Phase behavior of polyelectrolyte solutions
Abstract (Summary)Due to the presence of long-ranged electrostatic interactions, polyelectrolyte solutions are characterized by a length scale in addition to the radius of gyration and the correlation length, the Debye screening length. Contrasted to the behavior observed in neutral polymer solutions in which miscibility is controlled by molecular weight and temperature, the inverse-square Debye length additionally controls polyelectrolyte phase behavior. This thesis project experimentally investigated the influence of added barium chloride on both the collective and configurational properties of a model polyelectrolyte, sodium-poly (styrene sulfonate). Regarding the collective properties the crossover from mean field to Ising criticality close to the precipitation phase boundary was measured. This crossover was demonstrated for both salt-dependent and temperature-dependent thermodynamics. A mean field model qualitatively describes the collective behavior in polyelectrolyte solutions as a competition between a short-ranged chemical mismatch, governed by a Flory-Huggins interaction parameter, that disfavors miscibility and a repulsive screened-Coulombic interaction between monomers that favors miscibility. The addition of salt screens the electrostatic interaction such that it becomes short-ranged, leading to the observed precipitation at fixed temperature. Similarly, for a fixed salt concentration, the solvent quality is tuned and precipitation is observed upon lowering temperature. The configurational properties of labeled chains were also examined as a function of molecular weight, polymer concentration, and salt concentration. In solutions without any added salts, we observe scaling laws for low-ionic strength semidilute polyelectrolyte solutions in agreement with the double screening theory. These scaling laws, along with the adequate fits of the labeled chain structure factor with the Debye structure factor, highlight the concept of screening in semidilute solutions and polyelectrolytes obeys Gaussian chain statistics on length scales of the order of a renormalized Kuhn length. Significant coil contraction is measured upon the addition of the multivalent salt. Upon comparing the correlation length, the radius of gyration, and the Debye length, the radius of gyration remains the dominant length scale in the system, until a crossover is observed as the correlation length diverges and surpasses the labeled chain dimension with increased ionic strength. The double screening theory was applied to understand the dependence of size of the labeled chains as functions of polymer concentration and added multivalent salt. It was necessary to include the influence of ion-pairing into a salt-concentration de pendent degree of ionization. Such ion-pair formation is also necessary to calculate phase diagrams with better qualitative agreement with experimental data. These initial efforts should foster strong theoretical and simulation studies and further experimentation in the area of polyelectrolyte solutions.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2002