Surface Modification by Chemically Grafted Initiators

To modify the surface properties of inorganic materials, we successfully covalently attached a stable monolayer, with high grafting density, of N,N(diethyl)dithiocarbamoylbenzyl(trimethoxy)silane on the surface of silicon wafers and glass substrates, thereby creating a surface-grafted photoinitiator for “living” free radical photopolymerization. The surface-grafted initiators were utilized for the controlled growth of homopolymers and block copolymers from the surface of inorganic substrates. By using “living” free radical polymerizations, we were able to control the length of the grafted polymer chains and therefore the layer thickness up to ~100 nm. The increase in layer thickness after photopolymerization was directly measured by SFM using silicon wafers patterned with chromium. Furthermore, the samples were characterized by contact angle measurements, XPS, transmission IR, SEM, and TEM. The photopolymerizations of styrene and MMA monomers were used to demonstrate the surface-initiated “living” free radical photopolymerization representing a polymerization method in a “confined twodimensional space”. This is supported by (a) the linear increase of the polymer layer thickness, (b) the reinitiation of the photopolymerization after interruption of the process, (c) the occurrence of block copolymerization after switching from styrene to a methyl methacrylate monomer, and (d) the formation of a well-defined homopolymer monolayer or block copolymer bilayer. This method provides a novel approach to chemically modifying the surfaces between the active polymer and the electrodes in opto-electronic devices like polymer-based light-emitting diodes and photovoltaic cells. The work presented in this Chapter is covered by the following paper: ’“Living” Free Radical Photopolymerization Initiated from Surface-Grafted Iniferter Monolayers’, de Boer, B.; Simon, H. K.; Werts, M. P. L.; van der Vegte, E. W.; Hadziioannou, G., Macromolecules, 33, 349–356 (2000).