Stoichiometry of poly(N-vinylpyrrolidone)-poly(ethylene glycol) complex

Solutions of high-molecular-mass poly(N-vinylpyrrolidone) in liquid poly(et hylene glycol) with M = 400 were studied by FT-IR spectroscopy, DSC, and op tical microinterference. It was shown that hydrogen bonding between the ter minal hydroxyl groups of poly(ethylene glycol) and the carbonyl groups of m onomer units of poly(N-vinylpyrrolidone) leads to formation of a stoichiome tric complex. Over a wide interval of the reagent concentrations, no more t han 27 poly(ethylene glycol) macromolecules bearing 54 hydroxyl groups were involved in complex formation with 100 units of poly(N-vinylpyrrolidone). When complex formation was completed, the total amount of interacting poly( N-vinylpyrrolidone) units did not exceed 56%. It is suggested that the stro ngest, hydrogen bonds are formed by those poly(ethylene glycol) macromolecu les which are linked to poly(N-vinylpyrrolidone) through both terminal hydr oxyl groups, that is, are involved in formation of a network. It was establ ished that, in a dilute solution of this polymer in poly(ethylene glycol), 44% of poly(N-vinylpyrrolidone) units are not involved in complex formation . This finding can be explained by the fact that, for the carboxyl groups o f these units, the heat of hydrogen bonding cannot compensate the negative change in the noncombinatorial entropy of mixing due to the ordered orienta tion of poly(ethylene glycol) chains in the resulting network.