Superhydrophobic Polyimide via Ultraviolet Photooxidation: The Evolution of Surface Morphology and Hydrophobicity under Different Ultraviolet Intensities

Abstract

Ultraviolet (UV) photooxidation has recently been developed to fabricate superhydrophobic polyimide (PI) films in combination with fluoroalkylsilane modification. However, it remains unclear whether the surface morphology and hydrophobicity are sensitive to technical parameters such as UV intensity and radiation environment. Herein, we focus on the effects of UV intensity on PI surface structure and wettability to gain comprehensive understanding and more effective control of this technology. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results showed that UV intensity governed the evolutionary pattern of surface morphology: lower UV intensity (5 mW/cm2) facilitated in-plane expansion of dendritic protrusions while stronger UV (10 and 15 mW/cm2) encouraged localized growth of protrusions in a piling-up manner. Surface roughness and hydrophobicity maximized at the intensity of 10 mW/cm2, as a consequence of the slowed horizontal expansion and preferred vertical growth of the protrusions when UV intensity increased. Based on these results, the mechanism that surface micro/nanostructures developed in distinct ways when exposed to different UV intensities was proposed. Though superhydrophobicity (water contact angle larger than 150°) can be achieved at UV intensity not less than 10 mW/cm2, higher intensity decreased the effectiveness. Therefore, the UV photooxidation under 10 mW/cm2for 72 h is recommended to fabricate superhydrophobic PI films.