Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781118799499
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2). The difference between these values for the supercooled liquid and the glass,

Graph depicts the heat capacity of PVAc measured across the glass transition range by differential scanning calorimetry at the same rate of 1.2 °C/min first upon cooling (solid circle) and then upon heating (empty circle). Dashed lines: fits made from the heat capacities measured for the glass and supercooled liquid.

      From the equilibrium and actual configurational contributions, the variation of the configurational enthalpy ∆H conf and entropy ∆S conf, taken between two temperatures, are calculated with:

      where T1 = 360 K is in Figure 2 an arbitrarily selected reference temperature.

Graph depicts the difference between the configurational enthalpy of PVAc and a zero reference-value taken at 360 K. Actual value (solid circle) and equilibrium value (empty circle). Inset: magnification of Figure 4 showing extrapolated values of the glass and supercooled liquid of this differential configurational enthalpy intersecting at the point M, which defines the limiting fictive temperature TM = Tf′.

      Contrary to their equilibrium counterparts, which continue to decrease upon cooling, both the actual configurational enthalpy and configurational entropy level off in the amorphous state (Figure 4). Owing to the large width of the glass transition range, the heat capacity variations at the glass transition are much too smooth to be interpreted as reflecting the discontinuity of a second‐order phase transition. Such a discontinuity can nonetheless be identified at a temperature TM defined by the intersection of the extrapolated glass and supercooled liquid (Figure 4, inset). Both configurational enthalpy and entropy are thus continuous at that temperature, which separates the glass from the supercooled liquid. The same applies to other properties such as volume. Because entropy and volume are the first derivatives of the Gibbs free energy with respect to temperature and pressure, respectively, the following relations initially derived by Ehrenfest should hold when second‐order derivatives of the free energy vary discontinuously at this point M:

      (9a)equation

      (10)equation

Material Tg (K) ΔS0 (J/K/mol) PD ratio TK (K) T0 (K)
SiO2 1480 5.1 >103 1150 NA Arrhenius relaxation
CaAl2Si2O8 1109 36.2 1.5–22 815 805
Glucose 305 1.7 3.7 241 242
PVAc 301 NA No crystal 2.2 239

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