Structure-property relations in liquid crystalline and thermoplastic polymers: Crystal structure and morphology of liquid crystalline polyethers and the interaction of creep and physical aging in a semicrystalline thermoplastic matrix composite

Dean, Derrick Resha

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https://hdl.handle.net/2142/22486 Copy

Description

Title

Structure-property relations in liquid crystalline and thermoplastic polymers: Crystal structure and morphology of liquid crystalline polyethers and the interaction of creep and physical aging in a semicrystalline thermoplastic matrix composite

Author(s)

Dean, Derrick Resha

Issue Date

1994

Doctoral Committee Chair(s)

Geil, Phillip H.

Department of Study

Materials Science and Engineering

Discipline

Materials Science and Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Engineering, Materials Science

Language

eng

Abstract

• The crystal structure and morphology of a family of aromatic-aliphatic azomethine polyethers (where the aliphatic unit was composed of CH$\sb2$ units ranging in length from n = 1, 4, 7, 8, 9 and 10) has been examined primarily by Electron microscopy(EM) and Electron diffraction(ED).
• Chain folded single crystals with lamellar thicknesses of 35-100A, in which the molecular axis is perpendicular to the substrate, have been grown by slow cooling thin films from the nematic state on a glycerin surface for n = 4, 8 and 10 with AZMEP-7 yielding disclination domains with isolated regions of chain folded lamellae. For as-polymerized AZMEP-10, the lamellae were found to be 35A thick The thinness of the lamellae and their tilt relative to the substrate and molecular axis suggests a regular, adjacent re-entry type folding every three repeat distances. AZMEP-1 was found to crystallize as striated films of near uniform thickness; in these samples, the molecular axes are parallel to the substrate, yielding fiber-type ED patterns. AZMEP-9 samples yielded a similar morphology, but no ED patterns were obtained.
• Our findings on samples crystallized as thin films has added to earlier findings by S. Kent and P. H. Geil, which gave the first conclusive evidence of chain folding in lcps. Claims were also made by them, and is supported by our findings, of the presence of chain folding in the nematic state.
• Sheared samples with lamellae ca. 250A and more or less perpendicular to the shear direction have been obtained for n = 1, 4, 7, 8, 9 and 10. Annealing below T$\rm\sb{k-m}$ has been shown to increase the lamellar thickness and overall order. ED from sheared, annealed samples surprisingly yield h01 single crystal patterns rather than the expected fiber patterns, which have rotational symmetry. This finding is unique, as far as we know, for sheared polymer samples. The h01 single crystal nature of these samples was verified by dark field EM.
• Lamellar single crystals (ca. 100A thick) have been grown by the constrained film melt polymerization technique for n = 4, 8 and 9, yielding single crystals suitable for ED. The ED data from these samples, in conjunction with data obtained from the sheared samples, has permitted characterization of the unit cells.
• We have examined creep above and below the nominal Tg in both the viscoelastic and viscoplastic load ranges for AS4/J1 uniaxial composites. Surprisingly, long term creep in tension does not appear to be a problem; it was found to be compensated by shrinkage due to physical aging, resulting in negative creep in the viscoelastic load range, and near zero creep in the viscoplastic load range. This fact makes the time-temperature superposition principle invalid for long term predictions; it actually overpredicts it in tension. The fact that physical aging occurs above Tg in subcrystalline polymers is attributed to the presence of a distributed Tg.

Type of Resource

text

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Copyright 1994 Dean, Derrick Resha

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