Chain-End Attraction Causes Excess Scattering in Simple Polymer Solutions

For many decades, researchers have studied polymer solutions using small angle scattering techniques and attempted to explain deviations (i.e., excess scattering) from classical models. While these phenomena are sometimes explained away as uncontrolled experimental factors (e.g., dust), there can be a polymeric origin to the excess scattering, but few interpretable, physics-based models exist to describe it. For example, the Debye-Bueche model provides an ambiguous ‘correlation length’ but no physical origin of low-q excess scattering. In this contribution, we explain two observations of excess scattering in semi-dilute polystyrene solutions: (1) low-q excess scattering indicating long-range concentration fluctuations despite marginally good solvent conditions and (2) excess scattering near qR_g = 1, when crowded by contrast-matched chains. To explain these phenomena, we develop a liquid-state theory (PRISM) based scattering model which describes the scattering over all wavenumbers and all concentrations studied. With this model, we demonstrate that both phenomena can be explained by incorporating end-monomer interactions that account for the non-solvency of the acidic methanol end-groups of the polymer.