Effect of Nanofiller Size on the Mechanical Properties of Poly(acrylic acid)/Graphene Oxide Nanocomposites

Abstract

Two poly(acrylic acid) (PAA)/graphene oxide nanocomposite systems, characterized by the same graphene oxide (GO) weight fraction (13.5 wt %) but with different sizes of nanoflakes, are studied by means of molecular dynamic (MD) simulations and the solid-to-liquid-glass (SLG) transition model. The MD density data were described well by the SLG equation of state (SLG-EoS), and the PAA thermal expansion coefficient was predicted to decrease with the size of GO nanosheets. A new equation for the description of the bulk modulus is introduced and used for the estimation of the effects of the size of GO flakes on the mechanical properties of the composites. The mechanical behavior of the composites was found to depend on the size of the GO filler. In the case of larger-sized GO flakes, the chains form longer sequentially adsorbed configurations (trains) as well as loops, while in the presence of smaller-sized GO nanoflakes, the train configurations are shorter and the polymer bridging is extensive. The slow mobility of the large GO flakes results in more retarded dynamics of the PAA chains, explaining the higher viscosity and the enhanced mechanical properties compared to the small GO system. Furthermore, using a generalized Arrhenius equation (SLG model), the dynamic behavior at high temperatures is extrapolated to the glass-transition region, providing thus a means to describe dynamics at lower temperatures. Analysis of the configuration-specific bound-layer polymer dynamics, in hydrogen-bond-forming systems, reveals that chain immobilization is restricted to a layer surrounding the nanofiller rather than propagating in a way that would promote the formation of “glassy bridges” between nanofillers.