Solvent enhanced block copolymer *ordering in thin films
Diblock copolymer self-assembly of materials is emerging as a key element in the fabrication of functional nanostructured materials. By solvent casting or solvent annealing block copolymer thin films, we have demonstrated methods to produce diblock copolymer films with highly oriented, close-packed arrays of nanoscopic cylindrical domains with a high degree of long-range lateral order with few defects. The solvent imparts a high degree of mobility in the microphase-separated copolymer that enables a rapid removal of defects and a high degree of lateral order. Though the use of a selective cosolvent during solvent casting, it was found that the microdomain size and spacing could be increased, leading to a size-tunable system. Additionally, the presence of water also led to the ability to control the microdomain orientation during solvent annealing. Ionic complexation within cylinder-forming PS- b-EO block copolymer thin films was also investigated, where added salts bind PEO block as the minor component. Small amounts of added salts, on the order a few ions per chain, show large effects on the ordering of the copolymer films during solvent annealing. By using gold or cobalt salts, well-organized patterns of nanoparticles can be generated in the copolymer microdomains.
Topographically and chemically patterned surfaces were used as a route to sectorizing and controlling the lattice orientation of copolymer films. Topographically patterned surfaces allow well-defined boundaries to confine the copolymer microdomains on a surface and effectively direct the ordering and grain orientation of the copolymer microdomains. Chemically patterned surfaces provide a route to direct the block copolymer ordering on completely flat surface, which may have advantages in applications where adding additional topography may be undesirable.
To generate nanoporous templates from PS-b-PEO bases materials several routs were followed. The first route was through the addition and selective solvent removal of homopolymer PEO or PMMA. Second, we have incorporated a center block that is photodegradable by ultra violet radiation into PS- b-PMMA-b-PEO copolymers. Third, a tritylether junction was placed between the two blocks, which is cleavable by exposure to trifluoroacetic acid vapor.
Though the use of solvents in block copolymer thin films, were are able to markedly enhance the long range lateral ordering block copolymer films. Also, routes to sectorize surfaces to confine and direct the copolymer microdomains are shown. Also, three methods to generate nanoporous films from PS-b-PEO based copolymers are demonstrated. All of these results are important in the realization of addressable media from block copolymer nanolithography.