Molecular orientation within hierarchical high-χ block copolymers revealed by polarized resonant soft X-ray scattering

Hierarchical structure-within-structure assemblies offer a route toward complex and multifunctional materials while pushing the limits of block copolymer self-assembly. We present a series of high-χ block polymers (BP) synthesized via postmodification of a single poly(styrene)-block-poly(glycidyl methacrylate) (S-b-G) parent polymer with fluorinated alkylthiol pendant groups. This family of BPs self-assembles into a complex smectic-within-lamellae thin film structure. Perfluorinated moieties within the structure exhibit a liquid crystalline (LC-like) layered structure, with a degree of order that depends on moiety length.

A comprehensive characterization strategy was employed to meet the challenge of BP thin film structure measurement. Thermal analysis reveals LC-like transitions, and bulk small angle X-ray scattering (SAXS) indicates phase separation between the two blocks and within the fluorinated block. Important structural details of the thin film vertical lamellae were revealed with an emerging method, polarized resonant soft X-ray scattering (P-RSoXS), a small-angle scattering technique with unique radiolabel-free contrast modalities based on intrinsic chemical heterogeneity and bond orientation. The high contrast afforded by P-RSoXS reveals near-perfect orthogonality between the BP lamellar domains and the LC sublattice. Atomic force microscopy (AFM) and X-ray diffraction are consistent with a surprising bimodal distribution of sublattice orientations relative to the film normal.

The bond orientation sensitive contrast modalities of P-RSoXS indicate preferential orientation of the LC sublattice relative to lamellar domain boundaries. More surprisingly, the data also suggest significant preferential orientation in hydrocarbon glassy regions. Using a virtual P-RSoXS instrument (https://github.com/usnistgov/NRSS), we develop a rich description of the extent and nanoscale spatial variation of composition and orientation in this complex hierarchical material. The success of this characterization strategy provides a framework for measuring and optimizing the structure of high-χ block polymers and a powerful demonstration of the P-RSoXS method to access heretofore unmeasurable aspects of structure in the nanoscale amorphous regions of thin polymer films.