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Polymers
NRL STRATEGIC SERIES
Panel on Polymers Naval Studies Board Commission on Physical Sciences, Mathematics, and Applications National Research Council
NATIONAL ACADEMY PRESS Washington, D.C. 1995
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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the panel responsible for this report were chosen for their special competences and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Robert M. White is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. Robert M. White are chairman and vice chairman, respectively, of the National Research Council. This work was performed under Department of Navy Contract N00014-93-C-0089 issued by the Office of Naval Research under contract authority NR 201-124. However, the content does not necessarily reflect the position or the policy of the Department of the Navy or the government, and no official endorsement should be inferred. The United States Government has at least a royalty-free, nonexclusive, and irrevocable license throughout the world for government purposes to publish, translate, reproduce, deliver, perform, and dispose of all or any of this work, and to authorize others so to do. Copyright 1995 by the National Academy of Sciences . All rights reserved. Copies available from: Naval Studies Board National Research Council 2101 Constitution Avenue, N.W. Washington, D.C. 20418 Printed in the United States of America
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PANEL ON POLYMERS Eli M. Pearce, Polytechnic University, Chair Anna C. Balazs, University of Pittsburgh Georg G. Bohm, Bridgestone/Firestone, Inc. Robert H. Grubbs, California Institute of Technology Edward J. Kramer, Cornell University Sonja Krause, Rensselaer Polytechnic Institute James E. Mark, University of Cincinnati David W. McCall, Far Hills, New Jersey David A. Tirrell, University of Massachusetts C. Grant Willson, University of Texas at Austin Navy Liaison Representative Ronald N. Kostoff, Office of Naval Research Consultant Sidney G. Reed, Jr. Staff Ronald D. Taylor, Associate Director
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NAVAL STUDIES BOARD David R. Heebner, Science Applications International Corporation (retired), Chair George M. Whitesides, Harvard University, Vice Chair Albert J. Baciocco, Jr., The Baciocco Group, Inc. Alan Berman, Center for Naval Analyses Ruth M. Davis, Pymatuning Group, Inc. Seymour J. Deitchman, Institute for Defense Analyses John F. Egan, Lockheed Martin Corporation Ralph R. Goodman, Applied Research Laboratory, Pennsylvania State University Sherra E. Kerns, Vanderbilt University David W. McCall, Far Hills, New Jersey Irwin Mendelson, Singer Island, Florida George A. Paulikas, The Aerospace Corporation Alan Powell, University of Houston Herbert Rabin, University of Maryland Robert L. Silverstein, Northrop Grumman Corporation Keith A. Smith, Vienna, Virginia Robert C. Spindel, Applied Physics Laboratory, University of Washington H. Gregory Tornatore, Applied Physics Laboratory, Johns Hopkins University Richard H. Truly, Georgia Tech Research University, Georgia Institute of Technology J. Pace VanDevender, Sandia National Laboratories Vincent Vitto, Lincoln Laboratory, Massachusetts Institute of Technology Navy Liaison Representatives Paul Blatch, Office of the Chief of Naval Operations Ronald N. Kostoff, Office of Naval Research Lee M. Hunt, Director Ronald D. Taylor, Associate Director Susan G. Campbell, Administrative Assistant Mary (Dixie) Gordon, Information Officer
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COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND APPLICATIONS Richard N. Zare, Stanford University, Chair Richard S. Nicholson, American Association for the Advancement of Science, Vice Chair Stephen L. Adler, Institute for Advanced Study Sylvia T. Ceyer, Massachusetts Institute of Technology Susan L. Graham, University of California at Berkeley Robert J. Hermann, United Technologies Corporation Rhonda J. Hughes, Bryn Mawr College Shirley A. Jackson, Rutgers University Kenneth I. Kellermann, National Radio Astronomy Observatory Hans Mark, University of Texas at Austin Thomas A. Prince, California Institute of Technology Jerome Sacks, National Institute of Statistical Sciences L.E. Scriven, University of Minnesota Leon T. Silver, California Institute of Technology Charles P. Slichter, University of Illinois at Urbana-Champaign Alvin W. Trivelpiece, Oak Ridge National Laboratory Shmuel Winograd, IBM T.J. Watson Research Center Charles A. Zraket, Mitre Corporation (retired) Norman Metzger, Executive Director
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PREFACE
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Preface
To assist with its long-term strategic planning, the Naval Research Laboratory (NRL) requested that the Naval Studies Board (NSB) of the National Research Council (NRC) form a panel on polymer science. NRL's request for independent advice acknowledged that polymers are a highly important commodity for many applications. The great utility in these materials is due to the excellent properties that are introduced with polymer design and processing. This field clearly remains an important route for new materials. The subject of polymer science, including discussion of key science and technology questions and applications, has been addressed in the recent NRC report Polymer Science and Engineering: The Shifting Research Frontiers (National Academy Press, Washington, D.C., 1994). That report presents a comprehensive assessment of the field of polymer science and engineering and identifies the field's contributions to important national issues, such as international competitiveness, education, environment, energy, national defense, and so on. In response to NRL's request, the Panel on Polymers was formed and directed to identify, based on both NRL's strengths and opportunities in the field, how NRL could better focus its research program in polymer science. Accordingly, the panel was requested to identify selected research opportunities in the field as a whole and meet with NRL researchers working in the areas related to polymer science and receive briefings on existing and planned research efforts. NRL's efforts in polymers encompass a variety of approaches to polymer synthesis and characterization, including both traditional approaches and those based on biotechnology. NRL's research thrusts focus on specific polymeric materials such as elastomers, composites, and coatings. Owing to anticipation that modifications to the current program would draw on NRL's extensive experience and expertise in these areas, the panel also was requested to consider NRL's current capabilities in the areas overlapping those identified in the study. In this context, specific questions posed for the panel's consideration were the following: 1. What emerging approaches to advanced polymers are likely to present opportunities for NRL research within the next few years? What areas are being overlooked at NRL and should be incorporated into the ongoing programs? 2. What selection of polymers appears to represent the greatest opportunity for NRL given (a) the problems to which they may be best adapted within the Navy, and (b) the capabilities and strengths existing within NRL? 3. What selection of facilities is most appropriate at NRL, given the size and contributions of the effort, for the options identified in response to the above? During the course of the study, the panel met four times—September 21-22, 1993, at NRL and November 4, 1993, January 27-28, 1994, and September 27, 1994, in Washington, D.C.
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PREFACE viii
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CONTENTS
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Contents
Executive Summary
1
Chapter 1–
Introduction
2
Chapter 2–
Research Opportunities for NRL Polymer Surfaces and Interfaces Synthesis and Characterization Novel Syntheses Supramolecular Chemistry Polymer–Ceramic Composites Mechanical Properties and Failure Mechanisms Theory, Modeling, and Simulations Theoretical Modeling Computer Simulations Biomimetics, Biocomposites, and Biomedical Applications Biomimetics Biocomposites Biomedical Applications Electronic Properties Polymers with Other Special Properties Reduced Flammability Controlled Transpor
4 4 5 5 6 7 7 8 8 8 9 9 9 9 9 10 10 11
Appendix A–
Current and Promising Polymer Research Topics
13
Appendix B–
Present and Potential Future Uses of Polymers by the Navy
15
Appendix C–
Research in Progress at NRL Materials Materials with Specific Properties Classes of Materials Characterization
18 18 18 19 19
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CONTENTS x
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EXECUTIVE SUMMARY
1
Executive Summary
The Naval Research Laboratory (NRL) has a well-recognized program in polymer science and engineering. The program reinforces the Navy's recognition of the growing importance of polymeric materials to a broad range of applications. In the present study, areas of opportunity for research in polymers are identified and discussed in the context of Navy needs for advanced technology. The identified priority research opportunities are in the following areas: • Polymer surfaces and interfaces, • Polymer synthesis and characterization, and • Theoretical modeling and computer simulation of polymeric behavior and properties. These three topics were also recommended as high-priority, frontier areas for research in a recent National Research Council (NRC) study, Polymer Science and Engineering: The Shifting Research Frontiers (National Academy Press, Washington, D.C., 1994). (In addition, Polymer Science and Engineering recommended research on polymer processing and manufacturing, but the present panel concluded that work in this field was less directly suited to NRL.) Biological and biomedical applications of polymers are also identified as areas of rapid current development. Two opportunities in this area are suggested: • Biomimetics and biocomposites and • Biomedical applications. These application areas were also featured in Polymer Science and Engineering. Electronic applications of polymers are well established, but a variety of new uses, based on polymer electronic properties, are now emerging and give promise of future technological advantages. Three opportunities are suggested for this area: • Nonlinear optical materials, • Flexible display devices, and • Ultrahigh-density memory. In addition to opportunities in areas having broad applicability, the following research issues relate directly to the Navy's adoption of polymeric materials: • Reduced flammability, • Mechanical properties and failure mechanisms, • Controlled transport (release and membranes), and • Polymer composite materials, including polymer-ceramic composites. The panel believes that these areas of research are strongly relevant to the future technology needs of the Navy and that they are compatible with existing NRL strengths in personnel, programs, and facilities.
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INTRODUCTION
2
Chapter 1 Introduction
The Naval Research Laboratory (NRL) maintains a broad-based program of research and development that serves to ensure that the Navy will have advanced technological capability when it is needed. NRL has an active program in polymers, and the relevance of this work to Navy needs is direct and evident. As is often generally true of advances in science and technology, much of the research done at NRL also benefits the civilian domain. It should be noted that the panel was not charged with reviewing and evaluating NRL programs, but rather was asked to identify research opportunities. The remainder of this chapter briefly discusses frontiers in polymer science and engineering, and Chapter 2 discusses polymer research opportunities for NRL. Appendix A summarizes current and promising polymer research topics, Appendix B outlines present and potential future uses of polymers by the Navy, and Appendix C describes briefly the panel's understanding of polymer research in progress at NRL. The National Research Council (NRC) recently released the results of a comprehensive assessment of the polymer science and engineering field in the report Polymer Science and Engineering. The Shifting Research Frontiers (National Academy Press, Washington, D.C., 1994). Many of that study's main findings and conclusions are summarized below. Although the target audience for the present report (i.e., the Naval Research Laboratory) is much more specific than that for Polymer Science and Engineering, there is almost complete commonality of relevance on the research side and much that is pertinent in regard to applications. Thus the material in this report is closely related to the content and conclusions of Polymer Science and Engineering. Basic research in polymers is a major area of opportunity, highlighted by the following: • Synthetic methods are needed to provide more precise control of polymer compositions, and a number of promising techniques are being pursued, e.g., coordination catalysis, biocatalysis and enzyme synthesis, ring-opening metathesis, group transfer polymerization, hybrid organic-inorganic materials synthesis, and dendritic polymer preparations. Nature offers examples of precise polymer synthesis at good rates and mild conditions, and it remains for research to gain the understanding required to bring the fruits to commerce. • Polymer theory is another central aspect of current and future research. The recent revolutionary advances in computer power offer opportunities in modeling and simulation of structures, processes, and properties that were undreamed of a decade ago. The states of matter exemplified by polymers (including solutions, crystalline and amorphous morphologies, fibers, liquid crystals, and blends) will be classified and understood through the power of new theory supported by computation. Rheological behavior, mechanical properties, and electronic characteristics will become predictable by computation. Processing of polymers will be carried out by machines operating under total computer control. It is clear that these advances in understanding and control will be based on the combination of new theoretical methods (e.g., force-field and coarse-
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INTRODUCTION
3
grained simulations) and improved computer hardware and software. • Polymer characterization is a field that has profited greatly from advances in instrumentation and computation. Recent progress and opportunities are based on major improvements in traditional techniques and the introduction of new methods that are well adapted to the study of polymers. Examples include two-dimensional nuclear magnetic resonance (NMR), which has become a premier method for determining sequence distribution; several high-resolution microscopic techniques that are specifically suited to the study of dielectric surfaces and can be used without the requirement of high vacuum (e.g., near-field scanning optical microscopy); significantly improved scattering methods for solid-state, blend, and solution work, and advanced surface characterization techniques. Characterization methods are the basic tools that support the entirety of polymer science and engineering. No program can be at the forefront without broad-based access to and use of the most advanced methods. • Research is also needed to broaden the applications of polymeric materials. Materials with “tailored” properties based on blends, high-strength fibers, new matrices for composites, and improved stability of toughening additives are finding new uses as materials substitutes and in unique applications. This trend will accelerate as failure mechanisms become better understood. The areas for substitution of materials span automobiles, aircraft, boats, construction, machinery, and many other specialty items. While military applications are growing (e.g., body armor, uniforms, and aircraft), the field is ripe for rapid growth and penetration by polymeric materials. • Polymers are abundant in biological materials and are increasingly important in health, medicine, and biotechnology. Examples include implants, medical devices and diagnostics, controlled drug release, biological methods and mechanisms, and the techniques of biotechnology. This is an area of rapidly expanding understanding and application. • Even in electronics polymers are widely employed as dielectrics, resists, chip packaging, and electrophotographic media, and new applications of polymers are based on their electronic properties (e.g., synthetic metals, batteries, nonlinear optical materials, light-emitting diodes, displays, and holographic materials). All of these applications require research and development activity to ensure successful market performance. • Polymers may be employed in ways that are favorable from the standpoint of environmental acceptability. Polymers are generally environmentally benign, and research on recycling and disposal is progressing. Generally, polymers should be regarded as part of the environmental solution, not the problem. However, research on environmental aspects of polymers must be continued to ensure responsibility in materials choices. Environmental concerns are also of increasing importance to the Navy because of the need to comply with the MARPOL agreement, which restricts, and in some cases forbids, the earlier practice of disposal of refuse by dumping into the sea.
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RESEARCH OPPORTUNITIES FOR NRL
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Chapter 2 Research Opportunities for NRL
The panel has attempted to discuss briefly the potential directions and opportunities for NRL research and development related to polymers. The six major categories are (1) polymer surfaces and interfaces, (2) synthesis and characterization, (3) theory, modeling, and simulations, (4) biomimetics, biocomposites, and biomedical applications, (5) electronic properties, and (6) polymers with other special properties. In regard to the programmatic emphasis placed on these areas, the panel recognized that priorities are to be established by the NRL. In any case, regardless of choice, it is necessary that NRL maintain basic research competence in the core areas of theory, synthesis, and characterization. This approach would continue to provide an important Navy communication bridge with the advanced materials industry, government laboratories, and academia, where a number of innovative opportunities may develop. POLYMER SURFACES AND INTERFACES Polymer surfaces and interfaces are of great importance to the Navy. Such interfaces include the interface between polymer coatings and seawater, the interface between polymer matrices and inorganic fibers in composites, and polymer-polymer interfaces in rubber-modified polymer blends. The processes occurring at such interfaces control corrosion and biofouling of ship hulls, affect the reflection and scattering of electromagnetic and sound waves, and determine the mechanical properties (e.g., toughness) of structural materials. Polymers for coatings in contact with seawater are of obvious importance. Prospects exist for the synthesis of new polymers for such coatings which allow bulk and water interface properties to be separately tailored and optimized. Polymers with controlled molecular architecture, e.g., specific end groups or blocks, can promote molecular self-assembly at such an interface. A fundamental understanding of interface reconstruction and the molecular rearrangement of polymer interfaces over time is particularly desirable. Surfaces that are hydrophobic in air can become hydrophilic in water; interfaces that initially have low adhesion with marine organisms can reconstruct when they come into contact with such organisms to produce much stronger adhesion. The thermodynamics and kinetics of such reconstruction deserve special emphasis. Polymer-inorganic and polymer-polymer interfaces can also have a great impact on naval materials. Controlling adhesion at these interfaces often involves other polymer additives, e.g., block copolymers or endfunctional polymers, which organize themselves or react to strengthen the interface. Synthesis of new, more effective polymeric “coupling agents” is a worthwhile objective, as is the development of new theoretical and simulation methods for describing the segregation or reaction of these additives at such interfaces. Another area where it appears that rapid progress will be possible in the future is the friction and wear of polymer surfaces. This area is important in a number of practical applications ranging from the behavior of thin layers of polymers as lubricants for magnetic disk heads to the lifetime of polymer protective coatings for ship hulls affected by wear due to waterborne debris. In the related area of drag reduction, a currently dormant field, precise control of polymer surface structure or the use of slowly released soluble polymers, which might affect hydrodynamics, may offer new approaches. Much of the opportunity for advances in the above areas derives from the emergence of new
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RESEARCH OPPORTUNITIES FOR NRL
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characterization methods over the past decade. New depth profiling methods can give concentration versus depth information about polymeric additives labelled with deuterium with resolutions ranging from 1 to 100 nm. These methods include neutron and x-ray reflectivity, ion beam analysis (e.g., forward recoil spectrometry), and secondary ion mass spectrometry. When combined with older surface analytical methods, such as x-ray photoelectron spectroscopy and attenuated total reflection infrared spectroscopy and ellipsometry, these methods provide a powerful array of tools for polymer surface and interface characterization. A number of new surface analysis techniques have also become available with the advent of high-brightness synchrotron x-ray sources. Grazing incidence x-ray diffraction methods now allow one to elucidate the state of order in the 5 nanometers just below polymer surfaces, while near-edge absorption of polarized soft x-rays can interrogate the orientation of molecular segments of polymers at surfaces. New methods based on the contact mechanics technique promise to make it possible to measure simultaneously the work of adhesion, area of contact, and normal and lateral forces between two elastomer surfaces and thus offer the possibility of providing new insights on microscopic mechanisms of friction, adhesion, and wear. Finally, new scanning probe microscopies will make a substantial impact on our knowledge of polymer surfaces by revealing the lateral structure of these interfaces with unprecedented resolution. Scanning force microscopy (SFM) (also called atomic force microscopy) is now in common use to reveal surface topology. Lateral force microscopy, a variant of SFM, has been shown to be capable of imaging polymer surfaces with chemical, rather than just topological, resolution. Another variant, tapping-mode SFM, is capable of revealing local differences in near-surface elastic properties. Near-field scanning optical microscopy promises eventually to allow various optical spectroscopies to be done on polymer surfaces with lateral resolutions as small as 10 nm. Perhaps the most intriguing possible development is a scanning probe NMR spectrometer, an instrument that would make it possible to determine directly the lateral chemical structure of polymer surfaces. Since most of these scanning probe microscopies do not require vacuum, or even air, environments, they can be adapted to examine the structure of water-polymer interfaces as well. SYNTHESIS AND CHARACTERIZATION Novel Syntheses Polymer synthesis has seen major advances in regard to the preparation and controlled design of structure to obtain specific properties or improvements in properties. Structural detail and the tailoring of properties depend on factors such as the control of chain length, molecular weight distribution, sequencing of copolymer units, microstructure isomer control, and end groups. The past 10 years have seen developments such as group transfer polymerization, ring-opening metathesis for cyclic hydrocarbons, improved cationic and anionic techniques for molecular weight control, and new biosynthetic routes. Much remains to be done to more exactly control stereochemistry. The understanding and use of new catalysts for architectural control, and for coupled integrated syntheses from monomer through polymer and product, are significant opportunities for further research. The creation of new techniques for the synthesis of block, telechelic, and functional polymers will be essential for the study and development of new methods to control the interface of polymers with incompatible environments. Although there are now some techniques for the synthesis of block systems, new high-efficiency systems that allow the incorporation of blocks of standard polymers are required. Methods that produce polymers with controlled levels of functionality, as in telechelic polymers, are also needed.
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RESEARCH OPPORTUNITIES FOR NRL
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The ultimate control is that demonstrated by biological systems. Currently, only biological systems (in vivo or in vitro) can control monomer sequence efficiently. Professor Robert Bruce Merrifield was awarded the 1984 Nobel Prize in Chemistry for establishing a procedure that allows rather inefficient control of the monomer sequence in certain classes of materials. The ability to control sequence confers remarkable power. From only about 21 different amino acid monomers, nature is able to build the myriad proteins that make up functioning human organisms. Efficient and precise varying of the sequence of these monomers in biological systems leads to materials that function as structural materials, muscles, brains, or catalysts. The opportunity for the next great leap in polymer science is establishment of new synthetic methods that allow rational control of the sequence of monomers in complex copolymers. Advances in this area are being made by polymer synthetic chemists through enlistment of biological sequence control mechanisms. There is a very long way to go, but this frontier area clearly represents the great opportunity in polymer synthesis. Opportunities to develop high-performance materials will continue as new technologies become limited by the materials available and their costs. Economical preparation of new polymer structures capable of functioning in new and/or previously “hostile” environments will continue to be important. Major opportunities also exist for upgrading the less expensive commodity-type materials to compete more effectively with new engineered structures, and this will be done by stereochemical design and morphological control in homopolymer, copolymer, and blend systems. Cost and environmental issues related not only to syntheses but also to disposal or recycling are becoming increasingly important and will pose significant problems for the Navy and opportunities for NRL research. Opportunities in regard to supramolecular structures are discussed below. When new polymers are synthesized, their molecular characterization becomes imperative. NRL should continue to explore the newer methods of molecular characterization, including, for example, such methods as laser desorption mass spectrometry, which has been used to obtain molecular weight distributions for samples with molecules having molecular weights up to 105 grams per mole (g/mol). Supramolecular Chemistry The design and synthesis of polymeric materials as described above have traditionally focused on the covalent structure of the chain. With the development of new catalyst systems, living polymerization methods, and biosynthetic routes to new polymers, a high level of control of molecular architecture has been achieved. But because the materials properties of polymers depend on interactions among many contiguous chains in the solid state, it seems clear that a comprehensive view of the design process must include consideration of supramolecular phenomena, i.e., the exploitation of secondary interactions to control structure and properties on length scales greater than those of the single chain. At the same time, experimental and theoretical tools are emerging that enable a meaningful attack on problems of supramolecular chemistry. Particularly promising points of departure for studies of this kind are provided by ongoing investigations of hydrogen-bonded aggregates, liquid crystals, micellar and vesicular systems, and self-assembling membranes. For example, recent theoretical work has demonstrated the similarity of the thermodynamics that controls phase behavior and that which controls pattern selection in phospholipid bilayers and in multiphase block copolymers, and vesicular structures—known for many years in aqueous surfactant chemistry—have been identified in seemingly dissimilar styrene-diene-block copolymers in the absence of solvent. The size of supramolecular structures of concern is often of the order of the wavelength of light, and so
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RESEARCH OPPORTUNITIES FOR NRL
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the use of light-scattering techniques may be appropriate for NRL. NRL is well positioned to participate in the development of new materials based on supramolecular interactions. Current NRL investigations of ferroelectric liquid crystals and lipid tubules provide excellent examples of the kinds of novel structures and properties to be realized by taking proper account of both covalent and noncovalent bonds in controlling the architectural features of materials. These investigations have been unusually productive and merit high priority. Polymer–Ceramic Composites Ceramists have exploited the sol-gel chemical route to prepare high-performance ceramics. For example, the catalyzed hydrolysis of tetraethylorthosilicate coupled with an appropriate heat cycle can give useful silicabased ceramic materials. Various opportunities exist in exploiting this type of reaction system in the presence of organic-based polymers. For example, elastomers, particularly those that cannot undergo strain-induced crystallization, are compounded with a reinforcing filler such as silica to improve abrasion resistance, tear strength, and tensile strength. The processes and mechanisms for these improvements are still poorly understood. The NRL's elastomer research program might consider precipitating reinforcing fillers into polymers prior to and also after the formation of network structures. In addition to silicates, titanates, aluminates, and combinations of these are available for this approach. Reinforcing ability has been demonstrated by this approach, but much remains to be done in elucidation of properties, e.g., dynamic mechanical properties, as well as in processing. This approach would seem to have an advantage over the usual blending of separately prepared agglomerated filler into highmolecular-weight polymer, which is a difficult, time-consuming, and energy-intensive process, usually done with an elastomer that modifies a ceramic in which it is dispersed, leading to significant improved properties (e.g., impact strength). Also, the hydrolysis reaction can generate functional groups that form hydrogen bonds, leading to interesting inorganic-organic gels, which upon controlled pyrolysis can lead to foamed ceramics. Relatively little has been done in these areas, which suggests an opportunity for NRL involvement. In prepared composites, the adhesion between the phases is related to the distance between atoms and groups of atoms in the two phases, which are subject to specific interactions, as well as the effects of surface orientation, geometry, and roughness. Solidstate NMR can be used to determine these distances, and this kind of method could be explored further by NRL. Mechanical Properties and Failure Mechanisms The mechanical properties of polymers are of importance for most applications. For the Navy, polymer mechanical properties determine the acoustical impedance and damping of sound waves in sonar structures, on the one hand, and the fracture characteristics of polymer-based composite materials for aircraft structure, on the other. While the mechanical properties of elastomeric networks and polymer melts, especially those of miscible polymer blends, seem to be well represented in the present NRL research program, there is potential for extending this expertise to the mechanical properties of solid (glassy or semicrystalline) polymers and phaseseparated polymer blends (both those in the melt and solid state). Understanding of the fracture properties of these materials has increased dramatically recently, but the understanding of the mechanism of shear deformation is incomplete even in single-phase polymers and is particularly poor for phase-separated rubbermodified polymers—yet these polymers include the toughest polymeric materials available. New experimental techniques have been developed for the study of the fracture properties of these materials, but
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RESEARCH OPPORTUNITIES FOR NRL
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good methods for testing small quantities of polymer for screening purposes are badly needed. Since many of the properties of phase-separated polymer blends depend on the mechanical behavior of polymer interfaces, the relationship between the interface structure and the fracture properties is particularly important. Proper understanding of the fracture mechanism requires real-time observation of the process using a variety of techniques such as computer-assisted microscopies and x-ray scattering synchotron measurements. THEORY, MODELING, AND SIMULATIONS Theoretical Modeling Computer simulations provide insight into the behavior of polymers under a variety of circumstances and make it possible to probe problems that are analytically intractable. Theoretical modeling has the distinct advantage that the effects of various parameters can be explored more exhaustively than through simulations. That is, through mathematical models, we can determine the limits of certain behavior or map out phase diagrams for a system—those kinds of explorations that are more cumbersome and more time consuming through simulations (if at all feasible). One area where mathematical models have played a significant role is in polymer blends. In fact, NRL is conducting considerable experimental research on this topic. Combining this experimental program with theoretical modeling would facilitate and enhance both efforts. In particular, equation-of-state models could be used to predict the phase diagrams of polymers mixtures. These predictions could guide experimental efforts in designing new, miscible blends. Self-consistent mean field (SCF) models are useful in predicting the behavior of polymers at both penetrable and impenetrable surfaces. These techniques are now fairly standard; the analytical equations are solved numerically and can be carried out on workstations. The model can be used to examine the adsorption of copolymers onto surfaces and establish guidelines for synthesizing protective coatings and films. The technique can also be used to examine the behavior of chains at fluid-fluid interfaces. Consequently, the findings yield insight regarding the behavior of polymeric surfactants or compatibilizers. These studies would also complement the ongoing work on composites. Finally, the SCF approach can also be used to model interactions within lipid bilayers. Such calculations could complement the NRL work on lipid tubules and biomembranes. Computer Simulations Computer simulations have become an important research tool for determining the properties of polymers. With the advent of fast workstations, many computationally intensive simulations can now be carried out on reasonable time scales. Furthermore, recent studies have demonstrated the power of simulations to predict polymer properties, especially at interfaces and in solution. Thus, simulation can provide a valuable tool to supplement the ongoing NRL research on the interface between phase-separated domains and would complement current NRL studies in the area of blends and composites. There also exists at NRL considerable expertise in molecular dynamics techniques and other simulation methods. Consequently, a collaboration between the two groups in this area would provide a rich research opportunity. A specific area of investigation could be using computer simulations to model failure at an interface. An important goal would be to use the simulations to isolate signatures for a specific failure mode. These studies would be particularly helpful in designing high-strength composites. Another area in which computer simulations are useful is in modeling polymer self-assembly in solution. Current NRL studies on lipid tubules could be complemented by computer simulations to determine how the architecture of the lipid chains affects self-assembly or to
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RESEARCH OPPORTUNITIES FOR NRL
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investigate how these tubules interact with polymers in solution or other tubules. BIOMIMETICS, BIOCOMPOSITES, AND BIOMEDICAL APPLICATIONS Biomimetics Biomimetics involves studying structure-property-function relationships in biological materials and then utilizing this information to guide in the design of synthetic materials that mimic these properties or functions. Polymer scientists already know, for example, that the cross-linking of polymer chains is essential for rubber elasticity. Nature, however, is more sophisticated in designing such structures, so that bioelastomers have higher efficiencies than synthetic rubbers in storing elastic energy. Understanding and control of such structures could be used to design more efficient synthetic rubbers with reduced degradation from the heat buildup due to inefficient energy storage. Biocomposites Various types of naturally occurring biocomposites differ from most synthetic composites in one or more important respects. For example, while the hard reinforcing phase in synthetic composites may be either amorphous or crystalline, it is generally crystalline in biocomposites. Furthermore, this crystalline hard phase is carefully controlled with regard to amount, morphology, crystallite size, and crystallite size distribution. Moreover, the crystalline region orientation is fixed by the use of templates or epitaxial growth. Sometimes, an advantageous gradation of properties is built into the biomaterial as a result of changes in either chemical composition or physical structure. Large-scale ordering is often also present, and these complex laminated structures have specific interactions within the layers, resulting in excellent overall properties of the biocomposites. An understanding of the complexities of structure and morphology leading to these biocomposites should eventually be coupled with improved methods of synthesizing controlled polymer structures, thus translating the knowledge obtained from biosystems into the sphere of synthetic polymers. Although it may not be feasible for NRL to study biomaterials, researchers at NRL should be aware of the relevant research elsewhere and should probably be ready to use this information to design new materials. Biomedical Applications Polymeric materials are playing an increasing role in biomedical applications. These range from the polymers substituted in applications where glass and metal were traditionally used, to completely new technologies such as controlled release of drugs, drug delivery systems, dressings, and prostheses. Essential in most of these applications is the biocompatibility of the material and the useful lifetime of the device in the harsh environment of the body. Many of the same techniques and approaches that have been and are being developed by the Navy to study the interface of materials with nonbiological environments will be valuable in these studies. Many of the present NRL programs will provide materials that could have direct application in biomedical applications. ELECTRONIC PROPERTIES Organic polymers are widely employed in applications based on their excellent electrical insulating properties, coupled with mechanical toughness and flexibility. They also play a key role in the manufacture of integrated circuits as photoresists that define the microscopic patterns required. Polymers can be made to be photoconductive, piezoelectric, and pyroelectric, which makes them well adapted to xerography and sensor applications. In recent years, polymers have been discovered that have an array of electronic and optical properties that are promising for information processing, memory, displays, switching, and transmission. These rapidly evolving applications take polymeric materials into realms in which polymers have not been important in the past.
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RESEARCH OPPORTUNITIES FOR NRL
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NRL is well positioned to contribute in these emerging areas with selective choices of research topics and close coupling with other materials scientists and communications and information-processing engineers. The following research topics match NRL's existing expertise, equipment, and facilities and could offer special opportunities for NRL contributions that have clear-cut relevance to the Navy. • fabrication of integrated circuits. Photoresist materials are essential to the Advanced materials have an enormous impact on device function, cost, and reliability. For various reasons, U.S. industrial research in this field has diminished, and there is a danger that foreign interests could come to control lithographic materials, a critical requirement for progess in electronics. NRL polymer chemists, working with other divisions at NRL, could make important contributions in research on these enabling materials. Close collaboration with industrial and academic groups is essential. • Nonlinear optical materials are employed to construct optical circuits, and polymeric materials are prime candidates in this field. The nonlinear optical behavior is based on the dipolar nature of the polymers, and many promising structures have been synthesized. Key issues are the creation and the stability of dipolar order. • Flat panel display devices will have increasing importance to the Navy. Displays based on ferroelectric (chiral smectic) liquid crystals have the potential for providing a major reduction in the cost of such devices together with an improvement in performance. The NRL has the resources and the expertise to solve the critical problem in this technology—the stability of mesophase alignment. Display devices have also been demonstrated based on polymeric lightemitting diodes (LEDs). These devices are mechanically flexible and have color potential. • Polymers are favorable materials for applications in holography. Holographic devices offer applications in optical computing pattern recognition and very high density memory. • A wide range of sensor types are based on polymeric media. The sensor response may be based on the electrical or optical properties of the polymer, or may be controlled by a secondary factor such as permeability by a chemical entity to be detected. • Electronically conducting polymers offer many possible applications. To date, only novel batteries have been commercialized, but transistors have been demonstrated, and many possible opportunities for NRL could emerge. POLYMERS WITH OTHER SPECIAL PROPERTIES Reduced Flammability Flammability is a major problem associated with extended use of polymers in the Navy for structurally related components. The design of materials with greatly improved resistance to flammability would enhance their use by the Navy. At present, some work has been done by NRL in this regard based on phthalonitrile-based systems. Recent studies by others have indicated opportunities for reducing flammability by incorporation of new phosphorus-containing monomers and also of monomers incorporating structures capable of crosslinking at high temperatures. These operate primarily by the formation of barriers to heat, air, and pyrolysis products. This area is worthy of systematic investigation, and tools not previously applied are available now for investigating the chemical and physical nature of the chemistry involved and that of the barriers formed. The condensed phase reactions that occur at the elevated temperatures usually
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RESEARCH OPPORTUNITIES FOR NRL
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associated with polymer pyrolysis can be followed and studied. Intumescence is also an important factor in barrier efficiency as well as in the ability to achieve coherent and oxidation-resistant barriers. Such barriers include glass, fluorocarbon, and metallic surface coatings, in addition to char formation. Areas that could also be considered for exploration include the new catalytic modes of flame retardation—for example, oxidative dehydrogenation of polyolefins to form water and char. Recent studies have also indicated that blends including high-char-forming polymers have reduced flammability. Many commodity polymers could be made less flammable by this blending route and could thus be more acceptable for naval and civilian use, since lower amounts of additives would be needed, and also improved mechanical properties could be obtained. Possibilities exist for control of smoke and toxic vapor by-products by these approaches, which were not available in previous char-forming systems, e.g., the antimony oxide plus halogen systems. For many reasons—environmental, corrosion, and so on—it has become increasingly important to avoid use of halogens. The Navy Technology Center for Safety and Survivability at NRL can provide a unique and efficient way to test these materials under realistic conditions. Controlled Transport Controlled Release Controlled-release systems have been developed to deliver one or more drugs, insecticides, and fungicides at specific sites and at specific times. Controlled-release delivery is done by means of such mechanisms as osmosis, diffusion through the carrier material, and erosion of the carrier material. Until recently, areas related to medicine and agriculture have received major emphasis. There are some specific areas related to possible naval use where this technology is not only applicable but also requires further exploration. As one example, marine fouling by various organisms remains a significant problem with regard to ship speed loss and the cleaning and renewal of the fouled surfaces. A number of the marine antifoulant approaches based on controlled release principles are environmentally unsuitable since they release antifoulants that are considered pollutants. Several new approaches include (1) release of a nonpolluting agent specific to particular fouling organisms; (2) release of an agent that attacks the chemistry of the fouling organism's natural adhesive rather than the organism per se; and (3) release of an agent that keeps renewing a non-adhesive surface, e.g., silicone, which may slough off continuously. Another area of controlled release that could also be of interest involves flame retardants, which normally create deleterious effects in regard to the aging of materials. Tailored release systems in which the polymer structure is protected from deterioration by the flame retardant package might be useful for designing systems with improved properties. Release systems for patching or repair of structures would also be useful. Curing agents could be released under specific environmental situations, either in situ or in vitro. Membranes and Microfilters The Navy's current interest in membranes and microfilters is related to attempts to develop zero-discharge systems. The Navy is now equipping its new ships with reverse osmosis units for potable and cooling water. A major area of concern for reverse osmosis systems generally has been related to fouling, which seriously affects the efficiency in regard to conversion of waste water content to potable water and allows for the disposal of the dewatered residues. For example, industrially manufactured ultrafilters in reverse osmosis systems foul and must be cleaned frequently. As a result, such systems are treated by high-pressure and high-flow-rate procedures and with chemicals such as hypochlorite to decrease fouling and increase efficiency. In the process, the membrane and its properties are degraded, and systems must be replaced frequently. Understanding the
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RESEARCH OPPORTUNITIES FOR NRL 12
chemical and physical processes associated with fouling, and the hydrophilic-hydrophobic interactions with various polymers and their surfaces, allows opportunities for exploring new materials, surfaces, and membrane design to prevent and/or decrease the fouling problems.
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APPENDIX A
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Appendix A Current and Promising Polymer Research Topics
The following summary list of current and promising polymer research topics is necessarily quite brief. The field is large and will continue to have a great impact on all levels of society for the foreseeable future. • Growing use of polymers as biomaterials — Seasickness patches — Prostheses—hip cups, lenses, blood vessels, orthopedic implants, denture bases, fillings, sutures, heart valves, organs, vascular grafts, hernia mesh, catheters, syringes, diapers, blood bags, artificial limbs, ligaments, packaging — Controlled release — Diagnostics • Emerging electronic properties of polymers — Dielectrics — Synthetic metals and battery materials — Sensors — Lithographic resists — Photonic materials — Light-emitting diodes and displays — Electrophotography — Holography — Fuel cells — Solar cells • Emergence of synthetic means for control of polymer structures — Coordination catalysts — Biocatalysis, enzyme synthesis, biological organisms for synthesizing monomers and polymers — Ring-opening metathesis polymerization — Hybrid organic-inorganic materials synthesis, sol gel formation — Dendritic polymers — Composites with tailored transport, electrical, or optical properties • Growing use of blends and composites to obtain “tailored” properties — High-strength, high-modulus fibers — Enhanced matrix choices — “Tailored” mechanical properties — High-stability toughening additives — High-temperature options — Understanding of failure mechanisms • Enhanced characterization capability through computer and electronic advances — Molecular: colligative, light scattering, centrifuged separation, NMR, UV, FTIR, RAMAN — Solutions, melts: rheology, diffusion, neutron scattering — Solid state: synchrotron x-ray and electron spectroscopy, TEM, soft x-ray microscopy, mechanical testing — Surface analysis: XPS, depth profiling, SIMS, SFA, AMF, LFM — Folding: NMR — New microscopies: confocal and scanning tunneling • Evolution of polymer theory with emphasis on computer modeling and simulation — States of matter: solutions, crystalline, amorphous, LCs, blends, block polymers, copolymers, interfaces, surfaces
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APPENDIX A
— Dynamic properties: rheology, mechanical properties, electroactive • Methods: force field simulations, coarse-grained simulations • Continuing reduction of environmental threats — Elimination of toxic components — Replacement of plastics by natural materials — Recycling • Continuing search for viable recycling strategies — Collection problems: separation, contamination — Blending: properties of mixtures — Processing: to return to monomer or other feedstock type — Other means of disposal: incineration, landfill • Enhanced process control through computer and sensor applications — Molding — Extrusion — Film blowing — Coating • Continuing substitution of polymers for metals and other materials — Aircraft, space vehicles — Automobiles — Clothing — Machined parts — Construction — Electronics — Marine structures and vehicles • Growing use of polymeric materials for wide variety of military items — Bullet-proof clothing — Uniforms — Aircraft weight reduction • Growing understanding of structure-property relationships — Finite-element analysis — Flow modeling, rheology — Simulation of structures of composites, blends, crystalline polymers
14
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APPENDIX B
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Appendix B Present and Potential Future Uses of Polymers by the Navy
The use of polymers by the Navy reflects the unique performance advantages potentially realizable from these materials (summarized in Table B.1). While each application poses different opportunities and challenges, certain general directions are apparent. Following the trend set in consumer products, the Navy also makes increasing use of polymers for relatively simple parts, such as hoses, pipes, and gaskets. Light weight, corrosionresistance, and ease of manufacture provide the main impetus for the use of polymers in these applications, and the market share of polymers for such applications is anticipated to continue to grow steadily in the future. Table B.1 Properties of Polymers A DVANTAGES
C ONCERNS
High strength and stiffness/weight ratio Corrosion-resistance Low signature Manufacturing flexibility Variety of properties Flexibility Chemical stability Low cost
Survivability in combat Flammability and flame spreading Release of smoke and toxic gases Long-term durability Repair problems (i.e., joints) Resistance to high temperatures
The Navy also makes use of polymers in more critical and demanding applications, such as in load-bearing polymeric matrix composites, special coatings for signature control, coatings for corrosion reduction in wasteholding tanks, fuel storage tanks, and metal pipe linings, and so on, where polymers are often applied as part of a technology package to meet the needed performance criteria. This area provides significant opportunities for the Navy to improve the performance of its surface ships and submarines. For example, a lower-weight superstructure and hull mass would result in a lighter, smaller, and more stable vessel. In submarines, too, there are many potential applications for polymeric matrix composites, and there the achievement of a better balance in trim and an enhanced depth performance are often cited as potential advantages. Aside from the present use of advanced composites in bow domes, several applications external to the pressure hull will be evaluated, and several others, such as “intelligent” hulls with special embedded sensors, are under consideration by the Navy. An advantage of polymers for marine vessels is the low detectability by radar and the possibility of incorporating antifouling and/or drag-reducing agents. The potential advantages listed in Table B.1 provide the driving force for an increased use of polymers, but their application is also tempered by concerns, as well as currently perceived or documented performance deficiencies in both normal use and battle conditions. Damage tolerance, for example, is of paramount importance for the use of polymers in structural components, and specifications define the length of time that structures under load must be able to resist fire with no “holing” or collapse. Another concern is smoke and toxic gases liberated by combat-initiated fires, as most fire casualties occur from smoke inhalation and impaired vision that prevent escape. While Federal Aviation Administration (FAA) tests indicate that a variety of specially treated composites performed better in a fire environment relative to aluminum and steel, it is also known that burning composite resins can generate smoke and noxious fumes. Finally, the experiences encountered with combat-initiated fires during the Falkland Islands and Persian
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APPENDIX B
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Table B.2 Polymer Uses by the Navy P RESENT A PPLICATIONS
N EW A PPLICATIONS U NDER C ONSIDERATION
Composites
Radar domes Rocket motor casings Shipboard ventilation Pump casings and impellers Piping
Composite masts Acoustic isolation Ducts Piping for fluid handling Submarine pressure bottle Fairwater
Elastomers
Adhesives Sonar domes Hoses (water and fuel) Sealants (fuel and water tanks) Conductive sealants (electromagnetic shielding) Lubricants External tiles for submarine hulls Vibrational damping (engine and motor mounts) Electrical insulation Tires, belts, bushings, gaskets, seals Binders for propellants Foams
Protective cover for hydraulic actuators
Plastics
Deck houses Railings Shrapnel screens Body armor Insulating foams Interior fittings Floors
M ATERIALS Structural
Nonstructural Coatings
Anticorrosion Antistatic Nonskid Signature control (stealth) Interior lining of pipes Fuel storage tank lining
Biocompatible coatings Controlled drug release Anti fouling Drag-reducing
Films
Packaging Biodegradable bags for trash disposal
Skin grafts Membranes for soil Filtering Ultrafiltration
Textiles
Uniforms Cushions Bandages Ropes
Optical fibers
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APPENDIX B
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Gulf wars, and the fire-induced melting of an aluminum-based superstructure on the USS Belknap, a guidedmissile cruiser, following a collision with the USS Kennedy, have contributed to this continuing debate. As stated in Table B.2 , listing polymer uses by the Navy, polymeric matrix composites and advanced composites composing one or more stiff, high-strength reinforced fibers with a compatible resin system are already used for many Navy applications, such as radar domes, rocket motor casings, and aircraft structural parts. In many such cases, materials and manufacturing technology developed for and applied in the automotive, commercial aircraft, and consumer products industries can be, and in the past have been, usefully applied to meet the Navy's needs. However, many potential applications either have more demanding performance requirements or are unique to the shipbuilding industry and to the Navy, in particular. A successful use of advanced composites in more critical load-bearing shipboard structures will likely pose many scientific as well as technological challenges, including the further improvement of existing manufacturing methods as well as the development of new ones. In addition, there is a great need to supplement laboratory testing of potentially useful composite structures by realistic on-board evaluation using vessels dedicated for such purposes. Considerable experience has already been gained in Europe with mine hunter ships built with glass and polyester composite hulls, and the construction of a much larger ship from advanced composites is being undertaken in Japan. The U.S. Navy has build several minehunting ships with glass-reinforced plastic hulls fabricated under a license to U.S. industry from an Italian firm. The Navy also has informal and formal procedures for test and evaluation of research products and has designated two submarines for such purposes. There should be opportunities for early shipboard evaluation of new technology and materials, including advanced composite structures and other polymer-containing systems. Another area of much significance is coatings for corrosion-resistance (including biocorrosion), reduction of signature and drag, and other purposes. While such coatings are extensively used by the Navy, the panel believes that there are many new opportunities to improve the performance of coatings and to extend their range of applications, e.g., the reduction of flammability, as discussed above. Finally, new scientific advancements have opened the door for the use of polymers in biomedical applications, which may provide future benefits to Navy personnel. This is a relatively new field, but skin grafts, biocompatible coatings, and controlled drug release applications are already being tested.
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APPENDIX C
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Appendix C Research in Progress at NRL
This section summarizes polymer science and engineering research in progress at the NRL Chemistry Division and is based on the panel's on-site interactions with NRL researchers. Polymer research and development at NRL is involved with the synthesis, design, characterization, and improvement of novel polymer systems of interest to the Navy. Further understanding, prediction, and control of the behavior of these polymer systems in particular hostile environments such as seawater, on the one hand, and high temperatures in various propulsion systems or during fires, on the other hand, have been of particular importance. Research on potential innovative analytical test methods, especially nondestructive ones, has also been of interest. Much of the polymer-related research in progress at NRL can be placed in two categories: (1) materials— with specific properties and belonging to particular classes, and (2) characterization. MATERIALS Materials with Specific Properties Materials with specific properties that have been studied include acoustical absorbing and vibration damping materials. Various conventional elastomers, polyurethanes, and interpenetrating network polymers have been studied for such acoustical applications. In addition, polymers are being used in the design of new piezoelectric composite materials containing a ceramic phase. Polymeric materials that are being studied for use at high temperatures include phthalonitrile resins and composites using these resins. In addition, polymers made from high-carbon-content acetylenic monomers have been prepared and studied, especially with respect to pyrolysis, and inorganic-organic hybrid polymers have been prepared, especially for conversion to ceramics. Flame-retardant polymers are especially important for naval applications, making this an important research area. For example, it has been shown that the phthalonitrile polymers have self-extinguishing properties on exposure to fire and, when used in composites, show superior flame resistance. These types of materials can be tested by the Navy Technology Center for Safety and Survivability, another part of the NRL Chemistry Division. Another research area involves the study of different kinds of polymers that can be used as materials that have “low observable” properties with respect to radar. On the one hand, the high-frequency dielectric properties of such electronically conducting polymers as polyaniline and polypyrrole have been under investigation, and, on the other hand, a number of ultralow-dielectric-constant fluorinated polymers have been synthesized and studied. Polymers used in coatings of many kinds have been synthesized and studied by many groups at NRL. The materials studied include various epoxies, polyurethanes, and fluorinated polyurethanes. Applications include nonskid coatings for horizontal surfaces on naval ships; water-shedding coatings on antennas; solar-heatreflecting, anticorrosion, and antifouling coatings; and pipe, fuel storage, and sewage tank linings. Ferroelectric liquid crystals with very fast switching times have been prepared for eventual use in various sensing and switching devices. Other areas of materials research include the synthesis of spin-labeled polymers for sensing composite interface properties, the structure and function of polymer-stabilized synthetic membranes, and polymers for various sensor applications.
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APPENDIX C
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Classes of Materials In general, research on classes of materials is connected with that on materials with specific properties but includes somewhat more general research on composites, polyurethanes, epoxies, fluoropolymers, ferroelectric liquid crystals (especially those with fast switching times), polymer-polymer miscibility, double network elastomers, crystallization in polymers, polymeric Langmuir-Blodgett and other multilayer films, and polymerstabilized synthetic membranes. The application of composites to naval needs is the motif for research on joining techniques, hydrothermal effects and other damage and failure mechanisms, response to transient shock loading, and impact and underwater shock response. The research on ferroelectric liquid crystals includes their behavior as LangmuirBlodgett multilayers. Because of a general interest in polymer blends for various applications, some fundamental studies of such blends have been undertaken. For example, small-angle neutron scattering (SANS) and CP-MAS NMR studies showed that polyisoprene-polyvinylethylene (PIP-PVE) blends were miscible at molecular weights exceeding 106 and had a negative Flory interaction parameter. Also, segmental dynamics have been studied in various polymer-polymer mixtures to gain a general understanding of the effects of different diluents. The morphology of a freeze-dried dilute polymer solution was also studied. There are some ongoing general studies of surface modification of elastomers and studies on the synthesis and characterization of double-network elastomers. Research was done on the suppression of crystallization in blended natural rubber and neoprene. Other studies of crystallization, for example, on positron emission tomography (PET), have also been done. A number of rather complex fluorinated monomers and the resulting polymers have been synthesized and studied because of the Navy's interest in low-permittivity materials. The Navy's interest in coatings has led to the synthesis and study of various classes of polymers. For example, the curing reaction of epoxies with amidoamines has been studied, as has the polymerization of spirobislactones with epoxy resins. CHARACTERIZATION Materials mentioned above are characterized and their properties measured by standard approaches for which NRL is well equipped, and these are discussed in this report. However, some novel characterization methods have been developed or extended at NRL. The use of 129Xe NMR to probe phase separation in polymer blends has been particularly useful for probing miscibility in those polymer blends in which the components have comparable glass transition temperatures, making it difficult, if not impossible, to study miscibility by using methods such as differential scanning calorimetry. There are plans to use this NMR method to measure domain size in phase-separated polymer blends. Nondestructive characterization techniques such as NMR imaging of solid polymers are being studied. It has been possible to exploit methods that provide contrast discrimination based on molecular mobility. In addition, a method was recently demonstrated that provides an NMR image with contrast based on local polymer alignment in response to a strain field. Research is in progress to use electron spin resonance (ESR) to characterize the interior surfaces in composites. Spin probes are to be deposited near the interface of a fiber/polymer matrix composite, and ESR is to be used to monitor the local orientation of the spin probes. The goal is to assess the spatial range of the “interphase” and to understand how the mechanical load is transferred from the fiber to the matrix. Several groups at NRL have been working to better understand polymeric structure-property relationships with respect to particular uses of polymers. In addition, the composite
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APPENDIX C 20
material characterization includes use of a structural response simulator. It should also be noted that the NRL Center for Bio/Molecular Science and Engineering is involved in polymer research, e.g., in sensor design and fabrication, and in microlithography.
