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2
BACKGROUND
Relatively unhindered rotation about the single bonds in the backbone of
most macromolecules means that a random trajectory for the polymer chain is
the most common polymer conformation. Features of some special monomers
(resonance, stereochemistry, etc.), however, restrict internal degrees of
freedom in the polymers such that the mainchain is extended in space along an
almost linear trajectory. Extended polymer chains or chain segments can,
through excluded volume interactions, lead to long-range orientational
ordering of the macromolecules- liquid crystallinity in concentrated solution
or in the melt. Liquid crystals (LCs), sometimes also called mesoPhases. were
first recognized in low-molar-mass compounds a century ago, and they end oy
widespread technological applications because of their unique electro-optical
properties. Low-molar-mass LCs are highly anisotropic fluids that exist
between the boundaries of the solid state and the conventional isotropic
liquid state and exhibit features or both states. ~
macromolecules, orientational ordering of extended polymer chains is
sufficient to impart some crystal-like orientational ordering to their fluid
phases melts or polymer solutions. Although this orientation is very subtle
on a local scale (it is masked by the rapid and complex molecular dynamics
characteristic of all fluid phases), time-averaged attributes of these fluids
are anisotropic and therefore dramatically different from those same
attributes
1: =~ ~ ~
To Ah" ~= c" of
in ordinary isotropic liquids. Figure 2.1 exaggerates the
"~ between the melt of a conventional random coil polymer (top) and
that of a liquid crystalline polymer (LCP) (bottom) in order to pictorially
show the long range order in a fluid phase of the LCP. In LOP melts or
solutions this average anisotropy has dramatic consequences. When macroscopic
uniform alignment of local directors exists, such fluids exhibit bulk
anisotropic dielectric, magnetic, optical, transport, etc., properties.
Materials formed from polymers that are orientationally organized in the fluid
state retain this anisotropy in the solid state and frequently exhibit
ultrahigh strength and stiffness (modulus) along the machine direction
(parallel to the extended chains) because the organization and confirmational
preferences of the chain promote an extended-chain crystal habit. For these
13
OCR for page 14
Semi
| flexible
Ado
- chain ~
mer
+=
FIGURE 2.1 Schematic indication of the differences between an isotropic (top)
and a liquid crystalline (bottom) polymer fluid.
OCR for page 15
15
reasons, LCPs are being increasingly utilized in specialty and high-
performance applications. Historical reviews (Economy, 1989; Jackson, 1989;
Samulski, 1985; and Dobb and McIntyre, 1984), general introductions (Brows tow,
1988; Chapoy, 1985; Finkelmann, 1987; and S~mulski, 1982) , and contemporary
reviews of LOP properties (Calundann et al., 1988; and G. E. Williams, 1987)
abound.
LCs and LCPs may be divided into two broad categories, according to the
principal means of achieving fluidity. Lyotropic LCPs result from the action
of a solvent and hence are multicomponent polymer solutions polymer plus
solvents). Thermotropic LCPs are produced by heat and may be single (neat
polymer) or multicomponent melts.
Within each category, three distinctive supr~molecular organizational or
structural classes of LCs have been identified: the nematic, smectic, and
twisted nematic or cholesteric phases. Structural differentiation of these
phases is related to the packing aspect and dimensionality of the
translational organization of the molecules. In the examples of these phases
we limit consideration to mainchain LCPs. Nematic LCs are distinguished by a
unique director (optic axis) in the fluid; the nematic director is established
by the parallelism of the long axes of molecules ([average] polymer chain
axes). There is no translational order in this nominally uniaxial fluid
(Figure 2.2~.
Chain parallelism also characterizes the smectic phase, but translational
order is also present in the form of long-range stratification normal to the
chain axes (Figure 2.39. Mobility of the entire chain within the smectic
layers is possible, although this increased translational organization lowers
chain mobility relative to nematic phases. (In low-molar-mass LCs bulk
fluidity in the smectic structure involves the layers gliding past one
another; such a transport mechanism would be sharply attenuated in polymeric
analogs, wherein a single semiflexible chain traverses more than one layer;
smectics formed from rigid rod polymers with the layer spacing equal to the
rod length might exist.) (Wen et al., 1989) In nematic phases chain ends
(defects) are randomly distributed in the ordered fluid; there may be a
tendency for such defects to segregate between layers in smectic fluids
composed of semiflexible chains. Cholesteric LCs are similar to nematics in
organization, with the additional feature of a cumulative twist between
molecules, a result of the asymmetry of intermolecular forces. This asymmetry
is due to the presence of chiral centers in these mesogens. As a result, the
local nematic director twists into an inherently biaxial, helicoidal
supramolecular structure (Figure 2.4~. Although there is a large number of
applications of cholesterics in low-molar-mass LCs that exploit their optical
properties (temperature sensors, notch filters, etc.), there is little
evidence of widespread use of this biaxial superstructure in LCPs.
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16
FIGURE 2.2 An absence of translational order in the idealized nematic.
FIGURE 2.3 Smectic stratification (lateral registration) in a polymer
mesophase.
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FIGURE 2.4 Helicoidal cholesteric structure in a mainchain LOP.
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18
MACROMOLECULAR DESIGN AND SYNTHESIS
Thermotropics
Mainchain (linear) LCPs are generally synthesized by condensation
polymerization involving transesterification (Jackson, 1989) (Figure 2.5~.
The growth step is an ac~dolysis reaction yielding an ester connecting link
accompanied by the loss of acetic acid. The polymerization is usually
conducted in the melt, although in some cases the use of an inert suspending
medium is reported. The reaction in the melt is carried out either to
completion or first to low molecular-weight-oligomer followed by solid state
polymerization to high molecular weight. This general approach is employed
for the important high-temperature all-aromatic polymers such as Xydar~
(Amoco), Spectral (Hoechst-Celanese), Victrex~ SRP (ICI), and presumably the
recent polymers announced by Du Pont and Granmont/Montedison.
j CHi O~O 6 CH, + CHIN O¢6 OlI ~ HO 6 4~ 6 0lI |
T0~0346U' ¢~t
Polyester
FIGURE 2.5 Condensation polymerization involving acidolysis.
In one commercial case of an aliphatic- aromatic thermotropic LOP- the
lower-use-temperature polymer X7G (Eastman) - this condensation step is
preceded by the reaction of acetoxybenzoic acid with polytethylene
terephthalate) (PET) in an acidolysis reaction in which the PET chain is
cleaved. This results in one chain capped with a carbophenyl carboxylic acid
moiety and the other capped with an acetoxyphenyl moiety. This is then
followed by the acidolysis reaction of the acetoxy and carboxyl end groups,
with loss of acetic acid accompanied by other acidolysis reactions. This
sequence in short rebuilds the molecular weight with accompanying insertion of
oxyphenylcarbonyl mer sequences.
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19
Typical copolyester LCPs are shown in Figure 2.6.
Patented Aromatic LOP Polyesters
Amoco
(Carbons)
t
A <3~
fo~o]
J ~
3~ . touch
o o
. ·~C
, ~_
Hoec~Cehne~e t ~ _ ~ c:
to - }~44~c)
_fo~]
.~
DuPont -
J
hobo-a¢83 lo~~3}
.
- -3} ~
0 0
Jeff>
I
is ~ 5
r°~°~ r
OnzmonMo~e~bon t~ ~ ~ j- ~ o ~ :- ~ 04
l
J ~
_
FIGURE 2.6 Representative potential thermotropic copolyesters.
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20
LvotroDics
The lyotropic LCPs are prepared by solution polycondensation (Figure
2.7~. For many of the extended chain polyamides, solution polymerization in
amide solvents is the preferred method. For example, poly~p-phenylene
terephthalamide) is synthesized by reacting the appropriate aromatic diamine,
p-phenylenediamine in this case, with terephthaloyl chloride in an amphoteric
solvent, such as N-methyl-2-pyrrolidone containing a solubility-enhancing
salt. Lyotropic LC polyamides, depending on their composition, may be spun
directly from their reaction media, or they may be isolated, redissolved, and
spun from solutions containing strongly interacting acids, such as sulfuric
acid, oleum, etc. The use of a phosphorylation method for the preparation of
aromatic polyamides involves the direct condensation of aromatic amino acids
or aromatic diamines with aromatic diacids in the presence of an aryl
phosphite and organic base. Typical unit structures that yield polyamide
lyotropic LCPs are shown in Figure 2.8.
C16 ¢6C1
. =
H2N~NH2 ~
. ~
iN¢N-~ ¢8
.
FIGURE 2.7 Synthesis of polyarylamides.
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21
N~C
HAN\
HE
\,
A\
ASH
N
C~C\
FIGURE 2 . 8 Lyotropic polyamide unit structures .
b~c\
of \ C1
N~N~
HE \
H
O ~-C V ~
H \ OH
N
':~C
H: ~ ~ C
Con
~0
C
Poly(p-phenylene-2, 6-benzobisthiazole) (PBZT) (Figure 2.9) and poly(p-
phenylene-2,6-benzobisoxazole) (PBO) were initially prepared at the Air Force
Materials Laboratory at Wright-Patterson Air Force Base (see Wolfe, 1988, for
a review). PBZT was prepared by the reaction of 2,5-diamino-1,4-benzene-
dithiol dihydrochloride with terephthalic acid in polyphosphoric acid. PBO is
similarly derived from 4,6-diamino-1,3-benzenediol dihydrochloride. The
procedure also works well for A-B type monomers, such as 3-amino-4-
mercaptobenzoic acid and 3-~mino-4-hydroxybenzoic acids (Chow et al., 19899.
The preparation of "molecular complexes" from blends of polymers derived from
A-A+B-B monomers and polymers derived from A-B types has also been reported
(Wolfe, 1988~.
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22
HX~HH2
H2~-XH
r
~ H064/ \~6oH
t............
,X~N~ i~v
l
N~X~
-n
X = 0,S
Poly(p phenylene-2 ,6-benzobis " X "azole) rPBX]
~.
FIGURE 2.9 Synthesis of PBX polymers.
Sidechain Thermotropic LCPs
While most of the interest in LCPs is focused on mainchain a--
important class of LCPs contains the mesogenic groups as an appendage
(meso~enic core) on the polymer sidechain LCPs (see Attard and Williams,
. ~ ~ ~ TV ~ ~w . Synthetic routes to sidechain LCPs have traditionally
+~ ~ I ^~.rN
polymers, an
an appendage
~ car
1986, ~ .,
involved polymerization of a vinyl monomer (e.g., acry~ate or mernacry~are
under free radical conditions in solution (Figure 2.10~. (The only real
difficulty with this reaction is encountered when radical reactive groups are
located elsewhere in the monomer.)
It is also possible to produce sidechain LCPs through polycondensation.
For example, malonate monomers can be converted to polyesters in a
polyesterification reaction (Figure 2.11~. This reaction is of special
interest for radical reactive groups such as nitroaromatics and stilbenes,
which have application in nonlinear optics. Polycondensation of combined
sidechain and mainchain LCPs can also be utilized.
r - -~
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23
]
mesogea
spacer
AIBN
O=
~solvent
£~
FIGURE 2.10 Preparation of sidechain LCPs by free radical polymerization.
litOgOlit
~ HO _~OH
ID \_~oL
IS ( 0~)4
L ~
FIGURE 2.11 Preparation of sidechain LCPs by polyesterification.
-
~s
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38
ELECTRO-OPTICAL PROPERTIES
Herein the focus is on nonlinear optical (NL0) properties of organic
systems involving LCPs. There are, however, a number of recent phenomena
(generally linear electro-optic) that employ conventional polymers or LCPs
together with low-molar-mass LCs. The latter (guest) is dispersed in the host
polymer matrix as a microemulsion and the director responds to an applied
field thereby changing the refractive index difference between the guest LC
and polymer (LCP) host. The phenomena may be adapted to light attenuation and
optical switching (roan et al., 1986; Drzaic, 1986~. In the cases of NL0
phenomena, readers may readily sense the intensity of interest in organic
polymeric systems, in general (Boyd, 1989), and in LCPs (Williams, 1987), in
particular.
Relevant aspects of second- and third-order NL0 processes are reviewed
here so that readers may consider the potential of LCPs in this active
research area, one that is anticipated to yield technologically important
advances In the future (Williams, 1987~.
Second-Order NLO Processes
There are basically two categories of second-order NL0 processes: (a)
the linear electro-optic or Pockels effect and (b) parametric processes such
as second-harmonic generation (SHG) and sum or difference frequency
generation. In the former a d.c. electric field is applied to a medium, which
responds by altering its refractive index in proportion to the applied field.
In the latter the electric field associated with incident light produces
polarization components at other frequencies, which can act as a source of
electromagnetic radiation at those frequencies. For a material to exhibit
significant second-order NLO responses it must have a noncentrosymmetric
structure. In the case of polymers this implies that a polar symmetry axis
must be introduced into a medium that would otherwise be nonpolar because of
orientational averaging. Electric field poling of thermoplastic polymers at
elevated temperatures (above the glass transition temperature, Tg) leads to
the introduction of a polar axis (by biasing molecular dipoles in the
direction of the applied field) (Meredith et al., 1982; Le Barny et al., 1987;
DeMartino, 1988~. This induced polarity can be retained by cooling the
polymer to well below its To. The main advantage of introducing liquid
crystallinity into polymers for second-order NL0 applications is the
enhancement in the degree of polar molecular alignment it can provide; up to -
factor of 5 under certain processing conditions. The origin of this
enhancement is the effect of the local anisotropic potential associated with
the liquid crystalline director on the orientational distribution function of
the nonlinear chromophore. Enhanced alignment translates into up to a factor
of 5 larger nonlinear coefficient, which in turn can increase the efficiency
of processes such as second-harmonic generation by over an order of magnitude
For third-order NL0 materials uniaxial alignment associated with the liquid .
crystalline director can have a similar enhancement in the coefficient by
removing the spatial averaging effects of an isotropic environment on the
direction of largest nonlinearity in the chromophore. For commercial
applications the stability of retained alignment is of primary concern since
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39
critical device characteristics are determined by the stability of parameters
related to alignment. Liquid crystallinity can assist in the retention of
local alignment because of its highly anisotropic contribution to the local
orientational potential energy (D. J. Williams, 1987~.
An alternative approach to inducing noncentrosymmetry might be to
introduce chirality and the other structural requirements of the ferroelectric
smectic c* mesophase into a polymeric structure containing chromophores
capable of producing a nonlinear response (Goodby and Leslie, 1984~. Because
of its complexity, this approach has not yet been fully examined or exploited,
but it may be fruitful for achieving high degrees of order with excellent
stability.
Most devices designed to take advantage of the Pockels effect operate by
retarding the phase of light propagating through the medium with the field-
induced refractive index change. One figure of merit (FOM) quantifying the
suitability of a material for a particular electro-optic or integrated optic
application (Alferness, 1982) is
FOM ~ Xf21/n(£ + 1)
where x`2, is the first nonlinear coefficient, n the refractive index, and £
the dielectric constant of the electro-optical material. This FOM determines
the trade-off between the electric field and path length required to achieve a
particular degree of phase retardation. For very-high-speed devices where
electric fields are applied via microwave transmission lines, an additional
factor emerges. Here the velocity of the microwave pulse Vm = C/E~ must match
that of light, Ve = C/n, in the medium over the interaction length required
for a given amount of phase retardation to occur (DeMartino et al., 1987~.
From these considerations a large refractive index would favor phase
retardation. For high-bandwidth devices operating in the traveling wave mode,
n2 approximately equals £ of the electrodes at optical frequenc ies . For
electro-optic materials such as LiNbO3, n2 and E are very different, so that
velocity matching can only be achieved over short distances (Lytel et al.,
1988~. From a device design point of view, if the phase retardation must be
achieved over short distances, much higher voltages are required. A thorough
discussion of device-dependent requirements for electro-optic materials is
beyond the scope of this report, but a general list of requirements and
desirable characteristics compared to the properties of currently existing
materials is presented in Chapter 3.
For second-harmonic generation, a separate FOM is required that leads to
additional desirable material characteristics. Consider the fraction of power
converted to the harmonic frequency, P(2~/P(~), over a certain region in a
crystal. It is proportional to the following factors:
P¢2 '/p`~' ~ ~x`2, /n3~/~3/2] · P(~)L · f(AkL/2)
where n, A, and £ are, respectively, the refractive index, dipole moment, and
dielectric constant of the nonlinear materials, and f(AkL/2) is a phase
mismatch factor that is periodic in character and whose amplitude is reduced
by increasing periodicity. The periodicity of this function is determined by
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40
the mismatch in momentum Ak of the fundamental and harmonic waves. A FOM for
harmonic conversion is given by
FOM = (X ~ ~ /n (~/e ~ I
Here it is clear that low-refractive- index materials have a considerable
advantage relative to high-index materials, and the quadratic dependence of
power conversion efficiency on xt2, puts a tremendous premium on that factor.
The input power P(~) and interaction length L are parameters to be played off
against the material FOM. Other factors such as birefringence and geometrical
optical factors limit the length over which the interaction can be maintained.
In waveguided structures designed for optimized propagation of
fundamental and harmonic fields, the dependence on interaction length L
becomes quadratic, and optical fields can be propagated over long distances,
leading to high conversion efficiencies (Zyss and Chemla, 1987~. Because of
the lack of materials with suitable properties, as well as processes to
fabricate them into suitable waveguides, the potential technological
advantages of waveguide SHG have yet to be realized.
Third-Order NLO Processes
_
Third-order nonlinear processes arise from the nonlinearity in the
polarization response of all dielectric media, including conj ugated organic
systems . The x-electrons in conjugated organic systems , being loosely bound,
contribute much more strongly to the nonlinear response than the more tightly
bound core electrons (Rustagi and Ducuing , 1974~. Third-order processes fall
into two basic categories. The first is analogous to the Pockels effect,
where the refractive index change is quadratically dependent on the applied
field, which can be at d.c. or optical frequencies. This can lead to a
variety of interesting effects that are manifested in various device designs,
including bistable switches, power limiters, and optically driven modulators
(Stegeman et al., 19883. The second category of processes involves the
interaction of optical fields at different frequencies, where energy can be
exchanged between field components in a manner similar to second-order
parametric processes. The fields can all be at the same frequency (in
contrast to second-order processes, where one of the fields must be at the
harmonic, sum, or difference frequency) or at different frequencies. Third-
harmonic generation, degenerate four-wave mixing, and real-time holography are
examples of such effects (Shen, 1984~. If one of the frequencies or any
combination of them matches a resonant process in the molecule or medium,
large enhancements in nonlinear response can be achieved. In this case
dissipation of thermal energy and the temporal response of the resonant
process place constraints on the utility of the process. Momentum
conservation must be maintained and can be controlled by the interaction
geometry.
There is no symmetry restriction for third-order processes, unlike the
case for second-order processes, so they are exhibited by all media. In
conjugated polymers, where electron oscillations are much larger in the chain
direction than perpendicular to it, nonlinear responses are extremely large.
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41
Nonresonant third-order nonlinearities are larger in polymers such as
polyacetylene (Sinclair et al., 1987), polydiacetylene (Sauteret et al.,
1976), and other conjugated polymers than in any other class of materials,
including inorganic semiconductors. The response parallel to the chain
direction suggests that macroscopic orientation of the polymer should result
in a considerably larger response than in an isotropic system. The
enhancement factor can be shown to be a factor of five. Conjugated liquid
crystalline maincha~n polymers are also known to exhibit high degrees of
shear-induced uniaxial alignment, and the expected increases in nonlinear
coefficients in appropriate directions have been observed (Rao et al., 1986~.
All optical signal processing applications based on third-order NLO fall
into two basic categories: parallel and serial. There are two approaches to
parallel processing that enable the massive parallelism and interconnectivity
of optics to contribute to optical computing and information processing
(Gibbs, 19861. The first of these involves the use of simple spatial patterns
combined with the switching behavior of nonlinear etalon devices to perform
computational functions; these devices are simply miniature resonant cavities
where the thickness, refractive index, and reflectivity of the internal
surfaces are chosen to provide a destructive interference condition and
therefore low transmission through the device. The nonlinear contribution to
the refractive index of the medium, as illustrated by the equations below,
causes the transmission characteristics to be light-intensity-dependent and
capable of exhibiting bistable behavior.
The second approach to parallel processing involves the formation of
transient holograms generated by two counter-propagating beams in a bulk
nonlinear medium to alter the information content of a third beam interacting
with the grating thereby producing a new fourth beam. An example of
information processing by this method is associative memory (Yariv and Kwong,
1986~. Here an optical mode of a resonator containing a hologram with many
messages and a nonlinear medium can selectively amplify a particular message,
given only partial information from an input beam. For these applications the
primary requirement is for large X(3), where
xt3~/~ - n2nO~OC2/3
in MKS units and
n = nO ~ n2I
where nO is the material refractive index, n2 is the light-intensity-dependent
refractive index, and I is the light intensity and ~ is the absorption
coefficient. A large value of n2 maximizes the response of the material to
small amounts of energy. Because of the inherent parallelism, high degrees of
information throughput are generated, and response times in the nanosecond to
millisecond time frame are useful.
Because of their extremely large resonant nonlinearities, GaAlAs multiple
quantum wells and photorefractive crystals such as BaTiO3 have a tremendous
functional advantage for this class of applications. Investigations of NLO
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42
properties of organic systems, being in a much earlier stage, have not yet
demonstrated similar advantage.
Serial applications of third-order nonlinear optics involve the use of
the intensity-dependent refractive index in waveguided structures to perform
rapid switching of bit streams between two or more optical channels. The
importance of such devices can be appreciated by realizing that optical fibers
can be utilized to transmit information at rates approaching 1 THz. This rate
is beyond the capabilities of known light detectors and will require all-
optical signal processing before light signals are converted to electronic
ones. In waveguides, long interaction lengths can be used to generate phase
retardations needed for switching applications so that the premium for
materials is put on the speed of the nonlinear response and the optical losses
in the material caused by linear optical processes. The decay times of
nonlinear responses in semiconductors and quantum well structures, as well as
the response time in photorefractive materials, are in general too slow for
this type of application. Polymeric materials exhibiting extremely large
nonlinearities (albeit much smaller than the resonant nonlinear responses of
the inorganic materials that makes them more suitable for parallel processing)
may offer the best hope for this important class of applications '£ additional
gains can be made in their properties (Stegeman et al., 1987~.
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Representative terms from entire chapter:
refractive index
