Down the Line: Smart Adhesive Has an On-off Switch
Ferguson and Khongtong modified a 1-4 polybutadiene, a standard synthetic rubber, by treating it with aqueous permanganate, an oxidizing agent that triggers formation of carboxylic acids and other functional groups attracted to aluminum oxide. At room temperature, the molecular chains of the treated rubber extend and hook onto the metal oxide layer, Ferguson says. When heated, the molecular chains resume their disordered higher-entropy state, pulling away from the surface and weakening the polymer-metal bond.
When they heated the interface to 80 C and then quickly cooled it to room temperature, the researchers observed a 44 percent reduction in the adhesion bonding the two surfaces, confirming the temperature dependence of the bond. The interface regained its original stickiness 40 hours after the return to room temperature, and the reversibility persisted through multiple heating and cooling cycles. The scientists also report that the polybutadiene becomes more hydrophobic when it loses its adhesion at elevated temperature.
The elasticity of polybutadiene, like that of rubber, is temperature dependent. "Scientists have known for decades that this temperature dependence is a bulk property of rubber," says Ferguson. "We’ve demonstrated that this effect works at the surface as well."
Ferguson and Khongtong believe their discovery may prevent biofouling. This would be of particular interest to the U.S. Navy, which is seeking an environmentally friendly way to repel barnacles and other organisms that cling to ship hulls, creating drag that cuts speed and raises fuel consumption. It may also find application in chemical separation, water purification and medical use.