4.5. Neutron Reflectometry

Samples prepared for NR studies were assembled using two different designs. In the first design, used for observing the uptake and penetration of dQPC chains into hydrogenated matrices, films were assembled using 8 and 24 min immersions (PMAA₁₁^8min and PMAA₇^24min, respectively). Scattering densities (SLDs) for the hydrogenated stacks, upon penetration of a deuterated polyelectrolyte from solution, were held constant for all annealed samples, in fitting varying only the outermost layer’s thickness, interfacial roughnesses, and SLD. In another design, films contained a dPMAA block within the middle region of the film, i.e., had QPC₈/dPMAA₉/QPC₁₄^4min, QPC₈/dPMAA₉/QPC₁₄^8min, QPC₄/dPMAA₅/QPC₁₀^16min, or QPC₄/dPMAA₅/QPC₈^24min architecture.

NR measurements were performed at the Spallation Neutron Source Liquids Reflectometer (SNS-LR) at the Oak Ridge National Laboratory (ORNL). The reflectivity data were collected using a sequence of 3.4-Å-wide continuous wavelength bands (selected from 2.55 Å < λ < 16.7 Å) and incident angles (ranging over 0.6° < θ < 2.34°). The momentum transfer, Q = (4π sinθ/λ), was varied over a range of 0.008 Å⁻¹ < Q < 0.193 Å⁻¹. Reflectivity curves were assembled by combining seven different wavelength and angle data sets together, maintaining a constant relative instrumental resolution of δQ/Q = 0.023 by varying the incident-beam apertures. Scattering densities within hydrogenated and deuterated stacks were averaged over the 12 constituent bilayers, with each stack exhibiting its characteristic thickness, scattering-length density, and interlayer roughness. Those characteristic parameters were adjusted until the reflectivity curve was best fitted (minimized χ²).