Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 65

not accompanied by the latent heat, i.e., Δ H = 0 in Eq. 3.4. Then, the heat flow signal
is produced by the first term in Eq. 3.4. According to Eq. 3.3, the glass transition is
accompanied by a change in the heat capacity. At T g , its value changes from C P of
the glass to C P of the liquid, if the transition is measured on heating, or other way
around when it is measured on cooling. As a result, the glass transition manifests
itself as a step change between two nearly linear segments of the heat flow.
As follows from the above discussion, the solid state of matter can exist either
as the crystal or as the glass phase. The issue, however, can be further complicated
by the existence of more than one crystal phase for the same solid compound. This
phenomenon known as polymorphism [ 1 ] is widely encountered in inorganic [ 6 ,
7 ] and organic [ 8 ] compounds as well as in elements, for which it is referred to as
allotropy. The polymorphic solid-solid transitions can be of first and second order.
They are typically easy to measure by DSC.
Different liquid phases can be encountered in a single-component liquid. The
examples of the liquid-liquid transitions in isotropic liquids are quite rare [ 9 - 12 ].
Much more common are the transitions in liquid crystals, whose liquid state can
exist in disordered (isotropic) as well as in ordered (smectic, nematic, cholesteric)
phases [ 4 ]. The liquid crystalline phases are also called mesophases to emphasize
their intermediate character between the solid and liquid phases. For liquids involv-
ing more than one component (i.e., mixtures or solutions), a common liquid-liquid
phase transition is mixing and demixing (phase separation). The transition can be
caused by heating or cooling of a solution and results in its separation in the solvent-
rich and solute-rich phases. A very special case of a phase transition in a solution
is gelation [ 4 , 13 ]. It results in conversion of a liquid solution into a gel, which is a
network of cross-linked solute molecules that entrap a solvent. A gel is a soft solid
or a liquid that has lost its ability to flow. Most of the aforementioned transitions
in liquids are of first order and normally appear in DSC as well-defined peaks, al-
though some transitions [ 14 ] in liquid crystals can be of second order.
The following sections of this chapter provide a discussion about the kinetics of
most of the aforementioned transitions.

3.2 Vaporization and Sublimation

All existing things soon change, and they will either be reduced
to vapor, if indeed all substance is one, or they will be dispersed
Marcus Aurelius, Meditations

3.2.1 Background

Vaporization and sublimation are phase transitions in which the respective liquid
and solid compounds transform to the gas phase. Both processes are promoted by
heating that intensifies molecular motion and thus initiates breakage of the intermo-
lecular (cohesive) bonds that hold a compound in the condensed state. Depending