Warmth switches on a polymer's tackiness 

Bandages gummy enough to stay put often seem a bit too sticky when the time comes to pull them off. Now, researchers in Paris have created a glue that could take the "Ouch!" out of removing an adhesive strip. In a new study, they show that a polymer can switch from sticky to not-sticky with just a slight temperature change.

 The team, a joint research group of the National Center for Scientific Research and the chemical company Elf Atochem, created the new adhesive by combining two types of polymer molecules. Both have long carbon backbones with side groups poking out like the legs on a centipede. The side groups of one molecule contain hydrogen atoms while the other's contain mostly fluorine atoms. Because fluorinated polymers repel both oil and water, they provide the nonstick coating for cookware and the waterproofing for shoes and clothing.

 At room temperature, about 25@ C, the two molecules organize themselves in neat, alternating layers to form a hard material. Raising the temperature to 35@ C melts the polymer, which can be considered a liquid crystal.

 At the transition point, when the molecules lose their crystalline arrangement, the material turns from hard to sticky, says study coauthor Ludwik Leibler. He and his colleagues Guillaume de Crevoisier, Pascale Fabre, and Jean-Marc Corpart report their findings in the Aug. 20 Science.

"The change is very dramatic," Leibler says. The polymer switches to its sticky state as the temperature increases by just 2@ C. Conventional adhesives, by contrast, lose their tackiness only if they are cooled to about ^-40@ C.

 Because the transition happens close to body temperature, "I think this has high potential for bandage-type applications," says Richard P. Wool of the University of Delaware in Newark. With an adhesive derived from this polymer, "you could just cool [bandages] down and they'd pop off," he predicts.

 Bandages made with this material would also be easy to reposition: Simply warming them up would renew the stickiness of the adhesive.

 Leibler suggests that the polymer could also work as a coating on the grips of golf clubs or tennis rackets. The heat from an athlete's hand would improve his or her grasp, but dust and grime would fail to stick when the equipment is not in use.

 The researchers can control the temperature at which the transition occurs by either altering the ratio of polymers in the adhesive or controlling the composition and length of the side chains, says Leibler.

"That's a very nice aspect of this technology," says Wool. "If it's truly switchable, this is a very interesting material."

 The polymer's ability to wet a surface explains in part how it turns tacky. At the transition point, the material is no longer solid and flows into the crevices of a rough surface, making better contact. Improved contact with the surface in turn improves adhesion.

 Changes on a molecular level also contribute to the stickiness. At room temperature, "the fluorine groups will fend off the surface, and you get very low stick," says Wool. When the material heats up, however, the molecules become disordered, and "the backbone can start to play a role in making contact with the surface," he says.

 The polymer could serve as "a model system to understand the nature of adhesion," Leibler suggests. He and his group are now studying how changes in the chemical composition of the polymer components affect stickiness.