An investigation into the low temperature thermal behaviour of Vitamin E preparation USP using differential scanning calorimetry and low frequency dielectric analysis

The thermal and dielectric responses of Vitamin E Preparation USP have been examined to further understand the melting and solidification of this mate rial. A TA Instruments 2920 Differential Scanning Calorimeter was used to e xamine the thermal response of the sample at a range of scanning speeds. Is othermal dielectric studies were performed using a Novocontrol Dielectric S pectrometer over a range of temperatures down to -70 degrees C and a freque ncy range of 10(6)-10(-2)Hz. The differential scanning calorimetry (DSC) studies showed an anomalous res ponse whereby at slow heating rates (2 degrees C min(-1)) a small exotherm followed immediately by an endotherm was observed. This response was consid erably diminished in magnitude at higher rates (5 degrees C min(-1)) and wa s not observed at the fastest heating rate of 10 degrees C min(-1). No ther mal events were seen on cooling the sample to -60 degrees C. It was suggest ed that the material formed a glass on cooling, with a predicted transition temperature of approximately -100 degrees C. Further studies using a liqui d nitrogen cooling system indicated that the system did indeed exhibit a gl ass transition, albeit at a higher temperature than predicted (ca -63 degre es C). Low frequency dielectric analysis showed a clear relaxation peak in the loss component, from which the relaxation time could be calculated usin g the Havriliak-Negami model. The relationship between the relaxation time and the temperature was studied and was found to follow the Vogel-Tammann-F ulcher (VTF) modification of the Arrhenius equation. It is therefore concluded that Vitamin E Preparation USP is a glass-forming material that exhibits kinetically-hindered recrystallisation and melting behaviour. The study has also indicated that DSC and low frequency dielectr ic analysis may be powerful complementary tools in the study of the low tem perature behaviour of pharmaceuticals.