CHAMPAIGN, Ill. - Contrary to what scientists have long thought, recent experiments at the University of Illinois have revealed that flexible polymers stuck to a solid surface crawl around in a very different way than they would in the bulk. This is important because the properties of polymers at surfaces play a central role in issues as wide-ranging as adhesives, coatings and biomedical implants.
"A polymer in 'Flatland' may sound like science fiction, but every bulk phase is connected to adjoining ones through its surface," said Steve Granick, a professor of materials science at the UI. Flatland refers to the title of an 1880 novel by Edwin Abbott that takes place essentially on a flat surface. "By examining the characteristics of individual polymeric chains, we can better predict the behavior and performance of bulk polymers."
As reported in the July 13 issue of the journal Nature, Granick and his UI colleagues -- biophysicist Enrico Gratton, materials scientist Kenneth Schweizer and postdoctoral research associates Svetlana Sukhishvili, Yan Chen and Joachim Muller -- exposed a specially prepared solid surface to a dilute solution of polyethylene glycol chains of varying length. Then they used an extremely sensitive measurement technique called two-photon fluorescence correlation spectroscopy, developed by biophysicists but never applied previously to materials science problems, to monitor the motions of individual molecules.
"Conventional theory said that a molecule twice as long would move half as fast," Granick said. "But our study shows a stronger dependence on chain length. Now the theorists must resharpen their pencils and make sense of observations that don't agree with their preconceptions."
When a molecular chain moves on a surface, it can't just go in any direction at random, Granick said. There is a preferred motion that lies in the direction the chain is pointed. This restrained motion is known to occur in bulk flexible polymers, but only when the chains are very long and tangled together like noodles of spaghetti. "Because the noodles interfere with one another, you can only pull one out along a preferred direction," Granick said. "But in Flatland, nearby chains are not needed to see this effect.
"This may mean that an individual molecule 'entangles' in some way with the solid surface. The many points at which the chain sticks to the surface may play a role analogous to the points of entanglement that occur when many chains are present."
In their experiments, the researchers also found that each thread-like chain of polyethylene glycol attached to the surface weakly along its entire length, causing it to flatten like a pancake.
"The molecule moves because individual parts of the pancake -- different links in the chain -- are wandering around, hopping on and off the surface," Granick said. "When there is slack between sticking points -- for example, in the loops of a flexible polymer -- the chain might propagate along its length in a caterpillar-like fashion."
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