Volume Phase Transitions in Stimuli-Sensitive Core-Shell Microgels
L. Andrew Lyon
School of Chemistry and Biochemistry,
Georgia Institute of Technology, Atlanta, GA 30332
I will present investigations of environmentally responsive, poly-N-alkylacrylamide
hydrogel nanoparticles (microgels) possessing a core-shell type morphology.
Via chemical and morphological tuning of the core-shell structure, we have
demonstrated the ability to vary the position, magnitude and number of
volume phase transitions, as monitored by photon correlation spectroscopy
(PCS). Furthermore, differential scanning calorimetry (DSC) and nanosecond
T-jump studies have shown that simply adding hydrophobic “dopants” in the
shell can strongly influence the rate at which core-shell microgels collapse.
This added hydrophobicity however does not affect the thermodynamics of
the volume phase transition, suggesting that under certain conditions the
kinetics and thermodynamics of the phase transition can be decoupled. Finally,
by adding fluorescence donor-acceptor pairs to these microgels, nonradiative
energy transfer further elucidates particle collapse process; the volume
phase transition can be measured over a much smaller temperature range
with fluorescence spectroscopy than when probed with TP-PCS, suggesting
radial phase coexistence during collapse. The use of these particles in
the assembly of functional materials such as photonic crystals will also
be discussed with respect to hard vs. soft sphere pressure-volume phase
diagrams and the utility of responsive soft materials. Together, these
results indicate the strength of the core-shell microgel morphology in
the creation of value-added soft materials, where the thermodynamics, kinetics,
and optical properties can be exquisitely tuned via very small chemical
modifications.