The thermoresponsive behaviour of a poly(N-isopropylacrylamide) hydrogel with a D-π-A type pyran-based fluorescent dye

A thermoresponsive poly(NIPAM-co-fluorophore) hydrogel labeled with pyran-based fluorescent dye was prepared using conventional radical copolymerization. The low critical solution temperature behaviour was investigated using UV–vis spectroscopy which allowed the measurement of the phase transition over a wide range of temperature. The poly(NIPAM-co-fluorophore) in aqueous solution underwent a phase transition at 36–58 °C. The fluorescence intensities of the polymer in aqueous solution increased with increasing temperature across the low critical solution temperature transition range. The fluorophore displayed a blue shift in emission with increase in temperature due to destabilization of the excited state. Evidence from fluorescence and dynamic light scattering demonstrated that, >58 °C, the size of the polymer aggregates was 12,500 m. The morphology of the internal microstructure of the poly(NIPAM-co-fluorophore) hydrogel was observed using scanning electron microscopy.