Page 165 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology en012f-594 July 26, 2001 11:9

Polymers, Ferroelectric 673

rotated under an electric field above the glass transition. very sensitive to the processing conditions and the thick-
The ferroelectric transition in thiourea is essentially due ness of the specimen. The increase of the strain response as
to both crystalline domains and an “incommensurate the sample thickness is reduced is suggested to be caused
phase,” which are due to both hydrogen bonding and by a nonuniform electric field across the thickness direc-
dipole interactions. The D–E hysteresis was first ob- tion. In nonpiezoelectric materials, the field-induced strain
served in drawn polythiourea-9 above T g . The remanent can be caused by the electrostrictive and also the Maxwell
polarization is very small. This may be due to the fact stress effects. Experimental results show that the Maxwell
that only a small amount of chains can form hydrogen stress can significantly contribute to the strain response at
bonding. Therefore, the remanent polarization stabilized temperatures higher than the glass transition temperature,
by hydrogen bonding is small. and that the electrostrictive coefficient is much higher than
those of other materials.

D. Polyurethane
Polyurethanes have emerged as nonlinear optic, ferro- V. APPLICATIONS
electric, and piezoelectric materials in which molecular
structures can be tailored for specific applications. Poly- In recent years, many kinds of piezoelectric devices
urethanes have the chemical structure [–(CH 2 ) x OC(O) have been developed from organic polymer materials and
NH(CH 2 ) y NHCO(O) ]. The dipole moment of the uret- widely used in industrial settings as well as in medical
hane group is 2.8 D. Polyurethanes are usually com- instruments. These device applications can be grouped
posed of a polyester or polyether soft segment and a into sensors, medical instrumentation, robotics, low- or
diisocyanate-based hard segment. From the viewpoint audiofrequency transducers, ultrasonic and underwater
of the chemical structure of the hard segments, polyure- transducers, electroacoustic transducers, electromechan-
thanes can be classified into urethane polymers (PU), ical transducers, actuators, pyroelectric devices, and
which are formed by extending a diisocyanate with optical devices. The pyroelectric, biomedical, and robotic
low-molecular-weight diols, and urethane–urea polymers applications and the optical devices will not be discussed
(PUU), which are formed by extending a diisocyanate here.
with low-molecular-weight diamine. Polyurethanes The first commercial application of piezoelectric
undergo microphase separation due to the immiscibility polymer film was in audio transducers (tweeters) and
of the hard-segment and soft-segment. The hard segment loudspeakers. The merits of ferroelectric polymers for
domain acts as the physical crosslink as well as the filler piezoelectric devices over ferroelectric ceramics is their
particles for the soft-segment matrix. The driving force of softness, light weight, toughness, flexibility, and ability
the domain formation is the strong intermolecular inter- to be fabricated into large sheets. In addition to these
action of the hydrogen bondings between the hard–hard traits, they have a good electromechanical coupling factor
segments and the urea/urethane linkages. and much lower acoustic impedance (this property is
Both the temperature dependence and pressure–tempe- proportional to the product of density and stiffness) than
rature effects on the dielectric constant and relaxation ferroelectric ceramics. Therefore, they are suitable for
processes of aliphatic polyurethanes have been studied. acoustic applications in media such as air, water, and hu-
The relaxation times for both the I process, associated man tissue. Such applications include audio transducers
with the molecular motion in hard segments, and the α (headphones, microphones, loudspeakers), underwater
process, associated with the glass transition temperature, acoustic hydrophones, and biomedical transducers (sen-
increase with pressure, and that these shifts are much sors and probes, imaging systems, and acoustic sources).
more pronounced for the I process. Dielectric properties of Another merit of polymer piezoelectrics is their thin-
polyurethanes were also studied as a function of drawing film-forming ability. This could lead to very promising
and poling effects. applications, such as ultrasonic transducers or paperlike
The D–E hysteresis of polyurethanes can be obtained speakers for the future’s thin TVs, decorations, and interi-
above the glass transition temperature (at 70 C). Ferro- ors. The main disadvantages of polymer piezoelectrics are
◦
electricityoriginatedfromthecrystalregionbycontrolling their relatively low piezoelectric constant and relatively
the hydrogen bonding. Recently, there was a report on the poor dimensional stability.
origin of ferroelectricity that suggested it derives from the Devices based on the conversion of a mechanical in-
amorphous region above the glass transition temperature. put (stress or strain) into an electrical output signal are
The electrostrictive responses, which are proportional to often used as sensors to detect displacement, stress, vibra-
the square of the electric field, have been investigated in tion, and sound. Typical sensors are hydrophones, blood
polyurethane. The field-induced strain was found to be pulse counters, blood pressure meters, pressure sensors,