Phase Behavior of Poly(vinylidene fluoride)-graft-poly(diethylene glycol methyl ether methacrylate) in Alcohol–Water System: Coexistence of LCST and UCST

A thermoresponsive polymer poly­(diethylene glycol methyl ether methacrylate) (PMeO₂MA) is grafted from poly­(vinylidene fluoride) (PVDF) backbone by using a combined ATRC and ATRP technique with a high conversion (69%) of the monomer to produce the graft copolymer (PD). It is highly soluble polymer and its solution property is studied by varying polarity in pure solvents (water, methanol, isopropanol) and also in mixed solvents (water–methanol and water–isopropanol) by measuring the hydrodynamic size (Z-average) of the particles by dynamic light scattering (DLS). The variation of Z-average size with temperature of the PD solution (0.2%, w/v) indicates a lower critical solution temperature (LCST)-type phase transition (T_PL) in aqueous medium, an upper critical solution temperature (UCST)-type phase transition (T_PU) in isopropanol medium, and no such phase transition for methanol solution. In the mixed solvent (water + isopropanol) at 0–20% (v/v) isopropanol the T_PL increases, whereas the T_PU decreases at 92–100% with isopropanol content. For the mixture 20–90% isopropanol, PD particles having larger sizes (400–750 nm) exhibit neither any break in Z-average size–temperature plot nor any cloudiness, indicating their dispersed swelled state in the medium. In the methanol + water mixture with methanol content of 0–30%, T_PL increases, and at 40–60% both UCST- and LCST-type phase separations occur simultaneously, but at 70–90% methanol the swelled state of the particles (size 250–375 nm) is noticed. For 50 vol % methanol by varying polymer concentration (0.07–0.2% w/v) we have drawn a quasibinary phase diagram that indicates an approximate inverted hourglass phase diagram where a swelled state exists between two single phase boundary produced from LCST- and UCST-type phase transitions. An attempt is made to understand the phase separation process by temperature-dependent ¹H NMR spectroscopy along with transmission electron microscopy.