Micropolar-based modeling of size effects on stiffness and yield stress of nanoparticles-modified polymer composites

A new mechanical model for multiphase polymer matrix composites is proposed, accounting for size and size distribution effects. It is provided in three structural levels: nano, micro and macro. The polymer matrix is considered as a micropolar material and the hardening filler nanoparticles, as Cauchy media. Multi-steps homogenization procedures are applied to obtain the overall mechanical properties of the composite. A variation approach is used to evaluate the equivalent stress on macrolevel at the transition from micro to macroscale. The influence of the microstructure size parameters on the hardening behavior of the overall material is discussed. The model is applied to epoxy/glass and polypropylene/CaCO3 nanocomposites and theoretical predictions are compared with experimental data.