JPH04353831A - lcd shutter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal shutter, and more particularly to a liquid crystal shutter that reversibly exhibits an optically transparent state and a light scattering state by applying or blocking an alternating current electric field.

0002

[Prior Art] Liquid crystal elements can be made lightweight and thin, so they can be used as displays in pocketable calculators, testers, etc., or as decorations.
It is widely used as a device for displaying figures and characters mainly on a flat surface for OP purposes, and more recently it has been put into practical use as a liquid crystal television that displays moving images in full color using thin film transistors. The principle of shutter operation in these is called the twisted nematic mode (hereinafter referred to as TN type), which is a well-known technology (Kobayashi and Okano, eds., "Liquid Crystal", 1985, Baifukan).
.

However, in addition to being used as display elements, liquid crystals also have the potential to be used as functional materials that exhibit various functions, and are being extensively studied. As one of these possibilities, attempts are being made to research and develop larger-area light-shielding shutters. Specifically, for indoor use,
Applications are being considered for large vehicle inspection windows, plastic film curtains, blindfold windows, etc. From this point of view, the TN type requires alignment treatment using an organic material such as polyimide on a glass substrate and two polarizing plates, so it is not necessarily suitable as a technology for manufacturing large, lightweight, and thin shutters. This is because liquid crystals remain fluid and lack self-support, so they require a smooth and rigid substrate to support the weight of the liquid crystal itself, and glass or thick plastic are often used. For this reason, increasing the size not only increases the weight, but also makes it difficult to inject and align the liquid crystal, and also makes it difficult to align the liquid crystal phase itself. In addition, polarizing plates cause a significant decrease in light transmittance and durability, and are not suitable at all for liquid crystal shutters, etc., which require both high transparency and light-shielding properties as described above.

[0004] In order to create a thin and lightweight shutter with a large surface area, it is necessary to have an operation mode that does not require perfect alignment of liquid crystals or a polarizing plate, and also to be made of a self-supporting material, which requires printing technology and lamination technology in manufacturing. It is hoped that it can be used. Composites of liquid crystals and polymer binders have recently attracted attention as a solution to these demands. This is a polymer dispersed liquid crystal film (hereinafter simply referred to as PDLC).
．． W. DOANE et al. , Mol. Cryst
．． Liq. Cryst. , vol 165, 533 (1
988), Japanese Patent Application Laid-Open No. 60-252687). The structural feature of this is that liquid crystals are dispersed in a polymeric binder in grains of the size of visible wavelengths, or in a more complex network, as shown in Figure 4 (a). That's true. The state of dispersion depends on the manufacturing method. The origin of the shutter property is the alternating appearance of a light scattering state and a transparent state. As shown in FIG. 4B, when an alternating current electric field is applied, the refractive index of the binder and the dispersed liquid crystal become approximately the same, and the liquid crystal does not scatter light and becomes transparent. In the case of mismatch, light is scattered due to non-uniform refractive index distribution, as shown in FIG. 4(A).

However, the disadvantages of PDLC are that it is a two-phase composite; (1) the effective voltage applied to the liquid crystal portion decreases;
(2) Unreacted binder molecules remain in the liquid crystal, reducing its purity, which increases the operating voltage; (3) Due to the interaction between the liquid crystal and the binder, the binding to the liquid crystal is stronger than in normal TN cells. To compensate for this, the driving voltage is generally increased more than in pure TN operation. Furthermore, since the liquid crystal and binder are not completely separated, it is difficult to adjust the refractive index, resulting in a hazy appearance and complete transparency. These are essentially due to the fact that PDLC is a two-phase system with phase separation.

On the other hand, polymeric liquid crystals with excellent self-supporting properties are available as liquid crystalline materials that are lightweight and can be made into large-area films. Attempts have been made to operate a TN-type light-shielding shutter, but so far, the expected characteristics have not been achieved due to the high viscosity of synthesized liquid crystals, slow response speed, and high operating temperature range. do not have. Even if these problems are solved, it is difficult to increase the area in the TN mode due to the above-mentioned problems.

JP-A-64-6008 discloses an attempt to manufacture a film shutter using a siloxane-type polymer liquid crystal with ferroelectric mesogen as a side chain, but it is still very difficult due to problems such as liquid crystal alignment. At present, area shutters are far from being put to practical use. In general, the framework for achieving large-area shutter properties in polymer liquid crystals is not clear, and we are still searching in the dark.

The present inventors have discovered that a polymer liquid crystal/low molecular liquid crystal mixed system exhibiting a cholesteric phase can also be produced in the Grandjean state shown in FIG. 1 and the focal conic state shown in FIG. It is possible to take the homeotropic state shown in Figure 3 (referred to as the FC state), and we found that the FC state in particular has a much stronger light scattering ability than low-molecular liquid crystals and is stable over time. . This means that an optically transparent homeotropic state can be formed by applying an alternating electric field, so it can be used as a reversible optical shutter. The FC state has a light scattering ability as strong as that of PDLC, but because it is not a completely stable state like PDLC, the light scattering ability deteriorates in about a few minutes after the electric field is cut off, that is, it is a state with slightly higher transparency. transition to and stabilize. This situation is shown in the graph change in FIG. 6(a).

[0009] The light scattering ability in this final state increases proportionally as the thickness of the liquid crystal increases, but there are problems such as an increase in driving voltage. The degree of white turbidity required varies depending on the purpose of use, but there may be cases where very strong light-shielding properties are required. Transmittance) is shown in Figure 6 at the point (
It is necessary to maintain it at about the position M).

The light scattering ability in the equilibrium state also shifts to a state close to the Grandjean state shown in FIG. 1, or shifts from the FC state in which the helical axis of the FC state is a fingerprint state where it is flat with the substrate to the FC state where it is slightly inclined. Therefore, it will deteriorate in the long run. This tends to be accelerated as the temperature rises, and it is thought that this is because the viscosity of the liquid crystal decreases. Furthermore, when response speed is increased and low voltage operation is required, it is necessary to reduce the polymer liquid crystal component and increase the low molecular liquid crystal component. In this case, it is often difficult to maintain the light scattering state. Therefore, Figure 6
It is necessary to maintain a strong light scattering state similar to the position of the point (M) shown in FIG. 2 over a wide temperature range.

[0011]

Problems to be Solved by the Invention The present invention provides a polymer liquid crystal/low molecular liquid crystal mixed system exhibiting a cholesteric phase.
The objective is to provide a driving method that maintains the stability of the strong light-scattering focal conic state after the electric field is cut off, and makes the white turbidity (light scattering intensity) constant during light-shielding as a light shutter over a wide temperature range. be.

0012

[Means for Solving the Problems] That is, the present invention provides a method for preparing a mixture of a low-molecular liquid crystal and a polymer liquid crystal, or a mixture of a low-molecular liquid crystal, a polymer liquid crystal, and an optically active substance in a temperature range in which at least one of them exhibits a cholesteric phase. is sandwiched between substrates with transparent electrodes, which forms an optically transparent state by applying a high-voltage alternating current electric field, and maintains a light scattering state by applying an electric field in which an alternating current electric field is superimposed on a direct voltage. It is a liquid crystal shutter that features the following features:

[0013]

[Operation] In a mixed system in which a polymer liquid crystal has a concentration above the critical concentration and a low molecular liquid crystal, it is in the process of transitioning from the Grandjean state in Figure 1 to the homeotropic state in Figure 3 (voltage application), or vice versa. Although there may exist a state in which strong light scattering properties are shown to be fairly stable over time (electric field interruption), the present invention has made it possible to maintain this state semi-permanently over a wide temperature range.

[0014]

[Details of the Invention] The present invention has investigated in detail the electro-optical response of a liquid crystal system that has printability, can be made into a film, and exhibits high-speed response and a stable light scattering state over time. Based on results. A mixed system of high molecular liquid crystal and low molecular liquid crystal exhibiting a cholesteric phase can be easily formed as described in the following examples. Assuming the initial state of the mixed system as position (f) in FIG. 5, the voltage of 1Vrms/20
When an AC voltage is slowly applied at a step-up rate of sec (sine curve with a frequency of 100 Hz), the light transmittance starts from (f) and changes until it reaches a constant value (e), as shown in Figure 5 (a). reach.

What is important is that during the transition from the Grandjean state in FIG. 1 to the vertically oriented homeotropic state in FIG. F
There is a C state, which exhibits strong light scattering properties and a voltage range Vmin to Vmax (see FIG. 5) in which the transmittance is minimum. If we drop the voltage at the same rate from the optically transparent homeotropic state, (b)
The transmittance decreases along the route of and finally becomes (s). In other words, it does not match the route at the time of ascent and shows hysteresis. Hysteresis depends on the composition and becomes stronger as the polymer liquid crystal content increases. In this case, it is also possible to lower the holding voltage once the transparent state is formed. When the electric field is cut off in the homeotropic state, as shown in Figure 5 (m)
A state close to that of white turbidity is obtained, but this appears to be Vm
This state corresponds to voltage application in the range from in to Vmax.

The problem is that the degree of cloudiness in the state shown in FIG. 6(M) eases over time as shown in FIG. 6(a), and eventually reaches a constant value (s) with high transparency. It is to reach. The final cloudiness depends on the cell thickness, which affects the operating voltage. For home use, too high a voltage is undesirable. The present inventors focused on the fact that there was a region Vmin to Vmax showing strong light scattering in the response of the low voltage application part in FIG. 5, and after cutting off the electric field, successively applied a weak electric field within this voltage region. We investigated whether it was possible to maintain high white turbidity without relaxation.

According to this experiment, in general, when a very low voltage below Vmin was applied, the transparency gradually increased after a sufficiently long period of time had elapsed. This shows that this voltage cannot stop the transition to the equilibrium state shown in FIG. 6(a). However, even if the voltage is too high (several volts lower than Vmax), the transparency may increase, and this is thought to be due to a gradual change to a homeotropic state. In general, it was possible to maintain white turbidity by applying a voltage V (Vmin < V < Vmax) in between.

However, in this case, although the amount of relaxation decreased, it could not be completely suppressed. Therefore, we also investigated the relaxation behavior by applying an electric field in which alternating current was superimposed on a constant DC voltage. When the ratio of DC voltage to AC voltage was varied, it was found that when a value near Vmin was adopted as the AC voltage, relaxation was completely suppressed over a fairly wide range of DC voltages. This tendency was a universal effect that was independent of the composition ratio of polymer liquid crystal and low molecular liquid crystal, and independent of the frequency of alternating current. This is the content of claim 1. Since the same suppressive effect was not observed with low-molecular-weight cholesteric liquid crystals, it is thought that the effect originates from some kind of polymeric property or depends on the material used, but the detailed mechanism is unknown. It is thought that this is not a dynamic scattering phenomenon because there is no frequency dependence in the deterrent effect and no tissue flow is observed. This will be explained in detail below using examples.

[0019]

[Example] A copolymerized cholesteric type side chain polymer liquid crystal with an acrylic skeleton represented by the following structural formula (1) and a low molecular weight nematic liquid crystal 5CB (represented by the chemical formula 2) were mixed in a weight ratio of 1:9.
to 7:3. Chemical formula 1 is a copolymer type liquid crystal polymer consisting of optically active cholesteryl residues and cyanobiphenyl residues. The phase diagram is shown in FIG. 10, and a cholesteric phase was exhibited below the solid line.

[0020]

[Chemical formula 1]

[0021]

[Case 2]

Here, n:m=1:2, k=l=5. These values are not limited to the values listed here, but may be within a range that indicates a cholesteric phase. The weight average molecular weight of Chemical Formula 1 was about 15,000. (There was no essential difference in the following results even with polymeric liquid crystals with a molecular weight of approximately 110,000.) After dispersing 25 micron glass spacers in this mixed system to make a uniform ink, it was coated on a transparent film with electrodes. . Thereafter, it was sandwiched between films with counter electrodes to form a liquid crystal cell.

[0023] As the weight fraction of the polymeric liquid crystal increases, the viscosity becomes high and a transparent homeotropic alignment is obtained at a high voltage.
Moreover, even after turning it off, it took a long time to return to cholesteric status. On the contrary, as the low molecular weight component increased, the light scattering intensity became weaker, and the relaxation of the FC state mentioned above tended to become faster. The preferred composition is 3:7 to 5:5. 1V/20sec for a system with a mixing ratio of 4:6 (polymer: low molecule)
Figure 5 shows the change in transmittance when an electric field is applied at a speed of . A completely transparent state was exhibited at a voltage of 65 V for the composition of 3:7 and 90 V for the composition of 4:6.

[0024] When the voltage was cut off in this state, a very strong light scattering state was exhibited, and in all compositions, the haze value was about 95 and white turbidity. With a composition of 3:7, as shown in Figure 6(a), it gradually relaxes and reaches an equilibrium state after about 10 minutes.
The haze value at this time was about 80 (see (s) in FIG. 6). No relaxation of the light scattering state was observed from this state. The time change was measured by irradiating the cell with a He--Ne laser beam and converting the amount of transmission into voltage using a photodiode. Next, we investigated the effect of suppressing relaxation by applying a weak electric field again after the interruption.

FIGS. 6(b) and 6(b)' show the results of applying alternating currents of 20 V and 30 V (both 100 Hz) after the interruption. Although the relaxation is suppressed compared to when no application is applied, it is not complete. Generally, complete suppression was not possible by applying only alternating current. Therefore, the result of applying a DC voltage of 10 V and a voltage of 10 V, which is slightly lower than Vmin, is shown as the change in FIG. 6(c). In this case, there was no reduction in white turbidity immediately after shutoff, and an almost complete deterrent effect was observed. In order to investigate the effective DC voltage range, the equilibrium transmittance after 1000 seconds was determined by keeping the AC voltage constant and changing the DC voltage.
Figures 7 and 8 show the results of the investigation for the case of 6 (20V).
It was shown to. In either case, the minimum transmittance is obtained over a fairly wide voltage range. The higher the polymer weight, the wider the voltage range that provides the minimum, and when the AC voltage is increased, the DC voltage tends to shift to a lower value.

Next, for the composition of 4:6, a voltage of 30 V DC and 20 V AC was applied to obtain the lowest equilibrium transmittance, and then the temperature was raised to examine changes in transmittance. The results are shown in Figure 9. This composition showed almost no change up to about 48 degrees, and was found to be very stable against temperature changes. Finally, the DC voltages do not have to have the same polarity, and the polarity may be switched at appropriate time intervals, such as 5 minutes.

[0027]

According to the present invention, it is possible to provide a liquid crystal shutter that operates at a low voltage, has a large area, is lightweight, has extremely strong light shielding properties, and is stable against temperature changes. This eliminates the need for perfect alignment of the polarizing plate and liquid crystal, and can also be used as a liquid crystal display for large direct-view displays and projection displays.

[0028]

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a Grandjean state in which the helical axis is substantially perpendicular to the substrate surface, which the cholesteric phase can take in the present invention.

FIG. 2 is a schematic diagram showing a temporally stable focal conic state that the cholesteric phase can take in the present invention.

FIG. 3 is a schematic diagram showing the homeotropic state that the cholesteric phase can take in the present invention.

FIG. 4 is an example of a liquid crystal shutter of a polymer dispersed liquid crystal, in which (a) is an explanatory diagram showing that a light scattering state is exhibited when no voltage is applied, and (b) is an explanatory diagram showing that a light transmitting state is exhibited when a voltage is applied. FIG.

FIG. 5 is an explanatory diagram showing the principle of the liquid crystal shutter driving method of the present invention.

FIG. 6 is a graph showing the relaxation of the initial cloudy state as a function of time in the liquid crystal shutter of the present invention.

FIG. 7 is a graph showing the equilibrium transmittance in a cloudy state in the liquid crystal shutter of the present invention in which the composition ratio of polymer liquid crystal and low molecular liquid crystal is 3:7 as a function of DC voltage when AC voltage is constant. be.

FIG. 8 is a diagram showing the equilibrium transmittance in a cloudy state in a liquid crystal shutter of the present invention in which the composition ratio of polymer liquid crystal and low molecular liquid crystal is 4:6 as a function of DC voltage when AC voltage is constant. .

FIG. 9 is a graph showing temperature changes in light transmittance in a cloudy state obtained by superimposing alternating current on direct current.

FIG. 10 is a phase diagram showing a mixed system of polymer liquid crystal and low molecular liquid crystal used in Examples.

[Explanation of symbols]

400 Transparent electrode 401 Binder polymer 402 Randomly oriented liquid crystal 403 Transparent substrate 404 Homeotropically oriented liquid crystal 405 Liquid crystal grains 410 Incident light 411 Scattered light or reflected light 412 Transmitted light

Claims (1)

[Claims] Claim 1: A mixture of a low-molecular liquid crystal and a polymer liquid crystal, or a mixture of a low-molecular liquid crystal, a polymer liquid crystal, and an optically active substance,
It is sandwiched between substrates with electrodes, at least one of which is transparent, in a temperature range that exhibits a cholesteric phase, and an optically transparent state is formed by applying a high-voltage AC electric field, and an AC electric field is superimposed on the DC voltage. A liquid crystal shutter characterized by maintaining a light scattering state by applying an electric field.