Q.T. Nguyen

Novel hydrophilic membrane materials: sulfonated polyethersulfone Cardo

Abstract Sulfonated polyethersulfone Cardo was prepared by solvent-free reaction of concentrated sulfuric acid on the polymer. The polymer can be sulfonated to different degrees by control of the reaction temperature and time. The degradation of it main chain was shown to occur at high reaction temperature or at very long time. Higher sulfonation obtained at longer reaction times led to soluble polyelectrolytes. Polymers with an ion exchange capacity lower than 1.75xa0equiv.xa0kg −1 are not soluble in water while being hydrophilic. Asymmetric membranes can be prepared from the latter polymers by the wet phase-inversion method with water as the precipitation medium. They can be used as hydrophilic membranes for ultrafiltration or nanofiltration.

Clustering of solvents in membranes and its influence on membrane transport properties

Abstract Solvent clustering in membranes, if it occurs, could modify the sorption and diffusion behaviors of polymer membranes and therefore their transport properties. Solvent cluster formations in hydrophilic poly(vinyl accohol) (PVA), in hydrophobic poly(dimethylsiloxane) (PDMS) and intermediate poly(vinyl acetate) (PVAc) membranes were studied by infrared spectroscopy, differential scanning calorimetry, X-ray scattering techniques, and by visual observations. A water saturated PVAc membrane contains non-freezable (bound) and liquid-like water molecules in equivalent amounts (ca. 1.6 wt%); membrane infrared studies during the sorption process indicated that water penetrates first in the membrane as monomeric molecules which aggregate later to form clusters. In PVA membranes, water molecules can exist under three states, i.e. non-freezable, freezable bound and liquid-like states, depending on the water content in the membrane. The cluster formation seems to affect the membrane selectivity towards water and the ethanol diffusivities in PVA membranes. In PDMS membranes, solvent clusters were evidenced by infrared spectroscopy and visual observations. The clustering of alcohols in PDMS makes the sorption behavior of these solvents in the membrane deviate from the Flory-type behavior observed for other solvents: the sorbed amount increases drastically with the surrounding-solvent activity in the high activity range, but the solvent diffusivity decreases.

Pervaporation of aqueous ester solutions through hydrophobic poly(ether-block-amide) copolymer membranes

The pervaporation of three ethyl esters (aroma model compounds) in aqueous solutions was studied with poly(ether-block-amide) copolymer (PEBA) membranes of different compositions. The ester flux increases linearly with the ether-unit (tetramethylene oxide) content. The slope of the increase changes from a low to a higher value at 50 wt% polyether content. This behavior is explained by a change in the membrane structure from a continuous polyamide matrix structure to a polyether matrix one. The ester flux from saturated aqueous solutions increases from butyrate to propionate, then to acetate. This order differs from that of the poly(dimethylsiloxane) (PDMS) membrane whose flux was highest for butyrate and lowest for propionate ester. The PEBA membrane permeability and selectivity to the esters increase significantly with the polyether content. However, the best PEBA membrane had lower flux and selectivity to esters in aqueous solutions than the PDMS membrane. The activation energy for ester permeation was always lower than that for water permeation; its value was the highest for the PEBA membrane with the highest polyamide content.

The engaged species induced clustering (ENSIC) model: a unified mechanistic approach of sorption phenomena in polymers

Abstract The description of sorption equilibria of solvents in polymers is a key problem which has to be solved in order to achieve a correct understanding of transport processes through dense membranes. Up to now, this purpose is limited essentially to solvents in elastomers, which are described by Flory-Huggins thermodynamics theory, while sorption of gases in glassy polymers is usually based on a mechanistic one, the so-called dual mode model. Given the difficulties encountered as soon as polymer solvent peculiarities (non-regular mixing enthalpy, network elastic contribution) are taken into account, other types of mixtures can hardly be described by purely thermodynamic models unless complicated expressions are used. A simple mechanistic approach has been developed in order to circumvent these limitations; it is based on the assumption that insertion of a solvent molecule into the polymer solvent matrix will be governed by the intrinsic affinity of the solvent for either a polymer segment or an already sorbed solvent molecule. The model enabling cluster formation description has been named engaged species induced clustering (ENSIC). A series of data related to the most frequently used polymer dense membrane materials have been used in order to check the fitting efficiency of the newly developed expression; it is shown to give a very good description of sorption isotherms of many binary polymer solvent systems, including polar and apolar compounds, in either elastomeric, glassy or thermoplastic membrane materials. The physico-chemical interpretation of the two probabilistic insertion parameters used in the ENSIC model, as well as prospects concerning the extension to multicomponent systems or transport mechanisms simulations are discussed.

Application of Flory-Huggins theory to ternary polymer-solvents equilibria: A case study

Abstract The ternary equilibrium data of the following systems: benzene/heptane/NBR at 60 °C (system 1), heptane/isooctane/PE at 25 °C (system 2), ethanol/water/CA at 20 °C (system 3) and ethanol/water/ P(E-co-VAc) at 32 °C (system 4), already reported in the literature, have been used and experimental results compared to predictions offered by the Flory-Huggins theory applied to ternary mixtures (one polymer and two liquids), with constant interaction parameters and negligible elastic contribution. Polymer-solvent interaction parameters have been determined from swelling in pure liquid, while liquid-liquid interaction parameters have been estimated from liquid-vapour equilibrium data curve fitting. It is shown that the Flory-Huggins theory offers reasonable prediction in the case of apolar liquids in an elastomeric matrix (system 1), while approximate isotherm patterns with significant discrepancies are obtained with apolar liquids in a thermoplastic (system 2). The equilibrium data of polar liquids in either homopolymer or copolymer (systems 3 and 4) can hardly be achieved by Flory-Huggins theory, even when a variable liquid-liquid interaction parameter is used. Implications in ternary diagram simulations and possible prediction improvements are discussed.

Glass transition temperature regulation effect in a poly(vinyl alcohol)-water system

Abstract The temperature of crystallization and of melting of water in poly(vinyl alcohol) (PVA) membranes with various degrees of acetylation have been measured at different cooling and heating rates, and compared to the glass transition temperature (Tg) of the swollen materials. Above a certain critical concentration of water, c ∗ , when water begins to crystallize upon cooling, the Tg does not decrease according to the Fox equation, but remains constant. This phenomenon of regulation of the Tg and the origin of c ∗ are explained by the phenomenon of segregation of water in the amorphous phase during the process of crystallization. Finally, the effects of melting temperature depression and of broadening of the melting peak of ice in PVA—water systems, very similar to those observed in saccharose—water and alcohol—water mixtures, are explained by the phenomenon of dissolution, and not by the confinement effect.

Pervaporation of water-ethanol mixtures through a poly(acrylic acid) grafted polyethylene membrane. Influence of temperature and nature of counter-ions

Both the permeability and the selectivity to water of poly (acrylic acid) grafted polyethylene films increase when the counter-ion is changed from Li+ to Na+ or K+. The acid membrane behaves in a singular manner: high permeability at low water contents, slight selectivity to water for ethanol-rich mixtures, reversal in selectivity at 60 wt.% of water (at 60°C) and slight selectivity to ethanol in water-rich mixtures. Both partial water and ethanol fluxes increase with pervaporation temperature, according to an Arrhenius-like relationship, but with different activation energies. The activation energy for ethanol permeation is always higher than that for water permeation, but both of them decrease with increasing water content and when the counter-ion is changed from Li+ to Na+ or K+. As a consequence, selectivity to water is strongly reduced by an increase in pervaporation temperature, especially for the K+ membrane. Most of the results can be interpreted on the basis of the extent of ionization of the carboxylate groups, and the effects of swelling on the activation energy.

The solution–diffusion model: Order of magnitude calculation of coupling between the fluxes in pervaporation

Abstract In this paper, we develop an expression of Fick’s law to be used in the diffusion–solution model when coupling exists. To derive this relationship we have manipulated the generalised Stefan–Maxwell equation to get a more convenient generalised multicomponent Fick’s law in which clearly defined diffusivities and coupling coefficients appear: the two equations are wholly equivalent. In the general accepted frame of hypotheses when using the solution–diffusion model we have written Fick’s equation when coupling between the fluxes exists. The order of magnitude of coupling is calculated, using previously published experimental data, leading to the conclusion that coupling, in selective pervaporation process, can be neglected.

Ideal and non-ideal diffusion through polymers: Application to pervaporation

Abstract Using the approach developed in a previous paper, we integrate the generalised Stefan–Maxwell (SM) diffusion equations for a unique species in both Lagrangian and Cartesian coordinates. The dusty gas membrane model is considered. Two activity–concentration relationships are used: the Flory–Huggins equation and the Freundlich relationship. The ethyl acetate (EA)/PDMS system follows the Flory–Huggins equation and the computed interaction parameter allows to predict quantitatively the non-dimensional EA flux through a PDMS membrane when the Lagrangian coordinates are utilised. The prediction is less satisfactory when integrating the SM equation in the Cartesian coordinates. The system EA/PDMS can be qualified as an ideal system in diffusion. The water/ethanol/PVA-based membrane at 60°C system follows Freundlich’s equation at equilibrium. The bad fitting of the experimental water flux versus volume fraction of water at the feed side of the membrane suggests that this system cannot be described by the dusty gas/Stefan–Maxwell theory even when the Lagrangian coordinates are used. This type of system, defined here as non-ideal, obeys theories of the free volume type, as quoted in a recent paper.

Performances of poly(vinylpyrrolidone-co-vinyl acetate)-cellulose acetate blend membranes in the pervaporation of ethanol–ethyl tert-butyl ether mixtures: Simplified model for flux prediction

Abstract High performance pervaporation membranes for the selective removal of ethanol from ethyl t-butyl ether (ETBE) were prepared by blending cellulose acetate with poly(vinylpyrrolidone-co-vinyl acetate) in different proportions and studied in sorption and pervaporation of ethanol–ETBE mixtures of compositions ranging from 5 to 25xa0wt% ethanol. The membrane performances are improved when the copolymer content is higher due to strong flux enhancement. Both the ethanol and ETBE amounts sorbed by the membranes increase with the ethanol content in the mixture with which the membranes are in equilibrium, but that of ethanol is one order of magnitude higher than that of ETBE; this indicates an enhancement of ETBE sorption due to the interactions with ethanol sorbed by the membrane. We observed a similar behavior for the ethanol and ETBE fluxes. A simplified model with an ethanol flux increasing exponentially, and an ETBE flux increasing linearly with the ethanol content at the membrane upstream face, was developed and validated with the experimental data.