Liquid crystal elastomers

Details Liquid crystal elastomers Liquid crystal elastomers

1999-04-12

2001-11-06

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C428S001300, C252S299010

Reexamination Certificate

active

06312770

ABSTRACT:

This invention relates to liquid crystal elastomers and their use in devices and methods for making devices incorporating liquid crystal elastomers.
The unit that is the basic building block of a polymer is called a monomer.
The polymerisation process i.e. the formation of a polymer from its constituent monomers does not usually create polymers of uniform molecular weight, rather what is created is a distribution of molecular weights. In order to describe a sample of polymer it is necessary to state the average number of monomers in a polymer, this is called the degree of polymerisation (D.P). By how much the majority of polymer molecules differ from this average value (or to describe the spread of molecular weight) is called the polydispersity.
A number of different average molecular weights can be drawn from gel permeation chromatography (GPC) for a given sample including: Mn—number average molecular weight and Mw—weight average molecular weight. The value used to calculate D.P. is usually Mn and polydispersity is usually defined as Mw/Mn.
Polymers can be made from different types of monomers, in which case the polymer is called a co-polymer. If two types of monomer join in a random fashion then the polymer is called a random co-polymer. If the two monomers form short sequences of one type first which then combine to form the final polymer then a block copolymer results. If short sequences of one of the monomers attach themselves as side chains to long sequences consisting of the other type of monomer then the polymer is referred to as a graft copolymer.
In liquid crystal polymers the monomers can be attached together in essentially two ways. The liquid crystal part or mesogenic unit of the polymer may be part of the polymer backbone resulting in a main chain polymer, alternatively the mesogenic unit may be attached to the polymer backbone as a pendant group i.e. extending away from the polymer backbone; this results in a side-chain polymer. These different types of polymer liquid crystal are represented schematically below. The mesogenic units are depicted by the rectangles.
The side chain liquid crystal polymer can generally be thought of as containing a flexible polymer with rigid segments (the mesogenic unit) attached along its length by short flexible (or rigid) units. It is the anisotropic, rigid section of the mesogenic units that display orientational order in the liquid crystal phases. In order to affect the phases exhibited by the liquid crystal and the subsequent optical properties there are many features which can be altered, some of these features are particularly pertinent to side-chain liquid crystal polymers. One of these features is the flexible part that joins the mesogenic unit to the polymer backbone which is generally referred to as a spacer; the length of this spacer can be altered, its flexibility can also be altered.
A number of side-chain liquid crystal polymers are known, for example see GB 2146787 A.
Liquid crystal polyacrylates are known class of liquid crystal polymer (LCP). LCPs are known and used in electro-optic applications, for example in pyroelectric devices, non-linear optical devices and optical storage devices. For example see GB 2146787 and Makromol. Chem. (1985), 186 2639-47.
Side-chain liquid crystal polyacrylates are described in Polymer Communications (1988), 24, 364-365 e.g. of formula:
where (CH
2
)
m
—X is the side-chain mesogenic unit and R is hydrogen or alkyl.
Side-chain liquid crystal polychloroacrylates are described in Makromol. Chem. Rapid Commun. (1984), 5, 393-398 e.g. of fonnula:
where R is chlorine.
A method for the preparation of polyacrylate homo- or co-polymers having the following repeat unit is described in UK patent application GB 9203730.8
R
1
and R
2
are independently alkyl or hydrogen, R
3
is alkyl, hydrogen or chlorine, m is O or an integer 1-20, W is a linkage group COO or OOC, O and X is a mesogenic group,
One of the main problems with polymer liquid crystals is that they are extremely difficult to align in devices. Essentially there are two techniques which have been used for aligning polymer liquid crystals. It is possible to try to align the liquid crystal polymer in a similar manner as a low molar mass liquid crystal, which is described in more detail below. Alternatively, mechanical techniques can be used such as shearing. Typically, mechanical shearing is performed over hot rollers, this technique is generally only suitable for flexible substrates. It is possible to shear a sample between glass slides however the glass slides cannot be sealed in the conventional manner.
Materials and Assembling Process of LCDs by Morozumi in Liquid Crystals Applications and uses, vol 1 Ed. Bahadur, World Scientific Publishing Co, Pte. Ltd, 1990 pp 171-194 and references therein as the title suggests discusses methods for assembling liquid crystal devices.
The technique for aligning low molar mass liquid crystals is typically as follows. Transparent electrodes are fabricated on the surfaces of the substrates, the substrates typically being made of glass e.g. glass slides. In twisted nematic or super twisted nematic devices, for example, an alignment process is necessary for both substrates. A thin alignment layer is deposited to align the liquid crystal molecules, typically either organic or inorganic aligning layers are used, for example SiO deposited by evaporation is a typical inorganic alignment layer. One method to form the alignment layer involves rubbing the surface by textures or cloths. Polyimides have also been employed for the surface alignment layers. Polymide is coated onto the substrates bearing electrodes by a spinner and then cured to form a layer of approximately 50nm thickness. Then each layer surface is repeatedly rubbed in substantially one direction with an appropriate material. If the liquid crystal molecules are deposited on this layer they are automatically aligned in the direction made by the rubbing. It is often preferable if the molecules possess a small angle pre-tilt typically 2-3°. Higher pre-tilts are sometimes required.
The two substrates are then fixed together for example by adhesive and are kept separate by spacing materials. This results in uniform and accurate cell spacing. A typical adhesive is an epoxy resin. This sealing material is usually then precured. The electrodes may then be precisely aligned for example to form display pixels. The cell is then cured at, for example 100-150° C. At this point the empty liquid crystal cell is complete.
It is at this point that the cell is filled with liquid crystal material. The opening size in the sealing area of the liquid crystal cell is rather small therefore the cell can be evacuated, for example in a vacuum chamber, and the liquid crystal forced into the cell via gas pressure. More than one hole in the sealing area may be used. The empty cell is put into a vacuum chamber and then the vacuum chamber is pumped down. After the cell has been evacuated the open region of the sealant is dipped into the liquid crystal material and the vacuum chamber is brought back to normal pressure. Liquid crystal material is drawn into the cell as a result of capillary action, external gases can be applied to increase the pressure. When the filling process is complete the hole or holes in the sealant is/are capped and the cell is cured at a temperature above the liquid crystal material clearing point to make the liquid crystal molecular alignment stable and harden the capping material.
Polymer liquid crystal molecules tend to be more viscous than low molecular weight liquid crystal materials and are therefore more difficult to align and more difficult to fill into devices. Only liquid crystal polymers with low molecular weights can be flow filled into a cell, and once a degree of polymerisation greater than around 30 or 40 repeat units is reached, most liquid crystal polymers become so viscous that flow filling cells is extremely difficult. Much slower cooling is needed in order to try and align liquid crystal polymers and this usually results in p

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