Note: Descriptions are shown in the official language in which they were submitted.
I æ
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POLYMERIC LIQUID CRYSTALS
The present invention relates to liquid crystals
and more particularly to polymeric liquid crystals which
have fixed optical characteristics.
Background of the Invention
The distance of liquid crystalline materials has
been recognized since the late 1800's. The terms "liquid
crystal" or "misogyny" refer to a number of states of
matter which lie between solid crystals and isotropic
liquids, the latter being randomly ordered Liquid
crystalline materials possess some structural
characteristics of crystals, yet they may be viscous or
quite mobile liquids.
The varying degrees of order which are possessed
by liquid crystals give rise to three distinct types of
structures called mesophases. A liquid crystal, when in
the crystalline state, has a three-dimensional uniform
structure with orientation Al and positional order. As the
crystal is heated, it may initially lose one dimension of
its positional order. This is referred to as the smectic
mesophase, a phase in which the liquid crystal retains the
orientation Al order of the crystalline state, as well as
two directional positional order.
With further heating, the liquid crystal can
convert to the pneumatic mesophase. In this phase, the
remaining positional order is lost and the liquid
crystalline material retains only the one-directional
: `:
:
:
: ,
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orientation Al order of the crystalline state. The
molecular order ox pneumatic mesophases is characterized by
orientation of the molecules along an axis which coincides
with the long axis of the molecules. The centers of
gravity of the molecules are arranged randomly 80 that no
positional long-range order exists.
In the cholesteric mesophase, the molecular order
is characterized by orientation of the molecules along an
axis which coincides with the long molecular axis as in a
pneumatic phase, however, the axis changes direction in a
continuous manner along a second axis perpendicular to the
first. For this reason, cholesteric mesophases are often
referred to as twisted pneumatic mesophases. Optical
activity is necessary for a misogynic material to form a
cholesteric mesophase.
The term "cholesteric" is primarily of historical
significance because the first-discovered liquid
crystalline material which exhibited a cholesteric
mesophase was cholesterol bonniest. It has long been
recognized, however, that the presence of the cholesterol
moiety is not required, and that non-cholesterol
derivatives may also exhibit a cholesteric mesophase.
The P or art
Substantial interest has been shown in liquid
crystalline materials which exhibit cholesteric mesophases
because these materials exhibit unique optical properties
such as selective reflection of visible light to produce
iridescent colors, as well as circular dichroism. Thus,
for example, US. Patent 3,720,623 discloses mixtures of
cholesteric and pneumatic liquid crystals which are useful
in temperature-sensitive visual displays; US. Patent
3,766,061 discloses decorative films comprising solid
materials which are proportioned such that the composition
demonstrates cholesteric properties; US. Patent 3,923,685
discloses cholesteric materials which convert to the
pneumatic state upon exposure to an electric field;
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and US. Patent 3,931,041 discloses combinations of
pneumatic and potentially cholesteric material which are
useful in imaging and display devices.
Although the colored images produced using
cholesteric material are quite useful, most such images
are not permanent. Accordingly, there has been
substantial interest in preparing cholesteric materials in
which the color can be fixed. Thus, US. Patent
3,766,061, which was referred to above, discloses
decorative films in which the color is fixed by cooling.
In addition, US. Patent 4,293,435 discloses a polymeric
liquid crystal in which the color is fixed by lowering the
temperature of the polymer below the glass transition
temperature, thereby fixing the polymer in the solid state.
The use of temperature changes to fix the color
is not always practical, however, and there has been
interest in developing cholesteric materials whose color
can be fixed by other means, such as by
photo polymerization, whereby the resulting fixed color is
temperature insensitive. Applicant is aware of only one
such polymer. This was reported by a group of Japanese
workers who disclosed that poly(gamma-butyl-L-glutamate)
in trim ethylene glycol dimethacrylate could be
photo polymerized to fix the color such that it was
temperature insensitive.
Accordingly, one objective of the present
invention is to provide polymeric cholesteric liquid
crystalline materials having mixed, essentially
temperature-insensitive colors.
Yet another objective of the present invention is
to provide combinations of monomeric compounds which
provide variable optical responses over a variety of
temperature ranges.
Yet another objective of the present invention is
to provide polymeric films having fixed colors which are
useful in a variety of optical devices.
These and other objectives of the present
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invention will become apparent from the detailed
description of preferred embodiments which follow.
Summary of the Invention
The present invention concerns novel cholesteric
liquid crystalline monomers and combinations thereof with
materials that will support formation of a mixture which
demonstrates cholesteric liquid crystalline properties.
These materials may be formed as films, heated or cooled
to a desired temperature to cause the cholesteric film to
exhibit a desired optical response, and photo polymerized
to essentially fix the optical characteristics of the
resulting polymer.
Detailed Description of Preferred Embodiments
In one embodiment the present invention comprises
a composition suitable to provide a polymeric film having
fixed optical properties, said composition comprising a-
photopolymerizable monomer having the structure-- -
Jo
kiwi O OILY
YE
where Al = En or SHEA, A = -R2-, R30 or 4
R2 = an alkaline chain having 3-14 ethylene or lower
alkyl-substituted ethylene groups, R3 = an alkaline
chain having from 2~14 ethylene or lower alkyd-
substituted ethylene groups, R4 = an alkaline or lower
alkyl-substituted alkaline ether, dither or triether
having a total of from 3-14 carbon atoms in the alkaline
linkages, provided that the terminal alkaline linkage
adjacent the carbonate moiety comprises not less than
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two carbon atoms and y = O or l; and a suitable
photoinitia~or.
In a second embodiment, the present invention
comprises a polymeric film having a fixed optical
response, said film being obtained by photo polymerizing a
composition comprising a photopolymerizable monomer having
the structure
SCHICK
Y -
1 H or SHEA, A = -R2-, -R O or R O
R2 = an alkaline chain having from 3-14 ethylene or
lower alkyl-substituted ethylene groups, R3 = an
alXylene chain having from 2-14 ethylene or lower
alkyl-substituted ethylene groups, R4 = an alkaline or
lower alkyl-substituted alkaline ether, dither or
triether having a total of from 3-14 carbon atoms in the
alkaline linkages, provided that the terminal alkaline
linkage adjacent the carbonate moiety comprises not less
than two carbon atoms and y = O or l; and a suitable
photo initiator.
In a third embodiment the present invention
comprises a process for preparing films comprising
polymeric liquid crystalline materials having a fixed
optical response, said process comprising the steps of
preparing a film comprising a photopolymerizable monomer
having the structure
.
.
6 - LFM-7211
,
ye
SCHICK lo
H
H
where Al = H or C~3, A = -R2-, -R30- or -R40-,
R2 = an alkaline chain having from 3-14 ethylene or
lower alkyl-substituted ethylene groups, R3 = an
alkaline chain having from 2-14 ethylene or lower alkyd-
substituted ethylene groups, R4 = an alkylene-or lower
alkyl-substituted alkaline ether, dither or triether
having a total of from 3 14 carbon atoms in the alkaline
linkages, provided that the terminal alkaline linkage
adjacent the carbonate moiety comprises not less than two
carbon atoms, and y = 0 or 1; and a suitable
photo initiator; aligning said film; adjusting the
temperature of said film to obtain a desired optical
response and photo polymerizing said film
The cholesterol derivatives which may be used to
practice the present invention are cholesterol (where y =
0) and 5,6-dihydrocholesterol (where y = lo In addition,
a number of options are available in the three position
side chain. Thus, the polymerizable moiety of the side
chain can comprise an acrylate or methacrylate moiety
which is bridged to an ester or carbonate linkage. Where
an ester linkage it present, the bridge will comprise an
alkyd chain comprising from 3-14 ethylene or lower
alkyl-substituted ethylene groups. Lower alkyd as used
herein shall mean an alkyd group comprising from 1-4
carbon atoms. The methacrylate esters/ where Al = SHEA
and n - 5, 10 and 14, have been reported in the Russian
literature; however, these
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esters were prepared for use in solution polymerization
reactions and where was no appreciation of their utility
for preparing photo polymerized films as disclosed herein.
On the other hand, where a carbonate linkage is
present, the bridge may be more complex. Thus, it may
comprise from 2-14 ethylene or lower alkyl-substituted
ethylene groups, or an alkaline or lower alkyd-
substituted alkaline ether, dither or triether having a
total of from 3-14 carbon atoms in the alkaline linkages,
provided that the terminal alkaline linkage adjacent the
carbonate moiety comprises not less than two carbon
atoms. Examples of ether moieties which may be utilized
in practicing the present invention are those which are
analogous to ethylene glycol, diethylene glycol,
triethylene glycol, tetramethylene glycol,
3,3'-oxybis-l-propanol, 4,4'-oxybis-l-butanol,
l,l'-oxybis-2-propanol, and the like.
When in the pure state the compounds of the
present invention are somewhat difficult to work with
because they tend to crystallize at inopportune moments.
Furthermore, it is difficult to obtain colored polymers
from the pure monomers because the majority of them will
show either no colored cholesteric mesophase, or a very
narrow colored cholesteric mesophase. rrherefore, the pure
compounds of the present invention are limited in their
ability to produce polymeric films having desirable
optical responses.
Surprisingly, it has been discovered that these
limitations may be overcome and that colored and uncolored
films comprising a compound of the present invention and
either another compound of the present invention, or a
second material which is suitable to permit formation of a
film that exhibits cholesteric liquid crystalline
properties, can be prepared and photo polymerized in the
presence of a suitable photo initiator, thereby giving
films having fixed optical characteristics. If the film
is colored, the
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fixed color will preferably be substantially the sane 25
the color of the unpolymerized film; however, in certain
instances, it may be desirable to obtain â polymerized
film having a fixed color which differs from that of the
unpolymerized film. Thus, all such possibilities are
contemplated by the present invention. retails relating
to the preparation of the novel compounds used herein
are set forth in my Canadian cop ending application Serial
No. 431,936.
1 0
A preferred method of practicing the present
invention involves the preparation of a film which
exhibits a desired optical characteristic at a specific
temperature. For colored films, this has been
conveniently achieved, for example, by preparing a
mixture of the materials which provide the cholesteric
film and the photo initiator and, optionally, a
cross-linking agent, heating the mixture to obtain a
viscous liquid; spreading and aligning the liquid
between glass plates; submerging the plates in a
thermostatic water bath; and adjusting the temperature
to obtain a desired color. For uncolored films, the
optical characteristics must be determined
spectrophotometrically. The film is then irradiated
with a suitable radiation source, such as a mercury
lamp. The polymeric films thus obtained can remain
substantially unchanged even when exposed to high
temperatures for several weeks, depending on the
character of the second component as discussed in more
detail below.
Examples of photo initiators which will be
useful to practice the present invention are
benzophenone, 2,2-dimethoxy-2-phenylacetophenone,
2,2-diethoxy--acetophenone, 2-benzoyloxyacetophenone,
2-chlorothioxanthone and 2-hydroxs~cyclohexyl phenol
kitten, all of said compounds being provided by way of
illustration and not limitation.
Examples of optional cross-linking gents
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- 9 - LFM-7211
which will be useful to practice the present invention are
trimethylolpropane triacylate, trimethylolpropane
trimethacrylate, ethyleneglycol diacrylate, ethyleneglycol
dimethacrylate, dip and triethyleneglycol diacrylate and
dimethacrylate, l,6-hexanediol diacrylate and
dimethyacrylate, l,4-butanediol diacrylate and
dimethacrylate, similarly substituted acrylamides and
methacrylamides, and many others, said examples similarly
being provided by way of illustration and not limitation.
A wide variety of combinations may be made to
produce films having different optical characteristics,
and these will be largely a matter of choice to the
artisan. Nevertheless, several generalizations can be
made regarding combinations of the novel monomeric
compounds as described herein.
First, combinations of similar monomers will give
films which exhibit cholesteric mesophases over a
temperature range which is comparable to that of the
individual monomers. For example, if an acrylate/me-
thacrylate pair of cholesterol derivatives is prepared
wherein y = O and A = SHEA-, the methacrylate
(R1 = SHEA) exhibits a color range (monotropic only) at
55.8-55.3 C, whereas the acrylate (Al = H) exhibits a
color range at 57.8-59.2 C. A 1:1 mixture ox the two
exhibits a colored mesophase range of 56.5-55.9 C.
Secondly, combinations of similar monomers having
very different alkyd chain lengths provide mixtures with
substantially broadened mesophase ranges as compared to
the individual components. For example, if a pair of
acrylate monomers (Al = H and y - O) is prepared wherein
one monomer has A = -(SHEA-, and the other monomer
has A = -(SHEA-, the first monomer exhibits a color
range of 57.8 - 59.2 C whereas the second monomer
exhibits no color. A 1:1 mixture of the two exhibits a
substantially broader color range of 68 to -15 C, - 15 C
being the lower detection limit ox the
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testing apparatus which was used. Accordingly, it will be
seen that careful mixing o-f monomers can provide
mesophases which exhibit full optical response over a
variety of temperature ranges.
Thirdly, the addition of small amounts of
non-mesogenic materials to a mixture of misogynic
materials can lead to substantial changes in the optical
response ranges. Thus, for example, the addition of I of
a photo initiator or cross-linking agent can cause a
downward shift of 10 degrees or more in the color range
exhibited by a mixture of the pure misogynic materials.
so indicated above, an alternative method of
preparing photo polymerized films having fixed optical
properties is by combining a compound of the present
invention with a second material which is suitable to
permit formation of a film that exhibits cholesteric
liquid crystalline properties. It is not necessary that
the second component be either polymerizable or misogynic;
nevertheless it is preferred that it be photopolymerizable
in order to provide stable polymeric films. A wide
variety of materials will be suitable to provide
characteristic films. Examples of such materials, which
are provided by way of illustration and not limitation are
cholesterol oilily carbonate and 2-methyl-1,
4-phenylene-bis (4'-hexyloxybenzoate~, which are misogynic
but not polymerizable; p methoxyphenyl-_-
(6-methacryloyloxyhexyloxy)benzoate, which is
norl-mesogenic but polymerizable; and cholesterol
methacrylamido)undecanoate, which is both misogynic
and polymerizable. Illustrations of the utility of
certain of these compounds are provided in Example 9,
below.
The color intensity and uniformity which may be
. .
shown by various combinations of the present invention
will also be affected by the alignment. Thus, as is well
known in the art, some form of mechanical shearing must be
provided to yield the colored films. Such alignment has
been satisfactorily achieved by
.
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~2~5~12
LFM-7211
sandwiching the monomers between glass plates or polyester
films.
Although polymerization of the films can be
achieved by radical or thermal initiation, either in
solution or in bulk, in virtually all instances, no fixed
color or optical response is observed. Instead, the
polymers formed in solution or in bulk prefer to form
colorless smectic mesophases or amorphous polymers.
Accordingly, photo polymerization is required to achieve
the objects of the present invention. The way in which
photo polymerization is achieved may have an effect on the
optical characteristics of the resulting polymer. Thus,
where response duplication is desired, it appears
desirable to use a high intensity light source which
induces rapid polymerization. On the other hand, slower
; polymerization induced by lower intensity light may tend
to produce polymeric films in which the response is
j shifted toward the red end of the spectrum.
Multi-response films may also be produced
according to the present invention by sequential
photo polymerization of the unpolymerized films. For
example, a colored film can be placed under a mask and
irradiated to fix the color of the exposed areas. By
removing the mask and changing the temperature of the
partially cured film, a color change can be induced in the
non-polymerized portion of the film. Upon subsequent
irradiation, the second color can be fixed, thereby
providing a two-colored film. Of course, this technique
may be extended to provide films having multiple optical
responses, if desired by the artisan.
The unique ability of films of the present
invention to reflect specific wavelengths of light varying
from the near ultraviolet region into the infrared region
makes them remarkably useful. For example, their
insensitivity to changes in temperature makes them
especially suitable as filters, such as band pass, notch,
and circular polarization filters, in optical devices.
Further, they will be well suited for
:
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Lo
72ll
use in reflective displays and so-called "SchefLt-r
cells." In addition, where the films reflect in the
visible spectrum and show bright iridescent colors, they
isle be useful as replacements for dyes and pigments.
use for e~arrlple, they will be usable in floor and wall
coverings, textiles, mats, paper products, and in the
graphic arts in nonconventional inks.
he advantages and attributes of the present
invention will become more apparent from the following
examples which are intended to illustrate but not to
limit the scope of the present invention.
EXAMPLES
Compounds referred to herein by Roman numeral
designation have the following structures, the details
of their preparation being described in my cop ending
application which was referred to above. As used
herein, the temperature ranges are melting ranges unless
otherwise indicated by an asterisk (*) or by
parentheses. An asterisk signifies that the range is a
I mesophase range whereas parentheses indicate that the
range is a monotropic mesophase range, the latter being
measured as the temperature is decreased. With
materials that have ascertainable melting ranges, the
; monotropic mesophase range is often below the melting
range.
.
Yo-yo
I
"
R10 I
H
Jo H
`:
wow R2 chosen
:
.
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I`
elating or
espouse
Compound Al n_ my Range _ (C)
Via 13 10 0~54.5 - 71.5
Vb C~-13 10 0*58 - 64
Vc H 5 0*45.5 - 68.5
Ed SHEA 5 0*48 - 57.5
Vie H 3 068.5 - 70.5 (67.5)
Of SHEA 3 0 73 - 74 (56.0)
Vg H 3 1 41 - 43 (35.5)
Oh SHEA 3 1 43 - 45 (Below RUT)
Vi H 10 162.5 - 64.5 (58.0)
Oh SHEA 10 1*33.7 - 49.0
MY
SHUCKS -O IX
H
where A = R30 = (Sheehan, or
1`5 R40 = (Shoeshine
; R3_ Melting or Mesophase
Compound Rl(CH2JnO y Range I
Ida SHEA 6 0 58.5 - 60 ~51.0)
IXb SHEA 2 0 on - 81 (40.1)
; 20 IXc H 2 0 85.5 - 87 (56.0)
I; Id H 6 0 *52 - 62
I: :
: :
14 - LFM-7211
I`:
R~0 Melting or Mesophase
Compound R1( 2 - 2 on -Range (C)
Ire SHEA 2 048.5 - 52.9 (33.1)
If SHEA 3 no m. pt. (6.5)
Example 1
This example sets forth the color ranges of
various monomeric esters V of the present invention,
measured with a Lutz optical microscope using transmitted
light through cross-polars at 250X magnification. A
Mottler FP5 temperature regulator and a Mottler FP52 hot
, stage were used to control the temperature, cooling being
obtained by passing a nitrogen stream through a dry-ice
cooled copper coil and, subsequently, the FP52 hot stage.
Compound Color Range (C)
Via 57.8 - 59.2
Vb (55.8 - 55.3)
Vc (~8.5 - 33.0)
Ed (51.0 - 26.5)
Vie No Color
Of Jo Color
Example 2
This example describes the colored mesophase
ranges obtained for mixtures of previously described
paired monomers having identical alkyd chain lengths. The
measurements were made using the apparatus described in
Example 1, by heating a mixture of the monomers to a melt
and cooling. The components were 1:1 mixtures by weight.
optical
-2 or R30 Response
Components n Color Range Range (C)
Via - Vb 10 Violet - Red (56.5 - 55-9)
Vc - Ed 5 Violet - Orange (50.2 - 29.5)
Vie - Of 3 No Color 61.5 Mesophase
; Vg - Oh 3 No Color Not Measured
; Ida - Id 6 Violet-Blue Violet (51 - 1)
IXb - IXc 2 No Color 47 Mesophase
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Example 3
This example describes the colored mesophase
ranges obtained for ~wo-component mixtures of previously
described monomers having different alkyd chain lengths.
Color measurements were made as described in Example 1.
The components were 1:1 mixtures by weight.
Optical
Wrier RHO Response
Components n Color Range Range (C)**
Via volt - orange red (68 - -15)
Vie 3
Vb 10 green - orange (47.5 - -15)
IXc 2
**-15 C is the lower temperature limit of the
thermostat Ed water bath.
x mule 4
This example describes the colored mesophase
ranges obtained for mixtures ox previously described
monomers having different alkyd chain lengths. The
mixtures comprised Irgac~re 651 photo initiator and,
optionally other indicated components. Irgacure 651 is
2,2-dimethoxy-2-phenyl acetophenone. Color measurements
were made using a thermostat Ed water bath.
Optical
Response
Components Wig Color Range Range (C)
Vb 1.0
Vie 1.0 Violet - Red (50 - -5
Photo initiator 0.04
30Vb 0.25
Oh 0025 Blue Green - Red (40 - -5)
Photo initiator 0.01
.
. .
* Trademark
,
Jo
4~2
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.,
` Optical
Jo Response
Components Wt.tg) Color Range Range (C)
Ed 0.50
` Of 0.50 Green - Red (45 - 30)
Photo initiator 0.02
Vb 0.50
IXb 0.50
Photo initiator 0.02 Orange Green - Red (32 - o)
Methyl methacrylate 0.05
Ed 0.40
Vc 0.40
Via 0.20 Violet - Orange (16 - 6)
Photoinitator 0.Q2
Trimethylolpropane
triacrylate 0.06
Vb 0.5
Ire 0.5 Green - Red (37 - 0)
Photo initiator 0.01
Example 5
This example illustrates a colored polymeric film
derived from a film comprising a single monomer of the
present invention and 1% Irgacure 651 photo initiator. The
table lists "apparent absorbency" maximum (I Max,
percentage transmittance (IT) and half-width at
half-height (HOWE) of the film and the resulting polymer
when the film was photo polymerized at an indicated
temperature. The polymerizations in this and other
examples were achieved by exposing the film to a 450-watt
mercury arc lamp for about 30 seconds.
Film Monomeric Film Polymer
Temp. Max HOWE Max HOWE
I no IT (no) (no) IT (no)
Vg 25 738 53 45 738 55 50
Id I 438 49 27 441 46 33
~2~5~
E~m~le 6
This e~:arnple illustrates several colored
polymeric films derived from indicated monomer
compositioll5. All films contained 1% Irgacure 651
photo initiator. Also, fuller of the films contained 3%
trimethylolpropane triacrylate, excluding pair Vb:IXb
which contained 3% trimethylolpropane trimethacrylate.
Film Monomeric Film_ _ P lamer _
Composition Temp. Max HOWE Max HOWE
_ y weight) (C)_ (no) IT no) (no) IT (no?
Valve (1:1) 23 505 4918 505 5120
Vb:IXb (1:1) 25.5 58542 21 585 42 21
Vc:IXc (3:1) 25.5 56346 25 568 45 26
Vc:IXc* (1:1) 24.5 95053 50 950 53 50
15 IXc:IXd* (1:1)25.5 1260 59 112 1260 59 112
*Colorless mixture
FxaTnple 7
This example illustrates the differently
colored polymeric films which may be produced by
subjecting a monomeric mixture to different temperatures
and then exposing the colored film to US radiation. The
monomeric mixture described for this example comprises a
lo by eta mixture of compounds Vb and Of. The
apparent absorbency maximum and color are reported for
each film.
Film Temperature I Max (no) Color
. .
485 - blue - Green
515 Green
542 Iamb Green
11 605 Orange
A comparably experiment conducted with a 1:1 mixture of
compounds Via and Vie gave the following -results:
:' '.
: :
Jo
., : ,
. .
Film
Temp. Max Transmittance HUH
(C)_ no no Color
32 480 47 32 blue
23 505 51 20 Blarney
18 530 48 42 lime-yreen
574 48 40 orange
_xample_8
This example illustrates the effect of
non~mesogenic materials on a mixture of monomers. A 1:1
ho weight mixture of compounds Vc and Ed Yale a colored
mesophase range of 50.2 - 29.5C, as indicated in
Example 2. When 2% by weight of a photo initiator was
added, the colored Masonic range shifted to 43-20C~
Example 9
This example illustrates polymer films which
can be prepared from a compound of the present invention
and unrelated materials, as follows:
6~13 O I S I -clue
SHEA A
OH I C-O-- (OH ) -O-CO~-C-~<~-OCH3,
B
I
SHEA 2)7 C Cole C
H
Y
'
.
I
.
Compound A is a nernatic liquid crystalline retrial
which is not capable of participating in a
photo polymerization process. Compound B is a
nonmesoyenic material that is capable of participating
~;~ 5 in a photo polymerization reaction. Compound C is a
cholesteric liquid crystalline material which is not
capable of participating in a photo polymerization
I reaction. All three are suitable to permit formation of
a film that exhibits cholesteric liquid crystalline
properties. To illustrate this, films were prepared and
photopolyrnerized using 1% Irgacure 651 photo initiator
; and I trimethylolpropane trimethacrylate.
Composition Film Monomeric Film Polymer _
(weight Temp. Max HOWE Max HOWE
I _ ratio) _ I (no)_ % T (no) (no) % T no
Via (2:1) 24 355 43 30 350 41 35
Ebb (1:1) 25 388 52 15 400 42 40
Vg:C (4:1) 25 700 51 33 730 55 52
Although the film derived from pair Via demonstrates
suitable optical properties, it's not as stable as
other films in which both members of the pair are
polymerizable. For example, when this polymeric film
was heated at 60 C for one day, it underwent
crystallization to give an opaque colorless film.
This invention is not restricted solely to the
descriptions and illustrations provided above, but
encompasses all modifications envisaged by the following
claims.
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