Effect of Pendant Side-Chain Sterics and Dipole Forces on Short Range Ordering in Random Polyelectrolytes

Backbone-to-backbone spacing in polymers is known to be dictated by the length of the pendant side-chains. Dipole forces in random polyelectrolytes lead to ionic clusters with a characteristic spacing that can be observed by SAXS. Repulsion due to side-chain sterics will compete with dipole forces driving cluster formation in random polyelectrolytes. A model study on short range order in anion exchange membranes (AEMs) of quaternized P4VP-ran-PI is presented. Quaternization of P4VP with alkyl bromides having different numbers of carbons, C$_{n}$Br, introduces pendant side-chains as well as charges. X-ray scattering performed on PQ4VP-ran-PI(C$_{n}$Br) show that when n$<$5 the dipole forces dominate leading to the formation of ionic clusters. However, when n$>$4, the chains remain separated due to sterics, forming a distinct backbone-to-backbone spacing morphology. For n=3, both dipole clustering and backbone spacing can coexist. Crosslinking of the isoprene units increased the coexistence window from n=3 to n=6. Impedance measurements show that a maximum conductivity of 110mS/cm was obtained for PQ4VP-ran-PI(C$_{3}$Br). A discussion on short range order due to competition, or counter balancing, of steric repulsion and dipole forces will be presented.