Molecular anisotropy effects in carbonK-edge scattering: Depolarized diffuse scattering and optical anisotropy

Abstract

Some polymer properties, such as conductivity, are very sensitive to short-and intermediaterange orientational and positional ordering of anisotropic molecular functional groups, and yet means to characterize orientational order in disordered systems are very limited. We demonstrate that resonant scattering at the carbon K-edge is uniquely sensitive to short-range orientation correlations in polymers through depolarized scattering at high momentum transfers, using atactic polystyrene as a well-characterized test system. Depolarized scattering is found to coexist with unpolarized fluorescence, and to exhibit pronounced anisotropy. We also quantify the spatially averaged optical anisotropy from low-angle reflectivity measurements, finding anisotropy consistent with prior visible, x-ray absorption, and theoretical studies. The average anisotropy is much smaller than that in the depolarized scattering and the two have different character. Both measurements exhibit clear spectral signatures from the phenyl rings and the polyethylene-like backbone. Discussion focuses on analysis considerations and prospects for using this depolarized scattering for studies of disorder in soft condensed matter.

Spectral Analysis for Resonant Soft X-Ray Scattering Enables Measurement of Interfacial Width in 3D Organic Nanostructures

“…22 Sensitivity to transition dipole moments additionally enables correlative local measurements of molecular orientation. [26][27][28] To quantify these measurements, including BCP interfacial width, absolute scattering intensity is required. Unfortunately, while absolute SAS calibration standards have become increasingly available, 29 the low penetration depth of soft X-rays renders these resources unusable at absorption edges.…”

“…The error bars are dominated by poor beam stability reported previously. 28 Significantly higher accuracy could be achieved with improvements of the beamline and in beamlines currently under construction.…”

Combining theory and experiment for X-ray absorption spectroscopy and resonant X-ray scattering characterization of polymers

“…orientation fluctuations of functional groups, as shown for atactic polystyrene [77]. Anisotropic scattering patterns can reveal orientation in liquid crystalline materials, for example, the half-pitch in a bent core liquid crystal that forms a twisted smectic layer structure [78].…”

Resonant soft X‐ray scattering in polymer science

“…Measuring any polarization component separately at every pixel of a detector is difficult with current technology, but it is possible in special geometries such as the Brewster's angle. 65 Depolarized scattering will generally be weaker than polarized scattering because each excitation was a dipole interaction. Although these depolarized fields can be calculated, they further complicate a clean split between structural and spectral aspects.…”

“…See 30 for the breakdown of this in the uniaxial example. Measuring any polarization component separately at every pixel of a detector is difficult with current technology, but it is possible in special geometries such as the Brewster's angle 65 . Depolarized scattering will generally be weaker than polarized scattering because each excitation was a dipole interaction.…”