Conformation of Polyelectrolyte Chains in Dilute Aqueous Solutions Investigated by Conductometry, 2. Influence of Temperature, Chain Length and N-Alkyl Substituents on the Conformation of Exhaustively Alkylated Poly(N-alkyl-4-vinylpyridinium) Cations

Abstract

The temperature dependencies of the conductivity of salt-free dilute aqueous solutions of the vinylic polycations of poly(N-alkyl-4-vinylpyridinium) bromides (PAVP) were investigated. Samples of poly(N-ethyl-4-vinylpyridinium) bromides with different degrees of polymerization (DP), DP = 10–1300, and samples of PAVP with the same DP = 100 but with different N-alkyl substituents were investigated. The obtained data were evaluated using Manning's polyelectrolyte theory and the mean charge spacing b in the chains was calculated. The determined b values were used to assess the conformational changes of the macromolecules. The decrease of the chain length of the poly(N-ethyl-4-vinylpyridinium) cation to DP ≈ 100 was accompanied by a progressive growth of b indicating a transition from coil-like to rod-like regimes. Once this degree of polymerization was attained, further shortening of the chain did not affect the parameter b. These data suggest that the persistence length of this polycation corresponds to approximately 100 monomer units. However, even the chains with DP ≤ 100 did not adopt their fully stretched rod conformations at room temperature, since the heating of their solutions resulted in the increase of b. The temperature unfolding was most pronounced for the shortest chains and weakened drastically upon their elongation. The important role of the hydrophobic interactions in the stabilization of the chain conformation was revealed with the use of different PAVP samples. The values of b decreased proportionally with the increase of hydrophobicity of the N-alkyl substituent in the series —C2H4OH < —C3H6OH < —C2H5 < —C3H7 < —C4H9, indicating the transition from rather stretched rod-like chains to coils.