Contact angle of water on polystyrene thin films: effects of CO(2) environment and film thickness.

We examine the contact angle of water droplets on polystyrene (PS) thin films of varying thicknesses supported by silicon wafers under both air and pressurized carbon dioxide (CO2) environments. At 23 degrees C, the contact angle is found to increase upon increasing CO2 pressure in the vapor regime and then levels off in the liquid CO2 regime. A macroscopic model based on Young's equation and the geometric-mean method for interfacial tensions, and long-range van der Waals interactions, correctly predicts the trends and the magnitude of the contact angle dependence on pressure, although deviations occur at high CO2 activities. The contact angle was also found to depend on film thickness, h, when h was comparable to or smaller than 50 nm. Specifically, the contact angle decreases with decreasing PS film thickness. This behavior could be accounted for with the use of a model that incorporates the effects of film thickness, CO2 pressure, and the long-range van der Waals potential.

Polystyrene thin films in CO(2).

In air, or vacuum environments, liquid polystyrene (PS) thin films (thickness, h<100 nm ) supported by SiOx /Si substrates are structurally metastable or unstable, depending on film thickness. They rupture and eventually form droplets on the SiOx /Si substrates (dewet) due to the influence of destabilizing long-ranged van der Waals dispersion forces. We used scanning force microscopy to examine the structural stability of liquid PS films in the thickness range 5 nm

Pressure, temperature, and thickness dependence of CO2-induced devitrification of polymer films.

The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO2 pressure-induced devitrification transition, P(g), which is film thickness dependent, P(g)(h)=DeltaP(g)+P(bulk)(g). P(bulk)(g) is the bulk glass transition and DeltaP(g) can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent.