Page 613 - Carrahers_Polymer_Chemistry,_Eighth_Edition

576 Carraher’s Polymer Chemistry

Meters
10 –12 10 –9 10 –6 10 –3 10 –1

Atomic Meso Bulk

Organizational level
Nano Micro Macro
Shorter Change (reaction/interaction) Time Longer

FIGURE 18.2 Relationship between organizational level and size.
be easily realigned through application of processing forces such as extrusion. To be processed, such
polymers must be in the mesogenic state below their decomposition temperature. This can be achieved
through the use of a specific solvent or the introduction of special comonomer units that allow them to

melt (or soften), but that are introduced in such a manner as to preserve their LC character. As solids,
such materials exhibit high molecular orientation, high tensile moduli (near to theoretical), poor com-
pression (i.e., little unoccupied volume), poor shear behavior, and high tensile strengths (on the order
of 4 GPa). Such materials are anisotropic conductors and generally offer good liquid and gas barrier
properties. Properties are controlled by the inherent chemical structure, molecular orientation, defect
occurrences, and stress-transfer mechanisms. Defects often act as the “weak-links” in a chain limiting
mechanical properties so that defect detection, and elimination/curtailment are important and can be
dependant on the processing conditions. Only certain processing techniques are suitable.
Accessable mesogens are formed from polymers that are thermotropic (i.e., polymers that have a
phase organization that is temperature dependent) but have an accessible isotrophic phase below their
decomposition temperature. Such polymers can be processed either when the material is in its mesogen
or ordered state using LC-type processing forming strong well-ordered products, or at temperatures
where the ordered mesogen structure is absent. In temperature-assisted systems, the material is rapidly
cooled, quenched, preventing the mesogen structure formation producing a metastable isotropic glass
or rubber. The metastable material can be processed employing less energy and force followed by a
simple annealing and slower cooling that allows the formation of the ordered mesogenic structures
along with the appearance of associated properties. Examples of assembled mesogens include groups
of polymer coils and polymers with side chains that can form such mesogens. In the former case,
tertiary-mesophase structures can be formed when the bundles of coiled chains come together.
Transient mesogens are regions present in flexible, random coil polymers often caused by appli-

cation of external forces, including simple flowing/shearing. These regions occur through local seg-

mental chain movements that happen within the chain network at points of minimum chain entropy

such as sites of entanglements. They are fibrillar-like and appear to be the nucleating phase and key
to the row and shish-kabob-like structures in oriented polymer crystals.
These latter groups include many of the so-called crystalites and crystalline regions of common
polymers.
Understanding the factors that govern the formation of mesogens will assist in determining the
processing conditions for the production of materials with specified amounts, sizes, and distribution

of such crystalline microstructures. Mesophases can be local or permeate the entire structure. They
can be large or small, and present in a random or more ordered arrangement.

18.3 FIBERS

18.3.1 POLYMER PROCESSING—SPINNING AND FIBER PRODUCTION
18.3.1.1 Introduction
Most polymeric materials are controlled by the Federal Trade Commission (FTC) with respect to the

relationship between the name and the content, including fibers. While the FTC controls industry

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