On the Thermally Reversible
Dynamic Hydration Behavior
of Oligo(ethylene glycol) Methacrylate-Based Polymers in Water
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Abstract
Dynamic thermally reversible hydration behavior of a
well-defined
thermoresponsive copolymer P(MEO<sub>2</sub>MA-<i>co</i>-OEGMA<sub>475</sub>) in D<sub>2</sub>O synthesized by ATRP random
copolymerization of 2-(2-methoxyethoxy)ethyl methacrylate (MEO<sub>2</sub>MA) and oligo(ethylene glycol) methacrylate (<i>M</i><sub>n</sub> = 475 g/mol) was studied by means of IR spectroscopy
in combination with perturbation correlation moving window (PCMW)
technique and two-dimensional correlation spectroscopy (2DCOS). Largely
different from poly(<i>N</i>-isopropylacrylamide) (PNIPAM),
P(MEO<sub>2</sub>MA-<i>co</i>-OEGMA<sub>475</sub>) exhibits
a sharp change below LCST and a gradual change above LCST due to the
absence of strong intermolecular hydrogen bonding interactions between
polymer chains, and the apparent phase transition is mainly arising
from the multiple chain aggregation without a precontraction process
of individual polymer chains. Additionally, the self-aggregation process
of P(MEO<sub>2</sub>MA-<i>co</i>-OEGMA<sub>475</sub>) is
found to be mainly dominated or driven by the conformation changes
of oxyethylene side chains, which collapse first to get close to the
hydrophobic backbones and then distort to expose hydrophilic ether
oxygen groups to the “outer shell” of polymer chains
as much as possible. On the other hand, PCMW easily determined the
phase transition temperature to be ca. 32.5 °C during heating
and ca. 31 °C during cooling as well as the transition temperature
range to be 28.5–37 °C. 2DCOS was finally employed to
discern the sequence order of all the group motions during heating
and cooling. It is concluded that during the phase transition P(MEO<sub>2</sub>MA-<i>co</i>-OEGMA<sub>475</sub>) chains successively
experience “hydrated chains–dehydrated chains–loosely
aggregated micelles–densely aggregated micelles” four
consecutive conformation changes. The results were further confirmed
by temperature-variable <sup>1</sup>H NMR analysis and molecular dynamics
simulation