Eur. Phys. J. E (2005) 17: 129-137
https://doi.org/10.1140/epje/i2004-10134-9

Regular Articles

Amphiphilic telechelic poly(N-isopropylacrylamide) in water: From micelles to gels^⋆

¹ Department of Chemistry and Faculty of Pharmacy, University of Montreal, Succursale Centre Ville, CP 6128, H3C 3J7, Montreal, QC, Canada
² Institute for Chemical Research, Kyoto University, 611-0011, Uji, Kyoto, Japan
³ Department of Polymer Chemistry, Kyoto University, 615-8510, Katsura, Nishikyo-ku, Kyoto, Japan

^* e-mail: francoise.winnik@umontreal.ca

Received: 8 November 2004
Accepted: 22 March 2005
Published online: 6 May 2005

Abstract

We report the first study of aqueous solutions (0.025 gL^-1 to 46 gL^-1) of a telechelic poly(N-isopropylacrylamide) with octadecyl termini (C₁₈-PNIPAM-C₁₈, M _w: 37000, 320 NIPAM units, M _w/ M _n = 1.07) obtained by reversible addition-fragmentation chain transfer (RAFT) free radical polymerization of N-isopropylacrylamide. Static and dynamic light scattering measurements and fluorescence spectroscopy, using 8-anilino-1-naphthalenesulfonic acid (ANS) as probe, yielded the concentration dependence of the size and aggregation number of C₁₈-PNIPAM-C₁₈ micelles in cold ( 20^°C) dilute aqueous solutions. Concentrated solutions ( c > 20gL ^-1) form transient gels exhibiting an oscillatory shear behavior that can be approximated by a single-relaxation Maxwellian model. Aqueous solutions of C₁₈-PNIPAM-C₁₈ undergo a phase transition upon heating to 31.5^°C as determined by microcalorimetry. The heat-induced phase separation of dilute (0.025 gL^-1) C₁₈-PNIPAM-C₁₈ solutions yields a fluid that is colloidally stable at temperatures higher than 33^°C. The overall results are consistent with a model assuming the formation of flowerlike micelles in the dilute regime and a network of micelles connected by telechelic chains in the concentrated regime.