Scientists at Lehigh University, Pennsylvania, have created a new water-repellent glue that can alter its adhesive qualities depending on the temperature.
The rubbery polymer is said to cling to metals, but loosens its grip by 44% when warmed briefly to 80 degrees C. Heating also diminishes the affinity of the rubber surface for water.
Sureurg Khongtong and Gregory Ferguson developed the adhesive at Lehigh University in Bethlehem, Pennsylvania. They suggest that it could form an anti-fouling coating for boat hulls or for controlling cell adhesion in surgery.
At the molecular scale the smart rubber is like a kind of Velcro, with hooks connected to springs. The springs are loose and floppy when cool but are more tightly coiled when warm. This makes the hooks advance and retract from the material’s surface.
The basic material is a standard synthetic rubber called 1-4-polybutadiene. To make the rubber stick to metals, Khongtong and Ferguson treat it with an oxidising agent. This introduces chemical groups such as carboxylic acids that have an affinity for the thin coating of aluminium oxide, which forms as soon as aluminium is exposed to air.
The molecular chains of the rubber are crosslinked into a flexible network peppered with carboxylic acid groups. When the polymer comes into contact with aluminium, these groups wriggle to the surface, where they bond with the layer of metal oxide. This stretches and straightens the polymer chains, which otherwise adopt crumpled shapes like tangled balls of string.
Warming the polymer increases the tendency of its molecular chains to contract into blobs. This pulls the carboxylic acid groups away from the surface, weakening the bonding between the polymer and the metal. This decreased adhesion persists initially even when the polymer is cooled down again, making the rubber easier to peel off.
If the cooled laminate is left for many hours the carboxylic acid groups find their way slowly to the surface again, and the adhesion returns.
(Source: Philip Ball, Nature)