Measurements of viscoelastic properties by nanoindentation

Abstract

Scope and Method of Study: Nanoindentation technique was used to characterize the viscoelastic behavior of polymers. Methods were developed to measure linearly viscoelastic properties of polymers using nanoindentation. For polymers with Poisson's ratio assumed as a constant, viscoelastic functions in both time cand frequency domains were determined based on the viscoelastic contact analysis of axisymmetric indentation problems. Formulas were derived to calculate the viscoelastic functions such as creep compliance and relaxation modulus using nanoindentation load-displacement data. For highly viscoelastic materials with unknown Poisson's ratio varying with time, a method is developed to measure two independent viscoelastic functions using a Berkovich indenter and a spherical indenter.

Findings and Conclusions: Methods developed in this study were validated on several viscoelastic materials, such as polymethyl methacrylate (PMMA) and polycarbonate (PC), poly(vinyl acetate) (PVAc). The viscoelastic functions measured using the developed methods agree reasonable well with the data from conventional testing methods. The methods were applied to SWNT/polyelectrolyte films and tympanic membrane. Nanoindentation based on these methods have potential for use to measure viscoelastic properties of small amounts of time-dependent materials, such as thin polymeric films, polymer components or coatings on MEMS, and they can also be used to characterize local viscoelastic behavior of materials with properties varying spatially, such as bones and nanocomposites.