  Polymer brushes have found extensive applications as responsive surfaces,
particularly in achieving tunable friction in solvent environments. Despite
recent interest in extending this technology to air environments, little is
known about the impact of 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. However, water
absorption, structure, and swelling of the brushes followed continuous trends
with increasing humidity. In contrast, a humidity-induced glass transition
occurred which caused a shift from dry to fluid-like sliding with the
water-swollen brush acting as the lubricant. Below the glass transition,
friction is large without regard to water concentration; thus, friction
transitions sharply. This switching mechanism, shown to be a general property
of the hygroscopic polymer, provides new opportunities for switchable friction
or other surface actuation from vapor stimuli.
