Stronger materials for aircraft, farm equipment, medical devices and consumer products may result from University of Delaware research showing how hot polymer chains coil back and snap forward, snake-like, leaving a telltale, rippling "signature" wherever plastics are joined together.
The UD study--published in the July 28, 1998, issue of the journal, Macromolecules--confirms a popular but controversial scientific theory of polymer behavior, known as the "reptation model," and sets the stage for stronger composite materials and welded plastic seams, says Chemical Engineering Prof. Richard P. Wool.
Describing the serpentine motion of a single polymer chain trapped within a tangle or "tube" of neighboring polymer molecules, the reptation theory was proposed in 1971 by Nobel Prize winner Pierre-Gilles de Gennes and further developed by Sir Sam Edwards and M. Doi, explains UD doctoral candidate Keith A. Welp, lead author of the Macromolecules paper. Over the past 27 years, however, researchers have challenged the reptation model, arguing that polymer chains act in tandem with nearby molecules and, therefore, exhibit far more complex behaviors in hot environments.
"Critics of the reptation theory have been quick to point out that some snakes are sidewinders!" Wool says. "But our experiments clearly confirm de Gennes' vision of polymer chains slithering forward and back."
And, a mathematical formula expressing that behavior makes it possible to predict how quickly different polymer molecules completely mesh at weld interfaces. Such information should support many manufacturing uses--from strength testing of welds to efficient adhesives development-or perhaps even biological processes involving chains of DNA (deoxyribonucleic acid), Wool says.
"We can calculate the time it takes a molecule to diffuse over a given distance, relative to its size, or radius," he reports. "In this way, chemical company employees could more precisely determine, for instance, the size of molecules required to efficiently complete a particular welding application."
Ultimately, Wool says, manufacturing might be simplified even further, by incorporating the reptation formula into a software program, to support computer-based control on the factory floor. Meanwhile, Welp says, the reptation model suggests more efficient production processes. "If you don't assume the right dynamics theory during plastics manufacturing," he says, "then you're playing with fire--or snakes, as the case may be!"
Snakes On The Glass
To learn exactly how polymer chains move when plastic components are welded together, the UD research team--in collaboration with Jimmy Mays, professor, and S. Pispas, postdoctoral researcher, both of the University of Alabama at Birmingham--first "labeled" two types of polystyrene molecules. In the middle section of some chains, they replaced 50 percent of the molecules' hydrogen atoms with deuterium. Mirror-image molecules were then created by replacing hydrogen with deuterium on both ends, without disturbing the middle of the chain. "It would be like having one necklace with diamonds in the center and pearls on both ends," Wool explains, "and another necklace with pearls in the middle and diamonds at either end."
Labeling polystyrene with deuterium helped the researchers track the chains' movement during welding. Just as the end of a rope thrashes more freely than its center, Welp says, the exposed "head" and "tail" of a polymer chain exhibit greater flexibility. In experiments, therefore, more deuterium wound up on one side of a welded seam. That's because the confined, deuterium-loaded midsections of half the chains stayed put on one side, while the flapping, deuterium tips of the mirror-image molecules jumped across the interface. An experimental interface was created by spreading the hydrogen-deuterium-hydrogen (HDH) chains onto a platform of silicon, then covering clean, glass slides with the deuterium-hydrogen-deuterium (DHD) chains. Sliding the DHD samples carefully into water helped dislodge them from the glass, so that Welp could ease them directly on top of the HDH layer. Finally, Welp and Wool evaporated the water and heated their samples before investigating the molecular characteristics of the resulting weld.
With Sushil K. Satija of the Center for Neutron Research at the National Institute of Standards and Technology, welded samples were subjected to two types of tests: specular neutron reflectivity measurements and dynamic secondary ion mass spectroscopy. Bouncing neutrons off the interface to measure their reflection at different angles revealed the location of deuterium. And, drilling the surface with ions and analyzing molecular fragments at different depths helped the researchers trace the "smooth, rippling pattern" of deuterium atoms left by the snake-like motion of polymer chains.
Wool says he expects the UD findings to create "quite a stir" among reptation theory foes. But the experimental evidence provides clear support for de Gennes' model, and for similar theories inspired by it, he says. "The challenge now is to solve some of the fundamental viscosity problems with Sir Edwards' rheology theory, and to develop clever applications for the design of new adhesives, for better welds and for more accurate strength tests."
This research received support from the National Science Foundation, as well as the Delaware Space Grant College Fellowship Program, administered by the National Aeronautics and Space Administration. Some work was completed at the Center for Microanalysis of Materials at the University of Illinois, which is supported by the U.S. Department of Energy.
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