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 dose 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 °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 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-M_w thin films.