Sodium p-Styrene Sulfonate–1-Vinylimidazole Copolymers for Acid–Base Proton-Exchange Membranes

Abstract

Binary copolymers possessing thermal stability up to 290°C and suitable for the formation of ion-exchange membranes have been obtained by free radical copolymerization of sodium p-styrene sulfonate and 1-vinylimidazole with subsequent conversion to the H-form. The composition and structure of the copolymers are confirmed by elemental analysis and IR and NMR spectroscopy. Multicomponent membranes based on the copolymers of sodium p-styrene sulfonate and 1-vinylimidazole have been obtained using poly(vinyl alcohol) crosslinked by oxalic acid as a film-forming agent. It is shown by scanning electron microscopy that the surface and cut of the membranes have a uniform structure. The specific electric conductivity of the membranes with different concentrations of styrene sulfonic acid (20 to 80 mol % in the copolymer) increases with an increase in the fraction of styrene sulfonic acid and is 29.5 to 52.0 mS cm−1 at 80°C. The activation energy of proton transport of the membranes is 12.5 to 15.5 kJ mol−1. The water uptake of the membranes decreases with the increase in the concentration of the basic (imidazole) component in them, which may be associated with the increase in the density of ionic crosslinking due to the acid–base interaction of the sulfo groups and pyridine nitrogen atoms of the imidazole cycle. The formation of acid–base pairs results in the formation of continuous proton transport channels in addition to the sulfonic acid channels, which is confirmed by the high values of specific electric conductivity even in the case of a decrease in the concentration of sulfostyrene in the composition of the membrane. Increasing the concentration of 1‑vinylimidazole in the composition of the copolymer from 20 to 80 mol % leads to an increase in the relative elongation at break from 3 to 6%, an increase in the tensile strength at break from 4 to 7 MPa, a decrease in the Young modulus of the membranes from 127 to 102 MPa, and a decrease in the water uptake from 45 to 15.