Co-Nonsolvency Effect in Solutions of Poly(methyl methacrylate)-b-poly(N-isopropylacrylamide) Diblock Copolymers in Water/Methanol Mixtures

Abstract

The self-assembly of the thermoresponsive amphiphilic diblock copolymer PMMA21-b-PNIPAM283 is studied in different water/methanol mixtures. It consists of a short hydrophobic poly(methyl methacrylate) block and a long thermoresponsive poly(N-isopropylacrylamide) block. Adding methanol as a cosolvent causes the PNIPAM block, which is soluble in both pure water and pure methanol, to collapse due to the so-called co-nonsolvency effect. Meanwhile, the addition of methanol reduces the incompatibility of the PMMA block with water. By means of turbidimetry and differential scanning calorimetry, the solvent-composition-dependent phase diagram is constructed. Dynamic light scattering and synchrotron radiation-based small-angle X-ray scattering provide structural information at 20 °C in dependence on the solvent composition. In water-rich solvent mixtures, self-assembled spherical core–shell micelles are formed. The internal structure of the micelles is adjusted by the solvent compositions in two ways: methanol softens the PMMA micellar core, while it causes the shrinkage of the PNIPAM micellar shell. In methanol-rich solvent mixtures beyond the miscibility gap, the copolymers are molecularly dissolved chains. They are collapsed near the coexistence line, while they become random coils as the methanol content increases. We propose that the internal morphology of the micelles and the conformation of the dissolved chains depend strongly on the solvent composition, as a consequence of the superposed co-nonsolvency effect of PNIPAM and the overall enhanced solvation of PMMA when adding methanol.