Monotropic Polymorphism in Ester-Based Phase Change Materials from Fatty Acids and 1,4-Butanediol

This article reports on the synthesis,
thermal behavior, and morphology
of phase change materials based on diesters from 1,4-butanediol with
palmitic (BD16), stearic (BD18), or behenic (BD22) acid. The crystalline
contents and melting enthalpies of all substances exceed 90% and 180
J/g, respectively. The molecules display monotropic polymorphism.
The metastable β′ form is built from outstretched molecules,
whereas the denser, stable β form is composed of molecules with
a kinked conformation. BD18 and BD22 crystallize into the β′
form during cooling. Only BD18 transforms into the β form during
subsequent heating via a solid–solid transition. The resistance
of BD22 to convert into the β form is believed to originate
from the lack of mobility associated with longer aliphatic chains.
In contrast, the polymorphic conversion is thought to be very efficient
and to take place during cooling for the shorter BD16 molecules. The
hypothesis is put forward that the original BD16 β′ nuclei
are rapidly overgrown by the β phase by which crystal growth
is postponed to larger supercooling where the β phase can grow
from the melt. Consequently, BD16 only occurs in the β form.
The melt crystallization of BD18 and BD22 into the β′
form hardly requires any supercooling.