Investigation of the Micellization in Thin Films Using Resonant Soft X-Ray Scattering

The use of Resonant Soft X-rays Scattering (RSOXS) for the investigation of block copolymer micelles within a thin homopolymer film is demonstrated in this work. Thin films of polystyrene-block-polyisoprene, PS-b-PI, copolymer within a high molecular weight polystyrene, PS, matrix were coated onto Si3N4 substrates. The formation of copolymer micelles with a polyisoprene core and a polystyrene corona was verified by the RSOXS measurements. The thin film samples are difficult to study with hard x-rays or neutrons, and well-suited to the RSOXS approach having stronger cross-sections. The core radius of the PS-b-PI micelles was estimated from the experimental data and was found to be in good agreement with measurements in the bulk and with that theoretically predicted. Increasing copolymer concentration resulted in an increase in the number density of micelles in the films; however, no ordering of the confined micelles was observed, even at high copolymer concentrations, possibly because of the segregation of significant fraction of the PS-b-PI copolymer chains to the air/polymer and polymer/substrate interfaces. In order to overcome this problem polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, was utilized. A PS-b-PMMA/PS film of high copolymer concentration was spin-coated on a pre-treated Si3N4 substrate and RSOXS revealed the formation of dense micelle assemblies within the PS matrix.