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(151g) Dual-Responsive Block-Arm Star Copolymers for on-Demand Reduction or Enhancement of Friction and Adhesion

Authors: 
Tilton, R. D., Carnegie Mellon University
Riley, J. K., Carnegie Mellon University
Achieving low friction between moving parts undergoing sliding contact is of considerable and obvious importance in the efficient operation of machinery. It is equally desirable for the comfortable, low wear functioning of natural or artificial joints in the body. The importance of achieving low friction has motivated many studies of novel lubricating systems, including boundary lubrication by swollen polymer brushes. The attainment of very low friction coefficients by planar brushes in a good solvent is well established. There are situations where higher friction is the desirable condition. For example, greater traction between surfaces requires larger frictional forces. Consumer perception of the â??feelâ? of a surface is influenced by friction, and friction among suspended particles has been implicated as a possible source of shear thickening rheology. While it is possible to modulate the friction between planar brushes for a given load by modulating solvent quality, the dynamic range of accordingly modified frictional forces that have been reported is not large. We have developed a system based on adsorption of responsive block-arm star copolymers as friction control agents. These are designed to be triggered to reversibly decrease or increase friction in response to pH changes. As such, they include a weak polycation block among the arms. The magnitude of the frictional dynamic response to stimuli can be manipulated by controlling the non-equilibrium packing of the adsorbed layer, for example by controlling temperature during the initial adsorption of the star copolymers to the surfaces. An approximately two order of magnitude change in friction can be triggered for a given star copolymer system, with load-dependent friction forces that exceed those of the bare surfaces under one condition, transitioning to frictional forces much weaker than those of the bare surface under changed conditions. The response is based in part on the change in apparent friction coefficient and in part on the change in adhesion.

The manipulation of adhesion is particularly important. The full dynamic range is tuned by adjusting the ability of star copolymer arm segments to bridge between opposing surfaces. This takes advantage of the ability of adsorbed star copolymer arms to adapt their chain extensions both normal and parallel to the underlying surface. Star copolymers adsorbed at high coverage may fully block bridging access to arms emanating from star copolymers adsorbed to the opposing surface when the stars are swollen under lower pH conditions that yield strong polycation charging. Increasing pH to de-swell the star copolymers exposes underlying surfaces to bridging adhesion and large friction. A series of block-arm star copolymers was synthesized, consisting of an innermost temperature-responsive block of poly(di(ethylene glycol) methylether methacrylate), a second block of temperature- and pH-responsive poly(2-(dimethylamino)ethyl methacrylate), and in some cases an outermost zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine). The first two blocks are surface active on silica, the model surface for this study, and provide stimulus response. The latter block is not pH- or temperature-triggerable under the range of conditons examined here, and provides a high degree of hydration. The bulk swelling characteristics of these materials are monitored by dynamic light scattering, and adsorption is monitored by quartz crystal microbalance with dissipation and/or ellipsometry. Normal and tangential forces between opposing surfaces decorated by adsorbed star copolymers are measured by colloidal probe atomic force microscopy. This presentation will focus on the exploitation of thermal responsiveness to control adsorbed layer packing, the subsequent reversible manipulation of star copolymer swelling and bridging induced by pH changes and the resulting quantitative and qualitative changes in dynamic friction forces.