Influence of the composition of polymer inclusion membranes on their homogeneity and flexibility

This study involves the preparation of a number of polymer inclusion membranes (PIMs) using a variety of commercial solvent extraction reagents immobilized in cellulose triacetate (CTA), cellulose acetate, poly(vinyl chloride) (PVC), PVC co-polymerized with vinyl alcohol and vinyl acetate, and carboxylated PVC. The influence of the molecular structure and physicochemical properties of the extractants studied on their ability to form homogeneous, flexible and mechanical strong PIMs are discussed. The physicochemical properties found to be of most importance in this respect are the extractant’s lipophilicity, hydrogen bonding capacity, ability to participate in dipole–dipole interactions and its liquid aggregate state at room temperature.

This study has enabled a qualitative understanding of the properties an extractant should possess to form a successful PIM with the base polymers CTA and PVC without the use of an additional plasticizer. Successful membranes are obtained with both polymers if the extractant has a strong ability to form hydrogen bonds and/or a strong ability to participate in dipole–dipole interactions. With PVC, dipole–dipole interactions are more important but only if there is an atom in the extractant molecule that carries a partial positive charge. Also, lipophilicity plays a more important role with PVC membranes than for CTA based PIMs. Non-polar and strongly lipophilic molecules like high molar mass tertiary amines will not form membranes with CTA and PVC unless the nitrogen atom is protonated to form a quaternary ammonium cation. The incorporation of a plasticizer in addition to the extractant into the membrane composition is an important extension to this work as it has the potential to alter the compatibility of an extractant with a base polymer. The choice of the plasticizer by using the guidelines resulting from the current study may overcome some of the problems of poor compatibility observed with certain extractant/base polymer compositions.