Fabrication of Patterned High-Density Polymer Graft Surfaces. 1. Amplification of Phase-Separated Morphology of Organosilane Blend Monolayer by Surface-Initiated Atom Transfer Radical Polymerization Muhammad Ejaz, Shinpei Yamamoto, Yoshinobu Tsujii, and Takeshi Fukuda* Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received March 1, 2001; Revised Manuscript Received October 23, 2001 ABSTRACT: Patterned films of a low-polydispersity polymer densely end-grafted on a silicon substrate were fabricated for the first time by the combined use of the Langmuir-Blodgett (LB) and the surface- initiated atom transfer radical polymerization (ATRP) techniques: a blend monolayer of 2-(4-chlorosul- fonylphenyl)ethyltrimethoxysilane (CTS: ATRP initiator) and n-octadecyltrimethoxysilane (OTS: non- initiator) was immobilized on a silicon wafer by the LB technique, and then the ATRP of methyl methacrylate was carried out on the modified wafer in the presence of the Cu/ligand complexes. Atomic force microscopic studies revealed that the CTS/OTS blend was immiscible and phase-separated into two monolayer phases: most OTS molecules aggregate with each other, forming a condensed-type monolayer domain with CTS molecules excluded from there almost perfectly, and the remaining OTS molecules are incorporated in the matrix region. This 2-dimensionally phase-separated structure was successfully amplified by the controlled growth of a high-density graft layer only on the matrix phase of CTS as a main component. The amplification by the ATRP technique was characterized by a sharp boundary between the grafted and ungrafted domains; as a measure of the spatial resolution, the boundary sharpness Δw was evaluated to be ca. 100 nm. The domain size in the phase-separated structure was independent of the mole fraction of CTS, while it could be changed by changing the pH of the subphase water: namely, the higher was the pH, the larger was the domain size. It was deduced that a change in pH of the subphase water gave rise to a change in the hydrolysis rate of the methoxysilyl groups into silanol groups and hence a change in the rate of condensation of the silanol groups into domains. Introduction In recent years, surface modifications by polymers have been increasingly important for various applica- tions ranging from biotechnology to advanced mi- croelctronics. Graft polymerization starting with the initiating sites fixed on a surface is one of the most effective and versatile methods for this purpose, since surface properties can be widely changed by graft- polymerizing a variety of functional monomers. 1-7 Re- cently, some living polymerization techniques were successfully applied to surface-initiated graft polymer- ization to prepare a dense polymer brush. 8-16 We were the first to succeed in applying atom transfer radical polymerization (ATRP) 17 to the graft polymerization on a solid surface and yielding a graft layer of low- polydispersity polymer with the highest graft density reported to date. 9 Furthermore, we revealed that in such a graft layer polymer chains were highly extended in a good solvent, nearly to their full lengths, and that the properties of this high-density polymer brush were quite different and unpredictable from those of the “moder- ately dense” polymer brushes previously studied. 9c,d In addition to such parameters as graft density and the chain length and length distribution of graft polymer, the morphology of the grafted surface is also an impor- tant factor determining such surface properties as chemical reactivity, wettability, permeability, lubricity, biocompatibility, and electrical properties. For example, it was reported that the surface coated with a poly(2- hydroxyethyl methacrylate-b-styrene-b-2-hydroxyethyl methacrylate) triblock copolymer exhibits an excellent blood compatibility, possibly because of the unique surface morphology resulting from the microphase separation of the block copolymer. 18-23 In this paper, we attempt to control the surface morphology of a high-density graft layer by using the two-dimensional phase separation of an organosilane blend. The blend of organosilanes with different alkyl or fluoroalkyl chains was reported to form a phase- separated monolayer, which could be transferred on a substrate with silanol groups and immobilized by Si-O-Si covalent bonds. 24 2-(4-Chlorosulfonylphenyl)- ethyltrimethoxysilane (CTS) with an ATRP initiating group 17c,25,26 was blended with n-octadecyltrimethoxysi- lane (OTS) having a long alkyl chain. In this blend, OTS was expected to aggregate with each other to form a characteristic surface morphology. 24 After immobilizing the blend monolayer on a silicon substrate, the graft polymerization of methyl methacrylate (MMA) was carried out by the surface-initiated ATRP technique. The blend monolayer was prepared at different pHs of subphase water. Both CTS and OTS have a methoxysi- lyl group, which can be hydrolyzed to a silanol group. The hydrolysis rate of the methoxysilyl group is much smaller than that of the chlorosilyl group, and it is controllable by changing the pH of the subphase wa- ter. 27,28 For this reason, we expected that the growth rate of the domain structure and hence the domain size would be controllable by changing the pH of the sub- phase water. Another point of scientific importance is to clarify the spatial resolution of pattern amplification by the controlled growth of a high-density, low-polydis- persity graft layer. For this purpose, the phase- separated pattern formed by the blend of low-mass compounds on a fluid surface is suitable as a model * To whom correspondence should be addressed: e-mail fukuda@scl.kyoto-u.ac.jp. 1412 Macromolecules 2002, 35, 1412-1418 10.1021/ma010371f CCC: $22.00 © 2002 American Chemical Society Published on Web 01/12/2002