Frontiers of Polymers and Advanced Materials

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These polymers can be fabricated into specific structures in which their deformation or movement produces electrical signals that can be resolved. Electret materials, best exemplified by fluorinated polymers such as poly tetrafluoroethylene , can be fabricated into films, charged, and used to construct condenser-type acoustic transducers electret microphones. Ferroelectric polymers, such as poly vinylidene fluoride , can be poled by applying a strong electric field, and then used to construct acoustic, pressure, or thermal sensors.

They are applied in pyroelectric detectors, hydrophones, ultrasonic transducers, shock wave sensors, and tactile sensors for robotics. Often composites of these polymers with piezoelectric ceramics are used to provide enhanced performance. Polymers that emit light when exposed to ionizing radiation or high-energy particles are used as the active elements in radiation detectors scintillation detectors.

These polymer systems have advantages over liquid scintillation detectors because of their ease of fabrication and ruggedness with comparable sensitivity. For the last decade, the microelectronics industry has been engaged in a race to shrink the dimensions of semiconductor devices. The result of this effort is the continued improvement in the price-to-performance ratio of microelectronic devices and the myriad products that are produced from them see the vignette " Resists and Micromachines ". The market for silicon hardware will exceed.

Imagine a tiny robot—a micromachine—the size of a red blood cell, swimming through the arteries of a stroke victim until it reaches the blood clot in the victim's brain. The micromachine drills through the clot, restoring blood flow. The parts for such robots might one day be built using the same polymers that are used to stencil the incredibly complex pattern of an integrated circuit onto a silicon chip.

These polymers, called resists, react chemically when exposed to ultraviolet light, X-rays, or other energetic electromagnetic radiation. One polymer commonly used as a resist, poly methyl methacrylate , is better known to most people as Plexiglas. Tiny gears, for example, have already been made.

The process starts with a blank wafer of silicon, to which a thin layer of titanium has been applied as a sort of frosting. A layer of resist, as thick as the gear is supposed to be usually about a few microns thick, or much less than the thickness of a human hair , is applied to the titanium surface. The wafer is then bombarded with X-rays that have passed through a gold mask with many gear-shaped holes cut in it. Wherever the X-rays hit the wafer, the resist molecules become soluble. Wherever the wafer is shielded by the mask, the resist does not react and remains insoluble.

Washing the wafer in the solvent mixture leaves a gear-shaped hole in the resist to use as a form. The form is filled by electroplating copper into it—the titanium layer on the wafer is connected to a negatively charged electrode, and the wafer is immersed in a solution of copper ions. The copper deposits itself on the exposed titanium, but not on the resist, which does not conduct electricity. Once the resist is removed by an aggressive solvent or oxygen plasma, the free-standing copper gear remains on the titanium. Dunking the wafer, gears and all, into a bath of hydrofluoric acid dissolves the surface titanium, freeing the gears from the wafer.

Employing the pattern-making capability of polymer resists, it is feasible to make metal gears of the order of 1 micrometer in diameter. Living cells are tens of micrometers across, and a small blood vessel is about 50 micrometers in diameter. Thus, fairly elaborate machines appear possible, although other parts and assembly will require a great more development effort. The processes employed to manufacture the silicon hardware are intrinsically dependent on the polymeric materials that are used to define the patterns required for the many layers of circuitry.

In a typical process, several hundred steps are required to produce a wafer containing hundreds of chips. About two-thirds of these steps are devoted to pattern formation, a form of lithography. In the process as practiced by the semiconductor industry, the silicon wafer on which the devices e. Pattern-wise exposure of the resist to radiation of the appropriate wavelength results in a radiation-induced chemical reaction in the resist film, which renders the exposed areas more soluble in some developer solvent positive tone imaging or less soluble negative tone imaging.

The pattern is formed by passing the radiation. The result is a relief image consisting of regions of resist and regions of bare circuit. These relief images in the resist allow the underlying substrate to be processed selectively in those areas where the resist has been removed. The processes involved include etching, metal deposition, ion implantation, and oxidation of silicon. Virtually all production of semiconductor devices is accomplished by exposing the resist film to UV radiation through a projection lens system analogous to the familiar slide projector, although exposure tools shrink the projected image rather than expand the image of the slide.

Present systems employ UV radiation with a wavelength of nanometers nm , but nm systems are being introduced. Electron beam and X-ray radiation offer alternatives for the future. Each change in wavelength and radiation type requires development of new polymeric resist materials. Owing to the high cost of the exposure tools, it is important that the throughput of the machines e. The amount of light available at nm is only one-tenth that provided by the older machines operating in the near UV.

Therefore, the feasibility of moving to deep UV was entirely dependent on the ability of chemists to develop new generations of polymeric resists that are as much as times as sensitive as resists formerly used. These new resists derive their high sensitivity from exploitation of an acid-catalyzed reaction that converts an insoluble moiety to one that is soluble.

Exposure converts a neutral substance into an acid, thereby generating a latent image of the mask. The resulting films are then baked to provide the activation energy necessary to start the catalytic reaction in which the acid generated upon exposure facilitates a reaction of the resist polymer i. The radiation-created catalyst can convert many polymer groups, giving rise to the "chemical amplification" made necessary by the scarcity of deep-UV photons. Although there are many other factors involved in moving from near-to deep-UV lithography, both chemical and other, development of the chemically amplified polymeric resists was an essential contribution.

As the lateral dimensions of devices shrink, the width of the resist images required to define their component structures must shrink also. The thickness of the resist film does not shrink, however, owing to the necessity of being pinhole free and robust in subsequent processing e. Thus, the aspect ratio of the relief structure is increasing and could be as high as five by the end of the decade.

This is a very demanding requirement that will require a significant advance in resist technology. One promising approach to the production of high-aspect-ratio imaging at small dimensions is "top surface imaging. The transformation is designed to. If silicon is incorporated, subsequent anisotropic oxygen etching of the film results in rapid formation of a thin layer of silicon dioxide in the areas that reacted with the reagent. This thin oxide layer protects the polymer beneath while the unexposed, unprotected polymer areas are etched away by the oxygen plasma. The products of the etching are gaseous and are pumped away.

The aspect ratio of the image produced by this process is dependent on the anisotropy of the oxygen etching process. Aspect ratios exceeding five in polymer relief images have been achieved by this method. Although many features of the top surface imaging procedure remain to be worked out, it is a promising method. Some solution must be found that will provide support for the research and development required to produce these materials that are so critical to the continued advance of semiconductor technology.

Sematech, the U. A few U. It will be interesting to see how this conflict between the demand for small volumes of highly sophisticated specialty polymers and the high cost of developing such materials plays out over the next few years. Compact disks have emerged as the dominant recording medium for the musical entertainment field. Information is recorded as a series of pits on radial or concentric tracks that extend from the inner to the outer diameter of the disk. The pits are typically 0. The information is read by means of a laser beam that is reflected when it falls on the flat of the disk but is almost entirely deflected when it falls on a pit.

This digital bit stream is converted to an analog signal to reproduce the music. The disk itself is made of a polymer by means of a process that is technically demanding and economical. The manufacturing sequence consists of encoding the pit pattern onto a glass master.

This is accomplished by means of a lithographic process employing a polymer resist and an irradiating laser. The open areas thus formed are etched to form the pits. Nickel is then vacuum deposited, thickened, and formed into a negative "stamper. The molding process is carried out at very high pressures, and dust particles must be avoided. A class cleanroom is usually required to achieve quality replication and long stamper life.

Molded-in stress is a major consideration. When polycarbonate flows in the submillimeter thickness of optical disks for several centimeters, there is significant molecular orientation that manifests itself as birefringence or optical distortion. Material and process parameters have been refined to control birefringence and maintain replication integrity. New long flow grades of the materials have been developed specifically for the CD market. Photonics is a technology analogous to electronics in which the photon replaces the electron as the working particle.

Many of the applications now accomplished electronically, including transmission, switching, amplification, and modulation, can also be realized using photonics, and there are advantages to be gained by converting to a photon-based technology in some areas. Transmission of light in fiber-optic systems is the direct analogy of electrical transmission in coaxial cable systems. Fiber-optic systems are now in place all over the world, and they handle much of the world's long-distance telephone traffic.

The transmission medium of the fibers employed is based on inorganic glasses, but polymers are used for protective coatings and in cabling structures. Polymers can also be made into optical fibers, but the loss is considerably larger than with the inorganic fibers and only short-distance applications are realistic. The main advantage of polymer fibers is their flexibility when made in larger diameters, which are easier to splice.

Today, fiber-optic cables are generally terminated at the area substation level, where the optical signal is converted back to an electrical signal for transmission to the customer. This conversion process is necessary because the optical components needed to reach the individual telephone or terminal are not available at sufficiently low cost at this time. What is required to allow fiber to be connected to the home are inexpensive optical switches and amplifiers, which will enable the advantages of broad-band communications to be brought to every subscriber. Polymeric organic materials will play a major role in the realization of optical technology as fiber to the home becomes a reality.

Two kinds of optical technology need to be developed and commercialized before the photonics revolution can be fully realized. The first is linear optical technology, which includes not only the long-distance fibers mentioned above, but also shorter fibers and the optical equivalent of printed wiring boards of the electrical domain. These optical circuits can be created today by means of a photolithographic procedure in which lines of high refractive index are formed in thin polymer films by photochemical techniques. The circuit pattern is defined by irradiating a photoresist through a mask.

The substrate film bared by development of the resist is then exposed to light, which causes the chemistry, for example, the polymerization of monomers, that gives rise to the increase in refractive index needed to form an optical guide. Nonlinear optical materials will also be required for the manufacture of switches, modulators, and amplifiers, and this technology has not progressed as far as the linear domain.

Demonstration-of-principle devices have been fabricated. The necessary switches, amplifiers, and modulators can be made today with inorganic materials, but there is some question whether these realizations can be combined and manufactured in large volume at sufficiently low cost. Organic thin film technologies may fit the economic as well as the technological requirements, but many advances will be required and the outcome is uncertain. A nonlinear polymer in general has two components: the polymer itself and an optically nonlinear molecule a chromophore that is either chemically attached to the polymer or dissolved in it.

In order for the polymer-chromophore system to be optically nonlinear, the chromophores must be aligned such that on average they are all pointing in the same direction within the polymer matrix. This alignment is accomplished through a process called poling. The polymer is poled by cooling it through the glass transition temperature while it is in a very strong electric field, and the order induced by the field is frozen in. Poled polymeric systems have process and property advantages over their inorganic crystalline competitors.

The polymers can be formed into thin films and lithographically patterned, and they can be chemically modified to tailor and improve bulk properties. There are disadvantages as well, in that the orientation in the poled polymer systems tends to decay with time, a problem that can probably be overcome. Recently, light-emitting diodes LEDs based on conducting polymers have been achieved in a number of laboratories around the world.

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The active element is a thin film structure based on a modified poly phenylene vinylene PPV , with a metal film as the electron injector and polyaniline as the hole injector. Various colors have been demonstrated, and the operating characteristics are competitive with inorganic LEDs. Highly flexible devices have been fabricated supported on a poly ethylene terephthalate base. The possibility of making large-area displays exists.

Much research and development remains to be done. For example, low-work-function metals are required, and they are difficult to passivate. However, the simplicity of fabrication of the laboratory devices, involving spin casting from solution, is promising if the problem of limited device lifetime can be solved.

One of the major applications of polymers with tailored electronic and optical properties has been in electrophotography for copier, duplicator, and printer applications. In this application an electroactive polymer is used as one component of the light-sensitive element used for creating the latent electrostatic image. The image source can be light reflected from a document and focused onto the surface of the photoreceptor or a digital file of an original image, which is used to control a laser beam that is scanned over the surface of the photoreceptor.

The electrostatic image is rendered visible by dusting the surface of the photoreceptor with an electrostatic powder composed of a pigment-loaded thermoplastic polymer. The latent image can then be transferred to paper by a combination of pressure and electrical bias and then fused to the paper by heating. The photoreceptor itself was the key invention that enabled the development of electrophotography as a commercial success.

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The original photoreceptor materials were based on selenium and its alloys as well as group II-VI and other semiconductor materials. Because of the poor mechanical properties of selenium and its alloys, photoreceptors had to be fabricated on rigid metallic drums.

This, in turn, dictated relatively cumbersome and expensive copier machine architectures. These materials had a number of shortcomings, including degradation of photoconductive properties, instabilities in surface properties leading to incomplete toner transfer, and catastrophic abrasion.

Research efforts in several industrial and university research laboratories were successful in identifying polymeric materials that exhibited photoconductivity. The early photoconductive polymers were mainly sensitive to UV light. The copying process, however, requires differential reflectivity from the printed areas of the original document, which is very low for UV light but much higher for visible light sources.

The need for visible light sensitivity was therefore apparent. Eventually, the problems associated with spectral sensitivity and a variety of other technological requirements were solved, and it was clear that the polymeric materials could be used advantageously in copier and printer technology. The latent electrostatic image is formed by first depositing a layer of ionic charge from a corona discharge onto the photoreceptor surface. This induces an equal but opposite charge on the metal layer below, resulting in the formation of an electric field within the photoconductor layers.

As light passes through the transport layer and is absorbed by the photosensitive pigment layer, the pigment molecules are photoionized with the assistance of the internal electric field to form mobile charge carriers. The negative photogenerated charge in the film drifts under the influence of the electric field to the metal, and the positive charge drifts through the transport layer and neutralizes some of the ionic charge that was deposited on the surface.

Since photogeneration will occur only where light strikes the photoreceptor, a pattern of ionic charge corresponding to the original image is formed on the surface. The surface potential associated with this charge distribution is used to attract the toner as described above.

Frontiers of Polymers and Advanced Materials

Photoreceptor belts have been engineered to exhibit excellent mechanical properties, and this achievement has allowed the design of compact and cost-effective copier and printer architectures. Useful lifetimes, photosensitivities,. Photoreceptor wavelength sensitivities have now been extended to the near-infrared so that inexpensive diode lasers and light-emitting diode arrays can be used for digital printing applications. A light interference pattern comprising relatively large light intensity variations on a microscopic scale is created where two previously separate light beams from the same laser intersect.

A hologram is a physical record of such a pattern and is formed by exposing a photosensitive recording film to the interference pattern. When a hologram is illuminated with one of the two laser beams used in its recording, it produces a light beam that is essentially identical to the other recording beam. Familiar image holograms are usually produced from a simple collimated or diverging light beam, called a reference beam, and a beam formed by scattering light from a complex three-dimensional solid object.

When such a hologram is illuminated with the reference beam, it produces a light beam that appears to come from the solid object used in its recording. Holograms can also be made from light beams produced by conventional optical elements such as lenses and mirrors. The resulting holograms, called holographic optical elements HOEs , perform optical functions of the elements used in their recording. One HOE type, for example, is recorded with a collimated light beam and a light beam that converges to a focal point.

When illuminated with the collimated beam, the resulting hologram will produce a focused beam; it acts, therefore, as a holographic lens. HOEs have important advantages over conventional optical elements. They are lightweight and compact and can take the place of heavy and bulky glass elements. They can be made very large or very small. They can replace expensive conventional optics for the production of arbitrarily complex light beams.

They can be inexpensively mass produced. There is a wide variety of current and potential applications for holography. The use of holographic three-dimensional images is probably the most familiar application. These images are typically used on credit cards and for product advertisement and promotion. In these applications, holograms add both eye appeal and security.


Holographic images are also used in nondestructive testing. Holographic optical elements can be made in large thin films for use in solar lighting control and solar energy collection, and they can be made very small for use in optical communication systems. Narrow-band holographic mirrors may also be useful for laser eye protection. Optical computing, pattern recognition, and very-high-density information storage are other potential applications of holography. Conventional phase hologram recording materials have, unfortunately, limitations that have inhibited the growth of practical holography.

Phase hologram recording based on photopolymerization is a relatively recent development, and it promises to overcome important problems of current recording materials. Holographic photopolymer systems comprise, as major components, a film-forming polymer often called the binder , a photoinitiation system, and a monomer. The polymer binder aids in coating the appropriate substrate and helps to maintain film integrity during holographic exposure and subsequent processing. Properties of the binder can also strongly influence both the shelf life of the coated film and the rates and extents of photochemical reactions that occur during hologram formation.

Monomers join in a chain reaction during laser exposure. In fact, the relatively good light sensitivity of photopolymers results from the large number to 1, of monomer units that react per absorbed photon. The chemical and physical changes associated with monomer polymerization preserve the interference pattern created during exposure as a corresponding pattern of refractive index variation.

Numerous and diverse chemical and physical requirements greatly limit monomer choice. Shelf life and light sensitivity must be balanced. Film clarity and image stability are essential. Large refractive index changes are desirable. The monomer must also be compatible with the other components of the system. The ideal material does not yet exist. There is an excellent chance, however, that continued research with photopolymer systems will produce new holographic recording materials that will make practical many potential applications of holography.

The foregoing examples illustrate the breadth of application of advanced polymeric materials in applications that are not generally recognized. In these applications, the polymer is in some sense the active element that plays the central role. No other class of materials can rival its range of properties, flexibility in processing, and potential for low cost.

Quality of performance is an essential and challenging feature that is being demonstrated by polymeric materials in an impressive array of applications. And the polymer revolution in this arena is just beginning. The importance of polymers in advanced technology is a key factor in the future of materials development, as indicated in the following applications:. Polymer dielectrics in electronics offer the basis for the smallest circuits and the highest speed of operation. Conducting polymers have been commercialized in rechargeable batteries and offer the greatest promise for high energy storage with low weight.

Polymer sensors exist for chemical species, thermal and acoustic radiation, temperature, pressure, humidity, ionizing radiation, electric charge, and more. Buildings can be equipped with a network of optical fibers linking remote locations with a management console. The polymer sensors can be built into the optical fibers to report the presence of toxic gases or to turn off unneeded lights to conserve energy. Implanted sensors can detect the glucose level in blood and call for insulin injections by means of an implanted pump, as needed.

Electromagnetic shielding will become increasingly necessary, and conducting polymers offer solutions that are conveniently fabricated in complex shapes. Polymer resists are the basis for the microlithography that makes integrated circuit electronics possible. They are also the basis for the emerging field of micromechanics, which could produce machines smaller than a human cell.

High-density information storage is available through compact disk technology, and improved polymers will improve the performance of this medium. In the future, polymer-based holographic devices could revolutionize the storage and manipulation of information. Polymers offer solutions to critical economic problems facing the introduction of photonics, the light analog of electronics. The couplers, splitters, and other elements of photonic "circuit boards" all admit to polymeric solutions that may provide the economic breakthrough needed for the photonic revolution.

Broad-band communications can be brought directly to the home and office by polymer or glass fibers, using polymeric photonic circuits. The fabrication of liquid crystal display devices for computers and television can be facilitated and the robustness of the product enhanced by the incorporation of conductive, transparent polymer films.

Light-emitting diodes based on flexible polymeric films have been fabricated and are likely to find diverse applications in the future. Electrophotography is now based on polymeric photoactive materials, and these have made possible many improvements, such as compact and convenient machine architecture, durability of machines, and long-term print quality. Polymers are now the recording medium of choice for holography in many applications. This technology offers the promise of ultrahigh-density information storage.

The field is flourishing, and the future is bright. The United States must participate vigorously in this emerging area, from research to development to. Competition in the field is worldwide and moving rapidly ahead. Physicians' Desk Reference. Montvale, N. Sze, Simon. VLSI Technology. Polymers are used in everything from nylon stockings to commercial aircraft to artificial heart valves, and they have a key role in addressing international competitiveness and other national issues.

Polymer Science and Engineering explores the universe of polymers, describing their properties and wide-ranging potential, and presents the state of the science, with a hard look at downward trends in research support. Leading experts offer findings, recommendations, and research directions. Lively vignettes provide snapshots of polymers in everyday applications.

The volume includes an overview of the use of polymers in such fields as medicine and biotechnology, information and communication, housing and construction, energy and transportation, national defense, and environmental protection. The committee looks at the various classes of polymers--plastics, fibers, composites, and other materials, as well as polymers used as membranes and coatings--and how their composition and specific methods of processing result in unparalleled usefulness.

The reader can also learn the science behind the technology, including efforts to model polymer synthesis after nature's methods, and breakthroughs in characterizing polymer properties needed for twenty-first-century applications. This informative volume will be important to chemists, engineers, materials scientists, researchers, industrialists, and policymakers interested in the role of polymers, as well as to science and engineering educators and students. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

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    Novel Polymeric Composite Materials for Photonics. Paras N. Prasad, Maciek E. Properties and Applications of Polymers in Optics and Electrooptics. James T. Yardley, Karl W. Liquid Crystal Nonlinear Optics. Kajzar, F. Charra, J. Nunzi, P. Raimond, E. Idiart, M. Bosshard, H. Looser, P. Larry R. Plastics as Novel Optical Materials. Photoexcitations in Polydiacetylenes. Dellepiane, C.

    Cuniberti, D. Comoretto, G. Lanzani, G. Musso, A. Piaggi et al. Charles W. Sapochak, Larry R. Dalton, R. Sai Kumar. Polymers with Special Optical Properties. Rao, F.