Conformation of Polyelectrolyte Chains in Dilute Aqueous Solutions Investigated by Conductometry, 3. Influence of Charge Density on the Conformation of Partly Alkylated Poly(N-ethyl-4-vinylpyridinium) Cations and Ionenes

Abstract

The temperature dependencies of the conductivities of dilute salt-free aqueous solutions of partly alkylated poly(N-ethyl-4-vinylpyridinium) bromides (PEVP-β) and 2,n-ionene bromides with different degrees of polymerization (DP) were obtained. The charge density of the polycations was varied over a wide range by using PEVP-β with different degrees of alkylation β (%) in the range 25 < β < 95, and 2,n-ionenes with n values of 4, 8, and 10. On the basis of Manning's polyelectrolyte theory, the mean charge spacing (b) of the chains was calculated from these data. For the shortest PEVP-β chains, DP = 20, changing β had only a negligible effect on b in the whole β range studied. In contrast, the b values of the highly polymerized samples dropped significantly upon decrease of β in the range 95 > β > 60 indicating folding of the chains upon decrease of their charge density. Folding was supported by the fact that the persistence length of PEVP-95 estimated from the obtained dependency of b on DP far exceeded the estimated persistence lengths of PEVP-60 and 2,4-ionene having similar mean charge densities. Further decrease of β at β < 60 caused noticeable growth of b with a pronounced trend of convergence of the curves of all studied PEVP-β samples. These findings might be attributed to the conformational changes of the chains considered in the context of the scaling theory of polyelectrolytes. The obtained results suggest that the revealed trend in conformational changes of the chains upon variation of their charge density is common, irrespective of whether PEVP-β with a random distribution of the charges along the chains or ionenes with the regular charge distribution are under consideration.