Nanomechanical Quantification of Polymer Energy Absorption
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Nanomechanical Quantification of Polymer Energy Absorption Catherine A. Tweedie1, James F. Smith2 and Krystyn J. Van Vliet1 1 Department of Materials Science, Massachusetts Institute of Technology, MA, U.S.A. 2 Micro Materials Limited, Unit 3, Wrexham Technology Park, Wrexham, LL13 7YP, U.K. ABSTRACT Mechanical characterization of polymeric thin films and small volume structures is critical to device development in industrial applications ranging from low-k dielectric microelectronic packaging films to engineered and natural biological substrata. Although nanoindentation has the potential to quantify mechanical properties of polymeric systems, the established analyses developed for metals and ceramics (eg, calculation of hardness and Young’s modulus) do not capture key aspects of viscoelastoplastic deformation and are therefore not quantitatively applicable. Here, we present a set of complementary, nanoscale contact-based experimental approaches that together characterize specific energy absorption as a unique mechanical characteristic of polymers, and provide examples for a set of amorphous polymers. INTRODUCTION A quantitative, physics-based framework to interpret the mechanical behavior of timedependent materials is of increasing importance, given developments in areas such as nanocomposites, thin films and biological substrates/scaffolds. Despite established bulk polymer physics and rheology (eg, creep as a function of time and temperature [1]), there is currently no standard method for characterizing the mechanical behavior of polymers confined to small volumes. A standard semi-analytical model exists for time-independent materials, from which indentation elastic modulus E and indentation hardness H are calculated from the unloading portion of an instrumented indentation experiment [2]. As there exists no analytical parallel for time-dependent materials, this method is increasingly employed to quantify the mechanical properties of polymers [3-5]. However, it is difficult to derive general principles or polymer mechanical properties from these reported experiments for three reasons. First, the inherent time dependency of the polymer nanoindentation response necessitates a “hold period” at maximum load prior to unloading so that the unloading response can be approximated by a line of positive slope, as assumed and observed in the nanoindentation response of metals and ceramics. In the absence of such a holding segment in polymers, a so-called “nose effect” or initially negative slope upon unloading is observed. Second, the materials considered vary in basic molecular composition, molecular weight, and degree of crystallinity, all of which are well-known to affect polymer properties. Third, time-independent properties such as E and H are of limited utility in design of viscoelastoplastic polymers and polymer-based devices. Alternative experimental approaches address explicitly the time-dependence of deformation, including cyclic nanoindentation loading to measure the storage and loss moduli, E’ and E”, res
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