Nanostructured electrospun fibers : from superhydrophobicity to block copolymer self-assembly

Abstract

Electrospinning has emerged in recent years as a relatively easy, efficient and robust method to make ultrafine continuous fibers with diameter on the order of -100 nm from a variety of materials. As a result, numerous applications for electrospun fibers have already been demonstrated including the commercialized ones in the areas of filtration and tissue engineering. However, in most cases, the nanofibers are homogeneous; the development of external and internal nanostructures could significantly expand the applications of these fibers. The goal of this dissertation is therefore to develop controllable nanostructures for electrospun fibers with an emphasis on the understanding of structure formation and explore their unique applications. Specifically, this dissertation can be divided into two areas. The first part is related to superhydrophobic or "self-cleaning" surfaces. This has been a hot research area due to the wide applications of such materials. Electrospun fibers were first discovered in this dissertation to have sufficient surface roughness for superhydrophobic effect. In contrast to many conventional superhydrophobic surfaces, those composed of electrospun fibers are flexible, breathable and free-standing. It has been demonstrated that superhydrophobic fabrics can be made by either electrospinning a hydrophobic material or applying post-treatment to electrospun mats (e.g. through initiated chemical vapor deposition). Based on an understanding of the role of fibrous structure to create a surface of suitable topology, different strategies have been invented to enhance the superhydrophobic property and its robustness by carefully designing the external nanostructures of individual fibers using various methods such as layer-by-layer assembly. Other functionalities such as transparency and fluorescence were successfully incorporated into superhydrophobic surfaces. In particular, superhydrophobic fibrous membranes with structural colors as those displayed by some beautiful butterfly wings were produced. Besides making superhydrophobic materials from the externally nanostructured fibers, internally nanostructured electrospun fibers were also developed through the microphase separation of cylindrically confined block copolymer systems.

(cont.) This is the second part of this dissertation. Continuous nanofibers with long range order internal structure were obtained by two-fluid coaxial electrospinning in which the desired block copolymer is encapsulated as the core component within a polymer shell having a high glass transition temperature (Tg), followed by proper thermal annealing of the fibers. Various interesting, unusual and sometimes unprecedented self-assembly structures of block copolymers under cylindrical confinement have been observed. Based on quantitative analyses, the confinement was found to affect both phase type and fundamental domain sizes of the block copolymer. These internally nanostructured fibers have both practical and fundamental intellectual importance. For example, these nanofibers have unique potential for applications in optics, photonics, drug delivery, and other uses because of their small diameter, unique internal structure, and continuous filamentary nature.