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

Abstract

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 non-fluorescent poly(Nisopropylacrylamide) (PNIPAM(22K), Mn = 22,000 g/mol) was characterized using steadystate (SSF) and time-resolved (TRF) fluorescence. Turbidimetry studies indicated that all samples 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 <>. Plots of <>-versus-temperature also showed transitions at Tc and Tc22. The changes in behavior observed at Tc for both IE/IM and <> were consistent with those observed for solutions of solely Pyn-PNIPAM samples. The transitions found at Tc22 for the Pyn-PNIPAM solutions with PNIPAM(22K) were not observed in aqueous solutions of Pyn-PNIPAM without PNIPAM(22K). They were explained by invoking substantial mixing of labeled and unlabeled chains as temperature exceeded Tc22. This mixing could only occur if the mesoglobules composed of labeled chains were not "frozen" at temperatures above Tc22 despite forming stable entities in this temperature range. This phenomenon was rationalized by considering the difference in the characteristic reptation time of the chains found in a Pyn-PNIPAM and PNIPAM(22K) mesoglobule at temperatures larger than Tc22.

Surfactant Structure-Dependent Interactions with Modified Starch Nanoparticles Probed by Fluorescence Spectroscopy

“…As it turns out, this range of SDS concentration corresponds to the regime where the excimer decays showed a rise time reflected by negative A E– / A E+ values in Figure C, indicative of excimer formation by diffusion. In contrast, the blue shift in excimer emission found for SDS concentrations smaller than the CMC reflected the existence of ground-state dimers, as has been reported in earlier reports , and as would be expected from the absence of rise time in the excimer decays. The much less pronounced blue shift observed for the excimer fluorescence spectra of 3.1 mg/L Py(6.7)-SNP in water suggested that the pyrene labels in the absence of Np(48)-SNP were more mobile and generated aggregates where the pyrene labels were better aligned to form an excimer.…”

Time-Resolved Light Scattering of Atactic Poly(N-isopropylacrylamide) in Water: Physical Gelation and Spinodal Temperature of the Hydrogel

“…Aqueous solutions of poly­( N -isopropylacrylamide) (PNIPAM) exhibit the phase behavior of low critical solution temperature (LCST) with a sharp thermal response at a high temperature of about 32 °C. − Hydrogen bonds between the −CONH– group and the surrounding water molecules play the key role for the LCST behavior in the aqueous solution of PNIPAM. − Okada and Tanaka proposed the pearl-necklace chain model together with the cooperative hydration concept, which has been successfully applied to theoretically derive the “very flat” LCST phase boundary of PNIPAM/H 2 O. For a given PNIPAM chain, the majority of the −CONH– groups are hydrated with the cooperative H-bonded water molecules to form the necklace part; whereas some dehydrated segments will develop the hydrophobic pearls along the necklace.…”