Reconciling Chain Orientation in Polymer-Grafted Nanoparticles between Coarse-Grained Models and Resonant Soft X-ray Scattering.

Polymer chain stretching enables the plastic and elastic properties that make polymers unique and valuable engineering materials. Despite its importance, polymer chain orientation in amorphous regions remains very challenging to measure by conventional techniques because it is an intrinsically molecule-scale phenomenon lacking long-range order that is frequently heterogeneous across length scales of ≈ (1 to 100) nm. Polarized resonant soft X-ray scattering (P-RSoXS) is an emerging technique that has recently achieved the measurement of amorphous chain orientation with ≈2 nm spatial resolution. The advent of this measurement capability invites comparisons with computational results for which spatial variations in chain orientation are readily accessible, providing a powerful approach to computation validation. Here we forward simulate P-RSoXS patterns for polystyrene grafted gold nanoparticles from real-space representations incorporating spatial polymer backbone orientation heterogeneity directly extracted from coarse-grained modeling results. Agreement between the computation and P-RSoXS experiment is found to depend greatly on assumptions of phenyl ring conformation relative to the polymer chain backbone, because the orientation sensitivity of P-RSoXS relies on a bond-level transition dipole moment of the phenyl ring of polystyrene to report backbone orientation. By incorporating a statistical description of phenyl ring orientation based on atomistic calculations, we report excellent agreement between P-RSoXS data and forward-simulated patterns with no fitting variables.