Dynamic structure factor of diblock copolymers solutions in the disordered state. 3. The non-mean-field regime

The dynamic structure factor S(q,t) of seven diblock copolymers in a common solvent was measured by photon correlation spectroscopy as a function of concentration (φ) in the disordered state near the disordered-to-order transition (ODT). Both symmetric and asymmetric diblock copolymers of styrene-b-isoprene (SI) with molecular masses ranging from 0.95 to 3.6 million g/mol and their hydrogenated styrene-e-(ethylene-alt-propylene) (SEP) counterparts have been considered. The maximum of the static structure factor S(q*) and the characteristic spacing 1/q* do not conform to theoretical predictions over the whole φ range up to φ_ODT. The S(q,t) is determined mainly by the relative contribution of two relaxation processes one of which is diffusive (q²-dependent rate). When composition fluctuation corrections become important for S(q*) at φ_x < φ_ODT, S(q,t) exhibits a dynamic crossover from a dominant pure relaxational (q-independent rate) to a dominant diffusive character. This new dynamic behavior in the shape of S(q,t) of narrowly distributed diblock copolymer solutions is captured theoretically. Above about φ_x, the self-diffusion coefficient D_s(φ) obtained from the S(q,t) at low q/q* values becomes thermodynamically retarded. The real phase morphology of diblock copolymers is well complemented by their dynamic structure.