Fluorescence energy transfer studies of styrene-isoprene diblock copolymer solutions^†

Micelle formation in solutions of chromophore-labeled styrene-isoprene diblock copolymers in selective solvent systems has been investigated using nonradiative energy transfer techniques. Energy transfer donor (carbazolyl or naphthyl) or acceptor (anthryl) chromophores were attached to anionically polymerized block copolymers and used to study micellar solutions as parameters such as concentration, solvent composition, temperature, and molecular weight were varied. Dramatic increases in I_A/I_D, the ratio of acceptor to donor fluorescence intensity, accompanied the formation of micelles. As the solvent quality worsens for the labeled blocks through either a reduction in temperature or the addition of a nonsolvent, micelles are detected first in the more concentrated solutions, consistent with expectations of a phenomenological critical micelle concentration (cmc). The high sensitivity of energy transfer experiments was also demonstrated; multimolecular micelles are observed at very low copolymer concentrations, near 10^-3 g/L, where turbidimetry is insensitive to micelle formation. With the use of a two-state energy transfer model, it was determined for the block copolymer systems under investigation that at concentrations exceeding the cmc additional copolymer apparently partitions itself between micelles and the homogeneous solution phase. Single photon counting measurements were also performed to observe changes in fluorescence lifetimes of both donors and acceptors upon micellization. Measured lifetimes of anthryl units nearly doubled in micelles due to the time lag caused by the energy transfer process.