Liquid Crystalline Polymers (1990) / Chapter Skim
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4 PROBLEMS AND ISSUES: RECOMMENDATIONS FOR FURTHER WORK
4 PROBLEMS AND ISSUES: RECOMMENDATIONS FOR FURTHER WORK
Pages 71-100
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From page 71... ...
In this case and in general, mesomorphic structure is readily frozen into a glassy mesophase without crystallization as the polymer is cooled below its glass transition temperature (T~. Retention of mesophase positional alignment and dipolar orientation upon electric field poling while cooling below To can lead to bulk noncentrosymmetry in a mesomorphic system (Meredith et al., 1982)
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From page 72... ...
LCPs. Opportunities for exploiting the interplay of function and chemical structure in these anisotropic systems should be taken where their structural anisotropy and glassy mesophase formation can lead to enhanced performance.
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From page 73... ...
Future Cost Challenges The cost challenges inherent in developing future thermotropic LCPs will be either (a) to develop higher-temperature properties utilizing the lowercost hydroquinone or (b)
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From page 74... ...
The primary difficulty in developing a comprehensive picture of the thermodynamic aspects of LCPs lies in the fact that a high degree of coupling exists between all degrees of freedom in the system, internal and external to the polymer chains. Contrary to the situation in polymer melts and in amorphous systems, where chains can assume all conformations and the external constraints on a chain are essentially isotropic, in the liquid crystalline phase the necessity for the macromolecule to conform to the anisotropic spatial requirements of its environment reduces drastically its freedom in conformation (internal)
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From page 75... ...
. · Mixtures of ideal random coils and rigid rods also fractionate strongly (Flory, 1984)
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From page 76... ...
/X3 . Delis is a very rapidly decreasing function of the aspect ratio x ; long rigid rods rotate very slowly.
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From page 77... ...
is incomplete in that it does not contain any elastic contribution to the stress resulting from local director gradients; the Doi theory thus apparently cannot predict any phenomena associated with rapid spatial changes in orientation, although an extension of the theory that includes director gradient, has been reported (Beris and Edwards, 1990~. Neither the Leslie-Ericksen nor the Doi theory has been applied to the flow of fluids with domain-like textures; it is unlikely that the Doi theory in its present form can be applied here because of the limitations already cited.
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From page 78... ...
Molecular composites- a molecular dispersion of (lyotropic) rigid rod polymers in a (conventional)
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From page 79... ...
Herein we focus on novel processing possibilities for thermotropic LCPs. Solid-State Forming In studies of the extrusion of powder preforms of Xydar~ (a terpolymer of p-hydroxybenzoic acid, terephthalic acid, and biphenol)
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From page 80... ...
In the case of composites, the temperature resistance and tensile properties of fibers must also be combined with outstanding compressive properties and adhesion in fabrication schemes. For the LCP fibers to be effective in high-performance structural composites, they should also exhibit compressive strength that approaches the compressive properties of current carbon fibers.
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From page 81... ...
MECHANICAL PROPERTIES Tensile Properties Remote from major transitions, axial properties of solid-state filaments formed from LCPs are excellent and approach the theoretical limit in tensile modulus and exhibit extraordinary high tensile strength. Some typical properties of commercial fibers were shown in Figure 3.1.
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From page 82... ...
The outstanding problem is this: Can compressive properties of LCPs be improved without sacrificing desirable tensile properties? A major question in this regard is whether the fault is at the molecular or morphological level.
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in compressive properties were offset by substantial losses in tensile properties. No successful composite technology based on cross-linked fibers and no successful cross-linking of LCP fibers has been reported as yet.
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. RECOMMENDATIONS Conventional molding techniques exaggerate the weld line problem in · A significant research effort should focus on the compressive failure problem.
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From page 85... ...
For example, para-aramid composites are metal-like in ductile energy absorption and respond to compressive stress noncatastrophically, unlike carbon composites, which have a brittle or catastrophic response (International Encyclopedia of Composites, 1989~. The industry has recognized the benefits of combining the inherent damage tolerance of the LCP fibers with the compressive properties of carbon fiber in the form of hybrid composite structures.
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From page 86... ...
Kev~ar ~Thorny 300 Property 149 Aramid 49 Aramid Graphite Tensile Strength MPa 1450 1500 1420 Kpsi 210 218 206 Tensile Modulus GPa 107 79 133 Mpsi 15.6 11.5 19.3 % Conversion 99 99 97 Strain to Failure % 1.33 1.71 0.90 Compressive Strength MPa 193 234 Kpsi 28 34 Compressive Modulus GPa 73 66 Mpsi 10.6 9.5 Flexura] Strength MPa 634 655 1192 Kpsi 92 95 173 F]
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From page 87... ...
87 TABLE 4 .1B Properties of Unidirectional Epoxy Compos ite with 60% Fiber Loading* Property H.T.
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From page 88... ...
The potential utility of LCPs as a processing aid for high-viscosity conventional polymers was rigorously pursued by ICI in the early 1980s (Cogswell et al., 1983~. Although the desired viscosity lowering appears to be dominated by the ratio of the viscosities of the components of the mixture, LCPs are unique in possessing both high molecular weight and low viscosity.
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From page 89... ...
Blending is unlikely to solve the weld line problem (Kiss, 1987~. Mechanical properties of the blends, especially tensile modulus, follow expectations consistent with simple composite concepts in the absence of adhesion between matrix and reinforcement.
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From page 90... ...
As sparse as the data set describing mainchain nematic LCP blends with conventional polymers is, it is rich compared to the almost nonexistent data on the blending of other types of LCPs sidechain polymers, flexible spacer polymers, smectics, etc. LCP-LCP Blends Researchers have studied blends of thermotropic copolyesters with other thermotropic copolyesters, examining both blends of different copolymer ratios with identical chemistry (HBA-HNA type)
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From page 91... ...
The concepts underlying molecular composite physics are consistent with the concepts of miscible blends; the materials being produced, even at the very small sizes of rod structures observed, fit the definitions associated with immiscible LCPconventional polymer blends. Molecular composites can be treated with the already established framework of polymer-polymer mixing and do not require new concepts for accurate description.
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From page 92... ...
In the case of second-order NLO applications, some obstacles are these: · The uncertainty with respect to thermal stability of electric fieldinduced ordering in harsh environments, i.e., greater than 80°C. · The small published data base on NLO properties, physical properties, and processing conditions for making active and passive structures in polymer films, and the sparse literature on second-order NLO properties of LCPs.
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From page 93... ...
are unclear at this · The stringent linear optical properties, including optical uniformity, transparency, and low scattering losses, combined with large X(3) required for device applications , have not yet been demonstrated, and formidable obstacles associated with -C-H vibrational overtones in the 1.3-pm and 1.55-pm regions remain to be addressed.
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From page 94... ...
· For third-order NLO processes, LCPs offer ~ route to achieving uniaxial alignment through processing and the associated potential for a fivefold increase in nonlinear coefficient relative to an electronically identical amorphous system, although considerable progress in fundamental understanding of microscopic processes, materials design and synthesis, and processing will be required to take advantage of this. RECOMMENDATIONS ~ Research initiatives on organic and polymeric materials for secondorder NLO addressing the issues of stability and magnitude of alignment by electric field poling, spontaneous or self-alignment, physical property studies, processing, and exploratory device utility should be supported.
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From page 95... ...
1981. Molecular dynamics and rheological properties of concentrated solutions of rodlike polymers in isotropic and liquid crystalline phases.
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1981. Investigations on liquid crystalline polysiloxanes .3.
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1985. Liquid crystalline polymers as additives to enhance the device properties of low molecular mass liquid crystals.
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From page 98... ...
1985. On the theory of liquid crystalline ordering of polymer-chains with limited flexibility.
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From page 99... ...
Report to the Committee on Liquid Crystalline Polymers . Cambridge, Mass .
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