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|Title:||Poly(4-vinylpyridine)-filled microporous membranes: Factors affecting transport and selectivity|
|Advisor:||Childs, Ronald F.|
|Abstract:||<p>Pore-filled anion exchange membranes containing poly(4-vinylpyridinium) have been prepared by photochemically initiated free radical polymerization of 4-vinylpyridine and divinylbenzene in the pores of a polypropylene microporous membrane followed by alkylation or protonation of the incorporated poly(4-vinylpyridine). The mass increase of poly(4-vinylpyridine) incorporated in the polypropylene substrate can be controlled from 0 to 200%. Scanning electron microscope images show that the poly(4-vinylpyridine) is distributed evenly throughout the membrane. Membrane water contents range from 25% to 65%. The membranes have ion exchange capacities up to 4.8 meq/g of dry membrane and fixed charge concentrations up to 7.2 eq/kg of membrane water. The effects of mass gain, divinylbenzene content and the size of the quaternizing alkyl group on the rate of acid transport through the membranes and the selectivity of acid transport relative to salt transport have been determined. Activation energies as well as entropy terms for the diffusion of acid through the membranes have been determined which provide information regarding the mechanism of acid transport in the membranes. The mobility of the membrane water was determined by Pulsed-Gradient Spin-Echo NMR experiments which, combined with Differential Scanning Calorimetry experiments, provides information about the structure of the water in the membranes. An increase in divinylbenzene content or an increase in the length of the quaternizing alkyl group resulted in a decrease in acid transport and a dramatic increase in selectivity of acid over salt transport. The improved performance resulting from alkylation of the pyridine nitrogen atom compared to protonation was determined to be a result of an increase in water structure and resulting decrease in the mobility of the membrane water. It has been proposed that the structure of the water in a given membrane is of one type only and not found in the form of "bound" and "free" portions. The nature of the membrane water changes depending on the substituent on the pyridine nitrogen atom but is not present in more than one state in any given membrane. The work conducted in this thesis extends the understanding of acid transport through anion exchange membranes and the factors affecting acid/salt selectivity. It has been determined that acid transport is largely dependent on membrane water content and independent of the degree of structure of the membrane water. The acid/salt selectivity of the membranes was found to be a result of Donnan exclusion while the increased selectivity of the alkylated membranes compared to the protonated membranes was determined to be a result of the increase in membrane water structure in the former.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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