Probing substrate effects on relaxation dynamics of ultrathin poly(vinyl acetate) films by dynamic wetting of water droplets on their surfaces

Abstract

The dynamic wetting of water droplets on ultrathin poly(vinyl acetate) (PVAc) films supported by substrates with tunable surface hydroxyl group content was investigated at room temperature. The jumping angle of water droplets on the ultrathin PVAc films decreased with increasing hydroxyl content on the substrate surface, while a smooth evolution of the water droplet contacting line emerged for the film supported by a substrate with 100% hydroxyl groups. The critical thickness of ultrathin PAVc films, through which the interfacial effect from substrate chemistry on the dynamic wetting of water droplets on their surfaces will disappear, was measured by this novel method and the results showed that the critical thickness increased from ∼35 nm to ∼75 nm with increasing hydroxyl content from 41% to 100% on the substrate. This relationship between critical thickness and hydroxyl content was attributed to the intensities of the hydrogen bonds, resulting in lower chain mobility at the interface and a higher elastic modulus of the whole ultrathin film. This study demonstrates that the dynamic wetting of a liquid droplet on ultrathin polymer films is very sensitive and effective to access their mechanical properties, relaxation behavior and those properties affected by the substrate surface chemistry.