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Self-Association of the Thermosensitive Block Copolymer Poly(2-isopropyl-2-oxazoline)‑b‑poly(N‑isopropylacrylamide) in Water–Methanol Mixtures
journal contribution
posted on 2014-10-14, 00:00 authored by Rintaro Takahashi, Xing-Ping Qiu, Na Xue, Takahiro Sato, Ken Terao, Françoise
M. WinnikThe 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.