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Emerald Group Publishing Limited
Copyright © 2000, MCB UP Limited
Keywords: Recording, Optics, Polymers, Media
Applicant: Akzo Nobel NV (NL)Patent number: US5976638Publication date: 2 November 1999Title: Optical recording medium comprising a homoeotropically oriented liquid crystalline polymer film comprising dichroic acid
The invention pertains to an optical recording medium comprising a homoeotropically oriented liquid crystalline polymer, which comprises at least one dichroic dye, has a Tg between 75 and 100°C and a Tc between 110°C and 140°C. By "homeotropically oriented" is meant that the mesogenic groups of the liquid crystalline polymer are oriented perpendicular to the surface of the film. The perpendicular orientation along with the presence of the dichroic dye makes it possible to employ a different technique for writing out and reading data. In this case, contrast is obtained by the dichroic dye contained in the film being aligned in the same direction as the mesogenic groups of the liquid crystalline polymer. Dichroic dyes will absorb one polarization direction of the light to a much greater extent than the other ones. Generally this direction, averaged out on the basis of molecular movement, coincides with the long axis of the dye molecule. In a virgin film the mesogenic groups, and hence the dichroic dye molecules, are oriented perpendicular to the film's surface, and there is only low absorption of the incident light by these molecules. By local heating or irradiation the homeotropic orientation is converted to an isotropic one. As the irradiated or heated areas are cooled off rapidly (below the Tg of the liquid crystalline polymer), the isotropic orientation is frozen in. In the case of an isotropically written trace, the dichroic dye will likewise be isotropically oriented and frozen in, resulting in a substantially higher absorption of the incident light. The reading takes place as a function of the absorption maximum of the dichroic dye at visible light, ultraviolet light or infra-red light. Employing this technique in optical data storage makes it possible to attain a high contrast, even for thin films.