Melting characteristic
The evolution of the melting characteristic with time of MKF
crystallized at 25 °C without and with HIU application is given in Fig.
3. For the crystallization without HIU, the sample did not exhibit any
peak at t = 5 min (Fig. 3a). At 15-30 min, the sample started to melt at
27.6-27.7 °C and showed a relatively broad melting peak at 28-29 °C
(Table 2). At 45 and 60 min, the sample began to melt at higher
temperature (∼31 °C) and the melting peak became much broader, the
location of which moved to a significantly higher peak temperature of
∼35 °C (p < 0.05) with a decrease in the melting
enthalpy (ΔH) (p < 0.05). This suggests that there was
a phase transition of MKF from a less stable form to a more stable one
after 30 min of crystallization. With HIU (70% amplitude), MKF had
already started to crystallize at 5 min with one melting peak at ∼28 °C
(Fig. 3b and Table 2), indicating that, at the early stages, MKF in the
experiments without and with HIU solidified into the same polymorphic
structure (p > 0.05). At 15 min, the melting peak
location moved to ∼33 °C with a decrease in ΔH, indicating the
polymorphic transformation of MKF at this crystallization time and
suggesting that the phase transition of MKF that crystallized with the
application of HIU occurred sooner than MKF that crystallized without
HIU. At 45 and 60 min, the melting peak moved to higher temperature (∼35
°C) with an increase in ΔH from that of 15 min.