Interactions of Amphiphilic Polyelectrolytes and Neutral Polymeric Micelles: A Study by Nonradiative Energy Transfer

Abstract

The interactions of hydrophobically modified poly(N-isopropylacrylamides) (HM-PNIPAM) and unimolecular micelles formed by fluorescently labeled copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and N-n-dodecylmethacrylamide or N-adamantylmethacrylamide were investigated by fluorescence spectroscopy and dynamic light scattering measurements. The HM-PNIPAM copolymers were prepared by free-radical copolymerization of N-isopropylacrylamide (NIPAM) and either N-n-dodecylacrylamide or N-(1-adamantylacrylamide). Pyrene- and naphthalene-labeled HM-PNIPAM derivatives were prepared by reaction of 1-pyrenylmethylamine, 4-(1-pyrenyl)butylamine, or 1-(1-naphthyl)ethylamine with copolymers of NIPAM, the hydrophobic monomer, and N-(acryloxysuccinimide). In aqueous solutions, the HM-PNIPAM copolymers form multimolecular micelles ranging in size from 25 to 60 nm (25 °C), as determined by dynamic light scattering and fluorescence measurements based on the extent of nonradiative energy transfer (NRET) between naphthalene and pyrene in mixtures of naphthalene- and pyrene-labeled HM-PNIPAM derivatives. The interactions between HM-PNIPAM and hydrophobically modified PAMPS were studied by NRET measurements in solutions of (1) naphthalene-labeled HM-PNIPAM and pyrene-labeled PAMPS and (2) unlabeled HM-PNIPAM and a copolymer of AMPS carrying both naphthalene and pyrene chromophores. Each technique gave evidence for the formation of complexes between PAMPS micelles and HM-PNIPAM chains. The complexation was shown to depend on temperature, the ratio of the two polymers, and the molecular structure of the hydrophobic substituents.