Harry E. Johnson

A Simple Kinetic Model of Polymer Adsorption and Desorption

A model of the desorption and adsorption of a polymer layer at a planar surface indicates a transition from exponential kinetics at high temperatures to nonexponential kinetics (stretched exponential with index one-half) at lower temperatures where these processes are diffusion-limited. Measurements of polystyrene desorption through polyisoprene overlayers show this predicted transition. Corroborative results are obtained for polystyrene desorption through polymethylmethacrylate overlayers. This identification of two distinct kinetic regimes suggests a unifying perspective from which to analyze polymer and biopolymer mobility at surfaces.

New Mechanism of Nonequilibrium Polymer Adsorption

Nonequilibrium states of surface composition can be extremely long-lived when polymer chains adsorb competitively. In a model system (polymethylmethacrylate adsorbed from CCl4 onto oxidized silicon previously saturated with polystyrene), it is shown that a weakly adsorbing polymer was sterically pinned to a surface by a more strongly adsorbing polymer. The dynamical evolution of the surface composition was strongly nonexponential in time and non-Arrhenius in temperature; the phenomenology is analogous to bulk glasses. This interpretation offers a new mechanism to explain why weakly adsorbing chains may bind to surfaces, as well as a direction in which to look for a method to release them.

Overshoots as polymers adsorb

Abstract An overshoot in the time-dependent mass adsorbed from dilute solution was observed for poly(dimethylsiloxane) and cis -polyisoprene adsorbed onto single oxide surfaces. However, no overshoot was observed for poly(methyl methacrylate) and polystyrene, suggesting the possibility that the overshoot reflects transient surface-induced crystallization.

Regimes of polymer adsorption-desorption kinetics

Abstract We show, and support with experiments, two limiting regimes of polymer adsorption-desorption. When these processes are rate limited by diffusion they are expected to be non-exponential in time and the time constant is inversely proportional to the diffusion coefficient. When these processes are rate limited by the energetics of surface detachment, they are expected to be exponential in time and the time constant is exponential in the molecular weight. This suggests a unifying perspective from which to analyze polymer mobility at surfaces.