On the role of hydrodynamic interactions in block copolymer microphase separation

A melt of linear diblock copolymers (A(n)B(m)) can form a diverse range of microphase separated structures. The detailed morphology of the microstruct ure depends on the length of the polymer blocks A(n) and B-m and their mutu al solubility. In this paper, the role of hydrodynamic forces in microphase formation is studied. The microphase separation of block copolymer melts i s simulated using two continuum methods: dissipative particle dynamics (DPD ) and Brownian dynamics (BD). Although both methods produce the correct equ ilibrium distribution of polymer chains, the BD simulation does not include hydrodynamic interactions, whereas the DPD method correctly simulates the (compressible) Navier Stokes behavior of the melt. To quantify the mesophas e structure, we introduce a new order parameter that goes beyond the usual local segregation parameter and is sensitive to the morphology of the syste m. In the DPD simulation, a melt of asymmetric block copolymers rapidly evo lves towards the hexagonal structure that is predicted by mean-field theory , and that is observed in experiments. In contrast, the BD simulation remai ns in a metastable state consisting of interconnected tubes, and fails to r each equilibrium on a reasonable time scale. This demonstrates that the hyd rodynamic forces play a critical part in the kinetics of microphase separat ion into the hexagonal phase. For symmetric block copolymers, hydrodynamics appears not to be crucial for the evolution. Consequently, the lamellar ph ase forms an order of magnitude faster than the hexagonal phase does, and t hus it would be reasonable to infer a higher viscosity for the hexagonal ph ase than for the lamellar phase. The simulations suggest that the underlyin g cause of this difference is that the hexagonal phase forms via a metastab le gyroid-like structure, and therefore forms via a nucleation-and-growth m echanism, whereas the lamellar phase is formed via spinodal decomposition. (C) 1999 American Institute of Physics. [S0021-9606(99)51819-9].