Translational diffusion of small and large mesoscopic probes in hydroxypropylcellulose-water in the solutionlike regime

Abstract

Quasi-elastic light scattering spectroscopy was used to study the translational diffusion of monodisperse spheres in aqueous 1 MDa hydroxypropylcellulose (HPC) at 25 °C. Probe diameters d spanned 14–455 nm; HPC concentrations were 0⩽c⩽7g/L. Light scattering spectroscopy consistently found spectra having the form g(1)(t)=(1−Af)exp(−θtβ)+Af exp(−θftβf). Here θf and βf refer to the “fast” mode; θ and β describe the “slow” mode. We examine the dependence of θ, β, θf, βf, and Af on d, c, scattering vector q, and viscosity η. β=1 for large probes; elsewise, β and βf are ∈(0,1). The slow mode, with short-lived memory function, is diffusive; for large probes θ≈(dη)−1. The fast mode, with long-lived memory function, appears coupled to polymer chain internal dynamics. Probe behavior differs between “small” and “large” probes. Small probes have diameters d<Rh, Rh being the chain hydrodynamic radius. Large probes have d⩾Rg, Rg being the polymer radius of gyration.