Production of a ferroelectric or antiferroelectric or ferrielectric order in liquid crystals which solidify in a glass-like manner

The process according to the invention is a process for the production of aligned, ferroelectrically or ferrielectrically or antiferroelectrically ordered materials which solidify in a glass-like manner. In these layer structures with polar order, the molecular dipole moments are uncompensated, partially compensated or fully compensated, so that a dipole moment corresponding to the degree of compensation, acts externally.

The present invention relates to a process for the production of a 
ferroelectric or antiferroelectric or ferrielectric order in 
antiferroelectric, smectic, liquid-crystalline liquid crystals which 
solidify in a glass-like manner. 
There exist liquid-crystalline polymers in which the liquid-crystalline 
phase has been frozen in a nematic alignment or in an orthogonal layer 
structure (orthogonal smectic phases) or in a tilted layer structure 
(tilted smectic phases) (cf. DE-A 39 17 196). Low-molecular-weight, 
tilted, ferroelectric, smectic, liquid-crystalline materials which have 
solidified into a glass phase are also known. 
It is furthermore known that antiferroelectric or ferrielectric phases 
occur in low-molecular-weight liquid crystals (Jap. J. Appl. Phys., 28 
(1989), L1265), but these cannot be frozen in a glass-like manner. 
It is also known that antiferroelectric phases occur in polymeric 
liquid-crystalline phases (Third Int. Conference on Ferroelectric Liquid 
Crystals, Boulder, Colo., 24-28 Jun. 1991). 
The known materials can advantageously be used in the areas of integrated 
optics, opto-electronics and data storage, illustrative uses being in 
opto-electronic equipment, such as displays, opto-electronic shutters, 
opto-electronic diaphragms, memory elements, optical modulators, printer 
heads and multifocal lenses (H. Finkelmann in Polymer Liquid Crystals, 
Eds. A. Ciferri, W. R. Krigbaum, R. B. Meyer, Academic Press, 1982). 
However, the materials disclosed hitherto have disadvantages, for example 
unstable states in the glass state 
ferroelectric liquid-crystalline polymers are only capable of bistable 
switching between two states 
poor processing properties (for example in the production of devices) 
problems with switchability excessively long response times 
too narrow state ranges of the phases 
not freezable in the antiferroelectric phase structure. 
It is an object of the present invention to find novel liquid-crystalline 
states or phases which have solidified in a glass-like manner, have a 
polar order and do not have the disadvantages of the prior art. A further 
object of the present invention is to find a process for the production of 
liquid-crystalline states which have been frozen in a glass-like manner. 
We have found that these objects have been achieved by a process for the 
production of a ferroelectric or antiferroelectric or ferrielectric order 
in liquid crystals which solidify into a glass phase, by heating a 
mesogenic compound to above the glass transition temperature, which is 
above 25.degree. C., preferably above 35.degree. C., very particularly 
preferably above 45.degree. C., and subsequently cooling the compound to 
below the glass transition temperature. 
The materials to be used according to the invention are chiral compounds or 
chiral polymers which have liquid-crystalline states within certain 
temperature ranges and solidify into a glass phase below certain 
temperature ranges, a layer structure being frozen in the glass state of 
the polymer. Within a layer of this layer structure, the centers of 
gravity of the polymer side chains or the chiral compounds are in a random 
or ordered distribution. The director n, which defines the preferential 
direction of the polymer side chains, has a tilt to the layer 
perpendicular z, indicated by means of the angle between n and z. The 
angle is known as the tilt angle. The tilt angle can either be present 
inherently or induced by external forces, for example electrical and/or 
magnetic fields and/or shear forces. 
For example, an electrical field which has either a positive or negative 
sign and a strength greater than the critical field strength of from 3 to 
40 V/.mu.m to induce the desired tilt angle. The application of an 
electrical field of from less than 2 to 40 V/.mu.m, which is below the 
critical field strength and is thus of such a magnitude that no complete 
ferroelectric order is produced, so that the molecular dipole moments are 
not fully ordered, a macroscopic dipole moments results which is 
proportional to the applied electrical field. If little or no electrical 
field is applied during the cooling operation the molecular transverse 
dipole moments of the mesogenic compounds may be ordered, in macroscopic 
terms, so that their macroscopic dipole moment is zero or virtually zero. 
The materials to be used according to the invention, which are in a layer 
structure with induced or inherent tilt, should have polar properties in a 
microlayer, such as, for example, in a chiral smectic liquid-crystalline C 
phase (S.sub.c * phase). However, the polar properties of a microlayer 
should be partially or fully compensated by the polar properties of the 
directly adjacent layers. 
Such behavior is known as ferrielectric or antiferroelectric. 
These materials are preferably employed in the following areas: 
electrical, magnetic and/or optical storage systems, 
electrophotography 
as electronic components or as constituents of electronic components 
as electro-optical components or as constituents of electro-optical 
components 
in printing processes. 
Surprisingly, the materials to be used according to the invention, when 
subjected to the process according to the invention for the production of 
a ferroelectric or antiferroelectric or ferrielectric order in 
antiferroelectric, smectic, liquid-crystalline liquid crystals which 
solidify into a glass phase, have advantageous properties, for example 
shorter response times 
tristable switching 
broad state ranges 
freezability of the polar order 
field-dependent polar order.

EXAMPLES 
General experimental procedure 
Sample preparation: 
The sample was prepared in cells between two structured, plane-parallel 
glass plates which had electroconductive coatings and onto which a 
polyimide alignment layer had been applied by known methods. The layer 
thickness of the liquid-crystalline sample was on average 4 .mu.m and was 
determined for each measurement cell by interferometry. 
The measurement cell was filled with the substance in the isotropic phase 
by means of capillary forces. To this end, the cell containing the 
substance applied to the edge was heated to a temperature above the 
clearing point. Due to the capillary effect of the cell, the material was 
drawn into the cell cavity and was then slowly cooled into the 
liquid-crystalline phase. In combination with the effect of the alignment 
layer, this caused the desired planar edge alignment of the liquid 
crystal. In order to improve the alignment, an electrical and/or magnetic 
field can also be applied. Polarization and tilt angle measurements: 
In order to determine the tilt angle and the spontaneous polarization, the 
sample to be investigated was prepared and aligned in a cell by the 
abovementioned method. The temperature control was carried out using a 
Mettler FP 800/85 microscope heating stage. 
The spontaneous polarization was determined by the triangle method (K. 
Miyasato et al., Jap. J. Appl. Phys. 22 (1983), L661). The voltage signal 
used for this purpose was generated by means of a function generator 
(Wavetek 273) and amplified by means of a power amplifier (Krohn Hite 
7500). The current was recorded as a function of time by a storage 
oscilloscope (Hewlett Packard HP54501). The spontaneous polarization was 
then determined from the measured time dependence of the current. 
The sign of the polarization was determined in accordance with the 
convention of Lagerwall et al. (S. T. Lagerwall, I. Dahl, Mol. Cryst. Liq. 
Cryst., 114 (1984), 151). 
The tilt angle was determined by measuring the switching angle. 
In the initial state, the director n of the tilted smectic layer in an 
applied electrical field +E is parallel to the direction of the polarizer 
and perpendicular to the analyzer. The sample appears dark. If the 
electrical field is switched from +E to -E, the director n is rotated 
through an angle .alpha., the switching angle, which corresponds to twice 
the tilt angle. By rotating the microscope rotating stage beyond the layer 
perpendicular z until maximum extinction is obtained again, the switching 
angle, which corresponds to the angle of rotation of the microscope stage, 
is determined. 
The response time is determined from the change in transmission on changing 
the field strength from -E to +E. The following procedure was used: 
The sample was aligned between crossed polarizers so that the sample 
appeared dark with the field +E applied. A field of -E was then applied. 
The sample became bright. In order to determine the switching time, the 
field was rapidly changed from -E to +E, while at the same time the change 
in transmission was followed by means of a photodiode. The response time 
was determined as the 10%/90% value. 
Suitable materials to be used according to the invention are substances as 
described in DE-A 39 17 196 which corresponds to U.S. Pat. No. 5,187,248. 
The polymer materials are very highly suitable if the dispersity is low 
(&lt;1.2) and the molecular weight of the polymers is in the range from 2000 
to 10,000 g/mol. 
The materials to be used according to the invention have a uniaxial phase 
structure optically in the absence of an electrical field. 
The materials to be used according to the invention also have a pronounced 
jump behavior of the tilt angle within the liquid-crystalline phase and 
above the glass phase. This means that if a certain applied field 
strength, depending on the substance and temperature, is exceeded, the 
liquid crystal switches from the antiferroelectric or ferrielectric order 
to the ferroelectric order. 
This pronounced jump behavior of the tilt angle is associated with the flow 
of a polarization current corresponding to the change in dipole density. 
In all the examples, the samples were prepared as described above. 
The following liquid-crystalline material was used for Examples 1 to 3: 
##STR1## 
The material has the phase behavior G1 45 S.sub.c.sup.*.sub.A 134 S.sub.S 
160 I. 
EXAMPLE 1 
Preparation of an antiferroelectrically aligned glass 
No electrical field is applied to the cell filled with the abovementioned 
liquid-crystalline material during the cooling operation into the glass 
phase. The antiferroelectric order of the microlayers forms as described 
above. The cooling rate is unimportant. An optically uniaxial, orthogonal 
structure which has no surface polarization is formed. The optical tilt 
angle within the sample treated in this way is zero. 
EXAMPLE 2 
Preparation of a ferrielectrically aligned glass 
An electrical field is applied to the cell filled with the abovementioned 
liquid-crystalline material during the cooling operation into the glass 
phase. This electrical field is smaller than the transition field strength 
E.sub.t above which the ferroelectric order forms. The ferrielectric order 
of the microlayers forms as described above. The cooling rate is 
unimportant. An optically biaxial, tilted structure is formed. The tilt 
angle and thus also the dipole density within the sample treated in this 
way depends on the applied field strength E. The sign of the surface 
polarization is affected by the field direction. 
______________________________________ 
Surface 
Applied field Tilt angle 
polarization 
V/.mu.m grd Sign 
______________________________________ 
+25 10 + 
+37 15 + 
-25 10 - 
-37 15 - 
______________________________________ 
EXAMPLE 3 
Preparation of a ferroelectrically aligned glass 
An electrical field is applied to the cell filled with the abovementioned 
liquid-crystalline material during the cooling operation into the glass 
phase. This electrical field is greater than the transition field strength 
E.sub.t above which the ferroelectric order forms. The cooling rate is 
unimportant. 
An optically biaxial, tilted structure is formed. The tilt angle no longer 
depends on the applied field above the transition field strength. The 
dipole density within the sample treated in this way is constant and 
independent of the applied field strength E. The sign of the surface 
polarization is affected by the field direction. 
EXAMPLE 4 
Shorter response times in the antiferroelectric structure 
The following liquid crystal was used for Example 4: 
##STR2## 
The phase sequence of this substance is 
EQU G1 45 S.sub.c.sup.*.sub.A 160 S.sub.A 185 I 
The sample was introduced into the cell and aligned by the method described 
above. The response time experiments were carried out in the 
liquid-crystalline phase at 140.degree. C. The response time was 
determined at a field strength of 10 V/.mu.m and 17.5 V/.mu.m. The 
response time from a contrast of 10% to a contrast of 90% is 40 .mu.s at a 
field strength of 10 V/.mu.m (this is a ferrielectric alignment), whereas 
the response time from a contrast of 10% to a contrast of 90% is 115 .mu.s 
at a field strength of 17.5 V/.mu.m (this is a ferroelectric alignment). 
EXAMPLE 5 
Stability of the glass state 
The same material was used and the samples were prepared in the same way as 
in Examples 1 to 3. The samples were slowly cooled to the glass state at 
an applied field strength of 20 V/.mu.m. No change in the sample and the 
frozen glass state was detectable over a period of more than 3 months. 
Tilt angle after freezing: 10.degree. 
Tilt angle after 6 months: 10.degree.