Abstract
Semidilute solutions of ultrahigh molecular weight diblocks in a nonselective good solvent allow investigation of the dynamic structure factor S(q,t) by photon correlation spectroscopy for wavevectors q in the vicinity (and on both sides) of the maximum of the static structure factor q* (0.2 ≤ q/q* ≤ 2.1) as a function of copolymer concentration in the ordering regime. The relaxation rate of the short-range composition fluctuations at q*, Γ(q*), shows a significant slowing down relative to the respective long-range ones at low wavevectors; as the ordering transition is approached, this slowing down becomes very pronounced. Γ(q*) has been anticipated to influence the low-frequency rheological behavior of disordered diblocks. Additionally, a general theory for the S(q,t) of entangled polydisperse diblock copolymers is developed in the framework of the random phase approximation assuming reptation dynamics. Although both internal diffusion and ordinary interdiffusion contribute to the dynamics of long-range composition fluctuations, it is the internal diffusion at finite wavevectors near q*, which is affected by approaching the ordering transition from the disordered state. Nevertheless, composition polydispersity causes a coupling of these relaxation modes, which hinders their identification over the whole q range. The theoretical results are favorably compared with the experimental data.