Liquid crystal alignment layer

Abstract

This invention relates to a photopolymer mixture capable of use for a liquid crystal alignment layer and to an alignment layer formed from such a photopolymer mixture. The mixture comprises at least two polymerisable materials which are blended together in proportion so as to give a predetermined, preferably low, surface energy. A low surface energy, e.g. less than 4×10 −2 N/m can result in an alignment layer which imparts a particular orientation to liquid crystal molecules with out requiring any post cure treatments. The polymerisable materials may be monomers, oligomers or diluents that form long chain molecules when cured and the mixture may contain additives to lower the overall surface energy.

1 . A photopolymer mixture comprising at least a first polymerisable material and a second polymerisable material in proportion such that the mixture has a predetermined surface energy when cured.  
     
     
         2 . A photopolymer mixture as claimed in  claim 1  wherein the surface energy is such that, in use in a liquid crystal cell, the photopolymer imparts a predetermined orientation of the liquid crystal director with respect to the local surface normal.  
     
     
         3 . A photopolymer mixture as claimed in  claim 2  wherein the predetermined orientation is a homeotropic orientation.  
     
     
         4 . A photopolymer mixture as claimed in  claim 1  wherein the first short chainpolymerisable material is an oligomer or diluent.  
     
     
         5 . A photopolymer mixture as claimed in  4  wherein the second short chain polymerisable material is a monomer.  
     
     
         6 . A photopolymer mixture as claimed in  claim 1  wherein the mixture further comprises an additive.  
     
     
         7 . A photopolymer mixture as claimed in  claim 6  wherein the additive reduces the surface energy of the cured mixture.  
     
     
         8 . A photopolymer mixture as claimed in  claim 7  wherein the additive is an acrylate or an epoxy functionallised polydimethyl siloxane material.  
     
     
         9 . A photopolymer mixture as claimed in  claim 1  wherein the surface energy is less than4×10 −2  N/m.  
     
     
         10 . A photopolymer mixture as claimed in  claim 1  wherein the surface energy of the cured photopolymer has a dispersive energy of between 12 mN/m and 40 mN/m and a polar energy of betweenO. OmN/m and 15 mN/m.  
     
     
         11 . A photopolymer mixture as claimed in  claim 1  wherein the mixture comprises an immiscible component.  
     
     
         12 . A photopolymer mixture as claimed in  claim 11  wherein the immiscible component comprises one of the polymerisable materials.  
     
     
         13 . A photopolymer mixture as claimed in  claim 11  wherein the immiscible component comprises a solid particulate.  
     
     
         14 . A photopolymer mixture as claimed in  claim 1  wherein the proportion of first material to second material is such so as to give a predetermined viscosity and refractive index.  
     
     
         15 . A photopolymer mixture as claimed in  claim 12  wherein the refractive index is between 1.35 and 1.80.  
     
     
         16 . A photopolymer mixture as claimed in  claim 12  wherein the viscosity of the uncured photopolymer is between 5 centipoise and 10 poise.  
     
     
         17 . A photopolymer mixture comprising a first component and a second component together being capable of cured to form a polymeric material having a predetermined surface energy.  
     
     
         18 . (canceled)  
     
     
         19 . A liquid crystal alignment layer comprising a polymerised photopolymer bearing a surface profile characterised in that the polymerised photopolymer comprises a photopolymer mixture as claimed in  claim 1  which has been cured.  
     
     
         20 . A liquid crystal alignment layer according to  claim 19  wherein the alignment layer is formed on a substrate and surface energy of the alignment layer in contact with the substrate is greater than the surface energy of the alignment layer which, in use, contacts the liquid crystal material.  
     
     
         21 . (canceled)  
     
     
         22 . A liquid crystal cell comprising a liquid crystal material located between two cell walls wherein at least one of the cells walls carries an alignment layer according to  claim 19 .  
     
     
         23 . A liquid crystal cell as claimed in  claim 22  wherein the alignment layer has the same apparent refractive index as the liquid crystal material.  
     
     
         24 . A method of making an alignment layer for a liquid crystal material comprising the steps of; 
 i) taking a photopolymer mixture,    ii) introducing the photopolymer mixture to a substrate,    iii) taking a master bearing an inverse of the desired grating and impressing it into the photopolymer mixture on the substrate,    iv) curing the photopolymer mixture on the substrate, and v) removing the master from the cured photopolymer mixture characterised in that the photopolymer mixture comprises the photopolymer mixture according to  claim 1 .    
     
     
         25 . A method of making an alignment layer as claimed in  claim 24  wherein the photopolymer mixture has a component which has a lower surface energy than the rest of the mixture and the step of curing the photopolymer mixture involves curing the mixture such that there is a greater concentration of the lower surface energy component at one surface of the cured mixture than the other.  
     
     
         26 . A photopolymer mixture comprising a first polymerisable material and a second polymerisable material, the proportion of first polymerisable material to second polymerisable material being such that the photopolymer mixture has a predetermined viscosity and refractive index.