Effect of surface field on the morphology of a symmetric diblock copolymer under cylindrical confinement

Self-assembly of diblock copolymers confined in cylindrical nanopores

Self-assembly of AB diblock copolymers confined in cylindrical nanopores is studied using a simulated annealing technique. The pore diameter and surface preference are systematically varied to examine their effects on the self-assembled morphologies and the chain conformations. For bulk lamella-forming and cylinder-forming diblock copolymers, novel structures such as helices and concentric (perforated) lamellae spontaneously form when the copolymers are confined in cylindrical pores. The observed equilibrium morphologies are compared with that obtained from experiments, theory, and other simulations. A simple model is proposed for symmetric diblock copolymers, which gives a reasonable description of the layer thickness for the concentric lamellae. It is found that chains near the pore surfaces are compressed relative to the bulk chains, which can be attributed to the existence of the surfaces. The dependence of the chain conformation on the degree of confinement and strength of the surface preference are reasonably explained. The energetics is discussed qualitatively and used to account for the appearance of the complex phase behavior observed for certain intermediate conditions.

Self-assembled morphologies of diblock copolymers confined in nanochannels: Effects of confinement geometry

The self-assembly of diblock copolymers confined in channels of various shaped cross sections is studied using a simulated annealing technique with the "single-site bond fluctuation" model. In the bulk, the asymmetric diblock copolymers used in this study form hexagonally packed cylinders with period L0. The cross sections of the confining channels are of different shapes including regular triangles, rectangles, squares, regular hexagons, regular octagons, and ellipses. For a given geometry, the channel size (characterized by one or two lengths) is varied from very small to several times of L0. It is found that the geometry and size of the confining channels have a large effect on the structure and symmetry of the self-assembled morphologies. Multiple packed cylinders with the symmetry of the confining channels are the major morphologies for low-symmetry cross sections such as triangle, rectangle, and square. More complex structures such as helices or stacked toroids spontaneously form when the confining channels are shaped such as a regular hexagon, a regular octagon, or an ellipse. The domain spacing of the self-assembled structures can be altered by the shape and size of the confining channels. Our results are consistent with available experiments. These results indicate that the self-assembled structures of block copolymers can be manipulated by the shape of the confining channels.

Self-assembling of ABC linear triblock copolymers in nanocylindrical tubes

By employing Monte Carlo simulations for various tube diameters and preferences of the tube surface for the A, B, and C segments, the morphologies of A(5)B(5)C(5), A(5)B(10)C(5), and A(5)B(5)C(10) triblock copolymer melts confined in nanocylindrical tubes were examined. The interaction parameters between different segments were considered constant epsilon(AB)=epsilon(AC)=epsilon(BC)=0.3k(B)T, the tube diameter was changed from d=9xlattice parameter to d=33xlattice parameter, and the preferences of the tube surface for the segments A, B, and C (-epsilon(AS),-epsilon(BS), and -epsilon(CS)) were varied between 0.05k(B)T and k(B)T. ABCCBA alternately stacked disks were generated in most tubes when the preference of the tube surface for any of the segments was weak, and the morphologies tended to transform into curved lamellae in tubes with large diameters when the preference for one of the segments was high. Numerous novel morphologies, such as ABC double helixes, AB single helix+C double helixes, AB double helixes+C quadruple helixes, plate morphologies with fins, dendrites, etc., which were located in the phase diagram between the stacked disks and the curved lamellar structures, were identified. Additionally, the orientation parameters indicating the alignments of the polymer chains were calculated and correlated with the morphologies.

Symmetric diblock copolymers in nanopores: Monte Carlo simulations and strong-stretching theory

We have performed lattice Monte Carlo simulations to study the self-assembled morphology of symmetric diblock copolymers in nanopores. The pore diameter and surface preference are systematically varied to examine their effects on the chain conformations, structures of various morphologies, and their phase transition. Various ensemble-averaged profiles and quantities are used to provide detailed information about the system. The simulation results are also compared with the predictions of a strong-stretching theory commonly used in the literature. Such comparisons reveal the deficiencies of this theory in describing the morphologies under cylindrical confinement, and call for further theoretical studies using more accurate formalisms.

Electrospun polymer nanofibers with internal periodic structure obtained by microphase separation of cylindrically confined block copolymers

Continuous fibers are described having concentric layer or aligned sphere microphase-separated, styrene-isoprene block copolymer morphologies. The fibers are obtained by a two-fluid coaxial electrospinning technique in which the desired block copolymer is encapsulated as the core component within a polymer shell having a high glass transition temperature (Tg). The fibers range in diameter from 300 to 800 nm, and the block copolymer core ranges from 50 to 500 nm. Subsequent annealing of the fibers above the upper Tg of the block copolymer but below the Tg of the shell polymer results in microphase separation of the block copolymer under cylindrical confinement. The resulting fibers exhibit improved long-range order. This two-step strategy creates the opportunity to fabricate continuous nanofibers with periodic internal structure.

Morphology transitions of AB diblock copolymer melts confined in nanocylindrical tubes

By employing Monte Carlo simulations for various values for the interactions energies epsilonAB between the beads A and B, the selectivity of the surface for one of the beads, and the diameter d of the nanotube, the morphology transitions in A5B5 and A7B3 diblock copolymer melts were investigated. In symmetrical systems and for a negligible preference of the surface for A and B and not too large values of d, as epsilonAB increases, increasingly thicker stacked-disk macrodomains normal to the tube surface were formed. When epsilonAB became sufficiently large, a configuration transition to helixes occurred which became deformed at larger values of epsilonAB. The helixes contained an imperfect single helix of A coupled with an imperfect single helix of B or imperfect double helixes of A coupled with imperfect double helixes of B. When at constant and relatively large epsilonAB, the attractive interaction epsilonAS between the A bead and the surface was increased, a transition from a succession consisting of stacked disks and a helix to a helical one occurred, which changed to a circular lamellar structure at a sufficiently large attraction epsilonAS by the surface. When the diameter d was increased, in addition to the helixes already mentioned imperfect triple helixes of A coupled with triple helixes of B were identified. In the asymmetrical case, two kinds of helixes were observed, namely, those identified in the symmetrical case, as well as a helix formed by one kind of beads immersed in the matrix of the other one.

Monte Carlo Simulations of Cylinder-Forming ABC Triblock Terpolymer Thin Films

We systematically study the cylinder-forming ABC triblock terpolymer thin films using canonical ensemble Monte Carlo simulations. The simulated annealing procedure is applied to the self-assembling process. By judicious choice of the system dimensions, we elaborately investigate the effect of film thickness on the orientation of the cylinders. This confined triblock terpolymer system exhibits different phase behavior under the weak and strong surface fields. In addition, we also investigate the ensemble-averaged chain orientations and relative density profiles.