Polymer-particle interactions in aqueous solution

We characterize the interactions of polymers and colloidal particles by studying the particle aggregation behavior in the presence of adsorbing polymers. Our approach is to start with a system containing particles only, and then proceed to a polymer-mediated particle suspension system.; In the bare-particle system, we make use of the development of CONTIN, a constrained inverse Laplace Transform routine, and dynamic light scattering (DLS) to monitor structures formed for both diffusion-limited (DLA) and reaction-limited (RLA) aggregation processes. We combine the light scattering theory with the Smoluchowski rate equation to model the intensity distribution of light scattered by clusters. A fit of the model prediction to the CONTIN output provides fractal dimensions, d{dollar}sb{lcub}rm f{rcub}{dollar}. We find that d{dollar}sb{lcub}rm f{rcub}{dollar} increases with aggregation time in DLA, suggesting reorganization of particles after they aggregate; this phenomenon does not appear in RLA. Early in the aggregation process, the polydispersity exponent {dollar}tau{dollar} for the power-law mass distribution for RLA clusters is 1.5.; In the flocculation of colloidal silica by pyrene end-tagged poly(ethylene glycol) (Py-PEG-Py), we use both DLS and fluorescence spectroscopy techniques to measure aggregate sizes and pyrene-pyrene interactions, respectively. DLS probes length scales from a few nanometers to several tenths of a micron, enabling us to study the overall aggregate dimensions. By contrast, fluorescence is sensitive to changes on a molecular level, allowing us to investigate the mechanism of polymer-induced aggregation. Our results demonstrate that DLS and fluorescence techniques provide an ideal combination to study particle/polymer interactions in solution.; The average aggregate size, {dollar}langle Rrangle{dollar}, grows with ionic strength, due to the anticipated charge screening effect. In addition, {dollar}langle Rrangle{dollar} increases with Py-PEG-Py concentration, indicating that the hydrophobic interaction enhances flocculation. Fluorescence excitation spectra provide evidence of intensified pyrene-pyrene interactions as the degree of flocculation increases. The emission spectrum, analyzed in terms of the excimer-to-monomer ratio {dollar}rm (Isb{lcub}e{rcub}/Isb{lcub}m{rcub}),{dollar} exhibits a consistent increase with ionic strength, implying enhanced excimer formation as a result of increasing polymer layer overlap. We compare our emission data with a qualitative model based on the chain segment density distribution developed by Ploehn and Russel.