Relation of glass transition temperature to the hydrogen-bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers. Part 2. Effects of poly(ethylene glycol) chain length

A phenomenological approach has been developed to evaluate a variety of the characteristics of hydrogen bonding in poly(N-vinyl pyrrolidone) (PVP) miscible blends with short chain poly(ethylene glycol) (PEG), ranging in molecular weight from 200 to 1000 g mol−1. The approach is based on the analysis of experimentally measured composition dependence of the negative deviations in glass transition temperature, Tg, from weight-average values predicted by the Fox equation. The PVP–PEG miscibility is a result of hydrogen bonding between carbonyl groups in PVP repeat units and both terminal hydroxyls of PEG short chains. Because PEG macromolecules bear reactive hydroxyl groups only at both chain ends, the PVP–PEG complex has a network supramolecular structure. Influence of blend composition and PEG molecular weight on the mechanism of hydrogen bonding, the structure and the stoichiometry of the PVP–PEG complex have been studied. The significance of this work is two-fold. First, the validity of the approach suggested for determining the stoichiometry, network density and the thermodynamics of hydrogen-bonded complex formation in PVP–PEG systems has been confirmed by the results of independent measurements. Second, the nonequimolar stoichiometry of the hydrogen-bonded complex has been explained taking into account the counterbalancing contributions of the entropic loss caused by PEG chain immobilization by hydrogen bonding to PVP repeat units through both PEG chain-end hydroxyls, and the entropic gain due to the increase of the mobility of PVP chain segments between neighbouring hydrogen-bonded PEG crosslinks in the PVP–PEG network.