845 research outputs found
Self-Consistent Field Theory of Multiply-Branched Block Copolymer Melts
We present a numerical algorithm to evaluate the self-consistent field theory
for melts composed of block copolymers with multiply-branched architecture. We
present results for the case of branched copolymers with doubly-functional
groups for multiple branching generations. We discuss the stability of the
cubic phase of spherical micelles, the A15 phase, as a consequence of tendency
of the AB interfaces to conform to the polyhedral environment of the Voronoi
cell of the micelle lattice.Comment: 12 pages, 10 includes figure
Structure variation and evolution in microphase-separated grafted diblock copolymer films
The phase behavior of grafted d-polystyrene-block-poly(methyl methacrylate) diblock copolymer films is examined, with particular focus on the effect of solvent and annealing time. It was observed that the films undergo a two-step transformation from an initially disordered state, through an ordered metastable state, to the final equilibrium configuration. It was also found that altering the solvent used to wash the films, or complete removal of the solvent prior to thermal annealing using supercritical CO2, could influence the structure of the films in the metastable state, though the final equilibrium state was unaffected. To aid in the understanding to these experimental results, a series of self-consistent field theory calculations were done on a model diblock copolymer brush containing solvent. Of the different models examined, those which contained a solvent selective for the grafted polymer block most accurately matched the observed experimental behavior. We hypothesize that the structure of the films in the metastable state results from solvent enrichment of the film near the film/substrate interface in the case of films washed with solvent or faster relaxation of the nongrafted block for supercritical CO2 treated (solvent free) films. The persistence of the metastable structures was attributed to the slow reorganization of the polymer chains in the absence of solvent
Strong-Segregation Theory of Bicontinuous Phases in Block Copolymers
We compute phase diagrams for starblock copolymers in the
strong-segregation regime as a function of volume fraction , including
bicontinuous phases related to minimal surfaces (G, D, and P surfaces) as
candidate structures. We present the details of a general method to compute
free energies in the strong segregation limit, and demonstrate that the gyroid
G phase is the most nearly stable among the bicontinuous phases considered. We
explore some effects of conformational asymmetry on the topology of the phase
diagram.Comment: 14 pages, latex, 21 figures, to appear in Macromolecule
Interfaces in Diblocks: A Study of Miktoarm Star Copolymers
We study AB miktoarm star block copolymers in the strong segregation
limit, focussing on the role that the AB interface plays in determining the
phase behavior. We develop an extension of the kinked-path approach which
allows us to explore the energetic dependence on interfacial shape. We consider
a one-parameter family of interfaces to study the columnar to lamellar
transition in asymmetric stars. We compare with recent experimental results. We
discuss the stability of the A15 lattice of sphere-like micelles in the context
of interfacial energy minimization. We corroborate our theory by implementing a
numerically exact self-consistent field theory to probe the phase diagram and
the shape of the AB interface.Comment: 12 pages, 11 included figure
The non-centrosymmetric lamellar phase in blends of ABC triblock and ac diblock copolymers
The phase behaviour of blends of ABC triblock and ac diblock copolymers is
examined using self-consistent field theory. Several equilibrium lamellar
structures are observed, depending on the volume fraction of the diblocks,
phi_2, the monomer interactions, and the degrees of polymerization of the
copolymers. For segregations just above the order-disorder transition the
triblocks and diblocks mix together to form centrosymmetric lamellae. As the
segregation is increased the triblocks and diblocks spatially separate either
by macrophase-separating, or by forming a non-centrosymmetric (NCS) phase of
alternating layers of triblock and diblock (...ABCcaABCca...). The NCS phase is
stable over a narrow region near phi_2=0.4. This region is widest near the
critical point on the phase coexistence curve and narrows to terminate at a
triple point at higher segregation. Above the triple point there is two-phase
coexistence between almost pure triblock and diblock phases. The theoretical
phase diagram is consistent with experiments.Comment: 9 pages, 8 figures, submitted to Macromolecule
Phase diagram for diblock copolymer melts under cylindrical confinement
We extensively study the phase diagram of a diblock copolymer melt confined
in a cylindrical nanopore using real-space self-consistent mean-field theory.
We discover a rich variety of new two-dimensional equilibrium structures that
have no analog in the unconfined system. These include non-hexagonally
coordinated cylinder phases and structures intermediate between lamellae and
cylinders. We map the stability regions and phase boundaries for all the
structures we find. As the pore radius is decreased, the pore accommodates
fewer cylindrical domains and structural transitions occur as cylinders are
eliminated. Our results are consistent with experiments, but we also predict
phases yet to be observed.Comment: 12 pages, 3 figures. submitted to Physical Review Letter
Partial integration and local mean-field approach for a vector lattice model of microemulsions
A vector model on the simple cubic lattice, describing a mixture of water, oil, and amphiphile, is considered. An integration over the amphiphile orientational degrees of freedom is performed exactly in order to obtain an effective Hamiltonian for the system. The resulting model is a three-state (spin-1) system and contains many-site interaction terms. The analysis of the ground state reveals the presence of the water-oil-rich phase as well as the amphiphile-rich and the cubic phases. The temperature phase diagram of the system is analyzed in a local mean-field approach, and a triple line of water-rich, oil-rich, and microemulsion coexistence is obtained. For some values of the model parameters, lamellar phases also appear in the system, but only at finite temperature. The Lifshitz line is determined in a semianalytical way in order to locate the microemulsion region of the disordered phase
Block Copolymer at Nano-Patterned Surfaces
We present numerical calculations of lamellar phases of block copolymers at
patterned surfaces. We model symmetric di-block copolymer films forming
lamellar phases and the effect of geometrical and chemical surface patterning
on the alignment and orientation of lamellar phases. The calculations are done
within self-consistent field theory (SCFT), where the semi-implicit relaxation
scheme is used to solve the diffusion equation. Two specific set-ups, motivated
by recent experiments, are investigated. In the first, the film is placed on
top of a surface imprinted with long chemical stripes. The stripes interact
more favorably with one of the two blocks and induce a perpendicular
orientation in a large range of system parameters. However, the system is found
to be sensitive to its initial conditions, and sometimes gets trapped into a
metastable mixed state composed of domains in parallel and perpendicular
orientations. In a second set-up, we study the film structure and orientation
when it is pressed against a hard grooved mold. The mold surface prefers one of
the two components and this set-up is found to be superior for inducing a
perfect perpendicular lamellar orientation for a wide range of system
parameters
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