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Research Article

Nanoparticle amount, and not size, determines chain alignment and nonlinear hardening in polymer nanocomposites

H. Samet Varol, Fanlong Meng, Babak Hosseinkhani, Christian Malm, Daniel Bonn, Mischa Bonn, Alessio Zaccone, and View ORCID ProfileSapun H. Parekh
  1. aDepartment of Molecular Spectroscopy, Max Planck Institute for Polymer Research, 55128 Mainz, Germany;
  2. bCavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom;
  3. cSKF Engineering & Research Center, 3430DT Nieuwegein, The Netherlands;
  4. dInstitute of Physics, University of Amsterdam, 1098 XH Amsterdam, The Netherlands;
  5. eDepartment of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB2 3RA, United Kingdom

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PNAS first published April 4, 2017; https://doi.org/10.1073/pnas.1617069114
H. Samet Varol
aDepartment of Molecular Spectroscopy, Max Planck Institute for Polymer Research, 55128 Mainz, Germany;
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Fanlong Meng
bCavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom;
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Babak Hosseinkhani
cSKF Engineering & Research Center, 3430DT Nieuwegein, The Netherlands;
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Christian Malm
aDepartment of Molecular Spectroscopy, Max Planck Institute for Polymer Research, 55128 Mainz, Germany;
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Daniel Bonn
dInstitute of Physics, University of Amsterdam, 1098 XH Amsterdam, The Netherlands;
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Mischa Bonn
aDepartment of Molecular Spectroscopy, Max Planck Institute for Polymer Research, 55128 Mainz, Germany;
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Alessio Zaccone
bCavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom;
eDepartment of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB2 3RA, United Kingdom
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Sapun H. Parekh
aDepartment of Molecular Spectroscopy, Max Planck Institute for Polymer Research, 55128 Mainz, Germany;
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  • ORCID record for Sapun H. Parekh
  • For correspondence: parekh@mpip-mainz.mpg.de
  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved February 28, 2017 (received for review October 14, 2016)

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Significance

When straining materials (e.g., pulling a rubber band), they initially deform with a certain stiffness; if one pulls harder, some materials strengthen. This phenomenon, known as nonlinear strain hardening, is a critical feature of composite polymer materials—polymers with reinforcing filler particles—used in, e.g., car tires. Engineering properties such as modulus, toughness, and strength of nanocomposites have been traditionally optimized through trial and error by changing the size and amount of fillers. Our work elucidates the molecular origin of strain hardening in polymer nanocomposites, showing that filler amount, but not size, sets the strain-hardening properties based on interfiller chain elongation. The insensitivity to filler size provides a facile concept to independently tune linear and nonlinear mechanics in composites.

Abstract

Polymer nanocomposites—materials in which a polymer matrix is blended with nanoparticles (or fillers)—strengthen under sufficiently large strains. Such strain hardening is critical to their function, especially for materials that bear large cyclic loads such as car tires or bearing sealants. Although the reinforcement (i.e., the increase in the linear elasticity) by the addition of filler particles is phenomenologically understood, considerably less is known about strain hardening (the nonlinear elasticity). Here, we elucidate the molecular origin of strain hardening using uniaxial tensile loading, microspectroscopy of polymer chain alignment, and theory. The strain-hardening behavior and chain alignment are found to depend on the volume fraction, but not on the size of nanofillers. This contrasts with reinforcement, which depends on both volume fraction and size of nanofillers, potentially allowing linear and nonlinear elasticity of nanocomposites to be tuned independently.

  • nanocomposites
  • nonlinear elasticity
  • strain stiffening
  • polymer bridging
  • polymer chain alignment

Footnotes

  • ↵1To whom correspondence should be addressed. Email: parekh{at}mpip-mainz.mpg.de.
  • Author contributions: H.S.V. and S.H.P. designed research; H.S.V., F.M., C.M., and A.Z. performed research; B.H., M.B., and S.H.P. contributed new reagents/analytic tools; H.S.V., F.M., D.B., M.B., A.Z., and S.H.P. analyzed data; and H.S.V., F.M., C.M., D.B., M.B., A.Z., and S.H.P. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1617069114/-/DCSupplemental.

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Filler amount, not size, controls strain hardening
H. Samet Varol, Fanlong Meng, Babak Hosseinkhani, Christian Malm, Daniel Bonn, Mischa Bonn, Alessio Zaccone, Sapun H. Parekh
Proceedings of the National Academy of Sciences Apr 2017, 201617069; DOI: 10.1073/pnas.1617069114

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Filler amount, not size, controls strain hardening
H. Samet Varol, Fanlong Meng, Babak Hosseinkhani, Christian Malm, Daniel Bonn, Mischa Bonn, Alessio Zaccone, Sapun H. Parekh
Proceedings of the National Academy of Sciences Apr 2017, 201617069; DOI: 10.1073/pnas.1617069114
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