No abstract

Six techniques are used to measure the exciton diffusion length as a function of systematic chemical modifications.

Solvent evaporation has proven to be a remarkably successful tool for directing self-assembly in block copolymers, yet the microscopic mechanisms, processing history dependence and macroscopic control parameters influencing pattern selection remain poorly understood. Here, we leverage dynamical field theory simulations to clarify how copolymer self-assembly proceeds during evaporation. We find that cylinders in the vertical orientation tend to form under modest evaporation rates and relatively weak segregation strengths, and link this behavior to nontrivial, morphology-dependent density correlations present at the ordering front.

Recently there has been significant interest in manipulating the self-assembly behavior of block copolymers to obtain structures that are not observed in the bulk. Here we explore the conditions for which self-assembly in laterally confined thin block copolymer films results in tetragonal square arrays of standing up cylinders. More specifically, we used self-consistent field theory (SCFT) to study the equilibrium phase behavior of thin films composed of a blend of AB block copolymer and A homopolymer laterally confined in square wells. By using suitable homopolymer additives and appropriately sized wells, we observed square lattices of upright B cylinders that are not stable in pure 1 AB block copolymer systems. We further investigated the optimal conditions and parameters that lead to defect-free, in-plane tetragonal ordering. Considering the potential application of such films in block copolymer lithography, we also conducted numerical SCFT simulations of the role of line edge roughness at the periphery of the square well on feature defect populations. Our results indicate that the tetragonal ordering observed under square confinement is robust to a wide range of boundary perturbations.

We examine the effects of small-scale, hexagonal, lateral confinement on microdomain ordering in AB diblock copolymer thin films using self-consistent field theory simulations. Specifically, we examine a hexagonal confinement well with side length L approximately equal to five cylindrical microdomain lattice spacings. The commensurability constraints of the small-scale, lateral confinement, coupled with surfaceinduced effects allow the confining well to have a significant effect on the perfection of microdomain order. We identify commensurability windows in L that depend on the segment-wall interaction and the "temperature" annealing rate (modeled as a Flory χ ∼ 1/T annealing rate). The effect of added majority-block homopolymer is also explored.

Recently there has been a strong interest in the area of defect formation in ordered structures on curved surfaces. Here we explore the closely related topic of self-assembly in thin block copolymer melt films confined to the surface of a sphere. Our study is based on a self-consistent field theory (SCFT) model of block copolymers that is numerically simulated by spectral collocation with a spherical harmonic basis and an extension of the Rasmussen-Kalosakas operator splitting algorithm [J. Polym. Sci. Part B: Polym. Phys. 40, 1777 (2002)]. In this model, we assume that the composition of the thin block copolymer film varies only in longitude and colatitude and is constant in the radial direction. Using this approach we are able to study the formation of defects in the lamellar and cylindrical phases, and their dependence on sphere radius. Specifically, we compute ground-state (i.e., lowest-energy) configurations on the sphere for both the cylindrical and lamellar phases. Grain boundary scars are also observed in our simulations of the cylindrical phase when the sphere radius surpasses a threshold value R_{c} approximately 5d , where d is the natural lattice spacing of the cylindrical phase, which is consistent with theoretical predictions [Bowick, Phys. Rev. B 62, 8738 (2000); Bausch, Science 299, 1716 (2003)]. A strong segregation limit approximate free energy is also presented, along with simple microdomain packing arguments, to shed light on the observed SCFT simulation results.

Equation 6, which describes the fourth-order method used to solve the modified diffusion equation, contained a superfluous q n+1 term on the right-hand side and had mislabeled subscripts. The amended equation is

Abstract. Using a diblock copolymer melt as a model system, we show that complex Langevin (CL) simulations constitute a practical method for sampling the complex weights in field theory models of polymeric fluids. Prior work has primarily focused on numerical methods for obtaining mean-field solutions-the deterministic limit of the theory. This study is the first to go beyond EulerMaruyama integration of the full stochastic CL equations. Specifically, we use analytic expressions for the linearized forces to develop improved time integration schemes for solving the nonlinear, nonlocal stochastic CL equations. These methods can decrease the computation time required by orders of magnitude. Further, we show that the spatial and temporal multiscale nature of the system can be addressed by the use of Fourier acceleration.