After DSC and TGA, students generally have hypotheses
regarding the nature of the thermal behavior observed, but to
make final conclusions in the interpretation of events, the final
experiment uses thermal microscopy. The use of a transmitting
light microscope furnished with a polarizing attachment allowed
students to determine particle characteristics such as shape and
size, and also gain insight into the crystallinity of the material.
Crystalline material will exhibit birefringence under polarized
light in which, for an anisotropic crystal mounted under mineral
oil, the polarized light will be separated into two different
polarized beams that travel through the crystal at unequal speeds
and thus are unequally refracted. The use of thermal microscopy
augments the DSC/TGA by allowing students to visually
observe phase transitions, decomposition (if discoloration
occurs, or gases are evolved), crystallization/recrystallization,
or dehydration/desolvation in which bubbles emanate from the
crystals submerged in mineral oil. Thermal microscopy was used
to visualize the thermal events observed by DSC/TGA and
relate the behavior of crystals to that of the bulk material.