DIELECTRIC ANALYSIS IN THE CHARACTERIZATION OF AMORPHOUS PHARMACEUTICAL SOLIDS .1. MOLECULAR MOBILITY IN POLY(VINYLPYRROLIDONE) WATER-SYSTEMS IN THE GLASSY STATE

The effect of water on the relaxation behavior below the glass transition temperature (beta-retaxation) 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 (E(a)) of 36.5 kJ mol(-1). Water was found to significantly lower T-beta, increase the dielectric loss, and increase E(a). 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 approximate to 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 E(a) 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 an PVP has a distinct effect on the activation energy.