Fabrication of the 1-Dimensional Nanostructured Polymeric Materials Based on Wetting Phenomena and Their Analysis

Abstract

1-Dimensional (1-D) nanostructured materials have been studied due to their special properties and various applications in the field of optics, electronics and magnetics. Among lots of methods for the fabrication of 1-D nanostructure, polymer wetting in the nanoporous template is one of the most useful ways in the fabrication of polymer based 1-D materials. This wetting phenomenon, which can be described spreading coefficient (S), is related to the surface energy of the liquid state of the polymer and a substrate. Because most polymers can spread on the substrate having high surface energy, liquid state of polymers easily infiltrate into the nanoporous template. Various types of nanostructures can be made by adjusting the condition of the wetting, such as molecular weight, annealing temperature, and surface energies. Among several templates for infiltration of the polymers, anodized aluminum oxide (AAO) is widely used template having cylindrical nanopores. AAO with a highly areal density (up to 1011 pores/cm2) and narrow size distribution in a large area has received much attention because of simple and inexpensive control of structural parameters (diameter, height, interpore distance) and excellent thermal and mechanical stabilities. In this thesis, new types of structures which are difficult to be fabricated by previous wetting approach is studied and the application will be discussed.In chapter 2, hexagonally packed polymeric core-shell nanorods were fabricated by a sequential complete wetting from a reactive polymer bilayer inside the nanoporous alumina template. From a reaction between an epoxy group in polymethylmethacrylate (PMMA) and the carboxylic acid group in polystyrene (PS), in-situ PS-graft-PMMA copolymers were formed near the interface of the bilayer. These graft copolymers enhanced the complete wetting of the PMMA onto the pre-made PS wall, resulting in core-shell polymeric nanorods. Without formation of these graft copolymers at the interface, the core-shell nanorods were not formed because of negative spreading coefficient of PMMA and PS. Sequential complete wetting could be extended to various polymer pairs having negative spreading coefficients by simply introducing the functional groups to each polymer.In chapter 3, we successfully fabricated highly ordered nanoporous anodized aluminum oxide (AAO) template on various conducting substrates by surface modification of the substrates. Due to the excellent adhesion between AAO and the substrates, this template could be universally utilized to fabricate laterally long-range ordering and hexagonally packed arrays of free standing and vertically aligned metal, semiconductor, and conducting polymer nanorods by electrodeposition, even after the complete removal of the AAO template. A particularly important example is to fabricate the ultrahigh density array of free standing and vertically aligned poly (3-hexylthiopene) nanorods on a flexible substrate, which could be employed to achieve high power conversion efficiency of polymer solar cells on the flexible substrate. This highly ordered array of nanorods could be used for opto-electronics, high density data storage materials, sensors, and photovoltaic cells with high conversion efficiency. In chapter 4, an ultrahigh density array of vertically and laterally aligned poly (3-hexylthiophene) (P3HT) nanotubes on conducting substrates was successfully fabricated by solution wetting in the anodized aluminum oxide (AAO) template. After solvent annealing, the conductivity of P3HT nanotubes was significantly increased due to highly aligned P3HT chains along the nanotube direction (or perpendicular to the substrate). This approach also provides a facile route for the preparation of ultrahigh density array of various conjugated polymer nanotubes. The conducting polymer nanotube array could be used for high performance organic devices, such as sensors, organic photovoltaic cells, and electrochromic devices.