Polymer-dispersed liquid crystal composition

A novel PDLC film composition that contains a mixture of acrylate monomers that provide certain advantages when combined with a liquid crystal. The mixture of acrylate monomers contains one monofunctional monomer that gives rise to a homeotropic anchoring condition, and one monofunctional monomer that gives rise to homogeneous anchoring condition in the PDLC film made from the composition of the present invention at a selected temperature. The monomer mixture also contains a polyfunctional acrylate monomer. PDLC films made from these compositions have advantages over films made from compositions that have only one monofunctional acrylate monomer. Also, by using the process of the present invention, one can prepare a PDLC film that exhibits a low switching voltage in a desired temperature range.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention is directed to a polymer-dispersed liquid crystal 
composition. 
2. Art Background 
Polymer-dispersed liquid crystals (PDLCs herein) are compositions in which 
liquid crystal drops are dispersed in a polymer matrix. Liquid crystals 
are materials which are liquids in the conventional sense of being in a 
condensed state that is flowable. Liquid crystals are anisotropic 
molecules which exhibit long-range orientational order. The particular 
orientation affects the transmission of light through the liquid crystals. 
PDLC films can be used in applications such as flat panel displays because 
the light-scattering properties of PDLC films can be manipulated. One way 
to manipulate the light-scattering properties of PDLC films is to apply an 
electric field to the film. The light-scattering power of the film is 
different (typically smaller) when the liquid crystal in the drops are 
aligned by the electric field than when the liquid crystal is not so 
aligned (i.e. in the absence of the electrical field). Typically, a PDLC 
film is designed to scatter light in the absence of an electric field and 
be transparent to light when a voltage is applied across the film. In this 
case, it is desirable to choose the refractive index of the polymer matrix 
to be close to the ordinary refractive index of the liquid crystal when 
the voltage is applied, so that the film is transparent when the voltage 
is applied thereto. The degree of contrast between the transparent and 
scattering states of the PDLC film is one parameter that determines the 
quality of the image, i.e., the higher the contrast, the better the image 
quality. 
There are many ways to make PDLC films. In one scheme, the films are made 
by combining the liquid crystal composition with a radiation-curable 
monomer. When the liquid crystal/monomer mixture is exposed to radiation, 
the monomer polymerizes, thereby forming a polymer matrix. As the monomer 
polymerizes, the liquid crystal becomes less soluble in the mixture. Due 
to decreased solubility in the polymer matrix, liquid crystal droplets 
form during polymerization. The solidification of the matrix prevents the 
liquid crystal droplets from coalescing, thereby ensuring a stable droplet 
dispersion throughout the matrix. Typically, the polymer matrix for PDLC 
films is cured using ultraviolet (UV) radiation. The electro-optical 
properties of the resulting film depend upon the size distribution of the 
liquid crystal droplets, their shape, and the interactions between the 
matrix and the liquid crystal at the droplet surface. Since these 
parameters are determined, in large part, by the matrix in which the 
liquid crystal droplets are dispersed, it follows that selection of an 
appropriate matrix polymer is critical to the properties of the resulting 
PDLC film. 
Monomers that, when polymerized, provide a suitable matrix for liquid 
crystals for use as PDLC films, are known. Examples of such monomers 
include acrylates, methacrylates, thiols, and alkyl ethers. For example, 
PDLC films are formed by combining an alkyl acrylate such as 2-ethyl hexyl 
acrylate or n-hexyl acrylate with a photoinitiator, a polyfunctional 
acrylate such as, for example, trimethylol propane triacrylate, and a 
commercially available liquid crystal mixture, e.g. a liquid crystal 
mixture such as the one designated TL205 and obtained from E. M. 
Industries of Hawthorne, N.Y. The film is formed by placing the mixture 
between glass plates with an electrically conductive coating (e.g. a 
coating of indium tin oxide) thereon, and exposing the mixture to UV 
radiation. 
One of the difficulties with PDLC films is the high voltage required to 
switch them from the transparent to the opaque state. For convenience, 
switching from transparent to opaque is hereinafter referred to as 
switching from "off" to "on", or vice-versa. Current PDLC films have a 
thickness of about 10 .mu.m. Although thicker films are advantageous from 
the standpoint of contrast. between the "on" and "off" states of the film, 
thicker films also require a higher switching voltage than thinner films. 
Similarly, it is known that the contrast between the "on" and "off" states 
of the display is related to droplet size of the liquid crystal in the 
matrix. However, changing the drop size to improve contrast usually 
results in a higher switching voltage. Therefore, a PDLC film that 
provides a greater contrast between "on" and "off" states without a 
concomitant increase in switching voltage is sought. 
SUMMARY OF THE INVENTION 
The present invention is directed to a PDLC composition that contains a 
mixture of polymerizable acrylate monomers that provide certain 
advantages. The PDLC composition contains a liquid crystal in combination 
with the mixture of polymerizable acrylate monomers. Examples of suitable 
liquid crystals are described in International Patent Application No. 
PCT/EP93/00989 to Coates et al. which is hereby incorporated by reference. 
The liquid crystal content is about 50 weight percent to 90 weight percent 
of the PDLC composition, preferably about 75 weight percent to about 90 
weight percent. It is advantageous if the mixture of polymerizable 
acrylate monomers is about 10 percent to about 50 weight percent of the 
total PDLC composition. 
The polymerizable monomer mixture is, in turn, a mixture of at least two 
monofunctional acrylate monomers and at least one polyfunctional acrylate 
monomer. In the context of the present invention, monofunctional means 
that the monomer contains only one ethylenically unsaturated bond and that 
bond is contained in the acrylate moiety of the monomer. Polyfunctional 
means that the monomer contains more than one ethylenically unsaturated 
bond. It is advantageous if the polymerizable monomer mixture is about 70 
weight percent to about 95 weight percent of the monofunctional acrylate 
mixture, about 5 weight percent to about 25 weight percent of the 
polyfunctional acrylate, and about 0.01 weight percent to about 5 weight 
percent of a photoinitiator. 
The monofunctional acrylate monomers are, in turn, a mixture of acrylate 
monomers that provide films made of the PDLC composition of the present 
invention with advantageous properties such as low switching voltage. The 
mixture of monofunctional acrylate monomers contains at least two 
monomers. The two monomers have a combined effect on the surface 
interactions between the liquid crystal and the polymer matrix. 
These surface interactions are important because the surface energy between 
the liquid crystal droplet and the matrix is a function of what is 
referred to as the liquid crystal "director" with respect to the matrix 
surface. Certain director orientations are associated with lower surface 
energies than other director orientations. This relationship of surface 
energy and orientation is referred to as "surface anchoring." There are 
many types of surface anchoring. Two types of anchoring are of importance 
in the present invention: homeotropic anchoring and homogenous anchoring. 
For purposes of the present invention, homeotropic anchoring is defined as 
an anchoring condition where the droplet director prefers to align 
approximately normal to the matrix surface, i.e., the director orientation 
forms an angle greater than about 80 degrees with the plane of the 
interface. Homogeneous anchoring is defined as an anchoring condition 
where the droplet director prefers to align approximately with the plane 
of the matrix surface, i.e., the director orientation forms an angle of 
less than 15 degrees with the plane of the matrix surface. These surface 
interactions are temperature dependent as well as composition dependent. 
As previously noted, the PDLC composition of the present invention is 
formed by combining the liquid crystal with an acrylate mixture that 
contains at least two monofunctional acrylates. At least one of the 
monofunctional acrylates gives rise to a homeotropic anchoring condition 
at the desired operating temperature of the film formed from the PDLC 
composition. At least one of the monofunctional acrylates gives rise to a 
homogeneous anchoring condition at the desired operating temperature of 
the film formed from the PDLC composition. The anchoring condition 
resulting from use of a particular monofunctional acrylate at a particular 
operating temperature is determined empirically. This determination can be 
made in a variety of ways, such as, for example, by observing the effect 
of a change in temperature on the "fall time" of a PDLC film formed from a 
PDLC composition in which the monofunctional acrylate under investigation 
is the sole monofunctional acrylate in the composition. The fall time is a 
measure of the time it takes for the PDLC film to relax, i.e. to return to 
its "off" state scattering characteristics, after an electric field being 
applied to the film is turned off. A sharp peak in the plot of the fall 
time as a function of temperature indicates a transition between 
homeotropic and homogeneous anchoring. The temperature at which the film 
undergoes this transition is referred to as the transition temperature. 
Typically, a PDLC film undergoes a transition from homeotropic anchoring 
to homogenous anchoring at some temperature. Also, a film typically 
exhibits homeotropic anchoring below its transition temperature and 
homogeneous anchoring above its transition temperature. Once the 
transition temperature of a PDLC composition containing a single 
monofunctional acrylate is determined, that information is then used to 
select the mixture of monofunctional acrylates for the composition of the 
present invention. 
For example, if the operating temperature of a PDLC film is determined to 
be 25.degree. C. and an anchoring transition is desired at that 
temperature, then the mixture of monofunctional acrylates is prepared by 
selecting a first monofunctional acrylate that gives rise to a transition 
below 25.degree. C. (which means that a PDLC film made from a PDLC 
composition in which the first monofunctional acrylate was the sole 
monofunctional acrylate component of the PDLC composition and that film 
exhibited a transition temperature below 25.degree. C.) and a second 
monofunctional acrylate that gives rise to a transition temperature above 
25.degree. C. By way of specific example, a PDLC film made from a 
composition that is 80 percent by weight liquid crystal combined with 20 
percent by weight of a polymerizable acrylate mixture. The polymerizable 
acrylate mixture contains lauryl acrylate (85 weight percent of the 
mixture), trimethylol propane triacrylate (13.5 weight percent of the 
mixture) and a photoinitiator, Darocur 1173, (1.5 weight percent of the 
mixture), which is obtained from Ciba in Hawthorne, N.Y. The anchoring 
transition temperature of a film made from this PDLC composition is 
80.5.degree. C. The transition temperature of a film made from a 
composition that contains 2-ethyl hexyl acrylate instead of lauryl 
acrylate but was otherwise identical is 23.degree. C. If a PDLC film with 
an anchoring transition temperature of 30.degree. C. is desired, a PDLC 
composition of the present invention is prepared using lauryl acrylate and 
2-ethyl hexyl acrylate as the monofunctional acrylate mixture. The 
relative amounts of each of these monofunctional acrylates in the PDLC 
composition is determined by empirical methods that will be readily 
apparent to those skilled in the art. 
If the anchoring behavior resulting from use of a particular monofunctional 
acrylate cannot be determined in the above-described manner, an alternate 
method for making this determination is by observing the effect of the 
addition of the monofunctional acrylate on the transition temperature of 
film formed from a PDLC composition that contains the monofunctional 
acrylate monomer in question in conjunction with a second monofunctional 
acrylate monomer for which the anchoring transition temperature is known. 
If the transition temperature of the two-monofunctional monomer PDLC film 
is less than the known transition temperature of the single monofunctional 
monomer film, and the transition temperature decreases with the amount of 
the first monofunctional acrylate in the composition, then the first 
monofunctional acrylate gives rise to homogeneous anchoring at least down 
to temperatures below the lowest measured anchoring transition temperature 
of the film. If the transition temperature of the two-monofunctional 
monomer PDLC film is greater than the transition temperature of the single 
monofunctional monomer PDLC film, and the transition temperature increases 
with the amount of the first monofunctional acrylate in the composition, 
then the first monofunctional acrylate gives rise to homeotropic anchoring 
at least at temperatures above the highest measured anchoring transition 
temperature. 
A variety of monofunctional acrylates are contemplated as suitable for use 
in the PDLC composition of the present invention. Suitable monofunctional 
acrylates have a variety of substituent moieties. In this regard, alkyl 
acrylates, cycloalkyl acrylates, aryl acrylates and alkylaryl acrylates 
are contemplated as suitable. Examples of suitable monofunctional 
acrylates include lauryl acrylate, n-decyl acrylate, n-octyl acrylate, 
n-hexyl acrylate, 2-octyl acrylate, 2-ethyl hexyl acrylate, ethylbenzyl 
acrylate, and isobornyl acrylate. These acrylates are provided by way of 
example. The selection of a particular monofunctional acrylate for a 
particular composition is accomplished using the techniques previously 
described 
As previously stated, the liquid crystal, polyfunctional acrylate, and 
photoinitiator components of the composition of the present invention are 
well-known to one skilled in the art. One example of a suitable 
polyfunctional acrylate is trimethylol propane triacrylate. Examples of 
suitable photoinitiators include the benzoin-ether initiators, 
benzophenone-type initiators, and thioxanthone-type initiators.

DETAILED DESCRIPTION 
The PDLCs of the present invention are formed into films that have 
advantages over known PDLC films. The liquid crystal composition contains 
a standard, commercially available liquid crystal as described above in 
combination with a mixture of acrylate monomers that, when polymerized, 
forms a matrix that provides the resulting film with certain advantages. 
The PDLC compositions of the present invention are about 50 to about 90 
weight percent liquid crystal and about 50 weight percent to about 10 
weight percent of a polymerizable monomer mixture that is a mixture of a 
polyfunctional acrylate, a photoinitiator, and at least two monofunctional 
acrylates. One of the monofunctional acrylates gives rise to a homeotropic 
anchoring condition at the operating temperature of the film formed from 
the PDLC composition. The other monofunctional acrylate gives rise to a 
homeotropic anchoring condition at the operating temperature of the film 
formed from the PDLC composition. In a further embodiment of the PDLC 
composition of the present invention, the liquid crystal is about 75 
weight percent of the composition to about 90 weight percent of the 
composition and the polymerizable monomer mixture is about 10 weight 
percent to about 25 weight percent of the composition. 
The polymerizable monomer mixture is in turn, about 70 weight percent to 
about 95 weight percent of a mixture of monofunctional acrylates, about 5 
weight percent to about 25 weight percent of a polyfunctional acrylate and 
about 0.01 weight percent to about 5 weight percent of a photoinitiator. 
In order to select the monofunctional acrylates for the mixture to obtain 
the desired effect, it must be determined whether the monofunctional 
acrylate gives rise to a homogeneous anchoring condition or a 
heterogeneous anchoring condition at the temperature at which the PDLC 
film will operate. Since, typically, PDLC films operate at room 
temperature (20.degree. C. to 30.degree. C.), the monofunctional acrylate 
mixture is selected so that one of the acrylates gives rise to a 
homeotropic anchoring condition within this range and the other 
monofunctional acrylate mixture gives rise to a homogeneous anchoring 
condition within this range. 
Whether a particular monofunctional acrylate will give rise to a particular 
anchoring condition at a particular temperature can be determined by 
observing the behavior of a PDLC film made from a PDLC composition in 
which the polymerizable monomer mixture contains only one monofunctional 
acrylate monomer in conjunction with the polyfunctional acrylate and the 
photoinitiator. By observing the effects of temperature on properties of 
the film such as the time it takes for the film to respond to the 
cessation of an applied voltage and the light transmission through a film 
at a particular voltage, a transition temperature can be determined for 
many of these compositions. Exemplary methods for determining the 
anchoring condition provided by a particular monofunctional acrylate in a 
PDLC film are described below. These methods are provided by way of 
example only. Other equivalent methods are easily ascertained by persons 
skilled in the art. Examples of methods for making this determination for 
a particular monofunctional acrylate are provided below. 
EXAMPLE 1 
PDLC films were prepared by combining the following ingredients in the 
specified proportions: 
TABLE 1 
______________________________________ 
INGREDIENT AMOUNT 
______________________________________ 
liquid crystal 80 wt% 
polymerizable acrylate mixture 
20 wt% 
______________________________________ 
The polymerizable acrylate mixture was made from the following: 
TABLE 2 
______________________________________ 
INGREDIENT AMOUNT 
______________________________________ 
n-hexyl acrylate 83 wt % 
trimethyl propane triacrylate 
15.2 wt % 
photoinitiator 1.8 wt % 
______________________________________ 
The liquid crystal was TL205 which is obtained commercially from E.M. 
Industries of Hawthorne, N.Y. The initiator was Irgacure 184 which is 
obtained commercially from Ciba Corp. of Hawthorne, N.Y. The trimethylol 
propane triacrylate was commercially obtained from Aldrich Chemical Co. of 
Milwaukee, Wis. The acrylates specifically identified were obtained from 
Scientific Polymer Products of Ontario, N.Y. 
The above-identified composition was formed into a film by placing the 
composition between glass substrates with a coating of conductive indium 
tin oxide thereon which is transparent to the exposing radiation and 
visible light. Glass microspheres were placed between the glass plates to 
keep a distance of about 10 microns between the plates. The film was then 
exposed to a 17 mW/cm.sup.2 dose of UV radiation for 9 to 10 minutes using 
a mercury arc lamp. The resulting PDLC films were observed to have a 
"foam-texture" morphology, that is, the bulk of the film volume is made up 
of droplets with thin walls separating the droplets and the droplets 
themselves were in the 0.5 .mu.m to 4 .mu.m diameter range. 
The transition temperatures for the PDLC film was determined by the 
following procedure. Light from a helium-neon laser was propagated normal 
to the plane of the PDLC film. The light intensity was first modulated at 
100 kHz by a photoelastic modulator between crossed polarizers, then made 
circularly polarized by a quarter-wave plate. The collection half-angle 
for the transmitted light was set to 10.degree. by an iris. The 
forward-transmitted light was then focused by a lens onto a photodetector. 
The photodetector signal was amplified and demodulated by a lock-in 
amplifier, which used the photoelastic modulator signal as a reference. In 
this manner, background light was eliminated from the measurement. The 
signal from the lock-in amplifier was recorded by a digital oscilloscope. 
A gated, 1 kHz sinusoidal voltage was applied to the above-described PDLC 
film. The voltage was varied to determine the relationship between the 
applied voltage and the intensity and the film's light transmission. The 
applied voltage was varied in the range of 0 to 70 volts (rms). A pulse 
with a duration of about 500 ms was applied to determine the intensity of 
the film at that voltage. The duration of the pulse was increased if the 
intensity of the film was still visibly increasing at the end of 500 ms. 
At each voltage, the intensity of the signal from the film was recorded, 
as well as the time it took for the film to respond to commencement and 
cessation of the applied voltage. The time it takes for the transmitted 
light intensity from a PDLC film to go from 10 percent to 90 percent of 
the total change from the "off" and "on" states is referred to as the rise 
time. Similarly, the time it takes for the transmitted light intensity 
from a PDLC film to go from 10 percent to 90 percent of the total change 
between the "on" and "off" states is referred to as the fall time. By 
varying the voltage and observing the intensity of the light transmitted 
by the PDLC film, V.sub.90, the voltage required to achieve 90 percent of 
the total increase in transmittance between the "off" and "on" states, was 
determined. 
The PDLC film was heated and cooled in a chamber with nitrogen gas flow 
during electro-optical testing. The fall time, forward transmitted light 
intensity in the "off" state, and V.sub.90 were measured as a function of 
temperature. A graph of the response time (both rise and fall times) of a 
film as a function of temperature is illustrated in FIG. 1. The sharp peak 
at 64.degree. C. in FIG. 1 indicates that the transition temperature for 
the n-hexyl acrylate film is at 64.degree. C. A sharp peak in the forward 
light transmission (I.sub.o) at the same temperature also corresponds to 
the anchoring transition (FIG. 2), as does the cusp in V.sub.90 at the 
same temperature (FIG. 3). The sharp peak in the fall time (FIG. 1) is 
viewed as indicative of the transition temperature of the PDLC film under 
investigation. 
Polarized light microscopy was used to confirm the correspondence between 
the sharp peaks and cusps in the electro-optic data to the anchoring 
transition. The polarized light microscopy revealed many instances where 
the liquid crystal droplet texture was radial below 65.degree. C., and 
bipolar above 65.degree. C. These observed textures are indicative of 
homeotropic and homogeneous anchoring, respectively. The radial and 
bipolar textures are described in references such as Drzaic, P., "Liquid 
Crystal Dispersions," World Scientific, chap. 3 (1995). 
EXAMPLE 2 
The anchoring condition provided by various monofunctional acrylates was 
determined by preparing compositions in which the only monofunctional 
acrylate component in the polymerizable acrylate mixture was exclusively 
the monofunctional acrylate under evaluation. PDLC films were formed from 
these compositions and the transition temperature of the films was 
determined in the manner described in the previous example. The PDLC films 
were prepared as described above. Numerous compositions were prepared, 
each with a different monofunctional acrylate. The monofunctional 
acrylates used for this analysis are enumerated in Table 3 below. 
TABLE 3 
______________________________________ 
Monofunctional Acrylate 
Transition Temperature T.sub.t (.degree.C.) 
______________________________________ 
lauryl acrylate 
80.5 
n-decyl acrylate 
79.5 
n-octyl acrylate 
77 
n-hexyl acrylate 
64 
2-octyl acrylate 
48 
2-ethyl hexyl acrylate 
23 
isobornyl acrylate 
not observed 
______________________________________ 
The transition temperatures for the PDLC compositions were determined as 
described in the previous example. Generally, the films were formed from a 
composition that was 80 wt % liquid crystal and 20 wt % polymerizable 
acrylate mixture. The polymerizable acrylate mixture was 13.5 wt % of 
trimethylol propane triacrylate, 1.5 wt % Darocur 1173 and 85 wt % of a 
monofunctional acrylate. The composition containing 2-ethyl hexyl acrylate 
had a liquid crystal fraction of about 81 wt %, because this composition 
did not exhibit a transition when the liquid crystal fraction was 80 wt % 
or less. 
Although applicants do not wish to be held to a particular theory, 
applicants believe that the electro-optical signatures of the anchoring 
transition are related to droplet size. That is, these signatures are more 
difficult to detect as the droplet size decreases. When the droplet 
diameter is about 1 .mu.m or less, these signatures are typically not 
detectable. Therefore, if the anchoring behavior of a PDLC film is to be 
characterized by observing electro-optic signatures, it is advantageous if 
the iroplet size of the liquid crystal in the film is at least about 2 
.mu.m. If the film under nvestigation has a droplet size smaller than 2 
.mu.m, the electro-optic testing described above can be done using an 
analogous film with a larger liquid crystal droplet size. 
For example, a film with a larger liquid crystal droplet size is obtained 
by reducing the polymerization temperature of the matrix by about 
5.degree. C. to about 15.degree. C. A slight increase in the liquid 
crystal content of the composition (i.e. an increase from an 80 wt % 
fraction to an 81 wt % fraction) provides an increase in the liquid 
droplet size in a film formed from the composition with the increased 
amount of liquid crystal in the composition. Since the liquid crystal 
fraction affects slightly the transition temperature, the transition 
temperature at the lower fraction should be extrapolated from the 
transition temperature at the higher fractions. 
Another way to obtain larger droplets in a PDLC film is by reducing the 
rate of polymerization by reducing the concentration of photoinitiator or 
by reducing the intensity of the exposing radiation. However, if these 
methods are used, care must be taken to avoid incomplete polymerization. 
One observable trend from the information in Table 3 is that the structure 
of the alkyl acrylate is related to its transition temperature. In this 
regard it is observed that the transition temperature decreases as the 
number of carbon atoms in the alkyl chain decreases (i.e., the transition 
temperature for n-hexyl acrylate is less than the anchoring transition 
temperature of n-decyl acrylate). Acrylates with a branched alkyl side 
chain and secondary alkyl acrylates typically have a lower transition 
temperature than their corresponding unbranched or primary homologues 
(e.g., the transition temperature resulting from use of 2-ethyl hexyl 
acrylate is less than the transition temperature resulting from use of 
n-octyl acrylate). 
If the transition temperature of a particular monofunctional acrylate 
cannot be determined in the above-described manner, an alternate method 
for characterizing its effect on surface anchoring is by observing the 
effect of the addition of the monofunctional acrylate on the transition 
temperature of a film formed from a PDLC composition that contains the 
monofunctional acrylate monomer in question in conjunction with a second 
monofunctional acrylate monomer for which the anchoring transition 
temperature is known. This observation is also useful to determine the 
surface anchoring characteristics provided by a monofunctional acrylate 
where an anchoring transition temperature is observed but it is not known 
whether homeotropic anchoring (or homogeneous anchoring) occurs above or 
below the transition temperature. If the transition temperature of the 
films decrease with the addition of increased amounts of the first 
monofunctional acrylate in the PDLC composition then the first 
monofunctional acrylate gives rise to homogeneous anchoring at least down 
to the lowest measured anchoring temperature. If the transition 
temperature of the films increase with the addition of increased amounts 
of the first monofunctional acrylate in the PDLC, then the first 
monofunctional acrylate gives rise to homeotropic anchoring, at least up 
to the highest measured anchoring transition temperature. This technique 
is illustrated in the following example. 
EXAMPLE 3 
As noted in example 2, a transition temperature was not observed for 
isobornyl acrylate using the techniques described in the above example. A 
series of PDLC compositions were prepared in which the monofunctional 
acrylate component was a mixture of isobornyl acrylate and 2-ethyl hexyl 
acrylate. The amounts of the various components (e.g., the liquid crystal, 
polyfunctional acrylate, and photoinitiator) were the same as the amounts 
set forth in the previous example. Three compositions were prepared. The 
relative mass fractions of the 2-ethyl hexyl acrylate and isobornyl 
acrylate were different in each composition. The mass fractions were 
0.5:0.5, 0.75:0.25, and 1:0 2-ethyl hexyl acrylate to isobornyl acrylate. 
The compositions were formed into films and the transition temperatures of 
the films were determined as described in the previous example. The 
transition temperature of the film was observed to increase as the mass 
fraction of the isobornyl acrylate in the composition decreased. 
Since a PDLC film made from a composition in which the monofunctional 
acrylate component was solely 2-ethyl hexyl acrylate has a transition 
temperature 23.degree. C., and the transition temperature of the films was 
observed to decrease (which means that the range of temperatures at which 
the composition exhibited homogeneous anchoring, i.e., the temperature 
range above the transition temperature, increased) with an increasing mass 
fraction of isobornyl acrylate. This test established that isobornyl 
acrylate gives rise to a homogenous anchoring condition at least down to 
5.degree. C., which was the lowest observed anchoring transition 
temperature in the three films. 
After the anchoring condition associated with particular monofunctional 
acrylates is determined, an appropriate mixture of monofunctional 
acrylates for the composition of the present invention is determined. The 
following mixing rules are provided as one method to determine the 
appropriate amount of each monofunctional acrylate in the monofunctional 
acrylate mixture. An example of one incremental way to form a PDLC 
composition is to select a desired transition temperature for the 
composition. The selection of the transition temperature will depend upon 
the range of operating temperatures of the PDLC film. In certain 
applications, it is desirable for the PDLC composition to have a 
transition temperature within the operating range, since the anchoring 
strength of the composition is lower at the transition temperature. For 
example, if a film with a low V.sub.90 at a particular temperature is 
desired, then the goal will be to prepare a composition that exhibits a 
transition at that temperature. However, a PDLC composition in which the 
transition temperature is outside the operating temperature is also 
contemplated as useful because the switching voltage of PDLC films are 
often observed to be unusually insensitive to temperature changes over a 
wide temperature range above the anchoring transition temperature. 
Therefore, the PDLC compositions of the present invention allow one to 
design for temperature insensitivity over a chosen temperature range more 
readily than by using the one-monofunctional acrylate compositions. 
The selected monofunctional acrylate that has been determined to provide a 
homeotropic anchoring condition in a PDLC film operating at the desired 
temperature is combined with the selected monofunctional acrylate that has 
been determined to provide a homogeneous anchoring condition in a PDLC 
film operating at that same temperature. Whether a monofunctional acrylate 
will give rise to a homogeneous anchoring condition or a homeotropic 
anchoring condition in a particular temperature range can be determined in 
the above-described manner. 
After the particular monofunctional acrylates have been identified, the 
amount of each monofunctional acrylate in the composition that will give 
the desired effect is determined. The amount of each monofunctional 
acrylate monomer in the composition is determined in an iterative manner. 
That is, a composition is prepared and the transition temperature of a 
film made from the composition is observed. If the observed transition 
temperature is not the desired transition temperature, the composition is 
adjusted by adding more of one monofunctional acrylate monomer and 
commensurately less of the other. As a starting point, it is assumed that 
the transition temperature of a film made from the composition is linearly 
related to the relative amount of each monofunctional acrylate in the PDLC 
composition. That is, if a monofunctional acrylate designated as A is 
associated with a transition temperature T.sub.A and a monofunctional 
acrylate that is designated as B is associated with a transition 
temperature T.sub.B, the transition temperature is approximated from the 
sum of T.sub.A times the mass fraction of A and T.sub.B times the mass 
fraction of B in the monofunctional acrylate blend. This is represented by 
the following equation: 
EQU T.sub.goal =(T.sub.A .multidot.x)+(T.sub.B .multidot.(1-x)) 
wherein T.sub.goal is the desired transition temperature of the PDLC 
composition and x is the mass fraction of monofunctional acrylate A. Of 
course, this is only a starting point to determining the desired 
composition because in general the transition temperature is not linear 
with respect to composition. Once the composition is made and the 
transition temperature of the film formed from the composition is 
determined, the amounts of A and B in the composition may require further 
adjustment to obtain a composition with the desired transition 
temperature. 
For example, if the first iteration used a composition parameter x.sub.1 
that provided a transition temperature T.sub.1 that was unacceptably far 
from T.sub.goal, that information is used to characterize the relationship 
between the anchoring transition temperature and the composition of the 
monofunctional acrylate blend (presuming that the anchoring transition 
temperature associated with each of the monofunctional acrylates in the 
composition is known). A quadratic fit of the data then provides an 
approximation of the composition that will provide the desired anchoring 
temperature using the following equations: 
EQU T.sub.goal =T.sub.A +(T.sub.B -T.sub.A)x+c.multidot.x(1-x) 
where 
EQU c=T.sub.1 -T.sub.A -(T.sub.B -T.sub.A).multidot.x.sub.1 !/x.sub.1 
(1-x.sub.1)!. 
The above equations are then solved for x, which is the mass fraction of 
one of the monofunctional acrylate components in the mixture. 
If the anchoring transition temperature that is associated with a single 
monofunctional acrylate-containing composition cannot be determined, a 
different iteration is proposed based upon the effect of the blend of 
monofunctional acrylates on the transition temperature of a PDLC film made 
from a composition containing that blend of monofunctional acrylates. In 
this example, a transition temperature is observed for the film and is 
associated with the mass fraction, x, of one of the monofunctional 
acrylates in the two-monofunctional acrylate blend. At least two data 
pairs of data points (i.e. at least two transition temperatures, each 
associated with a particular mass fraction) are required to make this 
iteration. A linear fit of the data is then done which can be used to 
approximate the mass fraction that will provide a film with the desired 
transition temperature (T.sub.goal). Further iterations are performed 
using linear extrapolation of the data from the two compositions that 
exhibited an anchoring transition closest to T.sub.goal to obtain a 
composition that exhibits anchoring transition temperature even closer to 
T.sub.goal. 
Near the transition from homeotropic anchoring to homogeneous anchoring, 
the anchoring strength is expected to be weak. It is believed that it 
requires a lower electric field to switch films from "off" to "on" when 
the anchoring strength is low. Thus a PDLC composition in which the 
anchoring transition of the liquid crystal in the film occurs proximate 
(i.e. within about 10.degree. C.) to the typical operating temperature of 
the PDLC is believed to be advantageous. Since PDLCs typically operate at 
room temperature (20.degree. C. to about 30.degree. C.) it is advantageous 
if the anchoring transition of the liquid crystal in these films occurs 
just below this temperature range. Having the anchoring transition below 
the operating temperature range is advantageous over having the anchoring 
transition above the operating temperature range. This is because the 
temperature dependence of the switching voltage is often observed to be 
weak over a temperature range (typically 20-30.degree. C.) above the 
anchoring transition. The weak temperature dependence is desirable because 
it reduces the amount of temperature compensation required to operate the 
device. It is believed that the weak temperature dependence arises from 
two competing trends: 1.) with increasing temperature (above the 
transition) the anchoring strength increases; and 2.) the ratio of the 
nemato-elastic constants to the dielectric anisotropy decreases. The first 
causes an increase in switching voltage and the latter a decrease in the 
switching voltage. 
As stated previously, the PDLC composition of the present invention, when 
formed into a film, has a demonstrably lower switching voltage than films 
formed from PDLC compositions that do not have a mixture of monofunctional 
acrylates, one that gives rise to homeotropic anchoring and one that gives 
rise to homogeneous anchoring. This is illustrated by the following 
example. Furthermore, films formed from PDLC compositions of the present 
invention demonstrate this reduced switching voltage over a wide 
temperature range. It is advantageous if the films demonstrate this 
reduced switching over a temperature range of about 10.degree. C. to about 
50.degree. C. 
EXAMPLE 4 
A series of films with the following general liquid crystal composition 
described in Table 1 in Example 1 above were prepared. The monofunctional 
acrylate component of the composition was varied from film to film. In one 
composition, the monofunctional acrylate was 100% by weight 2-ethyl hexyl 
acrylate. In the other compositions, the 2-ethyl hexyl acrylate was 
combined with varying weight percents of n-octyl acrylate. Three 
compositions were prepared in which the amount of n-octyl acrylate was 2 
wt %, 5 wt %, and 9 wt % of the monofunctional acrylate component, 
respectively. 
PDLC films were prepared from these compositions as described in Example 1. 
The forward light transmittance through these films was measured while 
various voltages were applied to the films. The forward transmittance and 
the response time of the film were measured in the manner described in 
Example 1. 
FIG. 4 illustrates the switching voltage of PDLC films of these 
compositions as a function of temperature. As observed from FIG. 4, the 
switching voltage of the films formed from PDLC compositions in which the 
monofunctional acrylate component was 2 wt %, 5 wt % and 9 wt % n-octyl 
acrylate is significantly lower in the temperature range of 10.degree. C. 
to 80.degree. C. than the two films formed from liquid crystal 
compositions that contained only 2-ethyl hexyl acrylate and no n-octyl 
acrylate. 
FIG. 5 illustrates the fall time of the films as a function of temperature. 
FIG. 5 illustrates that the same two films that demonstrated a 
dramatically lower switching voltage in the 10.degree. C. to 80.degree. C. 
range also demonstrated an anchoring transition in this temperature range. 
The other films did not demonstrate a transition in this range. Although 
applicant does not wish to be held to a particular theory, applicant 
believes that PDLC films made from compositions that demonstrate a 
transition in the temperature range of 10.degree. C. to 80.degree. C. will 
also demonstrate a lower switching voltage in this range. Applicants 
attribute the presence of this transition to the mixture of monofunctional 
acrylates used in the liquid crystal composition. However, applicants also 
believe that compositions of the present invention will exhibit lower 
switching voltages without exhibiting this transition. 
EXAMPLE 5 
Three PDLC films were made from a composition that was 80 wt % TL205 liquid 
crystal and 20 wt % of a polymerizable mixture with the following general 
composition: monofunctional acrylate (85 wt %); trimethylol propane 
triacrylate (13.5 wt %); and a photoinitiator, Darocur 1173 (1.5 wt %). 
The monofunctional acrylate component was n-octyl acrylate in a first 
composition, isobornyl acrylate in a second composition, and a blend of 
isobornyl acrylate (68 wt %) and n-octyl acrylate (32 wt %) in a third 
composition. The amount of photoinitator was increased to 2.1 wt % in the 
second composition (which resulted in a commensurate reduction wt % of the 
other components in the composition) to compensate for the lower reaction 
rate of the isobornyl acrylate. Films were made from each of the three 
compositions in the manner described in Example 1. 
The anchoring transition temperature for the first composition (octyl 
acrylate only) was observed to be 77.degree. C. with homeotropic anchoring 
below the transition temperature and homogenous anchoring above the 
anchoring transition temperature. The anchoring transition temperature was 
determined using electro-optic analysis. An anchoring transition was not 
observed for the film made from the second composition (isobornyl acrylate 
only). The anchoring transition temperature of the films were observed to 
decrease with increasing amounts of isobornyl acrylate in the composition. 
The switching voltages of these films as a function of temperature is 
illustrated in FIG. 6. The cusp of the switching voltage for the third 
composition (the blend of n-octyl acrylate and isobornyl acrylate) 
corresponds to an anchoring transition temperature of about 18.degree. C., 
which is much lower than the transition temperature of a film formed from 
the first composition. This film also had a lower switching voltage than 
the films formed from the other compositions, especially over the 
temperature range of 12.degree. C. to about 50.degree. C.