JPS6111990B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a liquid crystal composition that is effective when applied to a display device that utilizes the field effect of a liquid crystal exhibiting a cholesteric liquid crystal phase. The electric field effect of the cholesteric liquid crystal phase (hereinafter referred to as Ch) has a structure ranging from a grandjean structure (hereinafter referred to as Gj) to a focal-conic structure (hereinafter referred to as Gj).
-conic) structure (hereinafter referred to as fc), and phase transition mode from Ch to nematic liquid crystal phase (hereinafter referred to as N).
mode) (hereinafter referred to as PTM). The latter has Ch
It also includes optical effects due to pitch changes. The present invention particularly relates to the latter PTM material. The electro-optical properties of this PTM are
It exhibits a light scattering phenomenon due to the fc of Ch, but above a certain critical electric field (Ec) it becomes homeotropically oriented N and becomes transparent. Display is performed using these two optical states. The main features of the electro-optical properties of this PTM can be expressed by Ec, transmittance Ts (%) at Ec, and response characteristics. The above response characteristic is τ _C representing the response from Ch to N.
_N , τ _NC representing the response from N to Ch, and τ ^M _N representing the response from N to Ch when a holding electric field (Eo) with E<Ec is applied. Here, τ _CN is the electric field (E) from Eo (〓Ec)
This is the time from when the optical effect changes to 2Eo (>Ec) until the optical effect reaches 90% of its saturation value. Figure 1a shows the change in the electric field (E), and Figure 1b shows the transmittance Ts.
(%) to illustrate τ _CN . τ ^M _N is the time when the optical effect decreases by 10% from the time when the electric field (E) changes from 2Eo to Eo, to 90% of the previous saturation value. _{Figure b shows the change in transmittance Ts (%) and illustrates τ MN} ^. τ _NC is the change in optical effect from the saturation value to other values from the time the electric field (E) changes from 2Eo or Eo to 0.
Figure 3a shows changes in electric field (E) and Figure 3b shows changes in transmittance Ts (%) to illustrate τ _NC . Furthermore, FIG. 4 shows the characteristics of the change in transmittance Ts (%) with respect to the electric field (E) of PTM. When this PTM is applied to a matrix display, it is desirable that the drive voltage (Vo=d·Eo) can be made low, and in particular, the value should be lower than the withstand voltage of the C/MOS type driver.
Here, d is the liquid crystal layer thickness, and this value has a minimum limit due to the element manufacturing aspect, and is about 5 μm. Therefore, in order to lower Vo, Ec, which is the upper limit of Eo, must be lowered. This Ec is determined by the liquid crystal material, and its dependence on physical property constants is expressed by the following equation. Ec=π ² (K ₂₂ ÷εaε ₀ ) ^1/2 ÷P
...(1) In (1), P is the helical pitch of Ch, and K ₂₂
is the torsional modulus, εa is the dielectric anisotropy, and ε ₀
is the vacuum constant. By the way, when driving with a 1:2 bias voltage, once N
If a display method is adopted in which writing is performed using fc of Ch, the upper limit of the number of lines M that can be driven in a matrix can be approximately written as follows using the value of the response characteristic. M〓(τ ^MN − _{τ CN} ) ÷ τ _NC ……(2) In other words, from equation (2), in order to increase M, τ
It is desirable to increase ^M _N while decreasing τ _CN and τ _NC . Of these, τ ^M _N mainly depends on the surface treatment state of the substrate holding the liquid crystal layer, and
The Ch liquid crystal composition is also closely related. Conversely, τ _NC and τ _CN depend on the Ch liquid crystal material rather than the surface state, and are as follows, where η is the viscosity of the Ch liquid crystal. τ _CN =C ₁ ηP ² ÷K ₂₂ ...(3) τ _NC =C ₂ ηP ² ÷K ₂₂ ...(4). Here, C ₁ and C ₂ are constants that do not depend on the substance. Now, by substituting (1), (3), and (4) into equation (2), we get M∝τ ^M _N ·εaEc ² /η ……(5). In other words, in order to increase M, surface treatment (for example, vertical alignment treatment) that increases τ ^M _N can be performed, and a liquid crystal material that has large values of εa and Ec and a small value of η can be selected. . Here, M includes Ec, and the decrease in Vo and M
This has a contradictory aspect to the increase in . Therefore,
In order to increase M without increasing Vo, it is necessary to select a material with a large εa and a small η. The present invention has been made in this regard, and it is an object of the present invention to provide a liquid crystal material having small values of τ _NC and τ _CN that have not been previously available. Table 1 shows various properties of mixed yarns of various liquid crystal materials.

[Table] Measurement conditions AC, 1 KHz, 25°C, cell thickness 6 μm τ _CN , τ _NC , τ ^M _N , Ts values are for Eo = Ec. Samples A to D have the following configuration. It can be seen from this table that Sample D is an example of the composition of the present invention and has good characteristics not found in other liquid crystal materials. In other words, τ _NC and τ _CN are _{equal to or extremely small compared to other values, τ MN} ^also has a very large value, and Ts also shows a small but good value, making it suitable for matrix driving. It is a characteristic that This is because conventional P-type Schiff liquid crystals, ester liquid crystals, biphenyl liquid crystals, etc. that have two or more benzene rings have a drawback of high viscosity, but the liquid crystal composition of the present invention is one made of P-n-alkylcyclohexyl-P'-cyanobenzene, or one whose viscosity is lowered by adding these to the above liquid crystal. The p-n-alkylcyclohexyl-p'-cyanobenzene liquid crystal was developed by Von Rudolf Eidenshink.
It is a well-known liquid crystal material, as described in Angew Chem. 89 , 103 (1977).
Furthermore, by mixing chiral nematic liquid crystal (+) P-2-methylbutyl-P'-cyanobiphenyl with low viscosity and high dielectric constant anisotropy, the mesomorphic range is widened and the visible A liquid crystal material with good display quality without light absorption in the optical region can be obtained. A specific composition of the present invention (sample D) is shown below. (Sample D) The composition of the present invention contains about 20 wt% to Pn-alkylcyclohexyl-P'-cyanobenzene.
Although it contains 90wt%, it also contains about 20% to 60% of the following compounds to form a P-type nematic liquid crystal composition with P-n-alkylcyclohexyl-P'-cyanobenzene. Also good. (R represents n-CmH _2n+1 or n-CmH _2o+1 O-, and n = 1 to 8.) In Table 1, the amounts of each component in samples A, B, and C are as follows. . Sample A contains 7wt of cholesteryl chloride.
%, B contains 7wt cholesteryl nonanoate
%, C is (+)-2 methylbutyl + P-[N-
(P-cyanobenzylidene)amino]cinnamate is added at 7 wt%, respectively. In addition, Ec and τ _C for the doping amount of (+)-P-2-methylbutyl-P'-cyanobiphenyl (ch liquid crystal) in sample D
The characteristics of _N τ _NC are shown in FIGS. 5 and 6. _τ ^MN
has almost no dependence on the doping amount of the ch liquid crystal and remains constant. From the results shown in Figures 5 and 6, when applied to display elements, the value of Ec is preferably 3 × 10 ⁴ V/cm or less, and the values of τ _CN and τ _NC are 100 msec (25°C).
The following is desirable, so the amount of (+)-P-2-methylbutyl-P'-cyanobiphenyl added is 10wt%
~16wt% is desirable. Further, the composition ratio of the P-n-alkylcyclohexyl-P'-cyanobenzene liquid crystal, which is a non-ch liquid crystal, can be varied from 20 wt% to 90 wt% depending on the application. As described above, the present invention provides a composition that is extremely effective as a liquid crystal material in which phase transition is applied to matrix drive.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining τ _CN , where Figure 1 a shows the electric field E, Figure 1 b shows the transmittance Ts, and Figure 2 explains τ ^MN , where Figure 2 a shows the electric field _E , Figure 2b is the transmittance
Figure 3 is a diagram explaining τ _NC .
Figure a shows the electric field E, Figure 3 b shows the transmittance Ts, Figure 4 shows the change characteristics of the transmittance Ts with respect to the electric field E, and Figures 5 and 6 show the characteristics of the liquid crystal composition of the present invention. Figure 5 shows the change in critical electric field Ec, and Figure 6 shows the change in τ _NC and τ _CN .

Claims (1)

[Scope of Claims] 1. A liquid crystal composition obtained by adding (+)-p-2-methylbutyl-p'-cyanobiphenyl to a p-n-alkylcyclohexyl-p'-cyanobenzene liquid crystal. 2. The liquid crystal composition according to claim 1, wherein the alkyl group of the p-n-alkylcyclohexyl-p'-cyanobenzene is represented by C _o H _2o+1 and n=3 to 7. 3. The liquid crystal composition according to claim 1, wherein the amount of (+)-p-2 methylbutyl-p'-cyanobiphenyl added is 10 to 25% by weight.