Date of Award

5-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Advisor

Perahia, Dvora

Committee Member

Stuart , Steve

Committee Member

Smith , Rhett C

Abstract

This research focuses on the structure and dynamics of a family of rigid polymers, poly(para-phenyleneethynylene)s (PPEs) in solutions and thin films using small angle neutron scattering, neutron spin echo, nuclear magnetic resonance, X-ray scattering, atomic force microscopy and fluorescence spectroscopy. The effects of the solvent and the side chain on the molecular conformation and the association of PPEs have been investigated.
In dilute solutions, the phase diagram of dinonyl PPE in toluene and cyclohexane exhibits three distinct phases. At higher temperatures, PPEs form molecular solutions with extended conformations in toluene and worm-like chains in cyclohexane. With decreasing temperatures, PPEs associate into flat aggregates, and then jam into fragile gels. The aggregates in cyclohexane are more densely packed than in toluene. The dynamic studies characterize the motion of all the components of the complex fluid in the molecular solution, the micellar phase and the gel. While toluene is coupled to the PPE molecule, cyclohexane is only slightly affected by the association. PPEs with bulky side chains in toluene form a gel by interlocking to each other and form a stable network of rigid rods.
In thin films, the morphology and structure are affected by the structure of the original solutions, the solvent evaporation rates and polymer structure including poly-dispersed length and nature of the side chains. Cast from the molecular solution and the gel at different temperatures, the thin films exhibit nanoaggregates and supramolecular structures of nanoribbons. Below their overlap concentration, PPEs collapse in thin films. PPEs with bulky or long side chains show a steric hindrance in the packing in thin films. The structure of PPEs in thin films is reflected in their fluorescence spectra. All the films exhibit different degrees of red-shifting in fluorescence spectra.

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