Thermotropic liquid crystals are generally crystalline compounds with significant anisotropy in shape. That is, at the molecular level, they are characterized by a rod-like or disc like structure. When heated they typically melt in a stepwise manner, exhibiting one or more thermal transitions from a crystal to a final isotropic phase. The intermediate phases, known as mesophases, can include several types of smectic phases wherein the molecules are generally confined to layers; and a nematic phase wherein the molecules are aligned parallel to one another with no long range positional order. The liquid crystal phase can be achieved in a heating cycle, or can be arrived at in cooling from an isotropic phase. A comprehensive description of the structure of liquid crystals in general, and twisted nematic liquid crystals in particular is given in “The Physics of Liquid Crystals,” P. G. de Gennes and J. Prost, Oxford University Press, 1995.
An important variant of the nematic phase is one wherein a chiral moiety is present, referred to as a twisted nematic or cholesteric phase. In this case, the molecules are parallel to each other as in the nematic phase, but the director of molecules (the average direction of the rodlike molecules) changes direction through the thickness of a layer to provide a helical packing of the nematic molecules. The pitch of the helix is perpendicular to the long axes of the molecules. This helical packing of anisotropic molecules leads to important and characteristic optical properties of twisted nematic phases including circular dichroism, a high degree of rotary power; and the selective reflection of light, including ultraviolet, visible, and near-IR light. Reflection in the visible region leads to brilliantly colored layers. The sense of the helix can either be right-handed or left-handed, and the rotational sense is an important characteristic of the material. The chiral moiety either may be present in the liquid crystalline molecule itself, for instance, as in a cholesteryl ester, or can be added to the nematic phase as a dopant, with induction of the cholesteric phase. This phenomenon is well documented, see e.g. H. Bassler, M. M. Labes, J. Chem. Phys., 52 p 631 (1970).
There has been significant effort invested in the synthesis and polymerization methods for preparing stable polymer layers exhibiting fixed nematic and/or cholesteric optical properties. One approach has been to synthesize monofunctional and/or polyfunctional reactive monomers that exhibit a nematic or cholesteric phase upon melting, formulate a low melting liquid crystal composition, and polymerize the liquid crystal composition in its nematic or cholesteric phase to provide a polymer network exhibiting stable optical properties of the nematic or cholesteric phase. Use of cholesteric monomers alone, as disclosed in U.S. Pat. No. 4,637,896, provided cholesteric layers with the desired optical properties, but the polymer layers possessed relatively weak mechanical properties.
Many efforts have been made to improve the physical properties and thermal stabilities by formulating twisted nematic monomer phases that are capable of crosslinking polymerizations to provide polymer networks. Examples of these crosslinking monomers are bis(meth)acrylates with ether groups (—O—) linking a core mesogen to flexible spacers and the polymerizable (meth)acrylates. Their synthesis and use in forming polymer networks are disclosed in Makromol. Chem. 190, 2255-2268 (1989); Macromolecules, 1988, 31, 5940; Makromol. Chem. 192, 59-74 (1991); WO 1998/047979; J. Polym. Sci.: Part A: Polym. Chem., Vol. 37, 3929-3935 (1999); Makromol. Chem. 190, 3201-3215 (1989); U.S. Pat. No. 5,833,880; DE 4,408,170; EP 261,712; EP 331,233 B1; EP 397,263 B1; and JP 1994/016616A. Although many of these references also claim ester groups (—C(O)—O—) linking the core mesogen to flexible spacers and the polymerizable (meth)acrylate, there is limited disclosure in any reference useful in relation to the teaching of how to make and use bis(meth)acrylates with ester groups (—C(O)—O—) linking the core mesogen to flexible spacers and the polymerizable (meth)acrylates. Furthermore, there is limited disclosure in relation to their specific physical or chemical properties.
The preparation of monofunctional (meth)acrylate liquid crystal monomers having an aliphatic ester group (—C(O)—O—) linking the mesogen to a flexible spacer and the polymerizable (meth)acrylate is disclosed by Shibaev, et al, in Polymer Bulletin, 6, 485-492 (1982), and similar compounds linking a (meth)acrylate to a cholesteryl mesogen via a flexible spacer and an ester moiety are disclosed in U.S. Pat. No. 4,614,619. However, since bis(meth)acrylate mesogens with ester linkages have not been prepared, any benefit of the utilities and properties previously thought to exist for them is difficult to realize.
A need thus remains for a process to make bis(meth)acrylates with esters linking the mesogen to a flexible spacer and the polymerizable (meth)acrylates. There is also a need for crosslinking monomers that exhibit nematic and/or cholesteric phases over broad temperature ranges, and there is a need for polymer networks that exhibit cholesteric optical properties.