Interphase percolation effects on the viscoelastic behavior of polymer nanocomposites

Recent experimental work demonstrated that by incorporating nanoscale inclusions, polymer nanocomposites can exhibit significant improvement in mechanical, electrical, thermal, and other physical properties in comparison to their parent polymer systems. In addition to the nanoinclusions themselves, the interphase, a special region of polymer chains in the vicinity of the nanofillers, also plays an important role in the improvement of polymer composite properties. Accurate prediction of effective properties of nanocomposites is impossible without fully understanding the impact of the interphase. The research work presented in this dissertation represents our efforts to develop appropriate modeling methods to aid the interpretation of current experimental observations regarding the interphase in nanocomposites, and understanding the influence of the interphase and its morphology on the overall viscoelastic behavior in nanoparticle reinforced polymers.;Finite element approach was employed because it can provide information at continuum scale that is directly comparable to experimental measurements, and is advantageous to address complicated structures and strong interactions which have been proven to be critical for the interphase in nanocomposites. Two-dimensional representative volume elements with periodic structure configuration were constructed to simplify the computational requirement but without losing any generality. Given the experimentally measured frequency domain response of the bulk polymer, the viscoelastic behaviors of the nanocomposites in both frequency and temperature domains have been calculated. The predicted pattern of the impact of the interphase on the overall performance of the nanocomposites is consistent with experimental observations. Furthermore, the simulation results reveal that presence of a geometrically percolating interphase greatly influences or even dominates the overall viscoelastic properties of polymer nanocomposites. It is also shown that agglomeration of nanoinclusions remarkably reduces the volume fraction of interphase and impedes the formation of a percolating interphase network inside the polymer matrix, and therefore undermines the potential enhancement by nanoinclusions. These findings not only help explain some experimental observations, but also draw attention to the importance of morphology control of interphase to further tune the thermo-mechanical properties of polymer nanocomposites.