HYDROPHOBICALLY-MODIFIED POLY(N-ISOPROPYLACRYLAMIDES) IN WATER - PROBING OF THE MICRODOMAIN COMPOSITION BY NONRADIATIVE ENERGY-TRANSFER

Fluorescently labeled copolymers have been prepared by free-radical copolymerization in dioxane of N-isopropylacrylamide (NIPAM) and (1) N-[2-(1-naphthyl)ethyl]-N-n-octadecylacrylamide (4) in 100:1, 200:1, and 400:1 molar ratios and (2) mixtures of the two labeled monomers 4 and N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide in 2000:6:1 and 1800:3:1 molar ratios. In aqueous solutions the copolymers form polymeric micellar structures ranging in diameter from 50 to 110 nm (25-degrees-C, 1 g L-1), as determined by quasi-elastic light scattering. Experiments based on nonradiative energy transfer between excited naphthalene (Na) and pyrene (Py) in solutions of the doubly labeled copolymers and in mixtures of the singly labeled copolymers carrying either the Na chromophore or the Py chromophore reveal the existence of interpolymeric micelles (critical aggregation concentration less-than-or-equal-to 4 ppm). The data are discussed in light of previous fluorescence probe experiments on solutions of copolymers of NIPAM and n-alkylacrylamides. The results described here highlight the power of the nonradiative energy transfer technique to detect interpolymeric aggregation.