Liquid crystalline film with broadened reflection bandwidth and process for preparation thereof

• US 20060083866A1
• Filed: 12/02/2005
• Published: 04/20/2006
• Est. Priority Date: 06/05/2003
• Status: Active Grant

• Current Assignee: Sicpa Holding SA
• Original Assignee: Consortium Fur Elektrochemische Industrie GmbH
• Inventors: Hanelt, Eckhard
• Granted Patent: US 7,311,952 B2
• Time in Patent Office: Days
• US Class Current: 428/1.100
• CPC Class Codes: C09K 2323/00 Functional layers of liquid crystal optical display excluding electroactive liquid crystal layer characterised by chemical composition
• CPC Class Codes: C09K 2323/03 Viewing layer characterised by chemical composition
• CPC Class Codes: C09K 2323/031 Polarizer or dye
• CPC Class Codes: G02B 5/3016 involving passive liquid crystal elements optical properties of liquid crystals G02F1/0063; polarising elements associated with active liquid crystal devices G02F1/133528

Abstract

Reflective liquid crystalline polymer films having increased bandwidth have a single layer of liquid crystalline material with short, long, and medium pitch of the helical molecular structure of the liquid crystalline polymer. The films are easily produced by a readily reproducible and economic process of low complexity.

Claims

1. A polymer film comprising a helical molecular structure which, in a direction at right angles to the film, contains a sequence of short, long and medium pitch of the helical molecular structure, the calculated pitch based on reflection wavelength/average refractive index, the film comprising a single layer of a polymerized mixture of polymerizable liquid-crystalline material, wherein the change between short and long pitch takes place in less than ten turns of the helical structure, and wherein layers with short, long and medium pitches have a layer thickness of at least three pitches in each layer.

2. The polymer film of claim 1, wherein the film has a reflection band which has been broadened by at least 50% compared to the reflection band in the unpolymerized state.

3. The polymer film of claim 1, wherein the thickness of the film is from 2 to 20 times the width of the reflection band divided by (n_e−

n_o) where n_e and n₀ are the refractive indices for the extraordinary (n_e) and the ordinary (n_e) beams in a birefringent medium.

4. The polymer film of claim 1, wherein the substrate is a polymer film which contains no additional oxygen barrier layer.

5. The polymer film of claim 1, wherein partial polymerization of the polymerizable mixture is effected at a temperature within a phase range having helical alignment of mesogens of the liquid-crystalline material.

6. The polymer film of claim 1, wherein an environment having an inhibiting action formed by a gas which comprises oxygen is present during a first, partial polymerization.

7. The polymer film of claim 1, wherein the mixture of polymerizable liquid-crystalline material comprises monomers or oligomers which differ in their reactivity with respect to polymerization.

8. The polymer film of claim 1, wherein polymerization is effected by actinic radiation acting through a mask in the course of a partial polymerization, in the course of a final fixing or in the course of both, and the mask is subsequently altered or replaced by a second mask, and this process step is repeated optionally with alteration of further process parameters in such a way that a portion of the material which is yet to be finally fixed is irradiated with actinic radiation.

9. An LC pigment having helical molecular structure, prepared by comminuting a polymer film prepared by the process of claim 1.

12. A wavelength-selective or polarization-selective optical element for electromagnetic radiation from the near ultraviolet to the infrared range, comprising at least one layer of the polymer film of claim 1.

13. A reflective linear polarizer, comprising in combination, a polymer film of claim 1, and a λ

/4 retardation film.

14. In a reflector or polarizer in a liquid-crystal display, visible or invisible marking, security element, reflector for heat or infrared radiation or for decorative purposes, wherein a reflective polymer film or pigment is employed, the improvement comprising employing as at least one film and/or at least one pigment, a film of claim 1 or a pigment prepared by comminuting a film thereof.

15. A projector, projection display or illumination body comprising at least one layer of a polymer film of claim 1.

17. A liquid-crystal apparatus having a liquid-crystalline material disposed between two electrodes, optionally further optical components selected from the group consisting of light source(s), mirror(s), polarizer(s), retarder(s), and color filter(s), and at least one film for selecting colored light comprising a polymer film of claim 1.

18. The projector, projection display, or illumination body of claim 15, wherein said layer of a polymer film has been commuted into particles.

10. A process for producing a polymer film having an adjustable pitch gradient of a helical molecular structure, comprising the steps of:

a) applying in a layer on a substrate, a mixture of polymerizable liquid-crystalline material which comprises

a)i) at least one monomer or oligomer, each of which has at least one mesogenic group and at least one polymerizable functional group, and

a)ii) at least one chiral compound,

b) aligning the material such that the mesogenic groups become arranged in a helical structure whose axis is transverse to the layer,

c) partially polymerizing the layer by the action of actinic radiation in an environment which has an inhibiting action on the polymerization, and

d) after the material has become newly aligned in a semipolymerized structure, final fixing by fully polymerizing the film or cooling the film into the glass state.

11. The process of claim 10, wherein the application of a mixture of polymerizable liquid-crystalline material and the subsequent alignment of the mesogens are carried out under cleanroom conditions with the exclusion of UV radiation.

16. A liquid-crystal apparatus having a liquid-crystalline material disposed between two electrodes, comprising at least one film for generating or increasing the yield of polarized light, and optionally further optical components selected from the group consisting of light source(s), mirror(s), polarizer(s), retarder(s), color filter(s) and combinations thereof.