Relationships between linear and nonlinear shear response of polymer nano-composites

Rheological analysis was used to understand the structure–property relations of polymer nano-composites based on ethylene vinyl acetate. Two geometrically different nano-particles (sphere of CaCO3 and platelet of montmorillonite) having the same energetic attractions with ethylene vinyl acetate were studied for concentrations between 2.5 and 15 wt%. Three phenomena were studied: the appearance of a solid-like behavior in the linear viscoelastic domain, the limits of linear viscoelasticity, and the presence of stress overshoot in step shear tests. In particular, stress overshoot was investigated based on the tube concept of polymeric chains. Also, differences related to nano-particle geometry (platelet vs. spherical) were investigated based on a filler-network mechanism. Due to higher physical contacting probability, platelet particles can better interact and create a network structure, which dominates the rheological response. On the other hand, although spherical particles can limit the motion of polymeric chains under flow, a strong physical network was not formed. For platelets, scaling behavior was well described by fractal model which considers direct aggregation, and such scaling was not observed for spherical particles. The filler-network mechanism was validated by image analysis.