Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 10 pps

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842 POLYMERS, BIOTECHNOLOGY AND MEDICAL APPLICATIONS
to hydrophilic below it has been successfully used for
detaching mammalian cells. Mammalian cells are nor-
mally cultivated on a hydrophobic solid substrate and are
detached from the substrate by protease treatment, which
often damages the cells by hydrolyzing various membrane-
associated protein molecules. The poly(NIPAAM)-grafted
surface is hydrophobic at 37
◦
C because this temperature
is above the critical temperature for the grafted polymer
and that cells that are growing well on it. A decrease in
temperature results in transition of the surface to the hy-
drophilic state, where the cells can be easily detached from
the solid substrate without any damage. Poly(NIPAAM)
was grafted to polystyrene culture dishes using an electron
beam. Bovine hepatocytes, cells that are highly sensitive
to enzymatic treatment, were cultivated for 2 days at 37
◦
C
and detached by incubation at 4
◦
C for 1 h. Nearly 100%
of the hepatocytes was detached and recovered from the
poly(NIPAAM)-grafted dishes by low-temperature treat-
ment, whereas only about 8% of the cells was detached from
the control dish (57). The technique has been extended
to different cell types (58,59). It is noteworthy that hep-

atocytes recovered by cooling retained their native form
had numerous bulges and dips, and attach well to the hy-
drophobic surface again, for example, when the tempera-
ture was increased above the conformational transition of
poly(NIPAAM). On the contrary, enzyme-treated cells had
a smooth outer surface and had lost their ability to attach
to the surface. Thus, cells recovered by a temperature shift
from poly(NIPAAM)-grafted surfaces have an intact struc-
ture and maintain normal cell functions (58).
The molecular machinery involved in cell-surface de-
tachment was investigated using temperature-responsive
surfaces (60). Poly(NIPAAM)-grafted and nongrafted sur-
faces showed no difference in attachment, spreading,
growth, confluent cell density, or morphology of bovine
aortic endothelial cells at 37
◦
C. Stress fibers, peripheral
bands, and focal contacts were established in similar ways.
When the temperature was decreased to 20
◦
C, the cells
grown on poly(NIPAAM)-grafted support lost their flat-
tened morphology and acquired aroundedappearance sim-
ilar to that of cells immediately after plating. Mild agi-
tation makes the cells float free from the surface without a
trypsin treatment. Neither changes in cell morphology nor
cell detachment occurred on ungrafted surfaces. Sodium
azide, an ATP synthesis inhibitor, and genistein, a tyrosine
kinase inhibitor, suppressed changes in cell morphology
and cell detachment, whereas cycloheximide, a protein syn-

thesis inhibitor, slightly enhanced cell detachment. Phal-
loidin, an actin filament stabilizer, and its depolymerizer,
cytochalasin D, also inhibited cell detachment. These find-
ings suggest that cell detachment from grafted surfaces
is mediated by intracellular signal transduction and re-
organization of the cytoskeleton, rather than by a simple
changes in the “stickiness” of the cells to the surface when
the hydrophobicity of the surface is changed.
One could imagine producing artificial organs using
temperature-induced detachment of cells. Artificial skin
could be produced as the cells are detached from the
support not as a suspension (the usual result of protease-
induced detachment) but preserving their intercellular
contacts. Fibroblasts were cultured on the poly(NIPAAM)-
collagen support until the cells completely covered the
surface at 37
◦
C, followed by a decrease in temperature to
about 15
◦
C. The sheets of fibroblasts detached from the
dish and within about 15 min floated in theculturemedium
(57). The detached cells could be transplanted to another
culture surface without functional and structural changes
(34). Grafting of poly(NIPPAM) onto a polystyrene sur-
face by photolitographic technique creates a special pat-
tern on the surface, and by decreasing temperature, cul-
tured mouse fibroblast STO cells are detached only from
the surface area on which poly(NIPAAM) was grafted (61).
Lithographed films of smart polymer present supports for

controlled interactions of cells with surfaces and can di-
rect the attachment and spreading of cells (62). One could
envisage producing artificial cell assemblies of complex ar-
chitecture using this technique.
Smart Surfaces—Temperature Controlled Chromatography
Surfaces that have thermoresponsive hydrophobic/hydro-
philic properties have been used in chromatography. HPLC
columns with grafted poly(NIPAAM) have been used for
separating steroids (63) and drugs (64). The chromato-
graphic retention and resolution of the solutes was strongly
dependent on temperature and increased as temperature
increased from 5 to 50
◦
C, whereas the reference column
packed with nonmodified silica displayed much shorter re-
tention times that decreased as temperature decreased.
Hydrophobic interactions dominate in retaining solutes
at higher temperature, and the preferential retention of
hydrogen-bond acceptors was observed at low tempera-
tures. The effect of temperature increase on the reten-
tion behavior of solutes separated on the poly(NIPAAM)-
grafted silica chromatographic matrix was similar to the
addition of methanol to the mobile phase at constant tem-
perature (65).
The temperature response of the poly(NIPAAM)-silica
matrices depends drastically on the architecture of the
grafted polymer molecules. Surface wettability changes
dramatically as temperature changes across the range
32–35
◦

C (corresponding to the phase-transition tempera-
ture for NIPAAM in aqueous media) for surfaces where
poly(NIPAAM) is terminally grafted either directly to the
surface or to the looped chain copolymer of NIPAAM and
N-acryloylhydroxysuccinimide which was initially coupled
to the surface. The wettability changes for the loop-grafted
surface itself were relatively large but had a slightly lower
transition temperature (∼27
◦
C). The restricted conforma-
tional transitions for multipoint grafted macromolecules
are probably the reason for the reduced transition tem-
perature. The largest surface free energy changes among
three surfaces was observed for the combination of both
loops and terminally grafted chains (30).
Introduction of a hydrophobic comonomer, buthyl-
methacrylate, in the polymer resulted in a decreased
transition temperature of about 20
◦
C. Retention of
steroids in poly(NIPAAM-co-buthylmethacrylate)-grafted
columns increases as column temperature increases. The
capacity factors for steroids on the copolymer-modified
silica beads was much larger than that on poly(NIPAAM)-
grafted columns. The effect of temperature on steroid
retention on poly(NIPAAM-co-buthylmethacrylate)-
grafted stationary phases was more pronounced compared
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POLYMERS, BIOTECHNOLOGY AND MEDICAL APPLICATIONS 843

to supports modified with poly(NIPAAM). Furthermore,
retention times for steroids increased remarkably as the
buthylmethacrylate content increased in the copolymer.
The temperature-responsive elution of steroids was
strongly affected by the hydrophobicity of the grafted
polymer chains on silica surfaces (63).
The mixture of polypeptides, consisting of 21–30 amino
acid residues (insulin chain A, β-endorphin fragment 1–27
and insulin chain B) could not be separated at 5
◦
C(below
the transition temperature) on copolymer-grafted matrix.
At this temperature, the copolymer is in an extended
hydrophilic conformation that results in decreased inter-
actions with peptides and hence short retention times in-
sufficient to resolve them. The mixture has been easily
separated at 30
◦
C, when the copolymer is collapsed, hy-
drophobic interactions are more pronounced, and reten-
tion times sufficiently long for resolving polypeptides (66).
Large protein molecules such as immunoglobulin G demon-
strate less pronounced changes in adsorption above and
below the transition temperature. Only about 20% of the
protein adsorbed on poly(NIPAAM)-grafted silica at 37
◦
C
(above the LCST) were eluted after decreasing tempera-
ture to 24
◦

C (below the transitiontemperature) (67). Quan-
titative elution of proteins adsorbed on the matrix via
hydrophobic interactions has not yet been demonstrated,
although protein adsorption onpoly(NIPAAM)-grafted ma-
trices could be somewhat controlled by a temperature
shift. A successful strategy for temperature-controlled
protein chromatography proved to be a combination of
temperature-responsive polymeric grafts and biorecogni-
tion element, for example, affinity ligands.
The access of the protein molecules to the ligands
on the surface of the matrix is affected by the transi-
tion of the polymer macromolecule grafted or attached to
the chromatographic matrix. Triazine dyes, for example,
Cibacron Blue, are often used as ligands for dye-affinity
chromatography of various nucleotide-dependent enzymes
(68). Poly(N-vinyl caprolactam), a thermoresponsive poly-
mer whose critical temperatureis about 35
◦
C interacts effi-
ciently with triazine dyes. Polymer molecules of 40000 MW
are capable of binding up to seven to eight dye molecules
hence, the polymer binds via multipoint interaction to the
dye ligands available on the chromatographic matrix. At
elevated temperature, polymer molecules are in a com-
pact globule conformation that can bind only to a few lig-
ands on the matrix. Lactate dehydrogenase, an enzyme
from porcine muscle has good access to the ligands that
are not occupied by the polymer and binds to the column.
Poly(N-vinyl caprolactam) macromolecules undergo tran-
sition to a more expanded coil conformation as temperature

decreases. Now, the polymer molecules interact with more
ligands and begin to compete with the bound enzyme for
the ligands. Finally, the bound enzyme is displaced by the
expanded polymer chains. The temperature-induced elu-
tion was quantitative, and the first reported in the litera-
ture when temperature change was used as the only elut-
ing factor without any changes in buffer composition (69).
Small changes in temperature, as the only eluting factor,
are quite promising because there is no need in this case to
separate the target protein from an eluent, usually a com-
peting nucleotide or high salt concentration in dye-affinity
chromatography.
Smart Surfaces—Controlled Porosity, “Chemical Valve”
Environmentally controlled change in macromolecular size
from a compact hydrophobic globule to an expanded hy-
drophilic coil is exploited when smart polymers are used
in systems of environmentally controlled porosity, so called
“chemical valves.” When a smart polymer is grafted to the
surface of the pores in a porous membrane or chromato-
graphic matrix, the transition in the macromolecule affects
the total free volume of the pores available for the solvent
and hence presents a means to regulate the porosity of the
system.
Membranes of pH-sensitive permeability were construc-
ted by grafting smart polymers such as poly(methacrylic
acid) (70), poly(benzyl glutamate), poly(2-ethylacrylic
acid) (71), poly(4-vinylpyridine) (72), which change
their conformation in response to pH. Thermosensitive
chemical valves have been developed by grafting poly(N-
acryloylpyrrolidine), poly(N-n-propylacrylamide), or

poly(acryloylpiperidine) (73), poly(NIPAAM) alone (33,74)
or in copolymers with poly(methacrylic acid) (74) inside
the pores. For example, grafted molecules of poly(benzyl
glutamate) at high pH are charged and are in extended
conformation. The efficient pore size is reduced, and
the flow through the membrane is low (“off-state” of the
membrane). As pH decreases, the macromolecules are
protonated, lose their charge, and adopt a compact confor-
mation. The efficient pore size and hence the flow through
the membrane increases (“on-state” of the membrane)
(71). The fluxes of bigger molecules (dextrans of molecular
weights 4400–50600) across a temperature-sensitive,
poly(NIPAAM)-grafted membrane were effectively con-
trolled by temperature, environmental ionic strength,
and degree of grafting of the membrane, while the flux of
smaller molecules such as mannitol was not affected by
temperature even at high degree of membrane grafting
(75). The on-off permeability ratio for different molecules
(water, Cl
−
ion, choline, insulin, and albumin) ranged
between 3 and 10 and increased as molecular weight in-
creased (76). An even more abrupt change of the on-off per-
meability ratio was observed for a membrane that had nar-
row pores formed by heavy ion beams when poly(NIPAAM)
or poly(acryloyl-
L
-proline methyl ester) were grafted (77).
Different stimuli could trigger the transition of the
smart polymer making it possible to produce membranes

whose permeabilities respond to these stimuli. When
a copolymer of NIPAAM with triphenylmethane leu-
cocianide was grafted to the membrane, it acquires
photosensitivity—UV irradiation increases permeation
through the membrane (78). Fully reversible, pH-
switchable permselectivity for both cationic and anionic
redox-active probe molecules was achieved by deposit-
ing composite films formed from multilayers of amine-
terminated dendrimers and poly(maleic anhydride-co-
methylvinyl ether) on gold-coated silicon (79).
When the smart polymer is grafted inside the
pores of the chromatographic matrix for gel permeation
chromatography, the transition of grafted macromolecules
regulates the pore size and as a result, the elution profile
of substances of different molecular weights. As the tem-
perature is raised, the substances are eluted progressively
earlier indicating shrinking of the pores of the hydrogel
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844 POLYMERS, BIOTECHNOLOGY AND MEDICAL APPLICATIONS
Glucose oxidase
Insulin
Insulin
Glucose
Glucose
H
+
H
+
Figure 6. Schematic of a “chemical valve.” Glucose oxidase is

immobilized on a pH-responsive polyacrylic acid grafted onto a
porous polycarbonate membrane: (a) poly(acrylic acid) is in an ex-
panded conformation that blocks insulin transport; (b) the oxida-
tion of glucose is accompanied by a decrease in pH and the transi-
tion of poly(acrylic cid) into a compact conformation that results in
opening of the pores and transport of insulin [redrawn from (82)].
beads composed of cross-linked poly(acrylamide-co-N-
isopropylacrylamide) (80) or porous polymer beads with
grafted poly(NIPAAM) (81).
When using a specific biorecognition element, which
recognizes specific substances and translates the signal
into a change of physicochemical properties, for exam-
ple, pH, a smart membrane that changes its permeability
in response to particular substances can be constructed.
Specific insulin release in response to increasing glucose
concentration, that is, an artificial pancreas, presents an
everlasting challenge to bioengineers. One of the potential
solutions is a “chemical valve” (Fig. 6). The enzyme, glu-
cose oxidase, was used as a biorecognition element,capable
of specific oxidation of glucose accompanied by a decrease
in pH. The enzyme was immobilized on pH-responsive
poly(acrylic acid) graft on a porous polycarbonate mem-
brane. In neutral conditions, polymer chains are densely
charged and have extended conformation that prevents
insulin transport through the membrane by blocking the
pores. Under exposure to glucose, the pH drops as the re-
sult of glucose oxidation by the immobilized enzyme, the
polymer chains adopt a more compact conformation that
diminishes the blockage of the pores, and insulin is trans-
ported through the membrane (82). Systems such as this

could be used for efficient drug delivery thatrespondstothe
needs of the organism. A membrane that consists of poly(2-
hydrohyethyl acrylate-co-N,N-diethylaminomethacrylate-
co-4-trimethylsilylstyrene) undergoes a sharp transition
from a shrunken state at pH 6.3 to a swollen state at
pH 6.15. The transition between the two states changes
the membrane permeability to insulin 42-fold. Copolymer
capsules that contain glucose oxidase and insulin increase
insulin release five fold in response to 0.2 M glucose. After
glucose removal, the rate of insulin release falls back to the
initial value (83).
Alternatively, reversible cross-linking of polymer
macromolecules could be used to control the porosity in
a system. Two polymers, poly(m-acrylamidophenylboronic
acid-co-vinylpyrrolidone) and poly(vinyl alcohol) form a gel
because of strong interactions between boronate groups
and the hydroxy groups of poly(vinyl alcohol). When
a low molecular weight polyalcohol such as glucose is
added to the gel, it competes with poly(vinyl alcohol) for
boronate groups. The boronate–poly(vinyl alcohol) com-
plex changes to a boronate–glucose complex that results
in eventual dissolution of the gel (84). In addition to a
glucose oxidase-based artificial pancreas, the boronate–
poly(vinyl alcohol) system has been used for constructing
glucose-sensitive systems for insulin delivery (29,85–87).
The glucose-induced transition from a gel to a sol state
drastically increases the release of insulin from the gel.
The reversible response to glucose has also been designed
using another glucose-sensitive biorecognition element,
Concanavalin A, a protein that contains four sites that can

bind glucose. Polymers that have glucose groups in the
side chain such as poly(vinylpyrrolidone-co-allylglucose)
(26) or poly(glucosyloxyethyl methacrylate) (88), are re-
versibly cross-linked by Concanavalin A and form a gel.
The addition of glucose results in displacing the glucose-
bearing polymer from the complex with Concanavalin A
and dissolving the gel.
Reversible gel-formation of thermosensitive block
copolymers in response to temperature could be utilized
in different applications. Poly(NIPAAM) block copolymers
with poly(ethylene oxide) which undergo a temperature-
induced reversible gel–sol transition were patented as
the basis for cosmetics such as depilatories and bleach-
ing agents (89). The copolymer solution is liquid at
room temperature and easily applied to the skin where
it forms a gel within 1 min. Commercially available
ethyl(hydroxyethyl)celluloses that have cloud points of
65–70
◦
C have been used as redeposition agents in wash-
ing powders. Adsorption of the precipitated polymer on the
laundry during the initial rinsing period counteracts read-
sorption of dirt when the detergent is diluted (90).
Liposomes That Trigger Release of the Contents
When a smart polymer is attached somehow to a lipid
membrane, the transition in the macromolecule affects the
properties of the membrane and renders the system sensi-
tive to environmental changes. To attach a smart polymer
to a lipid membrane, a suitable “anchor” which could be
incorporated in the membrane, should be introduced into

the macromolecule. This could be achieved by copolymer-
izing poly(NIPAAM) with comonomers that have large hy-
drophobic tails such as N,N-didodecylacrylamide (91), us-
ing a lipophilic radical initiator (92) modifying copolymers
(93), or polymers that have terminally active groups (94)
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POLYMERS, BIOTECHNOLOGY AND MEDICAL APPLICATIONS 845
with a phospholipid. Alternatively, smart polymers have
been covalently coupled to the active groups in the hy-
drophilic heads of the lipid-forming membrane (95).
Interesting and practically relevant materialsforstudy-
ing the behavior of smart polymers attached to lipid mem-
branes, are liposomes, self assembled 50–200 nm vesicles
that have one or more (phospho)lipid bilayers which en-
capsulate a fraction of the solvent. Liposomes are stable
in aqueous suspension due to the repulsive forces that ap-
pear when two liposomes approach each other. Liposomes
are widely used for drug delivery and in cosmetics (96).
The results of a temperature-induced conformational
transition of a smart polymer on the liposomal sur-
face depend significantly on the fluidity of the liposomal
membrane. When the membrane is in a fluid state at
temperatures both above and below the polymer transition
temperature, the collapse of the polymer molecule forces
anchor groups to move closer together by lateral diffusion
within the membrane. The compact globules of collapsed
polymer cover only a small part of the liposomal surface.
Such liposomes have a low tendency to aggregate because
the most of their surface is not covered by the polymer.

Naked surfaces contribute to the repulsion between lipo-
somes. On the other hand, when the liposomal membrane
is in a solid state at temperatures both above and below
the polymer transition temperature, the lateral diffusion
of anchor groups is impossible, and the collapsed polymer
cannot adopt a compact globule conformation but spreads
over the most of the liposomal surface (97). Liposomes
whose surfaces are covered to a large degree by a collapsed
polymer repel each other less efficiently than intact lipo-
somes. The stability of a liposomal suspension is thereby
decreased, and aggregation and fusion of liposomes takes
place, which is often accompanied by the release of the
liposomal content into the surrounding medium (98).
When the liposomal membrane is perturbed by the con-
formational transition of the polymer, both the aggregation
tendency and liposomal permeability for incorporated
substances are affected. Poly(ethacrylic acid) undergoes a
transition from an expanded to a compact conformation in
the physiological pH range of 7.4–6.5 (99). The pH-induced
transition of poly(ethacrylic acid) covalently coupled to the
surface of liposomes formed from phosphatidylcholine
results in liposomal reorganization into more compact
micelles and concomitant release of the liposomal content
into the external medium. The temperature-induced tran-
sition of poly(NIPAAM-co-N,N-didocecylacrylamide) (100)
or poly(NIPAAM-co-octadecylacrylate) (101), incorporated
into the liposomal membrane, enhanced the release of the
fluorescent marker, calcein, encapsulated in copolymer-
coated liposomes. Liposomes hardly release any marker
at temperatures below 32

◦
C (the polymer transition
temperature), whereas the liposomal content is released
completely within less than a minute at 40
◦
C. To increase
the speed of liposomal response to temperature change, the
smart polymer was attached to the outer and inner sides of
the lipid membrane. The polymer bound only to the outer
surface if the liposomes were treated with the polymer af-
ter liposomal formation. When the liposomes were formed
directly from the lipid–polymer mixture, the polymer was
present on both sides of the liposomal membrane (91).
Changes of liposomal surface properties caused by
polymer collapse affect liposomal interaction with cells.
Liposomes modified by a pH-sensitive polymer, partially
succinilated poly(glycidol), deliver calcein into cultured
kidney cells of the African green monkey more effi-
ciently compared to liposomes not treated with the poly-
mer (102). Polymeric micelles formed by smart polymers
and liposomes modified by smart polymers could be used
for targeted drug delivery. Polymeric micelles have been
prepared from amphiphilic block copolymers of styrene
(forming a hydrophobic core) and NIPAAM (forming a
thermosensitive outer shell). The polymeric micelles were
very stable in aqueous media and had long blood circu-
lation because of small diameter, unimodal size distribu-
tion (24 ± 4 nm), and, a low critical micellar concentration
of around 10 µg/mL. At temperatures above the polymer
transition temperature (32

◦
C), the polymer chains that
form an outer shell collapse, become more hydrophobic, and
allow aggregation between micelles and favoring binding
interactions with the surface of cell membranes. Thus, hy-
drophobic molecules incorporated into the micelles are de-
livered into the cellmembranes. Thesemicelles are capable
of site-specific delivery of drugs to the sites as temperature
changes, for example, to inflammation sites of increased
temperature (103).
Smart Polymers in Bioanalytical Systems
Because smart polymers can recognize small changes in
environmental properties and respond to them in a pro-
nounced way, they could be used directly as sensors of
these changes, for example, a series of polymer solutions
that have different LCSTs could be used as a simple ther-
mometer. As salts promote hydrophobic interactions and
decrease the LCST, the polymer system could “sense” the
salt concentration needed to decrease the LCST below
room temperature. A poly(NIPAAM)-based system that
can sense NaCl concentrations above 1.5% was patented
(104). The response of the polymer is controlled by a bal-
ance of hydrophilic and hydrophobic interactions in the
macromolecule. Using a recognition element that can sense
external stimuli and translate the signal into the changes
of the hydrophilic/hydrophobic balance of the smart poly-
mer, the resulting system presents a sensor for the stimu-
lus.If the conjugate of a smartpolymerandarecognitionel-
ement has a transition temperature T
1

in the absence and
T
2
in the presence of stimuli, fixing the temperature T in
the range T
1
< T < T
2
allows achieving the transition of a
smart polymer isothermally by the external stimulus (105).
An example of such a sensor was constructed using trans–
cis isomerization of the azobenzene chromophore when ir-
radiated by UV light. The transition is accomplished by an
increase in the dipole moment of azobenzene from 0.5 D
(for the trans-form) to 3.1 D (for the cis-form) and hence a
significant decrease of hydrophobicity. Irradiation with UV
light results in increasing the LCST from 19.4 to 26.0
◦
C for
the conjugate of the chromophore with poly(NIPAAM). The
solution of the conjugate is turbid at 19.4
◦
C < T < 26.0
◦
C,
but when irradiated, the conjugate dissolves because the
cis-form is below the LCST at this temperature. The sys-
tem responds to UV light by transition from a turbid to
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846 POLYMERS, BIOTECHNOLOGY AND MEDICAL APPLICATIONS
transparent solution. The termination of UV irradiation
results in a slow return of the system to its initial tur-
bid state (105). A few other light-sensitive systems were
proposed that use different chromophores: triarylmethyl-
cyanide (106) and leuconitriles (107).
The hydrophobicity of the recognition molecule was also
changed by chemical signals. Poly(NIPAAM) containing
11.6 mol% of crown ether 9 has a LCST of 31.5
◦
C in the
absence of Na
+
or K
+
ions, 32
◦
C in the presence of Na
+
,
and 38.9
◦
C in the presence of K
+
. Thus, the introduction
of both Na
+
and K
+
ions leads to the dissolution of the in-

soluble polymer at that temperature. At 37
◦
C, this effect is
achieved only by K
+
ions (108).
From better understanding of ligand–host interactions
and development of new highly selective binding pairs (e.g.,
by using combinatorial libraries to find ligands of high
affinity for particular biomolecules), one could expect that
smart polymer systems will be used as “signal amplifiers”
to visualize a physicochemical event, which takes place
in a recognition element, by a pronounced change in the
system—conversion of a transparent solution into a turbid
one or vice versa.
Antibody–antigen interactions present nearly ideal
analytical selectivity and sensitivity developed by nature.
Not surprisingly, they are increasingly used for a broad
variety of bioanalytical applications. Different analytical
formats have been developed. The common feature of
the most of them is the requirement for separating an
antibody–antigen complex from a nonbound antibody or
antigen. Traditionally, the separation is achieved by cou-
pling one of the components of antibody–antigen pair to a
solid support. The binding step is followed by washing non-
bound material. Interactions of the soluble partner of the
binding pair with the partner coupled to the support are
often accompanied by undesired diffusional limitations,
and hence, incubation times of several hours are required
for analysis. Because smart polymers can undergo tran-

sition from the soluble to the insoluble state, they allow
combining the advantages of homogeneous binding and,
after the phase transition of the smart polymer has taken
place, easy separation of the polymer precipitate from the
supernatant. The essential features of an immunoassay
that uses smart polymers (named PRECIPIA) are as
follows. The covalent conjugate of poly(NIPAAM) with
monoclonal antibodies to the κ-chain of human im-
munoglobulin G (IgG) are incubated for 1 h at room tem-
perature (below the LCST of the conjugate), and the IgG
solution is analyzed. Then plain poly(NIPAAM) (to facili-
tate thermoprecipitation of polymer–antibody conjugates)
and fluorescently labeled monoclonal antibodies to the γ -
chain of human IgG are added. The temperature is raised
to 45
◦
C, the precipitated polymer is separated by centrifu-
gation, and fluorescence is measured in the supernatant
(109). Immunoassay systems that use temperature-
induced precipitation of poly(NIPAAM) conjugates with
monoclonal antibodies are not inferior in sensitivity to the
traditional heterogenous immunoassay methods, but be-
cause the antigen–antibody interaction takes place in solu-
tion, the incubation can be shortened toabout1h(110,111).
The limitations of PRECIPIA as an immunoassay tech-
nique are essentially the same as those of affinity pre-
cipitation, namely, nonspecific coprecipitation of analyzed
protein when poly(NIPAAM) precipitates. Polyelectrolyte
complexes that have a low degree of nonspecific protein co-
precipitation have also been successfullyusedas reversibly

soluble carriers for PRECIPIA-type immunoassays (112).
The conjugate of antibody and polyanion poly(methacrylic
acid) binds to the antigen within a few minutes, and the
polymer hardly exerts any effect on the rate of antigen–
antibody binding. Subsequent addition of a polycation,
poly(N-ethyl-4-vinyl-pyridinium bromide) in conditions
where the polyelectrolyte is insoluble, results in quantita-
tive precipitation of the antibody–polymer conjugate
within 1 min. The total assay time is less than 15 min (10).
In principle, PRECIPIA-type immunoassays could be
used for simultaneous assay of different analytes in one
sample, provided that conjugates specific toward these
analytes are coupled covalently to different smart poly-
mers that have different precipitating conditions, for ex-
ample, precipitation of one conjugate by adding a polymeric
counterion followed by thermoprecipitation of the second
conjugate by increasing temperature.
Reversibly Soluble Biocatalysts
The transition between the soluble and insoluble state of
stimuli-responsive polymers has been used to develop re-
versibly soluble biocatalysts. A reversibly soluble biocat-
alyst catalyzes an enzymatic reaction in a soluble state
and hence could be used in reactions of insoluble or poorly
soluble substrates/products. As soon as the reaction is com-
pleted and the products are separated, the conditions (pH,
temperature) are changed to promote precipitation of the
biocatalyst. The precipitated biocatalyst is separated and
can be used in the next cycle after dissolution. The re-
versibly soluble biocatalyst acquires the advantages of im-
mobilized enzymes (ease of separation from the reaction

mixture after the reaction is completed and the possibility
for biocatalyst recovery and repeated use in many reaction
cycles) but at the same time overcomes the disadvantages
of enzymes immobilized onto solid matrices such as diffu-
sional limitations and the impossibility of using them in
reactions of insoluble substrates or products.
Biocatalysts that are reversibly soluble as a function of
pH have been obtained by the covalent coupling of
lysozyme to alginate (113); of trypsin to poly(acrolein-co-
acrylic acid) (114); and of cellulase (115); amylase (115);
α-chymotrypsin, and papain (116) to poly(methylmetha-
crylate-co-methacrylic acid). A reversibly soluble cofactor
has been produced by the covalent binding of NAD to
alginate (117). Reversibly soluble α-chymotrypsin, peni-
cillin acylase, and alcohol dehydrogenase were produced
by coupling to the polycation component of polyelectrolyte
complexes formed by poly(methacrylic acid) and poly(N-
ethyl-4-vinyl-pyridinium bromide) (118).
Biocatalysts that are reversibly soluble as a function
of temperature have been obtained by the covalent cou-
pling of α-chymotrypsin and penicillin acylase to a par-
tially hydrolysed poly(N-vinylcaprolactam) (119); and of
trypsin (120); alkaline phosphatase (121), α-chymotrypsin
(122), and thermolysin (123,124) to NIPAAM copolymers
that contain active groups suitable for covalent coupling of
biomolecules. Lipase was coupled to a graft copolymer com-
posed of NIPAAM grafts on a poly(acrylamide-co-acrylic
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POLYMERS, BIOTECHNOLOGY AND MEDICAL APPLICATIONS 847

acid) copolymer (125). No significant differences in bio-
catalytic properties were found for α -amylase coupled to
poly(NIPAAM) via single-point or multipoint mode. Both
enzyme preparations demonstrated increased thermosta-
bility and the absence of diffusional limitation when hy-
drolyzing starch, a high molecular weight substrate (126).
The temperature of a protein–ligand interaction was con-
trolled by site-directed coupling of terminally modified
poly(NIPAAM) to a specifically constructed site (close to
a biotin binding site) on a genetically modified strepta-
vidin (127).
Biocatalysts which are reversiblysolubleasafunctionof
Ca
2+
concentration were produced by covalent coupling of
phosphoglyceromutase, enolase, peroxidase, and pyruvate
kinase to α
s1
-casein. The enzyme casein conjugates are sol-
uble at a Ca
2+
concentration below 20 mM but precipi-
tate completely at a Ca
2+
concentration above 50 mM. The
precipitate redissolves when EDTA, a strong Ca
2+
-binding
agent is added (128).
The reversible flocculation of latices has been used to

produce thermosensitive reversibly soluble (more precisely
reversibly dispersible) biocatalysts using trypsin (129),
papain (130), and α-amylase (131). Latices sensitive to
a magnetic field have been used to immobilize trypsin
and β-galactosidase (132). Liposomes that have a polymer-
ized membrane, that reversibly aggregates on chang-
ing salt concentration have been used to immobilize
α-chymotrypsin (133).
The most attractive application of reversibly soluble
biocatalysts is repeated use in a reaction which is diffi-
cult or even impossible to carry out using enzymes im-
mobilized onto insoluble matrices, for example, hydroly-
sis of water-insoluble phlorizidin (134); hydrolysis of high
molecular weight substrates such as casein (123,130) and
starch (115); hydrolysis of insoluble substrates such as
cellulose (135) and raw starch (corn flour) (7,134,136–
138); production of insoluble products such as peptide,
benzyloxycarbonyl-
L-tyrosyl-N
ω
-nitro-L-arginine (116) and
phenylglycine (139).
The hydrolytic cleavage of corn flour to glucose is an
example of successfully using a reversibly soluble bio-
catalyst, amylase coupled to poly(methylmethacrylate-co-
methacrylic acid), in an industrially interesting process
(136). The reaction product of the process, glucose, inhibits
the hydrolysis. The use of a reversibly soluble biocatalyst
improves the efficiency of the hydrolysis which is carried
out at pH 5, at which the amylase–polymer conjugate is

soluble. After each 24 h, the pH is reduced to 3.5, the un-
hydrolyzed solid residue and the precipitated conjugate
are separated by centrifugation, the conjugate is resus-
pended in a fresh portion of the substrate at pH 5, and the
hydrolysis is continued. Theconversion achieved after 5 cy-
cles is 67%, and the activity of the amylase after the fifth
cycle was 96% of the initial value (136).
CONCLUSION
In the future, one looks forward to further developments
and the commercial introduction of new smart polymers
whose transition temperatures and pH are compatible with
physiological conditions orconditions for maximal stability
of target biomolecules, such as temperatures of 4–15
◦
C and
pH values of 5–8. Additional prospects will stem from a
better understanding of the mechanism of cooperative
interactions in polymers and increasing knowledge of
structure–property correlations to enable rational synthe-
sis of smart polymers that have predefined properties. Due
to the possibility of combining a variety of biorecognition
or biocatalytic systems and the unique features of smart
polymers, expectations are running high in this area. Only
time and more experimentation will determine whether
smart polymers will live up to their generous promises.
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J. Ferment. Bioeng. 69: 228–233 (1990).
139. Yu.V. Galaev, Prikl. Bioxim. Mikrobiol. 30: 167–170 (1994).
140. I.Yu. Galaev and B. Mattiasson, Trends Biotechnol. 17: 335–
340 (1999).
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PB091-P-DRV-II January 23, 2002 21:27
850 POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS
POLYMERS, FERROELECTRIC LIQUID
CRYSTALLINE ELASTOMERS
RUDOLF ZENTEL
University of Mainz
Mainz, Germany
INTRODUCTION
Ferroelectric materials are a subclass of pyro- and piezo-
electric materials (Fig. 1). They are very rarely found in
crystalline organic or polymeric materials because ferro-
electric hysteresis requires enough molecular mobility to
reorient molecular dipoles in space. So semicrystalline
polyvinylidene fluoride (PVDF) is nearly the only known
compound (1). On the contrary, ferroelectric behavior is
very often observed in chiral liquid crystalline materials,
both low molar mass and polymeric. For an overview of fer-

roelectric liquid crystals, see (2). Tilted smectic liquid crys-
tals that are made from chiral molecules lack the symmetry
plane perpendicular to the smectic layer structure (Fig. 2).
Therefore, they develop a spontaneous electric polariza-
tion, which is oriented perpendicular to the layer normal
and perpendicular to the tilt direction. Due to the liquid-
like structure inside the smectic layers, the direction of
the tilt and thus the polar axis can be easily switched in
external electric fields (see Figs. 2 and 4).
Here, we discuss materials (LC-elastomers) that com-
bine a liquid crystalline phase and ferroelectric properties
(preferable the chiral smectic C
∗
phase) in a polymer net-
work structure (see Fig. 3). The coupling of the liquid crys-
talline director to the network or the softness of the net-
work is chosen so that reorientation of the polar axis is still
possible. Thus densely cross-linked systems, that possess
a polar axis but cannot be switched (3) will be excluded.
Ferroelectric
Pyroelectric
Piezoelectric
P
S
E
Figure 1. Ferroelectric hysteresis that shows the spontaneous
polarization P
S
of a ferroelectric material reversed by an applied
electric field E.

It is the role of the network (1) to form a rubbery matrix
for the liquid crystalline phase and (2) to stabilize a direc-
tor configuration. LC-materials that have these properties
can be made either (see Fig. 3) by covalently linking the
mesogenic groups to a slightly cross-linked rubbery poly-
mer network structure (see Fig. 3a) (4–6) or by dispersing
a phase-separated polymer network structure within a low
molar mass liquid crystal (see Fig. 3b) (8,9). Both systems
possess locally a very different structure. They may show,
however, macroscopically similar properties.
LC-elastomers (see Fig. 3a) have been investigated in
detail (4–7). Although the liquid crystalline phase transi-
tions are nearly unaffected by the network, the network
retains the memory of the phase and director pattern dur-
ing cross-linking (7). In addition, it freezes fluctuations of
the smectic layers and leads to a real long range order
in one dimension (11). An attempt to change the direc-
tor pattern by electric or magnetic fields in LC-elastomers
leads to a deformation of the network and to an elastic
response (see Fig. 4). As a consequence of this, nematic
LC-elastomers could never be switched in electric fields, if
the shape of the elastomer was kept fixed. For freely sus-
pended pieces of nematic LC-elastomers, shape variations
in electric fields have been observed sometimes (12,13). In
ferroelectric liquid crystals, the interaction with the elec-
tric field is, however, much larger. Thus, it has been possi-
ble to prepare real ferroeletric LC-elastomers (see Fig. 4)
(14,15). In these systems, the polymer network stabilizes
one switching state like a soft spring. It is, however, soft
enough to allow ferroelectric switching. Therefore the fer-

roelectric hysteresis can therefore be measured in these
systems. It is, however, shifted away from zero voltage (see
Fig. 4).
SYNTHESIS OF FERROELECTRIC LC-ELASTOMERS
The ferroelectric LC-elastomers described so far (14–17,
44–46) are mostly prepared from cross-linkable ferroelec-
tric polysiloxanes (see Fig. 5), which are prepared by hy-
drosilylation of precursor polysiloxanes (18). The cross-
linking is finally initiated by irradiating a photoradical
generator, which leads to oligomerization of acrylamide or
acrylate groups (see Fig. 5). The functionality of the net
points is thus high (equal to the degree of polymerization)
and varies with the cross-linking conditions.
The advantage of this photochemical-initiated cross-
linking is that the crosslinking can be started—at willafter
the liquid crystalline polymer is oriented and sufficiently
characterized in the uncross-linked state (see Fig. 6). The
advantage of using polymerizable groups (acrylates) for
cross-linking is that small amounts of these groups are suf-
ficient to transform a soluble polymer into a polymer gel
and that the chemical reactions happens far away from the
mesogen. Cinnamoyl moieties, on the other hand (19), re-
quire a high concentration of these groups for cross-linking.
The dimers thus formed are, in addition, nonmesogenic.
Figure 7 summarizes the ferroelectric LC-elastomers dis-
cussed in this article. Two different positions of cross-
linkable groups are used. In polymer P1, the cross-linking
group is close to the siloxane chains, which are known to
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PB091-P-DRV-II January 23, 2002 21:27

POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS 851
P
E
E
P
n
z
θ
+
n
z
θ
−
Elektroden
Figure 2. Schematic of the bistable switch-
ing of a ferroelectric liquid crystal in the
“surface stabilized FLC” configuration.
(a)
(b)
Figure 3. Network: soft, can be transformed like rubber band, but retains its shape and couples to
director orientation because (a) director is preferably parallel (or perpendicular) to polymer chains
(LC-elastomer) (4–8). (b) Director aligns (parallel) to chains in oriented phase-separated polymer
network structure (low molar mass LC in LC-thermoset) (8,37).
microphase separate from the mesogenic groups (18,19).
Therefore, the crosslinking should proceed mostly within
the siloxane sublayers. In polymers P2 and P3, the cross-
linking group is located at the end of mesogens. There-
fore, the cross-linking should proceed mostly between dif-
ferent siloxane layers (see Fig. 7). A comparison of these
elastomers allows evaluating structure–property relation-

ships (17,23,26).
Properties and Characterization
Ferroelectric Characterization (Uncross-linked Systems).
Before cross-linking, polarization, tilt angle, and switch-
ing times can be determined in the usual way (17,18,21).
Figure 8 shows the temperature dependence of the spon-
taneous polarization for polymers P1–P3. For the ho-
mopolymer related to polymer P2, all relevant parameters
P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH
PB091-P-DRV-II January 23, 2002 21:27
852 POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS
(a)
(b)
E
(c)
Uncrosslinked
−80 −40
04080
Voltage/V
Optical response
Crosslinked
−200 −100
0 100 200
Voltage/V
Figure 4. Schematic of the ferroelectric LC-elastomer and its two switching states (14): (a) A
polymer chain acts as cross-linking point by connecting different mesogenic groups attached to the
main polymer chains. A ferroelectric switching in this elastomer extends polymer chains. (b) The
entropy elasticity arising from this acts like a spring that stabilizes one state. (c) For the uncross-
linked system (left) the hysteresis is symmetrical to zero voltage and both states are equal. After
cross-linking in one polar state (right), only that state is stable with no electric field, and the

hysteresis is no longer symmetrical to zero voltage.
were determined in a careful study by Kocot et al. (22).
It seems that the electroclinic effect is especially strong
in these polysiloxanes (15). This has implications for the
freezing of a memory of the tilt angle present during cross-
linking. Therefore, ferroelectric elastomers, which have
been crosslinked in the smectic A phase while applying
an electric field, produce a stable macroscopic polarization
(tilt) after cooling into the smectic C* phase (17).
Properties of Ferroelectric LC-Elastomers. The crosslink-
ing reactions of a series of copolymers analogs to polymer
P2, but differing in the amount of cross-linkable groups
were studied by FTIR spectroscopy (16). These measure-
ments show a decrease of the acrylamide double bond on
irradiation. Conversions between 60 to 84% were observed.
The uncertainity of the conversion, however, is high be-
cause only very few double bonds are present in polymer
P2 and they are visible in the infrared spectrum at rather
low intensity.
Mechanical measurements, which show how this photo-
chemical crosslinking (conversion of double bonds) leads to
an elastic response of the network are, however, still at the
beginning because photo-cross-linking can be performed
only in thin layers of some microns. It is best performed
between two glass slides to exclude oxygen.
AFM measurements of photo-cross-linked free stand-
ing films show changes in topology during stretching (see
Properties of Ferroelectric LC-Elastomers—AFM Imaging
of Thin Films) (23). They do, however, not allow measuring
elastic moduli.

The most promising approach to obtaining elastic data
for these ferroelectric elastomers is investigation of LC-
elastomer balloons (25,26). For this purpose, an experi-
mental setup was developed on the basis of an apparatus
P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH
PB091-P-DRV-II January 23, 2002 21:27
POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS 853
LC network
OCH
3
OCH
3
O
SiH
3
CH
O
SiH
3
CCH
3
O
(CH
2
)
9
OOC
O
CH
3

SiH
3
C
O
(CH
2
)
11
OO
SiH
3
CCH
3
O
CCH
3
O
H
2
NNH
2
THF
SiH
3
C
O
(CH
2
)
11

OOH
SiH
3
CCH
3
O
HO C
CH
CH C
2
H
5
OCH
3
HO C (CH
2
)
5
NC
O
O
SiH
3
C
(CH
2
)
10
OO
CH

2
CCHCHC
2
H
5
OCl CH
3
O
O
SiH
3
C
(CH
2
)
10
OO
CH
2
C
O
(CH
2
)
5
N
H
C
O
O

SiH
3
CCH
3
1 n
2.7 n
+
C
10
H
12
PtCl
2
toluene
1 n
2.7 n
1 n
2.7 n
∗∗
+
+
DCC, DMAP, THF, CH
2
Cl
2
0.9 n
∗∗
2.7 n
0.1 n
λ = 365 nm

in
smectic C*
Cl
O
H
Figure 5. Synthetic route to the cross-
linkable polysiloxane P2 and the follow-
ing preparation of the oriented smectic C*
network using UV light in the presence
of a photoinitiator (1,1-dimethoxy-1-phenyl-
acetophenone) (14).
P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH
PB091-P-DRV-II January 23, 2002 21:27
854 POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS
Polydomain
Monodomain
Electrical
field
ITO
Initiator
hν
electrical field
Oriented s
c
-network
Figure 6. Preparation of polar smectic C* monodomains (14,15)
(ITO: indium tin oxide).
designed to study smectic bubbles (25). Freely suspended
films of the uncross-linked material behave like ordinary
smectic films. They can be inflated to spherical bubblessev-

eral mm in diameter (the thickness of a smectic-layer skin
is about 50 nm). These bubbles are stabilized by the
smectic-layer structure and their inner pressure p is re-
lated to the surface tension and the bubble radius R by
the Laplace–Young equation, p ∝ 1/R. After exposure to
UV light, the material is cross-linked, and an anisotropic
elastomer is formed. When the cross-linked bubbles are
inflated/ deflated, the radius–pressure curve reverses its
slope and gives directaccessto the elastic moduli of the ma-
terial (26). Because the deformation during inflation of the
balloon is isotropic in the layer plane, the material should
contract in the direction of the layer normal.
Mechanical measurements of chemically crosslinked
LC-elastomers have been made extensively (4,5,27,28,
36,41–43). For these systems, it can be shown that stretch-
ing allows orientation of the liquid crystalline phase. In
ideal situations, it is thus possible to prepare aferroelectric
monodomain by stretching (28,30,36). This result can be
rationalized as a two-stage deformation process (see Fig. 9)
(36). This possibility of orienting or reorienting the polar
axis mechanically is the basis for the piezoelectric proper-
ties to be discussed later. Ferroelectric switching could not
be observed for any of the chemically crosslinked systems.
This may occur because chemically cross-linked films are
too thick (several 100 µm compared to about 10 µm for
photochemically cross-linked systems) and the electric
field applied is therefore too small. In addition, the cross-
linking density in chemically cross-linked systems is pre-
sumably higher.
Ferroelectric Properties LC-Elastomers. The ferroelectric

properties of the photochemically crosslinked elastomers
E1 to E3 differ significantly and depend on the topology
of the network formed. For the systems that have inter-
layer cross-linking (see Fig. 7, E2 and E3), the switch-
ing time is increased greatly. Therefore, spontaneous
polarization can no longer be determined by the triangu-
lar wave method. Slow switching is, however, still possi-
ble and therefore ferroelectric hysteresis can be measured
optically (see Figs. 4c and 10) (14,15). After photochem-
ical cross-linking in a ferroelectric monodomain, the fer-
roelectric hysteresis shows stabilization of the orientation
present during cross-linking. At zero external voltage, only
this state is stable. The second switching state can, how-
ever, be reached. Therefore, the network acts like a spring
that stabilized one state because switching to the other
state leads to a deviation from the most probable confor-
mation of the polymer chain (32) (see Fig. 10). Then, the
shift of the center of the hysteretic loop away from zero
voltage gives the magnitude of the electric field necessary
to balance the mechanical field of the network. The asym-
metry of the hysteresis increases with the cross-linking
density (17). For high cross-linking densities, switching
remains possible only if the spontaneous polarization is
rather high (17). Otherwise, the network prohibits switch-
ing. The asymmetry of ferroelectric switching could also
be proven by polarized FT-IR spectroscopy (33). Increasing
the temperature of this ferroelectric elastomerleadsto nar-
rowing of the hysteretic loop, which is lost at the transition
to the smectic A phase (see Fig. 10).
This behavior is best interpreted by plotting the liquid

crystalline potential, the elastic potential of the network,
and their superposition in one graph (15) (see Fig. 11). As
the network is formed in the smectic C* phase, an internal
elastic field is created, which has its minimum value for
the tilt angle and tilt direction during cross-linking. Other
tilt angles are destabilized.
The elastomer that has preferable intralayer cross-
linking (E1, see Fig. 7) shows completely different behav-
ior (see Fig. 12) (17,34). In this case, the switching time
increases by less than a factor of 2, the polarization can
still be determined, and measurement of the ferroelectric
hysteresis shows no stabilization of the switching state
present during cross-linking. Then, the coupling between
the orientation of the mesogens and the network confor-
mation is obviously very weak. The network stabilizes the
smectic layer structure (see Properties of Ferroelectric LC-
Elastomers—AFM Imaging of Thin Films), but it does not
stabilize the tilt direction. Therefore, the polar axis can be
switched easily. This is the result of the network topology
(see Fig. 7) in which interlayer cross-linking is rare.
Properties of Ferroelectric LC-Elastomers—AFM Imaging
of Thin Films. Freestanding films can be prepared from
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PB091-P-DRV-II January 23, 2002 21:27
OO
O
Cl
(CH
2
)

10
CH
2
Si
O
H
3
C
O
SiH
3
CCH
2
CH
2
CH
2
O
O
O
SiH
3
CCH
3
P1
*
*
0.9 n
0.1 n
2.7 n

Phase transitions [°C]: s
X
32 s
C
∗
60 s
A
92 i
n 30
∼
∼
O
O
Cl
(CH
2
)
10
CH
2
Si
O
H
3
C
O
SiH
3
CCH
2

O
SiH
3
CCH
3
(CH
2
)
10
OO
O
O
N
O
H
P2
O(CH
2
)
10
CH
2
Si
O
H
3
C
O
Si
OC

O
C*
NO
2
CH
3
H
C
6
H
13
H
3
C
O
CH
2
(CH
2
)
10
OOC
O
O (CH
2
)
6
O
O
SiH

3
CCH
3
P3
*
*
n 30
∼
∼
Phase transitions [°C]: s
X
29 s
C
∗
53 s
A
89 i
0.9 n
0.1 n
2.7 n
n 15
∼
∼
Phase transitions [°C]: s
X
32 s
C
∗
117 s
A

152 i
0.85 n
0.15 n
1.5 n
Crosslinking within siloxane sublayer (intra layer) Crosslinking between siloxane sublayers (inter layer)
P1
Photoinitiator Photoinitiator
P2, P3
E1
EE
hυ hυ
E2, E3
Figure 7. Chemical structure and phase transition temperatures of polymers P1–3 (17). (a) P1 is
designed to favor intralayer cross-linking. (b) P2–3 forming an interlayer network.
50−5−10−15−20−25−30−35−40−45−50
T-T
C
[°C]
0
20
40
60
80
100
120
140
160
P
S
[nC/cm

2
]
P1
P2
P3
Figure 8. Temperature dependence of the spontaneous polariza-
tion P
S
for the polymers P1–3 measured by the triangular wave
method (17).
First deformation Second deformation
Figure 9. Two-step deformation process of a chiral smectic C*
elastomer that displays macroscopic polarization at the end (36).
855
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PB091-P-DRV-II January 23, 2002 21:27
856 POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS
−200 −100 0 100
200
Optical response
Temperature/°C
42 44 46 48
−200 −100 0 100
200
Voltage/V
Voltage/V
Optical response
Temperature/°C
50 54 56
Figure 10. Temperature dependence of the optical hysteresis of

elastomer E2 (S
C
*49
◦
CS
A
) (15). (a) Ferroelectric behavior of the
S
C
* phase (42, 44, 46, and 48
◦
C, respectively). (b) Electroclinic
behavior of the S
A
phase (50, 54, and 56
◦
C, respectively).
−2000
0
10000
8000
6000
4000
2000
−30 −20 −10 0 10 20
30
Tilt angle Θ/deg
∆ g / J m
−3
Figure 11. Effect of network force on the free energy density (15)

2 K below the phase transition, S
C
* phase: (
◦
) calculated potential
of the S
C
*phase, (♦) force due to the network (
r
) superposition of
both.
25 20 15 10 5 0
0
10
20
30
40
50
P1 at 200 V / 10 µm
P1 + 1 wt % photoinitiator
E1
T-T
c
[°C]
τ [ms]
Figure 12. Temperature dependence of the switching time τ (de-
fined as 0–100% change in transmission) for P1 and E1 [see (17)
for comparison].
uncross-linked polymers (see also the smectic balloons in
this context). They can be photo-cross-linked and trans-

ferred to a solid substrate. Thereafter, the topology of the
films can be imaged by AFM, which gives direct visualiza-
tion of the smectic layer structure at low temperatures. The
uncross-linked polymers can only be imaged at low temper-
atures, deep inside the smectic phase and in the tapping
mode, which does not induce strong lateral forces. At higher
temperatures, the sample is too soft and mobile to allow
imaging. Cross-linked elastomers, on the other hand, are
mechanically stable, and films sustain the tapping mode
and also the contact mode of the atomic force microscope
(35). This holds both for intra- and interlayer cross-linked
systems. Because measurements can be done in all phases,
it is also possible to determine the change of the smec-
tic layer thickness at the phase transitions directly. For
elastomer E1, for example, the smectic layer thickness is
4.2 nm in the smectic C* phase (36
◦
C, tilt angle about 30
◦
).
It increases to 4.4 nm at 50
◦
C in the smectic A phase (35).
This corresponds to X-ray measurements.
To analyze the impact of the molecular structure on
network properties, elastomers are compared, which are
identical except for the molecular position of the cross-
linkable group: (1) elastomer E1 that has cross-linkable
groups attached to the backbone via a short spacer
(intralayer cross-linking) and (2) elastomer E2 where the

cross-linkable group is in the terminal position of a meso-
genic side group (intralayer cross-linking) (23,24). When
mechanical stress (stretching) is imposed on thin films in
homeotropic orientation, the two elastomers react differ-
ently to the deformation (23,24), as seen by AFM imag-
ing of the surface topology (see Fig. 13). For elastomer E1,
“intralayer” cross-linking results in two-dimensional net-
works in the backbone layers, separated by liquid-like FLC
side-group layers. Because there are practically no vertical
connections in this intralayer network, no vertical distor-
tions occur. Therefore, this elastomer can be stretched up
to 100%, the surface remains smoth, and the layers deform
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PB091-P-DRV-II January 23, 2002 21:27
POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS 857
(a)
(b)
(c)
(d)
(e)
(f)
Figure 13. Surface topography of as prepared and stretched transferred films of elastomer
E1
x=0.1
(a: λ = 0, b: λ = 10%), E2
x=0.07
(c: λ = 0, d: λ = 12%), and E2
x=0.25
(e: λ = 0, f: λ = 2%,).
Scale bars 1 µm, height scale 25 nm valid for all images. The surface of all polymers show plateau

patterns. In the E2 samples, the lateral strain leads to surface deformation (24).
affinely. In elastomer E2, a three-dimensional, “interlayer”
network is formed; the system reacts by distorting the
smectic layering. Therefore, only smaller stretching ratios
are accessible and the surfaceroughensand buckles during
stretching. The distortion strength increases with a higher
cross-linking density.
Piezoelectric Properties of Ferroelectric LC-Elastomers.
Because a ferroelectric material has to be piezoelectric (see
Fig. 1), observation of a piezoresponse is natural. It has
been observed for ferroelectric LC-elastomers (14,15,44–
46) and also for more densely cross-linked systems (36–
39) for which no ferroelectric switching could be observed.
For the elastomers described here it is, however, possible
to change the piezoresponse (14) by reorienting the po-
lar axis in an external field (see E2 in Fig. 14). For this
experiment, the polar axis was kept in one orientation
during cross-linking. This resulted in a positive piezore-
sponse (see Fig. 14). Thereafter, the direction of the polar
axis was inverted by applying an external field of opposite
direction. Then, the external field was removed and the
piezocoefficient was measured. At first, a piezoresponse of
opposite sign (negative) but identical value is determined
(see Fig. 14). In the field-free state, this piezoresponse con-
tinuously decreases, it goes through zero, increases again,
and finally reaches the original positive value. This ex-
periment is comparable to the hysteresis measurements
of Figs. 4 and 10 because it shows that two polar states
are accessible, but the one present during cross-linking is
stabilized.

The shape variation under application of an external
electric field was most intensively studied formicrotomized
P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH
PB091-P-DRV-II January 23, 2002 21:27
858 POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS
Charge
det.
Poling
200 V, 65°
Fast cooling
to room
temperature
d
33
−d
33
d
33
80 min.
−0.8
−0.6
−0.4
−0.2
0
0.2
0.4
0.6
0.8
0 20 40 60 80 100
Time/s

d
33
/pC N
−1
Figure 14. Relaxation of the piezocoefficient d
33
of elastomer E2
at room temperature after reversal poling at 65
◦
C(S
A
phase) (14).
piezes of ferroelectric elastomers, which had been oriented
by drawing (44–46). These experiments show only a small
shape variation if the field is applied parallel to the polar
axis of the monodomain. The effects become, on the other
hand, rather large if the smectic layer structure (chevron
h
0
E
= 0

for
E
= 0

for
E
> 0

for
E
< 0
E
≠ 0
z
smectic layers
z
smectic layers
Single smectic layer
Electric
field
E
∆
h
/
z
θ−θ
h
0
−∆
h
z
h
0
−∆
h
Figure 15. In the S
A
* phase, the mesogenic parts (depicted as

ellipsoids) of the elastomericmacromolecule stand upright (θ = 0
◦
)
inside the single smectic layers. By applying a lateral electric field
(perpendicular to the plane of the paper), a tilt angle θ that is
proportional to the electric field E can be induced (electroclinic
effect). Thesignof the tiltdepends on thesign of the electricfield E.
Hence, each smectic layer shrinks by h/z twice during one period
of the electricfield. The shrinkage hof thewhole film is measured
by the interferometer as an optical phase shift between the sample
beam and the reference beam.
Table 1.
Electrostrictive strain ε
Material (at electric field strength E) Ref.
Free-standing FLCE film 4% lateral strain (1.5 MV/m) 40
Spin-coated FLCE ∼1% lateral layer thickness 40
shrinkage (2.0 MV/m)
PMN
0.7
PT
0.3
0.15% (1 MV/m) 47
Lead magnesium
niobate—
Lead titanate
ceramics
P(VDF-TrFE) 4% (150 MV/m) 47
electron irradiated
poly(vinylidene
fluoride trifluoro-

ethylene) copolymer
PBLG monomolecular, 0.005% (300 MV/m) 49
grafted layer of
poly-γ -benzyl-
L-glutamate
texture) rearranges (44,46). To get a large electrostrictive
response, which can be understood on a molecular level, the
geometry presented in Fig. 15 was chosen (40). The appli-
cation of an electric field parallel to an smectic layer leads
to a tilt of the mesogens (electroclinic effect). Thereby, the
thickness of the layer decreases. For a stack of layers, the
effect sums up over all layers. As a result, the thickness
perpendicular to the smectic layers decreases if a field is
applied parallel to thelayers (see Fig. 15). Because the elec-
troclinic effect is relatively large in these polymers (15,33),
a large variation in thickness is expected.
X-ray diffraction measurements prove the electrically
induced shrinkage of single smectic layers. A freestand-
ing ultrathin (75 nm) film of a ferroelectric liquid crys-
talline elastomer (similiar to E2) was used to measure the
shape variation (electrostrictive response) associated with
this (40). It was measured by a high precision (± 3pm
at 133 Hz) Michelson interferometer. The measurements
(see Fig. 16) exhibit extremely high electrostrictive strains
of 4% in an electric field of only 1.5 MV/m, which is, to
our knowledge, a new world record for the correspond-
ing electrostrictive coefficient a. The effect exhibits typi-
cal electroclinic behavior, which means that it is caused
by an electrically induced tilt of the chiral LC molecules.
As a consequence of chirality, the primary strain is per-

pendicular to the applied field. Hence, a new material that
has a giant electrostriction effect is introduced, where the
effect can be fully understood on a molecular level. The
characterization of these materials is summarized in
Table 1.
CONCLUSION
Ferroelectric LC-elastomers represent an interesting class
of material because they combine the ordering of liquid
crystalline ferroelectric phases and the rubber elasticity
of polymer networks. Switching of the electric polarization
leads to deformation of the polymer network, equivalent to
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POLYMERS, FERROELECTRIC LIQUID CRYSTALLINE ELASTOMERS 859
0
0
1
2
3
4
250 500 750 1000 1250 1500
2
nd
harmonic
1
st
harmonic
55 60 65 70 75
Temperature (°C)
0

1
2
3
4
5
6
7
a (10000 nm
2
/V
2
)
∆
h
(nm)
U
AC

(V)
S
C
*S
A
*
Figure 16. Electrostriction of a ferroelectric
LC-elastomer (40). Big diagram: Thickness vari-
ation h as a function of the applied ac volt-
age U
ac
. Interferometric data were obtained

at the fundamental frequency of the electric
field (piezoelectricity, first harmonic: +) and
at twice the frequency (electrostriction, second
harmonic: o). Sample temperature: 60
◦
C.
Inset: Electrostrictive coefficient a (+) versus
temperature. At the temperature where the
non-cross-linked polymer would have its phase
transition S
C
*–S
A
* (about 62.5
◦
C), the tilt an-
gle of 0
◦
is unstable. That iswhythe electroclinic
effect is most effective at this temperature. An
electric field of only 1.5 MV/m is sufficient to in-
duce lateral strains of more than 4%.
stretching a spring, and creates a stress in the network of
polymer chains.
The interaction of mesogensandthe network can be var-
ied by using different topologies of net points: Crosslinking
is carried out either within the siloxane sublayers (lead-
ing to fast switching elastomers) or between the siloxane
sublayers (resulting in an elastomer that favors the ferro-
electric switching state in which the cross-linking reaction

took place).
Because the orientation of the smectic phase couples
to the polymer network, electromechanical measurements
show a piezoelectric effect. Mechanical deformation leads
to polarization or an external electric field to deformation of
the sample. Applying the electric field parallel to the smec-
tic layer structure leadstoanextremlyhighelectrostrictive
strain of 4% in an electric field of only 1.5 MV/m.
ACKNOWLEDGMENT
The work in this summary was possible only by close co-
operation with several groups from physics and physical
chemistry. We give special thanks to the groups of Profs.
F. Kremer and R. Stannarius (Leipzig). A. Helfer is than-
ked for setting up the manuscript.
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POLYMERS, PIEZOELECTRIC
J. S. HARRISON
NASA Langley Research Center
Hampton, VA
Z. OUNAIES
ICASE/NASA Langley Research Center

Hampton, VA
INTRODUCTION
Piezoelectric polymers have been known for more than
forty years, but in recent years they have gained repute as
a valuable class of “smart materials.” There is no standard
definition for smart materials, and terms such as intelli-
gent materials, smart materials, adaptive materials, active
devices, and smart systems are oftenused interchangeably.
The term “smart material” generally designates a material
that changes one or more of its properties in response to
an external stimulus.
The most popular smart materials are piezoelectric ma-
terials, magnetostrictive materials, shape-memory alloys,
electrorheological fluids, electrostrictive materials, and
optical fibers. Magnetostrictives, electrostrictives, shape-
memory alloys, and electrorheological fluids are used as
actuators; optical fibers are used primarily as sensors.
Among these active materials, piezoelectric materials
are most widely used because of their wide bandwidth,
fast electromechanical response, relatively low power re-
quirements, and high generative forces. A classical defini-
tion of piezoelectricity, a Greek term for “pressure elec-
tricity,” is the generation of electrical polarization in a
material in response to mechanical stress. This phe-
nomenon is known as the direct effect. Piezoelectric ma-
terials also display the converse effect: mechanical defor-
mation upon application of an electrical charge or signal.
Piezoelectricity is a property of many noncentrosymmet-
ric ceramics, polymers, and other biological systems. Pyro-
electricity is a subset of piezoelectricity, whereby the po-

larization is a function of temperature. Some pyroelectric
materials are ferroelectric, although not all ferroelectrics
are pyroelectric. Ferroelectricity is a property of certain
dielectrics that exhibit spontaneous electric polarization
(separation of the center of positive and negative electric
charge that makes one side of the crystal positive and the
opposite side negative) that can be reversed in direction by
applying an appropriate electric field. Ferroelectricity is
named by analogy with ferromagnetism, which occurs in
materials such as iron. Traditionally, ferroelectricity is de-
fined for crystalline materials, or at least in the crystalline
region of semicrystalline materials. In the last couple of
years, however, a number of researchers have explored the
possibility of ferroelectricity in amorphous polymers, that
is, ferroelectricity without a crystal lattice structure (1).
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POLYMERS, PIEZOELECTRIC 861
Table 1. Comparison of Properties of Standard Piezoelectric Polymer
and Ceramic Materials
d
a
31
g
a
31
(pm/V) (mV-m/N) k
31
Salient Features
Polyvinylidene fluoride 28 240 0.12 Flexible, lightweight, low

(PVDF) acoustic and mechanical
impedance
Lead zirconium titanate 175 11 0.34 Brittle, heavy, toxic
(PZT)
a
Values shown are absolute values of constants.
Characteristics of Piezoelectric Polymers
The properties of polymers are very different from those
of inorganics (Table 1), and they are uniquely qualified to
fill niche areas where single crystals and ceramics can-
not perform as effectively. As noted in Table 1, the piezo-
electric strain constant (d
31
) for the polymer is lower than
that of the ceramic. However, piezoelectric polymers have
much higher piezoelectric stress constants (g
31
) which in-
dicates that they are much better sensors than ceramics.
Piezoelectric polymeric sensors and actuators offer the
advantage of processing flexibility because they are
lightweight, tough, readily manufactured in large areas,
and can be cut and formed into complex shapes. Poly-
mers also exhibit high strength and high impact resis-
tance (2). Other notable features of polymers are low di-
electric constant, low elastic stiffness, and low density,
which result in high voltage sensitivity (excellent sensor
characteristic) and low acoustic and mechanical impedance
(crucial for medical and underwater applications). Poly-
mers also typically possess high dielectric breakdown and

high operating field strength, which means that they can
withstand much higher driving fields than ceramics. Poly-
mers offer the ability to pattern electrodes on the film
surface and pole only selected regions. Based on these
features, piezoelectric polymers possess their own estab-
lished area for technical applications and useful device
configurations.
Structural Requirements for Piezoelectric Polymers
The piezoelectric mechanisms for semicrystalline and
amorphous polymers differ. Although the differences are
distinct, particularly with respect to polarization stabil-
ity, in the simplest terms, four critical elements exist for
all piezoelectric polymers, regardless of morphology. These
essential elements are: (1) the presence of permanentmole-
cular dipoles, (2) the ability to orient or align the molecular
dipoles, (3) the ability to sustain this dipole alignment once
it is achieved, and (4) the ability of the material to undergo
large strains when mechanically stressed (3).
SEMICRYSTALLINE POLYMERS
Mechanism of Piezoelectricity in Semicrystalline Polymers
Semicrystalline polymers must have a polar crystalline
phase to renderthem piezoelectric. The morphologyof such
polymers consists of crystallites dispersed within amor-
phous regions, as shown in Fig. 1a. The amorphous region
has a glass transition temperature that dictates the me-
chanical properties of the polymer, andthemelting temper-
ature of the crystallites dictates the upper limit of the use
Electric
Field
Direction

1c. Electrically Poled
Crystalline
Region
1a. Melt Cast
Amorphous
Region
1b. Mechanically
Oriented
Stretch
Direction
Figure 1. Schematic illustration of random stacks of amorphous
and crystal lamellae in the PVDF polymer: (a) the morphology
after the film is melt cast; (b) after orientation of the film by me-
chanically stretching several timesitsoriginal length; and (c) after
depositing metal electrodes and poling through the film thickness.
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862 POLYMERS, PIEZOELECTRIC
temperature. The degree of crystallinity in such polymers
depends on the method of preparation and thermal his-
tory. Most semicrystalline polymers have several polymor-
phic phases, some of which may be polar. Mechanical ori-
entation, thermal annealing, and high-voltage treatment
are all it has been shown, effective in inducing crystalline
phase transformations. Stretching the polymer aligns the
amorphous strands in the film plane, as shown in Fig. 1b,
and facilitates uniform rotation of the crystallites by an
electric field. Depending on whether stretching is uniax-
ial or biaxial, the electrical and mechanical properties
(and therefore the transduction response) are either highly

anisotropic or isotropic in the plane of the polymer sheet.
Electrical poling is accomplished by applying an electric
field across the thickness of the polymer, as depicted in
Fig. 1c. An electric field of the order of 50 MV/m is typi-
cally sufficient to effect crystalline orientation. Polymers
can be poled by using a direct contact method or corona
discharge. The latter is advantageous because contacting
electrodes is not required and samples of large area can
be poled continuously. This method is used to manufacture
commercial poly(vinylidene fluoride) (PVDF) film. Some re-
searchers have also successfully poled large areas of poly-
mer films by sandwiching them between polished metal
plates under a vacuum. This method eliminates electri-
cal arcing of samples and the need for depositing metal
electrodes on the film surface. The amorphous phase of
semicrystalline polymers supports the crystal orientation,
and polarization is stable up to the Curie temperature.
This polarization can remain constant for many years if it
is not degraded by moisture uptake or elevated tempera-
tures.
Piezoelectric Constitutive Relationships
The constitutive relationships that describe piezoelectric
behavior in materials can be derived from thermodynamic
2
3
1
5
6
4
Figure 2. Tensor directions for defining the constitutive rela-

tionships.
principles (4). A tensor notation is adopted to identify the
coupling between the various entities through mechanical
and electrical coefficients. The common practice is to la-
bel directions as depicted in Fig. 2. The stretch direction is
denoted as “1.” The “2” axis is orthogonal to the stretch di-
rection in the plane of the film. The polarization axis (per-
pendicular to the surface of the film) is denoted “3.” The
shear planes, indicated by the subscripts “4,”“5,” and “6,”
are perpendicular to the directions “1,”“2,” and “3,” respec-
tively. By reducing the tensor elements and using standard
notations (5), the resulting equations can be displayed in
matrix form as follows:















S
1

S
2
S
3
S
4
S
5
S
6















=













d
11
d
12
d
13
d
21
d
22
d
23
d
31
d
32
d
33
d
41
d
42

d
43
d
51
d
52
d
53
d
61
d
62
d
63




















E
1
E
2
E
3







+
















s
E
11
s
E
12
s
E
13
s
E
14
s
E
15
s
E
16
s
E
21
s
E
22
s
E
23
s

E
24
s
E
25
s
E
26
s
E
31
s
E
32
s
E
33
s
E
34
s
E
35
s
E
36
s
E
41
s

E
42
s
E
43
s
E
44
s
E
45
s
E
46
s
E
51
s
E
52
s
E
53
s
E
54
s
E
55
s

E
56
s
E
61
s
E
62
s
E
63
s
E
64
s
E
65
s
E
66

















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












X
1
X
2
X
3
X
4
X
5

X
6















,
(1)







D
1
D
2

D
3







=







ε
T
11
ε
T
12
ε
T
13
ε
T
21
ε

T
22
ε
T
23
ε
T
31
ε
T
32
ε
T
33














E
1

E
2
E
3







+






d
11
d
12
d
13
d
14
d
15
d
16

d
21
d
22
d
23
d
24
d
25
d
26
d
31
d
32
d
33
d
34
d
35
d
36
















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
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X
1
X
2
X
3
X
4
X
5
X
6
















.
(2)
Piezoelectricity is a cross coupling among the elastic
variables, stress X, and strain S, and the dielectric vari-
ables, electric charge density D and electric field E. Note
that D is named in analogy to the B field in ferromag-
netism, although some authors also refer to it as dielec-
tric or electric displacement. There does not seem to be a
standard nomenclature; however, it is the opinion of the
authors of this article that electric charge density is a bet-
ter description of this property. The combinations of these
variables define the piezoelectric strain constant d, the ma-
terial compliance s, and the permittivity ε. Other piezo-
electric properties are the piezoelectric voltage constant g,
stress constant e, and strain constant h given by the equa-
tions in Table 2. For a given constant, the first definition in
the table refers to the direct effect, and the second refers to
the converse effect. The piezoelectric constants are inter-

related through the electrical and mechanical properties
Table 2. Definitions of Piezoelectric Constants
Equations Units
d = (dD/dX )
E
= (dS/dE)
X
(C/N or m/V)
e = (dD/dS)
E
=−(dX/dE)
S
(C/m or N/Vm)
g = (dE/dX )
D
= (dS/dD)
X
(Vm/N or m
2
/C)
h = (dE/dS)
D
=−(dX/dD)
S
(V/m or N/C)
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POLYMERS, PIEZOELECTRIC 863
of the material. Electric field strength and displacement
charge density are related through the dielectric constant,

εε
0
(where ε
0
is the permittivity of free space), and stress
and strain are related through the compliance according
to
d
ij
= ε
0
ε
i
g
ij
, (3)
e
ij
= s
ij
d
ij
. (4)
The polarization P is a measure of the degree of piezo-
electricity in a given material. In a piezoelectric material, a
change in polarization P results from an applied stress X
or strain S under conditions of constant temperature and
zero electric field. A linear relationship exists between P
and the piezoelectric constants. Due to material anisotropy,
P is a vector that has three orthogonal components in the

1, 2, and 3 directions. Alternatively, the piezoelectric con-
stants can be defined as
P
i
= d
ij
X
j
, (5)
P
i
= g
ij
S
j
. (6)
The electrical response of a piezoelectric material is a
function of the electrode configuration relative to the di-
rection of the applied mechanical stress. For a coefficient
d
ij
, the first subscript is the direction of the electric field
or charge displacement, and the second subscript gives the
direction of the mechanical deformation or stress. The C
2ν
crystallographic symmetry typical of synthetic oriented,
poled polymer film leads to cancellation of all but five of
the d
ij
components (d

31
, d
32
, d
33
, d
15
, and d
24
). If the film is
poled and biaxially oriented or unoriented, d
31
= d
32
, and
d
15
= d
24
. Most natural biopolymers possess D
∞
symme-
try which yields a matrix that possesses only the shear
piezoelectricity components d
13
and d
25
. Because the d
33
constant is difficult to measure without constraining the

lateral dimension of the sample, it is typically determined
from Eq. (7) which relates the constants to the hydrostatic
piezoelectric constant, d
3h
.
d
3h
= d
31
+ d
32
+ d
33
. (7)
The electromechanical coupling coefficient k
ij
repre-
sents the conversion of electrical energy into mechanical
energy and vice versa. The electromechanical coupling can
be considered a measure of transduction efficiency and is
always less than unity as shown here:
k
2
=
electrical energy converted to mechanical energy
input electrical energy
,
(8a)
k
2

=
mechanical energy converted to electrical energy
input mechanical energy
.
(8b)
−80
−60
−40
−20
0
20
40
60
80
−150 −100 −50 0 50 100 150
Electric field (MV/m)
P
r
(mC/m
2
)
P
r
E
c
Figure 3. Typical ferroelectric hysteretic behavior for PVDF.
Some k coefficients can be obtained from a measured d con-
stant as follows:
k
31

=
d
31

s
E
11
ε
T
3
. (9)
Ferroelectricity in Semicrystalline Polymers
At high electric fields, the polarization in semicrystalline
polymers such as PVDF is nonlinear with the applied elec-
tric field. This nonlinearity in polarization is defined as
hysteresis. The existence of spontaneous polarization toge-
ther with polarization reversal (as illustrated by a hystere-
sis loop) is generally accepted as proof of ferroelectricity.
Figure 3 is an example of the typical hysteretic behavior
of PVDF. Two other key properties typically reported for
ferroelectric materials are the coercive field and the rema-
nent polarization. The coercive field E
c
, which marks the
point where the hysteresis intersects the horizontal axis,
is about 50 MV/m at room temperature for many ferroelec-
tric polymers. The remanent polarization P
r
corresponds
to the point where the loop intersects the vertical axis. The

values of E
c
and P
r
depend on temperature and frequency.
The Curie temperature T
c
, is generally lower than but close
to the melting temperature of the polymer. Below T
c
, the
polymer is ferroelectric and above T
c
, the polymer loses its
noncentrosymmetric nature.
Although ferroelectric phenomenon has been well doc-
umented in ceramic crystals, the question of whether
polymer crystallites could exhibit dipole switching was
debated for about a decade after the discovery of piezo-
electricity in PVDF. Inhomogeneous polarization through
the film thickness that yielded higher polarization on the
positive electrode side of the polymer led to speculations
that PVDF was simply a trapped charge electret. These
speculations were dispelled when X-ray studies (6) demon-
strated that polarization anisotropy vanishes due to high
poling field strengths and that true ferroelectric dipole re-
orientation occurs in PVDF. Luongo used infrared to at-
tribute the polarization reversal in PVDF to 180
◦
dipole

rotation (7). Scheinbeim documented the same via X-ray
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864 POLYMERS, PIEZOELECTRIC
pole analysis and infrared techniques for odd-numbered
nylons (8).
State of the Art
Pioneering work in the area of piezoelectric polymers (9)
led to the development of strong piezoelectric activity in
polyvinylidene fluoride (PVDF) and its copolymers with
trifluoroethylene (TrFE) and tetrafluoroethylene (TFE).
These semicrystalline fluoropolymers represent the state
of the art in piezoelectric polymers and are currently the
only commercial piezoelectric polymers. Odd-numbered
nylons, the next most widely investigated semicrystalline
piezoelectric polymers, have excellent piezoelectric pro-
perties at elevated temperatures but have not yet been
embraced in practical application. Other semicrystalline
polymers including polyureas, liquid crystalline polymers,
biopolymers and an array of blends have been studied for
their piezoelectric potential and are summarized in the fol-
lowing section. The chemical repeat unit and piezoelectric
constants of several semicrystalline polymers are listed in
Table 3.
Polyvinylidene Fluoride (PVDF). Interest in the electrical
properties of PVDF began in 1969 when it was shown (9)
that poled thin films exhibit a very large piezoelectric co-
efficient, 6–7 pCN
−1
, a value about ten times larger than

had been observed in any otherpolymer. As seen in Table 3,
PVDF is inherently polar. The spatially symmetrical dis-
position of the hydrogen and fluorine atoms along the poly-
mer chain gives rise to unique polar effects that influence
the electromechnical response, solubility, dielectric proper-
ties, and crystal morphology and yield an unusally high di-
electric constant. The dielectric constant of PVDF is about
Table 3. Comparison of Piezoelectric Properties of Some Semicrystalline
Polymeric Materials
Polymer Structure
T
g
(
◦
C)
T
m
(
◦
C)
Max Use
Temp (
◦
C)
d
31
(pC/N) Ref.
PVDF −35 175 80 20–28 2
PTrFE
32 150 90–100 12 2

Nylon-11
68 195 185
3at
25
◦
C
14 at
107
◦
C
22
Polyrurea-9
50 180 ——28
12, which is four times greater than that of most polymers,
and makes PVDF attractive for integration into devices be-
cause the signal-to-noise ratio is smaller for higher dielec-
tric materials. The amorphous phase in PVDF has a glass
transition that is well below room temperature (−35
◦
C);
hence, the material is quite flexible and readily strained at
room temperature. PVDF is typically 50–60% crystalline,
depending on thermal and processing history, and has at
least four crystal phases (α, β, γ , and δ); at least three are
polar. The most stable, nonpolar α phase results upon cast-
ing PVDF from a melt and can be transformed into the po-
lar β phase by mechanical stretching at elevated temper-
atures or into the polar δ phase by rotating the molecular
chain axis in a high electric field (∼130 MV/m) (10). The
β phase is most important for piezoelectricity and has a

dipole moment perpendicular to the chain axis of 2.1 D
that corresponds to a dipole concentration of 7 ×10
−30
Cm. After poling PVDF, the room temperature polar-
ization stability is excellent; however, polarization and
piezoelectricity degrade as temperature increases and are
erased at its Curie temperature. Previously, it was believed
that polarization stability was defined only by the melt-
ing temperature of the PVDF crystals. Recently, however,
some researchers suggest that the polarization stability
of PVDF and its copolymers is associated with coulom-
bic interactions between injected, trapped charges and
oriented dipoles in the crystals (11). They hypothesize
that the thermal decay of the polarization is caused by
the thermally activated removal of the trapped charges
from the traps at the surface of the crystals. The role of
trapped charges in stabilizing orientation in both semicrys-
talline and amorphous polymers is still a subject that
needs further study. The electromechanical properties of
PVDF have been widely investigated. For more details, the
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POLYMERS, PIEZOELECTRIC 865
reader is referred to the wealth of literature that exists on
the subjects of piezoelectric, pyroelectric, and ferroelectric
properties (2,6,12,13), and the morphology (14–16) of this
polymer.
Poly(Vinylidene Fluoride–Trifluoroethylene and
Tetrafluoroethylene) Copolymers. Copolymers of polyvinyli-
dene fluoride with trifluoroethylene (TrFE) and tetraflu-

oroethylene (TFE) also exhibit strong piezoelectric,
pyroelectric, and ferroelectric effects. These polymers
are discussed together here because they behave sim-
ilarly when copolymerized with PVDF. An attractive
morphological feature of the comonomers is that they
force the polymer into an all-trans conformation that has
a polar crystalline phase, which eliminates the need for
mechanical stretching to yield a polar phase. P(VDF–
TrFE) crystallizes to a much greater extent than PVDF
(up to 90% crystalline) and yields a higher remanent
polarization, a lower coercive field, and much sharper
hysteretic loops. TrFE also extends the use temperature
by about 20
◦
C to close to 100
◦
C. Conversely, copolymers
with TFE exhibit a lower degree of crystallinity and a
suppressed melting temperature, compared to the PVDF
homopolymer. Although the piezoelectric constants for
the copolymers are not as large as those of the homopoly-
mer, the advantages of P(VDF–TrFE) in processibility,
enhanced crystallinity, and higher use temperature make
it favorable for applications.
Researchers have recently reported that highly ordered,
lamellar crystals of P(VDF–TrFE) can be made by anneal-
ing the material at temperatures between the Curie tem-
perature and the melting point. They refer to this mate-
rial as a “single crystalline film.” A relatively large single
crystal of P(VDF–TrFE) 75/25 mol% copolymer was grown

that exhibits a room temperature d
33
=−38 pm/V and a
coupling factor k
33
= 0.33 (17).
The result of introducing defects into the crystalline
structure of P(VDF–TrFE) copolymer on electroactive ac-
tuation has been studied using high electron irradiation
(18). Extensive structuralinvestigations indicate that elec-
tron irradiation disrupts the coherence of polarization do-
mains (all-trans chains) and forms localized polar regions
(nanometer-sized, all-trans chains interrupted by trans
and gauche bonds). After irradiation, the material ex-
hibits behavior analogous to that of relaxor ferroelectric
systems in inorganic materials. The resulting material is
no longer piezoelectric but rather exhibits a large electric
field-induced strain (5% strain) due to electrostriction. The
basis for such large electrostriction is the large change in
the lattice strain as the polymer traverses the ferroelec-
tric to paraelectric phase transistion and the expansion
and contraction of the polar regions. Piezoelectricity can
be measured in these and other electrostrictives when a dc
bias field is applied. Irradiation is typically accomplished
in a nitrogen atmosphere at elevated temperatures using
irradiation dosages up to 120 Mrad.
Polyamides. A low level of piezoelectricity was first re-
ported in polyamides (also known as nylons) in 1970 (19).
NH O C
NHCO

CH
2
H
2
C
CH
2
HN
O
H
2
C
CH
2
H
2
C
O
HN
OC
NH
CH
2
H
2
C
CH
2
HN
O

C
C
C
Nylon 4
(a)
NH O C
H
2
C
CH
2
H
2
C
CH
2
C
NH
O
H
2
C
OC
NH
H
2
C
CH
2
H

2
C
CH
2
NH
OC
CH
2
NH
H
2
C
CH
2
H
2
C
CH
2
C
NH
O
Nylon 5
(b)
Figure 4. Schematic depiction of hydrogen-bonded sheets show-
ing dipole directions in the crystal lattices of (a) even (nylon 4) and
(b) odd polyamides (nylon 5).
A systematic study of odd-numbered nylons, however, ini-
tiated in 1980 (20), served as the impetus for more than
20 years of subsequent investigations of piezoelectric and

ferroelectric activity in these polymers. The monomer
unit of odd nylons consists of even numbers of methy-
lene groups and one amide group whose dipole moment is
3.7 D. Polyamides crystallize in all-trans conformations
and are packed to maximize hydrogen bonding between
adjacent amine and carbonyl groups, as seen in Fig. 4 for
an even-numbered and an odd-numbered polyamide. The
amide dipoles align synergistically in the odd-numbered
monomer, resulting in a net dipole moment. The amide
dipole cancels in an even-numbered nylon, although re-
manent polarizations have been measured for some even-
numbered nylons, as discussed later in this article. The
unit dipole density depends on the number of methylene
groups present, and polarization increases from 58 mC/m
2
for nylon-11 to 125 mC/m
2
for nylon-5 as the number of
methylene groups decreases (8).
Polyamides are known hydrophilics. Because water ab-
sorption is associated with hydrogen bonding to the polar
amide groups, the hydrophilicity increases as the density
of amide groups increases. Water absorption in nylon-11
and nylon-7 has been shown to be as high as 4.5% (by
weight) and more than 12% for nylon-5 (21), whereas it
is less than 0.02% for PVDF and its copolymers. Studies
have shown that water absorption can have a dramatic ef-
fect on the dielectric and piezoelectric properties of nylons;
however, water does not affect the crystallinity or orienta-
tion in thermally annealed films (21). Thus, films can be

dried to restore their original properties.
At room temperature, odd-numbered nylons have
lower piezoelectric constants than PVDF; however, when