Nonequilibrium Liquid–Liquid Phase Separation of Poly(N-isopropylacrylamide) in Water/Methanol Mixtures

Abstract

At room temperature, poly(N-isopropylacrylamide) (PNIPAM) is soluble in water and in methanol. Within intermediate mixing ratios of the two solvents, the PNIPAM coils collapse into insoluble globules, a phenomenon known as co-nonsolvency. Visual observation of mixed PNIPAM/water/methanol systems (polymer concentration ≥10 g L–1, Mn 80 000 g L–1) revealed that mixtures of methanol volume fractions (ϕM) ranging from 0.57 to 0.65 undergo macroscopic liquid–liquid phase separation (MLLPS) at 21 °C. MLLPS took place over a wider composition range (0.25 < ϕM < 0.60) for water/methanol mixtures containing a polymer sample bearing n-butyl end-groups (PNIPAM-45K (10.0 g L–1, Mn 44 500 g mol–1), but systems containing a polymer with chloroethyl end-groups (PNIPAM-Cl) (10.0 g L–1, Mn 44 500 g mol–1) did not undergo MLLPS over the entire mixing ratio span. Observation by fluorescence microscopy of a demixed PNIPAM-45K/water/methanol sample containing trace amounts of pyrene-labeled PNIPAM revealed that the rim of the...