H. Strathmann

Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystalline polymers

Structural characteristics in membranes formed by diffusion induced phase separation processes are discussed. Established theories on membrane formation from ternary systems can be extended to describe the effects of high or low molecular weight additives. A mechanism for the formation of nodular structures in the top layer of ultrafiltration membranes is presented. In the last part structures arising from polymer crystallization during immersion precipitation are discussed.

CO2-induced plasticization phenomena in glassy polymers

A typical effect of plasticization of glassy polymers in gas permeation is a minimum in the relationship between the permeability and the feed pressure. The pressure corresponding to the minimum is called the plasticization pressure. Plasticization phenomena significantly effect the membrane performance in, for example, CO2/CH4 separation processes. The polymer swells upon sorption of CO2 accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. Fundamental understanding of the phenomenon is necessary to develop new concepts to prevent it. n nIn this paper, CO2-induced plasticization phenomena in 11 different glassy polymers are investigated by single gas permeation and sorption experiments. The main objective was to search for relationships between the plasticization pressure and the chemical structure or the physical properties of the polymer. No relationships were found with respect to the glass-transition temperature or fractional free volume. Furthermore, it was thought that polar groups of the polymer increase the tendency of a polymer to be plasticized because they may have dipolar interactions with the polarizable carbon dioxide molecules. But, no dependence of the plasticization pressure on the carbonyl or sulfone density of the polymers considered was observed. Instead, it was found that the polymers studied plasticized at the same critical CO2 concentration of 36±7 cm3 (STP)/cm3 polymer. Depending on the polymer, different pressures (the plasticization pressures) are required to reach the critical concentration.

Retention measurements of nanofiltration membranes with electrolyte solutions

Retention measurements with single salt solutions of CaCl2, NaCl and Na2SO4 revealed that the rejection mechanism of commercial polymeric nanofiltration membranes investigated in this study may be divided into two categories: n n1. Membranes for which Donnan exclusion seems to play an important role. n n2. Membranes for which retention is determined by both Donnan exclusion and size effects. n nIn category 1 both positively and negatively charged membranes were found. n nCeramic γ-Al2O3 ultrafiltration membranes with a pore size of 3 nm showed a same type of salt retention behavior as the positively charged polymeric membranes. n nThe extended Nernst–Planck equation in combination with the Donnan equilibrium has been used to model the flux-retention experiments for the salt solutions. The numerical calculations resulted in a good agreement with experimental data and acceptable values for the fixed charge densities have been determined. The effective membrane thicknesses calculated were higher than those observed by scanning electron microscopy.

Plasticization-resistant glassy polyimide membranes for CO2/CH4 separations

It is known that CO2 acts as a plasticizer in CO2/CO4 membrane separations at elevated pressures. The polymer matrix swells upon sorption of CO2, accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. To overcome this effect, plasticization should be minimized. We succeeded in stabilizing the polymer membrane by a thermal treatment. For this purpose the polyimide Matrimid 5218 is used as model polymer. In single gas experiments with CO2, the untreated membrane normally shows a minimum in its pressure dependence on permeability, whereas the treated membranes do not. Membrane performances for CO2/CO4 gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The heat treatment clearly suppresses this undesired methane acceleration. n nAdditionally to the pure and mixed gas permeation results, process calculations reveal valuable information as to what extent the stabilized membranes show improved membrane performance. The favourable performance of the stabilized membrane can be attributed to less methane loss and therefore a higher recovery, resulting in higher profit from gas sales.

Stability of Supported Liquid Membranes: State of the Art

Abstract This paper presents a state of art review on the stability of supported liquid membranes (SLM). The backgrounds of SLM instability phenomena are presented, and various mechanisms for explaining these phenomena are treated in detail. Several suggestions for stability improvement are discussed. ∗ Present address: Agrotechnological Research Institute (ATO-DLO), Fatty Acid and Membrane Technology, P.O, Box 17, NL-6700 AA Wageningen, The Netherlands.

Concentration polarization with monopolar ion exchange membranes: current-voltage curves and water dissociation

Concentration polarization is studied using a commercial anion and cation exchange membrane. Current?voltage curves show the occurrence of an overlimiting current. The nature of this overlimiting current is investigated in more detail, especially with respect to the contribution of water dissociation. pH measurements reveal that water dissociation is more pronounced in case of the anion exchange membrane than with the cation exchange membrane when the limiting current is exceeded. However, even with the anion exchange membrane it is found that the contribution of water dissociation is very low and more than 97% of the current is carried by the salt ions. Furthermore measurements are described showing that the membrane permselectivity (co-ion transport) remains constant in the overlimiting region. This means that the overlimiting current is virtually all carried by the salt counter ions for the two membranes investigated.

Limiting current density and water dissociation in bipolar membranes

The behaviour of bipolar membranes in NaCl and Na2SO4 solutions is discussed. The membranes are characterized in terms of their limiting current densities. Below the limiting current density the electric current is carried by salt ions migrating from the transition region between the anion and the cation exchange layer of the bipolar membrane. In steady state these ions are replaced by salt ions transported from the bulk solutions into the transition region by diffusion and migration due to the fact that the ion-exchange layers are not strictly permselective. When the limiting current density is exceeded, the salt transport from the transition region can no longer be compensated by the transport into the region and a drastic increase in the membrane resistance and enhanced water dissociation is observed. This water dissociation is described as being a combination of the second Wien effect and the protonation and deprotonation of functional groups in the membrane. The limiting current density is calculated from a mass balance that includes all components involved in the transport. The parameters used in the mathematical treatment are the diffusion coefficients of salt ions and water, the ion mobilities in the membrane, the fixed charge densitiy of the membrane, the pKb values of the functional groups and the solution bulk concentrations.

Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution

The preparation of polyethersulfone (PES) hollow fiber membranes has been studied using N-methylpyrrolidone (NMP) as solvent, polyethylene glycol 400 (PEG 400) as weak nonsolvent and water as strong nonsolvent. When PEG 400 is used as polymeric additive to the spinning dope the viscosity of the PES solution is strongly enhanced. Furthermore, it was observed that PEG 400 could be added to the solution in large amounts without causing phase separation (NMP/PEG ratio 1:9, PES concentration approximately 11 wt.%). Membranes prepared from a solution containing a NMP/PEG ratio of 1:1 results in higher fluxes than when a ratio of 1:4 is used. Similar fluxes were obtained for PES concentrations of 16 and 20 wt.%. Looking at the fiber cross-section it became clear that macrovoid formation could not be suppressed by the addition of PEG 400 alone, not even at concentrations as high as 38 wt.%. Only when relatively large amounts of water were added to the dope solution macrovoids disappeared and nice spongy structures were obtained. Variation of the bore liquid composition using the components NMP, PEG 400 and water showed to be a powerful method to control the pore size of the bore surface. Pores of 5–28 nm were obtained in combination with high pure water fluxes; e.g. a membrane with pores of 7 nm had a pure water flux of 940 l/(m2 h bar) and showed 100% BSA retention. When an air gap larger than 10 mm was applied the shell surface contained relatively large pores. Spinning directly in water (airgap=0) resulted in shell side pores of 8–10 nm, while an air gap of 10 mm resulted in pore sizes of 40–54 nm.

Chronopotentiometry and overlimiting ion transport through monopolar ion exchange membranes

In this paper chronopotentiometric measurements are described to study the overlimiting ion transport through a Neosepta CMX cation and AMX anion exchange membrane. This technique is used to characterise the fluctuations in membrane voltage drop observed in the overlimiting region of current?voltage curves and to investigate the structural inhomogeneity of the aforementioned membranes. Above the limiting current the measurements show large voltage drop fluctuations in time indicating hydrodynamic instabilities. The amplitude of these fluctuations is increasing with increasing applied current density. The fluctuations also occur when a set-up is used where there is no forced convection and the depleted diffusion layer is stabilised by gravitation. Experimental transition times are found to be smaller than calculated for an ideally permselective membrane and indicate a reduced permeable membrane area. The results can be related to the theory of electroconvection due to an inhomogeneous membrane structure.

Cation permeable membranes from blends of sulfonated poly (ether ether ketone) and poly (ether sulfone)

Sulfonated poly(aryl ether ether ketone), S-PEEK, is blended with non-sulfonated poly(ether sulfone) (PES) to adjust the properties of ion permeable and ion selective membranes. In this study, membranes are prepared from blends with (i) a S-PEEK content between 10 and 100 wt.% using one S-PEEK batch with a fixed degree of sulfonation and (ii) from batches of S-PEEK with a different degree of sulfonation, but with a fixed S-PEEK content in the blend. The transparent membranes are permeable for ions with selective transport of cations over anions. At contents of S-PEEK below 40%, phenomena related to a percolation threshold of the ion exchange functionalities are observed; the measured ion exchange capacity (IEC) indicates that not all functional groups are accessible in these blends. The transport properties of membranes with a S-PEEK content in the range of 50?80 wt.% are comparable to those known for commercial ion exchange membranes. In this range, a trade-off between resistance and selectivity with increasing IEC is observed. Both, the ion conductivity and the co-ion transport number increase with increasing IEC. This is mainly caused by the increased water content with increased IEC and the number of water molecules per fixed charge.