Measuring glassy and viscoelastic polymer flow in molecular-scale gaps using a flat punch mechanical probe.
This paper investigates molecular-scale polymer mechanical deformation during large-strain squeeze flow of polystyrene (PS) films, where the squeeze flow gap is close to the polymer radius of gyration (R(g)). 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 (M(w)) and deformation stress rate by over 2 orders of magnitude while temperatures ranged from 20 to 125 degrees C. In stress-strain curves exhibiting an elastic-to-plastic yield-like knee, the response was independent of M(w), as expected from bulk theory for glassy polymers. At high temperatures and long times sufficient to extinguish the yield-knee, the mechanical response M(w) degeneracy was broken, but no molecular confinement effects were observed during thinning. Creep measurements in films of 44K M(w) 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 M(w), extruded from an initial film thickness of 2R(g) to a residual film thickness of 0.5R(g), while showing stress-strain viscoelastic response similar to that of films of 900K M(w), 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-M(w) films, with deeper final extrusions for high loading rates than for low loading 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-M(w) thin films.
Rowland
Harry D
HD
Department of Mechanical Science and Engineering, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA.
King
William P
WP
Cross
Graham L W
GL
Pethica
John B
JB
D019245:Compressive Strength; D055119:Elastic Modulus; D005898:Glass / Q000737:chemistry; D006244:Hardness; D008422:Materials Testing / Q000295:instrumentation* / Q000379:methods*; D008567:Membranes, Artificial; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure; D036103:Nanotechnology / Q000379:methods*; D044367:Phase Transition; D011137:Polystyrenes / Q000737:chemistry*; D012212:Rheology / Q000295:instrumentation* / Q000379:methods; D014783:Viscosity
D008567:Membranes, Artificial; D011137:Polystyrenes