Smart polymers -a brief review.
As our understanding of polymers has increased, they have come to be regarded not just as large-volume, inexpensive materials, but increasingly as indispensable components of sophisticated systems such as fiber optics and liquid crystal displays, where the value of the polymer lies in its special characteristics, rather than just in its bulk mechanical properties. Polymers having a second valuable characteristic, such as optical clarity or electrical conductivity, have been labeled "functional polymers," and subsequently, "smart materials."
Progress in the use of polymers in a systems approach has been worldwide. Takayanagi of Kyushu University in Japan has detailed the origin, in 1972, of the concept of functional polymers. These compositions are described as having the ability to respond to stimuli such as stress, heat, light and electricity. An early example of functional materials providing information on their state is the well-known use of bire-fringence to monitor stresses in polymeric components. These ideas have led directly to the concept of intelligent, or smart materials. Their design has emerged from the potential for integration of sensing, information processing, and actuating functionalities within materials.
Continuous reinforcements in composites offer special opportunities for on-line evaluation of both stress exposure and life expectancy. Moreover, responses detected through fibers may be potentially utilized to adjust properties to the environment detected, that is, to enable the material to perform as a smart polymer.
Spectra of Fibers
Observed spectral shifts in organic fibers resulting from tension can represent an in situ strain gage for composite parts. Leaders in this new science include Robert Young of the University of Manchester, U.K., and Shaw Hsu of the University of Massachusetts, Amherst. Each uses Raman spectroscopy as a detector and has correlated spectral shifts with stress for several organic fibers. For example, frequency changes in Raman-active normal vibrations have been observed by each group as a function of strain in fibers of the DuPont aramid, Kevlar 49.
Compressive strain generated by residual thermal stresses has been spectroscopically observed by Hsu in fibers of polydiacetylene single crystals in an epoxy matrix. He has also reported the spectroscopic analysis of stress and strain distribution along fibers, with comparison made with finite-element analyses, in J. Polym. Sci., Polym. Phys. Ed., 30, 619 (1992). This spectral detection method can thus provide a direct, real-time measure of stress transfer from matrix to fiber--so crucial to composite performance.
Conduction of Fibers
On-line methods for fiber and composite evaluation may replace traditional off-line methods such as X-ray and ultrasonic analyses. Consider composites reinforced with continuous carbon fibers. A fraction of the fibers may be ionically doped and placed in electrical circuits to monitor deformation and rupture by changes in circuit inductance and resistance. Thus, direct and on-line cockpit read-out of aircraft damage appears achievable via the carbon fiber/epoxy composites that are used for the skin of high-performance U.S. military aircraft.
The conductance levels of carbon fibers can approach those of metals, particularly on a specific or weight basis. These fibers are also used for electromagnetic shielding and static charge dissipation, as well as for composite reinforcement.
The level and directional dependence of thermal conductivity can also lead to a smart polymer. Carbon fibers can exhibit both high thermal and electrical conductivities along the fiber or chain direction.
The surprise is that nonelectrically conducting fibers of polyethylene also have high thermal conductivity along their length. Choy of the Chinese University of Hong Kong has measured thermal conduction comparable with stainless-steel fibers along the fiber direction. Continuous polyethylene fibers can therefore possibly be used in composites as heat pipes or conduits. Such composites may be used for thermostatting homes by balancing house and underground temperatures, and for dissipating heat from electronic devices. The thermal conductivity transverse to the fiber is even less than that for a block of conventional polyethylene.
Optical fibers are already being evaluated for strain and damage detection in composites. The optical fiber can serve as both a strain gage for the fiber and for evaluation of matrix-fiber delamination. This optical method for detection again relies on continuous-fiber reinforcement; see Composites, 19, 288, 335 (1988).
S.R. Waite and co-workers at the City University, London, have demonstrated that a single glass optical fiber may fail before the composite in which it is embedded. The fiber can thus function as a strain-threshold detector. Moreover, the small, 140-micron-diameter optical fiber has a negligible effect on composite properties, representing less than 0.1% of the composite cross-sectional area. Optical fibers can also serve as detectors for geometric and photoelastic changes on application of stress. Strain changes to 0.1% have been evaluated using a linearly bire-fringent monomode optical fiber embedded in a composite. A schematic of Waite's test method is shown in Fig. 1.
Visual color changes, achieved by the time- and temperature-induced polymerization of substituted diacetylenes, can be made to parallel the spoilage rate of a packaged food product. Lifelines Technology Inc., Morris Plains, N.J., reverse prints a diacetylene-containing ink on a transparent poly(ethylene terephthalate) film that filters out UV and visible light. Once the polymer response that parallels the food spoilage rate is determined, the labels are printed and stored in a deep freeze until adhesion to the food package. As long as the polymer color is not darker than the color of the reference ring, the product is fresh. More than 10 million such labels are already in use.
R.J. Young and co-workers, Polymer, 35, 80 (1994), have developed an optical strain-sensitive coating by reacting diacetylene into a polyurethane network. The urethane copolymers, with polydiacetylene as the hard segment, produce an intense Raman spectrum. In particular, the remaining carbon-carbon triple bonds exhibit a sensitive stretching frequency at about 2090 |cm.sup.-1^. This frequency shifts by -20 |cm.sup.-1^ for 1% tensile strain. These tough and adherent copolyurethanes can be applied as a paint to a range of substrates. Moreover, stress concentrations around holes, cracks, and design features may be readily examined with a Raman spectrometer equipped with a microscope.
Spectroscopic, electrical, thermal and optical methods have been described for the on-line evaluation of strain in polymers and their composites. These four types of signals may be used to activate responses to strain in their compositions. Applications can include control of aircraft wing warp and modulation of vehicle suspension systems. For materials in biosystems, Dr. Aizawa of the Tokyo Institute of Technology has envisioned a set of sensors that can change inert polymer body implants into responsive artificial bones, muscles, and nerves.
In an interesting and broader definition of smart materials for polymers, Darrell Reneker and co-workers, in the new journal Smart Materials Structures, page 84 (1992), describe smart molecules as those stimulated by photons, other molecules, electromagnetic fields, and by pressure, temperature or shear to induce a useful effect. The effect may be absorption of a photon, a physiochemical reaction, or a conformational change that may lead to a useful change of color, refractive index, conductivity, or connectivity.
Smart molecules in membranes were discussed this year by Daly, Poche, and Negulescu in Progress in Polymer Science. Stimulus-response membranes can have applications in selective release and permeation by imposing a specific stimulus.
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|Author:||Porter, Roger S.|
|Date:||May 1, 1994|
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