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BACKGROUND OF THE INVENTION
The present invention relates to a multilayer birefringent interference
polarizer, and more particularly to a multilayer coextruded polymeric
device which can be designed to polarize selected wavelengths of light by
constructive optical interference.
Birefringent polarizers are generally known in the art and have been used
in the past to polarize and filter selected wavelengths of light. For
example, birefringent polarizers may be used to reject (reflect) specific
polarized narrow wavelength ranges while transmitting the remainder of the
incident light, to reduce glare from other light sources, and to act as
beam splitters.
Many naturally occurring crystalline compounds act as birefringent
polarizers. For example, calcite (calcium carbonate) crystals have well
known birefringent properties. However, single crystals are expensive
materials and cannot be readily formed into the desired shapes or
configurations which are required for particular applications. Others in
the art, such as Makas, U.S. Pat. No. 3,438,691, have fabricated
birefringent polarizers from plate-like or sheet-like birefringent
polymers such as polyethylene terephthalate incorporated into an isotropic
matrix polymer.
In many instances, polymers can be oriented by uniaxial stretching to
orient the polymer on a molecular level such as taught by Rogers et al,
U.S. Pat. No. 4,525,413. Multilayer optical devices comprising alternating
layers of highly birefringent polymers and isotropic polymers having large
refractive index mismatches have been proposed by Rogers et al. However,
the Rogers et al device requires the use of specific highly birefringent
polymers having certain mathematical relationships between their molecular
configurations and electron density distributions.
Accordingly, there remains a need in the art for birefringent interference
polarizers which can be readily produced using existing techniques and
readily available materials. Further, there still exists a need in the art
for birefringent interference polarizers which absorb little light.
Further, the need exists in the art for birefringent polarizers which can
be fabricated to polarize light of specific wavelengths as desired.
SUMMARY OF THE INVENTION
The present invention meets that need by providing a birefringent
interference polarizer in the form of a multi-layered sheet or film which
may be fabricated from readily available materials using established
coextrusion techniques. The polarizer of the present invention has a level
of light absorption near zero and can be fabricated to polarize and
reflect light of specific wavelengths while transmitting light of other
wavelengths. The polarizer will also polarize the transmitted light at
those wavelengths, while the remainder of the transmitted light remains
unpolarized.
Reference to polarizers, polarized light, and polarization as used herein
refers to a condition of light in which the transverse vibration of the
rays assume different forms in different planes. Polarization, as used
herein, includes the nonequal reflection of light in orthogonal planes and
encompasses elliptical and circular polarization of light as well as plane
polarization. By "light" we mean not only light in the visible spectrum,
but also ultraviolet and infrared light. When the plane of orientation of
the polymeric materials is discussed herein, we are referring to the
directions of orientation of the polymeric materials due to uniaxial or
biaxial stretching of the materials in the x and/or y direction to define
the polarizing effect of the materials. In other contexts, reference to
the plane that light enters or impinges upon the layers of polymeric
materials is a plane normal to the major surfaces of the layers (i.e., the
z direction), unless otherwise indicated.
In accordance with one aspect of the present invention, a birefringent
interference polarizer is provided comprising multiple alternating
oriented layers of at least first and second polymeric materials having
respective nonzero stress optical coefficients which are sufficiently
different to produce a refractive index mismatch between the first and
second polymeric materials in a first plane which is different from the
refractive index mismatch between the first and second polymeric materials
in a second plane normal to the first plane.
The birefringent polarizer of the present invention may also comprise three
or more alternating layers of diverse polymeric materials. For example, a
three layer pattern of repeating units ABCBA may be used, where the B unit
is a copolymer or miscible blend of the A and C repeat units. In some
instances, the B layer may not only contribute to the light polarization
properties of the invention, but also act as an adhesive layer to bond the
A and C layers together.
Also, the third polymer layer may be found as a surface or skin layer on
one or both major exterior surfaces for an ABABAB repeating body or as an
interior layer. The skin layer may be sacrificial, or may be permanent and
serve as scratch resistant or weatherable protective layer. Further, such
skin layers may be post applied to the polarizer after coextrusion. For
example, a skin layer may be applied as a sprayed on coating which would
act to level the surface of the polarizer to improve optical properties
and impart scratch resistance, chemical resistance and/or weatherability.
The skin layer may also be laminated to the multilayered polarizer.
Lamination is desirable for those polymers which are not readily
coextrudable.
In one embodiment of the invention, the first and second polymeric
materials have substantially equal refractive indices when unoriented. The
refractive index mismatch develops in the plane of orientation when the
materials are stretched. In another embodiment, the first and second
polymeric materials have differing refractive indices when unoriented.
Orienting the polymers by stretching causes the mismatch between
respective refractive indices in one of the planes to decrease, while the
mismatch in the other plane is maintained or increased. The polarizer may
be uniaxially or biaxially oriented.
In a preferred form of the invention, the first polymeric material has a
positive stress optical coefficient, while the second polymeric material
has a negative stress optical coefficient. Preferably, the refractive
index mismatch in the first plane is at least 0.03, and most preferably
0.05 or greater.
Preferably, the optical thickness of each polymeric layer is from about
0.09 micrometers to about 0.70 micrometers. Optical thickness, nd, is
defined as the product of the physical thickness of the layer (d) and its
refractive index (n). In a preferred form of the invention, the layers
increase in thickness monotonically through the thickness of the film to
produce a layer thickness gradient which reflects and polarizes a broad
range of wavelengths of light.
The two polymeric materials can be any of a number of different polymers
which possess nonzero stress optical coefficients which provide the
necessary refractive index mismatch when the materials are oriented. By
nonzero stress optical coefficient, it is meant that the refractive index
of the polymer changes in either a positive or negative direction when the
polymer is oriented. Isotropic materials possessing zero stress optical
coefficients lack birefringence.
For example, the first polymeric material may be a polycarbonate, such as a
bisphenol A based polycarbonate, or a polyethylene terephthalate, both of
which possess positive stress optical coefficients. The second polymeric
material may be a polystyrene which has a negative stress optical
coefficient. Either generally amorphous atactic polystyrenes or more
crystalline syndiotactic polystyrenes are suitable. Other suitable
polymers for the second polymeric material include copolymers of styrene
and acrylonitrile, copolymers of styrene and methyl methacrylate, and
polyethylene naphthalate, all of which possess negative stress optical
coefficients.
The polarizer of the present invention reflects and polarizes a portion of
the light incident on its surface while transmitting the remainder of the
incident light. During fabrication, it may be designed to transmit only a
narrow range of wavelengths while reflecting a broad range, or vice versa.
The polarizer of the present invention may also be designed to reflect and
polarize substantially all light incident in one plane of the device while
transmitting substantially all light incident in a plane normal thereto.
In some embodiments of the invention it may be desirable to incorporate
coloring agents such as dyes or pigments into one or more of the
individual layers of the birefringent polarizer. This can be done to one
or both of the outer or skin layers of the body, or alternatively, the
coloring agent may be incorporated into one or more interior layers in the
polarizer. The use of pigments or dyes permits the selective absorption of
certain wavelengths of light by the polarizer. While an unpigmented or
undyed multilayer film will reflect specific polarized wavelengths and
transmit the remainder of incident light, pigments and dyes can be used to
further control the bandwidth of reflected polarized light and the
wavelength range of transmitted light. For example, all transmitted light
may be absorbed by coextruding a black layer on the back side of the
birefringent polarizer. Furthermore, dyes may be used to narrow the
wavelength band of reflected polarized light and transmitted light by
absorbing selected wavelengths.
The polymers chosen will determine the refractive index mismatch,
respective stress optical coefficients, and glass transition temperatures.
The number of layers, degree of orientation, layer thicknesses, and use of
pigments or dyes may all be adjusted (controlled) to provide a polarizer
having the desired characteristics for a particular end use. This
contrasts to prior art devices which were limited both in design and
polarization characteristics.
In another embodiment of the invention, a tunable birefringent interference
polarizer is provided and comprises multiple alternating layers of first
and second elastomeric materials having respective nonzero stress optical
coefficients which are sufficiently different to produce a refractive
index mismatch between the first and second elastomeric materials in a
first plane which is different from the refractive index mismatch between
the first and second elastomeric materials in a second plane normal to the
first plane. Because the individual layers forming the polarizer are
elastomers, the polarizer variably polarizes wavelengths of light
dependent upon the degree of elongation of the elastomers. Additionally,
because the layers are elastomers, the polarizer is tunable and reversible
as the device is returned to a relaxed state.
The present invention also provides a method of making a birefringent
interference polarizer comprising the steps of coextruding at least first
and second polymeric materials having respective nonzero stress optical
coefficients in multiple layers. The layers may be stretched to orient the
polymeric materials and produce a refractive index mismatch in a first
plane which is different from the refractive index mismatch between the
first and second polymeric materials in a second plane normal to the first
plane. While many polymer combinations can be stretched at temperatures
above the glass transition temperature but below the melting temperature
of the polymers, some polymer combinations can be "cold drawn," where one
or more of the polymers can be stretched at a temperature below its glass
transition temperature.
In one embodiment of the invention, the first and second polymeric
materials have substantially equal refractive indices when unoriented,
with a refractive index mismatch in one plane developing upon orientation.
In another embodiment, when oriented, the first and second polymeric
materials have substantially equal refractive indices in one of the first
and second planes, but there is a refractive index mismatch in the other
plane. The orientation of the polymeric materials may be either uniaxial
or biaxial.
Preferably, the refractive index mismatch in the first plane is at least
about 0.03, and most preferably at least 0.05 or greater, with the optical
thickness of each layer being from about 0.09 micrometers to about 0.70
micrometers. In one embodiment, the layers increase in thickness
monotonically through the thickness of the film to provide a polarizer
which reflects a broad range of wavelengths. In a preferred form of the
invention, the first polymeric material has a positive stress optical
coefficient, and the second polymeric material has a negative stress
optical coefficient.
Accordingly, it is an object of the present invention to provide a
birefringent interference polarizer, and method of making, which may be
fabricated from readily available materials, using established coextrusion
techniques, to include having a level of light absorption near zero and be
fabricated to reflect and polarize light of specific wavelengths while
transmitting light of other wavelengths. This, and other objects and
advantages of the present invention will become apparent from the
following detailed description, the accompanying drawings, and the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of reflectance versus wavelength of light for a
multilayer optical interference polarizer made in accordance with the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides improved optical interference polarizers in
the form of multilayer films with a number of desirable properties
including the ability to tailor the device to polarize selected
wavelengths of light. The basic optical principles involved in the present
invention are those relating to the reflection of light by thin film
layers having differing refractive indices. These principles demonstrate
the dependency of the effect on both individual layer thickness as well as
refractive index of the material. See, Radford et al, "Reflectivity of
Iridescent Coextruded Multilayered Plastic Films", 13 Polymer Engineering
and Science 216 (1973).
A thin film is described in the literature as one whose thickness, d, is
less than about 0.5 micrometers or whose optical thickness, nd (where n is
the refractive index of the material) is less than about 0.7 micrometers.
Vasicek, Optics of Thin Films (1960) at pages 100 and 139.
Interference films which rely on the constructive optical interference of
light to produce intense reflected light in the visible, ultraviolet, or
infrared portions of the electromagnetic spectrum have been described in
the prior art. See, for example, Alfrey, Jr. et al, U.S. Pat. No.
3,711,176. Such interference films act according to the equation:
.lambda..sub.m -(2/m)(N.sub.1 D.sub.1 +N.sub.2 D.sub.2)
where .lambda..sub.m is the reflected wavelength in nanometers, N.sub.1 and
N.sub.2 are the refractive indices of the alternating polymers, D.sub.1
and D.sub.2 are the thickness of the respective layers of polymers in
nanometers, and m is the order of reflection (m=1, 2, 3, 4, 5). This is
the equation for light incident normal to the surface of the film. For
other angles of incidence, the equation will be modified to take into
account the angle, as is known in the art. The polarizer of the present
invention is operable for all angles of incident light. Each solution of
the equation determines a wavelength at which an intense reflection,
relative to surrounding regions, is expected. The intensity of the
reflection is a function of the "f-ratio" where,
##EQU1##
By proper selection of the f-ratio, one can exercise some degree of control
over the intensity of reflection of the various higher order reflections.
For example, first order visible reflections of violet (about 0.38.mu.
wavelength) to red (about 0.68.mu. wavelength) can be obtained with layer
optical thicknesses between about 0.075-0.25 micrometers.
However, light reflected from prior art thin layer interference films is
not polarized. The light reflected from the alternating polymeric layers
of the present invention is polarized principally due to the birefringent
nature of the film. Thus, in its preferred form, the birefringent
interference polarizer of the present invention comprises multiple
alternating oriented layers of at least first and second polymeric
materials having respective nonzero stress optical coefficients which are
sufficiently different to produce a refractive index mismatch between the
first and second polymeric materials in a first plane which is different
from the refractive index mismatch between the first and second polymeric
materials in a second plane normal to the first plane. This refractive
index mismatch is preferably at least about 0.03, and most preferably at
least 0.05 or greater. This construction results in a polarizer having
optical interference in a first plane, such as the plane of orientation,
and near zero optical interference in a second plane normal thereto.
Preferably, the optical thickness of each polymeric layer is in the range
of from about 0.09 to about 0.70 micrometers. Polymers suitable for use in
the practice of the present invention include generally transparent
thermoplastic polymers having stress optical coefficients which provide
the necessary refractive index mismatch in at least one plane when the
polymers are oriented. Additionally, it is desirable from a processing
standpoint that the polymers be compatible for coextrusion.
One example of a suitable polymer pair is polycarbonate and polystyrene.
Syndiotactic polystyrene is believed to be especially suitable.
Polycarbonate has a positive stress optical coefficient, while polystyrene
has a negative stress optical coefficient. Both have refractive indices
(unoriented) of approximately 1.6. Other generally transparent
thermoplastic polymers which are suitable for use in the present invention
include elastomers such as those described in commonly-assigned
application Ser. No. 339,267, filed Apr. 17, 1989, and entitled
"Elastomeric Optical Interference Films", now U.S. Pat. No. 4,937,134,
issued Jun. 26, 1990, the disclosure of which is hereby incorporated by
reference.
Additionally, other polymers and copolymers such as polyethylene 2,6
naphthalate, a copolymer based on 1,4-cyclohexanedimethylene terephthalate
(PCTG), and copolymers of gluterimide and methyl methacrylate (KAMAX
resins, available from Rohm and Haas), are useful in the practice of the
present invention. Further, miscible blends of polymers may be used to
adjust the refractive index, stress optical coefficient, and glass
transition temperature of the layers used in the polarizer. Other
exemplary thermoplastic resins, along with representative refractive
indices, which may find use in the practice of the present invention
include, but are not limited to: perfluoroalkoxy resins (refractive
index=1.35), polytetrafluoroethylene (1.35), fluorinated
ethylene-propylene copolymers (1.34), silicone resins (1.41),
polyvinylidene fluoride (1.42), polychlorotrifluoroethylene (1.42), epoxy
resins (1.45), poly(butyl acrylate) (1.46), poly(4-methylpentene-1)
(1.46), poly(vinyl acetate) (1.47), ethyl cellulose (1.47),
polyformaldehyde (1.48), polyisobutyl methacrylate (1.48), polymethyl
acrylate (1.48), polypropyl methacrylate (1.48), polyethyl methacrylate
(1.48), polyether block amide (1.49), polymethyl methacrylate (1.49),
cellulose acetate (1.49), cellulose propionate (1.49), cellulose acetate
butyrate (1.49), cellulose nitrate (1.49), polyvinyl butyral (1.49),
polypropylene (1.49), polybutylene (1.50), ionomeric resins such as Surlyn
(trademark) (1.51), low density polyethylene (1.51), polyacrylonitrile
(1.51), polyisobutylene (1.51), thermoplastic polyesters such as Ecdel
(trademark) (1.52), natural rubber (1.52), perbunan (1.52), polybutadiene
(1.52), nylon (1.53), polyacrylic imides (1.53), poly(vinyl chloro
acetate) (1.54), polyvinyl chloride (1.54), high density polyethylene
(1.54), copolymers of methyl methacrylate and styrene-(1.54), transparent
acrylonitrile-butadiene-styrene terpolymer (1.54), allyl diglycol resin
(1.55), blends of polyvinylidene chloride and polyvinyl chloride such as
Saran resins (trademark) (1.55), polyalpha-methyl styrene (1.56),
styrene-butadiene latexes such as Dow 512-K (trademark) (1.56),
polyurethane (1.56), neoprene (1.56), copolymers of styrene and
acrylonitrile such as Tyril resin (trademark) (1.57), copolymers of
styrene and butadiene (1.57), other thermoplastic polyesters such as
polyethylene terephthalate and polyethylene terephthalate glycol (1.60),
polyimide (1.61), polyvinylidene chloride (1.61), polydichlorostyrene
(1.62), polysulfone (1.63), polyether sulfone (1.65), and polyetherimide
(1.66).
Copolymers and miscible blends of the above polymers may also find use in
the practice of the present invention. Such copolymers and blends may be
used to provide an extremely wide variety of different refractive indices
which may be matched to provide optimum polarizing effects. Additionally,
the use of copolymers and miscible blends of polymers may be used to
enhance the processability of the alternating layers during coextrusion
and orientation. Further, the use of copolymers and miscible blends
permits the adjustment of the stress optical coefficients and glass
transition temperatures of the polymers.
Multilayer birefringent interference polarizing films in accordance with
the present invention are most advantageously prepared by employing a
multilayered coextrusion device as described in U.S. Pat. Nos. 3,773,882
and 3,884,606 the disclosures of which are incorporated herein by
reference. Such a device provides a method for preparing multilayered,
simultaneously extruded thermoplastic materials, each of which are of a
substantially uniform layer thickness. Preferably, a series of layer
multiplying means as are described in U.S. Pat. No. 3,759,647 the
disclosure of which is incorporated herein by reference may be employed.
The feedblock of the coextrusion device receives streams of the diverse
thermoplastic polymeric materials from a source such as a heat plastifying
extruder. The streams of resinous materials are passed to a mechanical
manipulating section within the feedblock. This section serves to
rearrange the original streams into a multilayered stream having the
number of layers desired in the final body. Optionally, this multilayered
stream may be subsequently passed through a series of layer multiplying
means in order to further increase the number of layers in the final body.
The multilayered stream is then passed into an extrusion die which is so
constructed and arranged that streamlined flow is maintained therein. Such
an extrusion device is described in U.S. Pat. No. 3,557,265, the
disclosure of which is incorporated by reference herein. The resultant
product is extruded to form a multilayered body in which each layer is
generally parallel to the major surface of adjacent layers.
The configuration of the extrusion die can vary and can be such as to
reduce the thickness and dimensions of each of the layers. The precise
degree of reduction in thickness of the layers delivered from the
mechanical orienting section, the configuration of the die, and the amount
of mechanical working of the body after extrusion are all factors which
affect the thickness of the individual layers in the final body.
After coextrusion, and layer multiplication, the resultant multilayer film
is stretched, either uniaxially or biaxially, at a temperature above the
respective glass transition temperatures of the polymers, but below their
respective melting temperatures. Alternatively, the multilayer film may be
cold drawn and stretched below the glass transition temperature of at
least one of the polymers in the film. This causes the polymers to orient
and produces a refractive index mismatch in at least one plane of the
polarizer due to the differences in stress optical coefficients and/or
refractive indices between the polymers.
Polarization of selected wavelengths of light is achieved by means of
constructive optical interference due to the refractive index mismatch in
at least one plane of the polarizer. The polarizer can be constructed so
that different wavelengths may be polarized as desired. Control of the
refractive index mismatch, relative layer thicknesses within the film, and
the amount of induced orientation in the film determines which wavelengths
will be polarized. As with other interference films, the wavelengths of
light which are polarized are also dependent on the angle of incidence of
the incoming light relative to the surface of the polarizer.
The birefringent interference polarizer of the present invention reflects
and polarizes a portion of the light incident on its surface while
transmitting the remainder of the incident light. Essentially no light is
absorbed by the polarizer. During fabrication, the layer thicknesses of
the alternating polymer layers may be controlled so that the polarizer
transmits only a narrow range of wavelengths while reflecting and
polarizing a broad range. For example, the layers in the multilayer film
may be arranged so that their thickness increases monotonically through
the thickness of the film to produce a layer thickness gradient. This
provides broad bandwidth reflective properties to the polarizer. Such a
polarizer can be used as a band pass filter which transmits only a narrow
range of wavelengths.
Alternatively, the film can be constructed to polarize and reflect only a
narrow wavelength range while remaining transparent to the remaining
portion of incident light. If white light is used as a source, the
polarizer of the present invention will reflect polarized light of
specific wavelengths in one plane dependent upon the optical thicknesses
of the layers, while transmitting the remaining light.
One end use for the polarizer of the present invention is installation on
an aircraft or vehicular windshield onto which a "heads-up" display is
projected. The polarizer will reduce the glare component from outside of
the aircraft or vehicle, or from within the aircraft or vehicle itself
which is at the same angle as the projected heads-up image. The use of the
present invention results in increased transmission of other incident
light over that which would be possible using conventional polarizers
which absorb at least some of the incident light. Another use for the
polarizer of the present invention is as a beam splitter.
In order that the invention may be more readily understood, reference is
made to the following example, which is intended to be illustrative of the
invention, but is not intended to be limiting in scope.
Example 1
Employing an apparatus as generally described in U.S. Pat. Nos. 3,773,882
and 3,759,647, a sheet of a birefringent interference polarizing film was
prepared. The sheet was approximately 0.003 inches in thickness and had
385 alternating layers (ABABAB) of polycarbonate (Calibre 300-15,
trademark of Dow Chemical Company) and polystyrene (Styron 685D, trademark
of Dow Chemical Company).
A 1" by 1" by 0.003" sample of the film was post stretched uniaxially at
160.degree. C. (above the glass transition temperature of the two
polymers) at 650 lb/in.sup.2 from its original 1" length to a final length
of 3" and then quickly quenched with water to orient the polymers. Final
sample thickness averaged 0.0015", and the minimum width of the sample was
0.50".
The post-stretch conditions were controlled to provide a final average
layer thickness of 856.8 angstroms for the polycarbonate layers and 873.1
angstroms for the polystyrene layers. These layer thicknesses were
calculated to provide a polarizing film which polarized light in the
middle of the visible spectrum (.lambda.=5500 angstroms) with an f-ratio,
as defined above, of 0.5.
Both polymers had measured refractive indices of about 1.6 in an unoriented
condition. However, the polycarbonate was measured to have a positive
stress optical coefficient of approximately +5,000 Brewsters, while the
polystyrene was measured to have a negative stress optical coefficient of
approximately -5,000 Brewsters. The degree of post-stretching was
controlled to provide a refractive index mismatch between the two polymers
of 0.03 in the plane of orientation.
To determine whether the film acted as a polarizer, two of the 385 layer
films were laminated prior to uniaxial stretching to orient the polymers
in the film. Reflectance at a given wavelength was measured along a plane
parallel to the uniaxial stretch and along a plane normal to the plane of
uniaxial stretch. As can be seen from the graph of FIG. 1, reflectance
differences in the parallel and perpendicular planes over a wide range of
wavelengths demonstrate that the film was functioning to polarize light.
While certain representative embodiments and details have been shown for
purposes of illustrating the invention, it will be apparent to those
skilled in the art that various changes in the methods and apparatus
disclosed herein may be made wi | | |
