below theaclear point); this is because the constraints imposed on the
molecules upon incorporation in the crystal lattice are less severe than for
the other polymorphs.
However, the molar free energy of the other crystals is lower than that
ofa: see Figure 15.20a. Only theb-form will be thermodynamically stable.
This results in thea-form generally being transformed into one of the more
stable polymorphs, even at constant temperature, while giving up heat. Only
exothermic transitionscan thus occur:a!b^0 ,a!b, andb^0 !b, as well as
b 2 !b 1 , etc. Endothermic transitions can only be from the crystal—
whatever the polymorph—to the liquid state, never to a ‘‘lower’’
polymorphic form.
In some cases, thebpolymorph is not observed, e.g., for PPO. In other
cases, some polymorphs can only form under special conditions. On the
other hand, several triglycerides show more than three forms. An example is
given in Figure 15.20b for SOS. Here also ag-form is seen and two different
b-forms. It depends on conditions, especially the cooling regime, what will
happen in practice.
Note In other systems, ag-form is observed having a higherG
value thana, at least at low temperature. A reversible transition
g>amay then occur. These phenomena need further study.FIGURE15.20 Gibbs free energyGversus temperatureTfor liquid and various
crystal forms of triglycerides. (a) General trend for the main polymorphs. (b)
Observed trends for SOS. (Results taken from K. Sato. Progr. Colloid Polym. Sci.
108 (1988) 58.)