Understanding molecular deformation and relaxation of ionomers by complementary small-angle scattering techniques

The molecular relaxation dynamics of rubidium neutralized sulfonated polystyrene ionomers after a large step uniaxial extension are investigated by small-angle neutron (SANS) and x-ray (SAXS) scattering techniques. Due to the different scattering contrast for neutrons and x-rays, the nonequilibrium structures of polymers and ionic clusters are revealed by SANS and SAXS, respectively. It is found that the single-chain structure becomes highly anisotropic upon deformation, whereas the ionic structures hardly show any distortions. This observation stands in stark contrast to the classical transient network picture, where the physical crosslinks formed by ionic clusters drive the molecular deformation of the polymer chains. Further analysis of the SAXS spectra indicates that small ionic clusters form fractal aggregates, which provide structural rigidity to resist the applied deformation. Moreover, quantitative analysis of the SANS using the spherical harmonic expansion technique shows that the presence of molecular association fundamentally alters the spatiotemporal dependence of the structural anisotropy relaxation relative to the near polymers, with much stronger wavenumber dependence for the associating chains.