Modification of polyimide wetting properties by laser ablated conical microstructures

Abstract

Laser texturing of Kapton® HN polyimide was performed by low-fluence ablation using a pulsed, frequency tripled (349nm) Nd:YLF laser. The laser was scanned in two dimensions in order to generate texture over a large area. The laser overlap percentage and fluence were varied and the resulting texture was studied. The texture features were inspected by electron microscopy and energy dispersive X-Ray spectroscopy (EDS), while the static contact angle of de-ionized water was measured by a contact angle goniometer. Rounded bump features were formed at all fluences, which decreased in areal density with fluence and number of laser pulses. Conical microstructures or “cones” were also formed at most fluences. Cones were larger than the bumps and thus had lower areal density, which increased as a function of the number of laser pulses. The polyimide was hydrophilic before texturing, with a contact angle of approximately 76°. For most of the experimental conditions the contact angle increased as a result of texturing, with the contact angle exceeding 90° for some textured surfaces, and reaching values as high as 118°. In general, the surfaces with significant increases in contact angle had high density of texture features, either bumps or cones. The surfaces that experienced a decrease in contact angle generally had low density of texture features. The increase in contact angle from a wetting (θ<90°) to a nonwetting surface (θ>90°) cannot be explained by texturing alone. EDS measurements indicate that textured regions had higher carbon content than the untextured regions due to depletion of oxygen species. The increase in carbon content relative to the oxygen content increased the native contact angle of the surface, causing the transition from hydrophilic to hydrophobic behavior. The contact angle of a textured surface increased as the relative spacing of features (diameter to spacing) decreased.