Probes of the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide)

Cloud point, microcalorimetric, and fluorescent probe methods were in good agreement in determining the lower critical solution temperature (LCST) of an aqueous solution of a high molecular weight (M n = 160,000), polydisperse (M w /(M n = 2.8) sample of poly(N-isopropylacrylamide) (PNIPAAM). The cloud point (32.2°C) concurred with the temperatures of abrupt changes observed in the emission spectra of micromolar quantities of solubilized fluorophores: the I 1 /I 3 ratio of pyrene and the emission maxima of 1-pyrenecarboxaldehyde (pycho), sodium 2-(N-dodecylamino)naphthalene-6-sulfonate (C 12 NS), and ammonium 8-anilinonaphthalene-1-sulfonate (ANS) successfully monitored the formation of the precipitated polymer phase. The observed microcalorimetric endotherm had a peak maximum of 32.4 ± 0.1°C and an enthalpy on the order of the strength of hydrogen bonds.