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 between 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 transition 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 terms of the entropy of water thermodesorption. Water sorption affects dramatically the state of crystalline (excess) PEG in blends, while the state of the amorphous phase, constituted by the PVP–PEG hydrogen-bonded complex, is practically unaffected by hydration.