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