Chemical Force Microscopic Study of UV Excimer Laser Irradiated Polyamide

Abstract

Recently, there has been great interest in physico-chemical treatment for modifying polymer surfaces. UV Excimer laser irradiation is of particular interest in morphological and chemical modifications. Depending on the laser energies used, effects of laser treatment fall into two groups: above the ablation threshold the ripple structures of micrometer size form (high-fluence), and below the ablation threshold the sub-micron structures emerge (low-fluence).

Traditional methods of studying chemical properties are Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, and mass spectroscopy. The ability of a relatively new technique involving chemical force microscopy (CFM) can be used to image and discriminate the areas exposing different functional groups on polymers. Gold-coated AFM tip modified with carboxylic acid (-COOH) terminated self-assembled alkanethiol monolayers (SAMs) was used to measure the adhesive forces between the tip and the laser treated samples in a water or hexane medium.

The CFM results showed that high-fluence laser treated polyamide has the highest adhesive force with the modified tip in a water medium when compared with the control and low-fluence ones. The adhesive force is due to electrostatic attraction between the negatively charged tips terminating in COO^- groups and the positively charged sample terminating in NH⁺ groups. This indicates that the high-fluence laser treated sample results in the formation of many amine end-groups on the polymer surface. In comparison, only low-fluence treated surface shows adhesive force with the modified tip in a hexane medium. This implies that the low-fluence laser treated polyamide has substantially more hydrophilic groups than the untreated and high-fluence laser treated samples. The adhesion force measurements by CFM allow one to have better understanding of surface chemical modifications induced by UV excimer laser.