Dynamic self-aggregation behavior of a PNIPAM-based nonlinear multihydrophilic block copolymer revealed by two-dimensional correlation spectroscopy

Abstract

IR spectroscopy in combination with the perturbation correlation moving window (PCMW) technique and 2D correlation spectroscopy (2DCOS) is employed to elucidate the dynamic self-aggregation behavior of a novel miktoarm star PNIPAM-based multihydrophilic block copolymer, poly(N-isopropylacrylamide)₂–[poly(N-vinylpyrrolidone)-b-poly(acrylic acid)]₂ ((PNIPAM)₂–(PVP-b-PAA)₂). At pH = 8, (PNIPAM)₂–(PVP-b-PAA)₂ tends to self-assemble into micelles with PNIPAM in the core and ionized PAA segments in the shell during heating. IR investigation shows that the AA segments exhibit a similar “phase transition” behavior to the PNIPAM segments, which can be ascribed to the indirect influence through the drastic content changes of water molecules along with the hydrophilic-to-hydrophobic transformation of PNIPAM segments. Boltzmann fitting and PCMW easily determine the transition temperature to be ca. 33 °C and the transition temperature range to be 29.5–35 °C. Moreover, 2DCOS discerns a sequential group motion from PNIPAM to PVP and PAA segments. It is concluded that the three polymeric segments have relatively independent phase behavior during the formation of PNIPAM-core micelles, and the chain conformation adjustment induced by hydrophilic-to-hydrophobic transformation of PNIPAM segments should be the driving force of the whole self-aggregation process. The dynamic self-aggregation process we proposed can be further confirmed by dynamic laser scattering (DLS) and zeta potential measurements.