Modifying Surface Fluctuations of Polymer Melt Films with Substrate Modification.

Area of Science:

• Materials Science
• Polymer Physics
• Surface Science

Background:

• Thin polymer films exhibit surface fluctuations that are sensitive to substrate interactions.
• Hydrodynamic continuum theory (HCT) describes these fluctuations based on bulk viscosity.
• Confinement effects become significant as film thickness decreases.

Purpose of the Study:

• To investigate the impact of plasma polymerized films on polystyrene (PS) melt film surface fluctuations.
• To understand how substrate modification affects confinement effects in ultrathin PS films.
• To correlate surface fluctuation behavior with adsorbed layer thickness and substrate surface energy.

Main Methods:

• X-ray photon correlation spectroscopy (XPCS) was used to measure surface fluctuation relaxation times.
• Experiments were conducted on 131 kg/mol linear PS films of varying thicknesses (ca. 4R and ∼3R).
• PS films were studied on both hydrogen-passivated silicon (H-Si) and plasma polymer modified silicon substrates.

Main Results:

• HCT accurately describes fluctuations for thicker PS films (ca. 4R) on both substrates.
• For ultrathin films (∼3R), surface fluctuations deviate from HCT predictions, indicating confinement effects.
• The confinement effect is more pronounced on H-Si than on the plasma polymer modified surface.
• The observed deviations are attributed to differences in the thickness of strongly adsorbed layers, influenced by substrate surface energy.

Conclusions:

• Plasma polymerized films significantly alter surface dynamics in confined polymer films.
• Substrate surface energy plays a crucial role in determining the adsorbed layer thickness and subsequent confinement effects.
• XPCS and HCT provide a framework for understanding polymer film behavior at the nanoscale, highlighting the importance of interfacial phenomena.