Aggregation behavior in the semidilute poly(N-vinyl-2-pyrrolidone)/water system

Dynamic and static light scattering experiments were performed on aqueous solutions of poly(N-vinyl-2-pyrrolidone) in the dilute and semidilute regime. In the dilute regime, typical good solvent behavior is found. In the semidilute regime, we observe two relaxation times in the autocorrelation functions and intense forward scattering. The short time relaxation has a typical q(2) dependence, characteristic of diffusion, and the long time relaxation an approximately q(3) dependence, characteristic of intraparticle dynamics. This behavior is not consistent with predicted semidilute, good solvent system behavior but instead indicates the presence of aggregates. Using combined static and dynamic scattering, we show these aggregates to be approximately 2-3 times the size of the dilute-solution radius of gyration. We attribute this anomalous behavior to the unique, tetrahedral hydrogen bond network of water and show that denaturing the water through the addition of denaturing agents known to disrupt the water structure significantly diminishes the aggregate size. We also show through pressure denaturation that the aggregation process is reversible.