Coherence of thermal transitions in poly(N-vinyl pyrrolidone)-poly(ethylene glycol) compatible blends 3. Impact of sorbed water upon phase behaviour

The state of sorbed water in hydrogels based on hydrogen-bonded complexes b etween poly(N-vinyl pyrrolidone) (PVP) and short-chain poly(ethylene glycol ) (PEG) has been examined by considering water vaporisation endotherms in d .s.c. heating traces and relating them to characteristics of the amorphous and crystalline phases, i.e. the thermodynamic parameters of glass transiti on and excess PEG melting. In compatible PVP-PEG blends water behaves as a third component in the PVP-PEG complex and is sorbed mainly by PVP, whereas PEG increases the mobility of sorbed water molecules evaluated in the term s of the entropy of water thermodesorption. Water sorption affects dramatic ally the state of crystalline (excess) PEG in blends, while the state of th e amorphous phase, constituted by the PVP-PEG hydrogen-bonded complex, is p ractically unaffected by hydration. (C) 2000 Elsevier Science Ltd. All righ ts reserved.