Simulation of Dynamic Scattering from Homopolymer and Symmetric Diblock Copolymer Solutions with the Bond Fluctuation Model

Abstract

The dynamic Monte Carlo algorithm is employed to explore the dynamic scattering of dilute and nondilute solutions of flexible linear chains. The chains are represented by the bond fluctuation model with and without attractions between nonbonded units placed at close distances, to describe different types of thermodynamic conditions (Θ or good solvents). This mimics the behavior of real chains in the different types of solvents. We also consider symmetric diblock copolymer chains in a nonselective good solvent, where the differences between the blocks are introduced in the scattering factors of the beads and, also, through a net repulsion between units belonging to different blocks. The analysis of the dynamic scattering functions for semidilute solutions reveals the presence of at least two different modes. In the homopolymer chains, one of the modes follows the behavior expected for the osmotic mode in the gel regime. The other mode seems to correspond to the structural relaxation, in the case of Θ systems at low values of the scattering, q. At higher q and for good solvents this mode is faster than expected from theory. It can tentatively be assigned to chain end effects. The total intensity at q = 0 gives results consistent with the scaling theory for good solvent. The Θ systems show a dramatic increase for this intensity, which is mainly manifested in the slowest collective mode, due to the proximity of critical conditions. For the copolymer chains, we observe a main internal mode, related with the longest Rouse relaxation, and a secondary faster mode that can be due to the accumulation of shorter Rouse motions. The introduction of a moderate repulsion between units in different blocks is manifested through a more prominent peak in the total intensity. Some decrease of the rates is also observed for the same q values, probably caused by the onset of the order-disorder transition.