Adaptation of a Styrene-Acrylic Acid Copolymer Surface to Water

Solid surfaces, in particular polymer surfaces, are able to adapt upon contact with a liquid. Adaptation results in an increase in contact angle hysteresis and influences the mobility of sliding drops on surfaces. To study adaptation and its kinetics, we synthesized a random copolymer composed of styrene and 11-25 mol% acrylic acid (PS/PAA). We measured the dynamic advancing (θ_A) and receding (θ_R) contact angles of water drops sliding down a tilted plate coated with this polymer. We measured θ_A ≈ 87° for velocities of the contact line <20 μm/s. At higher velocities, θ_A gradually increased to ∼98°. This value is similar to θ_A of a pure polystyrene (PS) film, which we studied for comparison. We associate the gradual increase in θ_A to the adaptation process to water: The presence of water leads to swelling and/or an enrichment of acid groups at the water/polymer interface. By applying the latest adaptation theory (Butt et al. Langmuir 2018, 34, 11292), we estimated the time constant of this adaptation process to be ≪1 s. For sliding water drops, θ_R is ∼10° lower compared to the reference PS surface for all tested velocities. Thus, at the receding side of a sliding drop, the surface is already enriched by acid groups. For a water drop with a width of 5 mm, the increase in contact angle hysteresis corresponds to an increase in capillary force in the range of 45-60 μN, depending on sliding velocity.