418

CHAPTER 11SMART MATERIALS

James A. HarveyUnder the Bridge Consulting, Inc.Corvallis, Oregon

1 INTRODUCTION 418

2 PIEZOELECTRIC MATERIALS 419

3 ELECTROSTRICTIVEMATERIALS 422

4 MAGNETOSTRICTIVEMATERIALS 423

5 ELASTORESTRICTIVEMATERIALS 424

6 ELECTRORHEOLOGICALMATERIALS 424

7 MAGNETORHEOLOGICALMATERIALS 424

8 THERMORESPONSIVEMATERIALS 425

9 pH-SENSITIVE MATERIALS 426

10 LIGHT-SENSITIVEMATERIALS 426

11 SMART POLYMERS 426

12 SMART (INTELLIGENT) GELS(HYDROGELS) 427

13 SMART CATALYSTS 428

14 SHAPE MEMORY ALLOYS 428

15 UNUSUAL BEHAVIORS OFMATERIALS 429

16 COMMENTS, CONCERNS, ANDCONCLUSIONS 429

17 FUTURE CONSIDERATIONS 430

REFERENCES 431

1 INTRODUCTION

The world has undergone two materials ages, the plastics age and the composite age, duringthe past centuries. In the midst of these two ages a new era has developed. This is the smartmaterials era. According to early definitions, smart materials are materials that respond totheir environments in a timely manner.1–4

The definition of smart materials has been expanded to materials that receive, transmit,or process a stimulus and respond by producing a useful effect that may include a signalthat the materials are acting upon it. Some of the stimuli that may act upon these materialsare strain, stress, temperature, chemicals (including pH stimuli), electric field, magnetic field,hydrostatic pressure, different types of radiation, and other forms of stimuli.5

The effect can be caused by absorption of a proton, a chemical reaction, integration ofa series of events, translation or rotation of segments within the molecular structure, creationand motion of crystallographic defects or other localized conformations, alteration of local-ized stress and strain fields, and others. The effects produced can be a color change, a changein index of refraction, a change in the distribution of stresses and strains, or a volumechange.5

Another important criterion for a material to be considered smart is that the action ofreceiving and responding to stimuli to produce a useful effect must be reversible. Another

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.Edited by Myer Kutz

Copyright 2006 by John Wiley & Sons, Inc.

2 Piezoelectric Materials 419

important factor in determining if a material is smart pertains to its asymmetrical nature.This is primarily critical for piezoelectric materials. Other types of smart materials exhibitthis trait. However, little research has been performed to verify this observation.

Also, it should be pointed out that the word ‘‘intelligent’’ is used to describe smartmaterials. The notation ‘‘smart’’ has been overused as a means to market materials andproducts.

From the purist point of view, materials are smart if at some point within their perform-ance history their reaction to a stimulus is reversible. Materials that formally have the labelof being smart include piezoelectric materials, electrostrictive materials, electrorheologicalmaterials, magnetorheological materials, thermoresponsive materials, pH-sensitive materials,UV-sensitive materials, smart polymers, smart gels (hydrogels), smart catalysts, and shapememory alloys. In this treatment of the subject we will be using some of these classifications;in some cases, however, the classification of a particular material may appear to be in error.This will be done to illustrate the rapid growth of the field of smart materials and therediscovery of the smart behavior of materials known for centuries. As we continue to betterunderstand smart materials, our definitions will change. In each material section there willbe discussions pertaining to the material definition, types of materials that belong to thatclass, properties of the members, and applications of the materials. In some cases a moredetailed discussion of application will be given to both illustrate the benefit of these materialsand simulate the use of these materials in new applications.

Another important feature of smart materials is their inclusion in smart structures, whichare simply structures with at least one smart material incorporated within its structure andfrom the effect of the smart material causes an action. A smart structure may have sensors(nerves), actuators (muscles), and a control (brain). Thus, the term biomimetric is associatedwith smart structures. Smart structures are being designed to make our life more productiveand easy. With the number of sensors, actuators, and control systems available, coupled withthe materials and the genius of scientists and engineers, these structures are becoming morecommonplace. Reading nontechnical magazines, watching television, and going to stores canverify this.

In a comical manner the growth of smart structures can be illustrated by an article thatappeared in the December 14, 2000, edition of the New York Times. This article describesthe ‘‘Big Mouth Billy Bass,’’ the singing fish, and how smart this novel toy is.

Examples of technical applications of smart structures are composite materials embed-ded with fiber optics, actuators, sensors, microelectromechanical systems (MEMSs), vibrationcontrol, sound control, shape control, product health or lifetime monitoring, cure monitoring,intelligent processing, active and passive controls, self-repair (healing), artificial organs,novel indicating devices, designed magnets, damping aeroelastic stability, and stress distri-butions. Smart structures are found in automobiles, space systems, fixed- and rotary-wingaircrafts, naval vessels, civil structures, machine tools, recreation, and medical devices.5,6

Another important feature related to smart materials and structures is that they encom-pass all fields of science and engineering. When searching for information on smart materialsand structures, there are numerous sources, web sites, and professional societies that dealwith this technology.

2 PIEZOELECTRIC MATERIALS

The simplest definition of piezoelectric materials can be obtained by first dividing the wordinto piezo and electric. Piezo is from the Greek word piezein, which means ‘‘to press tightlyor squeeze.’’ Combining piezein with electric, we have ‘‘squeeze electricity.’’

420 Smart Materials

The history of piezoelectric materials is relatively simple and only the highlights willbe presented. In 1880, Pierre and Paul-Jean Curie showed the piezoelectric effect in quartzand Rochelle salt crystals. Their first observations were made by placing weights on thefaces of particular crystal cuts, like the X-cut quartz plate, detecting charges on the crystalsurfaces, and demonstrating that the magnitude of the charge was proportional to the appliedweight. This phenomenon has become known as the direct-pressure piezoelectric effect. In1881 G. Lippmann predicted that a crystal such as quartz would develop a mechanical strainwhen an electric field was applied. In the same year the Curies reported the converse pressurepiezoelectric effect with quartz and Rochelle salt. They showed that if certain crystals weresubjected to mechanical strain, they became electrically polarized and the degree of polari-zation was proportional to the applied strain. The French inventor Langevin developed thefirst SONAR using quartz crystals in 1920. During the 1940s researchers discovered anddeveloped the first polycrystal line, piezoelectric ceramic, barium titanate. A significant ad-vantage of piezoelectric ceramics over piezoelectric crystals is their ability to be formed ina variety of shapes and sizes. In 1960, researchers discovered a weak piezoelectric effect inwhalebone and tendon that led to intense search for other piezoelectric organic materials.And in 1969, Kawai found very high piezoelectric activity in polarized polyvinylidene flu-oride (PVDF).7,8

The piezoelectric effect exists in a number of naturally occurring crystals, such as quartz,tourmaline, and sodium potassium tartrate. For a crystal to exhibit the piezoelectric effect,it must not have a center of symmetry. When a stress (tensile or compressive) is applied toa crystal with a center of symmetry, it will alter the spacing between the positive and negativesites in each elementary cell unit, thus causing a net polarization at the crystal surface. Theeffect is approximately linear. The polarization is directly related to the applied stress. It isdirection dependent. Thus, compressive and tensile forces will generate electric fields andvoltages of opposite polarity. The effect is also reciprocal; thus, when the crystal is exposedto an electric field, it undergoes an elastic strain changing its length based upon the fieldpolarity.

As previously mentioned, the ceramic-type materials play an important role in the areaof piezoelectrics. Piezoelectric ceramics are polycrystalline ferroelectric materials with aperovskite-crystal-type structure. The crystal structure is tetragonal / rhombohedral with aclose proximity to cubic in nature. Piezoelectric ceramics have the general formula ofA2�B4� where A represents a large divalent metal ion such as barium or lead and B is2�O ,3

one or more tetravalent metal ions such as titanium, zirconium, or manganese. These ceram-ics are considered to be masses of minute crystallites that change crystal forms at the Curietemperature. Above the Curie temperature, the ceramic crystallites have a simple cubic sym-metry. This form is centrosymmetric with positive and negative charge sites coinciding; thusthere are no dipoles present. The material is considered to be paraelectric. Below the Curietemperature, the ceramic crystallites have a tetragonal symmetry; this form lacks a center ofsymmetry with the positive and negative charge sites no longer coinciding, thus each unitcell has an electric dipole whose direction may be reversed and switched by the applicationof an electric field. The material is now considered to be ferroelectric.

The piezoelectric properties of ceramics can be enhanced by applying a large electricfield at an elevated temperature, thus generating an internal remnant polarization that con-tinues long after the removal of the electric and thermal fields. This technique is known aspoling. The poling of piezoelectric ceramics has eliminated the use of piezoelectric crystalsin many applications.

The vinylidene fluoride monomer CH2 CF yields the semicrystalline polymer (PVDF)——upon polymerization. This polymer was found to be highly piezoelectric. Polyvinylidenefluoride is manufactured in sheet form from the nonpolar �-phase film extruded from the

2 Piezoelectric Materials 421

melt. The extruded film is uniaxially stretched. This process rotates the long polymer chainsand forms the polar �-phase. The �-phase is needed for the high piezoelectricity. The finalstep consists of reorientation of the randomly directed dipoles associated with the stretched�-phase by applying a poling field in the direction normal to the plane of the film. Theresulting piezoelectric PVDF has orthotropic symmetry.

Copolymers and rubber and ceramic blends of PVDF have been prepared for use aspiezoelectric materials. The most common copolymer is one based on the polymerizationreaction of vinylidene fluoride and trifluoroethylene. Polyvinylidene fluoride and its copol-ymers with trifluoroethylene have low mechanical quality factors and high hydrostatic-moderesponses and their acoustic impedance is similar to water, making them ideal for underwaterhydrophone applications.9

A series of polyimides has been developed at NASA Langley for use in piezoelectricapplications. These polyimides have pendant trifluoromethyl (–CF3) and cyano (–CN) polargroups. Whenever these polyimides are exposed to applied voltages of the order of 100 MV/m at elevated temperatures, the polar groups develop a high degree of orientation, resultingin polymer films with high piezoelectric and pyroelectric properties. The piezoelectric re-sponse for these polyimides is in the same vicinity as those of PVDF at room temperature.However, the piezoelectric response of the polyimides is greater at elevated temperatures.10

Multiphase piezoelectric composites have been developed for their synergetic effectbetween the piezoelectric activity of monolithic ceramics and the low density of nonpiezo-electric polymeric materials. One class of piezoelectric composites that has been developedis the smart tagged composites. These piezoelectric composites are PZT-5A (soft piezoelec-tric ceramic) particles embedded into an unsaturated polyester polymer matrix and are usedfor structural health monitoring. Conductive metal-filled particles with polyimide films havebeen explored for microelectronic applications. The use of graphite-filled polymers hasshown potential as microsensors and actuators.

Piezoelectric materials have been used in thousands of applications in a wide variety ofproducts in the consumer, industrial, medical, aerospace, and military sectors. When a pie-zoelectric material is subjected to a mechanical stress, an electric charge is generated acrossthe material. The ability of a material to generate a charge or electric field when subjectedto a stress is measured by the piezoelectric voltage coefficient g.

The converse effect occurs when a piezoelectric material becomes strained when placedin an electric field. Under constant-stress conditions, the general equation for the piezoelectriccharge coefficient d can be expressed as the change in the strain S of the piezoelectricmaterial as a function of the applied electric field E.

In should be denoted that piezoelectric materials are also pyroelectric. They generatean electric charge as they undergo a temperature change. Whenever their temperature isincreased, a voltage develops having the same orientation as the polarization voltage. When-ever the temperature is decreased, a voltage develops with orientation opposite to the polar-ization voltage.11

Some of the applications of materials that utilize direct and converse piezoelectric effectsand the applications of PVDF piezoelectric film follow. Specific applications will be dis-cussed.

One of the uses of piezoelectric ceramics and polymers is in ink-on demand printing.Several commercial available ink-jet printers are based upon this technology.12,13 In thisapplication the impulse ink jet is produced using a cylindrical transducer that is tightly boundto the outer surface of a cylindrical glass nozzle with an orifice ranging from mils to mi-crometers in diameter. Several industrious researchers have expanded upon this technique byusing a printer as a chemical delivery system for the application of doped polymers fororganic light-emitting displays. Researchers from Princeton University have used a color ink-

422 Smart Materials

jet printer based upon piezoelectric technology with a resolution of 640 dots per line andreplaced the inks with polymer solutions.14 This technique has also been applied to themanufacture of color filters for liquid crystal displays.

Another application of piezoelectric polymer film involves the work of a group of re-searchers at the Thiokol Company and an earlier work15,16 to monitor adhesive joints. TheThiokol study showed that during the bonding process, the PVDF piezoelectric film sensorsmonitored the adhesive cure ultrasonically and qualitatively determined the presence of sig-nificant void content. The Thiokol study also indicated that normal bond stresses were quan-tified during cyclic loading of single lap joints and electrometric butt joints. The researchersrevealed that this was a significant step toward service life predictions. However, more andbetter understood monitoring techniques are needed.

An interesting application of piezoelectric ceramics can be illustrated by the hundredsof vertical pinball machines popular in the Pachinko parlors of Japan. The machines areassembled with stacks of piezoelectric disks that can act as both sensors and actuators. Whena ball falls on the stack, the force of impact generates a piezoelectric voltage pulse that inturn generates a response from the actuator stack through a feedback control system. Thestack expands, throwing the ball out of the hole and moving it up a spiral ramp through asequence of events and then repeating the sequence.17

3 ELECTROSTRICTIVE MATERIALS

Piezoelectric materials are materials that exhibit a linear relationship between electric andmechanical variables. Piezoelectricity is a third-rank tensor. Electrostrictive materials alsoshow a relationship between these two variables. However, in this case, it is a quadraticrelationship between mechanical stress and the square of electrical polarization. Electrostric-tion can occur in any material and is a small effect. One difference between piezoelectricand electrostrictive materials is the ability of the electrostrictive materials to show a largereffect in the vicinity of its Curie temperature. Elecrostriction is a fourth rank tensor propertyobserved both in centric and acentric insulators. This is especially true for ferroelectricmaterials such as the members of the perovskite family. Ferroelectrics are ferroic solidswhose domain walls have the capability of moving by external forces or fields. In additionto ferroelectrics, the other principal examples of ferroic solids are ferromagnetics and fer-roelastics, both of which have potential as smart materials. Other examples of electrostrictivematerials include lead manganese niobate–lead titanate (PMN–PT) and lead lanthanium zir-conate titanate (PLZT).

An interesting application of electrostrictive materials is in active optical applications.During the Cold War, the satellites that flew over the Soviet Union used active opticalsystems to eliminate atmospheric turbulence effects. Electrostrictive materials have the ad-vantage over piezoelectric materials of being able to adjust the position of optical componentsdue to the reduced hysteresis associated with the motion. Work on active optical systemshas continued. Similar multilayer actuators were used to correct the position of the opticalelements in the Hubble telescope. Supermarket scanners use actuators and flexible mirrorsto read bar codes optically.17

Other examples of this family may be included in the electroactive polymers. This namehas appeared in the technical literature for approximately a decade with an increasing interestin the last several years. Electroactive polymers include any polymer that is simulated byelectricity and responds to its effect in a reversible manner. This classification is sort of amelting pot of smart materials.18

4 Magnetostrictive Materials 423

4 MAGNETOSTRICTIVE MATERIALS

Magnetorestrictive materials are materials that have the material response of mechanicaldeformation when stimulated by a magnetic field. Shape changes are the largest in ferro-magnetic and ferromagnetic materials. The repositioning of domain walls that occurs whenthese solids are placed in a magnetic field leads to hysteresis between magnetization and anapplied magnetic field. When a ferromagnetic material is heated above its Curie temperature,these effects disappear. The microscopic properties of a ferromagnetic solid are different thanfor a ferromagnetic solid. The magnetic dipoles of a ferromagnetic solid are aligned parallel.The alignment of dipoles in a ferromagnetic solid can be parallel or in other directions.5,19,20

Magnetorestrictive materials are usually inorganic in chemical composition and are al-loys of iron and nickel and doped with rare earths. The most effective magnetorestrictivematerial is another alloy developed at the Naval Ordinance Laboratory, TERFENOL-D. It isan alloy of terbium, dysprosium, and iron. The full effect of magnetorestriction occurs incrystalline materials. One factor preventing magnetorestrictive materials from reaching com-mercial significance has been cost. Over the past three decades there has been a great dealof development of giant magnetorestrictive materials, colossal magnetorerestrictive materials,and organic and organometallic magnets.5,21

The giant magnetorestrictive effect was first observed in iron–chromium laminates in1988. These laminates consisted of alternating layers of 50-A-thick iron with chromiumlayers of various thicknesses. The iron layers oriented themselves with antiparallel magneticmoments. A magnetic field of 20 kOe applied in the plane of the iron layers will uncouplethis antiferromagnetic orientation. Since the magnetic orientation of one layer is controlled,another layer is free to rotate with an applied field; this change in orientation of the twolayers produces a macnetorestrictlve effect. This discovery of giant magnetorestrictive ma-terials made the increased sensitivities in hard drive heads more cost effective.

In the 1990s, researchers further enhanced the field of magnetorestrictive materials withthe development of colossal magnetorestrictive materials. Ratios of magnetorestrictive effectin excess of 100,000% were observed. These new materials were found to be epitaxiallanthanum–calcium–manganese–oxygen thin films, polycrystalline lanthanum–yttrium–calcium–manganese–oxygen, and polycrystalline lanthanum–barium–manganese–oxygen.These combinations enhanced the product performance at costs below that of original mag-netorestrictive materials.21

Magnets that exhibit commercial potential should have magnetic saturation and coercivefield properties that are operational at low temperatures. Magnets are useful below theircritical temperature. Most inorganic magnets have critical temperatures well above roomtemperature. The first organometallic magnet was the ionic salt complex of ferricbis(pentamethylcyclopentadienide) and tetracyanoethylene. This complex was a ferromagnetbelow its critical temperature of 4.8 K. The highest effective critical temperature of anorganometallic magnet was in the vicinity of 400 K.22

Organic magnets are different than organometallic magnets for two reasons. The first isobvious: Organic-based magnets contain metal atoms. This results in a rethinking of theprinciples of magnetism. The second difference refers to the fact that coupled spins of organicmagnets reside only in the p orbitals while coupled spins of the organometallic magnets canreside in either the p orbitals or the d orbitals or a combination of the two. A series oforganic magnets based upon the nitroxide chemistry was synthesized and their magneticbehavior studied between 0.6 and 1.48 K. The most studied compound of these nitroxide-based magnets is 4-nitrophenylnitronyl nitroxide. This compound showed a saturation mag-netization equivalent to one spin per molecule. This is indicative of ferromagneticbehavior.5,22

424 Smart Materials

5 ELASTORESTRICTIVE MATERIALS

This class of smart materials is the mechanical equivalent to electrorestrictive and magne-torestrictive smart materials. These smart materials exhibit high hysteresis between stressand strain. The motion of ferroelastic domain walls causes the hysteresis. This motion of theferroelastic domain walls is very complex near a martensitic-phase transformation. At thisphase change, two types of crystal structural changes occur. One is induced by mechanicalstress and the other by domain wall motion. Martensitic shape memory alloys have wide,diffuse phase changes and the ability to exist in both high- and low-temperature phases. Thedomain wall movements disappear with total change to the high-temperature phase.5,19,20 Theelastorestrictive smart material family is in its infancy.

6 ELECTRORHEOLOGICAL MATERIALS

Rheological materials comprise an exciting group of smart materials. Electrorheological andmagnetorheological materials can change their rheological properties instantly through theapplication of an electric or a magnetic field. Electrorheological materials (fluids) have beenknown for several centuries. The rheological or viscous properties of these fluids, which areusually uniform dispersions or suspensions of particles within a fluid, are changed with theapplication of an electric field. The mechanism of how these electrorheological fluids workis simple. In an applied electric field the particles orient themselves in fiberlike structures(fibrils). When the electric field is off, the fibrils disorient themselves. Another way to imag-ine this behavior is to consider logs in a river. If the logs are aligned, they flow down theriver. If they are disordered, they will cause a log jam, clogging up the river. A typicalexample of an electrorheological fluid is a mixture of corn starch in silicone oil. Anotherfluid that has been experimented with as a replacement for silicone oil is chocolate syrup.Another feature of electrorheological systems is that their damping characteristics can bechanged from flexible to rigid and vice versa. Electrorheological fluids were evaluated usinga single-link flexible-beam test bed. The beam was a sandwich configuration with electro-rheological fluids distributed along its length. When the beam was rapidly moved back andforth, the electrorheological fluid was used to provide flexibility during the transient responseperiod of the maneuver for speed and made rigid at the end point of the maneuver forstability. A practical way of viewing this behavior is to compare it with fly fishing.5 It hasbeen suggested that rheological fluids be used in the construction of fishing rods and golfclubs.

7 MAGNETORHEOLOGICAL MATERIALS

Magnetorheological materials (fluids) are the magnetic equivalent of electrorheological fluids.These fluids consist of ferromagnetic or ferromagnetic particles that are either dispersed orsuspended and the applied stimulus is a magnetic field. A simple magnetorheological fluidconsists of iron powder in motor oil. The Lord Corporation provided a clever demonstrationof magnetorheological fluids. It supplied an interlocking two-plastic-syringe system filledwith a magnetorheological fluid and two small magnets. The fluid flows freely, without themagnets placed in the middle of the two syringes. With the two magnets in place, the fluidflows completely.

An interesting adaptation of magnetorestrictive fluids is a series of clastomeric matrixcomposites embedded with iron particles. During the thermal cure of the elastomer, a strongmagnetic field was applied to align the iron particles into chains. These chains of iron

8 Thermoresponsive Materials 425

particles were locked into place within the composite through a crosslinked network of thecured elastomer. If a compressive force stimulated the composite, it was 60% more resistantto deformation in a magnetic field. If the composite was subjected to a shear force, itsmagnetic-field-induced modulus was an order of magnitude higher than its modulus in azero magnetic field.5,23

Recently attempts to enhance the properties of epoxies with magnetic fields showed thatat low conversion rates of the epoxy with a hardener, economically generated magnetic fieldshad an effect on the properties of the final composite.24 At the high conversion of thereactants there exists a need to drive the scarce unreacted glycidyl and amine functionalitiestogether. Only magnetic fields generated by superconducting electromagnets are capable ofthis.24

There continues to be a great deal of research into magnets and magnetism. A new areaof research involves magnetic nanocomposite films. Magnetic particles exhibit size effects.Below a critical size, magnetic clusters comprise single domains, whereas with bulk materialsthere are multiple domains. Nanomagnets show unusual properties of magnetism, such assuperparamagnetism and quantum tunneling. These unique properties of magnetic nanoclus-ters can lead to applications in information storage, color imaging, magnetic refrigeration,ferrofluids, cell storage, medical diagnosis, and controlled drug delivery. A nanocompositeis considered to be the incorporation of these nanoclusters into polymeric matrices such aspolyaniline.25

8 THERMORESPONSIVE MATERIALS

Amorphous and semicrystalline thermoplastic polymeric materials are unique due to thepresence of a glass transition temperature. Changes in the specific volume of polymers andtheir rate of change occur at their glass transition temperatures. This transition affects amultitude of physical properties. Numerous types of indicating devices could be developedbased upon the stimulus–response (temperature-specific volume) behavior. This chapter con-tains several examples of this type of behavior; however, its total impact is beyond the scopeof this chapter. To take advantage of this behavior in product development or material se-lection, it is necessary to consult the many polymer references.

A few unique examples that illustrate thermoresponsive behavior are included in thischapter. One example refers to the polyethylene/poly(ethylene glycol) copolymers that wereused to functionalize the surfaces of polyethylene films.5,26 One may refer to this illustrationas being a ‘‘smart surface’’ or functionally gradient surface. When the film is immersed inan aqueous dispersion of the copolymer, the ethylene glycol moieties attach to the polymerfilm surface, resulting in a film surface having solvation behavior similar to poly(ethyleneglycol) itself. Due to the inverse temperature-dependent solubility behavior of poly(alkeneoxide)s in water, surface-modified polymers are produced that reversibly change their hy-drophilicity and solvation with changes in temperatures.27 Similar behaviors have been ob-served as a function of changes in pH.28–31

Another interesting example of materials that respond smartly to changes in temperatureincludes cottons, polyesters, and polyamide/polyurethanes that are modified by poly(ethyleneglycol)s. A combination of the thermoresponsiveness of these fabrics with a sensitivity tomoisture resulted in a family of fabrics that can serve as smart pressure bandages. Whenexposed to an aqueous medium such as blood, these fabrics contract and apply pressure.Once the fabric dries, it releases the pressure.23

Polymers based upon the monomer vinyl methyl ether have the unique behavior ofshrinking upon heating to approximately 40�C. In the right design with normal behavingpolymers, one can construct a device that can grasp objects like a hand.

426 Smart Materials

9 pH-SENSITIVE MATERIALS

By far, the widely known chemical classes of pH-sensitive materials are the acids, bases,and indicators. The indicators fit the definition of smart materials by changing color as afunction of pH and the action is reversible.

Other examples of pH-sensitive materials include some of the smart gels and smartpolymers mentioned in this chapter. There are a large number of pH-sensitive polymers andgels that are used in biotechnology and medicine. Usually these materials are prepared fromvarious combinations of such monomers and polymers such as methacrylic acid, methylmethacrylate, carboxymethylethyl cellulose, cellulose acetate, cellulose phthalate, hydroxy-propylmethylcellulose phthalate, hydroxypropylmethylcellulose acetate, hydroxypropyl-methylcellulose succinate, diethylaminoethyl methacrylate, and butyl methacrylate.32

10 LIGHT-SENSITIVE MATERIALS

There are several different material families that exhibit different behavior to a light stimulus.Electrochromism is a change in color as a function of an electrical field. Other types ofbehavior for light-sensitive materials are thermochromism (color change with heat), photo-chromism (color change with light), and photostrictism (shape changes caused by changesin electronic configuration due to light).5,19,20

Electrochromic smart windows have been intensively researched over the past few dec-ades. There have been over 1800 patents issued for optical switching devices, with the bulkof these being issued in Japan. A typical switchable glass is multilayered with an electro-chromic device embedded inside. A window device may have glass with an interior con-ductive oxide layer both on the top and bottom. Inside the sandwich of glass and conductiveoxide is the electrochromic device. This device consists of an electrochromic layer, an ionstorage layer, and between these two layers an ion conductor.

An interesting light-sensitive material with both electro- and thermochromism behaviors,LixVO, was evaluated for a smart window application.33

Materials have been developed to exhibit both photochromic and photographic (irre-versible behavior to light) behaviors. One such system is based upon a substituted indoli-nospirobenzopyran embedded in a polystyrene matrix. This system performs as a pho-tochromic system at low exposure in the UV range and as a photographic system at highexposures. The image can be devisualized by heat and can be restored with UV irradiationmany times.34

11 SMART POLYMERS

The term smart polymers was almost dropped as a classification for smart materials in thistreatment of the subject. It is very confusing. Each field of science and engineering has itsown definition of a smart polymer, each definition can fit in another classification, and itsdistinction in smartness can be confusing at times. The term is being included in this chapterbecause several excellent articles on the subject have ‘‘smart polymer’’ in the title.

In medicine and biotechnology, smart polymer systems usually pertain to aqueous pol-ymer solutions, interfaces, and hydrogels. Smart gels, or hydrogels, will be treated separately.Smart polymers refer to polymeric systems that are capable of responding strongly to slightchanges in the external medium: a first-order transition accompanied by a sharp decrease inthe specific volume of the polymer. If the external medium is temperature, this transition is

12 Smart (Intelligent) Gels (Hydrogels) 427

known as the glass transition temperature of the polymer and several properties of the pol-ymer change. Among these properties are volume, coefficient of thermal expansion, specificheat, heat conductivity, modulus, and permeation. Manipulating the detection around theglass transition temperature of the polymer can develop in smart devices. There are numerousexamples of product development that has resulted in failure because the glass transitiontemperature of the polymer was not considered. It should be noted that as the polymer coolsdown from high temperatures to below its glass transition temperature its below-glass-transition properties are returned and vice versa.

Smart polymers can respond to stimuli such as temperature, pH, chemical species, light,UV radiation, recognition, electric fields, magnetic fields, and other types of stimuli. Theresulting response can be changes in phase, shape, optics, mechanical strength, electrical andthermal properties, reaction rate, and permeation rate.

12 SMART (INTELLIGENT) GELS (HYDROGELS)

In the literature you can find these smart materials under a variety of names, as reflected bythe title of this section. The concept of smart gels is a combination of the simple conceptof solvent-swollen polymer networks in conjunction with the material being able to respondto other types of stimuli. A partial list of these stimuli includes temperature, pH, chemicals,concentration of solvents, ionic strength, pressure, stress, light intensity, electric fields, mag-netic fields, and different types of radiation.35–39 The founding father of these smart gels,Toyochi Tanaka, first observed this phenomenon in swollen clear polyacrylamide gels. Uponcooling, these gels would cloud up and become opaque. Upon warming these gels regainedtheir clarity. Upon further investigation to explain this behavior, it was found that some gelsystems could expand to hundreds of times their original volume or could collapse to expelup to 90% of its fluid content with a stimulus of only a 1�C change in temperature. Similarbehavior was observed with a change of 0.1 pH unit.

These types of behaviors led to the development of gel-based actuators, values, sensors,control-led release systems for drugs and other substances, artificial muscles for roboticdevices, chemical memories, optical shutters, molecular separation systems, and toys. Otherpotential systems for the development of products with smart (intelligent) gels (hydrogels)include paints, adhesives, recyclable absorbents, bioreactors, bioassay systems, and display.

Numerous examples of the commercialization of these smart gels can be found in Ref.35. This chapter will only include a few examples of smart gels. One such smart gel consistsof an entangled network of two polymers, a poly(acrylic acid) (PAA) and a triblock copol-ymer of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) with a sequence ofPEO–PPO–PEO. The PAA portion is a bioadhesive and is pH responsive, the PPO moietiesare hydrophobic substances that assist in solubilizing lipophilic substances in medical ap-plications, and the PEO functionalities tend to aggregate, resulting in gelation at body tem-peratures. Another smart gel system with a fairly complex composition consists of citosan,a hydrolyzed derivative of chitin (a polymer of N-acetylglucosamine that is found in shrimpand crab shells), a copolymer of poly(nisopropylacrylamide) and poly(acrylic acid), and agraft copolymer of poly(methacrylic aid) and poly(ethylene glycol). This gel system wasdeveloped for the controlled release of insulin in diabetics.

Polyampholytic smart hydrogels swell to their maximum extent at neutral pH values.When such gels, copolymers of methacrylic acid 2-(N,N-dimethylamino)ethyl methacrylate,are subjected to either acidic or basic media, they undergo rapid dehydration.39

One very unusual smart gel is based upon the polymerization of N-isopropylacrylamide,a derivative of tris(2,2�-bipyridyl)ruthenium(II) that has a polymerizable vinyl group, and

428 Smart Materials

N,N�-methylenebisacrylamide. It is a self-oscillating gel that simulates the beating of theheart with color changes.40

13 SMART CATALYSTS

The development of smart catalysts is a new field of investigation and has shown a greatdeal of activity in universities of the oil-producing states. One such smart catalyst is rhodiumbased with a poly(ethylene oxide) backbone. Smart catalysts such as this one function op-posite to a traditional catalyst; that is, as the temperature increases, they become less soluble,precipitating out of the reaction solution, thus becoming inactive. As the reaction solutioncools down, the smart catalyst redissolves and thus becomes active again.5,23 Other smartcatalyst systems are being developed that dissociate at high temperatures (less active) andrecombine at low temperatures (more active).5,27

14 SHAPE MEMORY ALLOYS

The shape memory effect in metals is a very interesting phenomenon. Imagine taking a pieceof metal and deforming it completely and then restoring it to its original shape with theapplication of heat. Taking a shape memory alloy spring and hanging a weight on one endof the spring can easily illustrate this. After the spring has been stretched, heat the springwith a hot-air gun and watch it return to its original length with the weight still attached.

These materials undergo a thermomechanical change as they pass from one phase toanother. The crystalline structure of such materials, such as nickel–titanium alloys, entersinto the martensitic phase as the alloy is cooled below a critical temperature. In this stagethe material is easily manipulated through large strains with a little change in stress. As thetemperature of the material is increased above the critical temperature, it transforms into theaustenitic phase. In this phase the material regains its high strength and high modulus andbehaves normally. The material shrinks during the change from the martensitic to the aus-tenitic phase,5,19,20

Nickel–titanium alloys have been the most used shape memory material. This family ofnickel–titanium alloys is known as Nitinol, after the laboratory where this material was firstobserved (Nickel Titanium Naval Ordinance Laboratory). Nitinol has been used in military,medical, safety, and robotics applications. Specific applications include hydraulic lines on F-14 fighter planes, medical tweezers and sutures, anchors for attaching tendons to bones,stents for cardiac arteries, eyeglass frames, and antiscalding valves in water faucets andshowers.5,41,42

In addition to the family of nickel–titanium alloys there are other alloys that exhibit theshape memory effect. These alloys are silver–cadmium, gold–cadmium, copper–aluminum–nickel, copper–tin, copper–zinc, combinations of copper–zinc with silicon or tin or alumi-num, indium–thallium, nickel–aluminum, iron–platinum, manganese–copper, and iron–manganese–silicon.43 Not all combinations of the two or three elements yield an alloy withthe shape memory effect; thus it is recommended to review the original literature.

Several articles from Mitsubishi Heavy Industries describe the room temperature func-tional shape memory polyurethanes. To this writer, these papers only attest to the behaviorof a polymer at its glass transition temperature. Thus if you wish to describe a polymer’sbehavior at its glass transition temperature and since the free-volume change is reversible,you may call it a smart polymer, a shape memory material, or a thermoresponsive material.

15 Comments, Concerns, and Conclusions 429

The unique characteristic of these polyurethanes is that their transition occurs in the vicinityof room temperature.44,45

15 UNUSUAL BEHAVIORS OF MATERIALS

As one researches the field of smart materials and structures, one realizes that there are manysmart materials and there are many material behaviors that are reversible. The ability todevelop useful products from smart materials is left up to one’s imagination. For example,water is a very unique material. It expands upon freezing. As we know, the force generatedby this expansion causes sidewalks and highways to crack. Now, what if you surround waterpipes with a heating system that consists of a heater and water enclosed in a piezoelectricpolymer or elastomer container that is in a fixed space. As the temperature drops to thefreezing point of water, it expands and generates a force against the piezoelectric containerwhich in turns generate electricity, thus powering the heater and keeping the pipes fromfreezing.

As previously mentioned, sometimes it is difficult to classify the material and its be-havior. One such case involves a University of Illinois patent. The title of the patent is‘‘Magnetic Gels Which Change Volume in Response to Voltage Changes for Magnetic Res-onance Imaging.’’46 The patent teaches the use of a matrix that has a magnetic and preferablysuperparamagnetic component and the capability of changing its volume in an electric field.

Fullerenes are spherically caged molecules with carbon atoms at the corner of a poly-hedral structure consisting of pentagons and hexagons. The most stable of the fullerenes isa C60 structure known as a buckyball or buckminsterfullerene. The fullerenes have beenunder commercial development for the past decade. One application of the fullerenes as asmart material consists of embedding the fullerenes into sol–gel matrices for the purpose ofenhancing optical limiting properties.47

A semiconducting material with a magnetic ordering at 16.1 K was produced from thereaction of buckyball with tetra(dimethylamino)ethylene. This organic-based magnet didnot have the coercive or saturation magnetization to function totally as a ferromagnet. Thereplacement of buckyball with higher carbon number fullerenes in the reaction withtetra(diamethylamino)ethylene did not produce any complexes that showed magnetic order-ing.22

16 COMMENTS, CONCERNS, AND CONCLUSIONS

In dealing with smart materials and structures there is still much confusion over the nameof these materials and what makes a material or structure smart. Numerous products with‘‘smart’’ in the name do not meet the definition of being smart, that is, responding to theenvironment in a reversible manner. The scientist /engineer should not fault advertising pro-fessionals for using the term smart in a product description. But I must admit I chucklewhenever I see the magazine Smart Money. Does this mean that I can spend money and itwill return to me because it is reversible?

Confusion also exists within the smart materials community with the term smart poly-mers. Applications of a smart polymer center on the polymer’s glass transition temperature.As design engineers become more familiar with that term and its significance to a design,in some applications the term will be eliminated and replaced with terms like ‘‘workingsmartly with a polymer.’’

430 Smart Materials

We have not addressed the versatility of these smart materials. One such example mayinvolve the smart shock absorbers. Two literature sources discuss current research on vibra-tion suppression in automobiles using smart shock absorbers.3,17 These smart shock absorberswere developed by Toyota and consist of multilayer piezoelectric ceramics. These multilayerstacks are positioned near each wheel. After analyzing the vibration signals, a voltage is fedback to the actuator stack, which responds by pushing on the hydraulic system to enlargethe motion. Signal processors analyze the acceleration signals from road bumps and respondwith a motion that cancels the vibration.3,17 Such active piezoelectric systems are used tominimize excess vibrations in helicopter blades and in the twin tail of F-18 fighter jets. AnInternet source48 has discussed work at the University of Rochester with smart shock ab-sorbers using electrorheological fluids. The electrorheological fluids sense the force of abump and immediately send an electric signal to precisely dampen the force of the bump,thus providing a smoother ride. In an engine mount, electrorheological fluids damp out thevibrations of the engine, thus reducing wear and tear on the vehicle. Another application ofelectrorheological fluids is in clutches to reduce the wear between the plates as a driver shiftsgears. This can reduce the maintenance costs of high-duty trucks.47 Or one can dampen thevibrations of the road, engine mounts, and other sources on an automobile or heavy-dutytruck with magnetorheological fluids. The Lord Corporation has developed a series of trade-marked fluids and systems known as Rheonetic Fluids that are commercially available forvibration control as well as for noise suppression.49

17 FUTURE CONSIDERATIONS

The future of smart materials and structures is wide open. The use of smart materials in aproduct and the type of smart structures that one can design are only limited by one’s talents,capabilities, and ability to ‘‘think outside the box.’’

In an early work5 and as part of short courses there were discussions pertaining to futureconsiderations. A lot of the brainstorming that resulted from these efforts is now beingexplored. Some ideas that were in the conceptual stage are now moving forward. Look atthe advances in information and comforts provided through smart materials and structuresin automobiles.

Automobiles can be taken to a garage for service and be hooked up to a diagnosticcomputer that tells the mechanic what is wrong with the car. Or a light on the dashboardsignals ‘‘maintenance required.’’ Would it not be better for the light to inform us as to theexact nature of the problem and the severity of it? This approach mimics a cartoon thatappeared several years ago of an air mechanic near a plane in a hanger. The plane says‘‘Ouch’’ and the mechanic says ‘‘Where do you hurt?’’

One application of smart materials is the work mentioned earlier of piezoelectric ink-jet printer that serves as a chemical delivery to print organic light-emitting polymers in afine detail on various media. Why not take the same application to synthesize smaller mol-ecules? With the right set one could synthesize smaller molecules in significant amounts forcharacterization and evaluation and in such a way that we could design experiments withrelative ease.

A new class of smart materials has appeared in the literature. This is the group of smartadhesives. We previously mentioned that PVDF film strips have been placed within an ad-hesive joint to monitor performance. Khongtong and Ferguson developed a smart adhesiveat Lehigh University.50 They suggested that this new adhesive could form an antifoulingcoating for boat hulls or for controlling cell adhesion in surgery. The stickiness of the newadhesive can be switched on and off with changes in temperature. The smart adhesive also

Reference 431

becomes water repellent when its tackiness wanes.50 The term ‘‘smart adhesive’’ is appearingmore frequently in the literature.

A topic of research that was in the literature a few years ago was ‘‘smart clothes’’ or‘‘wearable computers’’ being studied at MIT. The potential of this concept is enormous. Thissounds wonderful as long as we learn how to work smarter, not longer.

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