Ion-doped block copolymers (BCPs) are a versatile, easy-to-process, and inexpensive class of materials with highly tunable structures. These attributes are promising for a variety of applications, including nanolithography and ion/electron transport. The practical adoption of ion-doped BCPs into these applications requires precise control over the local physics. For example, the distribution of ions throughout a BCP phase will affect the local polymer conformations, polymer/ion dynamics, and material performance. Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) doped with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) is a well-studied system for nanolithography and ion-conduction applications. Higher ion concentrations (e.g., 10-50 wt% EMIM TFSI relative to the mass of the ion-containing PMMA phase) in the BCP can be desirable toward accessing smaller feature sizes and sharper interfaces. However, theoretical predictions have suggested that as overall EMIM TFSI loading in PS-b-PMMA is increased, the EMIM TFSI becomes more concentrated in the center of the PMMA domain. Experimental assessment of EMIM TFSI distributions in PS-b-PMMA is crucial to evaluate the relevance of the theoretical predictions and connect how the local compositional heterogeneities impact material properties. In this work, EMIM TFSI distributions in PS-b-PMMA were studied using resonant soft X-ray scattering (RSoXS). RSoXS harnesses the energy-dependent absorption of soft X-rays by electrons in various organic functional groups to provide enhanced chemical contrast relative to hard X-ray and neutron scattering. This advantage is especially important in the characterization of thin films, for which RSoXS can provide high-contrast measurements without the need for deuteration or staining. The BCP thin-film samples were probed at the carbon, oxygen, and fluorine K edge, which provided good contrast conditions to deconvolute the contributions of the PS, PMMA, and EMIM TFSI to the scattering signal. Moreover, 3-4 Bragg peaks were identified at each energy, allowing rigorous analysis of the component distributions. The experimental data were compared to simulated scattering patterns generated from a soft X-ray scattering simulation framework (NRSS); these comparisons enabled the distribution of the ions in the BCP to be determined. The insights gleaned from this work provide a quantitative means to link BCP assembly to material performance.

Eliot Gann

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