MECHANISMS OF THE EXCHANGE OF DIBLOCK COPOLYMERS BETWEEN MICELLES AT DYNAMIC EQUILIBRIUM

The exchange dynamics of chains between micelles of diblock copolymers in dilute solution in a selective solvent has been studied by a semia nalytical approach. A close inspection of dynamic Monte Carlo simulati on trajectories showed that there are mainly two types of exchange mec hanisms, chain insertion/expulsion and micellar merger/splitting. The relative contributions of the two mechanisms to the overall exchange d ynamics are examined as a function of the following variables: the con centration of the diblock copolymers, the energy of interaction betwee n the insoluble block and the surroundings, and the size of the insolu ble block. A kinetic scheme, which incorporates the possible states of association and the mechanisms by which the exchange takes place, is constructed. The corresponding transition rates matrix, with the rate constants extracted from simulations, is used in a Master equation for malism to demonstrate that the two mechanisms involved in the exchange dynamics operate on time scales differing by at least 1 order of magn itude, in conformity with recent experimental observations. Applicatio n of a filtering technique to the transition rate matrix showed that t he high-frequency modes of the relaxation are from the chain insertion /explusion mechanism and the relatively slower modes, which dominate t he tail parts of the time decay of correlation functions, are from the micellar merger/splitting type of transitions. The analysis thus prov ides an explanation for the two processes observed in the recent chara cterization of the exchange dynamics of chains between micelles by the efficiency of nonradiative singlet energy transfer. The chain inserti on/expulsion mechanism is shown to be predominantly important at low c oncentrations and at high interaction energies between the insoluble b lock and the surroundings, while the micellar merger/splitting is acti vated at higher concentrations.