Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2002-07-03
2004-12-21
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C428S001200, C428S001300, C428S413000, C428S419000, C349S187000, C349S045000
Reexamination Certificate
active
06833166
ABSTRACT:
The invention pertains to an anisotropic composite comprising a mixture of a) an anisotropic oriented polymeric network with one or more binding moieties obtainable from a liquid crystalline monomer or a liquid crystalline mixture of monomers, at least one of the monomer comprising a polymerizable group selected from acrylate, epoxy, oxetane, vinyl ether, and thiolene, and b) an inorganic material, to a method for manufacturing this material and its intermediate product, and optical devices comprising said material.
The size quantization effects in semiconductor particles are of great interest. In particles of nanometer size, a gradual transition from bulk to molecular structure occurs as the particle size decreases. The particles which show these quantization effects are often called quantum dots. They show size dependant optical and electronic properties. For example, the band gap of these materials can show increase by several electron volts with respect to the bulk material with decreasing particle size. This is reflected in the absorption and the photo-luminescence spectra of the materials that shift hundreds of manometers with decreasing particle size. The band gap of these materials has been adjusted to produce composites to obtain electroluminescence. In the case of conductive metal particles optical, properties such as absorption become also size dependant. Various methods have been described for obtaining composites of quantum dots in polymer matrices. Liquid crystalline materials comprising an anisotropic oriented polymeric network with one or more binding moieties obtainable from monomers comprising a polymerizable group selected from acrylate, epoxy, vinyl ether, and thiolene are known in the art. For example, in U.S. Pat. No. 5,188,760 such polymeric networks and their synthesis have been disclosed. It is also known that such networks can form ionic complexes with divalent metal cations, such as disclosed in FR 2,738,249 (WO 97/09295). However, it was found that the order parameter of these network complexes is extremely low, making such complexes unsuitable for use in optical devices.
It is an object of the invention to obtain particles in an ordered polymeric matrix, which show size dependent electrical and/or optical effects. It is also an object of the present invention to obtain a new type of anisotropic composites. The invention pertains to an anisotropic composite of the aforementioned mixture of a polymeric network and an inorganic material wherein a mixture is comprised of a) an anisotropic oriented polymeric network with one or more binding moieties, obtainable from a liquid crystalline monomer or a liquid crystalline mixture of monomers, at least one of the monomers comprising a polymerizable group selected from acrylate, epoxy, oxetane, vinyl ether, and thiolene, and b) an inorganic material, characterized in that the network has an order parameter S greater than 0.01 and that the inorganic material is a substantially water-insoluble inorganic salt, a free metal particle, or a mixture thereof. The mixtures according to this invention differ from the prior art compounds, such as disclosed in FR 2,738,249 in that they do not contain ionic complexes of binding moieties, such as carboxylate groups, with divalent cations. The present mixture comprises polymeric networks that still contain a binding moiety, preferably stabilized with hydrogen bonds, and chemical cross-links thereby securing a high orientation, as can be expressed as a high order parameter S. Preferred networks comprises a COOH, pyridine, or polyether moiety as the binding group. The mole ratio of the liquid crystalline monomers: the inorganic salt and/or free metal particle is preferably 100:1 to 1:5. More preferably, the mol ratio is 10:1 to 1:1.
The anisotropic composite has a nematic, smectic, discotic, or chiral nematic structure. These network are obtained from monomers, including monomers with acrylate, epoxy, oxetane, vinyl ether, and thiolene groups which are polymerizable, usually by a photopolymerization reaction. When cholesteric networks are desired, the mixture of monomers should comprise an amount of a chiral monomer. Preferred chiral monomers are monomers comprising a chiral center in the spacer connecting the binding moiety and the polymerizable group. An example of such a monomer is C
6
CA (FIG.
1
). The chiral center may also be contained in a monomer without a polymerizable group. Preferably, the monomer or the mixture of monomers is in the thermotropic liquid crystalline phase.
Examples of other monomers are C
5
A and C
6
M (FIG.
1
), which are monoacrylate and diacrylate compounds, respectively, and due to the hydrogen bonding of the carboxylic groups can form a dimer. Such system shows liquid crystallinity. Other examples of suitable monomers comprising polyether moieties are exemplified in FIG.
2
. The examples of
FIGS. 1 and 2
can be prepared according to the methods disclosed in
Prog. Polym. Sci
., 21, 1165-1209 (1996). Monomers C
5
A and C
6
M and mixtures thereof are crystalline at room temperature and become nematic upon melting. With increasing temperature the ordinary refractive index (n
o
) tends to increase while the extraordinary refractive index (n
e
) shows a decrease. These changes are associated with the order parameter S that is related to the clearing temperature. Order parameter S can be calculated from the refractive indices as shown in equation (1)
S
≈
C
3
(
n
e
2
-
n
0
2
)
2
n
0
2
+
n
e
2
-
3
(
1
)
wherein C is a constant related to the polarizability of the molecules. Using equation 1, C can be estimated for various mixtures. In this way, the order parameter S can be determined as a function of reduced temperature (which affects the refractive indices). The order parameter S must be greater than 0.01. Preferably, the order parameter S is greater than 0.1, more preferably greater than 0.4.
The anisotropic networks contain physical (hydrogen bonds) as well as chemical cross-links. Anisotropic networks are formed in the oriented state upon polymerization of the acrylate, epoxy, oxetane, vinyl ether, or thiolene groups. For example, it was found that the birefringence (&Dgr;n=n
e
−n
o
) of pure C
5
A shows a large decrease upon polymerization. This decrease is associated with the loss of macroscopic order within the sample. This indicates that physical cross-links formed by hydrogen bonds are not strong enough to maintain the order within the system. It was also found that in a mixture containing 10 wt. % of diacrylate (C
6
M) the high birefringence is sustained upon polymerization. This proves that a small number of chemical cross-links are enough for maintaining the order within the system. Looking at the temperature dependence of the refractive index of the networks it was found that the birefringence of the network, specially those containing a small amount of C
6
M, shows a rapid decrease around 110° C. This behavior can be best expressed in terms of the order parameter of the network. It can be assumed that the constant C remains unchanged upon polymerization and its use in equation (1) together with the measured refractive indices was used to determine the order parameter of the network. The determination of C is known in the art and is disclosed in
Macromolecules
, 95(28), 3313-3327 (1995) and
Macromolecules
, 92(25), 4194-4199 (1992). It was found that polymerized pure C
5
A (hydrogen bonds only) has a low order parameter. Networks containing C
6
M (chemical cross-links) on the other hand show a relatively high order parameter at room temperature. It was also found that up to about 100° C. the order parameter remains almost constant before showing a rapid decrease at higher temperatures. The decrease observed at higher temperatures is lower for mixtures containing higher concentrations of C
6
M. This indicates that within the mixtures the decrease of the order parameter is caused by the hydrogen bonded C
5
A molecules. Two peaks which are associated with the free and hydrogen bonded carbonyl ba
Koninklijke Philips Electronics , N.V.
Sadula Jennifer R.
Wu Shean C.
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