I'm interested to know if a good glass former that has a given fragility, can be tuned into a fragile one by changing cooling speed of glass formation. I know that there are some data set concerning Tf/Tg, Tf=fictive temperature and Tg=glass transition temperature.
I would say no...because fragility is a measure of an equilibrated liquid. Maybe fragility of the glass from non-equilibrium veiscosity ?
Matthieu, thanks a lot for the answer, you're right. I was thinking in recent works of John Mauro; they talk about non-equilibrium properties. But maybe we can reverse the question. Suppose that you produce glasses with the same composition but different thermal history. Then you start heating waiting to reach equilibrium and look at viscosity, do you see the same fragility behavior even for different thermal history?
Look for example at this paper (below) with abstract attached
Best whises,
Gerardo
JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 17
1 MAY 2004
Fictive temperature, cooling rate, and viscosity of glasses
Yuanzheng Yue)
Section of Chemistry, Department of Life Sciences, Aalborg University, Sohngaardsholmsvej 57,
DK-9000 Aalborg, Denmark
Renate von der Ohe
Department of Production, Aalborg University, Fibigerstraede 16, DK-9220 Aalborg, Denmark
Søren Lund Jensen
Rockwool International A/S, DK-2640 Hedehusene, Denmark
͑Received 28 May 2003; accepted 3 February 2004͒
The physical correlation between the fictive temperature dependence of the cooling rate of the melts
and the temperature dependence of the equilibrium viscosity has been found by doing differential
scanning calorimetric and viscometric measurements on a silicate melt, and by performing finite
element simulations of the fiber drawing from that melt. This correlation is governed by a
correlation factor K c ͑in Pa K͒ which is constant and universal for silicate glasses. The factor K c is
obtained in the cooling rate range from 10Ϫ2 to 106 K/s and is in good agreement with that
theoretically predicted. The physical feature of the correlation is discussed in the paper. When the
fictive temperature equals the actual temperature, a linear relation exists between the cooling rate
and the Maxwell relaxation rate, the slope of which depends on the fragility of the glass melts. The
Avramov equation is extended to describe the cooling rate dependence of the fictive temperature.
The cooling rate equation contains only one adjusting parameter, i.e., the fragility parameter ␣.
© 2004 American Institute of Physics. ͓DOI: 10.1063/1.1689951͔
Any change in fragility would be minimal. Fragility is a property of the melt, i.e. the viscosity/temperature curve well above Tg. The activation energy for viscous flow is only slightly affected by thermal history and then only in the transformation range. The viscosity in the melt range does not reflect prior history in the transformation range.
Thanks a lot James. It is very reasonable although we were expecting more differences for fragile glass formers in the transformation range precisely due to stronger changes of activation energies.
Fragility is a property of the equilibrated glass former. Conventionally it is defined at the glass transition, whatever this means. The so-called "dynamic" Tg is the temperature with given relaxation time (normally between 1 and 1000 sec) or viscosity (normally 10^13 pois).
Given its definition, the fragility is completely independent of the thermal history below Tg, that is, the thermodynamic state of the glass.
For details see the following review article:
Article Dynamics and thermodynamics of polymer glasses
Fictive temperature (Tf) is a conceptual temperature supposed to fulfil the assumption that the glass structure approximates the frozen structure of the liquid if overcooled to that temperature. For a discussion on how Tf relates with the glass transition temperature (Tg), you are kindly invited to see: C.A. Queiroz, J. Šesták, Physics and Chemistry of Glasses - European Journal of Glass Science and Technology Part B, 51(3) 2010, 165-172.
Article Aspects of the non-crystalline state
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