Dynamics of the most probable composition fluctuations of "real" diblock copolymers near the ordering transition

Semidilute solutions of ultrahigh molecular weight diblocks in a nonselecti ve good solvent allow investigation of the dynamic structure factor S(q,t) by photon correlation spectroscopy for wavevectors q in the vicinity land o n both sides) of the maximum of the static structure factor q* (0.2 < q/q* <less than or equal to> 2.1) as a function of copolymer concentration in th e ordering regime. The relaxation rate of the short-range composition fluct uations at q*, Gamma (q*), shows a significant slowing down relative to the respective longrange ones at low wavevectors; as the ordering transition i s approached, this slowing down becomes very pronounced. Gamma (q*) has bee n anticipated to influence the low-frequency rheological behavior of disord ered diblocks. Additionally, a general theory for the S(q,t) of entangled p olydisperse diblock copolymers is developed in the framework of the random phase approximation assuming reptation dynamics. Although both internal dif fusion and ordinary interdiffusion contribute to the dynamics of long-range composition fluctuations, it is the internal diffusion at finite wavevecto rs near q*, which is affected by approaching the ordering transition from t he disordered state. Nevertheless, composition polydispersity causes a coup ling of these relaxation modes, which hinders their identification over the whole q range. The theoretical results are favorably compared with the exp erimental data.