Liquid crystals are applied to display media in which the reversible movement of liquid crystal molecules is made use of, such as display devices of various modes typified by twisted nematic (TN), vertical alignment (VA), and in-place-switching (IPS). Besides the application to display media, the liquid crystals, particularly those having a polymerizable functional group have been studied for applicability to optically anisotropic materials, such as a retardation film, a polarizer, a polarizing prism, a luminance-improving film, a low pass filter, various optical filters, and optical fibers, taking advantage of their anisotropy in physical properties, such as refractive index, dielectric constant, magnetic susceptibility, elastic modulus, and thermal expansion coefficient, as well as their alignment properties. It is important for the optically anisotropic material (polymer) obtained by the polymerization to have not only the optical anisotropy but other characteristics, such as polymerization rate, transparency, mechanical strength, coating properties, solubility, crystallinity, shrinking properties, water permeability, water absorption, melting point, glass transition point, clear point, chemical resistance, and heat resistance.
The optically anisotropic material (polymer) is obtained by, for example, uniformly aligning the molecules of a liquid crystal compound having a polymerizable functional group or a polymerizable composition containing the liquid crystal compound into a liquid crystal phase and irradiating the compound or the composition being in the liquid crystal phase with energy rays, such as ultraviolet rays, to cause photopolymerization. It is required to fix the aligned state of the liquid crystal compound uniformly and semi-permanently.
When the polymerizable composition has a high liquid crystal phase transition temperature, photopolymerization induced by energy rays may unintentionally invite thermal polymerization, which disturbs the uniform alignment of the liquid crystal molecules, making it difficult to fix a desired state of alignment. In order to facilitate temperature control during cure, a polymerizable composition showing a liquid crystal phase at or near room temperature is demanded.
The polymer is obtained by polymerizing the polymerizable composition in the form of coating film applied to a substrate. If the composition contains a non-polymerizable compound, the resulting polymer film may have insufficient strength or contain residual stress-induced strain. Removing a non-polymerizable compound using, e.g., a solvent can result in a failure to retain film homogeneity and cause unevenness. To obtain a polymer film with a uniform thickness, it is therefore preferred to apply a polymerizable composition in the form of a solution in a solvent to a substrate. Hence, it is desirable for a liquid crystal compound or a polymerizable composition containing it to have good solubility in a solvent.
To cope with the tendencies to reduction in thickness and weight of display devices, components making up the display devices including optical films have been requested to have a reduced thickness.
A polymerizable liquid crystal compound having an acryloyloxy group as a polymerizable functional group exhibits high polymerization reactivity, i.e., readily polymerizes through a convenient method (e.g., UV irradiation), and the resulting polymer has high transparency and has therefore been extensively studied for use as an optically anisotropic material (see, e.g., Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4). However, any of the polymerizable compositions disclosed in these reference documents has the disadvantage of difficulty in controlling film formation with a reduced thickness.
In general, formation of a thick film from a polymerizable composition containing a liquid crystal compound encounters with difficulty in controlling molecular alignment of the liquid crystal compound, which can cause problems, such as reduction in transmittance and coloration. On the other hand, a thin polymer film having satisfactory molecular alignment over the entire area can be obtained, but formation of a thin film has difficulty in thickness control, readily resulting in non-uniform surface condition or crystallization. Furthermore, the liquid crystal state resulting from alignment control shows poor stability and tends to be disturbed before curing by irradiation with, e.g., UV light. Conventional known polymerizable compositions have thus failed to provide a satisfactory polymer.
Although Patent Document 5 supra discloses an epoxy compound that cures on irradiation with light, it is not pertinent to utilization as a liquid crystal material, nor does it teach or even suggest the effect on control in film formation and fixation of liquid crystal alignment.    Patent Document 1: JP 2006-193596A    Patent Document 2: JP 2005-196221A    Patent Document 3: JP 11-148080A    Patent Document 4: JP 8-231958A    Patent Document 5: JP 2006-84985A