Detection of the rate of exchange of chains between micelles formed by diblock copolymers in aqueous solution

Abstract

A fluorescence method is described for the measurement of the rate of exchange of chains between micelles formed by diblock copolymers in aqueous solution. The method requires two samples of the diblock copolymer. One sample is labelled with a Forster donor, the other sample is labelled with a Forster acceptor. Successful application of the method is demonstrated with diblock copolymers composed of polystyrene and poly(ethylene oxide). The donor and acceptor are naphthalene and pyrene, respectively. The label is covalently attached to the copolymers at the junction points between the two blocks. Solutions with micelles are formed independently by the two labelled samples. At the time of mixing of the two solutions, no micelle contains both a donor and an acceptor. Micelles containing both types of labels may be formed at later times as a consequence of the exchange of labelled chains. The efficiency of nonradiative singlet energy transfer from naphthalene to pyrene is measured as a function of time after mixing of the two solutions. At 60° C the rate constant deduced from the time dependence of the fluorescence is on the order 10-5 s-1. At ambient temperature, however, no exchange can be detected, presumably because of the difficulty in extraction of a polystyrene block from the glassy core.