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Preface In the next millennium, the class of polymers labeled as smart or field responsive polymers will play a key role in the emergence of a wide array of new devices, such as those manufactured by aerospace, communications, and the expanding biomedical industry. A responsive or smart polymer is one that responds to an external stimulus in a controlled, reproducible, and reversible manner. This exter nal stimulus may be optical, electrical, mechanical, or environmental, such as a change in temperature or p H . The promise of smart polymeric materials results from their versatility and the relative ease by which structural changes can be synthesized into a polymer to create a desired functionality. Moreover polymers possess the processability that facilitates their incorporation into a variety of device configurations. In the past decade there has been a dramatic increase in the amount of research dedicated to the synthesis of novel responsive polymers. There are several markets just waiting to take advantage of these new materials. For example, in the telecommunications industry, low-loss, inexpensive optical wave guides and very-high-bandwidth, integrated modulators are two areas where improved N L O polymeric materials will provide breakthrough capabilities. Other technological concepts such as optical computing, microwave shielding, and trans mitting for stealth systems, synthetic enzymes, and targeted drug delivery have been theorized, but the materials for making them have not yet been optimized. In the paragraphs below, we have highlighted the enormous and exciting potential of field responsive polymers in the development of emerging technologies. We have illustrated by examples the breadth of applicability for responsive poly meric systems. A wealth of recent literature indicates that photorefractive polymeric materi als are key to the future technology of optical data storage and image processing. It is envisioned that photorefractive polymer technology may one day enable the storage of the entire Encyclopedia Britannica on a disk the size of a dime [see Dagani, R. C&E News, 28, February, 1995]. Such storage capability has the potential to revolutionize the manner and effectiveness of remote sensing and earth observation science missions by providing the capability to store large quantities of high-resolution digital images at high speed in a very compact device. The photorefractive effect is the spatial modulation of the index of refraction due to charge redistribution in an optically nonlinear material. The effect arises when charge carriers, generated by modulated light, separate via drift and diffu sion mechanisms and become trapped to produce a nonuniform space-charge
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distribution. The resulting internal space-charge electric field modulates the refractive index to create a phase grating that can diffract light. This threedimensional diffraction grating is a hologram that can be read with a reference laser beam and that will remain in the material when the light source is turned off. Several holograms can be stored in the same volume by tilting the sample or the laser source for each new hologram. The hologram can be readily erased by exposing the material to uniform light to evenly redistribute the charges. The hologram can also be inadvertently erased or altered if there is migration of trapped charges with time or variations in temperature. The exciting possibilities of responsive polymers have been demonstrated by many groups, and among the more fascinating examples were shown by Hoffman and co-workers [see Slayton, P. S.; Shimboji, T.; Long, C ; Chilkoti, A . ; Chen, G.; Harris, J. M . ; Hoffman, A . S. Nature, 1995,378,472]. The work involved is controlling the biotin-ligand recognition process by conjugation of a tempera ture-responsive polymer in the vicinity of the protein-binding pocket. The recogni tion process may be turned off and on by changing the temperature of the system; a change in the temperature causes the polymer to collapse and block the access of the biotin to the pocket. Such polymers have also been utilized in the important area of organic catalysis where Bergbreiter and co-workers have cleverly manipu lated temperature-responsive polymers. Bergbreiter successfully prepared smart catalysts whose reactivity may be regulated by a temperature-sensitive polymer support [see chapter 20]. B y increasing the temperature, the polymer collapses, thereby decreasing the accessibility of reactants to the active catalysts bound to the polymer, and hence the overall activity is decreased. Ostensibly the product may be easily recovered and the catalyst activity regenerated by decreasing the temperature. It is in the electronics industry where the research in the area of responsive polymers is currently most significant and visible. A number of recent studies have focused on the use of polymeric materials in smart microwave shielding or reflecting devices, which are important in stealth technology. These studies indi cate that conducting polymers are capable of variable microwave transmission [see Rupich, M . W.; L i u , Y . P.; K o n , A . B . Mat. Res. Soc. Symp. Proc. 1993,293, 163]. The setup used to demonstrate this consists of a multilayer structure where two conductive films are separated by a solid polymer electrolyte. The active conductive polymer is highly microwave transmitting when reduced and attenuat ing when oxidized. The passive conductive polymer is transmitting both in the reduced and the oxidized states. The microwave transmittance can be modulated by applying a voltage across the two conductive polymer layers. Responsive polymers whose microwave properties may be tuned upon the application of small bias voltages raises the possibility of developing newer types of responsive materials for use in the development of "smart skins", which are of interest to the avionics/defense industry [see Khan, S. M . ; Negi, S.; Khan, I. M . Polym. News, 1997,22, 414]. The aim of "smart skins" is to integrate antennas, sensors, transmit/receive (T/R) modules, preprocessors, and signal processors into the xii Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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platform of the skin during the structural design of an aircraft to yield a highly integrated, multifunctional, tunable structure [see Lockyer, A . J.; Kudva, J. N . ; Kane, D . M . ; H i l l , B . P.; Martin, C. A . ; Goetz, A . C ; Tuss, J. SPIE Proc, 1994, 2189, 172]. Multilayer polymeric structures have also been used to fabricate synthetic muscles or actuators [see Pei, Q.; Inganas, O. Synth. Met. 1993, 55-57, 3718]. A synthetic muscle or actuator functions by converting chemical energy into mechanical energy. For example, a multilayer structure capable of carrying out this function consists of a polyethylene layer, a thin gold layer, and a polypyrrole layer immersed in an electrolyte solution. Oxidation of the polypyrrole results in diffusion of the perchlorate ions from the electrolyte into the polymer and a net increase in the volume of the polymer, which causes deflection of the biopolymer strip. Undoping causes the perchlorate ions to be expelled from the polymer, and the multilayer structure returns to the original position. Thus, this electro chemical process has resulted in mimicking muscles of living organisms. Obvi ously, in this setup the liquid electrolyte is a drawback, and ideally an all-solidstate device is desirable. Wallace and co-workers have reported such an allpolymer solid-state electrochemical actuator [see Lewis, T. W.; Spinks, G . M . ; Wallace, G . G . ; De Rossi, C. E . ; Pachetti, M . Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 1991, 38(2), 520]. Large degrees of bending were obtained with this actuator composed of two polypyrrole layers separated by a solid poly mer electrolyte. In order for the full potential of these actuators to be realized, some of the drawbacks, such as slow response times and stability to recycling, must be solved. One method to decrease the response times of all polymer solidstate actuators may be to bring the solid polymer electrolyte and the electronic conductive polymer in intimate contact or within a few hundred angstroms of each other. This will significantly reduce the time required for the diffusioncontrolled doping-undoping process because the distance of ion transport will be decreased from the micrometer level to a few hundred angstom level. The microphase-separated mixed (ionic and electronic) conducting polymer system reported recently, in theory, will permit the fabrication of multilayer actuating structures with much faster response times [see Khan, I. M . ; L i , J.; Arnold, S.; Pratt, L . in Electrical and Optical Polymer Systems; Wise, D . L . et al. Eds.; Marcell Dekker, New York, 1998; p 331]. The future is indeed exciting and is dependent on the development of appro priate polymer systems to overcome drawbacks associated with some of the reported smart devices or structures. In addition to developing appropriate poly mer systems with the required properties, the processing properties of the poly mers must be addressed to allow fabrication of complementary polymers into appropriate smart device configurations. Polymers that respond to an external field in a controlled, reproducible, and reversible manner have tremendous application potential. A s we have highlighted in the preceding examples, the diverse utility of such polymers spans the gamut from biological to stealth to electro-optic systems. This highly interdisciplinary area includes research in chemical synthesis, property characterization, device xiii Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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development, and system integration. Chapters featured in this book divide the broader classification of responsive polymers into three more specialized catego ries: electroresponsive, photoresponsive, and responsive polymers in chemistry and biology. Within each of these categories some of the chapters offer an overview of specific types of responsive polymeric materials such as photorefrac tive and amorphous piezoelectric polymers. Other chapters present leading-edge research that encompasses a variety of materials, experimental techniques, theo ries, processes, and applications. The breadth of the information presented in this book should make it useful for materials scientists, polymer chemists, physi cists, and engineers in a broad spectrum of industries.
Acknowledgments Funding to support the symposium upon which this book is based was pro vided by the Polymer Chemistry Division of the American Chemical Society and P R F Grant # 32468-SE. We also greatly appreciate the help of Anne Wilson, Kelly Dennis, and Tracie Barnes of the A C S Books Department who provided the guidance and support to bring this book to a fruitful completion. Finally, we thank the contributors for their timely efforts and for sharing their valuable research results in this forum. ISHRAT M. K H A N
Department of Chemistry Clark Atlanta University Atlanta, G A 30314 JOYCELYN S. HARRISON
Langley Research Center National Aeronautics and Space Administration Hampton, V A 23681-0001
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