The modes of relaxation of composition fluctuations in disordered diblock copolymer systems have been experimentally and theoretically investigated for wavevectors q near the maximum of the static structure factor, q(c). By use of disordered semidilute solutions of a very high molecular weight diblock in a nonselective good solvent, the dynamic structure factor S(q,t) is probed in this q range by photon correlation spectroscopy over a broad time range. Two relaxation processes are observed to contribute to S(q,t) besides the cooperative diffusive relaxation of the total polymer concentration fluctuations : the well-anticipated internal copolymer relaxation, which is a single exponential process near q, and for which the effect of the proximity to the disorder-to-order transition (ODT) is evident both in the respective static light scattering intensity and in the relaxational dynamics by increasing the copolymer concentration in the range phi*

Keywords:RANDOM PHASE APPROXIMATION;BLOCK CO-POLYMERS;X-RAY-SCATTERING;MICROPHASE SEPARATION;LIGHT-SCATTERING;MUTUAL DIFFUSION;PSEUDOBINARY APPROXIMATION;COMPOSITION FLUCTUATIONS;SPINODAL DECOMPOSITION;DISORDER TRANSITION