Surface nano-structuring of materials by atmospheric pressure plasma-induced graft polymerization

Recent advances in polymer surface engineering have demonstrated the ability to direct and control nano-scale polymer growth from surfaces to create unique chemical functionalities and surface topography. Synthetic routes towards polymer surface structuring are necessary for the development of advanced materials for membrane separations and chemical sensors. Polymer surface modification has previously relied on spin-casting techniques. Such polymer layers, however, desorb (or delaminate) upon chemical, thermal and shear stresses. In contrast, single molecule polymer chains can be grown directly from the surface to create robust, grafted (i.e., covalently tethered to the surface) chains by graft polymerization. Two main challenges exist: (1) creation of a high number density of surface initiation sites, and (2) controlled growth of polymer chains from the surface. The classical approach, free-radical graft polymerization, relies on surface immobilized macroinitiators and results in polydisperse polymer growth. In the current work, atmospheric plasma surface activation was employed to create surface reactive groups, on organic and inorganic substrates, which effectively function as surface initiators. Polymer growth from plasma-initiated sites may still be subject to early termination or chain transfer, leading to a high polydispersity of polymer chain lengths. Therefore, the integration of plasma surface activation with controlled "living" radical graft polymerization has been investigated to create a high surface coverage of uniform length surface-bound polymers. Controlled "living" graft polymerization of styrene was achieved by establishing a reversible thermodynamic equilibrium between radical polymer chains and a chemical control agent. The control agent was a free-radical scavenger that reversibly binds to the growing polymer chains and thus retards chain termination and enables controlled chain growth. While there are various approaches to controlled "living" polymerization, the current approach has the advantage of not requiring surface macroinitiators or catalysts while enabling the synthesis of a polymer brush layer of a relatively low polydispersity. A series of graft polymerization studies were carried out to elucidate the polymer growth kinetics and film properties for classical and controlled "living" plasma-induced graft polymerization. The applications of the polymer layers toward the synthesis of thin film chemical sensing layers and low fouling separation membranes will be discussed and demonstrated.