This issue of the MRS BULLETIN is devoted to a class of materials undergoing a transition from a period in which they were viewed primarily as cheap substitutes for other materials into a new period where polymers are seen as high tech, value-added materials in their own right. The six articles included here focus on a portion of the wide range of topical areas concurrently at the frontiers of polymer materials science.

Polymers are molecules consisting of a large number of units (mers) covalently connected to form macromolecules of very high molar mass (upwards of 106). Polymer chemists have learned how to make an almost endless variety of highly complex yet well- defined macromolecules utilizing a wide variety of monomers. Once polymer physicists and materials scientists depended on industry to provide samples (which were far from model materials to work on). Today, significant improvements in chemical synthesis and a growing collaborative effort between polymer chemists and materials scientists have resulted in the availability of extremely well-defined materials (molecular weight distribution, composition, sequence of monomer types along the chain backbone, stereochemistry of these units and overall molecular architecture, e.g., branching vs. linear) for the attainment of novel properties and the investigation of structure-property relationships. Given the sophistication of current polymer synthesis, it is now possible to test structure-property hypotheses systematically and to rationally design macromolecules to form specified microstructures and provide desirable physical properties.

The typical mental image conjured by the word polymer is an entangled mass of cooked spaghetti. This is in fact very appropriate for the class of flexible chain polymers in the noncrystalline state. The pioneering work of P.J. Flory in elucidating the nature of such materials, e.g., polymer melts and amorphous polymers above their glass transition temperature, made crucial use of the essentially Gaussian behavior of the end-to-end distance vector of a flexible chain polymer in the condensed state.

Not much more than a decade ago, the plastics industry viewed itself as a mature branch of the heavy chemical industry. Its raison d'être was the mass production of four or five main-line polymers, and profits were equated to tonnage output, plant efficiency, and clever downstream processing such as film blowing. The chemistry was essentially simple and the monomer, of course, cheap. There was, however, a spark of new thinking. A trend was developing toward the design and manufacture of more complex, more expensive polymers, with special properties which could command a special price. Such products would sell advanced scientific know-how, not just engineering expertise which could all too easily be exported to the major oil producers in the form of a polymer plant.

Designing particular molecules to achieve desired properties is now a major theme of polymer producers. There is a move toward increasing the aromatic content of polymer backbones to achieve greater levels of chemical and thermal stability, while the development of new cross-linking systems remains as chemically intensive as ever. It is, however, the introduction of liquid crystalline polymers which, above all, has exploited the principles of molecular design, while at the same time challenging our understanding in a new area of polymer science.

A polymer is “liquid crystalline” where the chains are sufficiently rigid to remain mutually aligned in the liquid phase although the perfect positional periodicity of a crystal is no longer present. In other words there is a long-range orientational order without long-range positional order (Figure 1). Structurally, therefore, the phase is intermediate between a crystal and a liquid leading to the use of the term mesophase. Where the liquid crystalline phase forms on melting the polymer, it is known as thermotropic, but where it is achieved by solvent addition it is called Inotropic. Increasing temperature, or solvent concentration, will eventually lead to the reversion of the liquid crystal phase to the normal isotropic polymer melt.

Since the pioneering work of Carothers which led to the introduction of strong nylon fibers by DuPont in the 1930s, polymer scientists have pursued the development of high-performance fibers to replace natural or metallic products, both to improve mechanical properties and to reduce weight. That development was relatively slow and evenly paced until DuPont again revolutionized the field with the release of Kevlar™, an aromatic polyamide with unprecedented mechanical properties. Since then, the field has literally boomed with new developments, and now organic fibers are available with properties that compete with the best inorganics and are far superior to metal fibers.

Strong motivation for the invention of new organic fibers comes from the aerospace industry, which seeks fibers to use in reinforced composite structural materials. Composites bring new advances in stiffness (airplane wings can't bend too much!) in weight savings (every kilogram saved in the structure of an airplane saves $120 over its lifetime, and in a spacecraft $10,000), and in radically new ideas, such as radar-invisibility (stealth)5 and mission-adaptive wings (in-flight variable-shape wings). Hence, for a variety of specialty applications, otherwise commercially indefensible materials become viable.

It may be somewhat counter intuitive to materials scientists unfamiliar with polymers to expect polymer mechanical properties to be greater than in the best metals. The origin of high strength and stiffness in a polymer fiber is the covalent bond, especially when aligned in an ordered array of long chain molecules.

The importance of crystalline defects in determining the macroscopic properties of metals, ceramics, and semiconductors is well known. Crystalline polymers also exhibit defect structures, but analyzing of defects in polymers is more difficult because the topological connectivity of the covalently bonded polymer chains means that solid polymers typically exhibit larger amounts and types of disorder than crystals of low molar mass molecules. Imaging is also more complex because of the inherent electron beam sensitivity of these typically organic materials. Recent developments in synthesis and processing have made it possible to achieve very highly ordered polymer systems. Defects in these materials will undoubtedly play a critical role in determining mechanical properties, optical behavior, and transport properties such as electrical conductivity.

Direct imaging techniques are particularly important for studying defects because the typically low volume fraction of defects in the crystalline phase makes diffraction experiments difficult if not impossible. Also, in diffraction information about the relative orientation of individual crystallites is lost. It is not surprising, then, that High Resolution Electron Microscopy (HREM) has become extremely important for studying defects in inorganic materials systems. HREM studies of polymers have been limited by the beam sensitivity of organic compounds. Recently, however, high voltage instruments, low dose techniques, and image processing procedures have made it possible to obtain lattice images in systems with beam sensitivities as low as 0.003 C/cm2 (2 e-/A2).

This review discusses the nature of defects in crystalline polymer systems and illustrates how proper application of high resolution imaging techniques promises to answer questions in polymer morphology.

Polymeric materials are typically considered as insulators, and in fact important applications do rely on their poor conductivity— e.g., electrical cable insulation and charged dielectric films (electrets, electrical analogs of magnets), the latter finding use in microphones. Research in the last decade, however, has lead to the discovery of polymeric materials with extremely high conductivity, approaching that of copper. This brief article will highlight recent work in the synthesis, processing and applications of these novel materials.

Typical polymers, the oxidant (or “dopant”) used to create carriers, and room temperature conductivities are given in Table I. A key feature shared by these materials is delocalized (at least over a few repeat units) π-electron density. Such unsaturated polymers facilitate carrier generation because of the ability for resonance derealization of the resulting radical ions, which can also offer good intramolecular carrier mobility. In addition, the geometry of π-orbitals allows for good orbital overlap and encourages inter molecular carrier transport.

That the polymer chains are considerably shorter than typical sample dimensions indicates that intermolecular transport is dominant, especially in view of the disorder observed in most conducting polymer systems. However, as the number of defects (crosslinks, “twists” which inhibit conjugation) decreases, it might be anticipated that carriers can travel greater distances along a chain before a (presumably) higher activation energy, inter molecular electron transfer becomes necessary, thus affording higher conductivity. Indeed, it has been recently reported that samples of very high quality, oriented polyacetylene, when treated with iodine, exhibit conductivities approaching that of copper at room temperature.

Much of the anisotropic mechanical and thermal behavior exhibited by materials can be attributed to anisotropic orientation at the molecular level. In self-supporting thin films (5–10 microns) and those (0.01–1.0 microns) deposited on solid substrates, the role of molecular orientation is even more important since it will critically determine their two-dimensional behavior and their structural integrity as well. These two aspects are extremely important because if thin organic and polymeric films are to be competitive with existing materials for such diverse applications1 as chemical sensors and integrated optics, they must be mechanically robust and, ideally, defect free. These stringent constraints dictate that sophisticated characterization techniques, which can interrogate at the molecular level, be developed or refined so as to have the sensitivity to address these critical issues.

The development of nondestructive techniques for studying thin organic films has certainly lagged behind those developed for metallic and semiconductor films. Unfortunately, many of these same techniques cannot be simply applied to organic films because they are “invasive” and often alter the structure of the system they were designed to probe. This is especially so in organic and polymer films, and this awareness within the materials science community has led to the adaptation of many photon intensive techniques to the study of thin films. A number of these will be discussed in later sections with their relative merit put in perspective.

Certainly the origins of anisotropic structure in bulk materials are manyfold, but the spatial constraints in 2-D can lead to even more complex causes of orientation. In thin films on substrates the role of the surface is important in determining the ordering and orientation of the individual molecular segments which come into intimate contact with it. The extent of this orientation and order is still somewhat controversial but there is general agreement that it probably differs depending on the nature of the substrate.

Diffusion in polymer alloys or blends can be used to extract information on the fundamentals of the dynamics of individual polymer chains in the melt and the thermodynamics of the interaction between unlike polymer species. The dynamics of individual chains are available from measurements of the tracer diffusion coefficients, D*, of the various species while the thermodynamics of interaction, represented by the Flory parameter, x, can be obtained from measurements of the mutual diffusion or interdiffusion coefficient, D. We will show that these quantities can be measured conveniently by forward recoil spectrometry (FRES), an ion beam analysis technique that can determine the concentration versus depth profile of polymers labeled with deuterium diffusing into unlabeled polymer matrices.

For high enough molecular weight of the matrix, the tracer diffusion coefficient of both species in the blend scale as D0N−2, where N is the number of monomer segments per diffusing chain; the constant D0, however, can differ by more than 104 for chemically different molecules diffusing in the same blend, suggesting that conventional concepts of chain dynamics in melts, such as monomer friction coefficients, need to be reexamined. The mutual diffusion coefficient is controlled by the faster species in the blend (the one with the larger D*N product) in agreement with what was found in metallic alloys (but in sharp disagreement with the “slow” theory of mutual diffusion which predicts that the slower species controls). Since the combinatorial (ideal) entropy of mixing of polymers is low, the thermodynamic driving force for diffusion is dominated by enthalpy and excess entropy of mixing (x) to a degree unprecedented for atomic or small molecule systems. This means that one can observe not only a thermodynamic “slowing down” of diffusion when x becomes positive as one nears the spinodal but also a large thermodynamic “speeding up” of diffusion when x is negative. Measurements of mutual diffusion turn out to be one of the most sensitive methods available for measuring x.

Tom Read arrived at Columbia University on the same day that I did in 1948. He was a new professor in the School of Mines and I was a new graduate student. He was more than just a new professor. He was exceptional for that time. His father, T.T. Read, had been famous as an archeological metallurgist and professor at Columbia when the School of Mines was the premier school of its kind in the country. A measure of its eminence is that Irving Langmuir chose to study there rather than in a standard chemistry department.

The younger Tom Read had studied physics at Columbia under Shirley Quimby, one of the few solid-state physics professors of the time (pre-transistor). After graduation he worked at the Frankford Arsenal and at the Westinghouse Research Laboratories, where he and Frederick Seitz wrote their definitive review of the mechanisms of the plastic deformation of solids.

When he came back to Columbia as a professor, Tom Read's physics background made him almost unique among metallurgy professors. And, he had the zeal of a crusader — he was determined to teach fundamental knowledge rather than recipes. His techniques were often novel.

For example, one semester we were to learn about ferromagnetism. But he had trouble finding a good American text, so he announced that we would study both ferromagnetism and German using the famous book by Becker and Doring called Ferromagnetisms. As a result, I have never forgotten the essentials of ferromagnetism.