Resonant Soft X-ray Scattering (RSoXS)

Summary

Resonant soft X-ray scattering (RSoXS) is an emerging characterization technique that derives contrast from features in soft X-ray spectroscopy. RSoXS is a small-angle scattering technique with principles similar to small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS). Label-free contrast modes that highlight specific elements and specific bonds make this technique particularly powerful for soft material characterization. A unique strength of RSoXS among small-angle scattering techniques is its sensitivity to soft X-ray transition dipoles, which allows quantitative characterization of nanoscale molecular orientation in materials like polymer nanocomposites and membranes, opening up previously inaccessible processing-structure-performance correlations. 

Description

This project focuses on developing and demonstrating resonant soft X-ray data collection approaches that incorporate solids, fluids, and gases. We establish data analysis workflows for RSoXS data analysis, and a central focus is advancing the interpretation of RSoXS patterns. Our Resonant Soft X-ray Scattering Project works in close collaboration with NIST's new RSoXS beamline tool at the NSLS-2 synchrotron.

Although many early applications of RSoXS were focused around organic semiconductors, the technique has broad potential across many nanoscale soft matter characterization challenges. We have demonstrated this potential in self-assembled block copolymer systems, semicrystalline polymers, and nanocomposites. Some applications of RSoXS focus on its ability to derive contrast from chemical differences without labeling. Other applications leverage its unique sensitivity to molecular orientation at the nanoscale. Recent advances have enabled the measurement of solutions using RSoXS, and we anticipate sharing results from solution model systems soon.

 

How RSoXS measures composition

RSoXS contrast in a PS:PMMA block copolymer.

The dielectric function of most organic molecules varies dramatically across the carbon K-edge due to bond-specific resonant absorptions, the measurement of which is called Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Subtle chemical differences between molecules create significant contrast in RSoXS without radiolabeling. In this image, the contrast between poly(styrene) and poly(methylmethacrylate) in a block copolymer changes by orders of magnitude across the carbon K-edge. The maximum contrast exceeds what is possible with conventional SAXS and SANS.

For a comprehensive review of RSoXS principles and practical considerations, please read:

Collins, B. A.; Gann, E. Resonant Soft X-Ray Scattering in Polymer Science. Journal of Polymer Science 2021. https://doi.org/10.1002/pol.20210414.

Read more about the RSoXS of block copolymers:

Sunday, D. F.; Ren, J.; Liman, C. D.; Williamson, L. D.; Gronheid, R.; Nealey, P. F.; Kline, R. J. Characterizing Patterned Block Copolymer Thin Films with Soft X-Rays. ACS Appl. Mater. Interfaces 2017, 9 (37), 31325–31334. https://doi.org/10.1021/acsami.7b02791.

  How RSoXS measures molecular orientation

RSoXS of some fibrillar crystals

RSoXS measures the molecular orientation by the interaction of a linearly polarized electric field vector and NEXAFS transition dipole moments within the molecules. In this image, the scattering length densities of fibrillar crystals will vary with the angle of the electric field vector, which is shown as a rotating arrow. The formerly uniaxial film takes on biaxial qualities and scatters with a biaxial pattern that follows the electric field vector.

Modeling the dependence of the scattering anisotropy on electric field vector and energy can reveal the orientation of molecules within the scattering objects. Importantly, the orientation that is revealed is with respect to the scattering object, not with respect to the measurement frame. This is how RSoXS provides previously inaccessible information about molecular orientation at the nanoscale.

It is also possible for orientation contrast in RSoXS not to result in a biaxial pattern, depending on the shape of the scattering object. A nematic liquid crystal, for example, should result in a scattering pattern that does not vary with azimuth but appears due to orientational contrast.

 RSoXS for semicrystalline polymers

Voxel model of a semicrystalline polymer for RSoXS simulation.

Chain behavior at crystal-amorphous interfaces dictates the mechanical and electronic properties of semicrystalline polymers. Working collaboratively with the group of Prof. Harald Ade at NCSU, we demonstrated that polarized RSoXS is sensitive to this crystal-amorphous interphase.

Typical RSoXS patterns from semicrystalline polymers exhibit changes in pattern anisotropy with both incident energy and momentum transfer, q. In this work, we show that changes in pattern anisotropy with incident energy and momentum transfer, q, can only be explained with a 3-phase system, even for neat semicrystalline polymer. A proposed structural model containing an explicit crystal- amorphous interphase produces the expected pattern features.

Read more:

Mukherjee, S.; Gann, E.; Nahid, M. M.; McAfee, T.; Herzing, A. A.; DeLongchamp, D. M.; Ade, H. Orientational Ordering within Semiconducting Polymer Fibrils. Advanced Functional Materials 2021, 31 (28), 2102522. https://doi.org/10.1002/adfm.202102522.

 

 RSoXS for nanocomposites

Voxel model of a PGN for RSoXS pattern simulation.

Chain behavior in confined regions typically dictates the properties of nanocomposites as well. Recent work in collaboration with the Air Force Research Lab (AFRL) has resulted in the first measurement of nanoscale chain orientation in the condensed polymer brush region of polymer-grafted nanoparticles (PGNs).

This work marks the first demonstration of our powerful graphics processing unit (GPU)-accelerated RSoXS pattern forward simulation software, developed in conjunction with Prof. Baskar Ganapathysubramanian at Iowa State University.

We fit structure model parameters that separated the amount of polystyrene chain orientation from the spatial extent of the oriented region. The result is a nanoscale orientation map with ≈ 2.5 nm spatial resolution describing the orientation of polystyrene chains in the condensed polymer brush region. These orientation maps vary depending on the relative molecular mass of the polymer and the graft density, measuring heretofore inaccessible aspects of structure that are correlated to engineering properties and the fundamentals of polymer behavior in confinement.

Read more:

Mukherjee, S.; Streit, J. K.; Gann, E.; Saurabh, K.; Sunday, D. F.; Krishnamurthy, A.; Ganapathysubramanian, B.; Richter, L. J.; Vaia, R. A.; DeLongchamp, D. M. Polarized X-Ray Scattering Measures Molecular Orientation in Polymer-Grafted Nanoparticles. Nat Commun 2021, 12 (1), 4896. https://doi.org/10.1038/s41467-021-25176-4.