Free volume and nonlinear viscoelasticity in supercrystalline nanocomposites: A nanoindentation driven modelling analysis

• Supercrystalline nanocomposites (SCNCs) feature nonlinear viscoelasticity. • Organic crosslinking lowers the viscosity of SCNCs. • The free volume in SCNCs detected via PALS decreases with heat-treatment temperature. • The stress-dependent creep compliance can be predicted via the free volume theory. • The free volume-based creep model can predict the creep displacements of SCNCs. Supercrystalline nanocomposites (SCNCs) are a new class of hybrid materials consisting of organically functionalized nanoparticles that are arranged into periodic architectures, featuring multi-functional properties. While their mechanical behavior is starting to be assessed, the time-dependent aspects thereof, and especially creep, remain unexplored. This lack of understanding is an obstacle towards future implementation of SCNCs into devices. It is therefore imperative not only to capture experimentally the creep behavior of SCNCs, but also to develop models that accurately predict its evolution. Here, a model is proposed to capture the nanoindentation creep behavior of SCNCs, using both rheological models and free volume theory. The creep compliance derived from the rheological model shows a stress-dependent trend, indicating nonlinear viscoelasticity. The presence of free volume is experimentally detected in SCNCs via positron annihilation lifetime spectroscopy. It decreases in size with increasing degrees of crosslinking of the organic phase, a phenomenon attributed to the shrinkage of superlattices. The creep compliance is predicted by introducing a shift factor to account for the evolution of the relaxation time caused by the change in free volume. A free volume-based creep model is proposed to predict the creep behavior of SCNCs, and its applicability is validated through new nanoindentation creep tests at varying loads.