Alicja M. Mika

A new class of polyelectrolyte-filled microfiltration membranes with environmentally controlled porosity

Abstract A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine onto polyethylene and polypropylene microfiltration membranes. By imposing the rigid structure of the polyolefinic substrate on the highly charged but extremely flexible and swellable gel of polyelectrolyte, stable membranes with very high charge density (> 3.5 mequiv/g) have been developed. The membranes show an outstanding pH valve effect and the capability of rejecting small inorganic ions in the process of reverse osmosis. The pressure-driven transport and separation with these membranes differ markedly from those of conventional reverse osmosis membranes being governed by the properties of the polyelectrolyte. Salt rejection depends strongly on the feed concentration, decreasing substantially with increase in concentration. The most characteristic features of separation with these membranes are higher rejection of polyvalent co-ions and lower rejection of polyvalent counter-ions compared to monovalent ions.

Porous, polyelectrolyte-filled membranes: Effect of cross-linking on flux and separation

Abstract A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine (4VP) with varying amounts of divinylbenzene (DVB) onto polypropylene microfiltration membranes. Using the same irradiation conditions, it has been found that graft yield increases substantially in the presence of DVB. The increase in graft yield was shown to be accompanied by a substantial increase in the thickness of the grafted membranes and a small but significant decrease in water content. The composite membranes have very high charge densities and good mechanical properties. The membranes with various amounts of DVB were quaternized (methylated) and examined for reverse osmosis of various salts. In addition to an expected drop in flux due to the increased thickness and decreased water content, there was significantly different salt rejection for membranes with cross-linking. While, for example, there is practically no difference in rejection of NaCl by a membrane with 0.55% DVB and one having no cross-linker, the Na 2 SO 4 rejection by the cross-linked membrane is, on average, twice as high as that by the non-cross-linked one. Large differences between the cross-linked and non-cross-linked membranes were found in the ratios of pure water to NaCl permeate fluxes of the membranes at various pressures. The results are discussed in terms of the physicochemical nature of the membranes and conformational changes of the pore-grafted poly(4-vinylpyridine).

Acid recovery using diffusion dialysis with poly(4-vinylpyridine)-filled microporous membranes

Abstract Two series of membranes have been produced by photoinitiated polymerization of 4-vinylpyridine (4VP) and divinylbenzene (DVB) within the pores of polypropylene microfiltration membranes. The first series was comprised of membranes with varying mass gain and constant DVB content. The second series of membranes had similar mass gains but varying DVB content. The membranes were tested by diffusion dialysis of acid/salt solutions (HCl/NaCl/MgCl2) in order to determine the effects of both mass gain and degree of crosslinking on dialysis coefficients and acid/salt separation. It was found for the first series of membranes that the dialysis coefficients of the acid and salts decreased and then leveled off with increasing mass gain while separation increased and then also leveled off. The second series of membranes showed a decrease in acid and salt dialysis coefficients but a dramatic increase in separation as the DVB content was increased. These results are interpreted in terms of the fixed charge concentration and the water content of the membranes. A comparison is made with a commercial diffusion dialysis membrane.

Salt separation and hydrodynamic permeability of a porous membrane filled with pH-sensitive gel

The effect of pH on permeability and the pressure-driven ionic separation of a microporous membrane incorporating a pH-sensitive cross-linked poly(4-vinylpyridine) gel in the pores was examined. Membranes were prepared with cross-linked poly(4-vinylpyridine) constrained in the pores of a poly(ethylene) microporous host membrane. The degree of ionization (protonation) of poly(4-vinylpyridine) in the membranes as a function of pH was determined by potentiometric titration. The pressure-driven flux and salt separation of these membranes were monitored as a function of pH and, consequently, the degree of ionization of the pore-filling polyelectrolyte. The pure water flux was found to decrease reversibly by an order of magnitude when the pH of water was changed from 5.5 to 2.6 by the addition of HCl. Similar reversible flux changes were found with pH-adjusted municipal tap water used as a feed. On the other hand, the rejection of cations in tap water increased sharply with addition of HCl and increase of ionization of the incorporated poly(4-vinylpyridine). The results obtained in this study are explained by microphase transitions taking place in the gel network enmeshed with the porous structure of the support membrane.

Nanofiltration using pore-filled membranes: effect of polyelectrolyte composition on performance

Several series of membranes composed of microporous poly(propylene) substrates filled with polyelectrolyte gels of different chemical structure, polymer concentration and charge densities have been prepared in order to examine the effect of polyelelctrolyte composition on the performance of these membranes in nanofiltration. The membranes were made by two different routes involving either in situ chemical cross-linking of poly(4-vinylpyridine) or poly(vinylbenzyl chloride), or by in situ polymerization of acrylic acid with tetra(ethylene glycol) diacrylate or N,N-methylenebisacrylamide as cross-linking agents in the pores of the substrates. The resulting pore-filled membranes were characterized by the concentration (volume fraction) of the polyelectrolyte in the pores, ion exchange capacity (charge density), water content, and thickness. The different series of membranes were tested under pressure to determine their hydrodynamic permeabilities and salt separation properties (NaCl). It was found that there was a good correlation between hydrodynamic (Darcy) permeability and gel polymer concentration that holds irrespective of the gel polymer chemistry in the pore-filling gels. Membranes with different pore-filling gels, whether positively or negatively charged, followed the same relationship. The separation properties of the membranes are very good and the salt rejection was found to be practically constant over a wide range of permeabilities (gel concentrations). It was also found that the increase in the nominal charge density above approximately 0.2 mmol/cm3 of the swollen gel had a negligible effect on the separation properties of the gel-filled membranes. The results of this study provide a basis for the further design and optimization of polyelectrolyte filled membranes for nanofiltration applications.

Poly(4-vinylpyridine)-filled microfiltration membranes: physicochemical properties and morphology

Abstract The morphology and physicochemical properties of poly(4-vinylpyridine)-filled microfiltration membranes have been examined. These membranes, which were prepared by photoinitiated grafting of up to 125 mass% of 4-vinylpyridine into the pores of polypropylene (PP) microfiltration membranes, were characterized in terms of the amount of poly(4-vinylpyridine) incorporated (graft yield), ion-exchange capacity, water content, and thickness. The morphology of samples of the grafted membranes dehydrated by freeze substitution was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. While the ion-exchange capacities of the grafted membranes are a function of graft yield, ranging to 4.0 meq/g of dry membrane, the water contents of the grafted membranes in the free base form are essentially independent of graft yield. The porosity of the grafted membranes was shown to be almost the same as that of the starting base polypropylene membranes (60–80%). However, protonation of the grafted poly(4-vinylpyridine) was shown to lead to a further and very substantial increase of the void volume of the membranes as measured by their water content. The thickness of the grafted membranes was found to increase linearly with increasing incorporation of polyvinylpyridine. Ionization of the polyelectrolyte was shown to cause a further systematic increase in thickness which was partly reversible with reversion to the unprotonated form. These changes in thickness are attributed to the stretching of the mesh of the substrate microfiltration membranes.

Ultra-low pressure water softening: a new approach to membrane construction

Abstract The ultra-low pressure water softening properties of a series of novel membranes containing cross-linked poly(4-vinylpyridinium) salts incorporated into the pores of polyethylene or polypropylene microfiltration membranes have been examined. It has been shown that these membranes exhibit excellent separation properties with high permeabilities at low trans-membrane pressures. The separation mechanism in these pore-filled membranes is primarily due to Donnan exclusion, and as such bivalent cations are more strongly rejected than monovalent ions. Moreover, it has been shown that relatively large molecules such as sucrose are only rejected to a limited extent. The membranes have been shown to withstand free chlorine at ambient temperatures. The performance of the pore-filled membranes has been shown to compare favorably with existing state of the art thin-film composite (TFC) nanofiltration membranes (Osmonics Desal-51 and BQO1 and Hydranautics TFV-7450). In contrast to the TFC membranes, the separating layer in these pore-filled membranes has been shown to be thick, ranging from 80 to 160μm. These polyelectrolyte opre-filled membranes offer a new paradigm in the construction of high performance water softening membranes.

Pore-filled cation-exchange membranes containing poly(styrenesulfonic acid) gels

Abstract Pore-filled cation-exchange membranes containing poly(styrenesulfonic acid) gels have been prepared by a two-step procedure involving thermally initiated radical copolymerization of styrene and divinylbenzene in the pores of a microporous polyethylene) host followed by treatment with chlorosulfonic acid. The incorporated poly(styrenesulfonic acid) was shown to be evenly distributed throughout the thickness of the host membrane. A pore-contraction of the host membrane was observed during the incorporation of the cross-linked poly(styrene); however, the sulfonation step led to increases in the thicknesses and/or other dimensions of the membranes depending on mass loading ofthe incorporated polyelectrolyte. The ion-exchange capacities ofthe resulting membranes ranged up to 5.2 meq/g, depending on the mass of poly(styrene) initially incorporated into the host as well as the sulfonation temperature. The membranes were characterized in terms of water content, fixed charge concentration, and transport numbers. The pressure-driven performance of the membranes in reverse osmosis/nanofiltration applications was examined.