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

Abstract

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

Michael Andrew Fowlera, Jean Duhamel*a a Institute for Polymer Research, Waterloo Institute for Nanotechnology, Department of Chemistry, University of Waterloo, Waterloo, ON N2L 3G1, Canada

Xing Ping Qiub, Evgeniya Korchagina,b Françoise M. Winnik*b c b Department of Chemistry, Université de Montréal, CP 6128 Succursale Centre Ville, Montréal QC H3C 3J7, Canada c Faculty of Pharmacy and Department of Chemistry, University of Helsinki, Laboratory of Polymer Chemistry P.O.Box 55, Helsinki, FI-00014 Finland

* To Whom correspondence should be addressed. E-mail addresses: jduhamel@uwaterloo.ca; francoise.winnik@umontreal.ca

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(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 <>. 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.