Click for next page ( 6


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 5
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

OCR for page 5
6 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.

OCR for page 5
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.

OCR for page 5
8 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.

OCR for page 5
9 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.

OCR for page 5
10 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.

OCR for page 5
11 @ 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.

OCR for page 5