Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 73

Fig. 3.8 Enthalpy versus temperature diagram for the formation of two glass phases: glass 1 and
glass 2 . The glass 2 phase is formed at faster cooling rates than glass 1 and thus has a larger glass
transition temperature that is determined as intersection of the glass and liquid tangent lines
converted to glass subject to sufficiently fast rate of cooling. For slow crystalliz-
ing liquids such as the melts of some polymers, the glass can be formed on cooling
at tens of degrees per minute. Fast crystallizing liquids such as water may have
to be cooled at millions degrees per second to form the glass phase. Anyway, the
key reason of the glass formation is the limited rate of the molecular mobility that
slows down progressively as liquid is cooled. At certain point, the mobility be-
comes insufficient to maintain the equilibrium liquid structure so that a supercooled
liquid becomes a glass. The respective temperature is called the glass transition
temperature, T g . Since the glass is a nonequilibrium phase, its T g designates the
transition between the supercooled liquid and a specific glassy structure that de-
pends particularly on the cooling rate and generally on the overall thermal history.
Figure 3.8 demonstrates a change in the temperature dependencies of the enthalpy
for liquid and two glasses formed at different cooling rate. Obviously, the faster
liquid is cooled, the sooner it falls out of equilibrium and forms the glass phase.
Therefore, faster cooling produces the glass of a more nonequilibrium structure that
has larger glass transition temperature.
On reheating, the glass does not follow the same enthalpic trace as on cooling
(Fig. 3.9 ). The respective enthalpy values are lower because the glass is relaxing
continuously toward the supercooled liquid state. Another important feature of the
glass transition observed on heating is the “enthalpy overshoot.” Upon reaching
the equilibrium liquid line, the glass does not immediately convert to the liquid but
continues to follow the glass line for some time. The reason is that at this point the
molecular mobility of the glass is too slow to assume immediately the liquid struc-
ture. Therefore, it continues to maintain the glassy structure until the point when
temperature accelerates the molecular mobility to such extent that the glass can
quickly restore the liquid structure. For the glass formed at a certain cooling rate,
the use of faster heating rates results in increasing the magnitude of the enthalpy
overshoot. The heating and cooling traces are brought closer to each other when the
heating and cooling rates are equal.