Temperature-Controlled Interactions between Poly(N-isopropylacrylamide) Mesoglobules Probed by Fluorescence

The temperature-dependent behavior of aqueous solutions composed of a small amount of monodisperse poly(N-isopropylacrylamide) (PNIPAM) labeled at one or both ends with pyrene (Pyn-PNIPAM with n = 1 or 2) and a 10-fold excess of a nonfluorescent poly(N-isopropylacrylamide) (PNIPAM(22K), Mn = 22 000 g/mol) was characterized using steady-state (SSF) and time-resolved (TRF) fluorescence. Turbidimetry studies indicated that all solutions exhibited two temperature-induced transitions: one at Tc, the cloud point of the pyrene-labeled polymers, and one at Tc22, the cloud point of PNIPAM(22K). These two transitions were also inferred from a decrease in the excimer-to-monomer fluorescence intensity ratio, namely, the IE/IM ratio, obtained from SSF spectra. TRF decays of the pyrene monomer were acquired and fitted with a sum of exponentials to obtain the number-average lifetime ⟨τ⟩. Plots of ⟨τ⟩ versus temperature also showed transitions at Tc and Tc22. The changes in behavior observed at Tc for both IE/IM and ⟨τ⟩ ...