Time and Distance Dependence of Reversible Polymer Bridging Followed by Single-Molecule Force Spectroscopy Michael J. Serpe, ²,§ Monica Rivera, ‡,§, Farrell R. Kersey, ²§ Robert L. Clark,* ,‡,§ and Stephen L. Craig* ,²,§ Department of Chemistry, Department of Mechanical Engineering and Materials Science, and Center for Biologically Inspired Materials and Material Systems, Duke UniVersity, Durham, North Carolina 27708-0346 ReceiVed NoVember 2, 2007. In Final Form: February 18, 2008 Polymer bridging between surfaces plays an important role in a range of fundamental processes in the material and life sciences. Bridges formed by main-chain reversible polymers differ from their covalent analogs in that they can dynamically adjust their size and shape in response to external stimuli and have the potential to reform following bond scission. In this work, the time and distance dependence of main-chain reversible polymer bridge formation are studied using an atomic force microscope. The bridging process was studied using single-molecule force spectroscopy, and its dependence on the distance between surfaces and equilibration time was probed. The number of bridges formed decreases as the gap width increases, from ∼2 bridges per 14 s equilibration at separations of 5-15 nm to ∼0.5 bridges per 14 s equilibration at separations of 35-45 nm. The kinetics of bridge formation appear to be slightly faster at smaller separations. Introduction Polymer adsorption to, and between, surfaces has a dramatic impact on interfacial and colloidal properties. The bridging of polymers from one surface to another, for example, has important consequences in adhesion, tribology, and colloidal stability. 1 Many of these consequences are relatively well understood for covalent polymers (i.e., the competition between particle ag- gregation via depletion attractions and particle dispersion via steric stabilization). In recent years, the potential importance of surface interactions mediated by main-chain reversible polymers (also known as supramolecular, equilibrium, or living polymers) has been recognized. The monomers of main-chain reversible polymers (hereafter, reversible polymers) are not connected by covalent bonds, but rather they are held together via directional, weak interactions such as hydrogen bonding, 2 metal-ligand coordination, 3-8 and DNA base pairing. 9-11 van der Gucht et al. 12 have pointed out that both the structure and the dynamics of reversible polymers on and between surfaces, as well as the interfacial properties they mediate, are expected to differ significantly from those of their covalent counterparts. In particular, the number of bridging events deviates from that of covalent structures with similar molecular weight, and these bridging events are directly related to the attractive forces that lead to colloidal aggregation. 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