Directed Phase Separation of Polymer Blends on Binary-Patterned Polymer Brushes

Abstract

Self-assembly of well-defined polymer microstructures is of interest for applications such as polymer solar cells, light emitting diodes, microelectronics and biosensors. Chemically patterned substrates can direct the phase separation of thin films of polymer blends, producing controlled morphologies. This has been demonstrated using patterned self-assembled monolayers. Binary-patterned polymer brushes provide a robust, chemically and topographically patterned surface which can interact with the blend, potentially resulting in interesting new behaviour, and greater control over phase separation. Binary-patterned polystyrene/poly(methyl methacrylate) brushes were synthesised by a novel method. A self-assembled monolayer of triethoxysilane was patterned by exposure to ultraviolet light. This produced amine-terminated areas that could react with 2-bromoisobutyryl bromide to produce initiators for atom transfer radical polymerisation, allowing the synthesis of patterned polymer brushes. Dehalogenation of the first brush, followed by deprotection, modification and a second polymerisation produced binary-patterned brushes. Unpatterned and patterned polymer brushes were characterised using ellipsometry, x-ray photoelectron spectroscopy, contact angles, atomic force microscopy, lateral force microscopy, optical microscopy and secondary ion mass spectrometry. An alternative approach, based on direct microcontact printing of an atom transfer radical polymerisation initiator, 11-(2-bromo-2-methyl)propionyloxyundecyltrichlorosilane, was also investigated, although this approach was ultimately unsuccessful. The behaviour of thin films of polystyrene/poly(methyl methacrylate) blends on silicon, patterned self-assembled monolayers and binary-patterned polymer brushes was studied. The morphologies were investigated using atomic force microscopy, optical microscopy, nuclear reaction analysis and secondary ion mass spectrometry, in order to determine the effect of the binary-patterned polymer brushes on the domain structure of the blend. The blend morphology was complex and reflected interactions between the blend and the brushes (as well as other factors). When the natural length scale of the blend is commensurate with the underlying pattern, phase separation may be spatially directed by the substrate