Single-Molecule Tracking of Polymer Surface Diffusion Michael J. Skaug, Joshua N. Mabry, and Daniel K. Schwartz* Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States * S Supporting Information ABSTRACT: The dynamics of polymers adsorbed to a solid surface are important in thin-film formation, adhesion phenomena, and biosensing applications, but they are still poorly understood. Here we present tracking data that follow the dynamics of isolated poly(ethylene glycol) chains adsorbed at a hydrophobic solid-liquid interface. We found that molecules moved on the surface via a continuous-time random walk mechanism, where periods of immobilization were punctuated by desorption-mediated jumps. The dependence of the surface mobility on molecular weight (2, 5, 10, 20, and 40 kg/mol were investigated) suggested that surface-adsorbed polymers maintained effectively three-dimensional surface conformations. These results indicate that polymer surface diffusion, rather than occurring in the two dimensions of the interface, is dominated by a three- dimensional mechanism that leads to large surface displacements and significant bulk-surface coupling. ■ INTRODUCTION In lubrication 1 and adhesion phenomena, 2 at biointerfaces, and in thin-film formation processes, 3 polymer molecules adsorb to a solid surface, 4 and their dynamics govern subsequent relaxation and transport. While the motion of polymers in the “melted” state or in solution is fairly well understood, 5 the mechanisms by which polymers move on surfaces remain mysterious and a matter of debate. 6-12 It is clear, however, that polymer dynamics are significantly slowed near an attractive interface. 4,13 For example, surface diffusion coefficients are often orders of magnitude lower than bulk values, 4,14 but the available experi- mental evidence does not conclusively identify a dominant mechanism of polymer surface diffusion and suggests that the mechanism may depend on the surface and the chain length. 9,10 Part of the difficulty in understanding polymer surface dynamics is that polymer surface conformations may be different than bulk conformations and can vary depending on the polymer-surface interaction, the chain length, and surface coverage. 13 The conventional picture is that polymers adsorb to a solid surface with a “loop-train-tail” conformation in which adsorbed chain segments are “trains” separated by “loops” of unabsorbed monomers. 13 The bound fraction (the fraction of polymer segments adsorbed in trains) is one measure of the adsorbed chain conformation. The equilibrium bound fraction is predicted to depend on the monomer-surface interaction energy, χ s , and the chain length, N. For short chains (N < 10) or strongly adsorbing monomers (χ s >1kT), the bound fraction approaches unity, while for longer chains or weakly attractive monomers, the bound fraction is typically in the range 0.5 to 1. 6,15 Previous experiments have found bound fractions between 0.5 and 0.75 for polymers adsorbed at low surface coverage, suggesting a flattened two-dimensional conforma- tion. 9,16 However, this picture is further complicated if adsorbed chains relax toward equilibrium very slowly. 17,18 Whether an adsorbed chain is strictly two-dimensional or in a more three-dimensional conformation has a signi ficant influence on the possible mechanism by which a polymer moves across a surface. To uncover the detailed mechanism of polymer surface dif- fusion, we conducted a series of single-molecule tracking experi- ments to probe the behavior of isolated linear homopolymer chains at a hydrophobic solid-aqueous interface. Specifically, we studied a series of poly(ethylene glycol) (PEG) chains whose molecular weight varied by more than an order of magnitude. The primary driving force for PEG adsorption to the solid surface was the hydrophobic interaction. 9,19 Because the hydrophobic interaction is nonspecific and relatively long-ranged compared with other intermolecular forces, the system we studied represents the case of a delocalized, long-range polymer-surface interaction. We observed polymer surface transport characterized by desorption-mediated displacements that were interrupted by periods of immobility, qualitatively similar to a previous report for the surface diffusion of other molecular species. 20 A desorp- tion-mediated surface displacement is one where, instead of moving in the plane of the surface, the molecule desorbs, diffuses in the bulk liquid, and readsorbs at a new surface location. A specific example of a continuous-time random walk, 21 this mechanism can be described as “intermittent hopping” because each desorption-mediated surface displacement is separated by a random period of apparent surface immobilization. One con- sequence of the desorption-mediated mechanism is that large displacements are much more probable than if the process involved normal (Gaussian) Brownian motion within the plane of the surface. The prevalence of large surface displacements and the intermittency of the trajectories are predicted to dramatically influence the rate at which a polymer finds a surface target, 22,23 a key process in heterogeneous catalysis, biosensing, and other Received: July 18, 2013 Published: November 10, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 1327 dx.doi.org/10.1021/ja407396v | J. Am. Chem. Soc. 2014, 136, 1327-1332