Analytical ultracentrifugation of latexes

Abstract

Analytical Ultracentrifugation (AUC) is a fractionating technique based on the synchronized optical detection of the concentration profiles of the sample under a centrifugal acceleration of up to 300,000 g. The skilled choice of solvent, rotational speed and detection optics enables a variety of complementary AUC experiments due to the controlled interplay of centrifugal force, hydrodynamic friction, buoyancy, and diffusion. AUC resolves an individual peak width of less than 3%, such that AUC is the reference method for the polydispersity and shape of size distributions. If the parameters of particle density, solvent density and viscosity are known, AUC is an absolute technique that determines correct diameters and shares for 10-component latex mixtures from sub-10 nm to ~ 5 μm diameter. For standard polymers, the density is tabulated and most copolymer densities can be safely interpolated based on volume percentages; densities of exotic polymers are easily determined in Krattky gauges. Beyond size distributions, the dependence of sedimentation speeds on buoyant density is turned into a unique analysis opportunity by the methods of correlated size/density analysis and density gradients, AUC thus determines the chemical homogeneity of latexes via their density distribution. In the case of core-shell lattices and other nano-hybrids, the density approach is complementary to the size approach, but more precise and resolved with 0.002 g/cm³, corresponding to few-% resolution of chemical composition.