Two Lehigh scientists have developed a "smart glue" – a polymer-aluminum interface whose adhesiveness can be controlled with temperature. The findings may help the U.S. Navy, who is seeking an environmentally friendly way of repelling the organisms that cling to ship hulls, creating drag and driving up fuel consumption.
Gregory Ferguson and Sureurg Khongtong believe their discovery may find applications in coatings, particularly as a means of preventing biofouling or the costly buildup of barnacles and other marine organisms on ship hulls, and perhaps in chemical separations, water purification, and medical uses.
Ferguson is an associate professor of chemistry and also of materials science and engineering. Khongtong earned his Ph.D. in polymer science and engineering from Lehigh in January and is now a lecturer in polymer and rubber technology at Walailak University in Thailand.
They reported their discovery in an article published this year in the Journal of the American Chemical Society. Their work has also been featured in several scientific journals, including Nature and Scientific American, and in The Sunday Times of London, as well as in other news outlets and trade publications.
Ferguson and Khongtong modified 1-4-polybutadiene, a standard synthetic rubber, causing it to stick to an aluminum oxide coating at room temperature. When they heated the interface to 80 degrees Centigrade and then quickly re-cooled it to room temperature, they observed a 44-percent reduction in the adhesion bonding the two surfaces. Forty hours after the restoration of room temperature, the interface regained its original stickiness. The reversibility persisted through multiple cycles of heating and cooling.
The elasticity of polybutadiene, like that of rubber in general, is dependent upon temperature. This dependence, says Ferguson, can be demonstrated by heating a rubber band from which a weight is suspended. The heat increases the resisting force of the rubber, causing the rubber band to contract and the weight to be pulled up.
"Scientists have known for decades that this temperature-dependent effect is a bulk property of rubber," says Ferguson. "We’ve demonstrated that this effect works at the surface as well."
Ferguson and Khongtong induced the polybutadiene to stick to the aluminum surface by treating it with aqueous permanganate, an oxidizing agent, which triggers the formation of carboxylic acids and other functional groups that are attracted to aluminum oxide.
At room temperature, the molecular chains of the treated rubber extend and straighten and "hook into" the layer of metal oxide, says Ferguson. When heated, the molecular chains resume their disordered, or entropically favored state, pulling away from the surface and weakening the polymer-metal bond.
Ferguson and Khongtong also reported that the polybutadiene becomes more hydrophobic, or water-repellent, when it loses its adhesion at elevated temperature.
This is the second "on-off" switch that Ferguson has developed. Two years ago, in an article published in Angewandte Chemie and reviewed in Chemical & Engineering News, he and his colleague Michael Freund reported a method for using electrochemical pulses to form self-assembled monolayers (SAMs) selectively on one micro-electrode in the presence of a nearby microelectrode.