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Title: Computer modelling of block copolymers under external fields
Author: Dessi, Roberta
ISNI:       0000 0004 5919 0937
Awarding Body: University of Lincoln
Current Institution: University of Lincoln
Date of Award: 2016
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This thesis reports on modelling the morphological of Block Copolymers (BCPs) by using computer simulation. In this work, the Cell Dynamics Simulation (CDS) method is used, which is a good compromise between computational speed and physical accuracy. First, it has been performed a 3-dimensional study of diblock copolymer standing up cylinders in two different types of confinement: topographical and chemical patterns. In the case of square walls of small sizes the system is dominated by the walls, inducing a 2x2 square lattice of cylinders, while in large boxes the system is dominated by hexagonal packing. For intermediate sizes topographical confinement has a stronger influence than chemical pattern confinement and can induce a better system of 3x3 square lattice cylinders. For square confinement compatible with a 4x4 square lattice the tetragonal phase is observed to be a transient system leading towards a twisted hexagonal packing. Simulations have also been performed in a diamond lattice which can naturally accommodate hexagonal packing, and rectangular boxes which can induce better orientation of the hexagonal lattice along the direction parallel to the long side. Next, diblock copolymer cylinder-forming thin films confined between two parallel selective homogeneous walls have been investigated. By changing the value of the surface field and the value of the film thickness several morphologies have been observed. In order to tailor desired structures some of these morphologies are studied under a simple steady shear flow. Shear flow is observed to induce a better hexagonal packing of a monolayer of perforated lamellae and of a monolayer of cylinders perpendicular to the thin film plane. A monolayer of cylinders parallel to the thin film plane with random orientation is found to align perfectly in the shear flow direction. Further increase of the shear rate induces a phase transition: from one perforated lamellae layer to one lamellae layer; from a double layer of perforated lamellae to parallel cylinder layers; and from a monolayer of cylinders perpendicular to the thin film plane to two half parallel cylinder layers. In the end, self-assembly of lamellae-, cylinder-, bicontinuous, and sphere-forming diblock copolymers in spherical confinement is studied. The effects of different confine- ment size and selective surface are examined systematically. The simulations reveal that a rich variety of morphologies, ranging from onion-like structures, perforated lamellae, helices structures, and the coexistence of perforated and lamellae structures, can be formed spontaneously from a randomly generated initial state. The structure diagrams of lamellae-forming diblock copolymers show that the morphologies obtained with a selective surface are similar to those obtained with a negative selective surface, but different to those found with a neutral selective surface. The structural diagrams of bicontinuous-, sphere-, and cylinder-forming diblock copolymers show that the morphologies found for surfaces selective for different blocks are different from each other. In the case of bicontinuous-forming diblock copolymers the morphological behaviour obtained with a neutral surface is similar to those obtained when the surface is selective for the minority block. In the case of sphere-forming diblock copolymers the morphologies obtained with a neutral surface are similar to those found with surface selective for the majority block. Instead, for cylinder-forming diblock copolymers the morphology obtained under a neutral surface is totally different from those obtained when the surface is selective.
Supervisor: Zvelindovsky, Andrei ; Pinna, Marco Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available