Light and neutron scattering studies of excess low-angle scattering in moderately concentrated polystyrene solutions

The light scattering (LS) and small-angle neutron scattering (SANS) behaviour of semi-dilute solutions of polystyrene has been determined in both ‘good’ and ‘theta’ solvents. Above a critical concentration related to chain overlap, an excess small-angle scattering component is in evidence for scattering vectors, q, such that qR_g < 1. Application of a number of recent solution scattering theories fails to account for the small-angle scattering observed. The inter- and intramolecular scattering functions are measured experimentally through characterization of the SANS behaviour of solutions containing mixtures of polystyrene and perdeuteropolystyrene. The resultant intermolecular scattering functions depend on the fraction of labelled chains, indicating clearly that the solutions contain large scale fluctuations. LS studies support this hypothesis and further show that the presence of these fluctuations is reproducible, yet dependent on the solution preparation procedure. Similar behaviour is observed in screening length measurements. The excess low-angle scattering is well characterized by the Debye—Bueche random two-phase model, which is subsequently used to estimate the characteristic dimensions of the long-range fluctuations.