Nanostructured Polystyrene Films: Graft Polymerization and Organic Sorption Behavior
Gregory T. Lewis, Chemical and Biomolecular Engineering, University of California, Los Angeles, 5531 Boelter Hall, Los Angeles, CA 90095 and Yoram Cohen, Chemical & Biomolecular Engineering, University of California, Los Angeles, 5531 Boelter Hall, 420 Westwood Plaza, Los Angeles, CA 90095-1592.
Recent advances in polymer surface engineering have demonstrated the ability to direct and control nano-scale polymer brush growth from substrate 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, chemical sensors, and biotechnology. Polymer surface modification has, in the past, relied on traditional adsorption and spin-coating techniques. Such polymer layers, however, can 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, however: 1) creation of a high number density of surface initiation sites, and 2) controlled growth of polymer chains from the surface initiation sites. The classical approach, free-radical graft polymerization, relies on surface immobilized macroinitiators and results in polydisperse polymer growth. In the current work, atmospheric pressure plasma surface activation has been employed to create surface reactive groups, on both organic and inorganic substrates, which effectively function as surface initiators. Polymer growth from plasma-initiated surface 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/initiation with controlled "living" radical graft polymerization has been investigated, in the present study, to create a high surface coverage of uniform length polymer chains tethered to a inorganic substrate surface. Controlled “living” radical graft polymerization of styrene was achieved by establishing a reversible thermodynamic equilibrium between radical polymer chains and a chemical control agent (CCA). The CCA is a free-radical scavenger that reversibly binds to the growing polymer chains and thus retards the chain termination, thereby enabling control of chain growth. While there are various approaches of controlled "living" polymerization, the current approach has the advantage of not requiring the presence of surface macroinitiators or catalysts and 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 layer growth kinetics and polymer film properties for plasma-induced graft polymerization under both classical and nitroxide-mediated mechanisms. The potential application of the present nanostructured polymer layers toward the synthesis of thin film chemical sensing layers was evaluated via a series of organic sorption studies. The results of this latter part of the study indicated that a higher film diffusivities and organic sorption capacities were feasible with the grafted polymer layers, relative to physically adsorbed or spin-coated sensing layers.