Swelling in spin-coated methylcellulose ultra-thin films: effect on film structure, surface topography, and temperature-response property

Abstract

Methylcellulose (MC) thin films on a Si substrate, with a thickness of ∼200 Å (near the radius of gyration R_g of the polymer), were exposed to a saturated water vapor condition to study the moisture absorption and swelling behavior in situ and the changes in the film surface topography, structure and temperature-response nature after such swelling. In situ visual observation of the MC thin films showed a color change with the swelling to as much as a factor of 5 from the original dry thickness. After removal of the film under saturated water vapor condition, X-ray reflectivity measurements revealed no change in the film thickness and surface/interface roughness of the film which means that the film is robust and retains its original film structure. Surface topography images by AFM showed roughening of the film texture due to water cavities on the film surface. The presence of cavities is indicative of the slow relaxation process of the polymer chains relative to the transport time of the water vapor molecules. Temperature-dependent contact angle measurements of the swelled films showed a loss in the hydrophilic-to-hydrophobic switching property of the MC film surface.