(609b) Anion Exchange Membranes with Responsive Properties

Authors: 
Capparelli, C. - Presenter, The Pennsylvania State University
Hickner, M. A., The Pennsylvania State University
Fernandez Pulido, C. R., The Pennsylvania State University
Stimuli responsive membranes have been developed as sensors,1,2 actuators,3 and drug delivery systems4,5. In particular, polymers with reversible or “switchable” responses to changes in their environment are sought for advanced applications where membranes change their transport properties depending on conditions. With these capabilities, new polymer membraness and polymeric surfaces can increase the functionality afforded by current polymer membranes.

Various stimuli responsive membranes have been developed, especially those which respond to pH,6 temperature,7 and light.8 In most cases, porous membranes are modified by either “grafting from” or “grafting to” techniques, by incorporating the functional or responsive moiety to the surface of the pre-formed membrane. Consequently, the responsiveness of the film is usually due to changes in the size of the pores that arise from the swelling or shrinkage of the grafts. For example, quadri-stimuli responsive membranes were developed by surface initiated atom transfer radical polymerization of commercial Nylon-6 porous membranes.9 The responsive gates were designed as poly(N-isopropylacrylamide)-block-poly(methacrylic acid) (PNM) block copolymers. Because poly(N-isopropylacrylamide) presents a lower critical solution temperature (LCST) of 32ºC, it undergoes a reversible phase transition from swollen to shrinkable above or below its LCST. In the case of poly(methacrylic acid), the same occurs upon protonation/deprotonation. In consequence, the effective pore size and permeability change reversible at different pH and temperatures. pH responsive nanofiltration membranes for sugar separations have also been developed by grafting poly(acrylic acid) onto commercial NF membranes by UV initiated free radical polymerization.10 In this case, increased hydrogen bonding interactions of the poly(acrylic acid) with glucose compared to sucrose upon deprotonation are responsible for the different permeation rates of the two types of sugar at different pH levels.

Dense, ion containing membranes are also interesting candidates to present changes in their transport properties in response to changes in their environment. For example, an actuator was developed by plating both sides of a perfluorocarboxylic acid film with gold as electrode layers.11 The actuator bent in a 90º angle when a pulse voltage of 2.0 V between the gold electrodes was applied. It was proposed that the bending motion is driven by differences in water content in the membrane carried by the ions. Additionally, potentiometric humidity sensors have been prepared from anion exchange membranes and polypyrrole composite membranes.3 The strip prepared from the composite membrane generates an electrical potential dependent on the relative humidity and the anion exchanged.

Redox responsive membranes have also been proposed as candidates for drug and gene delivery.12 Glutathione is the most abundant reducing agent in most cells, thus, polymeric membrane with thiol/redox responsive properties have been developed for drug delivery that specifically release drugs upon entry to the cells. A redox sensitive microporous membrane was prepared by grafting viologen-containing side chains to poly(vinyldene fluoride) porous membranes.13 Viologen moieties are chemically stable and reversibly convert from a dicationic to a radical cationic state when immersed in a reducing agent. These porous membranes presented higher permeation rates of 4-styrenesulfonic acid sodium salt in the radical cationic state compared to the dicationic state. It was hypothesized that increased solubility of viologen in water in the dicationic state may cause the side chains to extend into the pores, reducing the free volume.

In this work, we present the development of anion exchange membranes that contain a viologen moiety which provides reversible changes in resistance and permselectivity at the reduced and oxidized state. Previously, anion exchange membrane with viologen moieties have been developed by Sata14,15, but transport properties such as ionic resistance and permselectivity haven’t been investigated. In our study, membranes were prepared by light induced free radical polymerization of poly(ethylene glycol diacrylate) (PEGDA, 30 wt %), diurethane dimethacrylate (DUDMA, 50 wt %), and vinyl benzyl chloride (VBC, 15 wt %) as a precursor for functionalization. Dipentaerythritol penta-/hexa-acrylate was used as crosslinker (5 wt %). To this base resin, 1 wt % of phenylbis(2, 4, 6-trimethylbenzoyl) phosphine oxide and 1 wt % of 1-hydroxycyclohexyl phenyl ketone were added as initiators. The resin was spread on a casting glass plate and light was projected to initiate the polymerization. Once the membrane was obtained, it was reacted with a 5 wt % solution of 4,4’-bipyridine in ethanol at 60ºC for 24 h. Later, in order to methylate unreacted amines, the membrane was immersed in a 2 M solution of iodomethane in ethanol at 60ºC for 24 h. The final membrane was washed with ethanol and stored in NaCl. The ionic resistance of the membranes was measured using a DC 4-point method and the permselectivity was measured in a similar cell arrangement by measuring the potential drop across the membrane. For each sample, both ionic resistance and permselectivity were measured in the oxidized (dicationic) and reduced (radical cationic) states. For all the samples, it was observed that the ionic resistance increased from the dicationic to the radical cationic state. As the membrane becomes less positively charged when changing from the oxidized to the reduced state, it is expected that the ionic resistance of the membrane increases, as observed in the results. Additionally, a decrease in permselectivity was observed when switching from dicationic to radical cationic state. Usually, in ion exchange membranes, permselectivity increases as ionic resistance increases due to a lower number of ions being transported through the membrane. However, in this case, the membrane also experiences changes in charge when redox cycling. Permselectivity is reduced as a consequence of a decrease in its charge density, which reduced the ion repulsion of co-ions, as indicated by the Donnan exclusion principle. A cycling experiment was also performed in which the membrane was reduced and re-oxidized 10 times, to observe the degree at which the stimuli responsive property is reversible. Overall, this works presents an anion exchange membrane with varying ionic resistance and permselectivity dependent on its redox state.

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