Patent Description:
In some aspects of the present description, a multilayer optical film including a plurality of polymeric layers arranged sequentially adjacent to each other is provided. A difference in thickness between spaced apart first and second polymeric layers in the plurality of polymeric layers is less than about <NUM>%. Each polymeric layer disposed between the first and second polymeric layers has a thickness less than about <NUM>. Each layer in a group of at least three polymeric layers in the plurality of polymeric layers that are disposed between the first and second polymeric layers has a thickness greater than an average thickness of the first and second polymeric layers by about <NUM>% to about <NUM>%. The group of at least three polymeric layers includes at least one pair of immediately adjacent polymeric layers.

In some aspects of the present description, a multilayer optical film including a plurality of optical repeat units arranged sequentially adjacent to each other along at least a portion of a thickness of the multilayer optical film is provided. Each optical repeat unit includes at least two layers and has a corresponding bandwidth. The bandwidths of spaced apart first and second optical repeat units in the plurality of optical repeat units overlap each other. At least a pair of adjacent optical repeat units in the plurality of optical repeat units that are disposed between the first and second optical repeat units have non-overlapping bandwidths. No optical repeat unit disposed between the first and second optical repeat units has a thickness less than an average thickness of the first and second optical repeat units by more than about <NUM>%. Each layer in the multilayer optical film disposed between the first and second optical repeat units has an average thickness less than about <NUM>.

In some aspects of the present description, a multilayer optical film including a plurality of alternating layers of first and second polymeric layers arranged sequentially adjacent to each other is provided. At least first through fourth sequentially arranged adjacent layers in the plurality of alternating layers of the first and second polymeric layers have intended average thicknesses t1 through t4, respectively. Each of t1 through t4 is less than about <NUM>. One of t2 and t3 is greater than t1, t4 and the other one of t2 and t3 by at least <NUM>%.

In some aspects of the present description, a multilayer optical film including a first multilayer stack, a second multilayer stack, and a third multilayer stack disposed therebetween is provided. Each of the first, second, and third multilayer stacks includes a plurality of polymeric layers. A total number of polymeric layers in each of the first and second multilayer stacks is at least <NUM>. The first and second multilayer stacks include respective first and second polymer layers immediately adjacent the third multilayer stack, where a difference in thickness between the first and second polymeric layers is less than about <NUM>%. The third multilayer stack includes at least one pair of immediately adjacent polymeric layers such that each polymeric layer in the at least one pair has a thickness greater than an average thickness of the first and second polymeric layers by at least about <NUM>%. The multilayer optical film is integrally formed and a minimum average peel force between first and second portions of the multilayer optical film is greater than about <NUM> N/cm, where the first and second portions include at least one polymeric layer of the first and second multilayer stacks, respectively.

In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. It is to be understood that other embodiments are contemplated.

The following detailed description, therefore, is not to be taken in a limiting sense. Prior art multilayer optical films, in addition to those listed below, can be found in <CIT>, and <CIT>.

Multilayer optical films that provide desirable transmission and/or reflection properties at least partially by an arrangement of microlayers of differing refractive index, are known. Such optical films have been demonstrated, for example, by coextrusion of alternating polymer layers, casting the layers through a film die onto a chill roll, and then stretching the cast web. See, e.g., <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), and <CIT>), and International Appl. No. <CIT>). In these polymeric multilayer optical films, polymer materials may be used predominantly or exclusively in the makeup of the individual layers. Such films are compatible with high volume manufacturing processes and can be made in large sheets and roll goods. By selecting suitable microlayers and suitable arrangement (e.g., thickness profile) of the microlayers, the multilayer optical film can be configured to be a broadband (e.g., a visible and/or near infrared wavelength range) reflective polarizer, a broadband mirror, a notch (e.g., having relatively narrow spaced apart reflection bands) reflective polarizer, or a notch mirror, for example.

In some cases, a multilayer optical film includes two or more optical stacks or optical packets of optical repeat units. An optical repeat unit includes two or more layers and is repeated across a stack or packet of the optical repeat units. An optical repeat unit has a first order reflection for a wavelength twice the optical thickness (thickness times refractive index) of the optical repeat unit. Each layer in a packet of optical repeat units may have a thickness less than <NUM>, or less than <NUM>, or less than <NUM>, or less than <NUM>. Each layer may have a thickness greater than about <NUM> or greater than about <NUM>. A multilayer optical film may include two packets of optical repeat units, where one packet is configured to reflect blue to green wavelengths and the other packet is configured to reflect green to red wavelengths, for example. One or more spacer layers may be included between the packets of optical repeat units. Conventionally, one or two optically thick (too thick to substantially contribute to a first order visible (e.g., wavelengths in a range of about <NUM> to about <NUM>) or near infrared (e.g., wavelengths in a range of about <NUM> to about <NUM>) light reflection by optical interference) spacer layers or protective boundary layers (PBLs) have been included. These PBL layers are typically included to prevent flow profiles in the coextruded web of alternating polymer layers from producing optical defects in the alternating polymer layers. According to the present description, it has been found that using thinner PBLs can provide improved resistance against delamination of adjacent optical packets, but that thinner PBLs can also result in optical defects. According to some embodiments of the present description, it has been found that using a larger number of thinner PBLs provides improved delamination resistance between optical packets without resulting in optical defects. According to some embodiments, these thinner PBLs are preferably less than about <NUM> thick. In some embodiments, three or more PBL layers are included between adjacent packets of optical repeat units. In some embodiments, at least some, and in some cases, all, of these PBL layers are optical layers. An optical layer in this context is a layer having a thickness in a range that that the layer can significantly contribute to first order visible or near infrared light reflection by optical interference. In some embodiments, the thickness profiles of the PBLs are chosen to prevent or reduce optical coherence from such reflections (e.g., different optical repeat units in the PBLs may have non-overlapping bandwidths) so that the PBLs do not substantially affect the reflectance of the multilayer optical film.

<FIG> is a schematic cross-sectional view of a multilayer optical film <NUM>. The optical film <NUM> includes a plurality of polymeric layers <NUM> arranged sequentially adjacent to each other. A difference in thickness between spaced apart first and second polymeric layers <NUM> and <NUM> in the plurality of polymeric layers less than about <NUM>%, or less than about <NUM>%, or less than about <NUM>%. The percent difference in thickness of the first and second layers <NUM> and <NUM> is |h1-h2| divided by the larger of h1 and h2 times <NUM>% where h1 is the thickness of first layer <NUM> and h2 is the thickness of second layer <NUM>. In the invention, each layer in a group <NUM> of at least three polymeric layers in the plurality of polymeric layers <NUM> that are disposed between the first and second polymeric layers <NUM> and <NUM> have a thickness (e.g., t1, t2, t3) greater than an average thickness ((h1+h2)/<NUM>) of the first and second polymeric layers by at least about <NUM>%, or at least about <NUM>%, or at least about <NUM>%. In the invention, each layer in the group <NUM> has a thickness that is greater than the average thickness of the first and second polymeric layers by no more than about <NUM>%, or no more than about <NUM>%, or no more than about <NUM>%. In the invention, the group <NUM> of at least three polymeric layers has an average thickness (e.g., (t1+t2+t3)/<NUM> in the illustrated embodiment) that is greater than the average thickness of the first and second polymeric layers <NUM> and <NUM> by about <NUM>% to about <NUM>%, or about <NUM>% to about <NUM>%, or about <NUM>% to about <NUM>%.

Typically, each layer in a multilayer optical film has a constant or approximately constant thickness. If there is a variation in the thickness of a layer in a multilayer optical film, the thickness of the layer refers to an average (unweighted mean) thickness of the layer, unless indicated differently. The average thickness of a set or group of layers is the arithmetic mean of the thicknesses of the individual layers in the set or group. The intended average thickness of a layer is a design or nominal thickness of the layer. In some embodiments, the intended average thickness of a layer is the same or substantially the same as the average thickness of the layer.

In some embodiments, the group <NUM> of at least three polymeric layers includes at least one pair of immediately adjacent polymeric layers (e.g., <NUM> and <NUM>, or <NUM> and <NUM>). In some embodiments, the polymeric layers in the group <NUM> of at least three polymeric layers are arranged sequentially adjacent to each other as schematically illustrated in <FIG>. In other embodiments, the additional polymeric layer(s) may separate some of the polymeric layers in the group <NUM>.

The optical film <NUM> may include many more layers than schematically illustrated in <FIG>. In some embodiments, the multilayer optical film <NUM> includes at least <NUM> layers, or at least <NUM> layers, or at least <NUM> layers. In some such embodiments or in other embodiments, the multilayer optical film <NUM> includes no more than <NUM> layers, or no more than <NUM> layers.

The polymeric layers <NUM> are the polymeric layers in the plurality of polymeric layers that are disposed between the first and second polymeric layers <NUM> and <NUM>. In some embodiments, a total number of polymeric layers <NUM> in the plurality of polymeric layers that are disposed between the first and second polymeric layers <NUM> and <NUM> is at least <NUM>, or at least <NUM>, or at least <NUM>. In the invention, a total number of polymeric layers <NUM> in the plurality of polymeric layers that are disposed between the first and second polymeric layers <NUM> and <NUM> is no more than <NUM>, or no more than <NUM>, or no more than <NUM>, or no more than <NUM>, or no more than <NUM>, or no more than <NUM>.

As described further elsewhere, the multilayer optical film <NUM> may have desired optical transmittances and reflectances for substantially normally incident light <NUM> (e.g., light normally incident or light incident within <NUM> degrees, or within <NUM> degrees, or within <NUM> degrees of normal) having a first polarization state <NUM> and for substantially normally incident light <NUM> having a second polarization state <NUM>.

<FIG> is a plot of layer thickness versus layer number for a multilayer optical film that may correspond to multilayer optical film <NUM>. <FIG> is a portion of the plot of <FIG> illustrating spaced apart first and second polymeric layers <NUM> and <NUM> and a group <NUM> of at least three polymeric layers disposed between the first and second polymeric layers <NUM> and <NUM>. Data points at integer layer numbers are shown. The lines between the data points are a guide to the eye. First layer <NUM> has a thickness of <NUM>, second layer <NUM> a thickness of <NUM>, and the layers in the group <NUM> have thickness of <NUM>, <NUM>, <NUM>, and <NUM>, respectively. Each layer in the group <NUM> had a thickness greater than an average thickness of the first and second layers <NUM> and <NUM> by about <NUM>% to about <NUM>% (e.g., (<NUM> - <NUM>)/<NUM> times <NUM>% is about <NUM>%).

In some embodiments, the group <NUM> or <NUM> of at least three polymeric layers in the plurality of polymeric layers is a group of at least four polymeric layers in the plurality of polymeric layers. In some embodiments, the group of at least three polymeric layers includes less than <NUM> polymeric layers, or less than <NUM> polymeric layers, or less than <NUM> polymeric layers.

In some embodiments, each of the first and second polymeric layers <NUM> and <NUM>, or <NUM> and <NUM>, is disposed between the group <NUM> or <NUM> of at least three polymeric layers and at least <NUM> other polymeric layers in the plurality of polymeric layers. For example, the groups of layers <NUM> and <NUM> schematically illustrated in <FIG> may each include at least <NUM> layers.

In some embodiments, each layer disposed between the first and second polymeric layers <NUM> and <NUM>, or <NUM> and <NUM>, has a thickness (e.g., average thickness of the layer) less than about <NUM> micron, or less than about <NUM>, or preferably less than about <NUM>, or more preferably less than about <NUM>, or even more preferably less than about <NUM> or less than about <NUM>.

In the invention, no layer disposed between the first and second polymeric layers <NUM> and <NUM>, or <NUM> and <NUM>, has a thickness (e.g., average thickness of the layer) that is less than the average thickness of the first and second polymeric layers by more than about <NUM>%, or by more than about <NUM>%. In other words, in some embodiments, no layer disposed between the first and second polymeric layers <NUM> and <NUM>, or <NUM> and <NUM>, has a thickness less than about <NUM>, or less than about <NUM>, times the average thickness of the first and second polymeric layers.

In some embodiments, the multilayer optical film <NUM> includes a first group <NUM> of polymeric layers arranged sequentially adjacent to each other along at least a portion of a thickness of the multilayer optical film <NUM>. In some embodiments, the first group <NUM> includes at least <NUM> polymeric layers (fewer layers are shown in the schematic illustration of <FIG>) arranged sequentially adjacent to each other along at least a portion of a thickness of the multilayer optical film <NUM>. For example, the first group <NUM> may correspond to a group of layers of <FIG> including at least layer numbers <NUM> to <NUM>. The first group <NUM> of polymeric layers includes the first and second polymeric layers <NUM> and <NUM> and the group <NUM> of at least three polymeric layers. Each layer in the first group <NUM> of at least <NUM> polymeric layers has a thickness (e.g., average thickness of the layer) less than about <NUM> micron, or less than about <NUM>, or preferably less than about <NUM>, or more preferably less than about <NUM>, or even more preferably less than about <NUM> or less than about <NUM>.

In some embodiments, a multilayer optical film <NUM> includes a plurality of optical repeat units (e.g., layer pairs <NUM>, <NUM>) arranged sequentially adjacent to each other along at least a portion of a thickness of the multilayer optical film <NUM>. Each optical repeat unit includes at least two layers <NUM> and <NUM> and has a corresponding bandwidth. For example, a first optical repeat unit <NUM>, <NUM> may have a bandwidth W1 between left and right wavelengths λ1L and λ1R and a second optical repeat unit <NUM>, <NUM> may have a bandwidth W2 between left and right wavelengths λ2L and λ2R as schematically illustrated in <FIG>. The left and right wavelengths can be understood to be the full-width at half maximum (FWHM) band edge wavelengths. In some embodiments, the bandwidths of spaced apart first and second optical repeat units <NUM>, <NUM> and <NUM>, <NUM> in the plurality of optical repeat units overlap each other. For example, λ2L is between λ1L and λ1R, and λ1R is between λ2L and λ2R. In some embodiments, at least a pair of adjacent optical repeat units (e.g., <NUM>, <NUM> and <NUM>, <NUM>) in the plurality of optical repeat units that are disposed between the first and second optical repeat units <NUM>, <NUM> and <NUM>, <NUM> have non-overlapping bandwidths. This is schematically illustrated in <FIG> which shows a bandwidth W3 between λ3L and λ3R for an optical repeat unit (e.g., <NUM>, <NUM>) and a bandwidth W4 between λ4L and λ4R for an adjacent optical repeat unit (e.g., <NUM>, <NUM>). The bandwidths are non-overlapping since there is no overlap in the range λ3L and λ3R and the range λ4L and λ4R. In the invention, no optical repeat unit disposed between the first and second optical repeat units has a thickness less than an average thickness of the first and second optical repeat units by more than about <NUM>%, or by more than about <NUM>%. The thickness of an optical repeat unit is the total thickness of the at least two layers in the optical repeat unit. In some embodiments, each layer in the multilayer optical film that is disposed between the first and second optical repeat units has a thickness (e.g., average thickness of the layer) less than less than about <NUM> micron, or less than about <NUM>, or preferably less than about <NUM>, or more preferably less than about <NUM>, or even more preferably less than about <NUM> or less than about <NUM>.

The bandwidth of an optical repeat unit (ORU) is the bandwidth of a first order reflection band that an infinite stack of ORU's of identical thickness would exhibit. This is readily calculated from the matrix elements of the characteristic matrix M as defined by Born and Wolf, "Principles of Optics", Edition <NUM>, page <NUM>.

The optical repeat unit may include two layers or may include more than two layers. For example, in some embodiments, each optical repeat unit includes at least three layers, or at least four layers. <FIG> is a schematic cross-sectional view of an optical repeat unit <NUM> including two layers <NUM> and <NUM>. In some embodiments, each of the layers <NUM> and <NUM> have an optical thickness of a quarter of substantially a same wavelength. In some embodiments, one or both of the layers <NUM> and <NUM> is replaced with two (or more) layers. In some embodiments, the two (or more) layers have a combined optical thickness of a quarter of a wavelength that the optical repeat unit is configured to reflect.

<FIG> is a schematic cross-sectional view of an optical repeat unit <NUM> including four layers <NUM>, <NUM>, <NUM>, <NUM>. In some embodiments, layers <NUM> and <NUM> are tie layers included to improve bonding to adjacent layers. In some embodiments, the layers <NUM> and <NUM> have a thickness in a range of about <NUM> to about <NUM>, or about <NUM> to about <NUM>, for example. In some embodiments, the layers <NUM> and <NUM> have a same composition. In some embodiment, each optical repeat unit <NUM> includes layers A (<NUM>), B (<NUM> or <NUM>), and C (<NUM>) arranged in an order ABCB. In some such embodiments, layers AB and CB have an optical thickness of a quarter of substantially a same wavelength.

In some embodiments, a multilayer optical film <NUM> includes a plurality of alternating layers of first and second polymeric layers <NUM> and <NUM> arranged sequentially adjacent to each other. At least first through fourth sequentially arranged adjacent layers (e.g., layers <NUM>, <NUM>, <NUM>, and <NUM>, respectively) in the plurality of alternating layers of the first and second polymeric layers <NUM> and <NUM> have intended average thicknesses t1 through t4, respectively, where one of t2 and t3 is greater than t1, t4 and the other one of t2 and t3 by at least <NUM>%, or by at least <NUM>%, or by at least <NUM>%, or by at least <NUM>%, or at least <NUM>%, or at least <NUM>%. In some embodiments, one of t2 and t3 is greater than t1, t4 and the other one of t2 and t3 by about <NUM>% to about <NUM>%; or about <NUM>% to about <NUM>%; or about <NUM>% to about <NUM>%, or to about <NUM>%, or to about <NUM>%, or to about <NUM>%. In some embodiments, each of t1 through t4 is less than about <NUM> micron, or less than about <NUM>, or preferably less than about <NUM>, or more preferably less than about <NUM>, or even more preferably less than about <NUM> or less than about <NUM>.

In some embodiments, the multilayer optical film <NUM> is a reflective polarizer. <FIG> is a schematic plot of the transmittance of an optical film versus wavelength for substantially normally incident for first and second polarizations states <NUM> and <NUM>. The wavelength range depicted in <FIG> may be at least <NUM> wide (e.g., at least from <NUM> to <NUM>, or from <NUM> to <NUM>). In some embodiments, for substantially normally incident light in a wavelength range of at least <NUM>, the multilayer optical film <NUM> has an average optical reflectance of at least <NUM>% for a first polarization state <NUM> and an average optical transmittance T2 of at least <NUM>% for an orthogonal second polarization state <NUM>. Often, the optical absorption is negligible so that the average optical reflectance is about <NUM>% minus T1.

In some embodiments, at least some of the polymeric layers (e.g., one of two alternating polymeric layers) are substantially uniaxially oriented. For example, in some embodiments, the multilayer optical film is a reflective polarizer that is a substantially uniaxially drawn film and has a degree of uniaxial character U of at least <NUM>, or at least <NUM>, or at least <NUM>, where U = (<NUM>/MDDR - <NUM>) / (TDDR<NUM>/<NUM> - <NUM>) with MDDR defined as the machine direction draw ratio and TDDR defined as the transverse direction draw ratio. Such substantially uniaxially oriented multilayer optical films are described in <CIT>), for example.

In some embodiments, the multilayer optical film <NUM> is a mirror film. In some embodiments, for substantially normally incident light in a wavelength range of at least <NUM>, the multilayer optical film <NUM> has an average optical reflectance of at least <NUM>% for each of mutually orthogonal first and second polarization states. For example, the transmittance for the first and second polarization states may each follow the curve labeled <NUM> in <FIG>.

The transmittance may be significantly different from the schematic illustration of <FIG>. For example, the transmittance may vary with wavelength rather than being constant or substantially constant over a wavelength range.

In some embodiments, the multilayer optical film <NUM> is a reflective polarizer having spaced apart reflection bands. <FIG> is a schematic plot of the transmittance of an optical film versus wavelength for substantially normally incident for first and second polarizations states <NUM> and <NUM>. In some embodiments, for substantially normally incident light and for each of first and second wavelengths λ1 and λ2, the multilayer optical film <NUM> reflects at least <NUM>% of the incident light having a first polarization state <NUM> and transmits at least <NUM>% of the incident light having an orthogonal second polarization state <NUM>. For at least a third wavelength λ3 disposed between the first and second wavelengths λ1 and λ2, the multilayer optical film <NUM> transmits at least <NUM>% of the incident light for each of the first and second polarization states <NUM> and <NUM>.

The transmittance may be significantly different from the schematic illustration of <FIG>. For example, the transmittance or reflectance in the reflection bands near λ1 and λ2 may vary within the bands and may have more gradual band edge transitions. As another example, the transmittance or reflectance may be significantly different for the reflection band near λ1 and the reflection band near λ2.

The multilayer optical films of the present description may be integrally formed. As used herein, a first element "integrally formed" with a second element means that the first and second elements are manufactured together rather than manufactured separately and then subsequently joined. Integrally formed includes manufacturing a first element followed by manufacturing the second element on the first element. An optical film including a plurality of layers is integrally formed if the layers are manufactured together (e.g., combined as melt streams and then cast onto a chill roll to form a cast film which is then oriented) rather than manufactured separately and then subsequently joined.

In some embodiments, a multilayer optical film <NUM> includes a first multilayer stack (layer <NUM> with group of layers <NUM>), a second multilayer stack (layer <NUM> with group of layers <NUM>), and a third multilayer stack (group of layers <NUM>) disposed therebetween. Each of the first, second, and third multilayer stacks include a plurality of polymeric layers. In some embodiments, a total number of polymeric layers in each of the first and second multilayer stacks is at least <NUM>, or at least <NUM>, or at least <NUM>, or at least <NUM>. In some embodiments, a total number of polymeric layers in the third multilayer stack is at least <NUM>, or at least <NUM>, or at least <NUM>, and, in the invention, no more than <NUM>, or no more than <NUM>, or no more than <NUM>, or no more than <NUM>, or no more than <NUM>, or no more than <NUM>. In some embodiments, each polymeric layer in the third multilayer stack has a thickness less than <NUM>, or less than <NUM>, or less than <NUM>. Each of the first and second multilayer stacks, and optionally the third multilayer stack, may be optical stacks including a plurality of optical repeat units as described elsewhere. The first and second multilayer stacks include respective first and second polymer layers <NUM> and <NUM> immediately adjacent the third multilayer stack (group of layers <NUM>). In the invention a difference in thickness between the first and second polymeric layers <NUM> and <NUM> is less than about <NUM>%. The third multilayer stack includes at least one pair of immediately adjacent polymeric layers (e.g., <NUM> and <NUM>, or <NUM> and <NUM>) such that each polymeric layer in the at least one pair has a thickness greater than an average thickness of the first and second polymeric layers by at least about <NUM>%. In some embodiments, each polymeric layer in the at least one pair has a thickness greater than an average thickness of the first and second polymeric layers <NUM> and <NUM> by no more than about <NUM> %. In some embodiments, each layer in the third multilayer stack (group of layers <NUM>) has a thickness in a range of <NUM>% to <NUM>% or to <NUM>% or to <NUM>% or <NUM>% of the average thickness of the first and second polymeric layers <NUM> and <NUM>. In some embodiments, the multilayer optical film is integrally formed and a minimum average peel strength between first and second portions of the multilayer optical film is greater than about <NUM> N/cm, where the first and second portions include at least one polymeric layer of the first and second multilayer stacks, respectively. In some embodiments, the minimum average peel strength is greater than about <NUM> N/cm, or greater than about <NUM> N/cm. In some embodiments, the minimum average peel strength is determined using a substantially <NUM>-degree peel test at a peel speed of about <NUM>/min, where the minimum average peel strength is the minimum of the peel strength averaged over an averaging time of about <NUM> seconds.

<FIG> schematically illustrates a peel test applied to an integrally formed multilayer optical film <NUM>, which may correspond to optical film <NUM>, for example. The optical film <NUM> may be cut into a standard size for testing (e.g., <NUM> inch (<NUM>) wide by <NUM> inch (<NUM>) strips). Double-sided tape <NUM> (e.g., <NUM> <NUM> Double Sided Tape available from <NUM> Company, St. Paul, MN) is attached to a plate <NUM> (e.g., a metal plate) and the film <NUM> is attached to the double-sided tape <NUM>. The film <NUM> is scored (e.g., with a razor blade) near an edge of the film along score line <NUM> which makes an angle α with a major surface of the plate <NUM> that is in a range of <NUM> to <NUM> degrees, or <NUM> to <NUM> degrees, for example. A tape <NUM> is applied to the film <NUM> such that the tape <NUM> covers at least the scored portion of the film <NUM> and such that a free end <NUM> of the tape <NUM> is available for peel testing. For example, the tape <NUM> may be an approximately <NUM> in (<NUM>) strip of <NUM> <NUM> tape available from <NUM> Company, St. The free end <NUM> used for gripping during peel testing may be folded onto itself to form a non-sticky tab (e.g., an approximately ½ inch (<NUM>) tab). A substantially <NUM>-degree peel test is then be performed by peeling from the free end <NUM>. For example, the angle β between the pull direction (schematically illustrated by the applied force F in <FIG>) and a direction parallel to a top surface of the plate <NUM> may be about <NUM> degrees. The peel test is carried out with a peel speed (speed of the free end <NUM> along the pull direction) in a range of about <NUM> to about <NUM>/min (e.g. about <NUM>/min). The peel test can be performed using an IMASS SP-<NUM> peel tester (IMASS Inc. , Accord, MA), for example. The peel strength is averaged over an averaging time of about <NUM> to about <NUM> seconds (e.g., about <NUM> seconds). The average peel strength can be determined for a single averaging time for each one of multiple samples (e.g., five film samples), or for multiple intervals of the averaging time for a single (e.g., longer) sample. The minimum of these average peel strengths is referred to as the minimum average peel strength.

The peel strength is between two portions <NUM> and <NUM> of the optical film <NUM> where each of the two portions <NUM>, <NUM> includes at least one layer of the multilayer optical film <NUM> (e.g., one of the outermost polymeric layers of the film). For example, during the peel test, the optical film <NUM> may delaminate at an interface between one of the outermost layers and an adjacent layer so that one of the two portions <NUM>, <NUM> includes the delaminated outermost layer and the other of the two portions <NUM>, <NUM> include the remainder of the optical film <NUM>. As another example, the optical film <NUM> may delaminate between first and second optical stacks. For example, when a conventional thick PBL layer is included between the first and second optical stacks, the peeling can occur through the bulk of this layer or at an interface between this layer and an adjacent layer. According to some embodiments, this failure mode is eliminated or substantially reduced by including a multiple, thinner PBLs between the first and second optical stacks. As another example, the optical film <NUM> may delaminate at an interface between two internal layers within one of the optical stacks. As still another example, the delamination may start at an interface between an outermost layer and an adjacent layer and then propagate into the internal layers of the optical film <NUM> so that each portion <NUM> and <NUM> comprise portions of an internal layer.

In some embodiments, the multilayer optical film includes a plurality of alternating high and low index layers. In some embodiments, the low index layers are formed from a blend of polycarbonate, PETG (a copolyester of polyethylene terephthalate (PET) with cyclohexane dimethanol used as a glycol modifier; available from Eastman Chemicals, Knoxville, TN) and PCTG (a copolyester of PET with twice the amount of cyclohexane dimethanol used as a glycol modifier compared to PETG; available from Eastman Chemicals, Knoxville, TN). The proportion of polycarbonate used can be selected to give a desired glass transition temperature. In some embodiments, the glass transition temperature may be selected to improve microwrinkling of the optical film as described further in co-owned Prov. No. <CIT> and titled "OPTICAL FILM AND OPTICAL STACK". In some embodiments, the high index layers are formed from polyethylene naphthalate (PEN) or a PEN/polyethylene terephthalate (PET) copolymer. Other polymeric materials known to be useful in polymeric multilayer optical films may alternatively be used.

Film samples were prepared and cut into <NUM> inch (<NUM>) wide by <NUM> inch (<NUM>) strips. Double sided tape (<NUM> <NUM> Double Sided Tape available from <NUM> Company, St. Paul, MN) was attached to a metal plate and a sample strip was attached to the double sided tape. The excess film was cut from one end of the plate so that the film was flush with that edge of the plate while the other edge was scored by cutting at a sharp angle with a razor blade. One end of an approximately <NUM> in (<NUM>) strip of tape (<NUM> <NUM> tape available from <NUM> Company, St. Paul, MN) was folded onto itself to form a ½ inch (<NUM>) non-sticky tab. The other end of the tape was applied to the scored edge of the film sample. A <NUM>-degree peel test was then performed using an IMASS SP-<NUM> peel tester (IMASS Inc. , Accord, MA) with a peel speed of <NUM> in/min (<NUM>/min) using a <NUM> second averaging time. Five strips were tested for each film sample. For the results given in the Examples, the minimum value is reported for sake of comparing weakest or lowest force required to delaminate layers from each other.

A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were co-extruded with each packet comprised of <NUM> alternating layers of polyethylene naphthalate (PEN) and a low index isotropic layer, which was made with a blend of polycarbonate and copolyesters (PC:coPET) such that the index is about <NUM> and remained substantially isotropic upon uniaxial orientation, where the PC:coPET weight ratio was approximately <NUM> wt % PC and <NUM> wt % coPET and has a Tg of <NUM> degrees centigrade. This isotropic material was chosen such that after stretching its refractive indices in the two non-stretch directions remains substantially matched with those of the birefringent material in the non-stretching direction while in the stretching direction there is a substantial mis-match in refractive indices between birefringent and non-birefringent layers. The PEN and PC/coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock, in which they were assembled into two packets of <NUM> alternating optical layers, plus a thicker protective boundary layer of the PC/coPET on the outside of the stacked optical packets, and in between the packets, <NUM> alternating inner protective boundary layers (see <FIG>), which were of optical thickness but were not in coherence, for a total of <NUM> layers. The multilayer melt was then cast through a film die onto a chill roll, in the conventional manner for polyester films, upon which it was quenched. The cast web was then stretched in a parabolic tenter as described in <CIT>) at approximately a <NUM>:<NUM> ratio in the transverse direction at a temperature of <NUM> °F.

The layer thickness profile for the optical film of Example <NUM> is shown in <FIG>. The outermost protective boundary layers are not included in the plots. The pass and block transmission at normal incidence were determined and are shown in <FIG>. The average transmission from <NUM>-<NUM> for block and pass polarizations was <NUM>% and <NUM>% respectively. The film of Example <NUM> had a resulting total thickness as measured by a capacitance gauge of approximately <NUM>. The minimum average peel force was <NUM> N/cm.

A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were co-extruded with each packet having of <NUM> alternating layers of <NUM>/<NUM> coPEN, a polymer composed of <NUM>% polyethylene naphthalate (PEN) and <NUM>% polyethylene terephthalate (PET) and a low index isotropic layer, which was made with a blend of polycarbonate and copolyesters (PC:coPET) such that the index was about <NUM> and remained substantially isotropic upon uniaxial orientation, where the PC:coPET weight ratio was approximately <NUM> wt % PC and <NUM> wt % coPET and had a Tg of <NUM> degrees centigrade. This isotropic material was chosen such that after stretching its refractive indices in the two non-stretch directions remains substantially matched with those of the birefringent material in the non-stretching direction while in the stretching direction there is a substantial mis-match in refractive indices between birefringent and non-birefringent layers. The PEN and PC/coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock, in which they were assembled into two packets of <NUM> alternating optical layers, plus a thicker protective boundary layer of the PC/coPET on the outsides of the stacked optical packets, and in between the packets, <NUM> alternating inner protective boundary layers (see <FIG>), which were of optical thickness but were not in coherence, for a total of <NUM> layers. The multilayer melt was then cast through a film die onto a chill roll, in the conventional manner for polyester films, upon which it was quenched. The cast web was then stretched in a parabolic tenter as described in <CIT>) at approximately a <NUM>:<NUM> ratio in the transverse direction at a temperature of <NUM> °F.

A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were co-extruded with each packet having <NUM> alternating layers of polyethylene naphthalate (PEN) and a low index isotropic layer, which was made with a blend of polycarbonate and copolyesters (PC:coPET) such that the index is about <NUM> and remained substantially isotropic upon uniaxial orientation, where the PC:coPET weight ratio was approximately <NUM> wt % PC and <NUM> wt % coPET and has a Tg of <NUM> degrees centigrade. This isotropic material was chosen such that after stretching its refractive indices in the two non-stretch directions remains substantially matched with those of the birefringent material in the non-stretching direction while in the stretching direction there is a substantial mis-match in refractive indices between birefringent and non-birefringent layers. The PEN and PC/coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock, in which they were assembled into two packets of <NUM> alternating optical layers, plus a thicker protective boundary layer of the PC/coPET on the outside of the stacked optical packets and a thicker protective boundary layer of the PC/coPET between the optical packets, for a total of <NUM> layers. The multilayer melt was then cast through a film die onto a chill roll, in the conventional manner for polyester films, upon which it was quenched. The cast web was then stretched in a parabolic tenter as described in <CIT>) at approximately a <NUM>:<NUM> ratio in the transverse direction at a temperature of <NUM> °F.

The layer thickness profile for the optical film of Comparative Example C1 is shown in <FIG>. The outermost protective boundary layers are not included in the plot. The center protective boundary layer thickness was <NUM>. The average transmission from <NUM>-<NUM> for pass and block polarizations was <NUM>% and <NUM>% respectively. The film of Comparative Example C1 had a resulting total thickness as measured by a capacitance gauge of approximately <NUM>. The minimum peel force throughout the entire film was found in between the packets and measured <NUM> N/cm.

A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were co-extruded with each packet comprised of <NUM> alternating layers of polyethylene naphthalate (PEN) and a low index isotropic layer, which was made with a blend of polycarbonate and copolyesters (PC:coPET) such that the index is about <NUM> and remained substantially isotropic upon uniaxial orientation, where the PC:coPET weight ratio was approximately <NUM> wt % PC and <NUM> wt % coPET and has a Tg of <NUM> degrees centigrade. This isotropic material was chosen such that after stretching its refractive indices in the two non-stretch directions remains substantially matched with those of the birefringent material in the non-stretching direction while in the stretching direction there is a substantial mis-match in refractive indices between birefringent and non-birefringent layers. The PEN and PC/coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock, in which they were assembled into two packets of <NUM> alternating optical layers, plus a thicker protective boundary layer of the PC/coPET, on the outside of the stacked optical packets, and in between the packets, <NUM> alternating inner protective boundary layers of optical thickness but not in coherence, for a total of <NUM> layers. The multilayer melt was then cast through a film die onto a chill roll, in the conventional manner for polyester films, upon which it was quenched. The cast web was then stretched in a parabolic tenter as described in <CIT>) at approximately a <NUM>:<NUM> ratio in the transverse direction at a temperature of <NUM> °F.

The layer thickness profile for the optical film of Example <NUM> is shown in <FIG>. The outermost protective boundary layers are not included in the plots. The average transmission from <NUM>-<NUM> for pass and block polarizations was <NUM>% and <NUM>% respectively. The film of Example <NUM> had a resulting total thickness as measured by a capacitance gauge of approximately <NUM>. The minimum peel force throughout the entire film was found near the outer layers and was <NUM> N/cm.

Terms such as "about" will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of "about" as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, "about" will be understood to mean within <NUM> percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about <NUM>, means that the quantity has a value between <NUM> and <NUM>, and that the value could be <NUM>.

Claim 1:
A multilayer optical film (<NUM>) comprising a plurality of polymeric layers (<NUM>) arranged sequentially adjacent to each other, a difference in thickness between spaced apart first and second polymeric layers (<NUM>, <NUM>) in the plurality of polymeric layers (<NUM>) less than about <NUM>%, each polymeric layer (<NUM>) disposed between the first and second polymeric layers (<NUM>, <NUM>) having a thickness less than about <NUM>, each layer in a group (<NUM>, <NUM>, <NUM>, <NUM>) of at least three polymeric layers in the plurality of polymeric layers (<NUM>) that are disposed between the first and second polymeric layers (<NUM>, <NUM>) having a thickness (t1, t2, t3) greater than an average thickness of the first and second polymeric layers (<NUM>, <NUM>) by about <NUM>% to about <NUM>%, the group (<NUM>, <NUM>, <NUM>, <NUM>) of at least three polymeric layers comprising at least one pair of immediately adjacent polymeric layers (<NUM>, <NUM>, <NUM>), wherein a total number of polymeric layers (<NUM>) in the plurality of polymeric layers (<NUM>) that are disposed between the first and second polymeric layers (<NUM>, <NUM>) is no more than <NUM>.