Uniaxial Recovery Perspective of Glassy Polymer Nanoindentation

Abstract

Sharp tip nanoindentation of glassy polymers is a constrained, localized viscoelastoplastic deformation. We interpret this complexity, in terms of the well-understood uniaxial deformation. From the uniaxial compression data in the literature, for PMMA, PC and crosslinked SU-8, we obtain their universal, yield-normalized recovery curves, with eps*=eps/eps_y, being one measure of the corresponding strain states (CSS). Nanoindentation recovery is determined from the 2sec constant rate unloading h-P data, modeled by a generalized power-law (variable power exponent). Comparing these data-sets, yields the correlation coefficient between the notional nanoindentation strain rate epsdot_N and strain and true strain rate and strain, c=epsdot_N/epsdot_t=eps_N/eps_t. The equivalent strain, eps, and the c value for any polymer, are within a narrow range, from the onset of indentation. Combining residual profiles via scanning probe microscopy with mathematical modeling of the indenter tip, provides the strain distribution beneath the tip. CSS measures examined here, indicate polymer-specific regions to regions common to glassy polymers, which are reached very early in the nanoindentation.