Phase separation behavior and fluorescence properties of poly(N-isopropylacrylamide) with chalcone side groups in mixed solvents

The phase separation behavior and fluorescence properties of linear PNIPAM with chalcone side groups (PNIPAM-C) in water-cononsolvent systems were studied as a function of solvent composition (x) and temperature. It was found that PNIPAM-C exhibited a single lower critical solution temperature (LCST) in water–methanol, water–acetone, and water–tetrahydrofuran; the addition of organic solvents initially decreased the phase separation temperature and a further addition increased it rapidly. The minimum LCST was −8 °C for the mole fraction of methanol x _methanol ≈ 0.31, 17 °C for x _acetone ≈ 0.14, and −2 °C for x _{tetrahydrofuran} ≈ 0.13. However, in water–ethanol solution, a simultaneous LCST and upper critical solution temperature (UCST) existed for lower and higher ethanol concentrations, respectively, and the phase separation temperature decreased drastically with increasing ethanol concentrations. Furthermore, the solvent composition-sensitive and temperature-sensitive fluorescence properties of PNIPAM-C were investigated. The results showed that the fluorescence emission wavelength (λ_max) underwent a blue shift and the intensity increased with the addition of a small amount of organic solvent. With further addition of organic solvent, λ_max and intensity were almost not changed. However, when the content of organic solvent exceeded a critical value, such as x _methanol > 0.17, λ_max blueshifted and the intensity increased with the increase of organic solvent. At low content of organic solvent, λ_max decreased greatly and the intensity increased rapidly as the temperature was raised from 20 °C to 50 °C because phase separation occurred when the temperature was elevated.