Polymer brushes produced by controlled surface-initiated polymerization provide a route to surfaces coated with well-defined thin polymer films that are covalently bound to the substrate. All of the major controlled polymerization techniques have been applied to the synthesis of polymer brushes and examples of each are presented here. Many examples of brush synthesis in the literature have used the living atom transfer radical polymerization (ATRP) system, and in this tutorial review a particular focus is given to examples of this technique.

We have achieved a significant breakthrough in the synthesis of polyelectrolyte brushes of controlled thickness and density, which has been demonstrated by the synthesis of triblock copolymer brushes composed of cationic, neutral, and anionic segments.

Polymers are ideal building blocks for nanofabrication and the engineering of surfaces because of their response in shape and size to environmental changes in polymer concentration, ionic strength, pH, temperature, and solvent. Furthermore, since they are easy to synthesize, as well as chemically and mechanically robust, polymers are ideal materials to bridge the gap between biological nanomachines and silicon-based devices. Of particular interest herein is the exploration of the use of polymer brushes as nanoactuators. [1] Polyelectrolytes, that is, polymers containing charged monomers, change from an extended conformation in a solution of low ionic strength to a coiled conformation in solutions of high ionic strength.[2] When polyelectrolytes are covalently attached to a surface to form a brush, the polymer chains show a strongly extended (stretched) conformation in pure water, as a result of both the repulsion between neighboring chains and the repulsion between the monomers. [3][4][5][6] In contrast, when polymer brushes are placed in electrolyte solutions, the charges of the pendant groups in the polymer chains are screened, and the minimization of the electrostatic repulsions leads to entropically more favorable collapsed conformations. [4][5][6][7][8] This collapse is comparable to transitions from extended to coil-like conformations commonly observed for polyelectrolytes in solution. [3,4] In the case of polymer brushes, there are constraints on the degrees of freedom related to conformational changes, and as a result the height and the amount of water on the inner brush environment are reduced. [5,8] Our aim is to use the ionic-strength-sensitive behavior of polyelectrolyte brushes [9][10][11] in combination with specific binding interactions and ion-size-exclusion effects to design smart polymer surfaces that can be switched between a permanently collapsed and an extended state, where both states have different surface properties. In addition, we aim to find a route toward a "fast switch" that reversibly alternates between different polymer brush conformations, and as a consequence gain control over surface properties [12,13] such as wettability, hydration, and stiffness. Reversibly manipulating the polymer-brush conformation in a chemically selective way (not only by changing the ionic strength) will be a crucial step toward the use of surface-confined polyelectrolytes as nanoactuators.Cationic brushes of [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) [14] were grown from gold surfaces, patterned into regions containing either initiator or methyl-terminated monolayers (Figure 1 a, b), by atom-transfer radical polymerization (ATRP) in an oxygen-free methanol/water (4:1) mixture. [15,16] The responsive behavior of the METAC brushes in different electrolyte environments (NaCl and MgSO 4 ) was studied under liquids using atomic force microscopy (AFM) and a quartz crystal microbalance with dissipation (QCM-D). [17] The conformational changes in the METAC brush in the presence of 1m NaCl electr...

Cyclic voltammetry and impedance spectroscopy were employed to probe the responsive properties of polyelectrolyte brushes. Poly[(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes over 100 nm thick on gold substrates were synthesized via surface-initiated atom-transfer radical polymerization and quaternized with methane iodide to obtain cationic brushes (Q-PDMAEMA). Q-PDMAEMA brushes respond to electrolytes by exhibiting swollen and collapsed states. Swollen brushes allow good permeability of electroactive probes, while collapsed states block electron transport. Electrolytes have different impacts on the electrochemical properties of Q-PDMAEMA. Some salts (NaNO3) cause brush collapse due to charge screening, while others such as those with more hydrophobic anions (ClO4-, PF6-, and Tf2N-) induce brush collapse because of solubility changes. The collapsed brushes exhibit intrinsically different resistance as probed with impedance. Charged screened brushes retain good permeability to electroactive probes. Strongly coordinating hydrophobic anions lead to insoluble brushes, resulting in a high resistance. These results show that electrochemical impedance spectroscopy is a powerful technique to probe the properties and structure of polyelectrolyte brushes.