Dielectric analysis in the characterization of amorphous pharmaceutical solids. 1. Molecular mobility in poly(vinylpyrrolidone)-water systems in the glassy state

Abstract

The effect of water on the relaxation behavior below the glass transition temperature (beta-relaxation) of an amorphous powder, poly(vinylpyrrolidone) (PVP, MW 30,000), was studied by subjecting the sample to dielectric analysis in the frequency range from 20 Hz to 20 kHz. The material stored at 0% relative humidity (RH) (containing 0.05% w/w H2O) exhibited a frequency dependent second-order beta-relaxation (T beta = -56 degrees C at 500 Hz). The peak frequency-temperature data could be fitted to the Arrhenius equation, yielding an activation energy (Ea) of 36.5 kJ mol-1. Water was found to significantly lower T beta, increase the dielectric loss, and increase Ea. The initial decrease in T beta was found to be quite significant, as little as 7% w/w H2O lowering T beta by 26 degrees C, followed by a more gradual decrease. PVP exposed to 69% RH (containing approximately 31% w/w H2O) exhibited T beta at -104 degrees C with an activation energy of 46.3 kJ mol-1. The observations that the beta relaxation was poorly visible when the water content was 0.05% w/w and that the change in Ea was from a low to a high value as the temperature is decreased suggest that thermally activated rotational diffusion of water molecules plays a major role in the beta-relaxation of PVP containing moderate to high water contents. The rate of increase in activation energy as a function of H2O/PVP mole ratio exhibited a minimum at unity, suggesting that water binding to one site on PVP has a distinct effect on the activation energy.