Humidity-induced glass transition of a polyelectrolyte brush creates switchable friction in air

Polymer brushes have found extensive applications as responsive surfaces, particularly in achieving tunable friction in aqueous environments due to their remarkably low friction underwater. Despite recent interest in extending this technology to air environments, little is known about the impact of water vapor absorption on friction. Considering polyelectrolyte brushes, we report findings that reveal, with increasing relative humidity, a trend of frictional shear forces decaying over two orders of magnitude only after achieving a critical humidity, which can be controlled by the polymer and counterions. Contrary to expectations based on the friction trends, water absorption, structure, and swelling of the brushes as measured by ATR-IR spectroscopy and ellipsometry followed continuous trends with increasing humidity. In contrast, mechanical properties and water absorption rate changed abruptly at the transition humidity, indicative of a humidity-induced glass transition. Below the glass transition humidity, slow relaxation dynamics of the polymer prevent fluid flow and promote dry sliding even for systems with 30-40 wt% water in the brush; thus, friction transitions sharply. This new switching mechanism, shown to be a general property of the polymer-solvent vapor system, provides new directions for fundamental research and creates opportunities for switchable friction and surface actuation from vapor stimuli.