From the above FT-IR spectral evidence, we regarded this exothermic peak at 125°C as a result of polymorphic phase transition from form III to form II, also consistent with Martino’s study.9) The en- dothermic peak at 159°C was attributed to the melting point of form II acetaminophen, which was confirmed by DSC
curves of other samples (Figs. 4c, d) and other studies.4,6,9,10) However, the enthalpy difference of the phase transition from form III to form II is quite small (8J/g) since the form II had strong type A hydrogen bonding but form III had strong type B hydrogen bonding.