Lateral nanopatterns in thin diblock copolymer films induced by selective solvents.

Block copolymers have long been studied for their theories and applications especially in nanotechnology.1-5 They have periodic structures with 10-100 nm long microphase separated domains which can be controlled by the molecular weight and composition of the block copolymer. Regulating these two parameters, one can prepare thin films with different morphologies over large areas. The patterns formed by the diblock copolymers could be used as templates for structures on the scale of tens to hundreds of nanometers. For the exploitation of block copolymer thin films especially in nanotechnology, the nanopatterns of films with perfectly ordered domains over large areas are prerequisite; therefore, many efforts are focused on controlling the orientation and perfection of ordering of the domains in thin films.6 Interfacial energies7-11 and commensurability12,13 determine the behavior of block copolymer thin films. Thus, changing interfacial energies is a general approach for preparing block copolymer films with perfect ordered domains. For instance, Russell and co-workers used random copolymer brushes to tune the interactions of symmetric and asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) copolymers with their boundaries to achieve perpendicular orientation of domains in thin block copolymer films,5,14 and this way is usually combined with electric fields.14d Electric fields were used to overcome interfacial interactions to orient nanoscopic domains within diblock copolymer films.15 Rockford et al. and Yang et al. employed chemically heterogeneous substrates to control nanoscale surface interactions and subsequent macromolecular ordering.6,16 Segalman et al. demonstrated graphoepitaxy to produce thin film arrays of spherical block copolymer with good long-range order.17 Fasolka et al. reported the use of the dependence of morphologies on the film thickness together with corrugated substratesbywhichthe lateralnanometerscalepatterning of diblock copolymer films was gained.18 Spatz et al. demonstrated the formation of surface-induced nanopattern in ultrathin polystyrene-b-poly(2-vinylpyridine) (PS-bP2VP) films caused by strong adsorption of one of the two blocks forming a quasi-two-dimensional coil while the other block dewets this adsorption layer.19,20 All the above routes obtaining laterally ordered microdomain structures in thin polymer films need thermal treatment. Recently, other nonequilibrium ways have been noted. The adsorption of diblock copolymers from a selective solvent onto a flat solid substrate results in the formation of laterally ordered microdomains.21-24 Apart from these, the solvent-influenced ordering has been used