<P> Polymers are one of the most prevalent types of molecules in modern life. These
long macromolecules make up everything from DNA to plastics to Jell-0™. An
interesting class of polymers are block copolymers, which are composed of two
(or more) chains, or blocks, of chemically distinct monomers covalently bonded
end-to-end to form a single polymer. Different types of polymers tend to avoid
each other, but since block copolymers are joined together the polymer species can
not macroscopically phase separate. Instead, they separate on the scale of the size
of the polymers, forming nanostructures. For a diblock copolymer melt, which is
made from two types of polymers, these nanostructures can be, depending on the
ratio of the length of one block to the other, spheres, cylinders, lamellae, or the
more bizarre gyroid phase. </p> <P> Self-consistent field theory (SCFT) as formulated by Helfand in 1975 has in recent
years been successfully applied to the study of the phase behaviour of diblock
copolymers. However, most of the studies assume that the polymers are monodisperse,
while almost all polymer melts are polydisperse. This work examines the
effect of polydispersity in the block lengths on phase behaviour of diblock copolymer
melts, by developing the SCFT for polydisperse block copolymers. The theory
is examined using a perturbation method, as well as the random-phase approximation
(RPA). The perturbation parameter is the ratio K of the weight-averaged
molecular weight and the number-averaged molecular weight, which is a common
measure of polydispersity. </p> <P> The results show polydispersity shifts the transition from a disordered phase
to an ordered phase to a higher temperature, and increases the period of the
nanostructures. It is also observed that polydispersity leads to larger non-lamellar
phase regions in the phase diagrams. Results from the RPA also suggest that
macrophase separation occurs for large polydispersities. </p> / Thesis / Master of Science (MSc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19241 |
| Date | January 2002 |
| Creators | Cooke, David |
| Contributors | Shi, A.C., Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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