Using epitaxy to form insoluble, ultrathin films of block copolymers

A new method of assembling functional films by epitaxy of diblock copolymers is reported. The fundamental architecture consists of a crystalline block that is used to tether the whole chain onto a substrate surface by epitaxial adsorption, and a noncrystalline block that provides functional groups. Epitaxial adsorption is realized by polymerization-induced epitaxy, where solution polymerization of monomers directly induces epitaxial growth of chains on the substrate surface. After cationic polymerization of tetrahydrofuran (THF) with the presence of graphite in a reaction mixture, a mixture of THF and 3,3-bis(chloromethyl)oxacyclobutane (BCMO) were introduced to the reaction mixture to form PTHF-block-P(THF-BCMO). Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of the block copolymer on graphite even after rigorous rinsing of the substrate surface. Atomic force microscopy scanned over micrometer ranges exhibited the same island features as that of PTHF homopolymer film. But these crystalline islands are covered by ultrathin, highly viscous layers that are insoluble. Molecular resolution images by scanning tunneling microscopy showed that the features on the copolymer film are the same as that of an epitaxially adsorbed monolayer of PTHF homopolymer. No film remained on the graphite surface if the graphite was simply immersed in the copolymer solution and was rinsed similarly. These results indicate that the insoluble film of diblock copolymer was formed during polymerization, in which the crystalline PTHF block binds to the surface by epitaxial adsorption and the noncrystalline P(THF-BCMO) block is free of surface attachment.