Effect of Solvent Condition on the Dynamic Response of Polymer Brushes
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ABSTRACT The frictional response of two opposing polymer brushes subjected to steady shear was studied as a function of solvent condition and degree of compression using the Surface Forces Apparatus (SFA). The brushes were made by preferential adsorption of polyvinylpryridinepolystyrene (PVP-PS) block copolymers adsorbed onto atomically smooth surfaces from a dilute solution of toluene. Extremely parallel lateral motion was imparted to one surface and the response was detected at the opposing brush-covered surface. When the brushes were bathed in a good solvent, it was necessary to strongly compress the layers in order to observe frictional forces transmitted between the surfaces. However, when the solvent was changed to a near-theta solvent, large frictional forces were measured at weaker compressions. The onset of these frictional forces occurred at distances comparable to the distance at which the opposing layers contact one another and rapidly increase as the brushes are compressed. Arguments are advanced that this behavior is attributable to frictional interactions between the polymer chains and the solvent. INTRODUCTION Many applications for polymers require that they adsorb at interfaces or surfaces. The ability to manipulate the structure and properties of the adsorbing layer provides a route to affect the adherence of materials, the rheology of colloidal suspensions, and the lubricative or wear properties of the surface. Furthermore, utilization of functionalized chains, coupled with the ability to manipulate the molecular-level structure and properties of the tethered layer, offers a potentially attractive means for making microvalves, sensors, or chemo-mechanical layers that are designed to respond to a particular chemical stimuli. In order to develop systems for these applications, it is of critical importance to understand the frictional response of the surface-tethered chains that are subjected to shear due to relative motion of the surrounding fluid. A-B diblock copolymers, which can be viewed as long-chained analogs of surfactant molecules, are extremely effective surface-modifying agents. The amphiphilic nature of these materials allows them to straddle phase boundaries and form "polymer brushes". Polymer brushes can be self-assembled onto a solid surface by using a solvent that is good for one block but poor for the other [1-5]. This scheme of tethering A-B diblock copolymers to a substrate is often referred to as preferential or selective adsorption. The non-solvated block adsorbs in a thin layer on a surface, driven by its desire to minimize its contact with the solvent. The adsorption of this "anchor" block tethers the well-solvated "buoy" block to the surface. When the distance between adjacent chains is less than the free-solution radius-of-gyration of the buoy block, the tethered layer becomes crowded. To alleviate the lateral crowding the well-solvated blocks stretch away from the surface, resulting in the brush structure depicted by Figure 1. Brushes can also be made using triblock copolym
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