Combination of optical elements

Abstract

The invention relates to a combination of optical elements comprising at least one optical retardation film and at least one broadband reflective polarizer, characterized in that the optical retardation film is comprising at least one layer of an anisotropic polymer material having an optical symmetry axis substantially parallel to the plane of the layer, said optical retardation film being obtainable by polymerization of a mixture of a polymerizable mesogenic material comprising: a) at least one reactive mosogen having at least one polymerizable functional group; b) an initiator; c) optionally a non-mesogenic compound having two or more polymerizable functional groups; and d) optionally a stabilizer, and relates to an optical retardation film used in said combination of optical elements and to a liquid crystal display comprising said combination of optical elements.

1 . A combination of optical elements comprising at least one optical retardation film and at least one broadband reflective polarizer, characterized in that the optical retardation film is comprising at least one layer of an anisotropic polymer material having an optical symmetry axis substantially parallel to the plane of the layer, said optical retardation film being obtainable by polymerization of a mixture of a polymerizable mesogenic material comprising 
 a) at least one reactive mesogen having at least one polymerizable functional group,    b) an initiator,    c) optionally a non-mesogenic compound having two or more polymerizable functional groups, and    d) optionally a stabilizer.    
     
     
         2 . A combination of optical elements according to  claim 1 , characterized in that the retardation of the optical retardation film is from 50 to 250 nm.  
     
     
         3 . A combination of optical elements according to  claim 1  or  2 , characterized in that the bandwidth of the wavelength band reflected by the broadband reflective polarizer is at least 100 nm.  
     
     
         4 . A combination of optical elements according to any of the  claims 1  to  3 , characterized in that it additionally comprises a compensation film comprising a layer of an anisotropic polymer material with a homeotropic or tilted homeotropic orientation, wherein the compensation film being can be positioned adjacent to either side of the optical retardation film.  
     
     
         5 . A combination of optical elements according to any of the  claims 1  to  4 , characterized in that it additionally comprises a linear polarizer, arranged in such a manner that the optical retardation film and, if present, the compensation film are positioned between the broadband reflective polarizer and the linear polarizer.  
     
     
         6 . A means to produce substantially linear polarized light comprising the following components 
 I) a combination of optical elements according to any of the  claims 1  to  5 ,    II) a radiation source, and    III) optionally a diffusor adjacent to the radiation source,    the components I to III being arranged in such a manner that the broadband reflective polarizer of the combination of optical elements I is facing the radiation source II or, if present, the diffusor III.    
     
     
         7 . An optical retardation film comprising at least one layer of an anisotropic polymer having an optical symmetry axis substantially parallel to the plane of the layer as used in the combination of optical elements according to any of the preceding claims, said optical retardation film being obtainable by 
 A) coating a mixture of a polymerizable mesogenic material comprising 
 a) at least one reactive mesogen having at least one polymerizable functional group,  
 b) an initiator,  
 c) optionally a non-mesogenic compound having two or more polymerizable functional groups, and  
 d) optionally a stabilizer  
 on a substrate or between two substrates in form of a layer,  
   B) aligning the polymerizable mesogenic material such that the optical symmetry axis is substantially parallel to the plane of the layer,    C) polymerizing said mixture by exposing it to heat or actinic radiation,    D) optionally repeating the steps A), B) and C) at least one more time,    and    E) optionally removing the substrate or, if present, one or two of the substrates from the polymerized material,    
     
     
         8 . An optical retardation film according to  claim 7 , characterized in that the substrate onto which the polymerizable mesogenic material is coated in step B) is a plastic sheet or film.  
     
     
         9 . An optical retardation film according to  claim 7  or  8 , characterized in that alignment of the polymerizable mesogenic material is achieved by directly rubbing at least one of the substrates onto which the polymerizable mesogenic material is coated in step B).  
     
     
         10 . An optical retardation film according to any of the  claims 7  to  9 , characterized in that the mixture of the polymerizable mesogenic material comprises at least one reactive mesogen having one polymerizable functional group and at least one reactive mesogen having two or more polymerizable functional groups.  
     
     
         11 . An optical retardation film according to any of the  claims 7  to  10 , characterized in that the reactive mesogens are compounds of formula I  
       P-(Sp-X) n -MG-R   I  
       wherein 
 P is a polymerizable group,  
 Sp is a spacer group having 1 to 20 C atoms,  
 X is a group selected from —O—, —S—, —CO—, —COO—, —OCO—, —OCOO— or a single bond,  
 n is 0 or 1,  
 MG is a mesogenic or mesogenity supporting group, preferably selected according to formula II  
 -(A 1 -Z 1 ) m -A 2 -Z 2 -A 3 -   II  
 wherein  
 A 1 , A 2  and A 3  are independently from each other 1,4-phenylene in which, in addition, one or more CH groups may be replaced by N, 1,4-cyclohexylene in which, in addition, one or two non-adjacent CH 2  groups may be replaced by O and/or S, 1,4-cyclohexenylene or naphthalene-2,6-diyl, it being possible for all these groups to be unsubstituted, mono- or polysubstituted with halogen, cyano or nitro groups or alkyl, alkoxy or alkanoyl groups having 1 to 7 C atoms wherein one or more H atoms may be substituted by F or Cl,  
 Z 1  and Z 2  are each independently —COO—, —OCO—, —CH 2 CH 2 —, —OCH 2 —, —CH 2 O—, —CH═CH—, —C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single bond,  
 m is0, 1 or2,  
 and  
 R is an alkyl radical with up to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH 2  groups to be replaced, in each case independently from one another, by —O—, —S , —NH—, —N(CH 3 )—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— in such a manner that oxygen atoms are not linked directly to one another, or alternatively R is halogen, cyano or has independently one of the meanings given for P-(Sp-X) n -.  
 
     
     
         12 . An optical retardation film according to  claim 10  or  11 , characterized in that the mixture of the polymerizable mesogenic material essentially consists of 
 a1) 15 to 95% by weight of at least one mesogen according to formula I having one polymerizable functional group,  
 a2) 5 to 80% by weight of at least one mesogen according to formula I having two or more polymerizable functional groups,  
 b) 0.01 to 5% by weight of an initiator,  
 c) 0 to 20% by weight of a non-mesogenic compound having two or more polymerizable functional groups,  
 d) 0 to 1000 ppm of a stabilizer, and  
 e) 0 to 5% by weight of a chain transfer agent.  
 
     
     
         13 . Mixture of a polymerizable mesogenic material according to any of the  claims 10  to  12 .  
     
     
         14 . A liquid crystal display device comprising a liquid crystal cell 10 and a means to produce substantially linear polarized light comprising a combination of optical elements, said combination comprising an optical retardation film according to any of the  claims 1  to  13 .