Filler aggregation as a reinforcement mechanism in polymer nanocomposites

Significant reinforcing effects that are often observed in polymer nanoparticulate composites are usually attributed to strong interfacial interactions over extended interfaces in these systems. Here, we study linear low density polyethylene (LLDPE) reinforced with 1–4% fumed silica nanoparticles. Nanocomposite modulus, evaluated as a function of filler volume fraction, significantly exceeds classical micromechanics predictions. Possible reasons for the observed discrepancy are evaluated experimentally and theoretically. It is concluded that primary nanoparticle aggregation rather than polymer–nanoparticle interaction at the interface is mainly responsible for the observed reinforcement effect. A simple micromechanics-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 nanoparticles 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 composites reported in the literature.

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► Elastic properties of LLDPE/fumed silica nanocomposites were modeled.
► Aggregation rather than interfacial interaction can explain the stiffening effect.
► A micromechanics model of a composite with primary particle aggregates is presented.
► A new structural parameter capable of predicting nanocomposites elastic behavior is introduced.
► The described mechanism may explain other property changes in nanocomposites.