Considering Viscoelastic Micromechanics for the Reinforcement of Graphene Polymer Nanocomposites

There has been much recent work investigating the reinforcement of glassy polymers with nanoparticles, and much excitement has been generated by some apparent synergies that suggest reinforcements greater than expected from elastic bound models. Here we show that it is necessary to consider the thermoviscoelastic response of the polymer matrix in nanocomposites (PNCs) to fully understand the reinforcement of the filler. This is especially so because polymer nanocomposites are frequently used at high fractions of the glass transition temperature T_g, where the time dependence of the polymer is significant. Therefore it is a conceptual error to examine the modulus behavior of PNCs via only elastic micromechanics. When the glass transition temperature increases due to the interactions between reinforcement and polymer, it is more reasonable to use a viscoelastic micromechanics approach to estimate the bounds on modulus behavior of PNCs. Here we use new results for grapheme oxide reinforced poly(ethyl methacrylate) (PEMA) and literature results for reinforced poly(methyl methacrylate) (PMMA) and show that the ultralow loading of graphene oxide raises the T_g of PEMA and PMMA significantly and leads to a large shift of the frequency-temperature properties of the polymer matrix. Our thermoviscoelastic approach shows that apparent extreme reinforcements can be attributed to the changing T_g of the polymer, and the corrected mechanical reinforcement from graphene oxide is much weaker than previously reported.