Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 85

Fig. 3.20 Relaxation of the
nonequilibrium glassy struc-
ture can occur under rising
temperature conditions as the
glass transition or at constant
temperature as aging
The kinetics of physical aging is of great practical importance because it deter-
mines for how long a glassy material can remain useful at temperature of opera-
tion. The stability of a glass increases with increasing the difference between the
temperatures of the glass transition and operation. For example, regular household
glassware made of silicate glass is used about 500 ﾰC below its glass transition tem-
perature. This makes it stable for all designed practical purposes.
However, even in the case of silicate glasses used far below T g , the signs of ag-
ing are detectable on the scale of decades [ 60 - 62 ]. An intriguing example [ 60 , 62 ]
includes data on aging of glass thermometers used by the renowned physicist James
Prescott Joule. Joule regularly calibrated his thermometers and noticed that what he
called the zero point was increasing systematically, shifting totally by 0.91 F over
23 years. The effect is explicable [ 60 , 62 ] by the glass shrinkage due to physical
aging.
The kinetics of physical aging is usually followed by measuring either volume
of enthalpy of a glassy sample. The heat flow released during physical aging is too
small to follow the process by regular DSC instruments in real time. The measure-
ments are thus conducted discretely, i.e., in steps. The idea is that the enthalpy lost
on aging can be recovered when heating an aged glassy sample through the glass
transition temperature. As seen from Fig. 3.21 , the sample held at aging temperature
T a will continue to lose its enthalpy until the glass reaches equilibrium, i.e., turns
into supercooled liquid. As discussed earlier (Fig. 3.9 ), reheating of unaged glass
results in the enthalpy overshoot. When glass ages, it assumes a denser and more
ordered structure that results in a decrease of the molecular mobility and an increase
of the relaxation time. For this reason, when aged glass crosses the equilibrium liq-
uid line, it takes longer to restore the liquid structure than for unaged glass.
Thus, the more glass aged (points B and C in Fig. 3.21 ), the more it overshoots
the liquid line. The inflection point on the enthalpy recovery line corresponds to the
temperature, T p , of the DSC peak that appears at the end of the glass transition step