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Contact creep compliance of viscoelastic materials via nanoindentation

Published online by Cambridge University Press:  01 June 2006

Catherine A. Tweedie
Affiliation:
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Krystyn J. Van Vliet*
Affiliation:
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
*
a) Address all correspondence to this author. e-mail: krystyn@mit.edu
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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 compliance Jc(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 to Jc(t), and demonstrate that Jc(t) correlates positively with molecular weight between entanglements or crosslinks of bulk, glassy polymers.

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Articles
Copyright
Copyright © Materials Research Society 2006

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References

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