Thermosensitive
Ionic Microgels via Surfactant-Free
Emulsion Copolymerization and in Situ Quaternization Cross-Linking
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Abstract
A type of thermosensitive ionic microgel
was successfully prepared
via the simultaneous quaternized cross-linking reaction during the
surfactant-free emulsion copolymerization of <i>N</i>-isopropylacrylamide
(NIPAm) as the main monomer and 1-vinylimidazole or 4-vinylpyridine
as the comonomer. 1,4-Dibromobutane and 1,6-dibromohexane were used
as the halogenated compounds to quaternize the tertiary amine in the
comonomer, leading to the formation of a cross-linking network and
thermosensitive ionic microgels. The sizes, morphologies, and properties
of the obtained ionic microgels were systematically investigated by
using transmission electron microscopy (TEM), dynamic and static light
scattering (DLS and SLS), electrophoretic light scattering (ELS),
thermogravimetric analyses (TGA), and UV–visible spectroscopy.
The obtained ionic microgels were spherical in shape with narrow size
distribution. These ionic microgels exhibited thermosensitive behavior
and a unique feature of poly(ionic liquid) in aqueous solutions, of
which the counteranions of the microgels could be changed by anion
exchange reaction with BF<sub>4</sub>K or lithium trifluoromethyl
sulfonate (PFM-Li). After the anion exchange reaction, the ionic microgels
were stable in aqueous solution and could be well dispersed in the
solvents with different polarities, depending on the type of counteranion.
The sizes and thermosensitive behavior of the ionic microgels could
be well tuned by controlling the quaternization extent, the type of
comonomer, halogenated compounds, and counteranions. The ionic microgels
showed superior swelling properties in aqueous solution. Furthermore,
these ionic microgels also showed capabilities to encapsulate and
release the anionic dyes, like methyl orange, in aqueous solutions