U.S. patent number RE39,605 [Application Number 10/947,488] was granted by the patent office on 2007-05-01 for combination of optical elements.
This patent grant is currently assigned to Merck Patent GmbH. Invention is credited to David Coates, James Hanmer, Mark Verrall, Jeremy Ward.
United States Patent |
RE39,605 |
Verrall , et al. |
May 1, 2007 |
**Please see images for:
( Certificate of Correction ) ** |
Combination of optical elements
Abstract
The invention relates to a combination of optical elements
comprising at least one optical retardation film and at least one
broadband reflective polarizer, characterized in that the optical
retardation film is comprising at least one layer of an anisotropic
polymer material having an optical symmetry axis substantially
parallel to the plane of the layer, said optical retardation film
being obtainable by polymerization of a mixture of a polymerizable
mesogenic material comprising: a) at least one reactive mesogen
having at least one polymerizable functional group; b) an
initiator, c) optionally a non-mesogenic compound having two or
more polymerizable functional groups; and d) optionally a
stabilizer; and relates to an optical retardation film used in said
combination of optical elements and to a liquid crystal display
comprising said combination of optical elements.
Inventors: |
Verrall; Mark (Taoyuan,
TW), Ward; Jeremy (Manchester, GB), Hanmer;
James (Southampton, GB), Coates; David (Wimborne,
GB) |
Assignee: |
Merck Patent GmbH (Darmstadt,
DE)
|
Family
ID: |
8223046 |
Appl.
No.: |
10/947,488 |
Filed: |
July 11, 1997 |
PCT
Filed: |
July 11, 1997 |
PCT No.: |
PCT/EP97/03676 |
371(c)(1),(2),(4) Date: |
January 25, 1999 |
PCT
Pub. No.: |
WO98/04651 |
PCT
Pub. Date: |
February 05, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09230335 |
Jan 25, 1999 |
06544605 |
Apr 8, 2003 |
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Foreign Application Priority Data
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Jul 26, 1996 [EP] |
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96112100 |
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Current U.S.
Class: |
428/1.3; 252/585;
349/117; 252/299.01 |
Current CPC
Class: |
G02F
1/13363 (20130101); C09K 19/38 (20130101); G02F
1/133543 (20210101); G02F 1/13362 (20130101); G02F
1/133633 (20210101); C09K 2019/0448 (20130101); C09K
2323/031 (20200801); G02F 1/133536 (20130101); C09K
2323/00 (20200801); C09K 2323/03 (20200801); G02F
2413/08 (20130101); G02F 2413/02 (20130101) |
Current International
Class: |
C09K
19/38 (20060101); C09K 19/52 (20060101); G02F
1/133 (20060101) |
Field of
Search: |
;428/1.3,1.31
;252/299.01,582,585 ;349/117 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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524028 |
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529813 |
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622789 |
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606940 |
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0606940 |
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643121 |
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0643121 |
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704513 |
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823442 |
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860455 |
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2306470 |
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2299333 |
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2306470 |
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WO 9016005 |
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WO 9422977 |
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Oct 1994 |
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WO |
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WO |
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WO |
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WO 9744409 |
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WO |
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WO 9744702 |
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Nov 1997 |
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WO |
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Primary Examiner: Miggins; Michael C.
Attorney, Agent or Firm: Millen, White, Zelano, Branigan,
P.C.
Claims
What is claimed is:
1. A combination of optical elements comprising at least one
optical retardation film and at least one broadband reflective
polarizer, wherein said optical retardation film comprises at least
one layer of an anisotropic polymeric material having an optical
symmetry axis which has a tilt angle relative to the plane of the
layer of 0.degree.-25.degree., said optical retardation film is
obtained by polymerization of a mixture of a polymerizable
mesogenic material comprising a) at least one reactive mesogen
having at least one polymerizable functional group, b) an
initiator, c) optionally a non-mesogenic compound having two or
more polymerizable functional groups, and d) optionally a
stabilizer.
2. A combination of optical elements according to claim 1,
.[.characterized in that.]. .Iadd.wherein .Iaddend.the retardation
of the optical retardation film is from 50 to 250 nm.
3. A combination of optical elements according to claim 1, wherein
the broadband of wavelength reflected by the broadband reflective
polarizer is at least 100 nm.
4. A combination of optical elements according to claim 1, further
comprising a compensation film comprising a layer of an anisotropic
polymeric material with a homeotropic or tilted homeotropic
orientation, wherein the compensation film is positioned adjacent
to either side of the optical retardation film.
5. A combination of optical elements according to claim 4, further
comprising a linear polarizer, and wherein the optical retardation
film and the compensation film are positioned between the broadband
reflective polarizer and the linear polarizer.
6. A device for producing substantially linear polarized light
comprising the following components I) a combination of optical
elements according to claim 1, II) a radiation source, and III)
optionally a diffusor adjacent to the radiation source, wherein the
components I to III are arranged whereby the broadband reflective
polarizer of component I is facing the radiation source of
component II or, optionally, the diffusor of component
.[.diffusor.]. III.
7. An optical retardation film comprising at least one layer of an
anisotropic polymeric material having an optical symmetry axis
which has a tilt angle relative to the plane of the layer of
0.degree.-25.degree., wherein said optical retardation film is
obtained by A) coating mixture of a polymerizable mesogenic
material comprising a) a least one reactive mesogen having at least
one polymerizable functional group, b) an initiator, c) optionally
a non-mesogenic compound having two or more polymerizable
functional groups, and d) optionally a stabilizer on a substrate or
between two substrates in form of a layer, B) aligning the
polymerizable mesogenic material such that the optical symmetry
axis has a tilt angle relative to the plane of the layer of
0.degree.-25.degree..[...]. .Iadd.,.Iaddend. C) polymerizing said
mixture by exposing it to heat or actinic radiation, D) optionally
repeating the steps A), B) and C) at least one more time, and E)
optionally removing at least one substrate from the polymerized
material.
8. An optical retardation film according to claim 7, wherein the
substrate onto which the polymerizable mesogenic material is coated
in step B) is a plastic sheet or film.
9. An optical retardation film according to claim 7, wherein
alignment of the polymerizable mesogenic material is achieved by
directly rubbing at least one of the substrates onto which the
polymerizable mesogenic material is coated in step B).
10. An optical retardation film according to claim 7, wherein the
mixture of the polymerizable mesogenic material comprises at least
one reactive mesogen having one polymerizable functional group and
at least one reactive mesogen having two or more polymerizable
functional groups.
11. An optical retardation film according to claim 7, wherein said
reactive mesogens are compounds of formula I
P--(Sp--X).sub.n--MG--R I wherein P is a polymerizable group, Sp is
a spacer group having 1 to 20 C atoms, X is a group selected from
--O--, --S--, --CO--, --COO--, --OCO--, --OCOO-- or a single bond,
n is 0 or 1, MG is a group according to formula II
--(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3-- II wherein
A.sup.1, A.sup.2 and A.sup.3 are independently from each other
1,4-phenylene, 1,4-phenylene in which one or more CH groups is
replaced by N, 1,4-cyclohexylene, 1,4-cyclohexylene in which one or
two non-adjacent CH.sub.2 groups is replaced in each case by O and
S, 1,4-cyclohexenylene or naphthalene-2,6-diyl, wherein these
groups are unsubstituted, or mono- or polysubstituted by halogen,
cyano, or nitro groups or alkyl, alkoxy or alkanoyl groups having 1
to 7 C atoms wherein one or more H atoms are in each case
optionally replaced by F or Cl, Z.sup.1 and Z.sup.2 are each
independently --COO--, --OCO--, --CH.sub.2CH.sub.2--,
--OCH.sub.2--, --CH.sub.2O--, --CH.dbd.CH--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond, m is 0, 1
or 2, and R is an alkyl radical with up to 25 C atoms which is
unsubstituted, or is mono- or polysubstituted by halogen or CN, it
being also possible for one or more non-adjacent CH.sub.2 groups to
be replaced, in each case independently from one another, by --O--,
--S--, --NH--, --N(CH.sub.3)--, --CO--, --COO--, --OCO--,
--OCO--O--, --S--CO--, --CO--S-- or --C.ident.C-- in such a manner
that oxygen atoms are not linked directly to one another, or R is
halogen, cyano or has independently one of the meanings given for
P--(Sp--X).sub.n--.
12. An optical retardation film according to claim 11, wherein the
mixture of the polymerizable mesogenic material consists
essentially of a1) 15 to 95% by weight of at least one mesogen
according to formula I having one polymerizable functional group,
a2) 5 to 80% by weight of at least one mesogen according to formula
I having two or more polymerizable functional groups, b) 0.01 to 5%
by weight of an initiator, c) 0 to 20% by weight of a non-mesogenic
compound having two or more polymerizable functional groups, d) 0
to 1000 ppm of a stabilizer, and e) 0 to 5% by weight of a chain
transfer agent.
13. In a liquid crystal display device comprising a liquid crystal
cell and a means to produce substantially linear polarized light
comprising a combination of optical elements, the improvement
wherein said combination of optical elements comprises an optical
retardation film according to claim 7.
14. A mixture of polymerizable mesogenic material comprising: a1)
15 to 95% by weight of at least one mesogen according to formula I
having one polymerizable functional group, P--(Sp--X).sub.n--MG--R
I wherein P is a polymerizable group, Sp is a spacer group having 1
to 20 C atoms, X is a group selected from --O--, --S--, --CO--,
--COO--, --OCO--, --OCOO-- or a single bond, n is 1, MG is a group
according to formula II
--(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3-- II wherein
A.sup.1, A.sup.2 and A.sup.3 are independently from each other
1,4-phenylene, 1,4-phenylene in which one or more CH groups is
replaced by N, 1,4-cyclohexylene, 1,4-cyclohexylene in which one or
two non-adjacent CH.sub.2 groups is replaced in each case by O or
S, 1,4-cyclohexenylene or naphthalene-2,6-diyl, wherein these
groups are unsubstituted, or mono- or polysubstituted by halogen,
cyano or nitro groups or alkyl, alkoxy or alkanoyl groups having 1
to 7 C atoms wherein one or more H atoms are in each case
optionally replaced by F or Cl, Z.sup.1 and Z.sup.2 are each
independently --COO--, --OCO--, --CH.sub.2CH.sub.2--,
--OCH.sub.2--, --CH.sub.2O--, --CH.dbd.CH--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond, m is 0, 1
or 2, and R is an alkyl radical with up to 25 C atoms which is
unsubstituted, or is mono- or polysubstituted by halogen or CN, it
being also possible for one or more non-adjacent CH.sub.2 groups to
be replaced, in each case independently from one another, by --O--,
--S--, --NH--, --N(CH.sub.3)--, --CO--, --COO--, --OCO--,
--OCO--O--, --S--CO--, --CO--S-- or --C.ident.C-- in such a manner
that oxygen atoms are not linked directly to one another, or R is
halogen or cyano, a2) 5 to 50% by weight of at least one mesogen
according to formula I' having two polymerizable groups P,
P--(Sp--X).sub.n--MG--R' I' wherein P, Sp, X, n, and MG are,
independently, as defined above and R' is a group of the formula
P--(Sp--X).sub.n-- wherein P, Sp X and n are, independently, as
defined above, b) 0.01 to 5% by weight of an initiator, c) 0 to 20%
by weight of a non-mesogenic compound having two or more
polymerizable functional groups, d) 0 to 1000 ppm of a stabilizer,
and e) 0 to 5% by weight of a chain transfer agent.
15. A combination of optical elements according to claim 1 further
comprising a linear polarizer, and wherein the optical retardation
film is positioned between the broadband reflective polarizer and
the linear polarizer.
16. An optical retardation film according to claim 10, wherein said
reactive mesogens are compounds of formula I
P--(Sp--X).sub.n--MG--R I wherein P is a polymerizable group, Sp is
a spacer group having 1 to 20 C atoms, X is a group selected from
--O--, --S--, --CO--, --COO--, --OCO--, --OCOO-- or a single bond,
n is 0 or 1, MG is a group according to formula II
--(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3-- II wherein
A.sup.1, A.sup.2 and A.sup.3 are independently from each other
1,4-phenylene, 1,4-phenylene in which one or more CH groups is
replaced by N, 1,4-cyclohexylene, 1,4-cyclohexylene in which one or
two non-adjacent CH.sub.2 groups is replaced in each case by O or
S, 1,4-cyclohexenylene or naphthalene-2,6-diyl, wherein these
groups are unsubstituted, or mono- or polysubstituted by halogen,
cyano, or nitro groups or alkyl, alkoxy or alkanoyl groups having 1
to 7 C atoms wherein one or more H atoms are in each case
optionally replaced by F or Cl, Z.sup.1 and Z.sup.2 are each
independently --COO--, --OCO--, --CH.sub.2CH.sub.2--,
--OCH.sub.2--, --CH.sub.2O--, --CH.dbd.CH--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond, m is 0, 1
or 2, and R is an alkyl radical with up to 25 C atoms which is
unsubstituted, or is mono- or polysubstituted by halogen or CN, it
being also possible for one or more non-adjacent CH.sub.2 groups to
be replaced, in each case independently from one another, by --O--,
--S--, --NH--, --N(CH.sub.3)--, --CO--, --COO--, --OCO--,
--OCO--O--, --S--CO--, --CO--S-- or --C.ident.C-- in such a manner
that oxygen atoms are not linked directly to one another, or R is
halogen cyano or has independently one of the meanings given for
P--(Sp--X).sub.n--.
17. An optical retardation film according to claim 11, wherein P is
CH.sub.2.dbd.CW--COO--, WCH.dbd.CH--O--, ##STR00010## or
CH.sub.2.dbd.CH-Phenyl-(O).sub.k--, W is H, CH.sub.3 or Cl, k is 0
or 1, and Sp is a linear or branched alkylene group having 1-20 C
atoms in which, optionally one or more non-adjacent CH.sub.2 groups
is in each case replaced by --O--, --S--, --NH--, --N(CH.sub.3)--,
--CO--, --O--CO--, --S--CO--, --O--COO--, --CO--S--, --CO--O--,
--CH (halogen)--, --CH(CN)--, --CH.dbd.CH-- or --C.ident.C--.
18. A combination of optical elements according to claim 1, wherein
said mixture of polymerizable mesogenic material further comprises
one or more surface active compounds.
19. An optical retardation film according to claim 7, wherein said
mixture of polymerizable mesogenic material further comprises one
or more surface active compounds.
20. A mixture of polymerizable mesogenic material according to
claim 14, further comprising one or more surface active
compounds.
21. A combination of optical elements according to claim 1, wherein
said at least one layer of anisotropic polymeric has an optical
symmetry axis with a tilt angle, relative to the plane of the
layer, of 0.degree.-15.degree..
22. A combination of optical elements according to claim 1, wherein
said at least one layer of anisotropic polymeric has an optical
symmetry axis with a tilt angle, relative to the plane of the
layer, of 0.degree.-10.degree..
23. A combination of optical elements according to claim 1, wherein
said at least one layer of anisotropic polymeric has an optical
symmetry axis with a tilt angle, relative to the plane of the
layer, of 0.degree.-5.degree..
24. A combination of optical elements according to claim 1, wherein
said reactive mesogens are compounds of formula I
P--(Sp--X).sub.n--MG--R I wherein P is a polymerizable group, Sp is
a spacer group having 1 to 20 C atoms, X is a group selected from
--O--, --S--, --CO--, --COO--, --OCO--, --OCOO-- or a single bond,
n is 0 or 1, MG is a group according to formula II
--(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3-- II wherein
A.sup.1, A.sup.2 and A.sup.3 are independently from each other
1,4-phenylene, 1,4-phenylene in which one or more CH groups is
replaced by N, 1,4-cyclohexylene, 1,4-cyclohexylene in which one or
two non-adjacent CH.sub.2 groups is replaced in each case by O or
S, 1,4-cyclohexenylene or naphthalene-2,6-diyl, wherein these
groups are unsubstituted, or mono- or polysubstituted by halogen,
cyano, or nitro groups or alkyl, alkoxy or alkanoyl groups having 1
to 7 C atoms wherein one or more H atoms are in each case
optionally replaced by F or Cl, Z.sup.1 and Z.sup.2 are each
independently --COO--, --OCO--, --CH.sub.2CH.sub.2--,
--OCH.sub.2--, --CH.sub.2O--, --CH.dbd.CH--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond, m is 0, 1
or 2, and R is an alkyl radical with up to 25 C atoms which is
unsubstituted, or is mono- or polysubstituted by halogen or CN, it
being also possible for one or more non-adjacent CH.sub.2 groups to
be replaced, in each case independently from one another, by --O--,
--S--, --NH--, --N(CH.sub.3)--, --CO--, --COO--, --OCO--,
--OCO--O--, --S--CO--, --CO--S-- or --C.ident.C-- in such a manner
that oxygen atoms are not linked directly to one another, or R is
halogen, cyano or has independently one of the meanings given for
P--(Sp--X).sub.n--.
25. A combination for optical elements according to claim 24,
wherein P is CH.sub.2.dbd.CW--COO--, WCH.dbd.CH--O--, ##STR00011##
or CH.sub.2.dbd.CH-Phenyl-(O).sub.k--, W is H, CH.sub.3 or Cl, k is
0 or 1, and Sp is a linear or branched alkylene group having 1-20 C
atoms in which, optionally one or more non-adjacent CH.sub.2 groups
is in each case replaced by --O--, --S--, --NH--, --N(CH.sub.3)--,
--CO--, --O--CO--, --S--CO--, --O--COO--, --CO--S--, --CO--O--,
--CH (halogen)--, --CH(CN)--, --CH.dbd.CH-- or --C.ident.C--.
26. An optical retardation film according to claim 16, wherein P is
CH.sub.2.dbd.CW--COO--, WCH.dbd.CH--O--, ##STR00012## or
CH.sub.2.dbd.CH-Phenyl-(O).sub.k--, W is H, CH.sub.3 or Cl, k is 0
or 1, and Sp is a linear or branched alkylene group having 1-20 C
atoms in which, optionally one or more non-adjacent CH.sub.2 groups
is in each case replaced by --O--, --S--, --NH--, --N(CH.sub.3)--,
--CO--, --O--CO--, --S--CO--, --O--COO--, --CO--S--, --CO--O--,
--CH (halogen)-, --CH(CN)--, --CH.dbd.CH-- or --C.ident.C--.
27. A mixture of polymerizable material according to claim 14,
wherein P is CH.sub.2.dbd.CW--COO--, WCH.dbd.CH--O--, ##STR00013##
or CH.sub.2.dbd.CH-Phenyl-(O).sub.k--, W is H, CH.sub.3 or Cl, k is
0 or 1, and Sp is a linear or branched alkylene group having 1-20 C
atoms to which, optionally one or more non-adjacent CH.sub.2 groups
is in each case replaced by --O--, --S--, --NH--, --N(CH.sub.3)--,
--CO--, --O--CO--, --S--CO--, --O--COO--, --CO--S--, --CO--O--,
--CH (halogen)-, --CH(CN)--, --CH.dbd.CH-- or --C.ident.C--.
28. A combination of optical elements according to claim 1, wherein
said optical retardation film provides a retardation of 50-250
nm.
29. A combination of optical elements according to claim 28,
wherein said optical retardation film has a thickness of 0.2-30
.mu.m.
30. A retardation film according to claim 7, wherein said optical
retardation film provides a retardation of 50-250 nm.
31. A retardation film according to claim 7, wherein said optical
retardation film has a thickness of 0.2-30 .mu.m.
32. An optical retardation film according to claim 11, wherein the
mixture of the polymerizable mesogenic material consists
essentially of components a1) 15 to 99% by weight of at least one
mesogen according to formula I having one polymerization functional
group, a2) 0 to 90% by weight of at least one mesogen according to
formula I having two or more polymerizable functional groups, b)
0.01 to 5% by weight of an initiator, c) 0 to 20% by weight of a
non-mesogenic compound having two or more polymerizable functional
groups, d) 0 to 1000 ppm of a stabilizer, and e) 0 to 5% by weight
of a chain transfer agent.
33. A film of claim 32, wherein the amount of component a2 is
0%.
34. A mixture of polymerizable mesogenic material comprising
components a1) 40 to 99% by weight of at least one mesogen
according to formula I having one polymerization functional group,
P--(Sp--X).sub.n--MG--R I wherein P is a polymerizable group, Sp is
a spacer group having 1 to 20 C atoms, X is a group selected from
--O--, --S--, --CO--, --COO--, --OCO--, --OCOO-- or a single bond,
n is 1, MG is a group according to formula II
--(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3-- II wherein
A.sup.1, A.sup.2 and A.sub.3 are independently from each other
1,4-phenylene, 1,4-phenylene in which one or more CH groups is
replaced by N, 1,4-cyclohexylene, 1,4-cyclohexylene in which one or
two non-adjacent CH.sub.2 groups is replaced in each case by O or
S, 1,4-cyclohexenylene or naphthalene-2,6-diyl, wherein these
groups are unsubstituted, or mono- or polysubstituted by halogen,
cyano, or nitro groups or alkyl, alkoxy or alkanoyl groups having 1
to 7 C atoms wherein one or more H atoms are in each case
optionally replaced by F or Cl, Z.sup.1 and Z.sub.2 are each
independently --COO--, --OCO--, --CH.sub.2CH.sub.2--,
--OCH.sub.2--, --CH.sub.2O--, --CH.dbd.CH--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond, m is 0, 1
or 2, and R is an alkyl radical with up to 25 C atoms which is
unsubstituted, or is mono- or polysubstituted by halogen or CN, it
being also possible for one or more non-adjacent CH.sub.2 groups to
be replaced, in each case independently from one another, by --O--,
--S--, --NH--, --N(CH.sub.3)--, --CO--, --COO--, --OCO--,
--OCO--O--, --S--CO--, --CO--S-- or --C.ident.C-- in such a manner
that oxygen atoms are not linked directly to one another, or R is
halogen or cyano, a2) >0 to 90% by weight of at least one
mesogen according to formula I' having two or more polymerizable
functional groups, P--(Sp--X).sub.n--MG--(X--Sp).sub.n--P I'
wherein P, Sp, X, n, and MG are each independently as defined
above, b) 0.01 to 5% by weight of an initiator, c) 0 to 20% by
weight of a non-mesogenic compound having two or more polymerizable
functional groups, d) 0 to 1000 ppm of a stabilizer, and e) 0 to 5%
by weight of a chain transfer agent.
35. A mixture of claim .[.33.]. .Iadd.34.Iaddend., wherein said
mixture contains two to eight different mesogens of formula I
having one polymerizable group.
36. A mixture of claim 34, where wherein P is in each case
independently CH.sub.2.dbd.CW--COO--, WCH.dbd.CH--O--,
CH.sub.2.dbd.CH-Phenyl-(O).sub.k--, or ##STR00014## W is H,
CH.sub.3 or Cl, k is 0 or 1, and Sp is a linear or branched
alkylene group having 1-20 C atoms in which, optionally one or more
non-adjacent CH.sub.2 groups is in each case replaced by --O--,
--S--, --NH--, --N(CH.sub.3)--, --CO--, --O--CO--, --S--CO--,
--O--COO--, --CO--S--, --CO--O--, --CH(halogen)-, --CH(CN)--,
--CH.dbd.CH-- or --C.ident.C--.
37. A mixture of claim 14, wherein MG is selected from formulae
II-1 to II-16: TABLE-US-00003 -Phe-Z.sup.2-Phe- II-1
-Phe-Z.sup.2-Cyc- II-2 -PheL-Z.sup.2-Phe- II-3 -PheL-Z.sup.2-Cyc-
II-4 -Phe-Z.sup.2-PheL- II-5 -Phe-Z.sup.1-Phe-Phe- II-6
-Phe-Z.sup.1-Phe-Cyc- II-7 -Phe-Z.sup.1-Phe-Z.sup.2-Phe- II-8
-Phe-Z.sup.1-Phe-Z.sup.2-Cyc- II-9 -Phe-Z.sup.1-Cyc-Z.sup.2-Phe-
II-10 -Phe-Z.sup.1-Cyc-Z.sup.2-Cyc- II-11
-Phe-Z.sup.1-PheL-Z.sup.2-Phe- II-12 -Phe-Z.sup.1-Phe-Z.sup.2-PheL-
II-13 -PheL-Z.sup.1-Phe-Z.sup.2-PheL- II-14
-PheL-Z.sup.1-PheL-Z.sup.2-Phe- II-15
-PheL-Z.sup.1-PheL-Z.sup.2-PheL- II-16
wherein L is F, Cl, CN, NO.sub.2, CH.sub.3, C.sub.2H.sub.5,
OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3, COC.sub.2H.sub.5, CF.sub.3,
OCF.sub.3, OCHF.sub.2, or OC.sub.2F.sub.5.
38. A .[.mixture of.]. .Iadd.combination of optical elements
according to .Iaddend.claim 25, wherein MG is selected from
formulae II-1 to II-16: TABLE-US-00004 -Phe-Z.sup.2-Phe- II-1
-Phe-Z.sup.2-Cyc- II-2 -PheL-Z.sup.2-Phe- II-3 -PheL-Z.sup.2-Cyc-
II-4 -Phe-Z.sup.2-PheL- II-5 -Phe-Z.sup.1-Phe-Phe- II-6
-Phe-Z.sup.1-Phe-Cyc- II-7 -Phe-Z.sup.1-Phe-Z.sup.2-Phe- II-8
-Phe-Z.sup.1-Phe-Z.sup.2-Cyc- II-9 -Phe-Z.sup.1-Cyc-Z.sup.2-Phe-
II-10 -Phe-Z.sup.1-Cyc-Z.sup.2-Cyc- II-11
-Phe-Z.sup.1-PheL-Z.sup.2-Phe- II-12 -Phe-Z.sup.1-Phe-Z.sup.2-PheL-
II-13 -PheL-Z.sup.1-Phe-Z.sup.2-PheL- II-14
-PheL-Z.sup.1-PheL-Z.sup.2-Phe- II-15
-PheL-Z.sup.1-PheL-Z.sup.2-PheL- II-16
wherein L is F, Cl, CN, NO.sub.2, CH.sub.3, C.sub.2H.sub.5,
OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3, COC.sub.2H.sub.5, CF.sub.3,
OCF.sub.3, OCHF.sub.2, or OC.sub.2F.sub.5.
39. A mixture of claim 27 wherein MG is selected from formulae II-1
to II-16: TABLE-US-00005 -Phe-Z.sup.2-Phe- II-1 -Phe-Z.sup.2-Cyc-
II-2 -PheL-Z.sup.2-Phe- II-3 -PheL-Z.sup.2-Cyc- II-4
-Phe-Z.sup.2-PheL- II-5 -Phe-Z.sup.1-Phe-Phe- II-6
-Phe-Z.sup.1-Phe-Cyc- II-7 -Phe-Z.sup.1-Phe-Z.sup.2-Phe- II-8
-Phe-Z.sup.1-Phe-Z.sup.2-Cyc- II-9 -Phe-Z.sup.1-Cyc-Z.sup.2-Phe-
II-10 -Phe-Z.sup.1-Cyc-Z.sup.2-Cyc- II-11
-Phe-Z.sup.1-PheL-Z.sup.2-Phe- II-12 -Phe-Z.sup.1-Phe-Z.sup.2-PheL-
II-13 -PheL-Z.sup.1-Phe-Z.sup.2-PheL- II-14
-PheL-Z.sup.1-PheL-Z.sup.2-Phe- II-15
-PheL-Z.sup.1-PheL-Z.sup.2-PheL- II-16
wherein L is F, Cl, CN, NO.sub.2, CH.sub.3, C.sub.2H.sub.5,
OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3, COC.sub.2H.sub.5, CF.sub.3,
OCF.sub.3, OCHF.sub.2, or OC.sub.2F.sub.5.
40. A mixture of claim 34, wherein MG is selected from formulae
II-1 to II-16: TABLE-US-00006 -Phe-Z.sup.2-Phe- II-1
-Phe-Z.sup.2-Cyc- II-2 -PheL-Z.sup.2-Phe- II-3 -PheL-Z.sup.2-Cyc-
II-4 -Phe-Z.sup.2-PheL- II-5 -Phe-Z.sup.1-Phe-Phe- II-6
-Phe-Z.sup.1-Phe-Cyc- II-7 -Phe-Z.sup.1-Phe-Z.sup.2-Phe- II-8
-Phe-Z.sup.1-Phe-Z.sup.2-Cyc- II-9 -Phe-Z.sup.1-Cyc-Z.sup.2-Phe-
II-10 -Phe-Z.sup.1-Cyc-Z.sup.2-Cyc- II-11
-Phe-Z.sup.1-PheL-Z.sup.2-Phe- II-12 -Phe-Z.sup.1-Phe-Z.sup.2-PheL-
II-13 -PheL-Z.sup.1-Phe-Z.sup.2-PheL- II-14
-PheL-Z.sup.1-PheL-Z.sup.2-Phe- II-15
-PheL-Z.sup.1-PheL-Z.sup.2-PheL- II-16
wherein L is F, Cl, CN, NO.sub.2, CH.sub.3, C.sub.2H.sub.5,
OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3, COC.sub.2H.sub.5, CF.sub.3,
OCF.sub.3, OCHF.sub.2, or OC.sub.2F.sub.5.
41. An optical retardation film of claim 7, wherein said film is a
quarter wave retardation film.
42. A device according to claim 6, further comprising a metallic or
nonmetallic reflector.
43. An optical retardation film according to claim 7, wherein the
polymerizable mesogenic material is coated and cured onto a
compensation film which serves as a substrate.
44. An optical retardation film according to claim 43, wherein the
compensation film that serves as a substrate comprises a layer of
anisotropic polymeric material with a homeotropic or tiled
homeotropic orientation.
45. An optical retardation film according to claim 7, wherein the
polymerizable mesogenic material is coated and cured directly onto
a reflective polarizer which serves as a substrate.
46. An optical retardation film according to claim 45, wherein the
reflective polarizer is a broadband reflective polarizer.
47. An optical retardation film in combination with a compensation
film comprising: an optical retardation film comprising at least
one layer of an anisotropic polymeric material having an optical
symmetry axis which has a tilt angle relative to the plane of the
layer 0.degree. to 25.degree., combined with a compensation film
comprising a layer of anisotropic polymeric material with a
homeotropic or tiled homeotropic orientation, wherein each of said
optical retardation film and compensation film are obtained by
polymerization of an oriented layer of reactive mesogens, and
wherein one of said optical retardation film and compensation film
is prepared on the other of said films which serves as a
substrate.
48. An optical retardation film in combination with a broadband
reflective polarizer compromising: an optical retardation film
comprising at least one layer of an anisotropic polymeric material
having an optical symmetry axis which has a tilt angle relative to
the plane of the layer of 0.degree. to 25.degree., combined with a
broadband reflective polarizer film, wherein each of said optical
retardation film and broadband reflective polarizer film are
obtained by polymerization of an oriented layer of reactive
mesogens, and wherein one of said optical retardation film and
broadband reflective polarizer film is prepared on the other of
said films which serves as a substrate.
49. In a liquid crystal display device the improvement wherein said
device comprises a combination of optical elements according to
claim 1.
50. In a liquid crystal display device the improvement wherein said
device further comprises a device according to claim 6.
51. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
7.
52. An optical retardation film according to claim 7 in combination
with a linear polarizer.
53. An optical retardation film according to claim 11 in
combination with a linear polarizer.
54. An optical retardation film according to claim 30 in
combination with a linear polarizer.
55. An optical retardation film according to claim 32 in
combination with a linear polarizer.
56. An optical retardation film according to claim 41 in
combination with a linear polarizer.
57. An optical retardation film according to claim 43 in
combination with a linear polarizer.
58. An optical retardation film according to claim 44 in
combination with a linear polarizer.
59. An optical retardation film according to claim 47 in
combination with a linear polarizer.
60. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
52.
61. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
53.
62. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
54.
63. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
55.
64. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
56.
65. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
57.
66. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
58.
67. In a liquid crystal display device the improvement wherein said
device comprises an optical retardation film according to claim
59.
.Iadd.68. A mixture of polymerizable mesogenic material according
to claim 34, further comprising one or more surface active
compounds..Iaddend.
.Iadd.69. An optical retardation film according to claim 11,
wherein said mixture of polymerizable mesogenic material further
comprises one or more surface active compounds..Iaddend.
.Iadd.70. A mixture according to claim 14, wherein MG is selected
from formulae IIa to IIn ##STR00015## ##STR00016## wherein L is F,
Cl, CN or an optionally fluorinated alkyl, alkoxy or alkanoyl group
having 1 to 4 C atoms, and r is 0, 1 or 2..Iaddend.
.Iadd.71. A mixture according to claim 34, wherein MG is selected
from formulae IIa to IIn ##STR00017## ##STR00018## wherein L is F,
Cl, CN or an optionally fluorinated alkyl, alkoxy or alkanoyl group
having 1 to 4 C atoms, and r is 0, 1 or 2..Iaddend.
.Iadd.72. A mixture according to claim 36, wherein MG is selected
from formulae IIa to IIn ##STR00019## ##STR00020## wherein L is F,
Cl, CN or an optionally fluorinated alkyl, alkoxy or alkanoyl group
having 1 to 4 C atoms, and r is 0, 1 or 2..Iaddend.
.Iadd.73. A mixture according to claim 70, wherein the group
##STR00021## .Iaddend.
.Iadd.74. A mixture according to claim 71, wherein the group
##STR00022## .Iaddend.
.Iadd.75. A mixture according to claim 14, wherein said at least
one mesogen according to formulas I and I' are selected from
formulae Ia to Ig ##STR00023## wherein x and y are each
independently 1 to 12, A is a 1,4-phenylene or 1,4-cyclohexylene
group, R.sup.1 is halogen, cyano or an optionally halogenated alkyl
or alkoxy group with 1 to 12 C atoms, and L.sup.1 and L.sup.2 are
each independently H, halogen, CN, or an alkyl, alkoxy or alkanoyl
group with 1 to 7 C atoms..Iaddend.
.Iadd.76. A mixture according to claim 34, wherein said mesogens
according to formulas I and I' are selected from formulae Ia to Ig
##STR00024## wherein x and y are each independently 1 to 12, A is a
1,4-phenylene or 1,4-cyclohexylene group, R.sup.1 is halogen, cyano
or an optionally halogenated alkyl or alkoxy group with 1 to 12 C
atoms, and L.sup.1 and L.sup.2 are each independently H, halogen,
CN, or an alkyl, alkoxy or alkanoyl group with 1 to 7 C
atoms..Iaddend.
.Iadd.77. A mixture according to claim 14, further comprising a
solvent..Iaddend.
.Iadd.78. A mixture according to claim 77, wherein said solvent is
toluene..Iaddend.
.Iadd.79. A mixture according to claim 34, further comprising a
solvent..Iaddend.
.Iadd.80. A mixture according to claim 79, wherein said solvent is
toluene..Iaddend.
Description
The invention relates to a combination of optical elements
comprising at least one optical retardation film and at least one
broadband reflective polarizer, characterized in that the optical
retardation film is comprising at least one layer of an anisotropic
polymer material having an optical symmetry axis substantially
parallel to the plane of the layer, said optical retardation film
being obtainable by polymerization of a mixture of a polymerizable
mesogenic material comprising a) at least one reactive mesogen
having at least one polymerizable functional group, b) an
initiator, c) optionally a non-mesogenic compound having two or
more polymerizable functional groups, and d) optionally a
stabilizer.
The invention further relates to a means to produce substantially
linear polarized light comprising a combination of optical elements
as described above. The invention also relates to an optical
retardation film used in such a combination of optical elements and
to a liquid crystal display comprising such a combination of
optical elements.
FIGS. 1a and 1b show a display device according to preferred
embodiments of the present invention.
FIG. 2 shows the retardation versus viewing angle of inventive
optical retardation films compared to a state of the art optical
retardation film.
FIG. 3 shows the measurement setup according to example B of the
present invention.
FIG. 4 shows the spectrum of a broad waveband reflective polarizer
that is used in combination together with an inventive optical
retardation film in a special embodiment of the invention.
FIG. 5 shows the relative luminance versus viewing angle for
inventive combinations of a broad waveband reflective polarizer and
an inventive optical retardation film compared to a combination of
a broad waveband reflective polarizer with a state of the art
optical retardation film.
FIG. 6 shows a method of rubbing a substrate for the use in the
process of preparing an inventive optical retardation film.
The European Patent Application EP 0 606 940-A1 discloses a
cholesteric reflective polarizer that produces circular polarized
light of a high luminance over a broad range of wavelengths.
However, since for most applications, e.g. in liquid crystal
displays, polarizers producing linear polarized light are needed,
EP 0 606 940 further describes the combined use of the broadband
cholesteric polarizer together with a quarter wave foil or plate
(QWF), which transforms the circular polarized light transmitted by
the cholesteric polarized into linear polarized light.
The laid open WO 96/02016 discloses a linear polarizer consisting
of a combination of the broadband cholesteric polarizer described
above and a quarter wave plate comprising a stretched film of an
isotropic polymer with negative birefringence.
However, when a liquid crystal display comprising a cholesteric
polarizer like those described in EP 0 606 940 and WO 96/02016 is
watched under an increasing viewing angle, its optical properties,
like e.g. the luminance and the contrast ratio usually
deteriorate.
It has therefore been desired to have available an optical
retardation film that, when used together with a broad waveband
cholesteric reflective polarizer, like e.g. those described in EP 0
606 940 and WO 96/02016 as mentioned above, produces substantially
linear polarized light and that improves the optical properties of
the reflective polarizer over a wide range of viewing angles.
Optical retardation films have been described in prior art. Usually
uniaxially stretched films of a prefabricated isotropic or LC
polymer like those described in the above mentioned WO 96/02016 are
used for this purpose.
Optical retardation films made of polymerized mixtures of reactive
mesogens have also been mentioned. Research Disclosure, May 1992,
p.411, No. 33799 describes an achromatic wave plate made of a stack
of two layers between glass substrates which under irradiation with
light shows a net retardation of 1/4 of the value of the wavelength
of light incident on the stack. Each of the layers consists of an
anisotropic polymer network which is obtained by curing an oriented
layer of a mesogenic diacrylate.
However, polymerizable liquid crystalline compositions containing
only one polymerizable compound as disclosed in the above mentioned
Research Disclosure in general exhibit high or even very high
melting points, which in turn requires high temperatures for
alignment and polymerization, which is a serious drawback when
manufacturing such layers.
Furthermore, the process of manufacturing a quarter wave plate as
described in the above mentioned Research Disclosure is complicated
as it requires that the two layers are coated, aligned and cured in
two separate steps. This is especially a disadvantage for mass
production, since the first of the films is used as a substrate for
producing the second film, which significantly increases the costs
in case production losses occur when manufacturing the second
layer, or requires more sophisticated production procedures and
controls.
Furthermore, the above mentioned document only discloses the use of
glass substrates for the production of the quarter wave plate, but
does not teach a method of producing of a quarter wave plate as a
flexible film with a large area, which is most desired for a large
scale production and for many applications.
Consequently there has been a considerable demand for an optical
retardation film that, when used together with a broad waveband
reflective polarizer, enhances the optical properties of the
polarizer over a wide range of viewing angles, that is easy to
fabricate in large scale as a flexible film with a large area and
does not have the disadvantages of the prior art optical
retardation films as discussed above.
One of the aims of the present invention is to provide an optical
retardation film having these properties. Another aim of the
invention is to provide a combination of optical elements
comprising such an optical retardation film and a broadband
reflective polarizer. Yet another aim of the invention is a liquid
crystal display device comprising a liquid crystal cell and such a
combination of optical elements. 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 these aims can be achieved by providing an
optical retardation film and a combination of optical elements
comprising such an optical retardation film and a broadband
reflective polarizer according to the present invention.
The object of the invention is a combination of optical elements
comprising at least one optical retardation film and at least one
broadband reflective polarizer, characterized in that the optical
retardation film comprises at least one layer of an anisotropic
polymer material having an optical symmetry axis substantially
parallel to the plane of the layer, said optical retardation film
being obtainable by polymerization of a mixture of a polymerizable
mesogenic material comprising a) at least one reactive mesogen
having at least one polymerizable functional group, b) an
initiator, c) optionally a non-mesogenic compound having two or
more polymerizable functional groups, and d) optionally a
stabilizer.
In a preferred embodiment of the present invention, the bandwidth
of the wavelength band reflected by the broadband reflective
polarizer is at least 100 nm.
In another preferred embodiment of the present invention the
retardation of the optical retardation film is from 50 to 250
nm.
In another preferred embodiment of the present invention, the
combination of optical elements additionally comprises a
compensation film comprising a layer of an anisotropic polymer
material with a homeotropic or tilted homeotropic orientation, the
compensation film being positioned adjacent to either side of the
optical retardation film.
In another preferred embodiment of the present invention, the
combination of optical elements additionally comprises a linear
polarizer, arranged in such a manner that the optical retardation
film and, if present, the compensation film are positioned between
the broadband reflective polarizer and the linear polarizer.
Another object of the present invention is a means to produce
substantially linear polarized light comprising the following
components I) a combination of optical elements comprising at least
one optical retardation film and at least one broadband reflective
polarizer and optionally a linear polarizer and a compensation film
as described above, II) a radiation source, and III) optionally a
diffusor adjacent to the radiation source, wherein the components I
to III are arranged in such a manner that the broadband reflective
polarizer of the combination of optical elements I is facing the
radiation source II or, if present, the diffusor III.
Another object of the present invention is an optical retardation
film which comprises at least one layer of an anisotropic polymer
with an optical symmetry axis substantially parallel to the plane
of the layer and which can be used in the combination of optical
elements as described above and below, said optical retardation
film being obtainable by A) coating a mixture of a polymerizable
mesogenic material comprising a) a least one reactive mesogen
having at least one polymerizable functional group, b) an
initiator, c) optionally a non-mesogenic compound having two or
more polymerizable functional groups, and d) optionally a
stabilizer on a substrate or between two substrates in form of a
layer; B) aligning the polymerizable mesogenic material such that
the optical symmetry axis is substantially parallel to the plane of
the, layer, C) polymerizing said mixture by exposing it to heat or
actinic radiation, D) optionally repeating the steps A), B) and C)
at least one more time, and E) optionally removing the substrate
or, if present, one or two of the substrates from the polymerized
material,
In a preferred embodiment of the present invention, the substrate
onto which the polymerizable mesogenic material is coated in step
B) is a plastic sheet or film.
In another preferred embodiment of the present invention, the
alignment of the polymerizable mesogenic material is achieved by
directly rubbing at least one of the substrates onto which the
polymerizable mesogenic material is coated in step B).
In another preferred embodiment of the present invention, the
mixture of the polymerizable mesogenic material comprises at least
one reactive mesogen having one polymerizable functional group and
at least one polymerizable mesogen having two or more polymerizable
functional groups.
In yet another preferred embodiment of the present invention, the
reactive mesogens comprised in the inventive mixture of the
polymerizable mesogenic material as described above and below are
compounds of formula I P--(Sp--X).sub.n--MG--R I wherein P is a
polymerizable group, Sp is a spacer group having 1 to 20 C atoms, X
is a group selected from --O--, --S--, --CO--, --COO--, --OCO--,
--OCOO-- or a single bond, n is 0 or 1, MG is a mesogenic or
mesogenity supporting group, preferably selected according to
formula II --(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3--
II wherein A.sup.1, A.sup.2 and A.sup.3 are independently from each
other 1,4-phenylene in which, in addition, one or more CH groups
may be replaced by N, 1,4-cyclohexylene in which, in addition, one
or two non-adjacent CH.sub.2 groups may be replaced by O and/or S,
1,4-cyclohexenylene or naphthalene-2,6-diyl, it being possible for
all these groups to be unsubstituted, mono- or polysubstituted with
halogen, cyano or nitro groups or alkyl, alkoxy or alkanoyl groups
having 1 to 7 C atoms wherein one or more H atoms may be
substituted by F or Cl, Z.sup.1 and Z.sup.2 are each independently
--COO--, --OCO--, --CH.sub.2CH.sub.2--, --OCH.sub.2--,
--CH.sub.2O--, --CH.dbd.CH--, --C.ident.C--, --CH.dbd.CH--COO--,
--OCO--CH.dbd.CH-- or a single bond and m is 0, 1 or 2, and R is an
alkyl radical with up to 25 C atoms which may be unsubstituted,
mono- or polysubstituted by halogen or CN, it being also possible
for one or more non-adjacent CH.sub.2 groups to be replaced, in
each case independently from one another, by --O--, --S--, --NH--,
--N(CH.sub.3)--, --CO--, --COO-- --OCO--, --OCO--O--, --S--CO--,
--CO--S-- or --C.ident.C-- in such a manner that oxygen atoms are
not linked directly to one another, or alternatively R is halogen,
cyano or has independently one of the meanings given for
P--(Sp--X).sub.n--.
Another object of the present invention is a liquid crystal display
device comprising a liquid crystal cell and a means to produce
substantially linear polarized light comprising a combination of
optical elements, said combination comprising an optical
retardation film as described above and below.
The retardation of the inventive optical retardation film is
preferably ranging from 50 to 250 nm, very preferably from 60 to
200 nm, most preferably from 70 to 170 nm.
The optical retardation film according to the present invention is
preferably used in a combination of optical elements together with
a broadband reflective polarizer. When using this combination,
light that is substantially linearly polarized can be produced.
The bandwidth of the wavelength band reflected by the reflective
polarizer according to the inventive combination of optical
elements is at least 100 nm, preferably at least 150 nm, most
preferably at least 200 nm, ideally 250 nm or larger.
Preferably the bandwidth of the reflective polarizer covers the
spectrum of visible light.
In another preferred embodiment of the present invention, the
optical retardation film according to the present invention is used
together with a broadband reflective polarizer, wherein the
retardation of the optical retardation film is substantially 0.25
times a wavelength reflected by the reflective polarizer, so that
the optical retardation film serves as a quarter wave retardation
film (QWF).
The term `a wavelength reflected by the reflective polarizer` in
this connection is indicating a wavelength of a value of the FWHM
(full width half maximum) of the wavelength band reflected by the
reflective polarizer.
In a preferred embodiment of the present invention this wavelength
is selected from the range of the center wavelength .+-.95%, in
particular .+-.70%, most preferably .+-.50% of the HWHM (half width
half maximum, which is 0.5 times the value of the FWHM) of the
wavelength band reflected by the reflective polarizer.
In another preferred embodiment of the present invention this
wavelength is ranging from the center wavelength plus 50% of the
HWHM to the centre wavelength plus 99% of the HWHM of the
wavelength band reflected by the reflective polarizer.
In another preferred embodiment of the present invention this
wavelength is ranging from the centre wavelength minus 50% of the
HWHM to the centre wavelength minus 99% of the HWHM of the
wavelength band reflected by the reflective polarizer.
Another preferred embodiment of the present invention is
characterized in that the combination of optical elements
additionally comprises a compensation film in order to compensate
the viewing angle dependence of the phase retardation of light
transmitted by the optical retardation film and/or the reflective
polarizer. The compensation film can be positioned adjacent to
either side of the optical retardation film.
Preferably a compensation film is used in which the phase
retardation is opposite in sign and substantially equal in
magnitude to the phase retardation of the reflective polarizer over
a wide range of viewing angles.
Particularly preferably a compensation film is used that comprises
a layer of an anisotropic polymer material with a homeotropic or
tilted homeotropic orientation.
Light incident on the broadband reflective polarizer is transformed
into circularly polarized light. However, due to the angle
dependence of the phase retardation of at least one of the optical
elements of the inventive combination comprising the reflective
polarizer, the optical retardation film and optionally the
compensation film a part of the light passing through these optical
elements will become elliptically polarized. This part of the light
can lead to undesired reduction of the contrast of the display.
Therefore in a preferred embodiment of the present invention a
linear polarizer is provided in the optical path of the display
behind the optical components of the combination mentioned above in
order to block the part of light that is not ideally polarized.
As a linear polarizer a commercially available polarizer can be
used. In a preferred embodiment of the present invention the linear
polarizer is a low contrast polarizer. In another preferred
embodiment of the present invention the linear polarizer is a
dichroic polarizer.
Preferably the linear polarizer is positioned such that the angle
between the optical axis of the linear polarizer and the major
optical axis of the inventive optical retardation film is in a
range from 30 to 60 degrees, especially preferably from 40 to 50
degrees.
The inventive optical retardation film comprises a layer of a
polymerized mesogenic material and is characterized by a
significantly high birefringence. Furthermore, the optical
properties of the optical retardation film, like e.g. the
birefringence, can be controlled by variation of the type and ratio
of the reactive mesogens in the polymerizable material.
For a liquid crystal display comprising a broad band reflective
polarizer and an optical retardation film of the state of the art,
like e.g. a quarter wave film (QWF) made of stretched PVA, the
luminance at normal incidence (viewing angle 0.degree.) and at low
values of the viewing angle is increased compared to a liquid
crystal display comprising the reflective polarizer alone without
an optical retardation film.
However, as the display comprising the a broad band reflective
polarizer and a QWF as mentioned above is viewed under an
increasing angle, the increasing phase retardation by the QWF
itself causes a reduction to the luminance, coinciding with the
value measured for the display comprising the reflective polarizer
as a single component at a certain angle. This angle is referred to
as the `cross-over angle` .alpha..sub.c.
When using an inventive optical retardation film instead of a
conventional QWF in the liquid crystal display, the cross-over
angle .alpha..sub.c increases significantly. In other words, the
brightness enhancement, i.e., the increase of luminance at low
viewing angles, that was achieved by using the reflective polarizer
is now extended also to large viewing angles.
The cross over angle .alpha..sub.c of a display comprising a
combination of optical elements comprising an optical retardation
film and a broadband reflective polarizer according to the present
invention is preferably 25.degree. or larger, particularly
preferably 30.degree. or larger, very particularly preferably
35.degree. or larger in all directions of observation.
The optical retardation films according to the present invention
comprise at least one layer of an anisotropic polymer having a
symmetry axis that is substantially parallel to the plane of the
layer. The term substantially parallel indicates in the foregoing
and the following, that the optical symmetry axis of said layer has
a tilt angle relative to the plane of the layer being in the range
from 0 to 25 degrees, preferably 0 to 15 degrees, in particular
from 0 to 10 degrees. Especially preferred are tilt angles from 0
to 5 degrees, in particular tilt angles of approximately 0
degrees.
Another object of the present invention is a means to produce
substantially linear polarized light comprising a combination of
optical elements as described in the foregoing and the following
and additionally comprising a radiation source, which is positioned
on the side of the reflective polarizer not facing the other
optical elements of the above mentioned combination.
As a radiation source preferably a standard backlight for liquid
crystal displays, like e.g. a side-lit or a meander type backlight,
can be used. These backlights typically comprise a lamp, a
reflector, a light guide and optionally a diffuser.
The radiation source can also consist of a reflector that reflects
radiation generated outside the means to produce substantially
linear polarized light. The display device according to the present
invention can then be used as a reflective display.
The function of the inventive combination of optical elements is
further explained by FIG. 1a, which shows a display device
according to a preferred embodiment of the present invention as an
example that should not limit the scope of the invention. The main
direction of light following the optical path is from the left side
to the right side. The display device 10 consists of a side-lit
backlight unit 11 with a lamp 12a and a combined light guide and
reflector 12b, a diffusor 13 and a polarizer combination consisting
of a reflective polarizer 14 comprising a layer of a liquid
crystalline material with a helically twisted molecular
orientation, the inventive optical retardation film 15, optionally
a compensation film 16 and a linear polarizer 17. The figure
further depicts a liquid crystal cell 18 and a second linear
polarizer 19 behind the display cell.
Light emitted from the backlight 11 interacts with the molecular
helix structure of the reflective polarizer 14 with the result that
50% of the intensity of the light incident on the reflective
polarizer is transmitted as circular polarized light that is either
right-handed or left-handed circular polarized depending on the
twist sense of the molecular helix structure of the reflective
polarizer, whereas the other 50% of the incident light is reflected
as circular polarized light of the opposite handedness. The
reflected light is depolarized by the backlight and redirected by
the reflector 12b onto the reflective polarizer 14. In this manner,
theoretically 100% of the light of a broad range of wavelengths
emitted from the backlight 11 is converted into circularly
polarized light.
The main part of the transmitted component is converted by the
inventive optical retardation film 15 into linear polarized light,
which is then compensated by the compensation film 16, if present,
and being-transmitted by the linear polarizer 17, whereas light
which is not completely transferred into linear polarized light by
the optical retardation film 15, such as elliptically polarized
light, is not transmitted by the linear polarizer 17. The linear
polarized light then passes through the display 18 and the second
linear polarizer 19 to reach the viewer 20.
FIG. 1b depicts a display device according to another preferred
embodiment of the invention having essentially the same
construction as that shown in FIG. 1a, with the modification being
that the inventive optical retardation film 15 is placed behind the
compensation film 16 when looking from the direction of incident
light.
As described above, the high efficiency of the reflective polarizer
is achieved by making use of the light reflected by the reflective
polarizer after it has been reversed, for example in the backlight
unit of the display, and redirected back again to the
polarizer.
One preferred embodiment of the present invention is characterized
in that the means to produce linear polarized light is comprising a
reflector in order to re-reflect circular polarized light reflected
by the reflective polarizer. This can be for example a metallic or
a non metallic reflector.
In case a metallic reflector is used the light coming from the
reflective polarizer is re-reflected as circularly polarized light
with opposite twist sense. This reflected circular polarized light
is then compatible with the molecular helix of the reflective
polarizer and is fully transmitted by the reflective polarizer.
In case a non metallic reflector is used, the light coming from the
reflective polarizer is depolarized and interacts again with the
reflective polarizer as described above. Depolarization of the
light reflected by the reflective polarizer can also occur due to
internal reflection and/or refraction in and/or between the optical
components of the display.
In another preferred embodiment of the present invention the means
to produce substantially linear polarized light comprises at least
one diffuser film or sheet situated between the backlight and the
reflective polarizer in order to optimize the angular distribution
of the light incident on the reflective polarizer and/or to
depolarize light redirected onto the reflective polarizer by the
reflector as described above.
The means to produce substantially linear polarized light according
to the present invention may also comprise one or more adhesive
layers provided to at least one of the components comprising the
reflective polarizer, the optical retardation film, the
compensation film, the linear polarizer and the diffuser
sheet(s).
The means to produce substantially linear polarized light according
to the present invention can further comprise one or more
protective layers provided to at least one of the components
comprising the reflective polarizer, the optical retardation film,
the compensation film, the linear polarizer, the diffuser sheet(s)
and the adhesive layer(s) in order to protect these components
against environmental influence.
The inventive optical retardation films are obtainable by coating
the mixture of a polymerizable mesogenic material on at least one
substrate in form of a layer, aligning the material and
polymerizing the aligned material. As a substrate for example a
glass or quartz sheet as well as a plastic film or sheet can be
used. It is also possible to put a second substrate on top of the
coated mixture prior to and/or during and/or after polymerization.
The substrates can be removed after polymerization or not. When
using two substrates in case of curing by actinic radiation, at
least one substrate has to be transmissive for the actinic
radiation used for the polymerization.
Isotropic or birefringent substrates can be used. In case the
substrate is not removed from the polymerized film after
polymerization, preferably isotropic substrates are used.
Preferably at least one substrate is a plastic substrate such as
for example a film of polyester such as polyethylene-terephthalate
(PET), a polyvinylalcohol (PVA), polycarbonate (PC) or
triacetylcellulose (TAC), especially preferably a PET film or a TAC
film. As a birefringent substrate for example an uniaxially
stretched plastic film can be used. For example PET films are
commercially available from ICI Corp. under the trade name
Melinex.
Planar alignment in the coated layer of the inventive mixture of
the polymerizable mesogenic material, i.e. an orientation wherein
the mesogenic material has a symmetry axis that has a low tilt
angle relative to the plane of the layer, can be achieved for
example by shearing the material, e.g. by means of a doctor blade.
It is also possible to apply an alignment layer, for example a
layer of rubbed polyimide or sputtered SiO.sub.x, on top of at
least one of the substrates.
An especially preferred embodiment of the present invention is
characterized in that planar alignment of the polymerizable
mesogenic material is achieved by directly rubbing the substrate,
i.e. without applying an additional alignment layer. This is a
considerable advantage as it allows a significant reduction of the
production costs of the optical retardation film. In this way a low
tilt angle can easily be achieved.
The term low tilt angle in connection with the aligned layer of the
mesogenic material before and/or after polymerization according to
the present invention is indicating in the foregoing and the
following that the mesogenic material has a symmetry axis with a
tilt angle relative to the plane of the layer that is preferably
smaller than 10 degrees, especially preferably smaller than 5
degrees, in particular smaller than 3 degrees and ideally
substantially zero degrees.
Preferably a plastic film, in particular a polyester film, e.g.
Melinex, or a TAC film are used as a substrate in this preferred
embodiment.
It is also possible to use a polymer film as a substrate, which is
annealed after rubbing near the glass transition temperature
T.sub.g of the polymer in order to reduce the tilt angle. For
example, when using a Melinex film (T.sub.g 140.degree. C.) as a
substrate, the substrate can be rubbed and subsequently annealed
for 20 to 40 minutes at a temperature of about 130 to 140.degree.
C.
When using an anisotropic substrate like e.g. a Melinex film, the
alignment quality is depending on the rubbing angle, i.e. the angle
between the major rubbing direction and the major optical symmetry
axis of the anisotropic substrate. Preferably rubbing is carried
out unidirectionally in a direction substantially parallel to the
major symmetry axis of the substrate.
For example rubbing can be achieved by means of a rubbing cloth or
with a flat bar coated with a rubbing cloth.
In another preferred embodiment of the present invention rubbing is
achieved by means of a at least one rubbing roller, like e.g. a
fast spinning roller that is brushing over the substrate, or by
putting the substrate between at least two rollers, wherein in each
case at least one of the rollers is optionally covered with a
rubbing cloth.
In another preferred embodiment of the present invention rubbing is
achieved by wrapping the substrate at least partially at a defined
angle around a roller that is preferably coated with a rubbing
cloth.
This method is exemplarily described by FIG. 6, that depicts a
substrate 1, like e.g a plastic web, which is wrapped at an angle a
around a rotating roller 2, with the arrow indicating the moving
direction of the web 1. The roller 2 may also be covered by a
rubbing cloth. An inventive polymerizable mesogenic mixture being
coated on the web 1 that was rubbed by this method shows planar
alignment of a high uniformity with a very low or even
substantially no tilt.
As rubbing cloth all materials can be used that are known to the
skilled in the art for this purpose. For example velvet of a
commercially available standard type can be used as a rubbing
cloth.
Preferably rubbing is carried out only in one direction.
The ability of the substrate to induce alignment in an inventive
polymerizable mesogenic composition coated on this substrate after
rubbing the substrate will depend on the process parameters of the
rubbing process, like the rubbing pressure and rubbing speed and,
in case a rubbing roller is used, on the rotational velocity of the
roller, the rubbing roller circumference and the tension on the
substrate.
The rubbing length in the rubbing process according to the above
described preferred embodiments is preferably from 0.2 to 5 meters,
in particular from 0.5 to 3 meters, most preferably from 1.0 to 2.5
meters.
Polymerization of the inventive polymerizable mesogenic mixture
takes place by exposing it to heat or to actinic radiation. Actinic
radiation means irradiation with light, X-rays, gamma rays or
irradiation with high energy particles, such as ions or electrons.
In particular preferably UV light is used. The irradiation
wavelength is preferably from 250 nm to 420 nm, especially
preferably from 320 nm to 390 nm.
As a source for actinic radiation for example a single UV lamp or a
set of UV lamps can be used. When using a high lamp power the
curing time can be reduced. The irradiance produced by the lamp
used in the invention is preferably from 0.01 to 100 mW/cm.sup.2,
especially preferably from 10 to 50 mW/cm.sup.2.
The curing time is dependening, inter alia, on the reactivity of
the polymerizable mesogenic material, the thickness of the coated
layer, the type of polymerization initiator and the power of the UV
lamp. For mass production short curing times are preferred. The
curing time according to the invention is preferably not longer
than 30 minutes, especially preferably not longer than 15 minutes
and very particularly preferably shorter than 8 minutes.
The polymerization is carried out in the presence of an initiator
absorbing the wavelength of the actinic radiation. For example,
when polymerizing by means of UV light, a photoinitiator can be
used that decomposes under UV irradiation to produce free radicals
that start the polymerization reaction. As a photoinitiator for
radicalic polymerization a commercially available photoinitiator
like e.g. Irgacure 651 (by Ciba Geigy A G, Basel, Switzerland) can
be used.
It is also possible to use a cationic photoinitiator, when curing
reactive mesogens with for example vinyl and epoxide reactive
groups, that photocures with cations instead of free radicals. The
polymerization may also be started by an initiator that initiates
the polymerization when heated above a certain temperature.
In addition to light- to temperature-sensitive initiators the
polymerizable mixture may also comprise one or more other suitable
components such as, for example, catalysts, stabilizers,
co-reacting monomers or surface-active compounds.
In a preferred embodiment of the invention, the polymerizable
mixture comprises a stabilizer that is used to prevent undesired
spontaneous polymerization for example during storage of the
mixture. As stabilizers in principal all compounds can be used that
are known to the skilled in the art for this purpose. These
compounds are commercially available in a broad variety. Typical
examples for stabilizers are 4-ethoxyphenol or butylated
hydroxytoluene (BHT).
The polymerizable mixture according to this preferred embodiment
preferably comprises a stabilizer as described above at an amount
of 1 to 1000, especially preferably 10 to 500 ppm.
Other additives, like e.g. chain transfer agents, can also be added
to the polymerizable mixture in order to modify the physical
properties of the inventive polymer film. For example when adding a
chain transfer agent to the polymerizable mixture, the length of
the free polymer chains and/or the length of the polymer chains
between two crosslinks in the inventive polymer film can be
controlled. When the amount of the chain transfer agent is
increased, polymer films with decreasing polymer chain length are
obtained.
In a preferred embodiment of the present invention the
polymerizable mixture comprises 0.01 to 10%, in particular 0.1 to
5%, very preferably 0.5 to 3% of a chain transfer agent. The
polymer films according to this preferred embodiment show
especially good adhesion to a substrate, in particular to a plastic
film, like e.g. a TAC film.
As a chain transfer agent for example monofunctional thiol
compounds like e.g. dodecane thiol or multifunctional thiol
compounds like e.g. trimethylpropane tri(3-mercaptopropionate) can
be used.
In some cases it is of advantage to apply a second substrate to aid
alignment and exclude oxygen that may inhibit the polymerization.
Alternatively the curing can be carried out under an atmosphere of
inert gas. However, curing in air is also possible using suitable
photoinitiators and high UV lamp power. When using a cationic
photoinitiator oxygen exclusion most often is not needed, but water
should be excluded. In a preferred embodiment of the invention the
polymerization of the polymerizable mesogenic material is carried
out under an atmosphere of inert gas, preferably under a nitrogen
atmosphere.
To obtain polymer films with good alignment the polymerization has
to be carried out in the liquid crystal phase of the polymerizable
mesogenic mixture. Therefore preferably a polymerizable mesogenic
mixture is used that has a low melting point, preferably a melting
point of 100.degree. C. or lower, in particular 60.degree. C. or
lower, so that curing can be carried out in the liquid crystalline
phase of the mixture at low temperatures. This is simplifying the
polymerization process as less heating of the mixture is required
and there is less strain of the mesogenic materials, the substrates
and the production equipment during polymerization. This is of
importance especially for mass production. Curing temperatures
below 100.degree. C. are preferred. Especially preferred are curing
temperatures below 60.degree. C.
The thickness of the inventive optical retardation film obtained by
the method as described above is preferably 0.2 to 10 .mu.m, in
particular 0.5 to 5 .mu.m, most preferably 1 to 3 .mu.m.
In another preferred embodiment of the present invention the
thickness of the inventive optical retardation film is 8 to 30
.mu.m, in particular 10 to 20 .mu.m.
In a particularly preferred embodiment of the invention the optical
retardation film is used together with a broadband reflective
polarizer and optionally a compensation film. The optical
retardation film may be connected to the reflective polarizer
and/or the compensation film as a separate optical element.
Preferably, the reflective polarizer and/or the compensation film
and the optical retardation film are integrated so that they form
an individual optical element. This can be done for example by
laminating the optical retardation film and the reflective
polarizer together and/or the compensation film after manufacturing
the optical retardation film.
The polymerizable mesogenic material can also be coated and cured
directly onto a reflective polarizer which serves as a substrate,
thus simplifying the production process.
Alternatively it is also possible that the polymerizable mesogenic
material is coated and cured onto a compensation film which serves
as a substrate.
In another preferred embodiment of the present invention, the
broadband reflective polarizer and/or the compensation film of the
inventive combination of optical elements are comprising a layer of
an anisotropic polymer material that is obtained by polymerizing an
oriented layer of reactive mesogens. Particularly preferably these
reactive mesogens have a similar structure like the reactive
mesogenic compounds of formula I as described above and below.
Thus, when using an inventive optical retardation film together
with a broadband reflective polarizer and/or a compensation film
according to this preferred embodiment, it is possible to adapt the
optical properties of the optical retardation film to those of the
reflective polarizer and/or the compensation film by using
materials comprising in principal a similar type of compounds. In
this way a combination of an optical retardation film and a
reflective polarizer and/or a compensation film with superior
optical performance can be obtained.
In a preferred embodiment the polymerizable mixture comprises
reactive mesogenic compounds having two or more polymerizable
functional groups (multifunctional compounds). Upon polymerization
of such a mixture a three-dimensional polymer network is formed. An
optical retardation film made of such a network is self-supporting
and shows a high mechanical and thermal stability and a low
temperature dependence of its physical and optical properties.
Thus, for example inventive optical retardation films can be
obtained that exhibit an excellent thermal stability of the optical
retardation, which does not change significantly when heating the
film up to 120.degree. C.
In another preferred embodiment the polymerizable mixture comprises
0 to 20% of a non mesogenic compound with two or more polymerizable
functional groups to increase crosslinking of the polymer. Typical
examples for difunctional non mesogenic monomers are
alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to
20 C atoms. Typical examples for non mesogenic monomers with more
than two polymerizable groups are trimethylpropanetrimethacrylate
or pentaerythritoltetraacrylate.
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 optical retardation
film, the thermal and mechanical stability or the solvent
resistance can be tuned easily.
The terms polymerizable or reactive mesogen, polymerizable or
reactive mesogenic compound, polymerizable or reactive liquid
crystal (compound) and polymerizable or reactive liquid crystalline
compound as used in the foregoing and the following comprise
compounds with a rodlike, boardlike or disklike mesogenic group.
These mesogenic compounds do not necessarily have to exhibit
mesophase behavior by themselves. It is also possible that they
show mesophase behaviour in mixtures with other compounds or after
polymerization of the pure mesogenic compounds or of the mixtures
comprising the mesogenic compounds.
Preferably the reactive mesogenic compounds exhibit mesophase
behaviour on their own.
In a particularly preferred embodiment of the present invention,
the reactive mesogens comprised by the mixture of the polymerizable
mesogenic material are compounds of formula I
P--(Sp--X).sub.n--MG--R I wherein P is a polymerizable group, Sp is
a spacer group having 1 to 20 atoms, X is a group selected from
--O--, --S--, --CO--, --COO--, --OCO--, --OCOO-- or a single bond,
n is 0 or 1, MG is a mesogenic or mesogenity supporting group,
preferably selected according to formula II
--(A.sup.1--Z.sup.1).sub.m--A.sup.2--Z.sup.2--A.sup.3-- II wherein
A.sup.1, A.sup.2 and A.sup.3 are independently from each other
1,4-phenylene in which, in addition, one or more CH groups may be
replaced by N, 1,4-cyclohexylene in which, in addition, one or two
non-adjacent CH.sub.2 groups may be replaced by O and/or S,
1,4-cyclohexenylene or naphthalene-2,6-diyl, it being possible for
all these groups to be unsubstituted mono- or polysubstituted with
halogen, cyano or nitro groups or alkyl, alkoxy or alkanoyl groups
having 1 to 7 C atoms wherein one or more H atoms may be
substituted by F or Cl, Z.sup.1 and Z.sup.2 are each independently
--COO--, --OCO--, --CH.sub.2CH.sub.2--, --OCH.sub.2--,
--CH.sub.2O--, --CH.dbd.CH--, --C.dbd.C--, --CH.dbd.CH--COO--,
--OCO--CH.dbd.C-- or a single bond and m is 0, 1 or 2, and R is an
alkyl radical with up to 25 C atoms which may be unsubstituted,
mono- or polysubstituted by halogen or CN, it being also possible
for one or more non-adjacent CH.sub.2 groups to be replaced, in
each case independently from one another, by --O--, --S--, --NH--,
--N(CH.sub.3)--, --CO--, --COO-- --O--OCO--, --OCO--O--, --S--CO--,
--CO--S-- or --C.ident.C-- in such a manner that oxygen atoms are
not linked directly to one another, or alternatively R is halogen,
cyano or has independently one of the meanings given for
P--(Sp--X).sub.n--.
Particularly preferred are polymerizable mixtures comprising at
least two reactive mesogenic compounds at least one of which is a
compound of formula I.
In another preferred embodiment of the invention the reactive
mesogenic compounds are selected according to formula I, wherein R
has one of the meanings of P--(Sp--X).sub.n-- as given above.
Bicyclic and tricyclic mesogenic compounds are preferred.
Halogen is preferably F or Cl.
Of the compounds of formula I especially preferred are those in
which R is F, Cl, cyano, alkyl or alkoxy or has the meaning given
for P--(Sp--X).sub.n--, and MG is of formula II wherein Z.sup.1 and
Z.sup.2 are --COO--, --OCO--, --CH.sub.2--CH.sub.2--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond. A smaller
group of preferred mesogenic groups of formula II is listed below.
For reasons of simplicity, Phe in these groups is 1,4-phenylene,
Phe L is a 1,4-phenylene group which is substituted by at least one
group L, with L being F, Cl, CN or an optionally fluorinated alkyl,
alkoxy or alkanoyl group with 1 to 4 C atoms, and Cyc is
1,4-cyclohexylene.
TABLE-US-00001 -Phe-Z.sup.2-Phe- II-1 -Phe-Z.sup.2-Cyc- II-2
-PheL-Z.sup.2-Phe- II-3 -PheL-Z.sup.2-Cyc- II-4 -Phe-Z.sup.2-PheL-
II-5 -Phe-Z.sup.1-Phe-Phe- II-6 -Phe-Z.sup.1-Phe-Cyc- II-7
-Phe-Z.sup.1-Phe-Z.sup.2-Phe- II-8 -Phe-Z.sup.1-Phe-Z.sup.2-Cyc-
II-9 -Phe-Z.sup.1-Cyc-Z.sup.2-Phe- II-10
-Phe-Z.sup.1-Cyc-Z.sup.2-Cyc- II-11 -Phe-Z.sup.1-PheL-Z.sup.2-Phe-
II-12 -Phe-Z.sup.1-Phe-Z.sup.2-PheL- II-13
-PheL-Z.sup.1-Phe-Z.sup.2-PheL- II-14
-PheL-Z.sup.1-PheL-Z.sup.2-Phe- II-15
-PheL-Z.sup.1-PheL-Z.sup.2-PheL- II-16
In these preferred groups Z.sup.1 and Z.sup.2 have the meaning
given in formula I described above. Preferably Z.sup.1 and Z.sup.2
are --COO--, --OCO--, --CH.sub.2CH.sub.2-- or CH.dbd.CH--COO--.
L is preferably F, Cl, CN, NO.sub.2, CH.sub.3, C.sub.2H.sub.5,
OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3, COC.sub.2H.sub.5, CF.sub.3,
OCF.sub.3, OCHF.sub.2, OC.sub.2F.sub.5, in particular F, Cl, CN,
CH.sub.3, C.sub.2H.sub.5, OCH.sub.3, COCH.sub.3 and OCF.sub.3, most
preferably F, CH.sub.3, OCH.sub.3 and COCH.sub.3.
Particularly preferred are compounds wherein MG is selected from
the following formulae ##STR00001## ##STR00002## wherein L has the
meaning given above and r is 0, 1 or 2.
The group ##STR00003## in this preferred formulae is preferably
denoting ##STR00004## furthermore ##STR00005## with L having each
independently one of the meanings given above.
If R as given in formula I is an alkyl or alkoxy radical, i.e.
where the terminal CH.sub.2 group is replaced by --O--, this may be
straight-chain or branched. It is preferably straight-chain, has 2,
3, 4, 5, 6, 7 or 8 carbon atoms and accordingly is preferably
ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, ethoxy,
propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy, furthermore
methyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl,
pentadecyl, methoxy, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy
or tetradecoxy, for example.
Oxaalkyl, i.e. where one CH.sub.2 group is replaced by --O--, is
preferably straight-chain 2-oxapropyl (=methoxymethyl),
2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3-, or
4-oxapentyl, 2-, 3-, 4-, or 5-oxahexyl, 2-, 3-, 4-, 5-, or
6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-,
7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8-, or 9-oxadecyl, for
example.
In addition, mesogenic compounds of the formula I containing a
branched group R can be of importance as comonomers, for example,
as they reduce the tendency towards crystallization. Branched
groups of this type generally do not contain more than one chain
branch. Preferred branched groups are isopropyl, isobutyl
(=methylpropyl), isopentyl (=3-methylbutyl), isopropoxy,
2-methylpropoxy and 3-methylbutoxy.
P in formula I is preferably selected form CH.sub.2.dbd.CW--COO--,
WCH.dbd.CH--O--, ##STR00006## or CH.sub.2.dbd.CH-Phenyl-(O)k-- with
W being H, CH.sub.3 or Cl and k being 0 or 1,
P is particularly preferably a vinyl group, an acrylate group, a
methacrylate group, a propenyl ether group or an epoxy group, very
particularly preferably an acrylate or methacrylate group.
As for the spacer group Sp in formula I, Ia and Ib all groups can
be used that are known for this purpose to the skilled in the art.
The spacer group Sp is preferably linked to the polymerizable group
P by an ester or ether group or a single bond. The spacer group Sp
is preferably a linear or branched alkylene group having 1 to 20 C
atoms, in particular 1 to 12 C atoms, in which, in addition, one or
more non-adjacent CH.sub.2 groups may be replaced by --O--, --S--,
--NH--, --N(CH.sub.3)--, --CO--, --O--CO--, --S--CO--, --O--COO--,
'CO--S--, --CO--O--, --CH(halogen)--, --CH(CN)--, --CH.dbd.CH-- or
--C.ident.C--.
Typical spacer groups Sp are for example --(CH.sub.2).sub.o--,
--(CH.sub.2CH.sub.2O).sub.r--CH.sub.2CH.sub.2--,
--CH.sub.2CH.sub.2--S--CH.sub.2CH.sub.2-- or
--CH.sub.2CH.sub.2--NH--CH.sub.2CH.sub.2--, with o being an integer
from 2 to 12 and r being an integer from 1 to 3.
Preferred spacer groups Sp are ethylene, propylene, butylene,
pentylene, hexylene, heptylene, octylene, nonylene, decylene,
undecylene, dodecylene, octadecylene, ethyleneoxyethylene,
methyleneoxybutylene, ethylene-thioethylene,
ethylene-N-methyliminoethylene and 1-methylalkylene, for
example.
In the event that R or Q.sup.2 is a group of formula P--Sp--X-- or
P--Sp-- respectively, the spacer groups on each side of the
mesogenic core may be identical or different.
In particular preferred are compounds of formula I wherein n is
1.
In another preferred embodiment, the inventive optical retardation
film is obtained by copolymerizing mixtures comprising compounds of
formula I wherein n is 0 and compounds of formula I wherein n is
1.
Typical examples representing polymerizable mesogenic compounds of
the formula I can be found in WO 93/22397, EP 0,261,712; DE
195,04,224; De 4,408,171 or DE 4,405,326. The compounds disclosed
in these documents, however are to be regarded merely as examples
that should not limit the scope of this invention.
Furthermore, typical examples representing polymerizable mesogenic
compounds are shown in the following list of compounds, which is,
however, to be understood only as illustrative without limiting the
scope of the present invention: ##STR00007##
In these compounds x and y are each independently 1 to 12, A is a
1,4-phenylene or 1,4-cyclohexylene group, R.sup.1 is halogen, cyano
or an alkyl or alkoxy group with 1 to 12 C atoms and L.sup.1 and
L.sup.2 are each independently H, Halogen, CN, or an alkyl, alkoxy
or alkanoyl group with 1 to 7 C atoms.
The reactive mesogenic compounds disclosed in the foregoing and the
following can be prepared by methods which are known per se and
which are described in the documents cited above and, for example,
in standard works of organic chemistry such as, for example,
Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag,
Stuttgart.
In a preferred embodiment of the present invention, the optical
retardation film is obtainable from a mixture of a polymerizable
mesogenic material comprising the following components a1) 15 to
95%, preferably 20 to 90% by weight of at least one mesogen
according to formula I having one polymerizable functional group,
a2) 5 to 80%, preferably 8 to 70%, in particular 10 to 50% by
weight of at least one mesogen according to formula I having two or
more polymerizable functional groups, b) 0.01 to 5% by weight of an
initiator, c) 0 to 20% by weight of a non-mesogenic compound having
two or more polymerizable functional groups, d) 0 to 1000 ppm of a
stabilizer, and e) 0 to 5% by weight of a chain transfer agent.
Mixtures according to this particularly preferred embodiment are
preferred that comprise one to eight, in particular one to six,
most preferably one to three different mesogens according to
formula I having one polymerizable functional group.
The mixture according to this particularly preferred embodiment
especially preferably contains less than 10% by weight, very
especially preferably none of the compounds of component c).
In another embodiment of the present invention, the mixture of the
polymerizable mesogenic material comprises a1) 15 to 99%,
preferably 40 to 99%, in particular 70 to 99% by weight of at least
one mesogen according to formula I having one polymerizable
functional group, a2) 0 to 90% by weight of at least one mesogen
according to formula I having two or more polymerizable functional
groups, b) 0.01 to 5% by weight of an initiator, c) 0 to 20% by
weight of a non-mesogenic compound having two or more polymerizable
functional groups, d) 0 to 1000 ppm of a stabilizer, and e) 0 to 5%
by weight of a chain transfer agent.
Mixtures according to this particularly preferred embodiment are
preferred that comprise one to eight, in particular one to six,
most preferably one to three different mesogens according to
formula I having one polymerizable functional group.
Further preferred are mixtures according this preferred embodiment
that comprise 15 to 99% by weight of at least two different
mesogens of component a1) and further comprises components b) and
optionally component a2), c), d) and e) as described above.
Mixtures according to this particularly preferred embodiment are
preferred that comprise two to eight, in particular two to six,
most preferably two to four different mesogens according to formula
I having one polymerizable functional group.
Further preferred are mixtures according to this particularly
preferred embodiment that comprise four or more, in particular four
to eight, very particularly four to six different mesogens
according to formula I having one polymerizable functional
group.
The ratio of each of the mesogens according to formula I having one
polymerizable functional group in the mixture according to this
particularly preferred embodiment is preferably 5 to 90%, in
particular 10 to 80%, very preferably 15 to 65% by weight of the
total mixture.
The mixture according to this particularly preferred embodiment
especially preferably contains less than 10% by weight, very
especially preferably none of the compounds of component a2).
In the mixtures comprising two or more different mesogens according
to formula I having one polymerizable functional group as described
above, preferably each of the different mesogens according to
formula I is different in at least one of the groups P, Sp, X,
A.sup.1, A.sup.2, A.sup.3, Z.sup.1, Z.sup.2 and R from each other
of the mesogens.
The mixtures of a polymerizable mesogenic material as described
above are another object of the present invention.
Without further elaboration one skilled in the art can, using the
preceding description, utilize the present invention to its fullest
extent. The following examples are, therefore, to be construed as
merely illustrative and not limitative of the remainder of the
disclosure in any way whatsoever.
In the foregoing and in the following examples, unless otherwise
indicated, all temperatures are set forth uncorrected in degrees
Celsius and all parts and percentages are by weight. The following
abbreviations are used to illustrate the liquid crystalline phase
behaviour of the compounds.
K=crystalline; N=nematic; S=smectic; Ch=cholesteric; I=isotropic.
The numbers between these symbols indicate the phase transition
temperatures in degree Celsius.
EXAMPLE 1
The following mixture was formulated
TABLE-US-00002 compound (1) 49.5% compound (2) 49.5% Irgacure 651
1.0% ##STR00008## ##STR00009##
The compounds (1) and (2) have been prepared in analogy to the
methods described in WO 93/22397 and DE195,04, 224. Irgacure 651 is
a photoinitiator for radicalic polymerization which is commercially
available from Ciba Geigy AG.
To prepare crosslinked polymer films, the mixture was dissolved in
toluene at a concentration of about 20% by weight and filtered to
remove impurities and small particles.
A sheet of PET (Melinex 401, available from ICI Corp.) was rubbed
unidirectionally 50 to 60 times with a flat aluminium bar coated
with velvet. The applied pressure was approximately 2 g/cm.sup.3,
and the rubbing length was approximately 1.5.+-.0.2 metres.
The toluene mixture was coated as a film with a thickness of
approximately 12 .mu.m on the PET sheet and the solvent was allowed
to evaporate at 55.degree. C. The mixture was then cured in a
nitrogen atmosphere at 55.degree. C. by irradiating with UV light
with a wavelength of 350 to 380 nm and an irradiance of 40
mW/cm.sup.2 for 4 minutes.
In this way, two crosslinked polymer films (1a, 1b) with different
thickness were obtained that can be used as a retardation film.
EXAMPLE A
The films 1a and 1b obtained as described in example 1 were removed
from the PET substrate and their retardation was measured on a
glass slide on an Olympus polarizing microscope using a Berek
compensator. The film 1a has a retardation value of 134 nm, and the
film 1b a retardation value of 154 nm.
FIG. 2 shows the change of the retardation depending on the viewing
angle for a sample of the inventive optical quarter wave
retardation films 1a (curve 2a) and 1b (curve 2b) in comparison to
a standard QWF based on PVA (curve 2c). It can be clearly seen that
the viewing angle dependence of both inventive optical quarter wave
retardation films is lower than that of the PVA film.
EXAMPLE B
The optical performance of the inventive retardation film was
determined in an inventive optical combination together with a
broad band cholesteric reflective polarizer.
The broad waveband reflective polarizer film consisted of a
polymerized mixture comprising chiral and achiral reactive
mesogenic compounds. The polarizer exhibited a cholesteric
structure with planar orientation with multiple pitch lengths of
the cholesteric helix and had a broad wavelength reflection band as
shown in FIG. 4 which is ranging from wavelength values of 500 to
800 nm with a bandwidth of about 300 nm.
The retardations of the films 1a (134 nm) and 1b (154 nm) are 0.25
times a value lying inside the waveband reflected by the broadband
reflective polarizer, therefore each of these two films, when used
together with the reflective polarizer, can act as a quarter wave
film.
FIG. 3 depicts the measurement setup. The luminance of light from a
commercial LCD backlight 50 passing through an assembly with the
reflective polarizer 51 and the inventive optical retardation films
1a and 1b of example 1 (52) was measured using a Minolta CS-100
colour camera 53 at a range of viewing angles (-60.degree. to
+60.degree.). The experiment was repeated with a similar assembly,
wherein the inventive optical quarter wave retardation films were
replaced by the standard QWF based on PVA as used in example A. The
measurement results are shown in FIG. 5.
Curve 5x depicts the luminance of the LCD backlight 50 together
with the reflective polarizer 51. Curves 5a, 5b and 5c show the
luminance of the LCD backlight 50 and a combination of the
reflective polarizer 51 together with an optical retardation film
52 that is either one of the inventive optical retardation films 1a
(curve 5a) or 1b (curve 5b) or the standard QWF (curve 5c).
The luminance of the assembly comprising the inventive optical
retardation films 1a (curve 5a) and 1b (curve 5b) is higher than
that of the assembly comprising the QWF based on PVA (curve 5c)
over the whole range of measured viewing angles, and the cross-over
angle .alpha..sub.c is increased by about 5 to 6 degrees.
The optical retardation film 52 and the broadband reflective
polarizer 51 in FIG. 3 according to example B were not optically
coupled. If they are laminated together, or if the reflective
polarizer is prepared by polymerization of a mixture of reactive
cholesteric mesogenic compounds using the optical retardation film
as a substrate, the cross over angle .alpha..sub.c is being further
increased.
The results of experiments according to example A and B clearly
demonstrate the improved properties of an inventive optical
retardation film compared to an optical retardation film of the
state of the art, especially when used in combination with a
broadband cholesteric reflective polarizer.
EXAMPLE C
A PET web substrate for planar alignment was prepared by rubbing
according to a preferred embodiment of the present invention.
Rubbing was carried out as depicted in FIG. 6 by wrapping the PET
web 1 at a wrap angle .alpha. of 65.degree. around a velvet coated
roller 2 with a circumference of 125 cm, which was rotating with a
velocity of 300 rpm, The web speed was 1750 cm/min and the tension
on the web was 5 lbs/inch. This resulted in a rubbing length of
1335 mm. An inventive polymerizable mesogenic mixture coated on
this PET substrate showed planar alignment of a high uniformity
with substantially no tilt.
The preceding examples can be repeated with similar success by
substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
From the foregoing description, one skilled in the art can easily
ascertain the essential characteristics of this invention, and
without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *