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Abstract
Responsive polymer brushes possess many interesting properties that enable them to control a range of important interfacial behaviours, including adhesion, wettability, surface adsorption, friction, flow and motility. The ability to design a macromolecular response to a wide variety of external stimuli makes polymer brushes an exciting class of functional materials, and has been made possible by advances in modern controlled polymerization techniques. In this review we discuss the physics of polymer brush response along with a summary of the techniques used in their synthesis. We then review the various stimuli that can be used to switch brush conformation; temperature, solvent quality, pH and ionic strength as well as the relatively new area of electric field actuation We discuss examples of devices that utilise brush conformational change, before highlighting other potential applications of responsive brushes in real world devices.
Highlights
Why Polymer Brushes? The Physics of Brush ResponsePolymer brushes are formed when polymer chains are tethered at one end to a surface at sufficient density to overlap and stretch away from the surface
Zhou and Huck [65] demonstrated that AFM microcantilevers grafted on one side with a poly[2-(methacrylolyloxy)ethyl] trimethyl ammonium chloride (PMETAC) strong polyelectrolyte brush layer could be actuated by the application of a voltage between the tip holder and a remote electrode within a liquid cell containing weak salt solutions
The name “polymer brush” can really refer to many things, though these varied systems all have the defining feature of having polymer chains grafted at one end to a substrate at sufficient density to overlap and stretch away from the surface
Summary
Polymer brushes are formed when polymer chains are tethered at one end to a surface at sufficient density to overlap and stretch away from the surface. These nanoscale surface layers are “smart”, i.e., their properties can be switched in response to external triggers [1], and are prime candidate. When poolymer chainns are grafted at one ennd to a surfface at sufficient densitty to allow the chains to t o overlap, theyy stretch aw way from thhe surface innto a brush-like conform mation [11]]. As the grafting density increases, and the grafting distance decreases, the polymer molecules are increasingly stretched away from the surface in a brush-like equilibrium conformation. In polyelectrolyte brushes, where electrical charges are present upon the polymer chains, electrostatic interactions become important either directly through repulsive interactions between charged monomers, or indirectly since they dictate the distribution and the osmotic pressure of the counterion cloud which remains bound to the brush to preserve electroneutrality
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