Computer modelling of block copolymers under external fields
Creators: name: Dessi, Roberta
Publisher: name: University of Lincoln
Year: 2023

description: This thesis reports on modelling the morphological of Block Copolymers (BCPs) byusing computer simulation. In this work, the Cell Dynamics Simulation (CDS) methodis 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 upcylinders 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, inducinga 2x2 square lattice of cylinders, while in large boxes the system is dominated byhexagonal packing. For intermediate sizes topographical confinement has a strongerinfluence than chemical pattern confinement and can induce a better system of 3x3square lattice cylinders. For square confinement compatible with a 4x4 square latticethe tetragonal phase is observed to be a transient system leading towards a twistedhexagonal packing. Simulations have also been performed in a diamond lattice whichcan naturally accommodate hexagonal packing, and rectangular boxes which can inducebetter orientation of the hexagonal lattice along the direction parallel to the longside.Next, diblock copolymer cylinder-forming thin films confined between two parallel selectivehomogeneous walls have been investigated. By changing the value of the surfacefield and the value of the film thickness several morphologies have been observed. Inorder to tailor desired structures some of these morphologies are studied under a simplesteady shear flow. Shear flow is observed to induce a better hexagonal packing of amonolayer of perforated lamellae and of a monolayer of cylinders perpendicular to thethin film plane. A monolayer of cylinders parallel to the thin film plane with randomorientation is found to align perfectly in the shear flow direction. Further increase ofthe shear rate induces a phase transition: from one perforated lamellae layer to onelamellae layer; from a double layer of perforated lamellae to parallel cylinder layers; andfrom a monolayer of cylinders perpendicular to the thin film plane to two half parallelcylinder layers.In the end, self-assembly of lamellae-, cylinder-, bicontinuous, and sphere-forming diblockcopolymers in spherical confinement is studied. The effects of different confine-ment size and selective surface are examined systematically. The simulations reveal thata rich variety of morphologies, ranging from onion-like structures, perforated lamellae,helices structures, and the coexistence of perforated and lamellae structures, can beformed spontaneously from a randomly generated initial state. The structure diagramsof lamellae-forming diblock copolymers show that the morphologies obtained with aselective surface are similar to those obtained with a negative selective surface, butdifferent to those found with a neutral selective surface. The structural diagrams ofbicontinuous-, sphere-, and cylinder-forming diblock copolymers show that the morphologiesfound for surfaces selective for different blocks are different from each other.In the case of bicontinuous-forming diblock copolymers the morphological behaviourobtained with a neutral surface is similar to those obtained when the surface is selectivefor the minority block. In the case of sphere-forming diblock copolymers themorphologies obtained with a neutral surface are similar to those found with surfaceselective for the majority block. Instead, for cylinder-forming diblock copolymers themorphology obtained under a neutral surface is totally different from those obtainedwhen the surface is selective.