Quantification of flow-induced phase separation in polymer blends by small-angle neutron scattering

Flow-induced mixing, demixing, and phase transition phenomena in polymeric liquids are ubiquitous and practically important. This study demonstrates how the flow-induced phase separation can be quantitatively studied by applying the spherical harmonic expansion technique to small-angle neutron scattering. Using binary polymer blends, it is shown that the emergence of the so-called butterfly patterns is caused by the change of sign in the leading anisotropic component of the small-angle spectrum, when the scattering is dominated by intermolecular correlation associated with viscoelastic phase separation. The increasing spatial fluctuation of concentration is evidenced by the enhancement of the isotropic component of the scattering spectrum in the zero-angle limit and peak shift of the leading anisotropic coefficients towards low wavenumbers. Additionally, the spherical harmonic expansion framework permits real-space analysis in a convenient form. The methodology described in this work provides a concrete venue for quantitative studies of phase transitions of polymeric fluids under deformation and flow via small-angle scattering techniques.