JPH0240625A - Liquid crystal electrooptic device - Google Patents

Abstract

PURPOSE:To recover the display function even when the liquid crystal electrooptic device is exposed to low temperature, then the temperature rises by constituting the liquid crystal electrooptic device by using ferroelectric liquid crystal having a crystal layer which does not decrease in smectic layer interval in a crystal phase. CONSTITUTION:The ferroelectric liquid crystal material 6 is charged hermetically between two electrode substrates 1 and 2 which are arranged at an interval of 2-10mum in parallel to each other. A liquid crystal cell is heated into an anisotropic liquid phase, then cooled on the whole gradually at a speed of 0.1-1.0 deg.C per minute into a chiral smectic C phase. Consequently, liquid crystal molecules are oriented as shown in a figure (b) and the layer direction of the smectic phase 20 crosses the liquid crystal molecules 10 almost at right angles. In such a state, an electric field is applied from the top side of the paper surface to the reverse side and then the liquid molecules tilt to the left at a tilt angle theta as shown in a figure (a). When, the electric field is applied from the reverse surface of the paper surface, the liquid crystal molecules are reoriented as shown in a figure (c). Thus, liquid crystal cells are sandwiched between a 102-directional polarizing plate and an orthogonal polarizing plate, so that a figure (b) shows a drak display and figures (a) and (c) show bright displays.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a liquid crystal electro-optical device, and for example, an optical shutter or a wall-mounted device is suitable for use in a flat display device such as a television.

[Conventional technology]

The mainstream of electro-optical devices using liquid crystals is a twisted nematic (TN) mode display in which a nematic liquid crystal has a twisted structure, which is used in watches, calculators, and automobile meters. - On the other hand, smectic liquid crystals, especially chiral smectic C and H phases that exhibit ferroelectricity, exhibit high-speed response.

Reports on phase stability (Appl, Pbys, Lett,
36,899° in 1980), research and development has been progressing to develop new application fields for liquid crystals.

Now, if we consider the operating temperature range of a display element using such a liquid crystal, first of all, in a TN liquid crystal display element, when used in automotive meters, the temperature range of the nematic phase is 130°C.
°C to +85 °C liquid crystal (e.g. ZL I-1565
(manufactured by Merck) is used. The inner surface of the glass substrate of a liquid crystal display device is subjected to alignment treatment to align liquid crystal molecules in one direction, and the liquid crystal molecules are aligned in one direction in a nematic phase. When the operating temperature drops below the nematic temperature, the liquid crystal undergoes a phase transition to a crystalline phase and loses its display function.

Next, when the operating temperature rises and a phase transition occurs from the crystalline phase to the nematic phase, the display function as a TN liquid crystal is restored as usual, and the display can be performed as before.

In other words, if the inner surface of the glass substrate of a display element is subjected to alignment treatment, the liquid crystal molecules will be oriented back to their original state as long as the temperature is returned to the original state after cooling and phase transition to crystals.

On the other hand, in the case of the ferroelectric liquid crystal element described later, for example, a ferroelectric liquid crystal material was synthesized for the first time by Meyer et al. (L, de Phys, L69, 1975).
P-decyloxybenzylidene P'-amino 2-methylbutyl cinnamate (DOBAMBC) was used as an alignment treatment method such as rubbing treatment or spacer edge method (Jpn, J
, Appl, Phys, 23. L211.19
(1984), a smectic layer is formed almost uniformly in the chiral smectic C phase almost perpendicular to the rubbing direction in the rubbing method and almost perpendicular to the spacer edge in the spacer edge method. Therefore, when observed under orthogonal polarizing plates, almost mono-domains are formed, and therefore, it shows brightness and darkness in the direction of the applied electric field. When this display element is subjected to a phase transition into a crystal by lowering the temperature in the same way as the TN element described above, the monodomain obtained in the chiral smectic C phase is broken and the display function is lost. When the temperature of this display element is raised again to cause a phase transition from the crystalline phase to the chiral smectic C phase, the monodomains in which the smectic layers previously obtained in the chiral smectic C phase are arranged almost uniformly remain broken, and the display Functionality will not be restored.

[Problems to be Solved by the Invention] Although it is desired to develop a liquid crystal electro-optical device using a ferroelectric smectic phase, it is particularly difficult to develop a liquid crystal electro-optical device for use in automobiles, and for liquid crystal electro-optical devices used in cold regions. If you think about it, there is a problem with the lower limit of the operating temperature range. In other words, as mentioned earlier, in a ferroelectric liquid crystal element, the smectic layer, which is a ferroelectric smectic phase and is arranged in a nearly uniform direction, decreases in temperature and undergoes a phase transition to a crystal, and then the temperature rises again and the smectic layer becomes ferroelectric. When a phase transition occurs to a smectic phase, the previously uniform alignment of the smectic layers is destroyed, resulting in a serious drawback in that the display function as a display element is lost.

In order to arrange this broken smectic layer in a nearly uniform direction, it is necessary to raise the temperature to an isotropic liquid phase and then gradually cool it down to a ferroelectric smectic phase at a rate of about 0.1 to 1°C per minute. be. In order to prevent the uniform orientation of this smectic layer from being broken due to crystallization, one possible countermeasure is to lower the lower limit of the ferroelectric smectic phase more than necessary.
It is difficult to synthesize liquid crystal materials that exhibit a ferroelectric smectic phase at low temperatures, and there is also a limit to lowering the temperature due to the blending effect of various types of liquid crystals.

Therefore, in the present invention, the uniformity of the smectic layer, which was uniformly oriented in the ferroelectric smectic phase, is preserved as it is even when the temperature decreases to the crystalline phase and the phase transitions to the crystalline phase. An object of the present invention is to provide a liquid crystal electro-optical device capable of displaying as a display element without destroying a smectic layer that has been oriented in advance even if the temperature is increased to a phase.

[Means to solve the problem]

To achieve the above object, the present invention provides a liquid crystal electro-optical device in which a ferroelectric liquid crystal is sandwiched between a first electrode substrate and a second electrode substrate arranged at a predetermined distance. , the ferroelectric liquid crystal is configured such that a voltage for forming an electric field is applied to the first and second electrode substrates, and the ferroelectric liquid crystal is a liquid crystal having a crystal phase in which the smectic layer spacing is not reduced; This liquid crystal is a ferroelectric smectic phase, which undergoes a phase transition to a crystalline phase while maintaining a smectic layer structure arranged in approximately one direction during the cooling process, and after the phase transition from the crystalline phase to the ferroelectric smectic phase during the heating process, The structure is such that the smectic layer structure arranged in one direction returns to its original state.

[Effect]

In the liquid crystal electro-optical device having the above configuration, since the ferroelectric liquid crystal has a crystal phase that preserves the smectic layer structure in the ferroelectric smectic phase, the liquid crystal electro-optical device can be heated to a temperature low enough to transition to the liquid crystal phase. After being exposed and returned to the ferroelectric smectic phase, the display function is restored as in the case of a conventional TN type liquid crystal electro-optical device, and display performance comparable to that before cooling can be exhibited.

〔Example〕

FIG. 1 shows the structure of a liquid crystal electro-optical device according to a first embodiment of the present invention. For example, two electrode substrates 1.
A ferroelectric liquid crystal material 6, which will be described later, is sealed between the holes 2 and 2. As shown in FIG. 1, the electrode substrate 1 includes an electrode 1a made of a transparent conductive film such as indium oxide or tin oxide along the inner surface of a transparent substrate 1C made of transparent glass or resin.
It is formed. The other electrode substrate 2 also has a similar configuration. On the inner surfaces of the transparent electrodes 1a and 2a, there is an alignment glib of a polymer film that has been subjected to alignment treatment to align liquid crystal molecules parallel to the substrate.

2b is placed. In addition, other techniques that generally align liquid crystals, such as rubbing the electrode substrate, obliquely vapor depositing silicon oxide or the like on the surface, or treating with a surfactant, can be applied. These electrode substrates 1 and 2
The liquid crystal cells are assembled in parallel so that the liquid crystals are aligned in one direction.

TFHPOBC represented by the following chemical formula was heated to an isotropic liquid phase and injected into this cell.

(4-(1-total 1jluor methyl hept
Oxy carbonyl Bhenyl) 4' 4ct
yl biphenyl-4-carboxylate
) The phase transition of this compound was measured using differential thermal analysis (DSC) and a polarizing microscope, and the result was that it became like a batter.

81.7°c 111.0°C120,7
°CCr S m C” ←---S m
A-1 Where, Cr: crystalline phase, SmC": chiral smectic C phase (ferroelectric liquid crystal phase), Sm
A: Smectic A phase, 1: Isotropic liquid phase. The DSC measurement results are shown in FIG. 4, and it can be seen that they closely approximate the polarization microscope results.

Now, after heating the liquid crystal cell containing the ferroelectric liquid crystal to make it into an isotropic liquid phase, the entire liquid crystal cell is heated at a rate of 0.1 to 1
．． It is slowly cooled at 0°C until it reaches the chiral smectic C phase. As a result of such cooling, the liquid crystal molecules 10 in the chiral smectic C phase are oriented as shown in FIG. 2(b). At this time, the layer direction of the smectic phase 20 is formed in a direction substantially perpendicular to the liquid crystal molecules IO. When an electric field is applied from the front side to the back side of the paper with the smectic layer formed almost uniformly in this manner, the liquid crystal molecules are tilted to the left by a tilt angle θ as shown in FIG. 2(a). Next, when an electric field is applied from the back to the front of the paper, the liquid crystal molecules will move as shown in Figure 2 (C
). 102 liquid crystal cells with this configuration
If it is placed between a polarizing plate in the direction of , and a polarizing plate in a direction perpendicular to the polarizing plate, it is possible to display the image in FIG. 2(b) in the dark state and in FIG. First, the contrast ratio between bright and dark when 30 V was applied at 100° C. was about 1:10.

Next, we left the liquid crystal cell, which can display bright and dark images by applying an electric field, at room temperature (22°C), and confirmed that the filled liquid crystal had transitioned to a crystalline phase, and that the liquid crystal molecules IO did not move due to the application of an electric field. I confirmed it. At this time, the liquid crystal cell was observed under a microscope, and it was found that the alignment state of the smectic layer in the chiral smectic C phase was maintained as it was.

Next, this liquid crystal cell was raised again to the chiral smectic C temperature range and an external electric field was applied. ), if an electric current is applied from the back of the paper to the front (
It was confirmed that the state was switched to state C). In addition, 1
The contrast ratio between bright and dark when applying 30V at 00°C is
The ratio was 1:10, which was almost the same as before cooling.

In order to investigate that the smectic layer structure, which was oriented almost in one direction in the chiral smectic C phase, is not superior even after undergoing a history of lowering the temperature and undergoing a phase transition to the crystalline phase, the X Layer spacing was measured by line diffraction. The X-ray diffraction peak of the polydomain of the ferroelectric liquid crystal TFHPOBC was measured using RU-200 (Rigaku Corporation).
Figure 5 shows the temperature change in the layer spacing calculated using the peak value. The position of the diffraction peak indicating the smectic layer spacing hardly changes in the measured temperature range, indicating that the smectic layer spacing is almost constant. That is, 29.5 to 31 through the crystalline phase, chiral smectic C phase, and smectic A phase.
．． 5 people, showing an almost constant value.

Next, another ferroelectric liquid crystal compound P-desyloxyhenzylidene P'-amino 2-methylbutylcinnamate (DO
BAMBC) C1゜t(z+0@-CI(=N勺-CH, CH-C-
0-CH□-G)[-(:2H5 was investigated. This ferroelectric liquid crystal exhibits the following phase change.

N Here, S□ indicates a smectic A phase.

Regarding this ferroelectric liquid crystal DOBAMBC, unlike the above embodiment, the smectic layer arranged in the chiral smectic phase undergoes a phase transition to the crystalline phase, and when it returns to the chiral smectic phase again, the smectic layer structure collapses.
It loses its function as a display element. The aforementioned DOBA
Regarding the temperature change in the smectic layer spacing of MBC, l
Idaka et al.
ns, 1984. vo14. P225-23
9, when looking at the temperature dependence of the interlayer spacing, it can be seen that the interlayer spacing changes significantly due to the transition from the chiral smectic C phase to the chiral smectic H phase. In this paper, the layer spacing was measured only up to 60°C, but as a result of measuring the layer spacing at room temperature (35°C), the old da
The data were almost equal to those of ka et al. In DOBAMBC, where the layer structure is broken in this way, the smectic interlayer spacing is crystalline, S
, , 5cSA phase differs greatly.

Next, a second embodiment of the present invention will be described.

A ferroelectric liquid crystal represented by the following chemical formula was heated to an isotropic liquid phase and injected into the same liquid crystal cell as in the first example.

(4-(1-Monofluor methyl no
Noxy carbon! phenyl)-4'-0
ctyl biphenyl-4-carboxyla
te) The phase transition of this compound was measured using DSC and a polarizing microscope, and the results were as follows.

S, H" Now, after heating the liquid crystal cell filled with the ferroelectric liquid crystal to cause a phase transition to an isotropic liquid phase, the entire liquid crystal cell is heated at 0 per minute.
．． Chiral smectic C
Cool to phase. At this time, the smectic layer 20 is aligned substantially in one direction as shown in FIG. 3, and the liquid crystal molecules 10 are aligned in the same direction as shown in FIG. 3(a) when an external electric field is applied.

It moves like (b). That is, when the external electric field is from the back of the paper to the front, switching occurs between two states as shown in FIG. 3(b), and when the external electric field is from the front to the back of the paper, as shown in FIG. 3(a).

At this time, (a), (b) when the polarizing plate is sandwiched between the polarizing axis in the direction shown in FIG. 3 102 and the polarizing plate in the direction orthogonal to this.
) had a contrast ratio of about 1:15. After lowering the temperature of this liquid crystal cell to the liquid crystal phase and causing a phase transition to the liquid crystal phase, the temperature was raised again and the orientation state 2 display characteristics of the liquid crystal molecules were investigated in the chiral smectic C phase in the same manner as before cooling. The state was the same as before the phase transition to the crystalline phase, there was no break in orientation, and the contrast ratio was as good as 1:15. Note that the X-ray diffraction results for the layer spacing are for the TF of the first example.
It showed the same tendency as HPOBC.

As is clear from the above, in a ferroelectric liquid crystal that has a crystal phase in which the smectic layer spacing does not decrease, even if there is a history of crystal phase transition during cooling, the smectic layers are aligned in the ferroelectric smectic phase. is not disturbed, and a --like orientation can be maintained.

This applies not only to the liquid crystal compound used in the above example, but also to all ferroelectric liquid crystals that have a crystal phase in which the smectic layer spacing is hardly reduced. The same applies to a liquid crystal mixture of two or more kinds of compounds.

Although not specifically mentioned in the above embodiments, the same applies even if the liquid crystal has dichroism or is a mixture with a dichroic dye.

Furthermore, in the structure of the liquid crystal cell or in the electrode substrates 1 and 2, a plurality of striped transparent electrodes are formed in parallel, and these substrates 1 and 2 are arranged so as to be perpendicular to each other. A matrix type display device can also be formed by connecting an external power supply including a circuit that can drive the display device.

[Effects of the Invention] The present invention as described above constitutes a liquid crystal electro-optical device using a ferroelectric liquid crystal that has a crystalline phase and a crystal layer in which the smectic layer spacing does not decrease. Even if the temperature rises after being exposed to a low temperature that causes a transition to a crystalline phase, the display function is restored and display performance comparable to that before cooling can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a side sectional view showing the configuration of a liquid crystal electro-optical device showing an embodiment of the present invention, and FIGS. 2(a), (b), (C)
3(a) and 3(b) are schematic diagrams explaining the motion of liquid crystal molecules in the second embodiment, and FIG. 4 is a schematic diagram illustrating the motion of liquid crystal molecules in the first embodiment. A characteristic diagram showing the results of differential thermal analysis (DSC) of the ferroelectric liquid crystal in the example of
FIG. 5 is a characteristic diagram showing the smectic layer spacing versus temperature of the ferroelectric liquid crystal in the first embodiment. 1... First electrode substrate, 2... Second electrode substrate, 4
゜5...Polarizing plate, 6...Ferroelectric liquid crystal, 10...
liquid crystal molecules. 20...Smectic layer.

Claims (1)

[Scope of Claims] A liquid crystal electro-optical device in which a ferroelectric liquid crystal is sandwiched between a first electrode substrate and a second electrode substrate arranged at a predetermined distance, comprising: The ferroelectric liquid crystal is configured such that a voltage for forming an electric field is applied to the second electrode substrate, and the ferroelectric liquid crystal is a liquid crystal having a crystal phase in which the smectic layer spacing does not decrease; The smectic layer structure, which is a smectic phase and is aligned in almost one direction, undergoes a phase transition to a crystalline phase while maintaining it during the cooling process, and after the phase transition from the crystalline phase to the ferroelectric smectic phase in the heating process, the smectic layer structure is aligned in almost one direction. A liquid crystal electro-optical device characterized in that the smectic layer structure is restored.