Reinforcing mechanisms in amorphous polymer nano-composites

Despite the large volume of experimental data on the peculiarities of the deformation response of polymer nano-composites published over the last 20 years, models addressing the principal physical reinforcing mechanisms taking into account the appropriate length and time-scales are scarce and not generally accepted. The characteristic length and time-scales in polymer nano-composites are also strongly temperature dependent, which further complicates the theoretical treatment due to principal differences in the behavior of polymers below and above their Tg. In addition, the chain mobility above Tg also affects nano-composite response below Tg due to effects of segmental immobilization on the formation of polymer structure during solidification. It seems evident that the principal reinforcing mechanism above the Tg is related to the chain immobilization induced by the presence of rigid high specific surface area inclusions. As a result, the length scale limits of validity of size independent continuum mechanics should be determined since the discrete nature of the matter at the nano-scale prohibits simple scaling down the micromechanics models. In this contribution, the existing reinforcing mechanisms are briefly analyzed in the view of the characteristic length and volume scales in polymers with nano-scale heterogeneities. The results are used to provide a conceptual framework for selecting a suitable molecular dynamics model accounting for molecular details relevant for deformation response of true nano-composites and an identification of possible bridging law between the micro- and nano-length scales.