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Temperature-Controlled Interactions between Poly(N-isopropylacrylamide) Mesoglobules Probed by Fluorescence
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DOI：10.1021/acs.macromol.8b00003.png)
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Abstract
En 中文
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 (Py-n-PNIPAM with n = 1 or 2) and a 10-fold excess of a nonfluorescent poly(N-isopropylacrylamide) (PNIPAM(22K), M-n = 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 T-upsilon the cloud point of the pyrene-labeled polymers, and one at T-c22, 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 I-E/I-M 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 (tau). Plots of (1) versus temperature also showed transitions at T-c and T-c22. The changes in behavior observed at T, for both I-E/I-M and (tau) were consistent with those observed for solutions of solely Py-n-PNIPAM samples. The transitions found at T-c22 for the Pyn-PNIPAM solutions with PNIPAM(22K) were not observed in aqueous solutions of Py-n-PNIPAM without PNIPAM(22K). They were explained by invoking substantial mixing of labeled and unlabeled chains as temperature exceeded Tc-22. This mixing could only occur if the mesoglobules composed of labeled chains were not frozen at temperatures above T-c22 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 Py-n-PNIPAM and PNIPAM(22K) mesoglobule at temperatures larger than T-c22.
Keywords:
PHASE-SEPARATION
DILUTE-SOLUTIONS
AQUEOUS-SOLUTIONS
POLYMER-SOLUTIONS
HYDROPHOBIC DRUG
SINGLE-CHAIN
WATER
TRANSITION
COPOLYMER
PYRENE
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