Measuring glassy and viscoelastic polymer flow in molecular-scale gaps using a flat punch mechanical probe

Harry D. Rowland, William P. King, Graham L.W. Cross, John B. Pethica

Research output: Contribution to journalArticlepeer-review

Abstract

This paper investigates molecular-scale polymer mechanical deformation during large-strain squeeze flow of polystyrene (PS) films, where the squeeze flow gap is dose to the polymer radius of gyration (Rg). Stress - strain and creep relations were measured during flat punch indentation from an initial film thickness of 170 nm to a residual film thickness of 10 nm in the PS films, varying molecular weight (Mw) and deformation stress rate by over 2 orders of magnitude while temperatures ranged from 20 to 125 °C. In stress - strain curves exhibiting an elastic-to-plastic yield-like knee, the response was independent of Mw, as expected from bulk theory for glassy polymers. At high temperatures and long times sufficient to extinguish the yield-knee, the mechanical response Mw degeneracy was broken, but no molecular confinement effects were observed during thinning. Creep measurements in films of 44K Mw were well-approximated by bulk Newtonian no-slip flow predictions. For extrusions down to a film thickness of 10 nm, the mechanical relaxation in these polymer films scaled with temperature similar to Williams - Landel - Ferry scaling in bulk polymer. Films of 9000K Mw, extruded from an initial film thickness of 2Rg to a residual film thickness of 0.5Rg, while showing stress-strain viscoelastic response similar to that of films of 900K Mw, suggestive of shear-thinning behavior, could not be matched to a constitutive flow model. In general, loading rate and magnitude influenced subsequent creep extrusion depth of high-Mw, films, with deeper final extrusions for high loading rates than for low leading rates. The measurements suggest that, for high-resolution nanoimprint lithography, mold flash or final residual film thickness can be reduced for high strain and strain rate loading of high-Mw thin films.

Original languageEnglish (US)
Pages (from-to)419-428
Number of pages10
JournalACS Nano
Volume2
Issue number3
DOIs
StatePublished - Mar 1 2008

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

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