Friday, December 7

Host:  MSG

 Andrew Lyon, Georgia Institute of Technology

Design and Assembly of Stimuli-Sensitive Microgels

Noon, Choppin 210


This talk will discuss two aspects of environmentally responsive, poly-N-alkylacrylamide hydrogel nanoparticles (microgels). First, the design of materials with tunable responsivities is accomplished by introducing a core-shell type morphology to the particles is discussed. 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 assembly of such microgels into responsive photonic crystals will also be presented. By taking advantage of the temperature responsivity of the component particles, we are able to thermally melt and recrystallize the assembly in a completely reversible fashion, thereby increasing the crystalline order. More importantly, the VPT offers a convenient route from the glassy (kinetically trapped) to the crystalline (thermodynamically preferred) portions of the packing phase diagram, allowing for much simpler assembly and processing of the photonic material. From a fundamental standpoint, the extreme chemical diversity of the microgel system also affords opportunities for tuning the kinetics and thermodynamics of the resultant assembly, melting, and recrystallization processes. Applications to environmentally sensitive photonics are also feasible, given the potential for designing analyte-induced swelling or shrinkage into the component particles, thereby obtaining a photonic crystal whose lattice constant is analyte sensitive.