Characteristics of hydrogen bond formation between sugar and polymer in freeze-dried mixtures under different rehumidification conditions and its impact on the glass transition temperature.

The characteristics of hydrogen bond formation between trehalose and polyvinylpyrrolidone (PVP) in amorphous mixtures at different hydration states were quantitatively investigated. Amorphous trehalose-PVP mixtures were prepared by freeze-drying and equilibrated at different relative humidities (RH). Infrared (IR) spectra of the trehalose-PVP mixtures were obtained by Fourier transform IR spectroscopy,(FTIR) and the IR band corresponding to C=O groups of PVP was deconvolved into the component bands responsible for C=O groups that were free and restricted by hydrogen bonds, to estimate the degree of the trehalose-PVP interactions. The FTIR analysis indicated that approximately 80% of the C=O groups of PVP formed hydrogen bonds with trehalose in the presence of more than 3 g of trehalose per gramme of PVP, independent of the RH. IR analysis of the O--H stretching vibration of the sugar demonstrated that the presence of PVP lead to an increase in the free hydroxyl groups of trehalose that did not form hydrogen bonds at RH 0%. On the other hand, the water sorption behavior of the trehalose-PVP mixtures suggested that rehumidification diminished the effect of PVP on increasing the free OH groups. Thus a peculiar relationship may exist between Tg, RH and the composition of the mixture: The presence of PVP increased Tg at RHs 0 and above 23% but decreased Tg at 11%.