Molecular mobility of the paracetamol amorphous form

The purpose of this paper is to study the molecular mobility of paracetamol molecules in their amorphous state below the glass transition temperature (T-g) in order to evaluate the thermodynamic driving force which allows the amorphous form to recrystallize under different polymorphic modifications. Samples were aged at temperatures of -15, 0, 6, and 12 degrees C for perio ds of time from 1 h to a maximum of 360 h, The extent of physical aging was measured by a DSC study of enthalpy recovery in the glass transition regio n. The onset temperature of glass transition was also determined (T-g). Ent halpy recovery (Delta H) and change in heat capacity (Delta Cp) were used t o calculate the mean molecular relaxation time constant (tau) using the emp irical Kohlausch-Williams-Watts (KWW) equation. Enthalpy recovery and onset glass transition temperature increased gradually with aging and aging temp eratures, Structural equilibrium was reached experimentally only at an agin g temperature of 12 degrees C (T-g-10 degrees C), according to the Delta H- infinity results. The experimental model used is appropriate only at lower aging temperatures, while at higher ones the complexity of the system incre ases and molecular polymorphic arrangement could be involved. When structur al equilibrium is experimentally reached, molecules can be arranged in thei r Lowest energy state, and the polymorphic form I formation is the one pref erred.