Deposition of block copolymer thin films onto polymeric substrates by adsorption from supercritical carbon dioxide

A novel method for the deposition of monolayer and submonolayer films of block copolymers onto polymeric substrates is described. We show that poly(styrene-b-dimethylsiloxane) films form spontaneously on polystyrene substrates by simply placing a sample of the block copolymer and the substrate into a pressure vessel that is charged with supercritical carbon dioxide. The modified polystyrene substrates are characterized by water contact angle and angle-dependent X-ray photoelectron spectroscopy (ADXPS) measurements performed after depressurization. Water contact angles indicate that a continuous PDMS surface layer forms when the pressure is 15 MPa or greater at 50degreesC, illustrating that the block copolymer self-assembles as a bilayer structure at the surface under these conditions. ADY-PS measurements confirm that monolayer to submonolayer coverage is produced as the PDMS layer thickness increases from ca. 0.8 nm at 15 MPa to a maximum of ca. 1.5 nm for pressures of 30 MPa and higher. The thickness data indicate that the PDMS layer forms a wet brush wherein PDMS chains adopt nearly unperturbed dimensions. The atomic concentration of silicon in the bulk, associated with PDMS from copolymer impregnated into the bulk phase, is negligible for pressures below about 30 MPa and rises to about 2% for pressures of 40 Pa and higher. Pressures in the range of 20-30 MPa produce a surface film of maximum thickness with little block copolymer dissolved in the bulk of the substrate, and are therefore optimal conditions for monolayer film fabrication by adsorption from supercritical carbon dioxide.