Page 149 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/MBS P2: GQT Final Pages
Encyclopedia of Physical Science and Technology En012j-597 July 26, 2001 11:8

656 Polymers, Electronic Properties

stage that potential applications have reached except to replicates the information to be produced either from an
indicate those that have already entered the marketplace original or from electronic input. This electrostatic pat-
and proved their worth. tern can be produced by selective photodischarging of a
Conductive plastics are presently used in, or being de- uniformly charged photoreceptor surface or by selective
veloped industrially for antistatic substances for photo- charging of a surface with an ion beam. The latent elec-
graphic film, shields for computer screen against elec- trostatic pattern must then be developed. In dry copying,
tromagnetic radiation, and for “smart” windows (that can as opposed to liquid ink development, the image is ren-
exclude sunlight on demand). In addition, semiconductive dered visible by moving a developer material across the
polymers have recently been developed for use in light- surface of the photoreceptor. This developer material is a
emitting diodes and solar cells and as displays in mobile fine powder that consists of black or colored toner parti-
telephones and miniformat television screens. cles, which are polymer spheres (∼10 µm in diameter)
A major commercial application of polymer electrets is in which a colorant such as carbon black is encapsulated.
as electroacoustic transducers such as microphones. Their These toner particles are charged by a triboelectric process
commercial acceptance is based on several advantages to a sign opposite to that with which the photoreceptor was
over condenser microphones. Among these are an insen- originally charged. (By proper choice of materials either
sitivity to mechanical shock and electromagnetic pickup. polarity of triboelectric charge can be obtained.) The op-
They are much simpler in design and less expensive. In positely charged toner particles are therefore attracted by
a typical microphone, a thin polymer electret is attached the latent electrostatic pattern and render it visible. This
to a back electrode and a front diaphragm. Incident sound toner pattern is then transferred to paper from the photore-
induces vibration of the diaphragm, and the vibration of ceptor and fixed. This is usually done thermally, by which
the electret thus generates an ac signal. This type of mi- process the polymer spheres melt and adhere to the paper.
crophone and variations are used in cassette recorders, Considering that billions of copies are made per year, the
hearing aids, stereo equipment, and sound level detection commercial significance of tribelectricity in polymers can
andtelecommunications.Currentworldwideproductionis be appreciated.
∼100 million units on an annual basis. The piezoelectric Another significant application of polymers as elec-
properties of PVF 2 have also led to its use in commercial tronic materials in electrophotography is as photorecep-
electroacoustic applications. In all cases, the chief advan- tors. Polymers that are electrically insulating in the dark
tages of polymers are their flexibility, toughness, and ca- but can transport nonequilibrium carriers produced by
pacity to be easily processed in large-area, thin films of light are well suited to function as photoreceptor ele-
practically any shape. They also possess low mechanical ments. Polymers such as PVCA or molecularly doped
impedance and therefore have the virtue of good acoustic polymers have strong intrinsic optical absorption only in
coupling to water and the human body. These two qual- the ultraviolet, and since visible light is employed in prac-
ities, together with the others, have suggested potential tice, the polymer photosensitivity must be extended into
important applications ranging from medical, biological, the visible. This can be done by (1) the formation of a
and geological to military. The good impedance match to charge-transfer complex with absorption in the visible,
the human body suggests use as real-time, in vitro heart (b) dye sensitization with an appropriately absorbing dye,
rate monitoring (e.g., fetal phonocardiographs). In marine or (c) the use of a thin, contiguous sensitizing layer such as
technology, the monitoring of underwater acoustic signals amorphous selenium or an organic pigment. The ability
whether from marine life or human-made objects such as to make large-area flexible polymer films at relatively
submarines becomes possible. The piezoelectric proper- low cost by solution coating accounts for the applica-
ties of PVF 2 allows its use as transducers in inkjet print- tion of these materials in electrophotography. Although in
ers, printing pressure monitors, blood pressure monitors, pendant-group polymers or polymers doped with aromatic
strain gauges, accelerometers, and the like. The pyroelec- molecules charge transport is a hopping process with in-
2
−1
tric properties have led to the use of PVF 2 as an infrared herently low mobilities, less than 10 −3 cm V −1 sec , this
detector with a fast response time because of the small is not a limitation for electrophotographic usage. In this
heat capacity achievable with thin films. These detectors process, the most important parameter, within limits, is
can also be produced in very large areas and therefore how far the carriers move before they are immobilized
have potential security applications such as burglar and rather than how fast they travel. For most process speeds,
fire alarms. such mobilities are adequate provided that the photogen-
The triboelectric properties of polymers have enormous erated carriers can traverse the total device thickness, that
applications in the area of electrophotography and dry-ink is, µEτ> sample thickness, where µ is the mobility, E the
electrophotography. The basic feature of electrophotogra- electric field, and τ the lifetime of carriers with respect to
phy is the production of a latent electrostatic pattern that deep traps. Remarkably, such polymers do exhibit large