2.4. Differential scanning calorimetry (DSC)
The DSC experiments were performed with a Q1000 DSC with a
refrigerated cooling system and an autosampler system . The DSC was calibrated with indium , azobenzene and undecane prior to analysis.
Nitrogen was used to purge the system. Samples were sealed in
hermetic pans and an empty pan was used as a reference. Three types
of experiments were performed: non-isothermal crystallization experiments,
isothermal crystallization experiments and stop-and-return experiments.
For the non-isothermal experiments, the following time–
temperature program was applied: holding at 80 °C (90 °C for pure
MP) for 10 min to ensure complete melting and to erase the crystal
memory, cooling at 10 °C/min to −40 °C, holding at −40 °C for
10 min and heating at 10 °C/min to 80 °C (90 °C for pure MP). For the
isothermal crystallization experiments, the procedure was adapted in
that the sample was only cooled to the crystallization temperature
(15, 18, 20, 25 or 28 °C) and was then held there for 120 min. For
the stop-and-return experiments, the procedure was basically identical
to the isothermal procedure except that the sample was heated
at 20 °C/min to 80 °C after a certain isothermal period . The isothermal
period prior to melting was varied in different experiments and
was maximally 90 min. A heating rate of 20 °C/min is fast enough
to prevent the occurrence of polymorphic transitions as much as
possible but slow enough to prevent thermal lag. The DSC profiles were analyzed with the Universal Analysis
software version 4.7A .