Kinetics of Solid-Phase Transitions

Enthalpy relaxation time, determined by differential scanning calorimetry,608 and mechanical relaxation, determined by dynamic mechanical analysis,609 can also be used as measures of molecular mobility of amorphous pharmaceutical solids. 

The Hancock-Sharp equation610 is often used to describe the kinetics of polymorphic transitions  

The polymorphic transition of carbamazepine from form I to form III and that of benoxaprofen from form I to form II exhibited m values of 2.23 and 2.24, respectively, indicating possible mechanisms involving two-dimensional growth of nuclei.61 1578 When m is approximately equal to 2, the reaction conforms to the Avrami-Erofe evequation  

The data for carbamazeine and benoxaprofen plotted according to Eq. (3.3) are shown in Figs. 143 and 144, respectively and the rate constants k obtained from the slopes gave linear Arrhenius plots, indicating that it might be possible to predict the polymorphic transition rates at other temperatures (Fig. 145). 

Polymorphic transitions of bromovalerylurea from form I to form II and from form III to form I conformed to mechanisms involving one-dimensional diffusion and two-dimensional nuclei growth processes, respectively. Both transitions also exhibited good Arrhenius behavior in the temperature range studied, as shown in Fig. 146.579 Transitions of phenyl- 

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