The relaxation dynamics of deformed polystyrene (PS)/silica nanocomposites with non-attractive polymer-nanoparticle interactions are studied by a combination of small-angle scattering techniques and rheology. Small-angle x-ray scattering measurements and rheology show clear signatures of nanoparticle aggregation that enhances the mechanical properties of the polymer nanocomposites (PNCs) in the linear viscoelastic regime and during the initial phase of stress relaxation along with accelerated relaxation dynamics. Small-angle neutron scattering experiments under the zero-average-contrast condition reveal, however, smaller structural anisotropy in the PNCs compared with the neat polymer matrix, as well as accelerated anisotropy relaxation. The degrees of anisotropy reduction and relaxation dynamics acceleration increase with increasing nanoparticle loading. While the hydrodynamic reinforcement picture holds, additional nonlinear relations are needed to account for the reduction of anisotropy and acceleration of relaxation dynamics: One is the nonlinear stress relaxation of entangled polymers at large deformation; the other is the nonlinear relation between the polymer structural anisotropy and the microscopic strain.

Stress relaxation in polymer nanocomposites through a combination of small-angle scattering and rheology

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