Structural Assembly in Glass Forming Melts. Randall E. Youngman, John Kieffer, and Jay D. Bass, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801

By combining the results of Brillouin light scattering experiments and molecular dynamic simulations, we were able to evidence distinctive structural rearrangements which occur upon the cooling of glass forming melts. The mechanisms and structural entities involved in these rearrangements correlate with the glass forming ability of the substance.

Brillouin scattering is used to determine the complex mechanical modulus of molecular scale structures, at GHz frequencies. The real part of this modulus reflects the structural integrity and the degree of networking, while the imaginary component provides a measure for the mechanical losses associated with aperiodic motions of structural moieties. With this method of investigation, elastic properties are probed on a length scale of about 100 nm. The technique is thus sensitive to the extended-range connectivity in the structure. For processes to be involved in the dissipation of energy at the GHz probing frequencies, they must achieve a significant degree of completion within this period. Brillouin scattering is therefore able to reveal elementary diffusive displacements and the structural rearrangements within unit building blocks. While the scattering experiment can discern the rates and activation energies for these processes, the modes of the underlying atomic displacements, which are obviously controlled by the topology of the structure, are more difficult to access. It is at this level that molecular dynamic simulations are used to identify possible mechanisms of structural rearrangement.

Complex moduli for the melts of a series of network oxides, including silica, germania, boron oxide and tellurium oxide, either pure or in mixture with alkali oxides, will be presented. The distinctiveness of the structural transition is most obvious in fragile glass formers, where rearrangements can be attributed to a drastic modification of the network building block. In strong glass formers, structural reorganization involves the cooperative motion of several building blocks. The temperature dependence of all mechanical data can be described by a multi-domain model involving a minimum of two structural states. Accordingly, the rearrangements are locally abrupt, but affect the entire system progressively, over a range of temperatures.