Dynamics of solutions of triblock copolymers in a selective solvent: Effect of varying copolymer concentration

We have used static and dynamic light scattering and pulsed field gradient NMR to study the effect of varying concentration on the dynamics of the tri block copolymer, polystyrene block poly(ethylene, butylene) block polystyre ne (PS-PEB-PS), dissolved in n-heptane, a selective solvent for the middle block. The correlation function for a dilute solution with c = 0.49 % (w/v) corresponds to the translational diffusion of micelles. At intermediate co ncentrations [1.1 less than or equal to c less than or equal to 2.6 % (w/v) ], the correlation functions can be fitted to the sum of a single exponenti al and stretched exponential functions. The slower mode is due to the diffu sion of polydisperse clusters formed by random association of triblock copo lymer molecules and the faster one again represents micelles. A complex beh avior is observed in the semidilute region [4.0 less than or equal to c les s than or equal to 6.9 % (w/v)]. Three dynamic processes can be extracted f rom the correlation function: (i) The fast diffusive mode is the collective diffusion mode in the physical gel, (ii) the middle, relaxational mode, is probably due to the local movement of insoluble domains trapped in the net work of the physical gel, and (iii) the slow diffusive mode implies the exi stence of large-scale inhomogeneities in the system. (C) 2000 John Wiley & Sons, Inc.