Development of a directly patterned low-surface-energy polymer brush in supercritical carbon dioxide.

Carbon dioxide (CO2) is a sustainable solvent because it is nonflammable, exhibits a relatively low toxicity, and is naturally abundant. As a selective, nonpolar solvent, supercritical CO2 (scCO2) is an ideal fit for the development of low-surface-energy polymers. The development of directly patterned poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) brushes in scCO2 was investigated. PTFEMA, in particular, was selected over other fluorinated polymers because of its very high electron-beam (e-beam) sensitivity. PTFEMA brushes were grown on silicon substrates via controlled surface-initiated atom-transfer radical polymerization of TFEMA. Surface analysis techniques including ellipsometry, contact-angle goniometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy were used to characterize the thickness, hydrophilicity, roughness, and chemical composition of the polymer brushes. PTFEMA brushes were directly patterned in a single step using e-beam lithography and were processed in an environmentally benign scCO2 solvent. Tapping-mode AFM imaging confirmed the successful e-beam patterning and development of these brushes. The sensitivity of PTFEMA brushes toward direct patterning with the e-beam, followed by scCO2 development, was studied and compared to development in tetrahydrofuran solvent. Using this direct-patterning method, followed by dry development in scCO2, highly resolved nanostructured polymer brush lines down to 78 nm could be prepared. This method can be generalized to prepare fluorinated low-surface-energy polymer brush surfaces in a single step for various applications.