Nanoindentation in polymer nanocomposites

This article reviews recent literature on polymer nanocomposites using advanced indentation techniques to evaluate the surface mechanical properties down to the nanoscale level. Special emphasis is placed on nanocomposites incorporating carbon-based (nanotubes, graphene, nanodiamond) or inorganic (nanoclays, spherical nanoparticles) nanofillers. The current literature on instrumented indentation provides apparently conflicting information on the synergistic effect of polymer nanocomposites on mechanical properties. An effort has been done to gather information from different sources to offer a clear picture of the state-of-the-art in the field. Nanoindentation is a most valuable tool for the evaluation of the modulus, hardness and creep enhancements upon incorporation of the filler. It is shown that thermoset, glassy and semicrystalline matrices can exhibit distinct reinforcing mechanisms. The improvement of mechanical properties is found to mainly depend on the nature of the filler and the dispersion and interaction with the matrix. Other factors such as shape, dimensions and degree of orientation of the nanofiller, as well as matrix morphology are discussed. A comparison between nanoindentation results and macroscopic properties is offered. Finally, indentation size effects are also critically examined. Challenges and future perspectives in the application of depth-sensing instrumentation to characterize mechanical properties of polymer nanocomposite materials are suggested.