Filler aggregation as a reinforcement mechanism in polymer nanocomposites

Significant reinforcing effects that are often observed in polymer nanoparticulat e composites are usually attributed to strong interfacial interactions over extended inter faces in these systems. Her e, we study linear low density polyethylene (LLDPE) reinforced with 1–4% fumed silica nanoparticles. Nanocomposi te modulus, evaluated as a function of filler volume fraction, significantly exceeds classical micromechanics predictions. Possible reasons for the observed discrepancy are evaluated experimen tally and theoretically. It is concluded that primary nanop article aggregation rather than polymer–nanoparticle interaction at the interface is mainly responsible for the observed reinforcement effect. A simple micromechan ics-informed model of a composite with primary particle aggregates is presented based on the model of secondary aggregation developed earlier. The model is shown capable of predicting nanocomposites behavior by introducing a single new structural parameter with a straightforward physical interpretation. As nanoparticl es are prone to agglomerate, their primary or secondary aggregates may be present in many nanocomposite systems and the aggregation state and its effects need to be thoroughly evaluated, along with the classical interfacial interactions. The described reinforcing mechanism may be responsible for other anomalous property changes in nanoparticulate comp osites reported in the literature.