Relation of glass transition temperature to the hydrogen-bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers. Part 1. Effects of hydroxyl group number in plasticizer molecule

The well-known Fox equation has been modified to express the glass transition temperature, Tg, as an explicit function of the number of polymer–plasticizer hydrogen bonds in miscible poly(N-vinyl pyrrolidone) (PVP) blends with ethyl alcohol, water, short-chain poly(ethylene glycol) (PEG), and glycerol. The plasticization effect has been found to be dependent on the fraction of hydroxyl groups in the blend rather than on the plasticizer weight fractions. Negative deviations of the blend Tg from the relationship predicted with the original form of the Fox equation were shown to be in direct proportion to the number of hydroxyl groups in the plasticizer molecule. The following quantities can be evaluated based on the Tg–composition profiles: binding degree, fraction of plasticizer hydroxyl groups forming hydrogen bonds to PVP repeat units, the fraction of plasticizer molecules crosslinking the polymer units by hydrogen bonding through two or more hydroxyl groups in their molecule. The dynamics of PVP–PEG hydrogen bonding over the entire compositional range has been evaluated in terms of hydrogen-bonded network density. The stoichiometric composition of the PVP–PEG hydrogen-bonded complex, determined from the Tg–composition relationship, corresponds to the data obtained with independent methods.