Probing the Influence of Water on the Molecular Mobility of PVP/VA using Terahertz Spectroscopy

The presence of water significantly impacts the physical stability of amorphous solid dispersions (ASDs) by altering polymer molecular mobility. This study investigates the influence of low levels of absorbed water on the molecular dynamics and glass transition behavior of amorphous poly(vinylpyrrolidone-co-vinyl acetate) (PVP/VA). Melt-quenched PVP/VA discs were conditioned at controlled relative humidities (RH 8.5 to 75%) and characterized using variable-temperature terahertz time-domain spectroscopy (THz-TDS, 80 to 420 K) and differential scanning calorimetry (DSC). DSC confirmed a significant plasticizing effect, with the glass transition temperature (T_g) decreasing from 367 to 275 K with increasing hydration. THz-TDS absorption correlated linearly with water content at low temperatures and exhibited complex temperature dependence. Even the nominally dry sample displayed a high-temperature (ca. 350 K) absorption plateau, indicating tightly bound residual water, while the onset temperature of bulk water evaporation decreased systematically with increasing hydration, suggesting a greater proportion of loosely bound water. Empirical fitting (α(T) = AT + B + C/T) revealed characteristic temperatures correlating with sub-T_g dynamics (T₁₂) and T_g (T₂₃), both sensitive to water content. Furthermore, the underlying polymer mobility rate was extracted from the THz data by accounting for evaporation; this analysis confirmed that water enhances mobility, potentially reaching a saturation limit at the higher hydration levels studied. This work highlights the capability of THz-TDS to provide detailed molecular-level insights into water-polymer interactions and dynamics, complementing thermal analysis and offering valuable information for assessing ASD stability.