Structure of stable H-bonded poly(N-vinylpyrrolidone) - Water complexes

The system poly(N-vinylpyrrolidone)-water was studied by physicochemical me thods (FTIR and electronic absorption spectroscopy, X-ray diffraction, and DSC) and using theoretical approaches (quantum chemistry). It was establish ed that poly(N-vinylpyrrolidone) and water are involved in complex formatio n, and the resulting stable H-bonded complexes are distinguished by stoichi ometry and the energy of complex formation. Under standard conditions, the maximum extent of hydration of solid poly(N-vinylpyrrolidone) makes up two water molecules per polymer unit. This corresponds to a cluster composed of four water molecules localized between two adjacent polymer units. It was demonstrated that the interaction of water with a solid polymer at temperat ures above T-g or in solution leads to a change in the macromolecular confo rmation (drawing together of pyrrolidone rings and extension of the backbon e). In this case, molecules of water are linked to a polymer matrix rather strongly (in the polymer-water system, charge transfer takes place). At T< T-g, when macromolecular conformation cannot alter, a bond formed between a polymer matrix and water sorbed by the polymer, is significantly weaker. H owever, drawing together of polymer units due to pressing or annealing of a glassy polymer also gives rise to an increase in the degree of water bindi ng. The data obtained provide an opportunity to gain insight into some obsc ure experimental facts such as the effect of prehistory of poly(N-vinylpyrr olidone) on its extent of equilibrium hydration.