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

Area of Science:

• Polymer Physics
• Materials Science
• Nanotechnology

Background:

• Understanding polymer mechanical deformation at the molecular scale is crucial for advanced manufacturing processes like nanoimprint lithography.
• Polystyrene (PS) films exhibit complex behavior under large-strain squeeze flow, influenced by molecular weight (M(w)) and deformation conditions.

Purpose of the Study:

• To investigate molecular-scale polymer mechanical deformation during large-strain squeeze flow of PS films.
• To determine the influence of molecular weight, temperature, and deformation rate on the mechanical response.
• To assess the potential for reducing residual film thickness in nanoimprint lithography.

Main Methods:

• Measured stress-strain and creep relations during flat punch indentation of PS films.
• Varied initial film thickness (170 nm to 10 nm), molecular weight (M(w)), and deformation stress rate.
• Conducted experiments across a temperature range of 20 to 125 °C.

Main Results:

• Stress-strain response was M(w)-independent at low temperatures, consistent with bulk glassy polymer theory.
• At high temperatures, M(w) degeneracy was broken, but no molecular confinement effects were observed.
• Creep measurements for lower M(w) films matched bulk Newtonian flow predictions.
• Mechanical relaxation scaled with temperature, similar to Williams-Landel-Ferry (WLF) scaling.
• High M(w) films exhibited shear-thinning behavior but could not be precisely modeled.
• Loading rate and magnitude affected extrusion depth in high-M(w) films.

Conclusions:

• The study provides insights into the molecular-scale deformation mechanisms of polystyrene films under squeeze flow.
• Findings suggest that optimized high strain and strain rate loading of high-M(w) thin films can reduce residual film thickness.
• This has implications for improving mold flash reduction and achieving thinner residual films in high-resolution nanoimprint lithography.