Mechanical reinforcement of nanoparticles: from dispersion to agglomeration

Polymer nanocomposites have broad applications. The size and shape of nanoparticles and their interaction with polymers determine the mechanical properties of polymer nanocomposites. In this work, we studied the mechanical enhancement and dynamics at different time scales in model polymer nanocomposites with different polymer molecular weights, particle sizes, and different surface properties of nanoparticles. We found the dynamic adsorption-desorption due to weak attractive interaction between nanoparticles and polymers. The adsorped chains may induce additional entanglements with the bulk chains. As a result, the increase of rubbery plateau modulus depends on the number of interaction sites (or effective surface area). These findings are general for both well-dispersed nanoparticles and agglomerated nanoparticles. At a longer timescale, the mechanical enhancement is ascribed to the particle motion or relaxation of agglomerates. A quantitative model was suggested to quantitatively describe the linear viscoelasticity of polymer nanocomposites over a wide range of timescales.