Side chain liquid crystalline polymers exhibiting nonlinear optical properties

This invention provides novel sidechain liquid crystalline polymers which exhibit nonlinear response, and which have utility as a transparent nonlinear optical component in optical light switch and light modulator devices. An invention sidechain liquid crystalline polymer is illustrated by poly[4-(8-hydroxyoctyloxybenzylideneamino)-4'-nitrostilbene methacrylate]: ##STR1##

BACKGROUND OF THE INVENTION 
It is known that organic and polymeric materials with large delocalized 
.pi.-electron systems can exhibit nonlinear optical response, which in 
many cases is a much larger response than by inorganic substrates. 
In addition, the properties of organic and polymeric materials can be 
varied to optimize other desirable properties, such as mechanical and 
thermoxidative stability and high laser damage threshold, with 
preservation of the electronic interactions responsible for nonlinear 
optical effects. 
Thin films of organic or polymeric materials with large second order 
nonlinearities in combination with silicon-based electronic circuitry have 
potential as systems for laser modulation and deflection, information 
control in optical circuitry, and the like. 
Other novel processes occurring through third order nonlinearity such as 
degenerative four-wave mixing, whereby real-time processing of optical 
fields occurs, have potential utility in such diverse fields as optical 
communications and integrated circuit fabrication. 
Nonlinear optical properties of organic and polymeric materials was the 
subject of a symposium sponsored by the ACS division of Polymer Chemistry 
at the 18th meeting of the American Chemical Society, September 1982. 
Papers presented at the meeting are published in ACS Symposium Series 233, 
American Chemical Society, Washington, D.C. 1983. 
The above-recited publications are incorporated herein by reference. 
Of general interest with respect to the present invention is prior art 
relating to azomethine derivatives, such as that described in U.S. Pat. 
Nos. 3,041,165; 3,253,022; 3,373,141; 3,483,131; 3,697,595; 3,742,054; 
3,872,140; 3,968,159; 3,973,830; 4,122,026; 4,173,544; 4,297,502; and 
4,370,502. 
There is continuing research effort to develop new nonlinear optical 
organic systems for prospective novel phenomena and devices adapted for 
laser frequency conversion, information control in optical circuitry, 
light valves and optical switches. The potential utility of organic 
materials with large second order and third order nonlinearities for very 
high frequency application contrasts with the bandwidth limitations of 
conventional inorganic electrooptic materials. 
Accordingly, it is an object of this invention to provide azomethine 
compositions which are characterized by a delocalized conjugated 
.pi.-electron system which can exhibit nonlinear optical response. 
It is another object of this invention to provide side chain liquid 
crystalline polymers which exhibit nonlinear optical response. 
It is a further object of this invention to provide high performance 
nonlinear optical media and devices. 
Other objects and advantages of the present invention shall become apparent 
from the accompanying description and examples.

DESCRIPTION OF THE INVENTION 
One or more objects of the present invention are accomplished by the 
provision of a side chain liquid crystalline polymer characterized by a 
recurring monomeric unit corresponding to the formula: 
##STR2## 
where m is an integer of at least 3; n is an integer between about 1-25; R 
is hydrogen or a C.sub.1 -C.sub.4 alkyl substituent; X is an 
electron-donating substituent; Z is an electron-withdrawing substituent; 
and Y is a substituent selected from 
##STR3## 
Illustrative of the X substituent in the above polymer formula are 
electron-donating divalent radicals such as --O--, --S--, and --NR.sup.1 
-- where R.sup.1 is hydrogen or methyl. 
Illustrative of the Z substituent in the above polymer formula are 
--NO.sub.2, --CN and --CF.sub.3. 
A present invention side chain liquid crystalline polymer typically has a 
weight average molecular weight between about 1000-500,000; contains 
mesogenic sidechains which exhibit a second order nonlinear optical 
susceptibility .beta. of at least about 1.times.10.sup.-30 esu as measured 
at 1.91 .mu.m excitation wavelength; has a glass transition temperature 
above about 40.degree. C.; and exhibits a thermotropic smectic mesophase. 
In addition to the recurring acrylate monomer unit represented in the 
formula above; the polymer can contain one or more copolymerized monomeric 
residues derived from vinyl monomers such as alkyl (meth)acrylate, 
(meth)acrylamide, N,N-dialkyl(meth)acrylamide, vinyl halide, vinyl 
carboxylate, acrylonitrile, methacrylonitrile, alkene, arylvinyl, and the 
like. Suitable vinyl monomers include vinyl chloride, vinyl acetate, 
ethylene, propylene, isobutylene, isoprene and styrene. 
The additional vinyl comonomer or comonomers can be incorporated in a 
proportion up to about 40 weight percent of a present invention liquid 
crystalline polymer. 
In another embodiment this invention provides a thermotropic side chain 
liquid crystalline polymer characterized by a recurring monomer unit 
corresponding to the formula: 
##STR4## 
where m.sup.1 is an integer of at least 5; n.sup.1 is an integer between 
about 2-20; R.sup.1 is hydrogen or a methyl group; X.sup.1 is --NR.sup.1 
--, --O-- or --S--; and Z.sup.1 is --NO.sub.2, --CN or --CF.sub.3. 
In another embodiment this invention provides a thermotropic side chain 
liquid crystalline polymer characterized by a recurring monomeric unit 
corresponding to the formula: 
##STR5## 
where R.sup.1, m.sup.1, n.sup.1, x.sup.1 and Z.sup.1 are defined above. 
In another embodiment this invention provides a thermotropic side chain 
liquid crystalline polymer characterized by a recurring monomeric unit 
corresponding to the formula: 
##STR6## 
where R.sup.1, m.sup.1, n.sup.1, X.sup.1 and Z.sup.1 are as defined above. 
In another embodiment this invention provides a thermotropic side chain 
liquid crystalline polymer characterized by a recurring monomeric unit 
corresponding to the formula: 
##STR7## 
where R.sup.1 ; m.sup.1, n.sup.1, X.sup.1 and Z.sup.1 are as defined 
above. 
In another embodiment this invention provides a transparent nonlinear 
optical medium comprising a thermotropic sidechain liquid crystalline 
polymer which is characterized by a recurring monomer unit corresponding 
to the formula: 
##STR8## 
where m is an integer of at least 3; n is an integer between about 1-25; R 
is hydrogen or a C.sub.1 -C.sub.4 alkyl substituent; X is an 
electron-donating substituent; Z is an electron-withdrawing substituent; 
and Y is a substituent selected from 
##STR9## 
A present invention nonlinear optical medium exhibits second order 
nonlinear optical response when the macroscopic molecular configuration is 
noncentrosymmetric. A present invention nonlinear optical medium exhibits 
third order nonlinear optical response when the macroscopic molecular 
configuration is centrosymmetric. 
A present inventoion nonlinear optical medium can have an external 
field-induced molecular orientation. 
A present invention nonlinear optical medium can contain a guest organic 
compound which exhibits nonlinear optical response. This type of 
guest/host blend is illustrated by a nonlinear optical medium in which the 
host liquid crystalline polymer corresponds to the formula: 
##STR10## 
where R, m, n, X and Z are as defined above; and the guest organic 
compound corresponds to the formula 
##STR11## 
when X, R, Y and Z are as defined above. 
In another embodiment this invention provides a light switch or light 
modulator device with an organic nonlinear optical component consisting of 
a transparent solid medium comprising a thermotropic sidechain liquid 
crystalline polymer which is characterized by a recurring monomeric unit 
corresponding to the formula: 
##STR12## 
where m is an integer of at least 3; n is an integer between about 1-25; R 
is hydrogen or a C.sub.1 -C.sub.4 alkyl substituent; X is an 
electron-donating substituent; Z is an electron-withdrawing substituent; 
and Y is a substituent selected from 
##STR13## 
An invention optical light switch or light modulator device typically will 
have a transparent solid medium of a thermotropic liquid crystalline 
polymer which has a stable orientation of an external field-induced 
alignment of mesogens. 
The term "transparent" as employed herein refers to an optical medium which 
is transparent or light transmitting with respect to incident fundamental 
light frequencies and created light frequencies. In a nonlinear optical 
device, a present invention nonlinear optical mediumis transparent to both 
the incident and exit light frequencies. 
Illustrative of a present invention optical device containing a polymeric 
nonlinear optical component as defined above is a laser frequency 
converter, an optical Pockels effect device, an optical Kerr effect 
device, a degenerate four wave mixing device, an optical interferometric 
waveguide gate, a wide-band electrooptical guided wave analog-to-digital 
converter, an optical bistable device, or an optical parametric device. 
Optical harmonic generating devices are described in Science, 216 (1982); 
and in U.S. Pat. Nos. 3,234,475; 3,395,329; 3,694,055; 3,858,124; and 
4,536,450. 
Optical Kerr effect devices are described in U.S. Pat. Nos. 4,428,873 and 
4,515,429; and references cited therein. 
Degenerate four wave mixing optical devices are discussed by Y. R. Shen in 
Chapter 15, "The Principles of Nonlinear Optics"; John Wiley & Sons, New 
York (1984). A nonresonant degenerate four wave mixing mirror device is 
described by J. Feinberg et al in Optics Letters, 5(12), 519 (1980). 
An optical interferometric waveguide gate device is described by A. Lattes 
et al in IEEE J. Quantum Electron, QE-19(11), 1718 (1983). 
A wide-band electrooptical guided-wave analog-to-digital converter device 
is described by R. A. Becker et al in Proceedings Of The IEEE, 72(7), 802 
(1984). 
Optical multiplexer-demultiplexer devices are described in U.S. Pat. Nos. 
3,532,890; 3,755,676; 4,427,895; 4,455,643; and 4,468,776. 
Optical bistable devices are described in U.S. Pat. Nos. 4,515,429 and 
4,583,818; and by P. W. Smith et al in Applied Physics Letters, 30(6); 280 
(1977) and in IEEE Spectrum, June 1981. 
Optical parametric devices are described in U.S. Pat. Nos. 3,371,220; 
3,530,301; and 3,537,020. 
A present invention optical device can be achieved by constructing one of 
the optical devices described in the technical literature, except that a 
present invention liquid crystalline polymer alone or in combination with 
a guest compound is utilized as the nonlinear optical component as 
described herein. 
In a further embodiment this invention provides an acrylic ester 
composition corresponding to the formula: 
##STR14## 
where n is an integer between about 1-25; R is hydrogen or a C.sub.1 
-C.sub.4 alkyl substituent; X is an electron-donating substituent; Z is an 
electron-withdrawing substituent; and Y is a substituent selected from 
##STR15## 
Preferably X is --NR.sup.1, --O-- or --S--; Y is divalent diphenyl or 
stilbene; and Z is --NO.sub.2, --CN or CF.sub.3. 
SYNTHESIS OF LIQUID CRYSTALLINE POLYMERS 
The preparation of a present invention thermotropic liquid crystalline 
polymer with azomethine mesogenic sidechains is illustrated by the 
following flow diagram: 
##STR16## 
NONLINEAR OPTICAL PROPERTIES 
The fundamental concepts of nonlinear optics and their relationship to 
chemical structures can be expressed in terms of dipolar approximation 
with respect to the polarization induced in an atom or molecule by an 
external field. 
As summarized in the ACS Symposium Series 233 (1983) listed hereinabove in 
the Background Of The Invention section, the fundamental equation (1) 
below describes the change in dipole moment between the ground state 
.mu..sub.g and an excited state .mu..sub.e expressed as a power series of 
the electric field E which occurs upon interaction of such a field, as in 
the electric component of electromagnetic radiation, with a single 
molecule. The coefficient .alpha. is the familiar linear polarizability, 
.beta. and .gamma. are the quadratic and cubic hyperpolarizabilities, 
respectively. The coefficients for these hyperpolarizabilities are tensor 
quantities and therefore highly symmetry dependent. Odd order coefficients 
are nonvanishing for all structures on the molecular and unit cell level. 
The even order coefficients such as .beta. are zero for those structures 
having a center of inversion symmetry on the molecular and/or unit cell 
level. 
Equation (2) is identical with (1) except that it describes a macroscopic 
polarization, such as that arising from an array of molecules in a liquid 
crystalline domain: 
EQU .DELTA..mu.=.mu..sub.e -.mu..sub.g =.alpha.E+.beta.EE+.gamma.EEE+(1) 
EQU P=PO+.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+ (2) 
Light waves passing through an array of molecules can interact with them to 
produce new waves. This interaction may be interpreted as resulting from a 
modulation in refractive index or alternatively as a nonlinearity of the 
polarization. Such interaction occurs most efficiently when certain phase 
matching conditions are met, requiring identical propagation speeds of the 
fundamental wave and the harmonic wave. 
A present invention liquid crystalline polymer medium typically is 
optically transparent and exhibits hyperpolarization tensor properties 
such as second or third harmonic generation. 
These theoretical considerations are elaborated by Garito et al in chapter 
1 of the ACS Symposium Series 233 (1983); and by Lipscomb et al in J. 
Chem., Phys., 75, 1509 (1981), incorporated by reference. See also Lalama 
et al, Phys. Rev., A20, 1179 (1979); and Garito et al, Mol., Cryst. and 
Liq. Cryst., 106, 219 (1984); incorporated by reference. 
EXTERNAL FIELD INDUCED LIQUID CRYSTAL ORIENTATION 
The term "external field" as employed herein refers to an electric, 
magnetic or mechanical stress field which is applied to a substrate of 
mobile organic molecules, to induce dipolar alignment of the molecules 
parallel to the field. 
Liquid crystals (including polymeric liquid crystals) may be aligned by the 
application of an external field to a matrix of liquid crystal molecules. 
The degree of orientation is determined by the orientational order 
parameter. For both nematic and smectic mesophases, the parameter is 
defined in terms of a director which is a vector parallel to the molecular 
long axis (and perpendicular to the plane of molecular layering in the 
case of the smectic mesophase). 
If theta is defined as the angle between the director and a chosen axis, 
then the orientational order parameter is defined as the average over all 
molecules of the second Legendre polynomial. The parameter ranges from 
-0.5 to 1.0 (1.0 corresponds to perfect uniaxial alignment along a given 
axis. 0.0 corresponds to random orientation, and -0.5 corresponds to 
random orientation confined in a plane perpendicular to a given axis). 
The order parameter thus defined does not distinguish between parallel and 
antiparallel alignment. Thus, a sample of asymmetric rod-like molecules 
would have an order parameter of 1.0 for both the cse in which the 
molecules are colinear and all pointed in the same direction, and the case 
in which the molecules are colinear and form antiparallel pairs. 
The application of an orienting external field to an array of nematic 
liquid crystal molecules results in an order parameter of approximately 
0.65. Deviations from ideal order are due to nematic fluctuations on a 
micron length scale which accommodate characteristic defects. These 
fluctuations may be dynamic for small molecule liquid crystals or frozen 
for polymeric liquid crystals. In either case, nematic fluctuations 
scatter light so that aligned samples appear to be hazy (particularly in a 
thick sample). 
Smectic liquid crystals may be aligned by application of an orienting 
external field, with a resulting order parameter exceeding 0.9. Unlike the 
nematic phase, characteristic defects are removed upon aligning the 
smectic phase and thereby forming a single liquid crystal phase. Such 
phases are known as monodomains and, because there are no defects, are 
optically clear. 
For both the nematic and smectic mesophases, application of a DC electric 
field produces orientation by torque due to the interaction of the applied 
electric field and the net molecular dipole moment. The molecular dipole 
moment is due to both the permanent dipole moment (i.e., the separation of 
fixed positive and negative charge) and the induced dipole moment (i.e., 
the separation of positive and negative charge by the applied field). 
The torque which results by the application of a DC electric field normally 
would only produce very slight alignment even for high dipolar and 
polarizable molecules at room temperature. The alignment torque is 
negligible in comparison with the disordering effect of thermally induced 
rotation (i.e., the Boltzman distribution of rotational eigenstates at 
room temperature). However, due to the unique associations developed by 
liquid crystalline molecules through intermolecular forces, long range 
orientational order on a micron length scale is present. Under these 
conditions, bulk orientation of the sample by applcation of aligning 
fields exceeding a few volts/cm can produce the degrees of alignment 
indicated above. 
Application of an AC electric field also can induce bulk alignment. In this 
case, orienting torque occurs solely due to the interaction of the applied 
AC field and the induced dipole moment. Typically, AC field strengths 
exceeding 1 kV/cm at a frequency exceeding 1 KHz are employed for the 
nematic phase. At these frequencies, rotational motion of aligned nematic 
regions is not sufficient to follow the field. As a direct result, torque 
due to the interaction of the applied field and any permanent dipole 
moment over time averages to zero. However, electronically induced 
polarization is a very rapid process so that the induced dipole moment 
changes direction depending upon the direction of the applied field 
resulting in a net torque. 
Application of a magnetic field also can effect alignment. Organic 
molecules do not possess a permanent magnetic dipole moment. In a manner 
analogous to AC electric field, a magnetic field can induce a net magnetic 
dipole moment. Torque results from the interaction of the induced dipole 
moment and the external magnetic field. Magnetic field strengths exceeding 
10 Kgauss are sufficient to induce alignment for a nematic phase. 
Alignment of nematics by electric or magnetic fields are accomplished 
simply by application of the field to the nematic material. Alignment of 
the smectic phase is more difficult due to a higher viscosity which 
decreases rotational freedom. Formation of aligned smectic monodomains can 
be achieved by orienting a material in the nematic phase, and cooling the 
material into the smectic phase while maintaining the aligning field. For 
materials which have only smectic and isotropic phases and which lack a 
nematic phase, alignment can be accomplished in the smectic phase at an 
elevated temperature near the smectic to isotropic transition temperature, 
e.g., sufficiently close to the transition temperature so that the medium 
may contain smectic domains in an isotropic fluid. 
Mechanical stress induced alignment is applicable to both the smectic and 
nematic mesophases. Strong aligning mechanical stress propagates 
throughout a solid liquid crystalline material due to the natural tendency 
of these media to self align. Specific mechanical stress methods nclude 
stretching a thin film, or coating a liquid crystalline surface with an 
aligning polymer such as nylon. Physical methods (e.g., stretching) rely 
upon the rigid and geometrically asymmetric character of certain liquid 
crystalline molecules to induce bulk orientation. Chemical methods (e.g., 
coating the surface with an aligning polymer) rely upon strong 
intermolecular interactions to induce surface orientation. All of the 
methods described above to produce oriented materials apply to both small 
molecule liquid crystals and polymeric liquid crystals. For polymers which 
possess a glass transition, the aligned liquid crystalline phase can be 
frozen by rapid cooling below the glass transition temperature. 
Publications relating to external field-induced liquid crystal molecular 
orientation include The Physics of Liquid Crystals, P. G. deGennes, p. 
95-97, Oxford University Press, 1974; J. Stamatoff et al, "X-Ray 
Diffraction Intensities of a Smectic--A Liquid Crystal", Phys. Rev. 
Letters, 44, 1509-1512, 1980; J. S. Patel et al, "A Reliable Method of 
Alignment for Smectic Liquid Crystals", Ferroelectrics, 59, 137-144, 1984; 
J. Cognard, "Alignment of Nematic Liquid Crystals and Their Mixtures", 
Mol. Cryst. Liq. Cryst.: Suppl., 1982; incorporated herein by reference. 
The following examples are further illustrative of the present invention. 
The components and specific ingredients are presented as being typical, 
and various modifications can be derived in view of the foregoing 
disclosure within the scope of the invention. 
EXAMPLE I 
This example illustrates the preparation of a thermotropic sidechain liquid 
crystalline polymer in accordance with the present invention. 
A. 4-(4-hydroxybenzylideneamino)-4'-nitrodiphenyl 
##STR17## 
A reactor is charged with toluene (100 ml), 4-amino-4'-nitrobiphenyl (4.28 
g, 0.02M) and p-hydroxybenzaldehyde (2.44 g, 0.02M). The solution is 
heated to reflux temperature, and the heating is continued for a period of 
about 18 hours, with continuous removal of water with a Dean-Stark trap. 
The product mixture is distilled to remove the solvent, and the residual 
solid is recrystallized from 50/50 ethanol/toluene to yield the azomethine 
product, m.p. 165.degree.-180.degree. C. 
The azomethine product can exhibit a second order nonlinear susceptibility 
.beta. of at least about 1.times.10.sup.-30 esu as measured at 1.91 .mu.m 
excitation wavelength. 
B. 4-(6-hydroxyhexyloxybenzylideneamino)-4'-nitrodiphenyl 
##STR18## 
Azomethine product (2 g) from procedure A is charged to a reactor 
containing ethanol (100 ml). 
The reactor is heated to reflux and a solution of potassium hydroxide (0.5 
g in 25 ml of H.sub.2 O) is added dropwise. After complete addition, 2 g 
of 6-iodohexanol is added and the mixture is heated overnight. 
The product mixture is distilled to remove the solvent medium, and the 
residual solid is recrystallized from 50/50 ethanol/toluene to yield 
purified product. 
C. 4-(6-hydroxyhexyloxybenzylideneamino)-4'-nitrodiphenyl methacrylate 
##STR19## 
A reactor equipped with a magnetic stirrer, addition funnel, thermometer, 
condenser and nitrogen inlet is charged with 10 g of 
4-(6-hydroxyhexyloxybenzylideneamino)-4'-nitrodiphenyl, 4 g of 
triethylamine and 500 ml of dioxane. The mixture is heated to 45.degree. 
C. and a solution of 3 g of methacryloyl chloride in an equal volume of 
dioxane is added dropwise. Stirring is continued 4 hours, then another 3 g 
of methacryloyl chloride is added and the mixture is stirred overnight at 
45.degree. C. 
After solvent removal by vacuum distillation, the residue is slurried in 
water, filtered, washed with water, and air dired. The solid residue is 
recrystallized from ethanol to yield a purified product. 
D. Poly[4-(6-hydroxyhexyloxybenzylideneamino)-4'-nitrodiphenyl 
methacrylate] 
##STR20## 
A reactor is charged with 2 g of 
4-(6-hydroxyhexyloxybenzylideneamino)-4'-nitrodiphenyl methacrylate and 25 
ml of toluene. After purging with nitrogen for 1 hour, 0.65 ml of 1 mole 
percent of azodiisobutyronitrile solution is added. The reactor is sealed 
and heated at 60.degree. C. overnight. The mixture then is mixed with 
methanol in a blender, and the solid polymer product is filtered, washed 
with methanol, and air dried. The polymer has a weight average molecular 
weight of about 10,000. 
The polymer is dissolved in dimethylacetamide and sprayed on the surface of 
optical glass to form a transparent coating. The coating exhibits third 
order nonlinear optical susceptibility .chi..sup.(3). 
EXAMPLE II 
This Example illustrates a poling procedure for producing a second order 
nonlinear optical side chain liquid crystalline polymer in accordance with 
the present invention. 
A. Poling Cell Construction 
A poling cell is constructed from electrically conductive glass plates, 
such as Donnelly Mirror PD 5007-7. The glass plates are washed with 
sulfuric acid, isopropanol, l-dodecanol, and isopropanol, with a distilled 
water rinse between each washing step. 
The poling cell is a sandwich type cell in which the conductive glass 
surfaces are in facing proximity and are separated by a polyimide film of 
approximately 25 micrometer thickness. A thin layer of epoxy adhesive is 
applied on the surfaces of the polyimide film to hold the glass plates. 
After the epoxy is completely cured, the cell is washed with isopropanol 
and rinsed with distilled water. After drying, the cell is stored in a dry 
box. 
B. Filling The Poling Cell 
Poly[4-(6-hydroxyhexyloxybenzylideneamino)-4'-nitrodiphenyl methacrylate] 
of Example I is placed in a vacuum oven and maintained in a melt phase at 
a temperature of about 120.degree. C. for about 4 hours to eliminate 
entrained air bubbles from the polymer melt. 
The liquid crystalline polymer melt is introduced into the space between 
the glass plates by charging a drop of the polymer melt to one of the 
openings of the poling cell space and placing the cell assembly in a 
vacuum oven maintained at a temperature approximately 10.degree. C. above 
the clearing temperature of the liquid crystalline polymer. The cell space 
fills gradually by capillary action. The space filling period is about 4 
hours for a 0.5 cm long space. The liquid crystalline polymer melt in the 
filled cell is bubble-free. 
C. Electric Field Induced Orientation 
Two lead wires are attached to each of the conductive glass surfaces using 
electrically conductive epoxy adhesive. The poling assembly is placed in a 
microscope hot stage (Mettler FP-82 with FP-80 Central Processor), and the 
sample is observed with a polarizing microscope (Leitz Ortholux Pol) for 
alignment. 
The microscope is switched into a photodiode (Mettler Photometer No. 17517) 
to record the change of light intensity upon application of an electric 
field. The two lead wires are connected to an AC voltage amplifier 
(Electro-Optic Developments LA10A), which amplifies the voltage signal 
from a signal generator (Hewlett-Packard No. 3310B). 
The poling cell first is heated to 85.degree. C. to bring the liquid 
crystal polymer to the isotropic phase. The assembly then is cooled at a 
rate of 0.2.degree. C./min. until the photodiode signal registers an 
abrupt increase which indicates that the melt has undergone a transition 
into a liquid crystalline phase. The temperature is further lowered by 
2.degree. C. and then maintained at this temperature. 
The AC voltage source is set at 500 V, and the frequency is set at 2000 Hz. 
The power to the poling cell is turned on to apply an electric field 
across the liquid crystalline sample. The field strength is calculated to 
be approximately 2.times.10.sup.5 V/cm. About three seconds after the 
electric field is applied, the photodiode signal drops close to the 
baseline, indicating that orientation development induced by the electric 
field is completed. At this point, the cooling is resumed until the 
temperature reaches 35.degree. C., and the poling assembly is disconnected 
from the power source. 
When the poling assembly is removed from the microscope hot stage, by 
visual observation the liquid crystalline polymer in the cell space is 
transparent. This is an indication that the molecular orientation is 
uniform and homogeneous throughout the sample. Orientation of the sample 
is further ascertained utilizing a wide angle X-ray diffraction technique, 
and the Hermann's orientation factor of the sample is approximately 0.9. 
D. High Field Poling For Symmetry Control 
The oriented liquid crystal sample is subjected further to a higher 
electric field to develop a noncentrosymmetric orientation of nonlinear 
optical moieties which are a part of the side chains of the polymer. 
The poling cell assembly is heated to approximately 5.degree. C. below the 
glass transition temperature of the polymer. Then the lead wires of the 
poling assembly are connected to a DC voltage source (Kepco OPS-3500) and 
the voltage is turned up slowly until it reaches 2000 V. At this point, 
the electric field strength across the sample is about 8.times.10.sup.5 
V/cm. The sample is maintained at this field strength level for 24 hours, 
then the sample is cooled and the voltage source is disconnected. A 
noncentrosymmetrically oriented liquid crystalline polymer transparent 
solid phase is obtained by this procedure. 
The noncentrosymmetry of the sample is determined from the wide angle X-ray 
diffraction measurement and the thermally stimulated electrical discharge 
measurement. The Hermann's orientation function from the X-ray measurement 
is approximately 0.9. 
From the measurements, there is an indication that a major proportion of 
the nonlinear optical moieties are aligned parallel to the electric field 
direction, and the rest are oriented antiparallel to the electric field 
direction.