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Liquid Crystalline Polymers.

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Liquid Crystalline Polymers
By Heino Finkelmann"
The liquid crystalline behavior of low molecular weight compounds has been known for
more than a century; synthetic polymers have been manufactured on a large scale for several decades, but just recently it was found possible to produce polymers using the structural principles of liquid crystalline compounds. The resulting materials have, as expected,
unusual properties. Numerous applications, not only in opto-electronics, are already anticipated for such materials.
1. Introduction
Technological progress is directly linked to the demand
for specific and efficient materials. Particularly, in the area
of macromolecular chemistry, developments in the past
few years have shown that specific modification of molecular structure can lead to materials with new potential applications. Alone for linear macromolecules, the number of
possibilities for varying the molecular structure is almost
inexhaustable. One can vary the chemical constitution of
the monomer units of homo- and copolymers or introduce
groups with specific chemical, photochemical or pharmacological characteristics. The possibilities are particularly
intriguing in the case of block copolymers, whose microphase-separated blocks can lead to ordered super structures.
But polymer behavior is not only determined by molecular architecture; much more crucial is the physical state in
which the macromolecular compound exists. For this reason, the crystalline (or partly crystalline) state, the liquid,
isotropic melt, and the glassy, amorphous state have been
intensively studied in the past decade. In the case of high
molecular weight compounds, the gaseous state is not
achievable without decomposition.
This list of possible physical states of a polymer is, however, still far from complete. For almost a century, it has
been known that certain low molecular weight compounds
d o not transform directly into the isotropic melt at their
melting point."] Under constant pressure and within a defined temperature range, these compounds exist in a state,
which on the one hand has the low viscosity of liquids, and
on the other displays the physical anisotropic characteristics of a crystalline solid. This aggregate phase is known as
the liquid crystalline phase, and the substances which form
such phases are known as liquid crystals. The liquid crystalline phase of low molecular weight compounds has been
intensively researched in the past twenty years, especially
after it was discovered that the unusual physical properties
of liquid crystals could be exploited in the field of optoelectronics.[''
It is obvious to pursue the question of whether macromolecular substances could have liquid crystalline phases.
[*] Prof. Dr. H. Finkelmann
lnstitut fur Makromolekulare Chemie der Universitat
Stefan-Meier-Strdsse 3 I . D-7800 Freiburg (FRG)
0 VCH Verlagsgeseilschaft mbH, D-6940 Weinheim. 1987
The combination of polymer-specific characteristics with
the anisotropic, physical characteristics of the liquid crystalline state promises not only interesting novel materials
but also opens u p new, multi-faceted theoretical, experimental and technological aspects. The chemist is thereby
not only faced with constructing specific molecular structures but also with producing a defined physical state of
the material.
The intention of this article is to show, with a knowledge
of the structural principles of low molecular weight liquid
crystals (Section 2), how various types of polymers with
potentially liquid crystalline phases can be synthesized
(Section 3). Each structural type shows in specific ways the
liquid crystalline phase and the properties of the polymer.
In Sections 4 to 6 , the peculiarities of selected examples
will be outlined and their properties and application perspectives will be discussed.
2. The Liquid Crystalline Phase
If an isobaric line is drawn through the phase diagram
of a liquid crystal (dashed line in Fig. l), one finds at lower
temperatures ( T < T,) the crystalline phase which is char-
Fig. 1. p T diagram of a substance which shows a liquid crystalline phase
(dashed area). s=crystalline; i=isotropic; g=gas; T,=melting point;
T,,, = liquid crystalline-isotropic phase transformation temperature.
acterized by a three-dimensional, long-range positional order of the molecule's center of mass in the crystalline lattice. Furthermore, in non-spherically symmetric molecules,
the superior molecular axes, which are usually the longest
molecular axes, are oriented with respect to each other
! 02.50/0
0570-0833/87/0909-0816 3
Angew. Chem. In!. Ed. Engl. 26 (1987) 816-824
(long-range orientational order of the longitudinal molecular axes). With non-cubic crystals, the positional and orientational long-range order lead to anisotropic physical
At the melting point T,, the long-range orientational order as well as the long-range positional order for most organic compounds are lost and a melt results in which only
short-range ordering between the molecules exists. The
physical properties are independent of direction (isotropic).
The long-range orientational and positional order of the
molecule need not always spontaneously and completely
disappear at the melting point. A step-wise decay with increasing temperature of the long-range positional order in
the first, second or third dimension and finally the longrange orientational order can also lead to an isotropic
melt. Hence, in the temperature range T, < T < T,,,_,aggregate phases (dashed area in Fig. 1) which show a reduced
long-range ordering with respect to the crystalline phase
can exist between the crystalline phase and the isotropic
phase. The loss of long-range positional order in at least
one dimension means, however, that the solid phase has
been lost and a liquid exists. The existence of long-range
positional order and/or long-range orientational order of
the molecules leads, on the other hand, to the anisotropic
physical characteristics typical for crystals with non-cubic
symmetry. This aggregate phase is therefore known as the
"liquid crystalline phase."
Systematic investigations have proven that the liquid
crystalline phase is found only in those substances whose
molecules have a pronounced shape anisometry. They are
rod- or disk-shaped molecules. Changes in conformation
should not substantially affect the shape anisometry;
therefore, a relatively rigid molecular structure is required.
Molecules which satisfy these structural principles are
known as mesogenic molecules o r compounds with mesogenic groups. Simple basic structures of low molar mass,
rod-shaped mesogenic compounds are given in Table l.l3]
The various liquid crystalline phases are differentiated
with respect to the state of order and the ordering possibilities of the rod- and disc-shaped molecules. The most important of these types for rod-shaped molecules are summarized in Figure 2. The phase structure is determined by
the chemical constitution of the mesogenic group and its
substituents. In general, for a homologous series of a class
of mesogenic compounds, the development of the highlyordered smectic phase increases with increasing chain
length of the flexible substituent located at the end of the
longitudinal axis of the m e ~ o g e n . [ ~ ]
Table I . Selected examples of some basic structures of liquid crystalline compounds with rod-shaped molecular structure.
coo -
3. Liquid Crystalline Polymers
The existence of the liquid crystalline state requires rigid, shape-anisometric molecules. With the knowledge of
these structural principles the construction of polymers
with potential liquid crystalline properties is immediately
obvious. The macromolecule as a whole must either have a
rigid rod-like o r disk-like structure or contain mesogenic
groups as monomer units. Polymers whose total structures
are mesogenic are only achievable when the polymer itself
is rod-shaped, for example via a rigid, helical secondary
structure. On the other hand, numerous structures are conceivable for polymers with mesogenic groups as monomer
units, whereby one can differentiate between two fundamentally different types (Fig. 3 ) :
1 . Those in which the mesogenic groups are located in the
polymer main chain. These polymers are known as liquid
crystalline main chain polymers (see Section 4).
2. Those in which the mesogenic groups are attached to
the polymer main chain in a side-chain-like manner. Such
polymers are known as liquid crystalline side chain polymers (see Section 5).
Liquid crystalline side chain elastomers are dealt with in
Section 6.
1111 1\11 11\Il111\
- \ I. \ ,
- I
Fig. 2. Phase sequence of a liquid crystalline substance. The arrow shows the preferred direction of the orientation of
the long axes of the rod-shaped molecules.
Angew. Chem. Int. Ed. Engl. 26 (1987) 816-824
LC side chain
Fig. 3. Types of liquid crystalline polymers derived
from rod-shaped and from disk-shaped mesogenic molecules. LC = liquid crystalline.
In comparison to low molar mass liquid crystals, attachment of the mesogenic groups to the polymer generally restricts the translational and rotational motions of the mesogenic groups. Therefore an influence on the temperature
range of existence and a change in the physical properties
of the liquid crystalline phase is to be directly expected.
From the list of possible polymer types given in Figure 3,
polymers which contain rod-shaped, mesogenic groups
have been synthesized and intensively investigated in the
past few years.[41 Polymers with disk-shaped monomer
units could only recently be synthesized.[s,61The same is
true for combinations of liquid crystalline main and side
chain polymer^^'-^] gnd other structural variations.[*l
In the following, therefore, only selected examples of
the synthesis of simple polymers with rod-shaped mesogenic groups in the main or side chain will be discussed. In
addition, their characteristic properties and scope of application will be outlined.
4. Liquid Crystalline Main Chain Polymers
4.1. Synthesis and Structure
As shown schematically in Figure 3, the longitudinal
axis of a mesogenic element can be laterally or linearly
bound to a macromolecule. In the first case, rotations
about the transverse axis and in the second case, rotations
about the longitudinal axis of the individual mesogenic
groups within the main chain are restricted due to polymerization. While linear attachment yields a more or less
rigid main chain, with lateral attachment a chain flexibility
(at least perpendicular to the mesogenic group) comparable to that of conventional polymers should result. Of
these two structural variations, almost exclusively polymers with linearly-attached mesogenic groups have thus
far been synthesized. The lateral attachment of mesogenic
groups has only recently been described for two examples.""
There is a wide variety of possible polycondensation
reactions for the synthesis of linear main chain polymers.
I n the simplest case, already known low molar mass liquid
crystals13' are functionalized with groups able to undergo
polycondensation (Scheme 1). This leads to polymers
which can be derived from simple basic types of mesogenic groups as shown in Table 1. Polycondensation reactions yielding polyesters, polyamides and polyethers are
the most preferred reactions.
mesogenic group
Scheme I
The direct, linear, rigid attachment of mesogenic groups,
however, leads to problems. Beginning with a low degree
of polymerization r ( r < lo), the liquid crystalline-isotropic
phase transformation temperature is quickly shifted to
higher temperature with increasing r. This is due to the
high anisometry of the molecule. Similarly, the melting
point rises to above the decomposition temperature of the
compound. As a result, the liquid crystalline phase often
cannot be reached in the case of oligomers. A typical example is the polyester of 4-hydroxybenzoic acid:" 'I
Although the low molar mass phenyl benzoates represent an important class of liquid crystals, the polymers of
4-hydroxybenzoic acid or analogous polycondensates of
terephthalic acid and hydroquinone decompose before
melting and are processable only as powders via a sintering method.
In order to obtain the liquid .crystalline state with these
types of polymers, there are three possible ways of modifying the rigid, rod-shaped basic s t r ~ c t u r e "Is'~ . to decrease
the melting point temperature:
Angew. Chem. In!. Ed. Engl. 26 (1987) 816-824
I . The linearity of the macromolecule can be reduced by a
less symmetric comonomer unit or a voluminous aromatic
side group.
2. The rigid mesogenic groups are attached via a “flexible
spacer,” for example an alkyl or alkoxy chain. With increasing length of the flexible chain between the mesogenic groups, not only is the melting point of the polymer
lowered, but the mesogenic groups have an increasing tendency to order in a layer-like manner. Above a specific
length of the flexible spacer, which is dependent on the
structure of the polymer, the development of a smectic
phase can occur.
3. Linear macromolecules are substituted laterally with
mesogenic groups via a flexible chain. These groups behave as “chemically-bound solvent molecules” and
Table 2. Liquid crystalline main chain polymers (examples).
I . Copolymers with non-linear chain segments
thereby reduce the polymer-polymer interactions and
hence the melting point.
A few examples arbitrarily selected from the several
polymers already synthesized are listed in Table 2.
Decreasing the melting temperature results in the lowering of the liquid crystalline-isotropic phase transformation
temperature. Through suitable combinations of different
monomer units, an adjustment of a desired phase transition temperature is possible.
Liquid crystalline main chain polymers build, in general, a
nematic phase above the melting point. An ordering of the
monomer units into a smectic layered structure is prevented, especially in the case of statistically irregular copolymers which are composed of two or more different monomer units. In analogy to low molar mass liquid crystals,
the development of the nematic phase is favored over that
of the higher ordered smectic phase only after flexible
spacers of increasing length have been introduced between
the mesogens.
The liquid crystalline phase can also be realized with
non-melting, rigid, rod-shaped polymers when a suitable
solvent is added to reduce the melting point. This behavior
was theoretically predicted in 1949 by On~ager[‘~I
and in
1956 by FI~ry.[~~l
Rod-shaped macromolecules favor the
development of nematic phases above a certain ratio of the
length of the longitudinal molecular axis (or of the statistical chain segment length) to the length of the transverse
axis in solution above a defined concentration of the
macromolecule. The suitable solvent is in many cases problematic. Polyesters, for example, are often only soluble in
substituted phenols. The most well-known polymer which
forms a liquid crystalline phase only in solution is Kevlar
(Du Pant).["] In concentrated sulfuric acid, it forms a nematic phase.
2. Polymers with flexible chains in the main chain
3. Polymers with laterally-attached flexible side-chain units
Angew. Chem. Inr. Ed. Engl. 26 (1987) 816-824
Polymers with laterally-ordered mesogenic molecules in
the polymer main chain are an interesting structural variation of the liquid crystalline main chain polymer. With
these polymers, the mesogenic group can rotate only about
its short molecular axis; rotation about the longitudinal
axis is impossible. This restriction of motion has consequences on the nematic phase structure, which will be discussed later with the corresponding side-chain polymers
(Section 5). Furthermore, for this type of polymer, the anisometry of the mesogenic unit does not increase with increasing r, so that the length of the macromolecule should
not influence the liquid crystalline phase behavior. Only
one such polymer has thus far been synthesized which has
a liquid crystalline
This phase structure has however not yet been clarified.
The characterization of liquid crystalline main chain polymers continues to be problematic. The polydispersity of
polymers and copolymers leads to very broad phase transitions. Due to this multicomponent system, isotropic, liquid
crystalline and crystalline phases can exist simultaneously.
In spite of that, homogeneous polymers with nematic and
smectic phases can be clearly identified and character819
ized.I4]The liquid crystalline phase leads to unusual properties, which will be discussed in the following by considering a model.
4.2. Properties
In the ideal case of the fully rigid polymer main chain,
the longitudinal axes of the macromolecules are ordered
more or less parallel to one another in the nematic melt.
The state of order of this long-range orientation is described through the order parameter
where the angle B gives the deviation of the longitudinal
axis of the molecule from the preferred direction of the
longitudinal axes of all the molecules. For S= 1, all molecules are ordered exactly parallel to one another, and for
the isotropic melt, S=O. The preferred direction of the
orientation of the longitudinal molecular axes is called the
director (director = symmetry axis of the orientation distribution function of the longitudinal molecular axes). The
position of the director oscillates due to thermal fluctuations within a probe so that over the macroscopic dimension of the probe, no single direction of the director develops. If, however, the polymer is exposed to a shear gradient and, for example, spun through a nozzle, a macroscopically singular orientation of the director-here in the
fiber direction-can be obtained (see Fig. 4). If vitrifica-
Fig. 4. I-low-orientation process of a liquid crystalline main chain polymer.
tion results upon cooling the flow-oriented polymer, the
macroscopically ordered nematic structure is maintained
in the glassy state. If this (ideally nematic) glass has the
chains ordered fully parallel (S= I), and if the degree of
polymerization is high (towards infinity), then a mechanical strength in the fiber direction results due to the strength
of the covalent bonds in the direction of the longitudinal
molecular axis. The van der Waals interactions between
the polymer chains then determine the mechanical properties perpendicular to the fiber direction.
In reality, unusually high mechanical properties in the
fiber direction have been found for these polymers; however, the theoretically possible values have not been
reached.['51The reason lies in the imperfect orientation of
the director, a degree of order which is less than one
( S < I), and the finite molecular weight of the polymer
which leads to defects. In addition, most polymers of this
type crystallize. Theoretically, this should not reduce the
mechanical characteristics of the above model, but apparently the crystallization does heavily influence the mechanical behavior due to defect structures.
Further technological interest in these polymers is based
on the lower viscosity of the nematic phase as compared to
the isotropic phase.[261The cause lies in the orientation of
the polymer in a shear gradient and in an anisotropic flow
process which can no longer be described by one viscosity
Due to these unusual mechanical and rheological properties, a series of liquid crystalline main chain polymers
has recently become commercially available. Besides their
outstanding mechanical proper tie^,"^] they are distinguished by their dimensional stability with a very low thermal expansion coefficient, high temperature stability and
low inflammability. Their possible areas of application extend therefore from fibers and composites to injection
moulded parts of high tolerance with high thermal stability. Tolerance and thermal stability of electronic and optical components are critical material demands which could
be fulfilled by liquid crystalline main chain polymers.
5. Liquid Crystalline Side Chain Polymers
5.1. Synthesis and Structure
In liquid crystalline side chain polymers, the mesogenic
molecules are bound as side-chains, usually via a flexible
spacer to a desired polymer main chain (Fig. 3). With rodshaped mesogenic molecules, it is significant whether the
liquid crystalline molecule is attached to the macromolecule along its longitudinal or transverse axis. Lateral (with
respect to the longitudinal molecular axis) substituents
considerably lower the stability of the liquid crystalline
state,'271while linear substituents are, in general, necessary
for the existence of the liquid crystalline state.''] In addition, motions of the mesogens are influenced differently
depending on their linkage to the backbone; this will be
discussed in detail later.
For the synthesis of liquid crystalline side chain polymers, each of the known low molar mass liquid crystalline
molecules can in principle be supplied with functional
groups through substitution reactions and then converted
into a polymer.
The active species of initiation and chain growth reactions of ionic, radical, insertion and group transfer polymerizations must be carefully selected with respect to the
chemical constitution of the mesogenic groups, which limAngew. Chem. Int. Ed. Engl. 26 11987) 816-824
its in most cases the choice of the type of polymerization.
N o polymerizable liquid crystal has yet been synthesized
which can be converted into a high molecular weight
polymer through ionic polymerization. Termination reactions in ionic polymerization lead only to oligomeric products.[".'"' On the other hand, ionic polymerization is of interest because it enables the study of the influence of the
tacticity of the polymer main chain on the liquid crystalline phase behavior. This question has yet to be clarified.
Most polymers have thus far been synthesized by radical
p ~ l y m e r i z a t i o n . 'Mesogenic
compounds which are esterified with acrylic, methacrylic o r chloracrylic acidL3" on the
terminal or lateral substituent of the mesogenic group are
converted without problem into high molecular weight
products via radical polymerization under usual experimental conditions (Scheme 2).
R = H , CH,
Scheme 2
Reactions analogous to polymerizations[31Jsuch as the
addition of liquid crystals with terminal C=C double
bonds on the mesogenic group to poly[oxy(methylsilylene)], (Scheme 3) afford access to liquid crystalline polysiIoxane~.'~~-~~]
These platinum compound-catalyzed reactions have
high reaction turnover and can be used to synthesize copolymers using monomer mixtures. Finally, liquid crystalline
side chain polymers can be synthesized by polycondensation reactions[35Jand group transfer reaction^.'^']
Based on experience collected thus far, a few simple
predictions can be made concerning the expected liquid
crystalline phase of a liquid crystalline side chain polymer
which is to be synthesized:
1. The conversion of monomers into a polymer leaves the
anisometry of the mesogenic groups unchanged. This is
contrary to the liquid crystalline main chain polymers with
linearly-ordered mesogenic groups. The specific volume of
the material, however, is lowered by polymerization; that
is, the packing density of the mesogenic groups increases.
Consequently, polymerization shifts the liquid crystallineisotropic phase transformation to higher temperatures, in
analogy to the increase in the packing density of low molar
mass compounds with increasing pressure. This effect as a
function of the degree of polymerization r of a polymer is
shown schematically in Figure 5. The phase transition temAngeu. Chem. Inr. Ed. Engl. 26 (1987) 816-824
peratures shift considerably in the oligomer region ( r < lo),
while for r > 100, dependence on the degree of polymerization is in general no longer observed.[37J
rFig. 5. Dependence of the liquid crystalline-isotropic phdse lranalbrmarion
temperature T,=.,and of the specific volume on the degree of polymerization r of the liquid crystalline side-chain polymer.
2. In general, the attachment of the mesogenic groups on a
po!ymer main chain tends to lead to the development of
the highly ordered smectic phase. The known dependence
of the polymorphology of the liquid crystalline phases on
the substituents of the mesogenic groups for low molar
mass liquid crystals is essentially valid also for polymers.
With increasing length of the terminal, flexible substituent,
development of the smectic phase is favored. This is
found, however, only for polymers with flexible spacers
which have a minimum length of three to four atoms
within the chain. With shorter spacers, packing problems
often lead to no liquid crystalline phase, o r the polymer
main chain forces a smectic ordering of the mesogenic side
groups. Under these conditions, nematic phases are in general not found in spite of the short-chain substituents on
the mesogenic
A further, theoretically interesting aspect is, as mentioned above, the manner in which the mesogenic group is
attached to the polymer main chain. Rotation of the
mesogenic group about the longitudinal axis is possible
with linear attachment of the mesogenic groups and the
spacers, while it is prevented by lateral attachment. Therefore, specific motions which influence the phase structure
can be affected by polymer fixation. This is exemplified by
the following nematic phase: For low molecular weight liquid crystals, the rather "board-shaped'' molecules can be
thought of as cylindrically symmetric because they rotate
nearly unrestricted about their longitudinal axis. A uniaxial phase structure results from the long-range orientational order of the cylindrically symmetric longitudinal
molecular axes. If now the rotation about the longitudinal
axis is prevented by the lateral attachment of the mesogenic groups to the polymer main chain, the molecular
shape can no longer be thought of as cylindrically symmetric. The molecule structure must be considered as being
"board-like.'' In 1970, it was theoretically predicted[39ithat
with restricted rotation about the longitudinal axes, the
short molecular axes could also develop long-range orientational order to one another. The result is a macroscopically biaxial nematic phase. However, because of the low
rotational barrier potential, a biaxial nematic phase for
low molar mass liquid crystals has not yet been demonstrated, although it does exist in polymers with laterally82 1
attached mesogenic group^.[^"^ Figure 6 shows a photograph, taken under the polarizing microscope, of a biaxial
nematic polymer between two surface parallel glass plates.
Through changes in the position of the director, which is
essentially determined by orientation effects of the polymer at the boundary surface, a characteristic texture of
the interference pattern under crossed polarizers results.
This texture differs in appearance from that of the uniaxial
nematic phase and can be used to identify the phase. This
example demonstrates that through the attachment of the
liquid crystalline molecule, specific motions can be influenced, resulting not only in new phase structures but
also theoretically interesting ones.
Fig. 6. Photographs, taken under the polarizing microscope, of the biaxially
nematic polymer poly[l-(ll-[2,5-bis@-anisoyloxy)phenyl]undecyloxycarbonyll-I-methylethylene](crossed polarizers, magnification 40 x ). Original colors: mainly red-violet, light-brown, lime-green.
5.2. Properties
Although the anisotropic phase structure of liquid crystalline main chain polymers results from the long-range ordering of the main chain, only the mesogenic side groups
determine the phase structure of side chain polymers. Thus
the polymer main chain does not itself need to have longrange ordering. The local chain conformation must only be
consistent with the liquid crystalline phase structure of the
side chains. For example, a deformed polymer coil in the
nematic phase was ascertained to have an axial length of
1.5 times its radius of
As a result, these polymers are similar to conventional amorphous polymers in
their mechanical characteristics. They do not show the
unusual mechanical properties of liquid crystalline main
chain polymers.
Of special interest is the phase behavior of these polymers when the liquid crystalline melt is cooled to lower
temperatures. This is shown schematically in Figure 7, in
which the order parameter S is given as a function of temperature. With decreasing temperature, the long-range
orientational order of the mesogenic side chain continually
increases until a defined temperature Tg,icbelow which it
then remains constant. Thermodynamic studies show that
at this temperature, the liquid crystalline melt transforms
into the glassy state.
and N M R spectroscopic
Fig. 7. Order parameter S as a function of temperature for a liquid crystallme
= liquid crystalside-chain polymer. T,,, = glass transition temperature; TIC.,
line-isotropic phase transformation temperature.
measurements[42'show that the liquid crystalline structure
remains unchanged in the glassy phase. Therefore it is possible to produce anisotropic glasses which display a nematic or smectic structure depending on the type of polymer. The glass transition temperature Tg,,=essentially depends on the chemical constitution of the polymer main
A further important aspect is the influence of an electric
or magnetic field on the position of the director in the liquid crystalline p h a ~ e . [ ~In~ analogy
- ~ ~ ] to modern electrooptical display elements of low molar mass liquid crystals,
the position of the director can be changed and macroscopically ordered by the application of an external field.
In the ordered phase, the optical properties of the polymer
correspond to that of a single crystal of the same dimension. By cooling the polymer into the glassy state, this ordered state is permanently frozen
The combination of these characteristics-the glassy
state and anisotropic physical characteristics-introduces
broad application perspectives for these polymers as optical components in linear and non-linear
Furthermore, optical memory (storage) elements which can utilize
changes in the most diverse physical properties can be realized. As an example, a recording element which gives
high resolution suitable for holographic photographs is
shown in Figure 8.[481A uniformly ordered polymer in the
Fig. 8. Recording element with a liquid crystalline side-chain polymer a) A
uniformly ordered polymer in the glassy state; b) a linear trans-dye compound isomerized by light radiation into the bent &-form.
glassy state (Fig. 8a) contains a dye which can undergo
trans-cis isomerization. The glassy polymer is warmed into
the liquid crystalline state by exposure to a laser for a short
time, and simultaneously the linear trans-dye isomerizes to
the bent cis-dye (Fig. 8b). Due to the bent conformation of
the dye, the local ordering of the phase within the radiated
area is reduced, thus changing the anisotropic optical
Angew. Chern. Int. Ed. Engl. 26 (1987) 816-824
properties. After irradiation, the polymer returns to the
glassy state and the given information is permanently
stored. It can be retrieved by a minimal laser output. The
process is reversible (for example, via a thermal reverse
reaction to the trans-dye in the glassy state, in which the
local state of order of the phase remains unaffected).
A completely different application follows from the anisotropic properties of the polymer and its negligible vapor
pressure at high temperatures. As stationary phases in gas
chromatography such polymers lead to higher separation
efficiencies than conventional material^.^^^.^^]
6. Liquid Crystalline Elastomers
6.1. Synthesis and Structure
In the liquid crystalline melt above the glass temperature, the linear polymer main chains of the liquid crystalline side chain polymers can move among themselves
through translational diffusion. If the linear chains are
made into a polymer network by a chemical cross-linking
reaction, this translational diffusion is prevented. Disregarding the cross-linkage sites, however, the high flexibility of the chain segments remains in spite of the cross-linking. I n addition, the mesogenic molecules which are attached to the network are not prevented from building liquid crystalline phase structures. The networks now show,
on the one hand, the anisotropic phase behavior characteristic of low molar mass liquid crystals and, o n the other,
the nondeformability and rubber elasticity characteristic of
conventional eIastomers.[5'.521
Liquid crystalline elastomers can be prepared by usual
procedures in macromolecular chemistry, but the synthesis
of defined networks is extremely difficult. The currently
known liquid crystalline elastomers have been synthesized
by simple, statistical copolymerization o r by statistical polymerization-analogous addition reactions with multifunctional cross-linking molecules. Another simple method is
the copolymerization of a mesogenic monomer with a
functionalized comonomer to give a linear liquid crystalline copolymer, which in a second reaction step with crosslinking agent B is transformed into a network (Scheme
Scheme 4.
Angew Chem. Int. Ed. Engl. 26 (1987) 816-824
6.2. Properties
Elastomers with nematic and smectic phases are known
which show the same dependence of the liquid crystalline
phase on the chemical structure of the monomer units as
the linear liquid crystalline polymers. At the glass transition temperature Tg,,=
the elastomer freezes into a glass
with the liquid crystalline structure.
Of special importance are the mechanical properties of
the elastomers. They behave at higher temperatures in the
isotropic phase like conventional rubbers, whereas below
the isotropic-liquid crystalline phase transformation the
mechanical properties are direction dependent. If one considers an undeformed elastomer in the liquid crystalline
phase, the preferred direction of the mesogenic side groups
varies due to thermal fluctuations, as it does in low molar
mass liquid crystals and linear liquid crystalline polymers
(Fig. 9). If the elastomer is mechanically deformed by a
Fig. 9. Orientation of the director of a liquid crystalline elastomer through a
mechanical stress (5. a) In the unstressed state, the orientation of the director
is non-uniform; b) through mechanical deformation. the director is uniformly ordered. The optical characteristics of the sample correspond to that
of a single crystal of the same dimensions.
retractive force, then one obtains a macroscopic ordering
of the mesogenic side groups. The orientation of the mesogenic side groups in the direction of the force is determined by the linkage between the mesogenic groups and
It has thus far been observed that, depending on the length of the linkage of the flexible spacer, the
preferred direction of the longitudinal axes of the mesogenic groups is either parallel to or perpendicular to the deformation axis; other angular positions are not found.
The elastomer is comparable in its optical properties to a
single crystal of the same dimension. A reorientation results from a change in the direction of the stress, in which
the direction of the stress and the direction of the longrange orientational order are always directly related.
Therefore mechanical deformation of the elastomers has
the same influence on the orientation of the liquid crystalline molecules as an electric or magnetic field has on the
orientation of low molar mass liquid crystals and linear liquid crystalline polymers.
Combination of the properties of anisotropic phase
structure, rubber elasticity and the ability to orient the op823
tical axis through mechanical deformation results in a new
class of materials whose scope of application extends
beyond the area of optics. For example, thin anisotropic
films can be used as separation membranes. Other detailed
studies on these new materials are therefore not only of
theoretical interest with respect to the interaction between
a deformed polymer network and the liquid crystalline
phase structure, but also provide the basis for new applications.
7. Summary and Outlook
Liquid crystalline phases always require a rigid, rod- or
disk-shaped mesogenic element. The introduction of these
structural elements into a polymer main chain or as side
groups on the monomer unit of a macromolecule enables
the synthesis of polymers which build thermodynamically
stable liquid crystalline phases within a defined temperature range. In the liquid crystalline phase, the main chain
segments or the mesogenic side groups which are structurally similar to low molar mass liquid crystals show longrange orientational and positional order. The liquid crystalline structure can be frozen into the glassy state of the
polymer, thus enabling the preparation of glasses with anisotropic physical properties.
The examples given in this article represent only a small
number of the wide spectrum of realizable or conceivable
liquid crystalline polymers. The intended aim of this article has been to point out that the combination of those
properties specific to polymers with the anisotropic properties of liquid crystailine structures enables the synthesis
of a wide variety of new materiafs. The liquid crystalline
polymers not only offer possibilities for new types of application. The interplay between anisotropic phase structure and the properties of the macromolecular system provides the chemist, physical chemist and physicist with
many interesting theoretical and experimental problems
which remain to be solved.
Received: April 23, 1987 [A 634 IE]
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