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) mis cible blends with short chain poly(ethylene glycol) (PEG), ranging in molec ular weight from 200 to 1000 g mol(-1). The approach is based on the analys is of experimentally measured composition dependence of the negative deviat ions in glass transition temperature, T-g, from weight-average values predi cted by the Fox equation. The PVP-PEG miscibility is a result of hydrogen b onding between carbonyl groups in PVP repeat units and both terminal hydrox yls of PEG short chains. Because PEG macromolecules bear reactive hydroxyl groups only at both chain ends, the PVP-PEG complex has a network supramole cular 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 st oichiometry, network density and the thermodynamics of hydrogen-bonded comp lex formation in PVP-PEG systems has been confirmed by the results of indep endent 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 h ydrogen bonding to PVP repeat units through both PEG chain-end hydroxyls, a nd the entropic gain due to the increase of the mobility of PVP chain segme nts between neighbouring hydrogen-bonded PEG crosslinks in the PVP-PEG netw ork. (C) 2000 Elsevier Science Ltd. All rights reserved.