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CONCLUSIONS AND RECOMMENDATIONS
The following are the major conclusions and recommendations. More
detailed information can be found in the following chapters of the report.
MACROMOLECULAR DESIGN AND SYNTHESIS
Thermotropic LCPs {Mainchain)
Conclusions
· Among the serious molecular design issues affecting copolyester
syntheses are the elucidation and control of the mer sequence along the
polymer backbone. Until these are accomplished, the structure-property
relationships associated with backbone sequence will be unclear and subsequent
design of improved polymers will be severely hampered. Future challenges
facing widespread utilization of thermotropic LCPs are economic in nature.
is necessary to develop higher-temperature properties utilizing lower-cost
monomers, i.e., to develop low-cost syntheses of key (naphthalene- and
biphenyl-based) monomers or to develop new low-cost (aromatic) monomers.
Recommenda Lions
· Develop new polyester polymerization techniques with the associated
catalysts and transesterification inhibitors that yield and maintain specific
mer sequences.
· Continue efforts to measure mer sequence distribution, e.g., by
multinuclei NMR.
· Develop high-temperature properties utilizing lower-cost aromatic
monomers, e . g., hydroquinone.
· Develop efficient direct coupling of simple monoaromatic functional
molecules to prepare low-cost biphenyl A-A and A-B monomers, e.g., A = -COOH
B = -OH.
5
It
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Thermotropic LCPs(Sidechain]
Conclusion
· Sidechain LCPs have great potential as functional polymers.
Development of properties dependent on noncentrosymmetric structures such as
certain nonlinear optical effects, piezoelectricity, and pyroelectricity is
possible via selective chemistry. Ferroelectric behavior has been observed in
smectic C sidechain LCPs. Since surface-stabilized ferroelectric liquid
crystals are in a sense "self-poling," a combination of ferroelectric and, for
example, nonlinear optical properties in a single material would be
attractive. In addition, sidechain LCPs as elastomers and as high-performance
coatings warrant more consideration.
Recommends Lions
· Create opportunities that explo it novel functional s idechain LCPs via
the interplay of function and chemical structure in these anisotropic systems,
particularly in areas where their structural anisotropy and elastomeric and/or
glassy mesophase formation can lead to new applications and enhanced
performance, e.g., optical storage and wave guide media.
· Encourage design and synthesis of sidechain LCPs for nonlinear optical
properties in integrated programs of characterization and evaluation.
· Develop synthesis of sidechain LCPs that exhibit novel properties such
as ferroelectricity, ferromagnetism, and elasticity.
· Evaluate sidechain LCPs as coatings on substrates and as films for use
as potential high-performance materials.
LYotropics
Concl usion
· Perhaps the biggest challenge in the future of advanced high-
performance materials from lyotropic polymers will be the development of low-
cost synthetic routes to these more complex polymer precursors, and subsequent
facile polymerization schemes. Both factors are essential for a consistent
and ample supply of polymer from commercial sources. Reliable and
reproducible supply is, in turn, a prerequisite for evaluating processing
procedures that are directed toward fabricating bulk sample alternatives to
uniaxial materials.
Recommends Lions
· Seek compositional modifications of aramid and benzobisoxazole
~ thiazole ~ polymers that enhance solubility in innocuous solvents while
maintaining properties of the polymers in the solid state.
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7
o Pursue compositional concepts that lower rigid monomer costs and/or
facilitate polymerization of rigid rod polymers.
THEORETICAL UNDERSTANDING
Single PolvmerSvstems
Conclusion
· Rigid-rod theories have been extended to include the influence of
macromolecular flexibility on the order parameter and phase transitions in
both lyotropic and thermotropic systems.
Recommends Lions
· Test predictions with unequivocal data from polymer systems that
conform to the theoretical assumptions.
· Extend the theories to deal with other, perhaps dominant features of
LCPs, such as polydispersity, the onset of "elation/aggregation, and
macroscopic heterogeneities.
Blends
Conclusion
· Blends of LCPs with LCPs and of LCPs with flexible polymers are of
potential importance. An adequate theoretical framework of blends to guide
experimental work is lacking. For example, the extent to which miscibility
would be enhanced or decreased because of differences in rigidity in the
component chains is not understood.
Reco~z~nenda Lion
· Perform further theoretical and experimental investigations of phase
equilibria of LCP-containing blends.
Dvnamics
Conclusion
· Chain dynamics dominate both phase transitions and the development of
structure during processing. Although there has been progress on the theory
of diffusion and rheology of rigid-rod polymers in dilute and semidilute
solutions, very little is known about these properties in liquid crystalline
solutions and bulk polymers. This lack of understanding of chain dynamics has
an impact on processing and property issues, such as adhesion, phase
decomposition and nucleation, and orientation development in complex molded
parts.
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Recommendations
· Investigate intensively the dynamic behavior of the anisotropic phase.
· Encourage theoretical studies of the rheology of LCPs.
PROCESSING
Lvotropics
Conclusion
· The technology for the formation of highly oriented fibers from
anisotropic polymers is well established, but fabrication processes for
biaxially oriented films and other multiaxially oriented shapes are still
under development. Process development appears to be feasible with existing
commercial lyotropic LCPs (e.g. , Kevlar@~; these processes should carry over
to new polymers with little modification.
Recommendation
· Continue process development (with better and more volatile solvents)
for high-performance multiaxially oriented shape formation using commercially
available LCPs, including post forming and/or other modifications for specific
applications.
Thermotropics
Conclusion
· Melt spinning of highly oriented fibers is under commercial
development by a number of companies . However, such efforts in injection
molding face problems because of the tendency to form highly oriented local
regions. This in turn leads to poor uniformity of macroscopic properties and
weak weld lines. The current meager understanding of melt flow precludes
systematic advances in mold design to overcome these problems. Solid-state
processing is one area offering promise for overcoming the problems associated
with flow-induced anisotropy.
Reco`~,ll~enda Lions
· Increase effort to understand the flow of LCPs; place emphasis on
efforts to overcome macroscopic heterogeneity and on studies that yield
sufficient background to design molds with mechanical solutions to the weld
line problem.
· Encourage research on solid-phase processing.
· Fund on a case-by-case basis specific fabrication processes to exploit
unique properties, e.g., poling in electro-optical applications.
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MECHANICAL PROPERTIES
Concl unions
· The general consensus is that poor compressive strength of uniaxial
LCP materials is a major limitation in their utilization in structural
materials. The situation is exacerbated by a poor understanding of how the
anisotropic strength (longitudinal covalent and lateral dispersion and/or
hydrogen bonding) manifests itself throughout the morphological hierarchy in
the solid state -macromolecule to bundling of fibrils. Efforts to fabricate
stronger materials via cross-linking are inconclusive. As with conventional
composites, adhesion of LCP fibers to the matrix is problematic.
· The influence of temperature on mechanical properties is more
important in melt-processible LCPs than in the case of the thermally stable
lyotropic LCPs.
· Since self-adhesion in LCPs is poor, the weld line problem in
injection-molded parts is exaggerated and may be insurmountable at the
molecular level. This also manifests itself as a tendency for fibrillation in
uniaxial films and limits their usefulness; efforts to superimpose biaxial
order on films are being pursued. Advances could be expedited by pursuing
film processing studies on readily available LCPs.
· Environmental exposure, whether natural or application-imposed, can
lead to deterioration of LCP-based structures and articles. This concern is
not fatal because adequate protection can be provided if the need is
recognized. For military materiel, the chemical warfare (COO) scenario is
considered to be part of the application-imposed environment and, although
some LCPs exhibit excellent barrier properties, exposure to CW liquids must be
included in any screening protocol during materials selection and
qualification.
Recommendations (in addition to recommendations cited under Processing
et al.)
· Encourage research directed toward identifying the compressive failure
mechanism in LCPs, in particular delineating the level (microscopic versus
macroscopic) at which the failure is initiated.
· Screen LCP materials to establish resistance to natural and end-use
environments that include exposure to CW liquids.
· Encourage the evaluation of diffusion and permeability in LCPs that
may be utilized as barrier materials.
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BLENDS AND COMPOSITES
Conclusion
· The combination of LCPs with other materials to control the balance of
properties and improve cost-effectiveness is clearly an important technology
area for increasing the overall utility of LCPs. The problems inhibiting the
rapid development of this technology are the same as those slowing LCP
acceptance in other areas. These include high property anisotropy in finished
parts, poor compressive strength, and poor adhesion to conventional and
mesogenic materials. This is not in contradiction to the observations that
fillers and blending can mitigate many of these effects but, is rather an
important hint into what the underlying controlling parameters of the science
must be. Hence, from both the scientific and technological points of view,
investigations of the behavior of LCPs in multicomponent systems should be
strongly supported.
Recommenda lions
· Investigate the effect of shape, surface chemistry and properties of
filler particles, as well as processing methods, on the orientation and
properties of the LCP phase.
· Understand and improve LCP adhesion to fillers, to conventional
polymers, and to other LCPs.
· Understand the dynamics and equilibrium of phase shapes, sizes, and
properties of mult~component systems that include LCPs.
NONLINEAR OPTICAL PROPERTIES
Conclusion
· Liquid crystallinity in combination with the unique electronic
properties that can be achieved in organic molecular solids and polymers may
lead to useful polymeric materials for second- and third-order nonlinear
optical applications. The advantageous property associated with liquid
crystallinity is anisotropic order, which is either essential to or enhances
nonlinear coefficients and imparts stability to the critical orientation of
chromophores in the films. The complex issues associated with the range of
properties that must be achieved will require substantial interdisciplinary
efforts.
Recommendations
· Support initiatives aimed at understanding and achieving stable,
uniform polar alignment in polymeric materials for second-order nonlinear
optical applications.
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@ Supporc polymeric design, synthesis, and processing studies aimed at
the unique properties of waveguide structures for third-order nonlinear
on hi r~
@ Focus research support in polymer nonlinear optics on
interdisciplinary programs Where molecular and polymer design and synthesis,
waveguide design and fabrication, and physical characterization are combined.
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Representative terms from entire chapter:
sidechain lcps
