Optical film and method of producing same
The optical film is an optical film including a cholesteric liquid crystal layer, in which first regions having a first film thickness and second regions having a second film thickness which is smaller than the first film thickness are alternately arranged with a period P of 0.5 μm to 2.0 μm in at least one direction in a plane, a difference in film thickness between the first film thickness and the second film thickness is in a range of 0.2 μm to 1.0 μm, and a helical pitch of a cholesteric phase in the first region is different from a helical pitch of a cholesteric phase in the second region.
1. An optical film comprising:
a cholesteric liquid crystal layer,
wherein the cholesteric liquid crystal layer is formed such that first regions having a first film thickness and second regions having a second film thickness which is smaller than the first film thickness are alternately arranged with a period P of 0.5 μm to 2.0 μm in at least one direction in a plane,
a difference in film thickness between the first film thickness and the second film thickness is in a range of 0.2 μm to 1.0 μm, and
a helical pitch of a cholesteric liquid crystal in the first region is different from a helical pitch of a cholesteric liquid crystal in the second region.
2. The optical film according to claim 1 ,
wherein, in a case where the helical pitch in the first region is set as p 1 and the helical pitch in the second region is set as p 2 , p 1 and p 2 have a relationship of p 1 ×0.80≤p 2 ≤p 1 ×0.95.
3. The optical film according to claim 1 ,
wherein each of widths of the first region and the second region in the one direction is half the period P.
4. The optical film according to claim 1 ,
wherein the number of rotations of the helix of the cholesteric liquid crystal in the first region is the same as the number of rotations of the helix of the cholesteric liquid crystal in the second region.
5. The optical film according to claim 1 ,
wherein the cholesteric liquid crystals in the first region and the second region are formed of the same material.
6. The optical film according to claim 1 , further comprising:
an optically isotropic layer provided on an uneven surface of the cholesteric liquid crystal layer which is generated due to the difference in film thickness between the first region and the second region.
7. The optical film according to claim 6 ,
wherein a refractive index of the optically isotropic layer is in a range of 1.4 to 1.7.
8. The optical film according to claim 1 ,
wherein the first regions and the second regions have the same stripe shape and are alternately arranged in a width direction of the stripe shape.
9. The optical film according to claim 1 ,
wherein the first regions and the second regions have the same rectangular shape and are alternately arranged vertically and horizontally.
10. A method of producing the optical film according to claim 1 , which forms a cholesteric liquid crystal layer including first regions having a first film thickness that correspond to opening portions of a mask and second regions having a second film thickness smaller than the first film thickness that correspond to non-opening portions, the method comprising:
a coating step of uniformly coating a support with a polymerizable liquid crystal composition which contains a polymerizable liquid crystal compound containing a cationic polymerization group and a photoradical polymerization group, a chiral agent, and a cationic polymerization initiator to form a coated film;
an aging step of aligning a liquid crystal in a cholesteric phase in the coated film;
a first entire surface exposure step of irradiating an entire surface of the coated film aligned in the cholesteric phase with ultraviolet rays to cause a photocationic polymerization reaction, and curing a part of the coated film to obtain a liquid crystal semi-fixed film;
an initiator coating step of coating a surface of the liquid crystal semi-fixed film with an initiator supply liquid containing a photoradical polymerization initiator;
a mask exposure step of irradiating the liquid crystal semi-fixed film with ultraviolet rays through a mask having non-opening portions and opening portions which are alternately arranged in at least one direction in a plane with a period P of 0.5 μm to 2.0 μm in a state in which the mask is positioned on the liquid crystal semi-fixed film;
a second entire surface exposure step of peeling off the mask from the liquid crystal semi-fixed film and irradiating an entire surface of the liquid crystal semi-fixed film with ultraviolet rays to form an ultraviolet cured film; and
a heat treatment step of performing a heat treatment on the ultraviolet cured film.
11. The method of producing the optical film according to claim 10 ,
wherein the polymerizable liquid crystal composition contains a polymerizable liquid crystal compound represented by Formula (1),
Q-Sp 1 -L 1 -M 1 -L 2 -Sp 2 -Ox (1)
in Formula (1), Q represents a polymerizable group; any one of Sp 1 or Sp 2 represents branched alkylene or alkylene containing at least one divalent linking group selected from the group consisting of —O—, —C≡C—, and —S— in a chain, and the other represents linear alkylene; L 1 and L 2 each independently represent a divalent linking group; M 1 represents a mesogenic group containing at least one divalent group selected from the group consisting of divalent groups represented by Formulae (2-1) to (2-12); and Ox represents a group represented by Formula (3),
in Formula (3), R 2 represents a hydrogen atom, a methyl group, or an ethyl group, X 1 represents —O—, —S—, —OCO—, or —COO—, X 2 represents a single bond or alkylene having 1 to 4 carbon atoms, and the symbol “*” represents a bonding site with respect to Sp 2 .