Structure and dynamics of polymer solutions: Role of solvent and strong specific interactions

Date of Completion

January 2000


Chemistry, Physical|Chemistry, Polymer




Macromolecules with strong specific interactions often exhibit anomalous or extraordinary properties in their solution behavior. This work encompasses a comprehensive study of the solution structure of polyelectrolytes with strong electrostatic interactions, and extends to light absorbing solutions of polyaniline that associate by hydrogen bonding. ^ Polyelectrolyte solutions were investigated in context of their “ordinary-extraordinary” transition by light scattering. A model system of sodium polystyrene sulfonate in N-methyl formamide (NMF) was chosen. Two diffusion coefficients corresponding to a “fast” and “slow” mode were observed at all salt concentrations. This “extraordinary” behavior persists into the “ordinary” regime calling for a reinterpretation of the transition. The Debye Huckel screening length (κ-1), was shown to be the yardstick that governs the solution dynamics. NMF has a high dielectric constant that varies strongly with temperature enabling us to independently vary the ionic strength and the dielectric screening by the medium. These two variables act in concert to cause the entire dynamic light scattering data to collapse onto single master curves with κ-1. The data underlines the electrostatic origins of “extraordinary” behavior. A new Fluorescence Recovery after Photobleaching (FRAP) instrument was built to measure the center of mass motion or self-diffusion. This single chain diffusion is completely distinct from the slow mode. This lends weight to the growing conviction that the slow mode is due to multi-chain domains or structure in polyelectrolyte solutions. ^ The next section addresses the anomalous dynamic light scattering data from highly absorbing and associating solutions of polyaniline. The intensity correlation functions are characterized by two exponential decays superposed with an oscillatory term. A new analytical treatment is proposed. We explain this behavior by heterodyning of scattered light from two distinct species in convective motion. Similar data were obtained for a complex of Cytochrome-C and Cytochrome-C Peroxidase in phosphate buffer. This suggests the generality of the phenomenon for polymers that both absorb light and aggregate, such as proteins and conducting polymers. Effect of laser power on convection, and thermal lensing and coning of the beam have been considered. The presence of the oscillations is shown to be a sensitive indication of aggregation. ^