Peculiarities of glass transition temperature relation to the composition of poly(N-vinyl pyrrolidone) blends with short chain poly(ethylene glycol)

Analysis of compositional Tg behaviour in poly(N-vinyl pyrrolidone) (PVP)–poly(ethylene glycol) (PEG) blends, performed in the terms of the equations offered by Gordon and Taylor, Couchman and Karasz, Kovacs and Braun–Kovacs provides an insight into the PVP–PEG complexation mechanism. Blending PVP with short chain PEG has been shown to be a two-stage process. At the first stage, the enhanced PVP–PEG interaction and excess free volume formation proceeds (the stage of formation of stoichiometric PVP–PEG hydrogen bonded complex) followed by nearly ideal mixing of the formed PVP–PEG stoichiometric complex with an excess amount of liquid PEG (the stage of weak favourable interaction in which no excess volume formation occurs). The first stage may be defined as PVP plasticization, whereas the second stage consists in gradual dissolving the plasticized PVP in excess PEG. The blend containing 36% of PEG-400 may be taken as an edge between the stages. This work complements the results of Tg analysis by including into consideration the effects of polymer chains mutual orientation, free volume and interaction within the PVP–PEG blends. To accomplish this aim, the Brekner–Schneider–Cantow (BSC) approach has been employed. In the ladder-like interpolymeric complexes, formed due to the interaction of complementary groups in repeating polymer chain units, the complexation is accompanied usually by the mutual orientation of polymer backbones and the decrease of blend free volume. In contrast, in PVP–PEG systems, the location of reactive hydroxyl groups at the PEG chain ends results in the increase of free volume and in formation of a carcass-like, flexible network. These particular features of PVP–PEG interaction have been demonstrated to be embedded in a specific compositional behaviour of the blend glass transition temperature.