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 PMMA 21 -b-PNIPAM 283 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 cononsolvency 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.

Role of chain architecture in the solution phase assembly and thermoreversibility of aqueous PNIPAM/silyl methacrylate copolymers

“…In solutions of responsive polymers, small changes in the solvent composition can lead to co-nonsolvency effects, which drastically alter the types of transitions and temperatures at which these transitions occur. 37,38,42,43 Small alterations to the polymer composition can also drastically impact the solution behavior. For example, the upper critical solution temperature of poly(methyl methacrylate) (PMMA) observed in water/ethanol solvent systems can be altered by up to 20 °C with low incorporation (6 mol%) of acrylamide comonomers.…”

Cononsolvency of thermoresponsive polymers: where we are now and where we are going

“…Cononsolvency is an intriguing phenomenon which has been widely researched through various experimental, simulation From. 149 Copyright 2021. Reproduced with permission from The American Chemical Society.…”

“…Besides linear chains of homopolymers, more complex systems have been investigated regarding their cononsolvency behavior such as cross-linked gels, 29,55,114,[141][142][143][144][145][146] block copolymer systems, 13,57,58,[147][148][149] thin films, 59,150 and grafted polymer brushes. 10,14,15,17,73,151,152 Microgels are macromolecular networks swollen by the solvent they are dissolved in.…”

“…The effect of methanol on the micellar structures and the single chain conformations of aqueous solutions of a poly(methyl methacrylate)- b -poly( N -isopropylacrylamide) diblock copolymer at 20 °C were investigated by Ko et al using synchrotron SAXS. 149 In water-rich solvent mixtures, self-assembled spherical core–shell micelles are formed (Fig. 17).…”

“…The full line guides the eye. Schematic representations of the micelles (left) and molecularly dissolved chains (right) at the points in the phase diagram having the same colors.From 149. Copyright 2021.…”