Dynamic Structure Factor of Diblock Copolymers in the Ordering Regime

Abstract

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, qc. 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 qc 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 φ* < φ < φODT; and an extra relaxation, faster than the internal, which is less influenced by the thermodynamics. A general theory for the dynamic structure factor for entangled diblock copolymer systems is presented which adequately describes the internal relaxation within the reptation model and predicts the existence of another relaxation mechanism due to Rouse-like motions related to curvilinear chain fluctuations inside their tubes. The theory can semiquantitatively account for the combined characteristics of the two processes.