Compositions – Liquid crystal compositions
Reexamination Certificate
2002-08-09
2004-07-20
Huff, Mark F. (Department: 1756)
Compositions
Liquid crystal compositions
C349S167000, C349S183000, C349S187000
Reexamination Certificate
active
06764614
ABSTRACT:
The invention relates to a stratified phase-separated composite and a method of manufacturing such a composite.
In Science vol.283 (1999) page 1903, Kumar et al. (see also U.S. Pat. No. 5,949, 508) disclose a phase-separated composite and a method of manufacturing such. The known composite is manufactured by providing, between a pair of opposed substrates, a layer of a photo-polymerizable prepolymer dissolved in an organic liquid, in particular a liquid crystal. The organic liquid and monomer are selected such that the liquid is poorly miscible with the photo-polymerized monomer. If so selected, phase-separation of the liquid and the photo-polymer takes place during photo-polymerization, a process known in the art as polymerization-induced phase separation (PIPS). In the composite of Kumar et al. the organic liquid is furthermore adapted to absorb the UV light used for photo-polymerizing the monomer. Therefore, according to Kumar et al, upon subjecting the layer to UV light, a light intensity gradient is set up in the layer in directions normal to the layer, the highest intensity occurring at the side layer facing the UV light source. Since the rate at which photo-polymerization takes place scales with the intensity of UV light, photo-polymerization and therefore phase-separation preferentially takes place at the side of the layer facing the light source. As a result the phase-separation takes place in a stratified manner, producing a composite comprising a predominantly polymeric layer formed at the UV light source side and a predominantly liquid layer at the side facing away from the UV light source.
A disadvantage of the known composite is that the stratification is not complete. In particular, the polymeric layer comprises small amounts of liquid crystal material. For many applications such inclusions of LC material may be undesirable. For example, the liquid crystal present in the polymeric layer may give rise to spurious switching effects and/or during the useful lifetime of the composite liquid crystal material may migrate to and merge with the liquid layer so as to affect the properties of the liquid crystal layer, such as its retardation or orientation.
It is an object of the invention, inter alia, to provide a stratified phase-separated composite which does not have these drawbacks or at least to a lesser extent. In particular, it is an object to provide a stratified-phase-separated composite which is well-stratified. In particular, the number of liquid molecules present in the polymeric layer (regardless the form in which such liquid molecules are present, ie dispersed as droplets or dissolved on a molecular scale) is to be low.
In accordance with the invention, these objectives are achieved by a stratified phase-separated composite comprising a crosslinked polymeric layer and a liquid layer, the composite being obtainable by crosslinking a layer of a crosslinkable, stratified-phase-separable composition comprising a crosslinkable material and a liquid.
The stratified-phase-separated composite in accordance with the invention has a separate liquid layer and a chemically crosslinked polymeric layer. In contrast to polymer dispersed liquid crystals the liquid layer is a continuous layer. The composite in accordance with the invention is well stratified in that only a small number of liquid crystal molecules is present in the crosslinked polymeric layer indicating that the phase-separation is more complete. Upon crosslinking of the crosslinkable composition, phase-separation occurs in a stratified manner to produce a layer of a crosslinked polymer and a layer of liquid. A crosslinked polymer, in the art also referred to as a network polymer, has a polymer network extending in three dimensions, the network structure rendering the polymer relatively rigid compared to corresponding linear chain polymers.
Although not wishing to be bound by any theory, it is believed that, compared to linear chain polymers, crosslinked polymers are particularly effective in squeezing out liquid molecules from the polymeric layer being formed. Specifically, in the early stages of the stratification process it is observed that a polymeric layer is formed which extends almost across the entire layer of stratified-phase-separable material. In the early stages this polymeric layer has no to very few crosslinks and is swollen with liquid molecules. Upon further crosslinking, the number of crosslinks increases, the polymer network becomes more dense and, as a result, the polymeric layer contracts, its thickness becoming smaller, thus squeezing out the liquid molecules. As a result, the crosslinked polymeric layer contains less liquid material and the stratified phase-separation is more complete compared to a polymeric layer made of a linear polymer.
Further advantages of crosslinked polymers are improved temperature and mechanical resistance and resistance to chemicals such as solvents, the latter being convenient if the polymeric layer is to serve as a substrate for wet deposition of further layers. Moreover, being crosslinked, the polymer has a high strength and may, if used to support further layers or to serve as a substrate, be applied in thinner layers and still provide adequate support.
The composite is obtainable by stratification of a single homogenous layer. The stratification is by means of phase-separation. In the present invention, phase-separation is induced by crosslinking. Other methods of phase-separation are known in the art and, in order to optimize the extent of phase-separation, crosslinking is used in combination with such other method, such other method being, for example, solvent-induced phase-separation or thermally-induced phase-separation. Dynamically stabilized or metastable mixtures of non-miscible liquid and crosslinkable material may also be used to induce phase-separation. Preferred however is to use crosslinking in combination with polymerization-induced phase-separation (PIPS). PIPS may thermally initiated or photo-initiated. Preferred is photo-initiated, that is photo-polymerization-induced phase-separation.
Stratification, that is phase-separation in a stratified manner, may be brought about in various ways.
Stratification may be achieved by making use of differences in surface tension in particular differences in wetting. The composite is then obtained by supplying the stratified-phase-separable composition on a substrate, the liquid having a spreading contact angle on the substrate and the liquid being capable of wetting the substrate significantly better than the crosslinkable material being polymerized. When phase-separation is induced by means of crosslinking, optionally in combination with other phase-separation means described hereinabove, it is energetically favorable for the liquid to spread on the substrate and push crosslinkable material (being polymerized) adjacent the substrate away from the substrate thus forming a liquid layer directly adjacent to that substrate and a polymeric layer on the other side.
Differential wetting may be employed using a single substrate but may be used and even more effectively so if the composition is provided between two substrates.
Differences in wettability can be achieved by subjecting the substrate surface(s) to a wetting treatment such as plasma or UV ozone treatment or apply a wetting layer or mix the stratified-phase-separable material with a wetting agent. Such layers, agents and treatments are all well known in the art.
An alternative method of stratification, one which may be applied independent of differential wetting but preferable is applied in conjunction with differential wetting comprises applying a crosslinkable, stratified-phase-separable composition which is photo-polymerizable and which has absorbing means to set up a light intensity gradient with respect to the light being used to photo-polymerize. The composition may be rendered absorbing by using an absorbing liquid or an absorbing photo-crosslinkable monomer or by adding a separate photo-polymerization dye.
For further details regarding the us
Broer Dirk Jan
Klink Stephen Isadore
Penterman Roel
Huff Mark F.
Sadula Jennifer R.
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