Self-Association of the Thermosensitive Block Copolymer Poly(2-isopropyl-2-oxazoline)‑b‑poly(N‑isopropylacrylamide) in Water–Methanol Mixtures

The dehydration and self-association of a novel thermosensitive diblock copolymer consisting of poly­(2-isopropyl-2-oxazoline) and poly­(N-isopropylacrylamide) (PIPOZ-b-PNIPAM) upon heating were studied in water and water–methanol (MeOH) mixtures by differential scanning calorimetry (DSC), turbidimetry, small-angle X-ray scattering (SAXS), and fluorescence depolarization. Although the difference of the phase-separation temperatures of the PIPOZ and PNIPAM homopolymers solutions was enhanced as the MeOH content in the mixed solvent increases, the DSC thermograms of PIPOZ-b-PNIPAM in water–MeOH mixtures were not bimodal, which indicates that the dehydration of each block does not occur independently. As a result, the amphiphilicity of this copolymer is weaker in the amphiphilic condition than that of the related thermosensitive block copolymer of PIPOZ and poly­(2-ethyl-2-oxazoline) (PIPOZ-b-PEOZ) examined previously, which undergoes phase separation in hot water under the same conditions. Both PIPOZ-b-PNIPAM and PIPOZ-b-PEOZ solutions undergo a temperature-induced liquid–liquid phase separation, but the formation of the spherical micelle in the coexisting dilute phase is more difficult in the former solution. The colloidal stability of the coexisting concentrated phase depends on the mixed solvent composition. The colloids flocculate in conditions for which the copolymer is of intermediate amphiphilicity, as observed previously also in the PIPOZ-b-PEOZ solution, possibly due to the viscoelastic effect experienced by colloidal particles bearing on their interface chains of a more hydrophilic block.