Structure and dynamics of associative water-soluble polymer aggregates as seen by F-19 NMR spectroscopy

The microstructure and the molecular dynamics of aqueous solutions of an as sociative polymer (AP), consisting of a polyelectrolyte backbone and attach ed perfluorocarbon side chains, have been studied by F-19 NMR. The spectral shape and the distributions of the molecular self-diffusion coefficient an d of the transverse and the longitudinal relaxation rates along the F-19 sp ectrum have been measured, together with the magnetic field dependence of t he relaxation rates. A comprehensive analysis yields that the spectral line s are broadened by a chemical shift distribution that reflects the distribu tion of the average hydrophobicity of the environment within the aggregates in which the perfluorocarbon side chains reside. A fraction of the AP chai ns remain unaggregated with all their side chains immersed in water. As sho wn by the NMR data, the exchange time of the APs among different states of aggregation is long, > 1 s, while the exchange time of individual side chai ns among aqueous and hydrophobic environments within the same aggregate is fast, approximately microseconds. Among the aggregates, there is a consider able heterogeneity in the average hydrophobicity. Some implications of the F-19 NMR findings for H-1 NMR studies of APs with hydrocarbon side chains a re discussed.