Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1998-09-16
2001-04-17
Yoon, Tae (Department: 1714)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S504000, C428S001400, C252S299600, C252S299700
Reexamination Certificate
active
06217948
ABSTRACT:
The invention relates to a polymer film with a helically twisted molecular structure exhibiting at least two maxima of the selective reflection wavelength, said polymer film being obtainable by a process comprising the following steps
A) coating a thermochromic polymerizable mesogenic composition comprising the following components
a) at least one achiral polymerizable mesogenic compound,
b) at least one chiral compound that is optionally also polymerizable and/or mesogenic,
c) a polymerization initiator, and
d) optionally a dye component,
on a substrate or between two substrates in form of a layer,
B) aligning the polymerizable mesogenic composition so that the axis of the molecular helix extends transversely to the layer,
C) optionally heating at least a part of the aligned composition to a defined temperature,
D) polymerizing at least a part of the aligned composition by exposure to actinic radiation,
E) optionally repeating step D) alone or together with step C) and/or steps A) and B) at least once, and
F) optionally removing the substrate or, if two substrates are present, one or both of the substrates from the polymerized material,
characterized in that the temperature is varied in step D).
The invention further relates to a process of such a polymer film and to the use of such a polymer film for optical data storage, photomasks, decorative pigments, cosmetics, security applications or active and passive optical elements such as polarizers, optical retarders or color filters.
Thermochromic materials, such as thermochromic compounds or compositions are characterized in that they show a change of the reflective wavelength upon temperature variation. They are well known in the prior art and usually consist of cholesteric liquid crystals (CLCs) with a helically twisted molecular structure, wherein the pitch p of the molecular helix is related to the reflected wavelength &lgr; and the average refractive index n of the liquid crystal by equation (1)
&lgr;=
n·p
(1)
If the reflected wavelengths are inside the visible range, the thermochromic material undergoes a visible colour change upon variation of the temperature.
Thermochromic compositions are disclosed for example in the international application WO 90/02161.
For many applications, such as optical information storage, security applications or the preparation of optical films like e.g. color filters, it is desirable that the reflected wavelength of the thermochromic material can be permanently fixed to remain constant over a broad range of temperatures. Furthermore, some applications like e.g. security devices or patterned color filters require that the material should exhibit more than one reflection maximum, in particular two or more reflective colors in the visible wavelength range, which furthermore should be temperature independent.
European Patent Application EP 0 661 287 A1 discloses cholesteric polymerizable liquid crystalline siloxanes with a temperature dependent reflection wavelength, and further proposes to prepare a structured color film by heating these materials to a given temperature and photopolymerizing them through a mask.
However, linear polymers and in particular siloxane polymers like those disclosed in EP 0 661 287 most often exhibit low glass transition temperatures and show only limited temperature stability. When heating these polymers, their optical properties are often deteriorating 1.
Consequently there is still a demand for a polymer film that exhibits two or more reflection maxima remaining almost constant over a wide temperature range, that has a high temperature stability, is easily obtainable and does not have the disadvantages of the materials of prior art as discussed above.
One of the aims of the present invention is to provide a polymer film having these properties. Another aim of the invention is to provide a process of preparing such a polymer film. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It has been found that the above mentioned aims can be achieved by providing a polymer film that is obtainable from a polymerizable thermochromic mesogenic composition with a helically twisted molecular structure by a process according to the present invention. In this process, the thermochromic composition is coated onto a substrate as a thin layer and aligned in a planar orientation. The helical pitch and thereby the reflection wavelength of the layer are then adjusted to the desired value temperature variation, and the molecular structure of the layer is fixed by polymerization, wherein the temperature is changed during the polymerization reaction so that the layer exhibits various reflective wavelengths which are subsequently fixed. This process yields a polymer film with two or more reflection wavelengths that remain stable over a wide temperature range.
One aspect of the present invention is a polymer film with a helically twisted molecular structure exhibiting at least two maxima of the selective reflection wavelength, said polymer film being obtainable by a process comprising the following steps
A) coating a thermochromic polymerizable mesogenic composition comprising the following components
a) at least one achiral polymerizable mesogenic compound,
b) at least one chiral compound that is optionally also polymerizable and/or mesogenic,
c) a polymerization initiator, and
d) optionally a dye component,
on a substrate or between two substrates in the form of a layer
B) aligning the polymerizable mesogenic composition so that the axis of the molecular helix extends transversely to the layer,
C) optionally heating at least a part of the aligned composition to a defined temperature,
D) polymerizing at least a part of the aligned composition by exposure to actinic radiation,
E) optionally repeating step D) alone or together with step C) and/or steps A) and B) at least once, and
F) optionally removing the substrate or, if two substrates are present, one or both of the substrates from the polymerized material,
characterized in that the temperature is varied in step D).
Preferred embodiments of the present invention relate to
a polymer film obtainable by a process as described above, wherein the temperature in step D) is varied continuously between a maximum and a minimum value,
a polymer film obtainable by a process as described above, wherein in step D) the thermochromic polymerizable mesogenic composition is covered at least partially with a photomask,
a polymer film obtainable by a process as described above, wherein the photomask exhibits at least one part having a transmission of the actinic radiation used in step D) that is different from at least one other part of the photomask,
a polymer film obtainable by a process as described above wherein component b) of the thermochromic polymerizable mesogenic composition comprises at least one polymerizable mesogenic chiral compound,
a polymer film exhibiting at least two different reflection maxima in the visible wavelength range.
Another aspect of the present invention is a process of preparing a polymer film with a helically twisted molecular structure exibiting at least two maxima of the selective reflection wavelengths as described above.
Another aspect of the present invention is a method of using a polymer film with a helically twisted molecular structure as described above for optical data storage, photomasks, decorative pigments, cosmetics, security applications or active and passive optical elements such as polarizers, optical retarders or color filters.
The terms polymerizable mesogen, polymerizable mesogenic compound or polymerizable liquid crystal or liquid crystalline compound as used in the foregoing and the following comprise compounds with a rod-shaped, board-shaped or disk-shaped mesogenic group (i.e. a group with the ability to induce mesophase behaviour in a compound comprising such a group). These compounds do not necessarily have to exhibit mesophase behaviour by themselves. In a preferred embodiment of the present inv
Coates David
Verrall Mark
Merck Patent Gesellschaft mit
Millen White Zelano & Branigan P.C.
Yoon Tae
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