Conformation of Polyelectrolyte Chains in Dilute Aqueous Solutions Investigated by Conductometry. 4. Influence of Molecular Mass and Charge Density of the Chains on Conformation of Symmetrical Aliphatic Ionene Bromides

摘要

Samples of 3,3-, 4,4-, 6,6-, and 10,10-ionene bromides of different degree of polymerization (DP) were synthesized, fractionated, and characterized. Salt-free dilute aqueous solutions of the samples were investigated by means of conductometry. On the basis of Manning's polyelectrolyte theory, the mean charge spacing b of the polyion chains was calculated from these data. The calculated b values were used for the estimation of the conformational changes of the polyions. The decrease in the chain length of 3,3-ionene bromide to DP 50 was followed by a growth of b indicating a transition from coillike to rodlike regimes. Once this DP was attained, further shortening of the chain did not affect the parameter b suggesting the persistence length (P) of approximately 50 monomer units. This value proves to be significantly higher than P estimated by the same approach for 2,4-ionene bromide having the same mean charge density. This implies that the distribution of charges along the ionene chain (symmetrical or nonsymmetrical) could noticeably affect the flexibility of the chain. For 3,3-ionene bromide and poly(N-ethyl-4-vinylpyridinium) bromide, a linear correlation between the determined P value and the corresponding Debye screening lengths, κ~(-1), was established. P values estimated from the calculated κ~(-1) values on the basis of this correlation successively decreased in the series 3,3- > 4,4- > 6,6- > 10,10-ionene bromides and were in good agreement with the trends of the corresponding b vs DP curves. The obtained data suggest that for all studied ionene samples including the least charged 10,10-ionene bromide, the chain conformation is determined primarily by the electrostatic repulsion of the quaternized nitrogen atoms in the ionene backbone.