Paracetamol is known to have three polymorphs :
- Stable form I (monoclinic)
- Metastable form II (orthorhombic) and
- Unstable form III
Form I has a lower heat capacity than form II. Below -120°C, the form II modification becomes more stable than form I . In addition, form II contains well-defined slip planes in its crystal structure which may facilitate direct compression for tablet production .
Thermal Behavior of Paracetamol
In the following example, 2.09 mg of paracetamol was heated twice from room temperature to 200°C in aluminum crucibles and a nitrogen atmosphere. The cooling step in between was also performed at 10 K/min. In the 1st heating step, an endothermal effect is visible with an extrapolated onset temperature of 169°C. This correlates well to the melting point of form I (6). During the subsequent controlled cooling step (not shown here), no crystallization takes place. This means that the paracetamol is still amorphous at the beginning of the 2nd heating step. In the 2nd heating, an ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermal effect (with a peak temperature of 82°C) appears first which is related to a cold or post crystallization process. But with increasing heating, it turns out that there is not the same modification present as before because the corresponding melting effect is shifted to lower values. The extrapolated onset temperature of 157°C is characteristic for form II . Thus, the metastable form II was formed during post crystallization.
More information about application possibilities of thermal analysis in the pharmaceutical field can be found here. Literature:  https://en.wikipedia.org/wiki/Paracetamol  Bashpa, K. Bijudas, Anjali M Tom, P.K. Archana, K.P. Murshida, K. Noufala Banu, K.R. Amritha, K. Vimisha, PolymorphismPolymorphism is the ability of a solid material to form different crystalline structures (synonyms: forms, modifications).Polymorphism of paracetamol: A comparative study on commercial paracetamol samples, International Journal of Chemical Studies, Vol. 1, Issue 6 (2014), p 25 -29  Sacchetti, Thermodynamic Analysis of DSC Data for Acetaminophen Polymorphs, J. Therm. Anal. and Cal., 63; p 345 – 350  Kachrimanis, D. Braun, U. Griesser, Quantitative analysis of paracetamol polymorphs in powder mixtures by FT-Raman spectroscopy and PLS Regression, J Pharm Biomed Anal. 2007; 43; p 407 –412  Polymorphism in the Pharmaceutical Industry, edited by R. Hilfiker and M. van Raumer, pp 435/436, Wiley-VCH, 2019