Cincinnati -- A team of researchers in the University of Cincinnati College of Engineering has experimentally pinpointed conditions for a key transition in glass formation: the switch from a "floppy" polymer state to a rigid network.
Punit Boolchand, a professor in the department of electrical and computer engineering at UC, reported results from three different chalcogenide glasses March 18 as an invited speaker at the annual meeting of the American Physical Society (APS) in Los Angeles.
"There is a great deal of interest in this, because it touches many areas of science. The reason we're interested in this, is because in nature, the best glass compositions occur right at this transition."
Boolchand's group reported in Physical Review Letters (Vol. 78, Number 23) last summer that the onset of rigidity or stiffness threshold closely matched the theoretical predictions of Jim Phillips and Michael Thorpe. Phillips and Thorpe's calculations indicated the transition would occur when each atom in the network had an average of 2.4 atoms as "neighbors." This value represents the mean coordination number of the network. However, Boolchand's experiments showed the actual mean coordination value for germanium-selenium glasses was 2.46.
Boolchand explained what happens on a structural level this way. "If you have a chain, a chain is floppy. If you cross-link it, at some point, the whole network gets stiff. Just at the time it gets stiff...the first time...when you add one more cross- link, that is the onset of rigidity."
During the Los Angeles APS meeting, Boolchand discussed results from the germanium-selenium system and two others: germanium-sulfur-iodine and silicon-selenium glasses.
"In this work, we have observed for the first time very clearly the stiffness transition, and we observed it using two experimental techniques," said Boolchand. The first technique is Raman scattering. The other one is called temperature modulated scanning calorimetry.
Interestingly, according to Boolchand, the point of onset changes for each glass system and differs measurably from what the Phillips-Thorpe theory predicts.
"In a sense, I love it, because it might be telling us something new about what's going on," said Boolchand. "The current theory assumes a random network model of these glasses. These results should push theorists to rethink the variables."
Boolchand's work is supported by two grants from the National Science Foundation.