Contact creep compliance of viscoelastic materials via nanoindentation

Abstract

The creep compliance of viscoelastic materials such as synthetic polymers is an established metric of the rate at which strain increases for a constant applied stress and can, in principle, be implemented at the nanoscale to compare quantitatively bulk or thin film polymers of different structures or processing histories. Here, we outline the evolution of contact creep compliance analysis and application for both conical and spherical indenter geometries. Through systematic experiments on four amorphous (glassy) polymers, two semi-crystalline polymers and two epoxies, we show that assumptions of linear viscoelasticity are not maintained for any of these polymers when creep compliance is measured via conical indentation at the nanoscale, regardless of the rate of stress application (step or ramp). Further, we show that these assumptions can be maintained to evaluate the contact creep complianceJc(t) of these bulk polymers, regardless of the rate of stress application, provided that the contact strains are reduced sufficiently through spherical indentation. Finally, we consider the structural and physical properties of these polymers in relation toJc(t), and demonstrate thatJc(t) correlates positively with molecular weight between entanglements or crosslinks of bulk, glassy polymers.