Schizophrenic Core–Shell Microgels: Thermoregulated
Core and Shell Swelling/Collapse by Combining UCST and LCST Phase
Transitions
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Abstract
A variety
of slightly cross-linked poly(2-vinylpyridine)–poly(<i>N</i>-isopropylacrylamide) (P2VP–PNIPAM) core–shell
microgels with pH- and temperature-responsive characteristic were
prepared via seeded emulsion polymerization. Negatively charged sodium
2,6-naphthalenedisulfonate (2,6-NDS) could be internalized into the
inner core, followed by formation of (P2VPH<sup>+</sup>/SO<sub>3</sub><sup>2–</sup>) supramolecular complex through the electrostatic
attractive interaction in acid condition. The thermoresponsive characteristic
feature of the (P2VPH<sup>+</sup>/SO<sub>3</sub><sup>2–</sup>)–PNIPAM core–shell microgels was investigated by laser
light scattering and UV–vis measurement, revealing an integration
of upper critical solution temperature (UCST) and lower critical solution
temperature (LCST) behaviors in the temperature range of 20–55
°C. The UCST performance arised from the compromised electrostatic
attractive interaction between P2VPH<sup>+</sup> and 2,6-NDS at elevated
temperatures, while the subsequent LCST transition is correlated to
the thermo-induced collapse of PNIPAM shells. The controlled release
of 2,6-NDS was monitored by static fluorescence spectra as a function
of temperature change. Moreover, stopped-flow equipped with a temperature-jump
accessory was then employed to assess the dynamic process, suggesting
a millisecond characteristic relaxation time of the 2,6-NDS diffusion
process. Interestingly, the characteristic relaxation time is independent
of the shell cross-link density, whereas it was significantly affected
by shell thickness. We believe that these dual thermoresponsive core–shell
microgels with thermotunable volume phase transition may augur promising
applications in the fields of polymer science and materials, particularly
for temperature-triggered release