Dynamics of Chain Aggregates of Carbon Nanoparticles in Isolation and in Polymer Films: Implications for Nanocomposite Materials

Composite materials composed of polymers blended with reinforcing fillers play a major role in various technologies. An important example, commercial rubber, incorporates carbon black fillers, which significantly enhance mechanical properties such as Youngʹs modulus and ultimate strength. The carbon black fillers consist of nanoparticle chain aggregates (NCAs). Because the mechanisms behind the filler effects are poorly understood, we have studied the dynamics of NCAs composed of carbon particles that we synthesize by laser ablation. In situ stretching of the NCA was carried out in the transmission electron microscope (TEM), first in the absence of a supporting polymer matrix and then embedded in a polymer film. The samples studied were first deposited across a micronsized slit mounted on a commercial specimen holder in the TEM. For an isolated NCA, after initial reorientation and unfolding of the chain, a large strain led to its elongation, plastic deformation, and breakage, followed by fast recoil of the broken segments. On the other hand, a small strain could be partially recovered by subsequent contraction, displaying elastic behavior. Similar stretching and elastic behavior were observed for NCA embedded in polystyrene and neoprene films. Reorientation of chains within the film indicated load transfer to the former through the polymer matrix, which may contribute to the enhanced mechanical properties. The interactions within these nanocomposite films could be qualitatively arranged in the increasing order of polymer-polymer < particle-particle < polymerparticle. Finally, commercial carbon black fillers embedded in a poly(acrylic acid) polymer film also demonstrated stretching and reorientation similar to those of the NCA-polymer films. Our results therefore point to a possible mechanism of enhanced mechanical properties of carbon black nanocomposites: namely, separate contribution to the overall properties by the stretching and elastic properties of filler materials embedded in the polymer matrix