Fluorescence Investigations of "Smart" Microgel Systems

Abstract

Fluorescence techniques, including lifetime, quenching, and time-resolved anisotropy measurements (TRAMS), were used to study microgel systems based upon N-isopropylacrylamide (NI-PAM) using pyrene as a fluorescent probe. These experiments have revealed that poly(N-isopropylacrylamide) (PNIPAM) nanoparticles undergo a phase transition at a lower critical solution temperature (LCST), of ca. 34°C, which involves collapse of the particles into compacted, hydrophobic spheres. A degree of control over the LCST has been achieved by copolymerization of NIPAM with varying amounts of dimethylacrylamide (DMAC). Incorporation of DMAC into the gel has the effect of changing the hydrophobic to hydrophilic balance and shifts the LCST to a higher temperature. Fluorescence methods indicate that the NIPAM/DMAC gels are of a more open, water-swollen nature above the LCST than that of their PNIPAM counterparts.