Method of preparing an anisotropic polymer film on a substrate with a structured surface

Abstract

An anisotropic polymer film with improved alignmentis prepared by coating onto a substrate with a structured surface. The polymer films thereby obtained are useful in optical and electrooptical devices for decorative and security applications.

Description

[0001] The invention relates to a method of preparing an anisotropic polymer film with improved alignment on a substrate with a structured surface, to polymer films thereby obtained and their use in optical and electrooptical devices, and for decorative and security applications.BACKGROUND AND PRIOR ART[0002] Anisotropic polymer films comprising a polymerized liquid crystal material with uniform orientation are known in prior art. They are usually prepared by coating a thin layer of a polymerizable liquid crystal mixture onto a substrate, aligning the mixture into uniform orientation and polymerizing the mixture.[0003] For specific applications it is required to induce planar alignment in the liquid crystal layer, i.e., where the liquid crystal molecules are oriented substantially parallel to the layer. The alignment is then frozen in by polymerizing the liquid crystal mixture in situ. For example, oriented films or layers of polymerized nematic liquid crystal material with planar a...

AI Technical Summary

Benefits of technology

[0055] The above embodiments are particularly suitable for use as false-proof security markings on banknotes or documents of value, by which the document is easy to authenticate when viewed through one polariser against a dark or reflective background or when viewed between two polarisers.

[0057] If the polymerisable LC material comprises polymerisable mesogenic compounds having two or more polymerisable functional groups (di-or multireactive or di-or multifunctional compounds), upon polymerisation a three-dimensional polymer network is formed, which is self-supporting and shows a high mechanical and thermal stability and a low temperature dependence of its physical and optical properties. By varying the concentration of the multifunctional mesogenic or non mesogenic compounds the crosslink density of the polymer film and thereby its physical and chemical properties such as the glass transition temperature, which is also important for the temperature dependence of the optical properties of the polymerised film, the thermal and mechanical stability or the solvent resistance can be tuned easily.

Problems solved by technology

However, the methods of the prior art have several drawbacks.

The use of alignment layers or additives in the liquid crystal material leads to increased costs.

The formation of patterned films comprising regions of different orientation is difficult with these methods.

Rubbing of a substrate also implies several drawbacks when used in mass production of liquid crystal polymer films.

Whereas rubbing and thus alignment of the liquid crystal molecules can easily be achieved by conveying the substrate over a rubbing roller or between two rollers, alignment at any desired angle to the substrate moving direction requires more complicated rubbing stages, where the rollers can be positioned at an angle to the conveying direction of the substrate.

Also, rubbing in a direction at right angles to the substrate conveying direction is not possible with this method.

However, these documents do not mention polymerizable liquid crystal materials or the formation of polymer films with uniform or patterned orientation in large scale production.

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