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Unearthing the Power of Small-Angle Neutron Scattering for Molecular Rheology of Polymers

Yangyang Wang (Center for Nanophase Materials Sciences, Oak Ridge National Laboratory)

Flow of macromolecules, a ubiquitous phenomenon in both nature and industry, needs to be understood at both macroscopic and microscopic scales. Here we present the formulation of a general mathematical framework, which could be used to extract, from scattering experiments, the molecular deformation of flowing polymers. By combining and modestly extending several key conceptual ingredients in the literature, we show how the anisotropic single-chain structure factor can be decomposed by spherical harmonics and experimentally reconstructed from its projections on different scattering planes. The resulting wavenumber-dependent expansion coefficients constitute a characteristic fingerprint of the macromolecular deformation, permitting detailed examinations of polymer dynamics in flow at the microscopic level. We apply this approach to survey the molecular deformation of polymers by performing small-angle neutron scattering experiments on uniaxially-stretched entangled polystyrenes. One of the central ideas of the tube model . chain retraction within affinely deformed tube . gives rise to unique scattering patterns in terms of the spherical harmonic expansion coefficients. However, our scattering experiments, spanning over a wide parameter space, have found no evidence of such predicted behavior. This work provides an important experimental observation, as well as a powerful methodology, for improving our understanding of macromolecular flows.

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