On the indentation recovery and fleeting hardness of polymers

Accurate mechanical characterization of viscoelastoplastic materials in small volumes is required for the development of polymeric thin film, nanocomposite, and biomedical applications. Instrumented indentation-based approaches are increasingly implemented to quantify the resistance to permanent deformation of such systems via time-independent analyses. Here, we quantify the significant post-indentation recovery of several bulk polymers via time-lapsed scanning-probe microscopy under ambient conditions, indicating up to 80% recovery of both indentation depth and volume within 48 h. This viscoelastic response demonstrates that indentation hardness values for these polymers are accurate within 10% for less than 5 min to 3.5 days post-indentation, neglecting any other analytical or experimental errors. Further, although the extent and rates of volumetric recovery depend strongly on loading history and polymer structure/physical properties, deformation resistance inferred from indentation hardness does not quantitatively or qualitatively predict recoverable work or residual deformation of polymer surfaces.