it is clearly shown that the CF 4 plasma treated layer improves drastically the barrier effects.