Surface coatings of PEO–PPO–PEO block copolymers on native and polystyrene-coated silicon wafers

Abstract

Adsorbed layers of three PEO–PPO–PEO triblock copolymers on a hydrophilic SiO 2 and a hydrophobic polystyrene surface were studied by atomic force microscopy, X-ray reflectometry, ellipsometry, and contact angle measurements. Adsorption from aqueous solutions at the hydrophilic surface is found to depend in a delicate way on N EO and N PO, the total numbers of hydrophilic EO and hydrophobic PO groups of the polymers, as quantified by the asymmetry parameter β = N EO 3/5/ N PO 1/2. Only for the polymer of medial β value, F127 ( β = 2.76), an adsorbed layer is retained after extensive rinsing with pure water. At the hydrophobic surface all polymers, L64 ( β = 1.2), F127 ( β = 2.76), and F68 ( β = 3.47), form stable ad-layers and their thickness scales with the number of EO units of the hydrophilic blocks. From this result and the observed decrease of the contact angle of water droplets caused by the ad-layer, it is conjectured that the polymers adsorb with their PPO block to the hydrophobic substrate and the PEO blocks directed outwards. At the hydrophilic surface the ad-layer of F127 is oriented in the opposite way, i.e., with the PPO blocks oriented outwards. In all cases, the ad-layers form uniform coatings on a micrometer scale, with a thickness in the range 1.0–2.3 nm, again suggesting monolayer coverage of the surface.