Bin Yu, University of South Carolina


The Self-Assembly of Block Copolymers in Confinement: A Simulated Annealing study

Abstract: Block copolymers are typical soft matter and ideal systems for investigating the behavior of self-assembly. Besides, they are connected intimately with life science. Block copolymer molecules consist of two or more chemically distinct polymer chains (blocks) covalently bonded at one end. Block copolymers have the most fascinating characteristic of most soft matter, that is, they can form rich variety of microstructures at nanoscale via self-assembly. Potential applications of block copolymer microstructures include nano-lithographic, drug delivery vehicles, templates for nanowires and high-density magnetic storage media. The bulk equilibrium morphologies of the simplest case of diblock copolymers, which are linear polymers composed of two different subchains, have been extensively studied experimentally and theoretically. A variety of ordered bulk phases, including lamellae, hexagonally packed cylinders, body-centered-cubic spheres, and a bicontinuous network structure called gyroid are observed in the diblock copolymer melts. Recently, nanoconfinement is frequently used in experiments to induce long range ordering of microstructures resulting from the self-assembly of block copolymers. On the other hand, confinement is a powerful tool in breaking the symmetry of a structure, thus allowing materials to demonstrate new behavior. It has been shown that nanoconfinement of block copolymers can be used to produce novel morphologies with potentially novel applications. Another way of producing novel morphologies is increasing the number of blocks in block copolymers. For example, ABC triblock copolymers, made up of three chemically distinct blocks connected in an A-B-C sequence, can form many microstructures that cannot form from AB diblock copolymers. We systematically investigated the self-assembly behavior of block copolymers under various confined states using simulated annealing technique. On one hand, based on the idea of confinement inducing broken of symmetry of a structure, we predict novel morphologies that cannot form in the bulk, through imposing confinement with plane or curved surfaces of different sizes and shapes. On the other hand, we systematically studied the influence of confinement effects, including the degree of structural frustration and surface-polymer interactions, on the self-assembled microstructures. We elucidated the underlying rules that the self-assembled structures vary with the various parameters. We also elucidated the origin of the confinement-induced morphologies and the mechanism for the morphological transitions between different morphologies based on energetics.

Mentor: Qi Wang (University of South Carolina).