Heal Thyself

Researchers develop 'self-healing' polymers at the nanoscale.

Imagine a hip replacement covered with a nanometersthin biocompatible layer on its outer surface, where it contacts the body, while the rest of it is designed with the strength to cope with any stress the body might deliver.

One could also imagine a multi-layered coating for a doorknob. The outer layer is designed to be microbially resistant, while the remaining layer contains properties that adhere to the doorknob. When an individual with a cold touches the doorknob, the anti-microbial agents immediately kill the bacteria before they can spread. Meanwhile, the adhesive properties keep the coating in place.

A collaboration of polymer scientists at ORNL is using the Liquids Reflectometer at the Spallation Neutron Source to study the dynamics of polymer mixtures. The mixtures comprise repeating large molecules connected by covalent chemical bonds that hold promise for applications as diverse as biocompatible films for human implants; semiconductors; substrates for electronic displays; children's toys; and durable, self-repairing aircraft body materials.

Polymers in nature include cellulose, the main constituent of wood and paper. Familiar synthetic polymers include nylon, Teflon and silicone. Mark Dadmun, professor of chemistry at the University of Tennessee and a Joint Faculty appointee in the Chemical Sciences Division at ORNL is exploring what he calls "self-healing materials"—polymer mixtures in which one critical component moves quickly to the surface while the matrix (the understructure) gives structural rigidity. Specifically, Dadmun is looking at the dynamics of a copolymer (the targeted, surface material) in a matrix (the homopolymer, the bulk of the material). "We design our process so that the copolymer comes to and saturates the surface. We retain a portion in the matrix so that if we lose it at the surface, we simply force the copolymer to the surface again."

Dadmun works with instrument scientist John Ankner at SNS and materials scientist Joe Pickel at the Center for Nanophase Materials Sciences, as well as UT & ORNL Distinguished Scientist Jimmy Mays. Pickel and Mays synthesized the polymer samples. In the self-healing materials, they target the key properties of biocompatibility, microbial resistance, adhesion and flammability.

"Our goal is to design a system in which the majority of the component has the stability we need and the strength to be a suitable matrix," Dadmun says. "We have a separate polymer designed to bloom to the surface, with the potential to provide the surface-sensitive property we need. Because we started with a mixture and forced it to the surface, if the polymer is washed off a reservoir of material would continue to rise to the surface."

Dadmun stresses that finding a copolymer that migrates to the surface is not difficult. More difficult is finding one that gets there fast enough, a process that involves the material's thermodynamics. What the researchers seek to learn is how the specific structure of the copolymer affects the speed with which it migrates to the surface.

The researchers began the experiment with a silicon wafer. They coated the wafer with a thin film, a mixture of deuterated polymethyl methacrylate as the matrix polymer, and a branched copolymer of methyl methacrylate and ethylene oxide. As they heated this sample, allowing the mixture to approach thermal equilibrium, the graft copolymer containing ethylene oxide diffused to the surface. The process enabled the measurement of the water contact angle to verify that the copolymer segregated to the surface.

"We could determine the presence of additional ethylene oxide from the copolymer in the mixture at the surface," Dadmun says. "Our ultimate goal is to use the liquids reflectometer to extract information on how quickly it gets to the surface."

Neutrons are ideally suited to study the copolymer's dynamics because, Dadmun says, "with neutrons we are able to label the material selectively." In Dadmun's samples the matrix is deuterated polymethyl methacrylate and the copolymer is an undeuterated polymethyl methacrylate, grafted to undeuterated ethylene oxide. The various neutron scattering properties of the deuterated and undeuterated materials enable researchers to observe the location and movement of the copolymer in the composite material as a function of the "annealing time," or the heating and slow cooling.

As it is heated, the copolymer tends to migrate to the surface. The experimenters observe the time dependence of the intensity of neutrons scattered from the copolymer near the surface, which can be analyzed to provide detailed dynamics of the copolymer diffusion process. "We thus can analyze the data to determine the diffusion coefficients, as well as other precise dynamic information about the surface segregation process, including the volume and speed."

Dadmun knows from previous experiments that the polymer chain is actually collapsing. "The polymer is changing its conformation away from conventional behavior in the homopolymer because of the repulsive interaction between the polymers. The changes cascade from repulsive interaction, to conformation, to dynamics, which ultimately changes the properties. The cascading effect enables us to correlate the structure and thermodynamics of the copolymer to its dynamics," he says.

"If you think of a polymer chain with long arms, those long arms make it very difficult to move. But because of this odd repulsive interaction they might actually be retracted, and, therefore, may be moving faster. We do not yet have clear evidence of this phenomenon, but this is one of the things our team is trying to determine."

Dadmun is cautious about predicting the future for these polymers. "All of the applications may not be commercially viable, because ultimately it may take too long for the polymers to get to the surface. If the process does prove viable, however, the result will be a wide range of applications. That, after all, is why we do the research."

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