Effect of Polydispersity on Fluorescence Quenching in Micelles Formed by Telechelic Associative Polymers

Time-resolved fluorescence quenching studies, using pyrene as a probe for micellar environments in solutions of a telechelic associating polymer based on PEG with n-C12 (dodecyl) hydrophobic end groups, have been carried out to determine the aggregation number(s) of the micelles. At low [pyrene]/[hydrophobe] values, η, quencher-average aggregation numbers, Nq, calculated from the fitting of the fluorescence decay curves to the Infelta equation were found to decrease rapidly as η increased. By fitting the Nq vs η plots to a cubic curve [Warr, G. G.; Grieser, F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1813], this observation was shown to be consistent with a highly skewed, polydisperse distribution of micelle aggregation numbers, described by a weight-average aggregation number of 75 and a standard deviation of 86. For a given polymer concentration in the range 1.0−2.5 g dL-1, the fluorescence decay curves did not produce parallel log(I/I0) vs time plots, as would be expected for a monodisperse array of micelles. Instead, the plots showed a characteristic intersection. Simulation of these curves using Infelta-type equations could not reproduce this behavior, indicating that a more refined model may be needed. This detailed study of the distribution of aggregation number in associative telechelic polymers is discussed in the context of recent investigations [Meng, X.-X.; Russel, W. B. Macromolecules 2005, 38, 593], from which it is suggested that polydispersity of micelle size is to be expected as a normal feature of such systems