Simulation of interphase percolation and gradients in polymer nanocomposites

Experimental data suggests that well dispersed nanoparticles within a polymer matrix induce a significant interphase zone of altered polymer mobility surrounding each nanoparticle, which can lead to a percolating interphase network inside of the composite. To investigate this concept and the nature of the interphase, a two-dimensional finite element model is developed to study the impact of interphase zones on the overall properties of the composite. Thirty non-overlapping identical circular inclusions are randomly distributed in the matrix with layers of interphase surrounding the inclusions. The simulation results clearly show that the loss moduli of composites are either broadened or shifted corresponding to the absence or presence of a geometrically percolating interphase network. Our numerical study correlates well with experimental data showing broadening of loss peaks for unfunctionalized composites and a large shift of the loss modulus for functionalized nanotube polymer composites. Further, our results indicate the existence of a gradient in properties of the interphase layer and that incorporating this gradient into modeling is critical to reflect the behavior of polymer nanocomposites.