Origin of cononsolvency, based on the structure of tetrahydrofuran-water mixture

The origin of poly(N-isopropylacrylamide) (PNIPAM) cononsolvency in tetrahydrofuran-water (THF-water) mixture was studied from the point of view of mixed solvent structure. The dynamic equilibrium of THF-water composition fluctuation in the mixed solvent system was found to be the main variable for this cononsolvency effect. Temperature and THF content dependences of composition fluctuation were studied by a combination of small angle neutron scattering (SANS), dynamic laser light scattering, and viscometry. A lower critical solution temperature (LCST) type phase diagram for THF-water mixture was established by SANS. The composition fluctuation in THF-water system reaches the maximum at about 20 mol % THF content at constant temperature and increases with temperature as getting closer to the phase boundary. This kind of composition fluctuation induces PNIPAM cononsolvency. When the THF content is lower than 4.5 mol %, the composition fluctuation influence of the THF-water structure is quite weak and most of water structure is not disturbed. Then, at low THF content, poly(N-isopropylacrylamide-co-ethylene glycol) (PNIPAM-co-PEG) microgel can still form hydrogen bonds with water and exist in the swollen state. The basic phase transition behavior of the microgel in THF-water is relatively similar to that in pure water, except for the shift of LCST to lower temperature. With THF content increasing to 20 mol %, the influence of composition fluctuation in the THF-water mixture becomes dominant. Solvent-solvent interaction is stronger than mixed solvent-polymer interaction. So PNIPAM does not dissolve in the mixed solvent, and the microgel is in the collapsed state. Further increase in THF content abates the contribution of composition fluctuation, and the structures of mixed solvents tend to be that in pure THF. PNIPAM becomes soluble again via Van der Waals interaction between THF and polymer.