Elastic behavior of nanoparticle chain aggregates: A hypothesis for polymer-filler behavior

Electron microscopy studies in our laboratory have shown that nanoparticle chain aggregates (NCAs) of inorganic oxides have elastic properties. Measur ements were made with titania, alumina, and iron oxide NCAs generated by la ser ablation. Primary particles were 5-10 nm in diameter, and the mobility diameters of the NCAs studied were about 0.5 mu m. NCA stretching appeared to begin with the rotation and/or sliding of adjacent nanocrystals. This le d to a small change in the NCA length but allowed for chain straightening. Most of the NCA lengthening resulted from the separation of kinked chain se gments held together by weak, probably van der Waals (vdw), forces. NCA str ains up to 90% were observed. Calculated values for NCA. deformation energi es per unit volume were compared with those for conventional polymers; unde r certain conditions, the two deformation energies were of the same order o f magnitude. These results may help explain the remarkable effects that nan oparticle reinforcing fillers such as carbon black and silica have on comme rcial rubber. It may be possible to improve the properties of composites of molecular polymers and NCAs through the use of NCAs with prescribed primar y particle sizes, vdw-bond numbers, chain lengths, and morphological proper ties. Synthesizing such NCAs will require the use of modern concepts of aer osol aggregate formation. (C) 2000 John Wiley & Sons, Inc.