Ferroelectric liquid crystal devices

Details Ferroelectric liquid crystal devices Ferroelectric liquid crystal devices

1998-09-09

2001-06-26

Kelly, C. H. (Department: 1756)

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

C252S299630, C252S299660, C252S299610, C252S299670, C252S299680, C428S001100

Reexamination Certificate

active

06251301

ABSTRACT:

This invention relates to ferroelectric liquid crystal devices and ferroelectric liquid crystal mixtures.
Liquid crystal devices commonly comprise a thin layer of a liquid crystal material contained between two glass slides. Optically transparent electrodes are formed on the inner surface of both slides. When an electric voltage is applied to these electrodes the resulting electric field changes the molecular alignment of the liquid crystal molecules. The changes in molecular alignment are readily observable and form the basis for many types of liquid crystal device.
In ferroelectric liquid crystal devices the molecules switch between different alignment directions depending on the polarity of an applied electric field. These devices often exhibit bistability where the molecules tend to remain in one of two states until switched to the other switched state. This allows the multiplex addressing of quite large and complex devices.
One common multiplex display has display elements, ie pixels, arranged in an x, y matrix format for the display of eg, alpha numeric characters. The matrix format is provided by forming the electrodes on one slide as a series of column electrodes, and the electrodes on the other slide as a series of row electrodes. The intersections between each column and row form addressable elements or pixels. Other matrix layouts are known, eg seven bar numeric displays.
There are many different multiplex addressing schemes. A common feature involves the application of a voltage, called a strobe voltage to each row or line in sequence. Coincidentally with the strobe applied at each row, appropriate voltages, called data voltages, are applied to all column electrodes. The differences between the different schemes lies in the shape of the strobe and data voltage waveforms.
Other addressing schemes are described in GB-2,146,473-A; GB-2,173,336-A; GB-2,173,337-A; GB-2,173,629-A; WO 89/05025; Harada et al 1985 S.I.D. Paper 8.4 pp 131-134; Lagerwall et al 1985 I.D.R.C pp 213-221 and P Maltese et al in Proc 1988 IDRC p 90-101 Fast Addressing for Ferro Electric LC Display Panels.
The material may be switched between its two states by two strobe pulses of opposite sign, in conjunction with a data waveform. Alternatively, a blanking pulse may be used to switch the material into one of its states. Periodically the sign of the blanking and the strobe pulses may be alternated to maintain a net d.c. value.
These blanking pulses are normally greater in amplitude and length of application than the strobe pulses so that the material switches irrespective of which, of the two data waveforms is applied to any one intersection. Blanking pulses may be applied on a line by line basis ahead of the strobe, or the whole display may be blanked at one time, or a group of lines may be simultaneously blanked.
It is well known in the field of ferroelectric liquid crystal device technology that in order to achieve the highest performance from devices, it is important to use mixtures of compounds which give materials possessing the most suitable ferroelectric smectic characteristics for particular types of device.
Devices can be assessed for speed by consideration of the response time vs pulse voltage curve. This relationship may show a minimum in the switching time (t
min
) at a particular applied voltage (V
min
). At voltages higher or lower than V
min
the switching time is longer than t
min
. It is well understood that devices having such a minimum in their response time vs voltage curve can be multiplex driven at high duty ratio with higher contrast than other ferroelectric liquid crystal devices. It is preferred that the said minimum in the response time vs voltage curve should occur at low applied voltage and at short pulse length respectively to allow the device to be driven using a low voltage source and fast frame address refresh rate.
Typical known materials (where materials are a mixture of compounds having suitable liquid crystal characteristics) which do not allow such a minimum when included in a ferroelectric device include the commercially available materials known as SCE13 and ZLI-3654 (both supplied by Merck UK Ltd, Poole, Dorset). A device which does show such a minimum may be constructed according to PCT GB 88/01004 and utilising materials such as eg commercially available SCE8 (Merck UK Ltd.). Other examples of prior art materials are exemplified by PCT/GB/86/00040, PCT/GB87/00441 and UK 2232416B.
It is the aim of this invention to provide devices having a shorter switching time and/or a lower voltage than previously achieved.
According to this invention a ferroelectric liquid crystal device (eg multiplex addressed) comprises two spaced cell walls each bearing electrode structures and treated on at least one facing surface with an alignment layer, a layer of a smectic liquid crystal material enclosed between the cell walls, a minimum in its response time versus voltage curve, characterised in that the liquid crystal material consists essentially of two components; A and B, where the two components are given by:
Component A being present in the range of 0.1-50 wt % and is one or more optically active compounds capable of imparting a spontaneous polarisation to the material and is given by the following general formula:
in which X is a group having a general structure:
and B is alkyl containing 1-12 carbons, a chiral group, or a group having a general structure:
where R and R
1
are independently hydrogen or C
1-12
alkyl, alkoxy, alkylcarbonyloxy or alkoxycarbonyl, each of the rings (A) and (D) may be the same or different and are each independently selected from optionally alkyl-, cyano- or halogen-substituted phenyl, transcyclohexyl, pyridyl, pyrimidyl, bicyclo (2,2,2) octyl or dioxan each A and D may be the same or different and is independently selected from a single bond, COCO, OOC, CH═N, N═CH, CH
2
O, OCH
2
, CH
2
, CH
2
CH
2
, CH(CH
3
) or a combination of two of such groups, in which each a and d is independently O and 1, in which Y is selected from —COO—, —OOC—, —O— or a single bond, in which W is selected from a single bond, —(—CH
2
—)—
n
or —(CH
2
)
m
CH(Z
1
)— —(CH
2
)
p
— where n, m and p are independently 0 to 10; in which Z or Z′ are independently selected from CN, Cl, F, Br and CF
3
; provided that when Z is Cl or CN, then when B is alkyl, X—Y— is not
Compounds of formula I in which the unit —YWCH(Z) is COOCH(CN) may be prepared for example by a number of widely applicable routes 1-5 shown schematically in
FIGS. 1-5
. Compounds of formula I in which —YWCH(CN) is —OCH(CN) may be prepared for example by route 6 shown in FIG.
6
.
In routes 1-6 the group B is in some cases introduced into the compound of formula I using an alpha-hydroxy carboxylic acid of formula:
or an alpha-amino acid of formula:
Routes 1-6 are of general suitability but are particularly suited to cases where B is alkyl, phenyl or cyclohexyl.
Some of these acids are commercially available in an optionally pure enantiomeric form, eg the hydroxy carboxylic acids lactic acid (B=CH
3
) and mandelic acid (B=phenyl), and the series of commercially available or naturally occurring amino acids, eg alanie, valine, leucine, isoleucine, butyrine, alloisoleucine, norvaline, norleucine and phenylalanine—As many such acids are of biochemical origin, they are often available in optically pure forms of one or more enantioners or antipodes, thus yielding optically pure products, whilst at the same time being relatively cheap. The use of these acids where possible is therefore preferred.
Alternatively these acids may be synthesised, to lead to a wider range of B— groups. The alpha-hydroxy acids may in fact be prepared from the corresponding amino acids by reaction of the amino acid with nitrous acid at low temperatures:
(eg “Tetrahedron” (1979), 35, 1603 and J.A.C.S (1956) 78, 2428).
Other methods of preparation of alpha-hydroxy carboxylic acids are well known, for example as described in “Chemistry of Carbon Compounds” ed D H Dodd (pvb Elsevier) (1952),
IB
, p 780-781 (R

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