Kutz M. Handbook of materials selection

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402 SMART MATERIALS

within the molecular structure, of a creation and motion of crystallographic de-

fects or other localized conformations, of an alteration of localized stress and

strain ﬁelds, and of others. The effects produced can be a color change, a change

in index of refraction, a change in the distribution of stresses and strains, or a

volume change.

Another important element of a smart material is that the action of receiving

stimuli and responding to the stimuli to produce a useful effect is that it must

be reversible. Another feature that is an important factor in determining if a

material is smart pertains to its asymmetrical nature. This is primarily critical

for the piezoelectric materials. Other types of smart materials exhibit this 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

smart materials. The notation smart has been overused as a means to market

materials and products.

From the purist point of view materials are smart if at some point within their

performance history they act reversible to a stimulus. Materials that formally

have the label of being smart include piezoelectric materials, electrostrictive

materials, electrorheological materials, magnetorheological materials, thermo-

responsive materials, pH-sensitive materials, ultraviolet-(UV) sensitive materials,

smart polymers, smart gels (hydrogels), smart catalysts, and shape memory

alloys. In this treatment of the subject we will be using some of these classiﬁ-

cations and in some cases, 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 the rediscovery of the smart behavior of materials known

for centuries. As we continue to better understand the smartness of materials,

our deﬁnitions will change. In each material section there will be discussions

pertaining to the material deﬁnition, types of materials that belong to that class,

properties of the members, and applications of the materials. In some cases a

more detailed discussion of application will be given to both illustrate the beneﬁt

of these materials and to simulate the use of these materials in new applications.

This is being done in the spirit of this handbook.

Another important feature of smart materials is their inclusion in structures.

These are known as smart structures. Smart structures are simply structures with

at least one smart material incorporated within its structure and that from the

effect of the smart material cause an action. A smart structure may have sensors

(nerves), actuators (muscles), and a control (brain). Thus, you can see the term

biomimetric associated with smart structures. Smart structures are being designed

to make our life more productive and easy. And with the number of sensors,

actuators, and control systems available, coupled with the materials and the

genius of scientists and engineers, these structures are being more noticeable

and numerous. Reading nontechnical magazines, watching television, and going

to stores can verify this.

In a silly way the growth of smart structures can be illustrated by an article

that appeared on page D11 of the Thursday 14, 2000, issue of the New York

Times. This article describes the ‘‘Big Mouth Billy Bass,’’ the singing ﬁsh. The

article described the smartness of this novel toy.

Examples of technical applications of smart structures are structures such as

composite materials embedded with ﬁber optics, actuators, sensors, microelec-

2 PIEZOELECTRIC MATERIALS 403

tromechanical systems (MEMS), vibration control, sound control, shape control,

product health or lifetime monitoring, cure monitoring, intelligent processing,

active and passive controls, self-repair (healing), artiﬁcial organs, novel indicat-

ing devices, designed magnets, damping, aeroelastic stability, and stress distri-

butions. Smart structures are found in automobiles, space systems, ﬁxed- and

rotary-wing aircrafts, naval vessels, civil structures, machine tools, recreation,

and medical devices.

5,6

Another feature that is important about smart materials and structures is that

they encompass all ﬁelds of science and engineering. Thus when searching for

information on smart materials and structures, there are numerous sources, web

sites, and professional societies that deal with the technology.

2 PIEZOELECTRIC MATERIALS

The simplest deﬁnition of piezoelectric materials can be obtained by ﬁrst break-

ing the word into piezo and electric. Piezo is from the Greek word piezein that

means to press tightly or squeeze. Combining piezein with electric we have

‘‘squeeze electricity.’’ And that to the ﬁrst estimation is a very good deﬁnition.

The history of piezoelectric materials is relatively simple and only the high-

lights will be presented. In 1880, Pierre and Paul-Jean Curie showed the pie-

zoelectric effect in quartz and Rochelle salt crystals. Their ﬁrst observations were

made by placing weights on the faces of particular crystal cuts, like the X cut

quartz plate, detecting charges on the crystal surfaces, and demonstrating that

the magnitude of charge was proportional to the applied weight. This phenom-

enon has become to be known as the direct pressure electric piezoelectric effect.

In 1881 G. Lippmann made the prediction that a crystal such as quartz would

develop a mechanical strain when an electric ﬁeld was applied. In the same year

the Curies reported the converse pressure electric piezoelectric effect with quartz

and Rochelle salt. They showed that if certain crystals were subjected to me-

chanical strain, they became electrically polarized and the degree of polarization

was proportional to the applied strain. The French inventor Langevin developed

the ﬁrst SONAR, using quartz crystals in 1920. During the 1940s researchers

discovered and developed the ﬁrst polycrystalline, piezoelectric ceramic, barium

titanate. One of the signiﬁcant advantages of piezoelectric ceramics over pie-

zoelectric crystals is their ability to be formed in a variety of shapes and sizes.

In 1960, researchers discovered a weak piezoelectric effect in whalebone and

tendon that lead to intense search for other piezoelectric organic materials. And

in 1969, Kawai found very high piezoelectric activity in polarized polyvinylidene

ﬂuoride.

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 to a crystal with a center of symmetry, it will alter

the spacing between the positive and negative sites in each elementary cell unit,

thus causing a net polarization at the crystal surface. The effect is approximately

linear. The polarization is directly related to the applied stress. It is directional

dependent. Thus, compressive and tensile forces will generate electric ﬁelds and

voltages of opposite polarity. The effect is also reciprocal; thus when the crystal

404 SMART MATERIALS

is exposed to an electric ﬁeld, it undergoes an elastic strain changing its length

based upon the ﬁeld polarity.

As previously mentioned, the ceramic-type materials play an important role

in the area of piezoelectrics. Piezoelectric ceramics are polycrystalline ferro-

electric materials with a perovskite crystal-type structure. The crystal structure

is tetragonal/rhombahedral with a close proximity to cubic in nature. Piezoelec-

tric ceramics have the general formula of A

⫹

The letter A represents a

⫺

large divalent metal ion such as barium or lead, and B is one or more tetravalent

metal ion such as titanium or zirconium or manganese. These ceramics are con-

sidered to be masses of minute crystallities that change crystal forms at the Curie

temperature. Above the Curie temperature, the ceramic crystallities have a simple

cubic symmetry. This form is centrosymmetric with positive and negative charge

sites coinciding, thus there are no dipoles present. The material is considered to

be paraelectric. Below the Curie temperature, the ceramic crystallities have a

tetragonal symmetry; this form lacks a center of symmetry with the positive and

negative charge sites that no longer coincide, thus each unit cell now has an

electric dipole that may be reversed and switched to certain direction by the

application of an electric ﬁeld. The material is now considered to be ferroelectric.

The piezoelectric properties of the ceramics can be enhanced by applying a

large electric ﬁeld at an elevated temperature, thus generating an internal remnant

polarization that continues long after the removal of the electric and thermal

ﬁelds. This technique is known as poling. The poling of piezoelectric ceramics

has eliminated the use of piezoelectric crystals in many applications.

The vinylidene ﬂuoride monomer, yields the semicrystalline pol-

—

CH CF ,

—

ymer, polyvinylidene ﬂuoride (PVDF), upon polymerization. This polymer was

found to be highly piezoelectric. Polyvinylidene ﬂuoride is manufactured in

sheet form from the nonpolar

␣

-phase ﬁlm extruded from the melt. The extruded

ﬁlm is uniaxially stretched. This process rotates the long polymer chains and

forms the polar beta phase. The beta phase is needed for the high piezoelectricity.

The ﬁnal step consists of reorientation of the randomly directed dipoles asso-

ciated with the stretched

␤

phase by applying a poling ﬁeld in the direction

normal to the plane of the ﬁlm. The resulting piezoelectric polyvinylidene ﬂu-

oride has orthotropic symmetry.

Copolymers and rubber and ceramic blends of polyvinylidene ﬂuoride have

been prepared for used as piezoelectric materials. The most common copolymer

is one based on the polymerization reaction of vinylidene ﬂuoride and triﬂuoro-

ethylene. Polyvinylidene ﬂuoride and its copolymers with triﬂuoroethylene have

low mechanical quality factors, high hydrostatic mode responses, and their

acoustic impedance is similar to water, thus making them ideal for underwater

hydrophone applications.

A series of polyimides has been developed at NASA Langley for use

in piezoelectric applications. These polyimides have pendant triﬂuoromethyl

(—CF3) and cyano (—CN) polar groups. Whenever these polyimides are ex-

posed to applied voltage of the order of 100 MV/m at elevated temperatures,

the polar groups developed a high degree of orientation thus resulting with pol-

ymer ﬁlms with high piezoelectric and pyroelectric properties. The piezoelectric

response for these polyimides is in the same vicinity to those of polyvinylidene

2 PIEZOELECTRIC MATERIALS 405

Table 1 Applications of Piezoelectric Materials Utilizing the

Converse Effect

Process Application

Load cells Force measurements

Pressure measurements

Velocity sensors Velocity measurements

Accelerometer Acceleration measurements

Vibration monitoring

Structural monitoring Detection of acoustic emissions

Hydrophones Monitoring of marine life

Monitoring of the heart

Monitoring of circulatory systems

Igniters Gas ignition in welders, barbeques, lighters

Remote control Sensor in commercial remote controls

Microphones Detection of audible frequencies

ﬂuoride at room temperature. However, the piezoelectric response of the poly-

imides is greater at elevated temperatures.

Multiphase piezoelectric composites have been developed for their synergetic

effect between the piezoelectric activity of monolithic ceramics and the low

density of nonpiezoelectric polymeric materials. One class of piezoelectric com-

posites that has been developed is the smart tagged composites. These piezo-

electric composites are PZT-5A particles embedded into an unsaturated polyester

polymer matrix and are used for structural health monitoring. Conductive metal-

ﬁlled particles with polyimides ﬁlms have been explored for microelectronic

applications. The use of graphite-ﬁlled polyimides has shown potential as mi-

crosensors and actuators.

Piezoelectric materials have been used in thousands of applications in a wide

variety of products in the consumer, industrial, medical, aerospace, and military

sectors. When a piezoelectric material is subjected to a mechanical stress an

electric charge is generated across the material. The ability of a material to

generate a charge or electric ﬁeld when subjected to a stress is measured by the

piezoelectric voltage coefﬁcient (g).

The converse effect occurs when a piezoelectric material becomes strained

when placed in an electric ﬁeld. Under constant stress conditions, the general

equation for the piezoelectric charge coefﬁcient (d ) can be expressed as the

change in the strain (S) of the piezoelectric material as a function of the applied

electric ﬁeld (E).

In should be denoted that piezoelectric materials are also pyroelectric. They

generate electric charge as they undergo a temperature change. Whenever their

temperature is increased, a voltage develops having the same orientation as the

polarization voltage. Whenever the temperature is decreased, a voltage develops

with an opposite orientation opposite to the polarization voltage.

Tables 1–3 show some of the applications of materials that utilize direct and

converse piezoelectric effects and the applications of polyvinylidene ﬂuoride

piezoelectric ﬁlm. Speciﬁc applications will be discussed.

One of the uses of piezoelectric ceramics and polymers is their application

in ink-on-demand printing. Several commercially available ink jet printers are

406 SMART MATERIALS

Table 2 Applications of Piezoelectric Materials Utilizing the Converse Effect

Process Application

Welding Welding rigid thermoplastics

Seam welding ﬁlm and fabric

Metal-in-plastic insertion

Metal microbonding

Lap welding of high electrical conductivity and dissimilar materials

Seam welding sheets

Machining Prophylaxis teeth dental treatment

Vibration-assisted rotary machining of hard, brittle materials

Impact grinding with abrasive slurries

Vibration-assisted drilling, tapping, and turning

Forming Drawing thin-wall metal tubing of large diameter-to-wall ratios

Drawing small-diameter wire from difﬁcult to form metals

Cutting Vibration-assisted cutting of ﬁbrous and spongy materials

Cleavage Cleaving crystals and laminated objects

Densiﬁcation Compaction of powder

Table 3 Applications of Polyvinylidene Fluoride Piezoelectric Film

Medical

Diagnostics: Apnea monitor, blood pressure cuff, pulse counter, stethoscope, sleep disorder

monitor, respiratory airﬂow, patient bed monitor

Ultrasound: near-ﬁeld imaging, prostrate, transdermal, transluminal, coronary arterial, breast,

lithotripter, hydrophone calibration

Handicapped aides: switches, braille reader, hearing aid, speech intensiﬁcation

Implantables: pacemaker activity monitor, vasecular graft monitor, micropower source

Instrumentation: intravenous drop counter, IV air bubble detector, laser switch / monitor

Automotive: accelerometers, occupancy seat sensor, compartment switches, horn, fuel level,

tire rotation, security, keyless entry, motion (theft) sensor

Consumer

Musical instruments: pick-up, drum trigger

Sports equipment: target location, reaction time, foul time, force, sweet spot

Toys / games: switches, proximity (air ranging, pyro), novelty speakers, target scoring

Audio: tweeter, balloon speaker, novelty speaker (visor, poster), microphone, speaker distor-

tion feedback, electronic piano keys

Appliance: washer imbalance, vacuum soil sensing, dishwasher

Computer input/ output: keypads, digitizers, mouse, joystick, pen, ink jet droplet generator, ink

jet droplet detector, toner and ink jet level, coin counting, disk drive shock sensing

Industrial

Switches: solid state, snap action, cantilever beam, keypad, vandal proof

Robotics: tactical sensor, micropositioner, safety mats and switches, bumper impact

Security and energy management: glass break detection, ﬂoor / mat sensors, penetration de-

tection, ﬂame detection

Flow/ level: fan monitor, ﬂuidic oscillator, doppler ultrasound, solid-state ﬂuid level, laminar /

turbulent layer

Instrumentation

Machine health monitor: accelerometer, contact microphone, strain gauging

Weather sensors: rain intensity, hail detection, wind velocity

Active vibration damping: strain gauge array, actuator array

Nondestructive testing: acoustic emission

Ranging: distance, safety

Adaptive optics: ﬁber optic shutter, deformable mirror, laser scanner

Oil exploration: hydrophone, seismic geophone

3 ELECTROSTRICTIVE MATERIALS 407

based upon this technology.

12,13

In this application the impulse ink jet is produced

using a cylindrical transducer that is tightly bound to the outer surface of a

cylindrical glass nozzle with an oriﬁce ranging from mils to microns in diameter.

In addition to this application several industrious researchers have expanded

upon this technique by using a printer as a chemical delivery system for the

application of doped polymers for organic light-emitting displays. A group of

researchers from Princeton University has used a color ink jet printer based upon

piezoelectric technology with a resolution of 640 dots per line and simply re-

placed the inks with polymer solutions.

This technique has also been applied

to the manufacture of color ﬁlters for liquid-crystal displays.

Another application of the use of piezoelectric polymer ﬁlm involves the work

of a group of researchers at the Thiokol Company and an earlier work

15,16

monitor adhesive joints. The study showed that during the bonding process, the

polyvinylidene ﬂuoride piezoelectric ﬁlm sensors monitored the adhesive cure

ultrasonically and qualitatively determined the presence of signiﬁcant void con-

tent. The study also indicated that normal bond stresses were quantiﬁed during

cyclic loading of single lap joints and electrometric butt joints. The researchers

revealed that this was a signiﬁcant step toward service life predictions. But, more

and better-understood monitoring techniques are needed.

An interested application of piezoelectric ceramics can be illustrated in the

hundred of vertical pinball machines popular in the Pachinko parlors of Japan.

The machines are assembled with stacks of piezoelectric disks that can act as

both sensors and actuators. When a ball falls on the stack, the force of impact

generates a piezoelectric voltage pulse that in turns generates a response from

the actuator stack through a feedback control system. The stack expands, thus

throwing the ball out of the hole and moving up a spiral ramp through a sequence

of events, and ﬁnally repeating the sequence.

3 ELECTROSTRICTIVE MATERIALS

Piezoelectric materials are materials that exhibit a linear relationship between

electric and mechanical variables. Piezoelectricity is a third-rank tensor. Elec-

trostrictive materials also show a relationship between these two variables. How-

ever, in this case, it is a quadratic relationship between mechanical stress and

the square of electrical polarization. Electrostriction can occur in any material

and is a small effect. One difference between piezoelectric and electrostrictive

materials is the ability of the electrostrictive materials to show a larger effect in

the vicinity of its Curie temperature. Elecrostriction is a fourth-rank tensor prop-

erty observed both in centric and acentric insulators. This is especially true for

ferroelectric materials such as the members of the perovskite family. Ferroelec-

trics are ferroic solids whose domain walls have the capability of moving by

external forces or ﬁelds. In addition to ferroelectrics the other principal examples

of ferroic solids are ferromagnetics and ferroelastics, both of which have poten-

tial as smart materials. Other examples of electrostrictive materials include lead

manganese niobate: lead titanate (PMN: PT) and lead lanthanium zirconate ti-

tanate (PLZT).

An interesting application of electrostrictive materials is in active optical ap-

plications. During the Cold War, the satellites that ﬂew over the Soviet Union

used active optic systems to eliminate the atmospheric turbulence effects. Elec-

408 SMART MATERIALS

trostrictive materials have the advantage over piezoelectric materials in adjusting

the position of optical components due to the less amount of hysteresis associ-

ated with the motion. Work on active optic systems has continued. Similar mul-

tilayer actuators were used to correct the position of the optical elements in the

Hubble telescope. Supermarket scanners use actuators and ﬂexible mirrors to

read bar codes optically.

Other examples of this family may be included in the electroactive polymers.

This name has appeared in the technical literature for approximately a decade

with an increasing interest in the last couple of years. Electroactive polymers

include any polymer that is simulated by electricity and responds to its effect in

a reversible manner. This classiﬁcation is sort of a melting pot of smart mate-

rials.

4 MAGNETOSTRICTIVE MATERIALS

Magnetorestrictive materials have the material response of mechanical defor-

mation when stimulated by a magnetic ﬁeld. Shape changes are the largest in

ferromagnetic and ferrimagnetic materials. The repositioning of domain walls

that occur when these solids are placed in a magnetic ﬁeld leads to hysteresis

between magnetization and an applied magnetic ﬁeld. When a ferromagnetic

material is heated above its Curie temperature, these effects disappear. The mi-

croscopic properties of a ferromagnetic solid are different than a ferrimagnetic

solid. The magnetic dipoles of a ferromagnetic solid are aligned parallel. The

alignment of dipoles in a ferrimagnetic solid can be either parallel or in other

directions.

5,19,20

Magnetorestrictive materials are usually inorganic in chemical composition

and alloys of iron, nickel, and doped with rare earths. The most effective mag-

netorestrictive material is another alloy developed at the Naval Ordinance Lab-

oratory, TERFENOL-D. It is an alloy of terbium, dysprosium, and iron with a

formula of Tb

⫺

Fe with x between 0.27 and 0.30 and y between 1.90 and

1.98. The full effect of magnetorestriction occurs in crystalline materials. One

of the hindrances preventing magnetorestrictive materials reaching commercial

signiﬁcance has been cost. Over the past three decades there has been a great

deal of development of giant magnetorestrictive materials, colossal magneto-

restrictive materials, and organic and organometallic magnets.

5,21

The giant magnetorestrictive effect was ﬁrst observed in iron-chromium lam-

inates in 1988. These laminates consisted of alternating layers of 50-A

-thick iron

with chromium layers of various thicknesses. The iron layers oriented themselves

with antiparallel magnetic moments. A magnetic ﬁeld of 20 kOe applied in the

plane of the iron layers will uncouple this 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 two layers produces a

magnetorestrictive effect. This discovery of giant magnetorestrictive materials

enhanced the increased sensitivities in the hard disk drive heads at a cost-

effective price.

In the 1990s, researchers further enhanced the ﬁeld of magnetorestrictive ma-

terials with the development of colossal magnetorestrictive materials. Ratios of

magnetorestrictive effect in excess of 100,000% were observed. These new ma-

terials were found to be epitaxial lanthanum–calcium–manganese–oxygen thin

6 ELECTRORHEOLOGICAL MATERIALS 409

ﬁlms, polycrystalline lanthanum–yttrium–calcium–manganese–oxygen and

polycrystalline lanthanum–barium–manganese–oxygen. These again enhanced

the product performance at costs less that the original magnetorestrictive mate-

rials.

Magnets that exhibit commercial potential should have magnetic saturation

and coercive ﬁeld properties that are operational at low temperatures. Magnets

are useful below their critical temperature. Most inorganic magnets have critical

temperatures well above room temperature.

The ﬁrst organometallic magnet was the ionic salt complex of ferric

bis(pentamethylcyclopentadienide) and tetracyanoethylene. This complex was a

ferromagnet below its critical temperature of 4.8 K. The highest effective critical

temperature of an organometallic magnet was in the vicinity of 400 K.

Organic magnets are different than organometallic magnets for two reasons.

The ﬁrst is obvious. Organic-based magnets do not contain metal atoms. This

causes a great deal of rethinking the principles of magnetism. The second dif-

ference refers to the fact the coupled spins of the organic magnets reside only

in the p orbitals while the coupled spins of the organometallic magnets can reside

in either the p orbitals or the d orbitals or a combination of the two. A series

of organic magnets based upon the nitroxide chemistry was synthesized and their

magnetic behavior studied between 0.6 and 1.48 K. The most studied compound

of these nitroxide-based magnets was 4-nitrophenylnitronyl nitroxide. This com-

pound showed a saturation magnetization equivalent to one spin per molecule.

This is indicative of ferromagnetic behavior.

5,22

5 ELASTORESTRICTIVE MATERIALS

This class of smart materials is the mechanical equivalent to electrorestrictive

and magnetorestrictive smart materials. These smart materials exhibit high hys-

teresis between stress and strain. The motion of ferroelastic domain walls causes

the hysteresis. This motion of the ferroelastic domain walls is very complicated

and complex near a martensitic phase transformation. At this phase change, two

types of crystal structural changes occur. One is induced by mechanical stress

and the other is 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. The domain wall movements disappear with total change to

the high-temperature phase.

5,19,20

The elastorestrictive smart material family is

in its infancy.

6 ELECTRORHEOLOGICAL MATERIALS

The rheological materials are an exciting group of smart materials. Electrorheo-

logical and magnetorheological materials can change their rheological properties

instantly through the application of either an electric or magnetic ﬁeld. The

electrorheological materials (ﬂuids) have been known for several centuries. The

rheological or viscous properties of these ﬂuids, which are usually uniform dis-

persions, or suspensions of particles within a ﬂuid are changed with the appli-

cation of an electric ﬁeld. The mechanism of how these electrorheological ﬂuids

work is 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 imagine this behavior is to consider logs in a river.

410 SMART MATERIALS

If the logs are aligned, they ﬂow down the river. If they are disordered, they

will cause a log jam thus clogging up the river. A typical example of an elec-

trorheological ﬂuid is a mixture of corn starch in silicone oil. Another ﬂuid that

has been experimented with as a replacement for the silicone oil has been choc-

olate syrup. Another feature of electrorheological systems is that their damping

characteristics can be changed from ﬂexible to rigid and vice versa. Electro-

rheological ﬂuids were evaluated using a single-link ﬂexible-beam test bed. The

beam was a sandwich conﬁguration with electrorheological ﬂuids distributed

along its length. When the beam was rapidly moved back and forth, the elec-

trorheological ﬂuid was used to provide ﬂexibility during the transient response

period of the maneuver for speed and made rigid at the end point of the ma-

neuver for stability. A practical way of viewing this behavior is to compare it

with ﬂy ﬁshing.

It has been suggested to use rheological ﬂuids in the construc-

tion of ﬁshing rods and golf clubs.

7 MAGNETORHEOLOGICAL MATERIALS

The magnetorheological materials (ﬂuids) are the magnetic equivalent of elec-

trorheological ﬂuids. These ﬂuids consist of either ferromagnetic or ferrimag-

netic particles that are either dispersed or suspended within; and the applied

stimulus is a magnetic ﬁeld. A simple magnetorheological ﬂuid consists of iron

powder in motor oil. The Lord Corporation provided a clever demonstration of

magnetorheological ﬂuids. They supplied an interlocking two-plastic-syringe

system ﬁlled with a magnetorheological ﬂuid and two small magnets. The ﬂuid

ﬂows freely, without the magnets placed in the middle of the two syringes. With

the two magnets in place, the ﬂuid ﬂow completely stops.

An interesting adaptation of magnetorestrictive ﬂuids is a series of elastomeric

matrix composites embedded with iron particles. During the thermal cure of the

elastomer, a strong magnetic ﬁeld was applied to align the iron particles into

chains. These chains of iron particles were locked into place within the com-

posite through the cross-linked network of the cured elastomer. If a compressive

force stimulated the composite, it was 60% more resistant to deformation in a

magnetic ﬁeld. If the composite was subjected to a shear force, its magnetic

ﬁeld induced modulus was an order of magnitude higher than its modulus in a

zero magnetic ﬁeld.

5,23

A recent well-thought article described attempts to enhance the properties of

epoxies with magnetic ﬁelds. The study did show the low conversion rates of

the epoxy with the hardener; economically generated magnetic ﬁelds had an

effect on the properties of the ﬁnal composite. At the high conversion of the

reactants there exists a need to drive the scarce unreacted glycidyl and amine

functionalities together. And only the magnetic ﬁelds generated by supercon-

ducting electromagnets are capable of this.

There continues to be a great deal of research into magnets and magnetism.

A new area of research involves magnetic nanocomposite ﬁlms. Magnetic par-

ticles exhibit size effects. Below a critical size, magnetic clusters comprise single

domains, whereas with bulk materials there are multiple domains. Nanomagnets

show unusual properties such as superparamagnetism and quantum tunneling of

magnetization. These unique properties of magnetic nanoclusters can lead to

applications in information storage, color imaging, magnetic refrigeration, fer-

9 pH-SENSITIVE MATERIALS 411

roﬂuids, cell storing, medical diagnosis, and controlled drug delivery. A nano-

composite is considered to be the incorporation of these nanoclusters into

polymeric matrices such as polyaniline.

8 THERMORESPONSIVE MATERIALS

Amorphous and semicrystalline thermoplastic polymeric materials are unique

due to the presence of a glass transition temperature. At their glass transition

temperatures, the speciﬁc volume of polymer and its rate of change changes.

This transition affects a multitude of physical properties. Numerous types of indi-

cating devices could be developed based upon the stimuli-response (temperature-

speciﬁc volume) behavior. This chapter contains several examples of this type

of behavior; however, its total impact is beyond the scope of this chapter. To

take advantage of this behavior in product development or material selection it

requires reviewing the necessary requirements in the many polymer references.

A few examples that illustrate thermoresponsive behavior that are a little

unique are being included in this chapter. One refers to the polyethylene/

poly(ethylene glycol) copolymers that were used to functionalize the surfaces of

polyethylene ﬁlms.

5,26

One may refer to this illustration as being a ‘‘smart sur-

face’’ or functionally gradient surfaces. When the ﬁlm is immersed in an aqueous

dispersion of the copolymer, the ethylene glycol moieties attach to the polymer

ﬁlm surface resulting in a ﬁlm surface having solvation behavior similar to

poly(ethylene glycol) itself. Due to the inverse temperature-dependent solubility

behavior of poly(alkene oxide)s in water surface modiﬁed polymers are produced

that reversibly change their hydrophilicity and solvation with changes in tem-

peratures.

Similar behaviors have been observed as a function of changes in

pH.

28–31

Another interesting example of materials that respond smartly to changes in

temperature are the fabrics based upon poly(ethylene glycol)-modiﬁed cottons,

-modiﬁed polyesters, and -modiﬁed polyamide/polyurethanes. A combination of

the thermoresponsiveness of these fabrics with a sensitivity to moisture resulted

in a family of fabrics that can serve as smart pressure bandages. When exposed

to an aqueous medium such as blood, these fabrics contract and apply pressure.

Once the fabric dries, it releases the pressure.

Polymers based upon the monomer vinyl methyl ether have the unique be-

havior of shrinking upon heating to approximately 40

⬚C. In the right design with

normal behaving polymers, one can construct a device that can grasp objects

like a hand.

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 colors, as a function of pH and the action is reversible.

Other examples of pH-sensitive materials include some of the smart gels and

smart polymers mentioned in this chapter. There are a large number of pH-

sensitive polymers and gels that are used in biotechnology and medicine. Usually

these materials are prepared from various combinations of such monomers and

polymers such as methacrylic acid, methyl methacrylate, carboxymethyethyl-

cellulose, cellulose acetate, cellulose phthalate, hydroxypropylmethylcellulose