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OPTICAL DEVICE HAVING CONTINUOUS
AND DISPERSE PHASES
This application claims priority from Provisional Application No. 60/158,867, filed Oct. 12, 1999.
FIELD OF THE INVENTION
The present invention relates generally to optical devices such as polarizers, diffusers, and mirrors, and more particularly to improvements in the materials used to make such devices.
Various optical films and devices are known to the art which rely upon refractive index differentials, sometimes produced by strain-induced birefringence, to achieve certain optical effects, such as the polarization of randomly polarized light. Such films and devices may be in the form of a multilayer stack in which index differentials between adjacent layers in the stack give rise to certain optical properties, as in the films disclosed in U.S. Pat. No. 5,882,774 (Jonza et al.). Other optical devices comprise a disperse phase which is disposed in a continuous matrix, and derive their optical properties from refractive index differentials between the continuous and disperse phases. The materials disclosed in U.S. Pat. No. 5,825,543 (Ouderkirk et al.) are representative of this type of a system. Various hybrids of the aforementioned systems are also known, such as the multilayer optical films disclosed in U.S. Pat. No. 5,867,316 (Carlson et al.), wherein the film comprises a multilayer stack having a repeating layer sequence in which at least one of the layers has a continuous phase/disperse phase morphology. Various other optical films and devices are also known to the art, and are described in U.S. Pat. No. 5,831,375 (Benson, Jr.), U.S. Pat. No. 5,825,542 (Cobb, Jr. et al), U.S. Pat. No. 5,808,794 (Weber et al), U.S. Pat. No. 5,783,120 (Ouderkirk et al.), U.S. Pat. No. 5,751,388 (Larson), U.S. Pat. No. 5,940,211 (Hikmet et al), U.S. Pat. No. 3,213,753 (Rogers), U.S. Pat. No. 2,604,817 (Schupp, Jr.), Aphonin, O. A., "Optical Properties of Stretched Polymer Dispersed Liquid Crystal Films: Angle-Dependent Polarized Light Scattering", Liquid Crystals, Vol. 19, No. 4, pp. 469-480 (1995), Land, E. H., "Some Aspects of the Development of Sheet Polarizers,©1951 Optical Society of America, Reprinted from Journal of the Optical Society of America, Vol. 41(12), 957-963, (Dec. 1951), pp. 45-51 and 2244 Research Disclosure (1993), July, No. 351, Emsworth, GB, "Polarizer", pp. 452-453.
In the past several years, a number of advances have been made in the materials sciences, especially in the area of block copolymers, which have resulted in the development of new and interesting materials and methods for making and using these materials to various ends. In some cases, these advances have led to applications in the field of optical films and devices. Thus, for example, Urbas et al., "OneDimensional Peroidic Reflectors from Self-Assembly Block Copolymer-Homopolymer Blends," Macromolecules, Vol. 32, pages 4748-50 (1999), report the formation of well ordered photonic crystals similar to a multilayer quarter wave stack comprising self assembling blends of block copolymers optionally containing homopolymers. One embodiment describes the formation of a narrow band reflector. Also summarized is the use of neat block copolymers as well as copolymers comprising liquid crystalline materials as means of producing periodicities in block copolymer materials.
U.S. Ser. No. 08/904,325 (Weber et al.)(corresponding to WO 9906203) discloses the transesterification or reaction of polyesters lying in adjacent layers of a multilayer optical stack for the express purpose of improving interlayer adhe
5 sion. It is assumed that the thickness of the interface comprising the reacted materials is sufficiently thin so as not to otherwise affect the optical properties of the optical stack except at the interface.
U.S. Ser. No. 09/006,455 (Merrill et al.)(corresponding to
1° WO 9936812) discloses the use of transesterified blends of PEN and PET within a single layer in a multilayer optical stack for the purpose of producing optical devices such as polarizers and mirrors.
U.S. Pat. No. 3,546,320 (Duling et al.) discloses transes
15 terification methods for preparing a semicrystalline composition comprising 94 to 60 weight percent polyalkylene terephthalates, 6 to 40 weight percent polyalkylene naphthalene-2,6-dicarboxylate, and at least 5 weight percent of a block copolymer comprising discrete polymer segments
20 of the percent polyalkylene terephthalate and the polyalkylene naphthalene-2,6-dicarboxylate. The block copolymer is prepared by melt transesterification of the individual homopolymers, and the degree of transesterification is controlled by the mixing time. Duling demonstrates a total loss
25 of crystallinity of the block copolymer after extensive transesterification, depending on the composition.
U.S. Pat. No. 3,937,754 (Sagamihara et al.) discloses a biaxially oriented polyethylene-2,6-naphthalate (PEN) film
3Q containing a polyester resin other than PEN in an amount of 0.5 to 10 percent by weight based on the PEN, and a process for its production. The reference notes that when the PEN resin (1) is blended in the molten state with a polyester resin (2), the softening point of the blended mixture decreases
35 gradually from the softening point of the PEN until it finally reaches a certain point, referred to as an equilibrium softening point. The reference teaches that this softening point coincides with the softening point of a PEN copolymer obtained by copolymerising monomers of the same compo
4Q sition and proportion as the monomers which constitute the PEN resin (1) and the polyester resin (2). From this fact, the reference presumes that reaction occurs via a stage of forming a block copolymer, where given enough reaction time a copolymer will be obtained.
45 Research Disclosures 28,340 and 29,410 disclose transesterified products of PEN, PET, and other polymers comprising dibasic acids. Typical dibasic acids include isophthalic, adipic, glutaric, azelaic, and sebacic acid and the like. The PEN based polymers are generally based on
5Q 2,6-naphthalene-dicarboxylic acid but may be based on 1,4-, 1,5-, or 2,7-isomers or mixtures of these isomers. These teachings primarily address the ability to control mechanical and physical properties such as modulus, gaseous permeabilities, and glass transition temperatures.
55 WO 92/02584 (Cox et al.) disclose the use of phosphite materials to control the rate of transesterification during solid state polymerization, primarily for the intended use of improving physical and mechanical properties, such as gaseous diffusion, in the final product application. The
go reference discloses blends of PEN and PET homopolymer pellets, which are held at a temperature range between the higher glass transition temperature and the lower melting temperature.
Despite the many advances noted above in the area of 65 optical films and devices, a number of problems still persist in the art. For example, it is often desirable to rely on strain-induced birefringence to achieve desirable optical
properties in an optical film, since the film can be conveniently oriented in a controlled manner on a laboratory stretcher in accordance with well established methodologies and principles. However, these methodologies do not work equally well for all materials selections. In particular, prob- 5 lems are frequently encountered with the use of thermodynamically immiscible polymers whose interfacial strength is not large, because the resulting film cannot always be stretched to a high enough draw ratio to achieve an optimal level of birefringence. In the case of a continuous/disperse 10 phase system, for example, orienting such a film to the draw ratios required for optimal birefringence may lead to voiding at the interface between the two phases, thereby compromising the desired optical properties (e.g., polarizing properties) of the system. Voiding of this type is described 15 in U.S. Pat. No. 5,811,493 (Kent), where it is used to produce paper-like films which are diffusely reflective to both polarizations of light. Unfortunately, if lower draw ratios are used to prevent voiding, the resulting film may have a lower degree of birefringence and less than optimal 20 optical properties.
There is thus a need in the art for a method for achieving a desired degree of birefringence in an optical film or device while reducing the draw ratio normally required to achieve the desired level of birefringence. There is also a need in the 25 art for a method for making optical films and devices from thermodynamically immiscible polymers whose interfacial strength is not large, wherein the films and devices are capable of being oriented to the higher draw ratios frequently required to achieve a higher degree of birefringence 30 and optimal optical properties. These and other needs are met by the present invention, as hereinafter described.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plot of engineering stress as a function of apparent draw ratio;
FIG. 2 is a plot of refractive index in the principle draw direction, Nx, for a positively birefringent material as a function of true draw ratio; 40
FIG. 3 is a plot of % increase in on-axis gain as a function of % randomness; and
FIG. 4 is a plot of engineering stress as a function of apparent draw ratio.
In one aspect, the present invention relates to a method for making a continuous/disperse phase optical device, such as a mirror, polarizer, or diffuser, and to optical devices so 50 made. In accordance with the method, at least one of the continuous and disperse phases is fabricated from a blend of homopolymers under conditions that allow the homopolymers to inter-react (e.g., through transesterification or transamidization) to form a copolymer which can be used to 55 make an optical device having improved physical and optical properties.
In another aspect of the invention, the present invention relates to continuous/disperse phase optical devices made from a blend of homopolymers that are capable of inter- 60 reacting, such devices having better physical and optical properties as compared to an analogous system in which the blend of homopolymers is replaced by a copolymer of the same monomers in the same ratios, but not made from homopolymers. Some of the improved physical and optical 65 effects observed with such a system include increased gain, higher intrinsic viscosities, and an earlier onset of strain
hardening (that is, the optical devices of the present invention exhibit a higher level of birefringence for a given level of strain than their random copolymer counterparts). The later property is especially advantageous in systems in which the materials of the continuous and disperse phases have poor adhesion, because it allows such systems to achieve a desired degree of birefringence at a lower draw ratio, thereby avoiding or minimizing voiding between the two phases. The degree of inter-reaction may be manipulated through control of processing conditions, such as residence time, so as to achieve desirable properties in the resulting optical device, but the degree of randomness will typically be less than 70%.
The use of a blend of homopolymers in accordance with the method of the present invention allows for the attainment of higher molecular weights (and, therefore, higher intrinsic viscosities) than those achievable with the aforementioned statistically random copolymer analogs. Surprisingly, continuous/disperse phase optical devices which are made with such blends exhibit increased gain as compared to their statistically random analogs, even though the average particle size of the disperse phase in systems made with the blend materials was not observed to be smaller than the average particle size of the disperse phase in systems made with the random copolymer analogs, as might have been expected had there been a larger difference in intrinsic viscosities of the final films.
In another aspect, the present invention relates to a method for making an improved optical device, such as a mirror, polarizer, or diffuser, from a continuous/disperse phase system, and to the optical devices so produced. Surprisingly, applicants have discovered that, when at least one of the continuous and disperse phases comprises a block copolymer, the degree of randomness of the monomeric units of the copolymer can be manipulated to maximize the strain-induced birefringence achievable in the system. In particular, through proper manipulation of the degree of randomness in the copolymer, a higher degree of birefringence can be obtained under the same stretching conditions than is achievable for a similar system in which the sequence lengths of the monomeric units are statistically random.
In a related aspect, the present invention relates to a method for improving the physical and optical properties of a continuous/disperse phase optical body in which at least one of the phases comprises a statistically random copolymer of two or more monomers, incorporated into optical devices made according to the method. In accordance with the method, the statistically random copolymer is replaced with a blend of homopolymers of the same monomers, such that the ratios of the monomers remains the same.
DETAILED DESCRIPTION OF THE
As used herein, the following abbreviations have the following meanings:
"T" refers to dimethyl terephthalate.
"N" refers to naphthalene dicarboxylate.
"E" refers to ethylene glycol.
"coPEN" refers to a copolymer based on naphthalene dicarboxylate and dimethyl terephthalate and ethylene glycol.
"PEN" refers to polyethylene naphthalate.
"PET" refers to polyethylene terephthalate.
"NDC" refers to naphthalene dicarboxylate.
"DMT" refers to dimethyl terephthalate.
"EG" refers to ethylene glycol.
"I.V." refers to intrinsic viscosity.
"An" refers to birefringence, and is denned as the index of refraction in the principle draw direction minus the index of refraction in a perpendicular direction. Where referred to herein, indices of refraction are measured at 632.8 nm, as the index of refraction typically increases with decreasing wave- 5 length due to dispersion.
In addition to its use in creating block copolymers, solid state polymerization is a process commonly used to increase the molecular weight of polyesters including polyethylenen- io phthalate (PEN) and polyethyleneterephthalate (PET). As has been taught in prior applications describing continuous and disperse phase optical devices, such as, for example, U.S. Pat. No. 5,825,543 (Ouderkirk,et al), U.S. Pat. No. 5,783,120 (Ouderkirk, et al.), and U.S. Pat. No. 5,867,316 15 (Carlson et al.), the contents of which are herein incorporated by reference in their entirety, the particle size of the disperse phase is an important parameter to control in optimizing these devices. Matching the viscosities of the two phases is one method of minimizing and controlling the 20 particle size of the disperse phase. During solid state polymerization, polymer pellets comprising polyesters of the type described are crystallized and then typically raised to a temperature of 235° to 255° C. under vacuum of less than 5 torr to drive off the polycondensation by-product ethylene glycol and thus increase molecular weight. Below temperatures of 210° C, solid state polymerization of aromatic polyesters becomes impractical due to extremely slow reaction rates and thus long polymerization times. Long ^ polymerization times can make the process and/or materials cost prohibitive. Since the melting point of some random copolyesters are lower than 210° C, the use of higher temperatures will cause an agglomerate of the pellets into an undesirable solid mass. Thus, it is impractical and cost 3J prohibitive to solid state polymerize random copolyethylenenaphthalates with less than about 75 mole percent naphthalate content. Copolymers of polyethylenenaphthalate having less than 75 mole percent naphthalate are desirable, however, for use in optical devices such as polarizers and mirrors due to their improved color, lower dispersion, reduced degradation by light, including Lw light, in the range of about 380 to 400 nm, and lower cost. Advantages of low naphthalate content resins are described in applicants copending U.S. patent application Ser. No. 09/416462 filed on even date herewith under attorney docket number 55028USA1A, which is herein incorporated by reference.
High molecular weight copolyethylenenaphthalates with less than about 75 mole percent naphthalate can be created by extrusion blending and transesterifying high molecular 50 weight polyethylene terephthalate (PET) with high molecular weight polyethylene naphthalate (PEN). One can achieve higher viscosities of a copolymer of PET and PEN than can be obtained during polymerization in conventional reactor processes since solid state polymerized PET and PEN with 55 higher viscosities can be used as starting homopolymers. In accordance with the present invention, the birefringence and other optical and physical properties of a continuous/ disperse phase system comprising a statistically random, or nearly random, copolymer of two or more monomers may be 60 improved by replacing the random or nearly random copolymer with a copolymer which comprises the same ratios of the individual monomers, but whose degree of randomness is less than that of a statistically random copolymer. Preferably, the replacement copolymer is a blend of con- 65 densation homopolymers of the individual monomers which are capable of inter-reacting with each other (e.g., by under
going transesterification, transamidization, or similar reactions) to a degree that can be controlled so as to result in a copolymer whose degree of randomness is less than that of a statistically random copolymer.
Polarizers, mirrors, diffusers, and other optical elements made from the continuous/disperse phase systems of the present invention have several advantages over analogous systems which utilize statistically random copolymers. In particular, the monomer ratios and the degree of transesterification or other inter-reaction can be conveniently controlled at the time of extrusion, thereby allowing one to optimize the amount of birefringence achievable under a given set of stretching conditions. Moreover, the continuous/ disperse phase systems of the present invention can be fabricated from homopolymers which are typically less expensive and more readily available in higher molecular weights than is typically the case with custom copolymers. Additionally, in contrast to a process that requires pre-made copolymer materials, the process of the present invention allows for greater compositional flexibility of the copolymer, in that the composition can be easily controlled or changed during extrusion.
In an optical device comprising a continuous and disperse phase system, one of the methods of improving the optical performance of the device is to maximize the birefringence of at least one of the phases in at least one of three orthogonal directions while minimizing the refractive index mismatch between the two phases along at least another of the orthogonal directions. It has been found in the present invention that a given level of birefringence can be achieved sooner (e.g., at a lower draw ratio) for block copolymers and/or statistically non-random copolymers of the type described above than is the case with a statistically random copolymer analog. The ability to achieve a given degree of birefringence at a lower draw ratio is particularly advantageous when it is desirable to utilize for the continuous and disperse phases fhermodynamically immiscible polymers whose interfacial strengths are not large, since the use of a lower draw ratio has less of a tendency to compromise the interfacial contact between the two phases (e.g., by inducing voiding).
In the context of the present invention, transesterification, transesterifying and transesterifies are meant to include reaction of condensation polymers such as polyesters, polyamides, copolyesteramides, and certain methine moieties intended to provide color, UV stability, or other desirable properties. Materials useful in the present invention include polyesters, polyamides, copolyesteramides, as well as other materials, for example those mentioned in U.S. Patent No. 4,617,373 (Pruett et al).
In many embodiments of the present invention, the degree of transesterification is controlled in the thermoplastic component(s) of at least one phase of a continuous/disperse phase optical device such that a statistically non-random copolymer results which preferably inter-reacts to an extent such that the degree of randomness is less than about 70%, preferably less than about 50%, and more preferably about 40% or lower. Methods available to achieve the desired degree of randomness include melt processes, as well as solid state polymerization processes which can occur prior to melt processing. Various parameters may be used to control the rate of the inter-reaction, including, but not limited to, the molecular weight of the individual blocks and of the entire block copolymer, the temperature of reaction, the state of matter in which the reaction is performed (e.g., whether it is performed in the solid or molten state), and the time allowed for the reaction to complete. Optionally, any of