ORIGIN OF CONONSOLVENCY, BASED ON THE STRUCTURE OF TETRAHYDROFURAN-WATER MIXTURE

摘要

Cononsolvency is the rare phenomenon that a mixture of two good solvents for a polymer forms a poor one.1-7 For example, PNIPAM is soluble in (cold) water and THF, but insoluble in mixtures of these two solvents under certain conditions. In our search, the origin of poly(N-isopropylacrylamide) (PNIPAM) cononsolvency in THF-water mixture was studied from the mixed solvent structure point of view. The structure of THF-water system has been studied by a combination of small angle neutron scattering (SANS), dynamic laser light scattering (DLS) and viscometry. A Lower Critical Solution Temperature (LCST) type of 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 with temperature controlled as constant. Then, the conformation change of poly(N-isopropylacry lamide-co-ethylene oxide) (PNIPAM-co-PEO) microgel in THF-water mixture has been studied by DLS, and static laser light scattering (SLS). When the THF content is lower than 4.5 mol% (at low temperature), the composition fluctuation influence of THF-water structure is quite weak, thus PNIPAM-co-PEO microgel can form hydrogen bonding with water and exists in swollen state. With THF content increases, the influence of composition fluctuation in THF-water mixture becomes dominant. Solvent-solvent interaction is stronger than mixed solvent-polymer interaction. Therefore when THF content reaches to 20 mol%, the influence of composition fluctuation in THF-water becomes overwhelming and as a result, the soluability of microgel turns to be the worst. Further increase of THF content abates the contribution of composition fluctuation, and PNIPAM becomes soluble again via Van der Waals interaction between THF and polymer. By our research, we proposed that the origin of PNIPAM cononsolvency is attributed to this kind of composition fluctuation.