Concentration-Dependent Long-Range Repulsive Interactions of Adsorbed Associative Polymers

Colloidal atomic force microscopy is used to study the quasi-static and velocity-dependent normal force-separation response of adsorbed layers of associative and non-associative olefin copolymers (OCP). OCP containing a small fraction of a polar group forms transient bonds, leading to a stronger concentration-dependent viscosity than native OCP. This functional group also imparts a strong surface affinity to OCP, leading to strong adsorption. Quasi-static measurements show that the surfaces with native OCP have minimal interaction on approach, and strong adhesion on retraction due to bridging. In contrast, surfaces with associative OCPs exhibit repulsive interactions on approach with the magnitude and onset increasing with the polymer concentration. The interaction distances are long range, approaching 10x the unperturbed polymer dimension. Adhesion on retraction is greatly reduced and occurs over a range close to the contour length of the polymer. Increasing the approach velocity leads to long range repulsion for both polymers due to hindered solvent drainage. The magnitude of repulsion is highest for the associating OCP, but the onset of interaction is comparable for both. These results correlate well with impact of these polymers on friction in the boundary regime.