Nanofiller Aggregation as Reinforcing Mechanism in Nanocomposites

A new approach is proposed to model the elastic properties of polymer nanocomposites taking into account agglomeration effects. In particular, the stiffening effect provided by rigid nanoparticles forming primary aggregates is modelled on the hypothesis that part of the polymer matrix is mechanically constrained within the aggregates. To validate the model, linear-low-density polyethylene (LLDPE)/silica micro- and nano-composites have been prepared by melt compounding followed by hot pressing. Electron microscopy observations indicated that the microstructure of the resulting nanocomposites clearly manifested primary aggregation of nanoparticles. Concurrently, thermal calorimetry and X-ray diffraction analyses proved that the crystallization behaviour was not affected by the presence of the filler. Dog-bone specimens have been mechanically tested under uniaxial tension and the data used to validate the model. A good agreement between theoretical predictions and experimental data was demonstrated. The results coupled with the propensity of nanoparticles to form aggregates could explain significant modulus increases in many nanocomposites systems reported earlier. © 2011 Published by Elsevier Ltd.

High density polyethylene was melt compounded with various untreated (hydrophilic) or surface treated (hydrophobic) fumed silica nanoparticles, having different surface areas. The thermo-mechanical properties of the resulting nanocomposites have been thoroughly investigated. Field emission scanning electron microscopy revealed that nanofiller aggregation was more pronounced as the silica surface area increased, while nano filler dispersion improved with a proper filler functionalization. The homogeneous distribution of fumed silica aggregates at low filler content allowed us to reach remarkable improvements of thermal stability, evidenced by an increase of the degradation temperature and a decrease of the mass loss rate with respect to neat matrix, especially when surface treated nanoparticles were utilized. Interestingly, the stabilizing effect produced by fumed silica nanoparticles was accompanied by noticeable enhancements of the ultimate tensile mechanical properties, both under quasi-static and impact conditions. Concurrently, a progressive enhancement of both elastic modulus and tensile stress at yield with the filler amount, was observed.