Morphology and crystal structure of random, main chain, semi-flexible, thermotropic liquid crystal polymers
Kent, Susan Louise
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https://hdl.handle.net/2142/21202
Description
Title
Morphology and crystal structure of random, main chain, semi-flexible, thermotropic liquid crystal polymers
Author(s)
Kent, Susan Louise
Issue Date
1992
Doctoral Committee Chair(s)
Geil, Phillip H.
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Polymer
Engineering, Materials Science
Plastics Technology
Language
eng
Abstract
"The crystal structure and morphology of random, main chain, thermotropic liquid crystal polymers (LCP's) of equimolar amounts of the following structural units (with n equal to 4, 5, 6 and 7 in ""A"") have been examined by electron microscopy and diffraction:(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)"
"When crystallized from the liquid crystal state in the form of thin films, the polymer forms a 50-70A lamellar texture. Electron diffraction reveals an orthorhombic unit cell (a = 8.34A and b = 5.10A). Similar results were obtained with the 4, 6 and 7 carbon spacer LCP's. The observations can only be interpreted in terms of chain folding, the flexibility of the ""spacer"" along the polymers backbone permitting folding to occur. High temperature electron diffraction shows little change in positional ordering above the crystal-liquid crystal transition temperature. Rapid quenching from the liquid crystal state produces a ribbon-like morphology with the molecules oriented perpendicular to the long direction of the ribbons."
Shearing the 7 carbon spacer LCP in the liquid crystal state results in $\sim$150A lamellar crystals normal to the shear direction. The lamellae readily shear over one another, suggesting the presence of adjacent reentry chain folding.
Single crystals of the 7 carbon flexible spacer LCP have been grown from dilute solution. This is the first report of solution grown single crystals of a liquid crystalline polymer. The crystals thicken (from $\sim$100A to 190A) upon annealing below their crystal-liquid crystal melting temperature. Annealing in the liquid crystalline state produces unusual and unique morphologies.
"Extruded pellets of the 7 carbon flexible spacer LCP resemble a composite material with 0.2 $-$ 1.8$\mu$m ""fibers"" oriented parallel to the extrusion direction. Etching revealed a lamellar morphology with the lamellae (50-1,800A) oriented perpendicular to the fiber extrusion direction. Thinner lamellae (50-130A) were observed on the surface of compression-molded articles with thicker, presumably extended chain lamellae, observed in the interior. Selected area electron diffraction confirms that the molecular axes are perpendicular to the lamellae."
The results suggest significant degrees of chain folding in the liquid crystal state since it is unexpected that previously extended, nematically packed chains would fold during the liquid crystal-crystal transition.
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