Doctoral Dissertations

Date of Award

5-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Mark D. Dadmun

Abstract

This dissertation presents work that examines the use of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer thin films as nanoporous templates. Diblock copolymer thin films are able to microphase separate to produce microdomains on the nanometer length scale. Cylindrical domains can be perpendicularly oriented when cast as a thin film on a neutral surface. From the perpendicularly oriented cylindrical microdomains, nanoporous templates can be created.

Cylindrical domain block copolymer templates can lead to the production of nanostructures, but the lack of long range ordering prevents the use of diblock copolymer template for some industrial applications. Graphoepitaxy utilizes a patterned substrate to direct epitaxial growth of long range order of the cylindrical microdomains. The role of graphoepitaxial parameters including trough width and mesa height on the ordering process of cylindrical domains in diblock copolymers thin films is monitored in this study. The quantification of order was achieved by the calculation of the order parameter from the Fast Fourier transform. An increase in order was observed in samples prepared on the mesas and in troughs of widths up to 20 μm, and mesa heights in the range of 1.0 to 5.0 L0. The role of molecular weight on the kinetics of the ordering process of cylindrical domains in diblock copolymers thin films is also examined. The kinetics of the ordering process is quantified for lower molecular weight copolymers (Mn = 63,000), but no ordering is observed for the same time scales (up to 144 hours) for larger copolymers (Mn 230,000).

The reduction of the rate of formation of long-range order is attributed to the impeded diffusion of higher molecular weight polymers. These results demonstrate that there exists an upper limit on the molecular weights of diblock copolymers that can be used to create nanoscale templates with long range order, which also translates to an upper limit in pore size and spacing in these templates.

While diblock copolymer templates are well established as a method to produce nanoscale arrays, the size scale of the template is limited by the molecular weight of the copolymer. For larger molecular weight polymers the diffusion rate is retarded which prevents the development of long range order. This work explores the use of solvent annealing to improve long range order of high molecular weight diblock copolymer templates. By annealing copolymer thin films in a solvent rich environment increases the diffusivity of the high molecular weight copolymer increases, which improves long range order over experimental time scales.

Diblock copolymer templates are an effective method for patterning nanoscale features that are able to produce vertically aligned carbon nanofibers. We report the ability to control the size and spacing of patterned nickel nanoparticles by controlling the molecular weight of the diblock copolymer template. These nanoporous templates are able to create catalytic nickel nanoparticles with diameters as small as 18 nm with a center to center spacing of 32nm. From these catalytic nanoreactors, arrays of vertically aligned carbon nanofibers are grown via plasma enhanced chemical vapor deposition.

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