Photoinduced Phase Transitions in Poly(Nisopropylacrylamide) Microgels

Photoresponsive microgels have been prepared by precipitation polymerization of the thermoresponsive polymer poly(N-isopropylacrylamide) followed by covalent conjugation of the temperature-jump dye malachite green. The photoresponsivity of these dyelabeled microgels was characterized by a pump-probe optical setup. A HeNe laser is used for exciting the dye molecules and a near-IR-diode laser is used to simultaneously measure the turbidity of the colloidal dispersion. Irradiation of malachite green increases the temperature of the sample through rapid nonradiative decay, thereby causing the polymer chains to aggregate. On deswelling, a decrease in the intensity of transmitted light is observed due to scattering. It is also observed that the photoresponsive behavior of the microgels is dependent on the concentration of the dye, intensity of the laser, and bath temperature.