43743-GB10
Dynamic Light Scattering in Network-Forming Glasses
A dynamic light scattering technique known as photon correlation spectroscopy (PCS) has been used to investigate the dynamics of supercooled liquids at temperatures above their glass transition. PCS provides a direct measure of the liquid's dynamic structure factor and for supercooled liquids near Tg exhibits a viscoelastic relaxation that is a non-exponential (parameterized by smallness of b) and whose average relaxation time displays a non-Arrhenius temperature dependence (parameterized by largeness of the fragility, m).
In this project, we have examined a series of phosphate glass forming liquids in which the chemical structure is systematically altered through the addition of alkali oxides. To date, we have succeeded in studying: neat P2O5, which forms a continuous random network of covalent bonds possessing three linkages per each PO4 unit; the two alkali metaphosphate liquids LiPO3 and NaPO3, for which the added alkali result in a reduction of linkages to only two per phophate unit; and a series of compositions ranging from the metaphosphate to 40 mol% Na2O.
We find significant differences in the liquid dynamics arising from the destruction of the continuous random network. The fragility of P2O5 is only m = 20, while alkali addition produces greater non-Arrhenius behavior with m Å 90 for the two metaphosphates. Most significant among our finding this year was how intermediate compositions exhibit variations in fragility which exactly match those seen for covalently-bonded chalcogenide glasses when described in terms of the mean coodination number. Differences in the non-exponentiality of the relaxation are also found. For P2O5, b decreases from about 0.85 seen at high temperatures to around 0.55 near Tg. In comparison, NaPO3 (and to a lesser degree, LiPO3) display b Å 0.3 at high temperatures that increases to near 0.55 at Tg. For the intermediate compostions, b shows only a weak temperature dependence comparable to that seen in the metaphosphates.
Sample preparation continued to be troublesome this year, and at the time of this report, we have not yet been able to obtain viable samples at concentrations below 40 mol% Na2O. We continue to resolve this issue, and remain confident that we can complete the remaining compositions of (Na2O)x(P2O5)1-x, for x = 0.1, 0.2, and 0.3 within the year.
To date, three graduate students and one undergraduate have benefited from the financial support of the grant: Ms. Jessica Changstrom and Mr. Mark Durante both completed M.S. theses over the summer. Mark performed measurement on aqueous maltose as a companion study to that done on glucose. Together, these two studies form the preliminary work for a recently submitted NIH proposal aimed at using dynamic light scattering to study the cryopreservation of proteins in sugar solutions. Jessica is now pursuing graduate work at Kansas State University and Mark is teaching in the Omaha public school system. Mr. Roberto Fabian is currently working on obtaining the last ultraphosphate compositions, and plans to complete his M.S. thesis in May 2009. Mr. Brandon Rodenburg, who obtained his B.S. in May 2007 is currently pursuing graduate work in optics at the University of Rochester.
All of the students presented posters at the recent Glass and Optical Materials Division Meeting of the American Ceramic Society (Jessica won 3rd prize in 2007 and Mark won 2nd prize in 2008 in poster competition among graduate students) and all have made oral presentations at the regional meeting of the Nebraska Academy of Sciences.
