Interfaces in self-assembling diblock copolymer systems:: Characterization of poly(isoprene-b-methyl methacrylate) micelles in acetonitrile

The kinetics of dipolar nonradiative energy transfer (DET) between dyes confined to the core-corona interface region of poly(isoprene-b-methyl methacrylate) block copolymers (PI-PMMA) in acetonitrile was analyzed using a new distribution model for energy transfer in spherical micelles. The distribution of block junction points was described by the model of Helfand and Tagami (HT) for the strong segregation limit, adapted for the spherical geometry of the core-corona interface. We used this model to analyze experimental fluorescence decay curves for block copolymer micelles made up of polymers containing a donor dye or an acceptor dye covalently attached to the PI-PMMA junction. The analysis yielded an interface thickness between the PI core and the PMMA corona of delta = (0.9 +/- 0.1) nm. In the past, the experimental fluorescence decay curves measured for similar systems have been fitted with the Klafter and Blumen (KB) equation for energy transfer, which has a stretched exponential form. To relate these results to topological characteristics of the system, we simulated donor decay profiles for different interface thickness values using the new distribution model for energy transfer and a modified HT equation. Subsequent analysis by the stretched exponential KB equation proved that the magnitude of the fitted exponent is directly related to the interface thickness between the blocks for a given dye concentration in the core-corona interface. Within a certain range of interface thickness values, this relation can be used to determine the interface thickness from the fitting parameters of the KB equation.