Forces between polymer surfaces and self-assembled monolayers

Gold-coated atomic force microscope (AFM) tips functionalized with amine-, hydroxyl-, carboxylic acid-, and methyl-terminated alkanethiol molecules were used to probe the adhesive forces of polystyrene and poly(acrylic acid) films in dry air (relative humidity < 0.5%). X-ray photoelectron spectroscopy (XPS) and contact angle measurements confirmed the quality and uniformity of similarly treated gold surfaces and the polymer films. XPS indicated that the amine-functionalized thiol films were protonated and comprised of multilayers. Contact angle data were used to calculate surface free energies, and DMT theory yielded the works of adhesion and interfacial free energies for the tip-substrate combinations. In the case of polystyrene, the work of adhesion followed the order methyl > carboxylic acid > hydroxyl > amine. For poly(acrylic acid), the observed order was hydroxyl > amine > carboxylic acid > methyl.