Light scattering characterization of polystyrene latex with and without adsorbed polymer

Dynamic light scattering (LS) is sometimes used to determine the thickness of adsorbed layers of polymers by comparing the hydrodynamic radii determined with and without the adsorbed layer. Here we point out a number of pitfalls with this method for one particular polystyrene latex. For example, even without an adsorbed layer, the hydrodynamic diameter determined by dynamic LS is 152 nm while that determined by TEM is 140 nm. Moreover, static LS also yields a larger diameter (150 nm) after subtracting a 3 nm correction for the difference between Mie and Rayleigh scattering. Both the angular dependence of dynamic LS and a cumulants analysis of the autocorrelation function at each angle suggest some polydispersity in size which explains the larger diameter from LS: TEM yields the number-average diameter whereas LS yields a z-average (heavily weighted toward larger particles). The greater polydispersity deduced from LS than reported with TEM might arise from doublet formation. The small difference between diameters determined by dynamic and static LS might be due to electroviscous effects or finite particle concentration. Non-sphericity of particles is not important. The polydispersity with adsorbed polymer (F108 Pluronic) was found to be larger than without. The layer thickness inferred from the difference in average hydrodynamic radius (11 nm) is overestimated owing to the increase in polydispersity with polymer adsorption.