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642 Carraher’s Polymer Chemistry

chloride) are used because they resist rapid degradation. When only short-term retention is needed
natural materials such as cotton are used. Geotextiles are not always made from fibers but include

film material such as polyethylene and PP sheets used to retain moisture but retard weed growth in

gardens.
Geotextiles perform a number of functions. They help control fl uid transmission such as helping

to direct the flow of rain water to an outlet. They are used to separate materials for containment and

to stop mixing such as highway embankment retention. It can be used for filtration control that allows
for liquid and small particulates to pass through but to retain larger materials such as rocks. In sand-

bags for flood control, geotextiles simply hold together a material such as sand for breach control.
19.8 SMART MATERIALS

We have had smart materials as materials for a long time though the term is relatively new. Some

of the first smart materials were piezoelectric materials, including poly(vinylene fluoride), that emit
an electric current when pressure is applied and change volume when a current is passed through it.
Most smart materials are polymeric or have a critical portion of the smart system that is polymeric.
Today research involves not only the synthesis of new smart materials, but also on the application
of already existing smart materials. Much of the applications of smart materials involve the assem-
blage of smart materials and envisioning uses for these smart materials. Thus, it is possible that
since application of pressure to a piezoelectric material causes a discharge of current, a portion of a
wing could be constructed such that apparent “warpage” of a wing would result form an “electronic
feedback” mechanism employing a computer coupled with a complex system of electronic sensing
devices. Almost instantaneous, self-correcting changes in the overall wing shape would act to allow

safer and more fuel-efficient air flight. Piezoelectric sensors could also be used to measure applica-
tion of “loads” through reaction of the piezoelectric sensors to stress–strain.
Smart materials are materials that react to applied force—electrical, stress–strain (including
pressure), thermal, light, and magnetic. A smart material is not smart simply because it responds to
external stimuli, but it becomes smart when the interaction is used to achieve a defi ned engineering
or scientific goal. Thus, most materials, including ceramics, alloys, and polymers, undergo volume

changes as they undergo phase changes. While the best known phase changes involve changes in state

such as melting/freezing, many materials, specifically polymers, offer more subtle phase changes. For
polymers, the best known subtle phase change is associated with the glass transition, T , where local
g
segmental mobility occurs. Volume changes associated with T are well known and used as a measure
g
of the amount of crystallinity present in a polymer. Thus, when this volume change is used to effect
some desired change, such as switching on and off an electric circuit, the polymer becomes a smart
material. Multiple switching devices can be constructed using polymers with varying T values.
g
The use of smart materials as sensing devices and shape-changing materials has been enhanced
because of the increased emphasis on composite materials that allow the introduction of smart
materials as components.
A smart material assembly might contain the following:

• Sensor components that contain smart materials that monitor changes in some parameters
such as temperature, light, magnetic field, and/or current

• Communications networks that relay changes detected by the sensor components through

fiberoptics or conductive “wire”
• Actuator parts that react to the external stimuli such as changes in temperature, current,
and so on

The actuator part may also be a smart material such as a piezoelectric bar placed in a wind foil
that changes orientation according to a current imputed by the computer center allowing a machine
such as an automobile to handle better and be more fuel effi cient.

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