Spinodal Decomposition in a Binary Polymer Mixture: Dynamic Self Consistent Field Theory and Monte Carlo Simulations

We investigate how the dynamics of a single chain influences the kinetics of early stage phase separation in a symmetric binary polymer mixture. We consider quenches from the disordered phase into the region of spinodal instability. On a mean field level we approach this problem with two methods: a dynamical extension of the self consistent field theory for Gaussian chains, with the density variables evolving in time, and the method of the external potential dynamics where the effective external fields are propagated in time. Different wave vector dependencies of the kinetic coefficient are taken into account. These early stages of spinodal decomposition are also studied through Monte Carlo simulations employing the bond fluctuation model that maps the chains -- in our case with 64 effective segments -- on a coarse grained lattice. The results obtained through self consistent field calculations and Monte Carlo simulations can be compared because the time, length, and temperature scales are mapped onto each other through the diffusion constant, the chain extension, and the energy of mixing. The quantitative comparison of the relaxation rate of the global structure factor shows that a kinetic coefficient according to the Rouse model gives a much better agreement than a local, i.e. wave vector independent, kinetic factor. Including fluctuations in the self consistent field calculations leads to a shorter time span of spinodal behaviour and a reduction of the relaxation rate for smaller wave vectors and prevents the relaxation rate from becoming negative for larger values of the wave vector. This is also in agreement with the simulation results.Comment: Phys.Rev.E in prin

Influence of confinement on the orientational phase transitions in the lamellar phase of a block copolymer melt under shear flow

In this work we incorporate some real-system effects into the theory of orientational phase transitions under shear flow (M. E. Cates and S. T. Milner, Phys. Rev. Lett. v.62, p.1856 (1989) and G. H. Fredrickson, J. Rheol. v.38, p.1045 (1994)). In particular, we study the influence of the shear-cell boundaries on the orientation of the lamellar phase. We predict that at low shear rates the parallel orientation appears to be stable. We show that there is a critical value of the shear rate at which the parallel orientation loses its stability and the perpendicular one appears immediately below the spinodal. We associate this transition with a crossover from the fluctuation to the mean-field behaviour. At lower temperatures the stability of the parallel orientation is restored. We find that the region of stability of the perpendicular orientation rapidly decreases as shear rate increases. This behaviour might be misinterpreted as an additional perpendicular to parallel transition recently discussed in literature.Comment: 25 pages, 4 figures, submitted to Phys. Rev.

Mesoscopic phase separation dynamics of compressible copolymer melts

In this paper we extend the dynamic mean-field density functional method, derived from the generalized time-dependent Ginzburg-Landau theory, to the mesoscopic dynamics of compressible polymer liquids. We discuss and compare different classes of compressibility models: exactly incompressible, the Helfand's harmonic penalty model, and a cell model. We present numerical results and show that the penalty model is a very practical and easy to use solution. In the current nVT ensemble dynamics algorithms application of the cell model leads to a variation of the pressure and, depending on conditions, the system develops liquid-gas transitions. We show that the morphology of a phase separated diblock copolymer melt around a gas bubble has intruiging structures, with lamellar phases oriented towards the gas-liquid interface

Functional Langevin models for the mesoscopic dynamics of surfactant aggregation in solution

We discuss a time-dependent potential model for the simulation of surfactant aggregation In solution. The numerical model is derived from a generalization of time-dependent Ginzburg-Landau theory or conserved order parameters. An element in our coarse-grained approach is that we retain important aspects of molecular detail by inclusion of single-chain density functionals. Representative results of simulations of concentrated dioctadecylamine solutions are discussed. We find that multicomponent coarse-grained simulations are indeed feasible, and may increase our understanding of a wide variety of mesoscopic aggregation processes in complex surfactant solutions. A conspicuous result is that thermal fluctuations greatly influence the formation of the aggregate structures