Ronald F. Childs

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.

Formation and characterization of highly crosslinked anion-exchange membranes

Highly crosslinked/hyperbranched anion-exchange membranes have been prepared by anchoring poly(vinylbenzyl chloride) (PVBCl) within the pores of poly(propylene) microporous base membranes by in situ crosslinking of PVBCl with a diamine 1,4-diazabicyclo[2.2.2]octane (DABCO). The resulting PVBCl-filled precursor membranes were converted to anion-exchange membranes by reacting these with (i) excess of DABCO followed by alkylation with α,α′-dibromo-p-xylene (DBX) (membrane A), and (ii) with excess of tetraethylenepentamine (TEPA) (membrane B). A third membrane C was synthesized by alkylating membrane B with DBX. The chemical analyses indicated that these anion-exchange membranes consist of highly crosslinked/hyperbranched anionic gels within the pores of host microporous membranes. These anion-exchange membranes were characterized in terms of water-uptake capacities, ion-exchange capacities and thermal stability. The physical structures of the membranes were examined by small angle X-rays scattering (SAXS) analysis. The study of SAXS profiles of the dry and water equilibrated membrane A samples indicated that microstructure of anionic gel within the pores of membrane was changed significantly on water equilibration. However, no significant change in the SAXS profile was observed in wet samples of membranes B and C with respect to their dry samples. Thus, the crosslinking generated in membrane A was flexible and very rigid in membranes B and C. The self-diffusion coefficient of I− ions and transport numbers of Cl− ions were measured to examine the effects of crosslinking on transport properties of the membranes.

Formation of pore-filled ion-exchange membranes with in situ crosslinking: Poly(vinylbenzyl ammonium salt)-filled membranes

Robust, polyelectrolyte-filled, microporous membranes were prepared by the introduction and crosslinking of a preformed polymer within the pores of a poly(propylene) host membrane. Specifically, poly(vinylbenzyl chloride) (PVBCl) was reacted with piperazine or 1,4-diaminobicyclo[2.2.2]octane in an N,N-dimethylformamide (DMF) solution contained in the pores of the microporous base membrane. The remaining chloromethyl groups were reacted with an amine, such as trimethylamine, to form positively charged ammonium sites. This simple two-step procedure gave dimensionally stable, anion-exchange membranes in which the degree of crosslinking and the mass loading were determined by the concentration of PVBCl and crosslinker in the starting DMF solution. The incorporated polyelectrolyte gel was evenly distributed within the pores of the host membrane with no surface layers present. The membranes are fully characterized.

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.

Tuning the acid recovery performance of poly(4-vinylpyridine)-filled membranes by the introduction of hydrophobic groups

Abstract Two series of anion-exchange membranes have been produced by photo-initiated copolymerization of 4-vinylpyridine and divinylbenzene (DVB) within the pores of polypropylene microfiltration membranes followed by quaternization of the pyridine nitrogen atoms. The membranes were tested for use in acid recovery applications by performing diffusion dialysis of acid/salt solutions (HCl/NaCl/MgCl 2 ) and determining dialysis coefficients and acid/salt selectivity. The nature of the water present in the membranes was examined by differential scanning calorimetry (DSC). The first series was comprised of membranes with varying DVB content and all were quaternized with a methyl group. When compared to similar protonated membranes, this series showed a decrease in fixed charge concentration, an increase in dialysis coefficients, an increase in selectivity of HCl over NaCl and a slight decrease in selectivity of HCl over MgCl 2 . The second series had different quaternizing groups. It was found for this series that increasing the size of the N -alkyl substituent resulted in an increase in the membrane fixed charge concentration, a decrease in dialysis coefficients and a dramatic increase in selectivity of acid over salts. The results are interpreted in terms of the effects of the hydrophobic alkyl groups on the water structure in the membranes. The acid recovery performance of the N -alkylated membranes was shown to be better than a commercial diffusion dialysis membrane.

A novel method to enhance the stability of alginate-poly-L-lysine-alginate microcapsules

Implantation of microencapsulated recombinant cells is an alternative approach to gene therapy. These genetically-engineered cells enclosed in microcapsules to deliver therapeutic recombinant products have been effective in treating several murine models of human diseases. However, the most commonly used microcapsules fabricated from alginate ionically cross-linked with calcium suffer from loss of long-term mechanical stability. We now report on a method to improve their stability by introducing additional polymers to provide covalent linkages via photopolymerization. Vinyl monomers and a photoinitiator were allowed to diffuse into the initially formed calcium–alginate microcapsules. In situ photopolymerization in the presence of sodium acrylate and N-vinylpyrrolidone substantially enhanced their mechanical strength. After four months of storage in saline, > 70% of these capsules remained intact in the osmotic pressure test, while the un-modified alginate microcapsules totally disintegrated. Tests of their permeability to polyethylene glycol of different molecular weight and their ability to support cell survival showed that these properties remained unaffected by the photopolymerization. Hence, these microcapsules modified by adding a network of vinyl polymers are promising candidates to use for long-term delivery of recombinant gene products in this cell-based method of gene therapy.