Dielectric Analysis in the Characterization of Amorphous Pharmaceutical Solids. 1. Molecular Mobility in Poly(vinylpyrro1idone)-Water Systems in the Glassy State

Abstract

The effect of water on the relaxation behavior below the glass transition temperature (β‐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 β‐relaxation (Tβ = –56 °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 lowerTβ, increase the dielectric loss, and increaseEa. The initial decrease inTβwas found to be quite significant, as little as 7% w/w H2O loweringTβby 26 °C, followed by a more gradual decrease. PVP exposed to 69% RH (containing ∽ 31% w/w H2O) exhibitedTβat – 104 °C with an activation energy of 46.3 kJ mol−1. The observations that the β relaxation was poorly visible when the water content was 0.05% w/w and that the change inEawas 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 β‐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.