Thermal analysis of the dehydration process of cross-linked polyvinylpyrrolidone and its mixtures with naproxen

Ways of finding kinetic parameters useful for pharmaceutical formulators from thermal analytical methods (TGA, DSC) under both isothermal and non-isothermal conditions are suggested. We considered as an example those followed to determine the first-order kinetics of dehydration of cross-linked polyvinylpyrrolidone (PVP XL). PVP XL samples differing in particle size (granulometric fractions in the 250 μm to 63 μm range) and/or equilibrium moisture content (12 to 32 gH2o/100 g) show activation energy values in the 43 to 54 kJ mol−1 (non-isothermal TGA) and 48 to 61 kJ mol−1 (non-isothermal DSC) ranges. The weight fraction of water present may influence the activation energy of the dehydration process whilst the particle size of PVP XL seems not to affect this parameter. The low interaction energy between PVP XL and water suggests that the interaction mainly occurs within the porous agglomerates of the polymer particles, at the intraparticulate surface level (surface-type interaction). The influence of an admixed drug (naproxen) on the thermodynamic and kinetic parameters of PVP XL dehydration suggests that analogous surface phenomena are probably involved in the solid-state interaction between the amorphous polymer and the crystalline drug.