Reducing Block Copolymer Interfacial Widths through Polymer Additives
Daniel F. Sunday, Regis J. Kline
There is a need to design new materials to achieve smaller pitches and reduced interfacial widths for use in the nanopatterning of block copolymers (BCPs). One option is the use of blends, where the addition of a homopolymer which selectively associates to one of the blocks results in the increase in the effective Flory-Huggins interaction parameter (χ) between the two phases. In order to explore the effect of this approach on the interfacial width between BCP components, poly(vinyl phenol) (PVPH) was added to polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA). Multilayers of this blend were characterized using resonant soft x-ray reflectivity (RSoXR), a measurement that allows the contrast between PS, PMMA, and PVPH to be selectively tuned by varying the beam energy. RSoXR measurements confirmed that PVPH is uniformly distributed throughout the PMMA block. The interfacial width of the block was reduced by 20 % upon the addition of a mass fraction of 8 % PVPH. The interfacial width of homopolymer bilayers was also investigated in order to probe the same effect at higher PVPH concentrations. A blend of 70:30 PMMA:PVPH capped by a PS layer resulted in an interfacial width of (2.75 ± 0.1) nm, corresponding to a 475 % increase in the effective interaction parameter between the two layers.