Simulation of the collective dynamics scattering of diblock copolymers with heterogeneous composition in the semidilute regime

Abstract

The dynamic behavior of symmetric diblock copolymer chains in semidilute solution with heterogeneous composition is simulated by means of the bond fluctuation model with repulsive interactions between monomers of different types. We have considered two different lengths at a given concentration close to the order–disorder transition. We have computed the radius of gyration, the translational diffusion coefficient and the first Rouse relaxation time, but most of the analysis is devoted to the results for the collective dynamic scattering functions. As in the simpler case of copolymers without compositional heterogeneity, we can fundamentally distinguish two modes in these functions, but the behavior of the main, slower, mode is clearly altered by heterogeneity. According to the present simulations, this mode reflects the slowest Rouse internal motion together with a purely diffusive heterogeneity contribution. In some cases, the two contributions of the main mode can be separated. However, the retardation of the slower mode in the proximity of the intensity maximum and other subtle features predicted by the random phase approximation theory are not clearly visible in the simulation data from these particular systems. In most cases, we have found a less intense and significantly faster mode, also appearing in previous simulations for which the heterogeneity feature was not included. The discussion also takes into account existing experimental data for diblock copolymer solutions.