Compositions – Liquid crystal compositions
Reexamination Certificate
1998-04-22
2001-01-09
Kelly, C. H. (Department: 1721)
Compositions
Liquid crystal compositions
C252S299650, C252S299660, C252S299670
Reexamination Certificate
active
06171518
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of preparing an oriented cross-linked polymer, wherein a polymerizable liquid crystal composition, comprising at least a liquid crystal monomer, is oriented and subsequently polymerized. The invention also relates to liquid crystal compositions suitable for use in such a method and to cross-linked polymers obtainable by using such a method. The invention further relates to optical elements comprising cross-linked polymers obtainable by using such a method.
Oriented cross-linked polymers are useful materials, inter alia, for lighting and display applications, because they are optically anisotropic. Optically anisotropic materials can be used to manipulate the characteristics of light beams, such as polarizable, direction, intensity, color and splitting. Optical elements which can be made thereof are, for example, (polarizing) beam splitters, retardation plates, (reflective) polarizers and compensation foils.
A method of the type mentioned in the opening paragraph is described in U.S. Pat. No. Specification 5,042,925. In the known method, a uniaxially oriented liquid-crystal diacrylate monomer is photopolymerized using a photoinitiator and UV radiation so as to obtain a layer of a uniaxially oriented polymeric network or, in other words, a cross-linked polymer. The polymer is optically anisotropic, its birefringence being approximately 0.12.
A disadvantage of said known method is that it has limited means to adjust the optical properties, such as the (extra)ordinary refractive index or the birefringence, to desired values. Adjustment of the birefringence is achieved by adjustment of the temperature at which the polymerization is performed. In this way, only a small range of birefringence values is reliably accessible. In particular, low degrees of optical anisotropy are difficult to obtain since this requires polymerizing at a temperature close to the clearing temperature (that is the temperature at which the transition between the liquid crystalline and isotropic phases occurs) of the liquid-crystal composition. At such critical temperatures, the optical properties are rather sensitive to small changes in temperature, the orientation is easily lost and undesirable biphasic structures easily develop.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention, inter alia, to provide a method of preparing an oriented cross-linked polymer which does not have the above-mentioned disadvantages. In particular, it is an object of the invention to provide a method of the type mentioned in the opening paragraph which has novel means for adjusting the degree of optical anisotropy to a desired value. The method should allow the degree of optical anisotropy to be adjustable over a wide range. Further, it should allow low degrees of optical anisotropy to be attained in a simple and reliable manner.
This object is achieved by a method of the type mentioned in the opening paragraph which, according to the invention, is characterized in that the polymerizable liquid-crystal composition further comprises an order-reducing monomer, which reduces the order of said liquid-crystal composition during polymerization.
The order-reducing monomer represents a novel means for adjusting the degree of optical anisotropy. Using said monomer in the method according to the invention allows the degree of optical anisotropy to be adjusted over a wide range. By simply varying the nature of the order-reducing monomer and/or the concentration in which it is used, the degree of optical anisotropy can be accurately and reliably adjusted. In particular, low degrees of optical anisotropy can be attained. In a typical example, the birefringence of a uniaxially oriented cross-linked polymer could be varied from 0.101 to a value as low as 0.009 by varying the liquid-crystal monomer to order-reducing monomer ratio from 3:1 to 1:3.
In addition, the temperature of polymerization can be used to adjust the degree of optical anisotropy.
The purpose of the order-reducing monomer is to reduce the order of the liquid-crystal composition during polymerisation and, consequently, the optical anisotropy. As a result, a cross-linked polymer is obtained which has a degree of optical anisotropy which is significantly lower than the degree of optical anisotropy of the liquid-crystal composition from which it is obtained. The presence of the order-reducing monomer may also lower the degree of optical anisotropy of the liquid-crystal composition before polymerization, but the invention does not relate to this side-effect per se. Preferably use is made of an order-reducing monomer having intrinsically a high degree of optical anisotropy so as to obtain a polymerizable liquid-crystal composition with a high degree of optical anisotropy. In this case, it has surprisingly been found that the degree of optical anisotropy of the polymer which is obtained after polymerization is lower than the degree of optical anisotropy which can be obtained with oriented samples made from each of the monomers separately, both before and after polymerization with the proviso that the order-reducing monomer itself exhibits liquid-crystalline behaviour.
The presence of the order-reducing monomer neither precludes the liquid crystal composition from being oriented, nor leads to a substantial change of the clearing temperature of the liquid crystal composition, if it is used in moderate amounts.
The method according to the invention can be used to obtain any type of oriented cross-linked polymer, such as a uniaxially, biaxially, or helicoidally oriented polymer. The orientation can also be tilted.
In the context of the invention, an oriented polymer is considered to be a polymer whose type of orientation is the same throughout a certain volume, and whose size is selected so as to achieve a specific technical effect. If throughout an oriented sample the type of orientation is the same, it does not mean that the (optical) properties may not vary from position to position. For example, the pitch of a helicoidally oriented polymer may vary across the sample. In another example, local variations in the properties of an oriented polymer can be attained by photoinitiating the polymerization by patternwise irradiation.
The polymerizable liquid-crystal composition is conveniently oriented using conventional means such as exposure to an electric field or magnetic field or, preferably, by bringing the composition into contact with an orientation-inducing surface of a substrate. Such a surface can be provided with an orientation-inducing layer such as an obliquely sputtered silicon-dioxide layer or, preferably, a polyimide layer rubbed with a velvet cloth.
In contrast to inorganic optically anisotropic materials, such as quartz and calcite, the cross-linked polymers can be easily given any desired shape using simple replication techniques.
The polymerization is conveniently initiated by adding to the polymerizable liquid-crystal composition a small amount, generally less than 2 wt %, of an initiator which is activated thermally. Preferably, an initiator which can be activated by means of actinic radiation such as, in particular, (near) UV radiation, i.e. a photoinitiator, is used. One of the resultant advantages is that polymerization may be performed according to a desired pattern by a patternwise exposure using for example a mask. Optical elements such as gratings can be easily obtained in this manner without the use of photolithography with a photoresist.
The polymerization product is a cross-linked polymer. The polymerizable liquid-crystal composition should therefore be cross-linkable, that is, it should contain a cross-linking agent. If acrylate, vinyl ether or epoxide monomers are polymerized, diacrylates, divinylethers, diepoxides or monomers having even more than two groups serve as cross-linking agents. The cross-linking agent may be added as an additional component or, alternatively, the liquid crystal monomer and/or the order-reducing monomer may serve as a cross-linking agent. In the latter c
Hikmet Rifat A. M.
Lub Johan
Bartlett Ernestine C.
Kelly C. H.
U.S. Philips Corporation
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