WO2009058396A1

WO2009058396A1 - Negative birefringent optical films with flat or reversed birefringence wavelength dispersions for liquid crystal displays

Info

Publication number: WO2009058396A1
Application number: PCT/US2008/012435
Authority: WO
Inventors: Frank W. Harris; Limin Sun; Dong Zhang; Jiaokai Jing; Xiaoliang Zheng; Katsuyoshi Kubo
Original assignee: Akron Polymer Systems Inc.; Daikin Institute Of Advanced Chemistry And Technology, Inc.
Priority date: 2007-11-02
Filing date: 2008-11-03
Publication date: 2009-05-07

Abstract

A negative optical compensation film, which formed from a polymer selected from the group consisting of polyesters, polyamides, polycarbonates, polyimides, polyaryletherketones, and polysulfones, having a flat or reversed birefringence wavelength dispersion curve when formed by solution casting and without subjecting the film to a stretching process.

Description

NEGATIVE BIREFRINGENT OPTICAL FILMS WITH FLAT OR REVERSED BIREFRINGENCE WAVELENGTH DISPERSIONS FOR LIQUID CRYSTAL

DISPLAYS

FIELD OF THE INVENTION

This invention relates to a novel negative birefringent optical film, which can be used in liquid crystal displays to enhance viewing angle and reduce color distortion problems. The novel compensation film is comprised of a polymer having both positive birefringent units and negative birefringent units. The film birefringence wavelength dispersion may be adjusted by changing the composition and the ratio of the positive and negative birefringent units.

BACKGROUND OF THE INVENTION

Compensation films are widely used in liquid crystal displays to compensate for ^'the inherent optical birefringence in the display and, thus, improve viewing angle and image quality. It is highly desirable that the compensation film provide the same compensation effect throughout the whole visible wavelength spectrum (400-700nm) or provide decreased compensation at shorter wavelengths. The dependence of the value of birefringence on the wavelength of light is called the wavelength dispersion.

Compensation films often have "normal" wavelength dispersions, i.e., the absolute value of birefringence decreases as the wavelength of light increases. In many cases, such phase retardation dependence on wavelength (λ) may result in light leakage and color shift problems for liquid crystal displays. In order to overcome such problems, compensation films with reversed dispersion (i.e., the absolute value of birefringence increases with increasing wavelength) or flat dispersion (birefringence is independent of wavelength) are desirable. Thus, liquid crystal displays with different cell configurations require different compensation values and wavelength dispersions to provide optimum picture quality. Exemplary shapes of these wavelength dispersion curves are shown in Figure 1 and 2. In Fig. 1, curves (b) and (c) are considered normal curves, with curve (b) being a positive normal curve, while curve (c) is a negative normal curve. Curves (a) and (d) are reversed curves, with curve (a) being a positive reversed curve and curve (d) being a negative reversed curve. In Fig. 2, curve (c) is a desirable negative flat or reversed curve, which in fact is shown as a reversed curve, but is substantially flat.

Negative birefringent compensation films have been widely used in liquid crystal display to enhance the image quality. The compensation films are generally prepared by precision stretching of polymer films uniaxially or biaxially. The precise control of the film stretching to get exactly required birefringence is difficult, and it is particularly hard to get uniformity for the large area films. Further more, there can be residual stress in the stretched film which can cause distortions in the corner of the compensation films during long-term use.

Negative birefringent films have also been directly prepared by simple solution casting. U.S. Patent 5,344,916 to Harris first disclosed a class of organo-soluble polyimides, which when used to cast films, undergo a self-orientation process whereby the polyimide backbone becomes more or less aligned parallel to the film surface. This in-plane orientation, which can be controlled by varying the polyimide backbone linearity and rigidity, results in a film that displays out-of-plane negative birefringence. Negative birefringent films based on these polyimides have been commercially used in LCD TVs. However, all these films have normal wavelength dispersions that may cause color distortion problem in the display.

U.S. Patent 6,937,310 to Elman et al. discloses a class of amorphous polyesters that can also be solution cast into negative birefringent films. These films also exhibit normal wavelength dispersions.

U.S. Patent Nos. 5,750,641; 5,969,088; 6,074,709; 6,383,578, all to Ezzel et al., describe solution cast negative birefringent films prepared from polyimides containing at least a small amount of 9,9-bis(aminoaryl)fluorene. These polyimide films also exhibit normal wavelength dispersions.

The use of multiple films to achieve the reversed wavelength dispersion compensation film have been proposed and demonstrated as shown in U.S. Patent Application 2008/0241427 to Harris et al. However, more films mean more process steps and more cost for products.

U.S. Patent 6,565,974 to Uchiyama et al. discloses a single stretched oriented polycarbonate film having a smaller retardation value at a shorter wavelength at a measuring wavelength of 400-700 nm. The polycarbonate is composed of both monomer unit with positive refractive index anisotropy and a monomer unit with negative refractive index anisotropy. Such configuration of the polymer will only give small birefringence since the cancellation of the positive and negative anisotropy. Precise stretching is necessary to get the required optical properties.

Japan Patent Application: 2006-178401 to Nitto Denko discloses polyimides containing cardo structures and films from these polymers display an extremely small positive birefringent characteristic. These films must be stretched to provide useful optical properties.

U.S. Patent Application 2007/0153178 to Rao et al. discloses a method in which infrared dyes are added to a polymer solution, which is then cast as a film with reverse wavelength dispersions. However, the miscibility and stability of the low molecular infrared dyes in polymers is very poor since the resulting films display only a small negative birefringence.

Negative birefringent compensation films with reversed or adjustable wavelength dispersions are highly desirable. However, such compensation films have been prepared only by stretching single films or by stacking multiple films to provide a net benefit from the accumulated films.

SUMMARY OF THE INVENTION

The present invention provides an optical compensation film that displays negative birefringence and reversed or flat wavelength dispersions. The film can be used to increase the viewing angle and contrast of a liquid crystal display. More importantly, such a compensation film can be used to minimize color distortion problems that are difficult to eliminate using conventional compensation films with normal wavelength dispersions. The novel compensation film is made from an organo-soluble copolymer by solution casting. The copolymer contains both positive birefringent monomer units and negative birefringent monomer units combined in such a manner to result in an overall negative birefringence. By simply adjusting the amounts of the positive and negative units, any birefringence value between that of the intrinsic birefringences of the homopolymers of the two kinds of monomer units can be achieved. The wavelength dispersion of the copolymer depends on the amounts and wavelength dispersions of the monomer units. Therefore, the positive birefringent monomer units and the negative birefringent monomer units can be selected based on their wavelength dispersions. While the composition range is not critical or considered limiting, the amount of positive birefringent monomer units most often varies from about 5 to 70 mol percent. In general, an increase in the amount of positive birefringent monomer units decreases the overall negative birefringence of the copolymer.

Selecting the positive and negative birefringent monomer units can be done by determining the wavelength dispersions of the homopolymers of the positive birefringent monomer units and the negative birefringent monomer units. In the next step, the monomers are polymerized to form a copolymer that is then solution cast into a film having a thickness of from about 1 micron to about 100 microns. The copolymer film will have a substantially flat wavelength dispersion when the wavelength dispersions of the homopolymer of the positive birefringent monomer unit is an approximate mirror image of the wavelength dispersion of the homopolymer of the negative birefringent monomer unit. The dispersion curves should lie on opposite sides of the x-axis of a plot of birefringence vs. light wavelength where the x-axis is Δn equals zero, as shown in Figure 2. When the negative slope (|dΔn/dλ|) of the dispersion curve of the positive birefringent monomer unit is somewhat larger than the positive slope (|dΔn/dλ|) of the negative birefringent monomer unit at the same short wavelengths, a negative birefringent copolymer film may be obtained with a reversed wavelength dispersion.

A simple way to screen the positive and negative birefringent monomer units is to determine the UV absorption spectra of the homopolymers of the corresponding positive and negative birefringent monomer units. The higher the λ* of the spectrum of the polymer, the higher the dΔn/dλ of the polymer's wavelength dispersion curve. The λ* is mean resonance wavelength, and it can be roughly approximated as the mean UV absorption wavelength. In fact, for many simple polymer systems, the mean UV absorption wavelength is roughly equal to the maximum UV absorption wavelength (λmax). In other words, the wavelength dispersion curve slope dΔn/dλ can be estimated by the λmax of the polymer.

Another way to choose positive and negative birefringent monomer units is to measure the UV absorption spectra of the monomers themselves. In order to get a negative birefringent optical film with a reversed wavelength dispersion, the positive monomer unit should have a higher λ* than that of the negative birefringent monomer unit in the copolymer. Thus, the positive birefringent monomer units should have higher UV absorption λmax than that of the negative birefringent monomer units.

It will be appreciated that the composition of the positive and negative birefringent monomer units may vary widely because they are selected and combined according to the wavelength dispersion curve shapes of their homopolymers. If a copolymer film with a substantially flat wavelength dispersion curve is desired the comonomers must be selected such that the wavelength dispersions of the homopolymer of the positive birefringent monomer unit is an approximate mirror image of the wavelength dispersion of the homopolymer of the negative birefringent monomer unit. If a copolymer film with a reversed dispersion curve is desired, the comonomers must be selected such that the negative slope (|dΔn/dλ|) of the dispersion curve of the positive birefringent monomer unit is somewhat larger than the positive slope (|dΔn/dλ|) of the negative birefringent monomer unit at the same short wavelengths. It should be understood that more than one positive and more than one negative birefringent monomer units may be used to attain the desired wavelength dispersions.

One embodiment of this invention is a polyimide copolymer. The polyimide is prepared by condensation polymerization of dianhydride and diamine monomers. At least one of the monomers (either the diamine or the dianhydride) should comprise a substituted cardo structure such as that shown below. The λmax of the substituted cardo monomer should be longer than the λmax of the other monomers used to prepare the copolymer.

Where A, A2, Bl, and B2 can be the same or different and can be H, halogen, alkyl, phenyl, substituted phenyl, biphenyl, substituted biphenyl, naphthyl, phenyl ethynyl, benzoyl, while R1-R4 can be the same or different and can be H, halogen, phenyl, or alkyl.

Another embodiment of this invention is a polyester copolymer. The polyester is prepared by condensation polymerization of dicarboxylic dichloride and bisphenol monomers. At least one of the monomers (either the dicarboxylic dichloride or the bisphenol) should comprise a substituted cardo structure as shown above. The λmax of the substituted cardo monomer should be longer than the λmax of the other monomers used to prepare the copolymer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which:

Figure 1 is the wavelength dispersion curves of birefringent optical films, (b) and (c) are typical normal dispersion curve of positive C and negative C birefringent film respectively; (a) and (d), are reversed positive C and negative C dispersion curve respectively;

Figure 2 illustrates that combination of positive C (a) and negative C (c) can give relative flat wavelength dispersion (b);

Figure 3 is a graph of the wavelength dispersion curves for the polymers listed in Table 3; _. Figure 4 is a graph of the wavelength dispersion curves for the polymers listed in Table 4;

Figure 5 is a graph of the wavelength dispersion curves for the polymers listed in Table 5;

Figure 6 is a graph of the wavelength dispersion curves for the polymers listed in Table 6;

Figure 7 is a graph of the wavelength dispersion curves for the polymers listed in Table 6 where the data has been normalized;

Figure 8 is a graph of dispersion curves and normalized dispersion curves of polyesters without cardo monomer;

Figure 9 is a graph of dispersion curves of polyester copolymers based upon IPC/TMBP/MePh; and

Figure 10 is a graph of dispersion curves and normalized dispersion curves of polyester copolymers based upon IPC/TMBP/MeBz.

DETAILED DESCRIPTION OF THE INVENTION

This invention makes possible the preparation of negative birefringent polymer film with flat or reversed wavelength dispersions.

Generally, the property of an optical compensation film can be characterized with three refraction indices n_x, n_y and n_z, wherein n_x and n_y represent in-plane (x and y direction) indices and n_z represent the film thickness direction (out-of-plane) refractive index. A negative birefringent optical film (negative C plate) is an optically anisotropic film satisfying the relation of nz<nx=ny. The out-of-plane birefringence is usually defined by Δn_th=^rn_z-(n_x+n_y)/2 and in-plane birefringence is defined by Δnj_n=n_x-n_y. Accordingly, out-of-plane retardation Rth is related to birefringence by

where d is the film thickness and in plane retardation by Rin= Δnj_n ^• d. For an ideal negative C plate, the Δnj_n=n_x-n_y=0 and Δn_th=n_z-(n_x+n_y)/2=n₂-n_x==n_z-n_y. Since the refraction indices are functions of wavelength (λ), the birefringence Δn also depends on wavelength. The dependence of birefringence on the wavelength at which it is determined is called the wavelength dispersion. In this document, if not specified, all birefringence values (Δn) are those values obtained using a wavelength of 633nm (An₆₃₃). The wavelength dispersion curve of a compensation film is very important for optimizing the performance of a liquid crystal display. For example, negative compensation films with normal wavelength dispersions may improve the viewing angle but not eliminate all the color distortion problems in vertically aligned (VA) liquid crystal displays. However, these problems may be avoided through use of the negative compensation films with flat or reversed wavelength dispersions of this invention. The invention compensation films, which are cast from solutions of a single copolymer, can be used as cast. No stretching process is required to obtain their targeted optical properties.

The copolymers of this invention are made from both positive birefringent monomer units and negative birefringent monomer units. The positive birefringent monomer unit can be used to prepare homopolymers that can be solution cast into films that could display positive birefringence. The negative birefringent monomer unit can be used to prepare homopolymers that can be solution cast into films that display negative birefringence. The overall birefringence (Δn) of such a copolymer film is roughly given by An = v_pAn_p + v_NAn_p , wherein v_p and v_N representing the volume fractions of the positive component and negative component respectively. And the wavelength dispersion (dΔn/dλ) of this copolymer should have the following relationship:

dAn dAn_p dAn_N

If An = V_pAfi_p + v_N An p ≠ 0 , and = v_p — + v_N — ≡ 0 , then either negative or dλ dλ dλ positive birefringent compensation film with substantially flat wavelength dispersion curve is obtained. r, i dAn dAn_p dAn_{N Λ} ,

If Δ;; = v _p An _p + v_NAn_p < 0 , and = v,, — + v_N — < 0 , then negative dλ dλ dλ birefringent compensation film with reversed wavelength dispersion curve is obtained.

_Λ , dAn dAn_p dAn^ _Λ ,

If An = V_pAn _p + v_NAn_p > 0 , and = v_p + v, — > 0 , then positive dλ dλ dλ birefringent compensation film with reversed wavelength dispersion curve is obtained. In order to simply characterize the wavelength dispersion of a compensation film, the dispersion coefficient is defined as K _M5 ^₂=An (λl)/Δn(λ2) = Rth (λl)/Rth(λ2), where 400nm< λl<λ2<700nm. If K u_, λ₂~l, the wavelength dispersion curve is substantially flat. IfK χι_.

u <!, the wavelength dispersion is reversed. There are various K w. u values depending on the choosing of λl and λ2, for even the same dispersion curve. Unless specially stated, all the K parameters used in this document refer to K₄₅OmTi, ssonm- The term R450/R550 is another expression of K4₅o_nm,_55θnm that is used. In this invention, if the value of the K is between 1.0 and 1.10, the dispersion curve is considered flat, and if the value of the K is less than 1.0, the dispersion curve is considered reversed. The overall negative birefringence is estimated by the Δn value determined at 633nm (An₆₃₃).

In the paper by Shin-Tson Wu, "Birefringence dispersions of liquid crystals," Physical Review A 33(2), 1270, (1986), the relationship between birefringence at different light wavelengths [Δn (λ)] and the mean resonance wavelength (λ*) of liquid crystal molecules has been given as:

An(λ) = G(T)- x\xy

I 1 * \ 2

G(T) is a constant once the chemical structure and order parameter is fixed at a certain temperature. The equation suggests that Δn is directly related to λ*. The higher λ* will give higher absolute values for Δn (λ) and dΔn/dλ, when λ>λ*. The λ* itself is difficult to be determined exactly, but can be roughly treated as the mean UV absorption wavelength. For many polymer systems, the mean UV absorption wavelength is roughly their maximum UV absorption wavelength (λmax). This suggests that the magnitude of the wavelength dispersion curve slope (dΔn/dλ) can be estimated by the λmax of the polymer. A longer λmax suggests that the slope of the wavelength dispersion curve (dΔn/dλ) will be steeper.

In one embodiment of present invention, we determine the UV absorption spectra of homopolymers and use the λmax of these spectra to estimate their mean resonance wavelength λ*. The positive birefringent monomer units and negative birefringent monomer units are then chosen according to the λmax of their homopolymers. The λmax of the monomer units can also be used in the selection of appropriate monomers for copolymers with targeted optical properties.

By using the same approach described in this invention, one can prepare a positive birefringence film with flat or reversed dispersion curves by adjusting the relative amounts of the positive birefringent monomer units and negative birefringent monomer units. In this case, however, the λmax of the positive birefringent monomer unit must be shorter than that of the negative birefringent monomer unit.

In one embodiment of the invention, the positive birefringent unit is a cardo containing monomer, preferably substituted cardo containing monomer. The substituted cardo structure can be expressed as:

where Al, A2, Bl, and B2 can be the same or different and can be H, halogen, alkyl, phenyl, substituted phenyl, biphenyl, substituted biphenyl, naphthyl, phenylethynyl, or benzoyl; and R1-R4 can be the same or different and can be H, halogen, phenyl, or alkyl.

When the polymer is a polyimide copolymer, the polyimide is prepared by condensation polymerization of dianhydride and diamine monomers. At least one of the monomers (either the diamine or the dianhydride) should comprise substituted cardo structure. Normally, cardo diamines are used. Examples of such diamines include the following but not limited to:

C13 C14 In one example embodiment of the present invention, the diamine negative birefringent monomer units that can be used in combination with cardo based positive birefiϊngent monomer units are:

D5 D6

UT D8

When the polymer is a polyester copolymer, the polyester is prepared by condensation polymerization of dicarboxylic dichloride and bisphenol monomers. At least one of the monomers (either the dicarboxylic dichloride or the bisphenol) should comprise a substituted cardo structure. Normally, cardo bisphenols based are used. Further, the ultraviolet absorption of the substituted cardo monomer should have a longer wavelength at its maximum absorption peak λmax than that of the polyimide made from other monomers without this cardo containing monomer. Examples of cardo bisphenols are:

E4 E5

E6 E7

E8

E9 In one example embodiment of the present invention, the other bisphenol monomers that can be used in pair with cardo based bisphenol monomers are:

F2

F5 F6 F7

F8 F9

The polymers of the present invention, especially the polyimide or polyester polymers can be dissolved in conventional solvents, such as dimethyl formamide (DMF), tetrahydrofuran (THF), Chloroform, cyclopentanone, cyclohexanone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), toluene, and the like, and can be solution cast into films. The films can be cast on an inert substrate and laminated with other optical films or can be directly cast on other optical films such as cellulose triacetate (TAC) films. The present invention is not to be limited in scope by the specific examples described herein which are intended as illustrations of various aspects of the invention, and functionally equivalent methods and components are within the scope of the invention. Such modifications are intended to fall within the scope of the claims. Various publications are cited herein, the contents of which are hereby incorporated, by reference, in their entireties

Examples

In the following experiments, the UV spectra were measured with a UV/VIS spectrophotometer (UV-2450) from Shimadzu. The birefringence of the polymer films were measured with a Metricon® 2010 Prism Coupler. The wavelength dispersion curves were measured with a VASE® Ellipsometer from J. A. Woollan.

Example 1

This example illustrates the preparation of 9, 9-Bis(3-methyl-4-aminophenyl)-2, 7-dibromofluorene.

91.6%

The synthesis of 2, 7-Dibromo-9-fluorenone (3): To a IL three-necked flask equipped with a mechanical stirrer, a condenser, and a thermometer ware added fluorenone (I) (4Og, 0.222 mol), water (300 ml), and potassium iodide (0.4g). The mixture was stirred vigorously. After the mixture was cooled to 10°C with ice bath, bromine (2) (28 ml) was added slowly with an additional funnel. The mixture was allowed to warm to room temperature by removing ice bath and then heated to reflux by an electrical heating mantle. After 10.5 hrs, the mixture was cooled to room temperature, sodium hydroxide (21.7g) and an additional portion of potassium iodide (0.5g) were added. After more bromine (10 ml) was added, the mixture was heated to reflux again for another 14.5 hrs. Upon cooling, the mixture was filtered. The solid collected was washed with water. Re-crystallization from DMF/water mixture once afforded 68.7 g yellow powder: yield —91.6%.

The synthesis of 9, 9-Bis(3-methyl-4-aminophenyl)-2, 7-dibromofluorene (C5): To a 1 L three necked round bottom flask equipped with a thermometer, a mechanical stirrer, and Dean-Stark trap were added o-toluidine (400 ml), benzene ( 100 ml), 2, 1- dibromo fluorenone (54g, 0.16 mol), and concentrated hydrochloric acid (40 ml). The mixture was heated to reflux and water was removed through the trap. The mixture was further heated up to 175°C by removing benzene through the trap. After stirred overnight, the trap was replaced with a distillation head, and about 300 ml of o-toluidine was removed under reduced pressure. After cooled to room temperature, the residual mixture was neutralized by addition of a sodium bicarbonate aqueous solution. A dark purple liquid was isolated by remove of the aqueous solution, which was added to methanol. Large amount of fine crystals precipitated were obtained by filtration. Further washed with methanol following drying afforded fine powder (68g). Yield: -80%.

Similarly, when replaced o-toluidine with o-cresol, 9, 9-Bis(3-methyl-4- hydroxyphenyl)-2, 7-dibromofluorene (E4) (MeBr) was obtained.

E4

Example 2

This example illustrates the preparation of a substituted cardo diamine using a Suzuki Coupling reaction.

C5 C8

Synthesis of 1.5. 9, 9-bis(3-methyl-4-aminophenyl), 2, 7-diphenylfluorene (PHME) :

To a 1000 ml three necked round bottom flask equipped with a mechanical stirrer, a condenser, and nitrogen inlet were added dibromo-dimethyl cardo diamine (5.84g, 11.0 mmol) and toluene (300 ml). The mixture was heated until all solid was dissolved. After the mixture was cooled, sodium carbonate (5.12g, 48.4 mmol) in water (32 ml) and tetrakis(triphenylphosphine)-palladium (0) (0.76g, 0.66 mmol) were added. The mixture was stirred for another half hour. After a solution of phenylboronic acid (3.2g, 26.4 mmol) in ethanol (20 ml) was added, the mixture was heated, and refluxed overnight. Upon cooling, the mixture was filtered through Celite to remove any solid. The filtrate was washed with water in a separation funnel, and extracted with chloroform. After the solvents were removed over a rotary evaporator under reduced pressure, the residual was purified by column chromatography on silica gel with a mixture of hexanes and ethyl acetate as eluent to give 3.33 g (~60% yield) white powder. The product was verified by TLC and proton NMR.

9, 9-bis(3-methyl-4-hydroxyphenyl), 2, 7-diphenylfluorene (E6) (MePh) was synthesized via a similar way:

E4 E6

Example 3

This example illustrates the preparation of a substituted cardo diamine using a Sonogashira Coupling reaction:

(PhCN)₂PdO₂

9CP/o

Synthesis of 9, 9-bis(3-methyl-4-aminophenyl), 2, 7-di(phenylacetyl)fluorene (ACME)

(12):

To a 500 ml three necked round bottom flask equipped with a mechanical stirrer, a condenser, and a nitrogen inlet were added 9, 9-Bis(3-methyl-4-aminophenyl)-2, 7- dibromofluorene (5_) (10.68g, 0.02 mol), bis(benzonitrile)-dichloropalldium (II) (6) (0.025g), triphenylphosphine (7) ( 1.3Ig), Toluene (9) (200 ml), diisopropylaniline (DIPA, U)) (20 ml), copper (II) acetate monohydrate (ϋ) (O.lg). The mixture was heated to 100⁰C until a homogenous solution was obtained. After the mixture was cooled to the room temperature, phenylacetylene (8) (4.9g) was added slowly to the mixture. The mixture was heated at reflux overnight. Upon cooling, large amount of crystals precipitated, which were collected by filtration, followed with methanol washing and dried under reduced pressure. Recrystallization from toluene afforded 10.35g of pale crystals. Yield: 90%.

Example 4

This example illustrates the preparation of 9, 9-bis(3-methyl-4-hydroxyphenyl)-2, 7-dibenzoyl-fluorene (E9) (MeBz)

Step 1. To a 1 L three necked round bottom flask equipped with a mechanical stirrer, a condenser and an additional funnel were added fluorene 1 (49.86g, 0.30 mol), nitromethane (350 ml), and dry aluminum chloride (10Og, 0.75 mol). After the mixture was cooled below 10°C, benzoyl chloride 2 (92.77g, 0.66 mol) was added to the mixture dropwise. The mixture then heated to 70⁰C overnight, then poured into cold diluted HCl water solution. The precipitate was collected by filtration, and dried. The crude product 3 was about 98.34g (87% yield). Recrystallization from acetic acid twice afforded fine crystals: 61.55g, 55% yield.

Step 2. To a 1 L three necked round bottom flask equipped with a mechanical stirrer, a condenser and a thermometer were added 3_ (37.4g, 0.1 mol), acetic acid (400 ml), and sodium dichromate dihydrate (124g, 0.42 mol). The mixture was heated to reflux for 5 hrs. The solid were dissolved first, and then large amount of yellow precipitate appeared. Upon cooling, the yellow precipitate was isolated by filtration. After washed with methanol several times, the product was dried. This procedure afforded 4 with bright yellow powder: 35.63g, 92.0% yield.

Step 3. To a 500 ml three necked round bottom flask equipped with a mechanical stirrer, a condenser and a thermometer were added 4 (23.28g, 0.06 mol), o-cresol (46g, 0.36 mol), dichloroethane (200 ml) and methylsulfonic acid (15 ml). The mixture was heated to reflux overnight. Upon cooling, dichloroethane was removed over an evaporator. The residual was poured into water. Methylsulfonic acid was removed first by decant the water phase. O-cresol was removed by washing the residual with hot water several times. The solid formed was washed with acetic acid, and ethanol. The product was dried under reduced pressure. This procedure afforded the product 5_: 29.5g, 83.8% yield. Example 5

This example illustrates the preparation of a polyimide copolymer containing a positive birefringent monomer unit and a negative birefringent monomer unit.

mm

mCreεd

Fblyimde Copolymer Rm,n

The polyimide copolymer synthesis:

To a 250 ml three necked round bottom flask equipped with a mechanical stirrer, a water trap, and a nitrogen inlet, were added PFMB (3.3305g, 13x0.8 mmol), ACME (1.4994g, 13x0.2 mmol) and dry m-cresol (100 ml). The mixture was heated until all the monomers dissolved. After the mixture was cooled to the room temperature, 6FDA (5.7746g, 13 mmol) was added. The mixture was heated to 120⁰C. Once the mixture became clear, it was cooled to room temperature and stirred for 3~4 hrs. After several drops of isoquinoline were added, the mixture was heated to 205⁰C overnight. Upon cooling, the polymer was precipitated with methanol. The polymer was soaked with methanol several times and dried. The polymer was finally post baked at 205⁰C under reduced pressure overnight.

Example 6

This example illustrates the preparation of a polyester copolymer containing a positive birefringent monomer unit and a negative birefringent monomer unit.

F2 E9 m+n m

Polyester copolymer

The polyester copolymer synthesis:

In a four neck 100 mL flask was equipped with a mechanic stirring, a nitrogen inlet, stopper and a pressure equalizing addition funnel. On the top joint of the addition funnel was connected to an oil bubbler. The whole system was oven dried and cooled under nitrogen flow. HMBP (F2) (3.7852g, 0.014 mol) and MeBz (E9) (3.52Og, O.OOβmol) were heated to dissolve in a mixture solvent containing cyclopentanone (6 mL), triethylamine (6 mL) and 1 ,2-dichloroethane (DCE) (20 mL). The solution was then cooled with ice-water bath. After 10 min, IPC (4.1416g, 0.0204mol) in 7 mL of 1,2-dichloroethane was added to the solution in 3 hours. The addition funnel was rinsed with 3 mL of DCE and added to the flask after an additional hour stirring. Let the temperature gradually increase to room temperature and was allowed to stir overnight. The following day, the viscous polyester solution was washed with 5% hydrochloric acid, water and then precipitated into methanol to isolate the polymer as fiber. The fiber was soaked with methanol four more times and dried in a vacuum oven.

Example 7

This example illustrates the UV absorption λmax of some polyimide homopolymers based on different diamines and dianhydrides. Listed in Table 1 are the birefringence values and the UV absorption λmax of negative birefringence films made from polyimides. Listed in Table 2 are the birefringence values and UV absorption λmax of birefringence films made from polyimides based on 6FDA and substituted cardo diamines.

Table 1, Birefringence and the UV absorption λmax of polyimides based on non-cardo diamines and 6FDA

Table 2, Birefringence and the UV absorption λmax of polyimides based on 6FDA and substituted cardo diamines

As shown in Table 1, polyiniides can be prepared from 6FDA and rigid diamines that can be solution cast into negative birefringent films. However, when cardo diamines are used as shown in Table 2, the negative birefringences become much smaller. In fact, some systems even display a small positive birefringence. The methyl substituents located at the ortho-positions next to the amino groups disrupt the in plane packing of the polymers, thus, further lowering the value of the negative birefringence. However, at the same time they are contributing to the flattening or reversing of the wavelength dispersion curve. Substitutents at the 2,7 positions on the cardo ring also result in less negative birefringence.

The following examples will illustrate how to use the UV absorption λmax of polyimide homopolymers to determine which diamine monomers can be combined to give negative birefringent films with flat or reversed birefringence wavelength dispersions.

Example 8

As shown in Tables 1 and 3, the polyimide based on 6FDA and PFMB (No.l) has a λmax of 250 nm and the polyimide based on 6FDA and PhMe-FDA (No.1 1 ) has a λmax of 330nm, which is longer than 250nm. According to our invention, they can be combined to form a copolymer which gives an overall negative birefringence (Δri₆₃₃) and a wavelength dispersion that can be adjusted by varying the molar ratio of the PFMB and PhMe-FDA. Here the positive birefringent monomer unit is PhMe-FDA and the negative birefringent monomer unit is PFMB. The birefringence values and R450/R550 of solution cast films of several different copolymers, which were prepared by the copolymerization of various amounts of the monomers, are listed in Table 3. The wavelength dispersion curves of the films are shown in Figure 3.

Table 3 Birefringence and R450/R550 values of polyimide copolymers based on

6FDA/PFMB/PhMe-FDA

Example 9

The polyimide based on 6FDA and BPMe-FDA (No.16) has a λmax of 345nm, which is longer than that of 6FDA/PFMB (250nm) polyimide. According to our invention, they can be combined to form a copolymer which gives an overall negative birefringence (Δri₆₃₃) and a wavelength dispersion that can be adjusted by varying the molar ratio of the PFMB and BPMe-FDA. Here, the positive birefringent monomer unit is BPMe-FDA and the negative birefringent monomer unit is PFMB. The birefringence values and R450/R550 of solution cast films of several different copolymers, which were prepared by the copolymerization of various amounts of the monomers, are listed in Table 4. The wavelength dispersion curves of the films are shown in Figure 4.

Table 4 Birefringence and R450/R550 values of polyimide copolymers based on

6FDA/PFMB/BBMe-FDA

Example 10

The polyimide based on 6FDA and 2NAMe-FDA (No.15) has a λmax of 345nm, which is longer than that of 6FDA/PFMB (250nm) polyimide. According to our invention, they can be combined to form a copolymer which gives an overall negative birefringence (Δri₆₃₃) and a wavelength dispersion that can be adjusted by varying the molar ratio of the PFMB and 2NAMe-FDA. Here the positive birefringent monomer unit is 2NAMe-FDA and the negative birefringent monomer unit is PFMB. The birefringence values and R450/R550 of solution cast films of several different copolymei's, which were prepared by the copolymerization of various amounts of the monomers, are listed in Table 5. The wavelength dispersion curves of the films are shown in Figure 5.

Table 5 Birefringence and R450/R550 values of polyimide copolymers based on

6FDA/PFMB/2NAMe-FDA

No. 6FDA PFMB 2NAMe-FDA R450/R550 Δn 633

Example 11

The polyimide based on 6FDA and ACME (No.17) has a λmax of 355-360nm, which is longer than that of 6FDA/PFMB (250nm) polyimide. According to our invention, they can be combined to form a copolymer which gives an overall negative birefringence (Δri₆₃₃) and a wavelength dispersion that can be adjusted by varying the molar ratio of the PFMB and ACME. Here the positive birefringent monomer unit is ACME and the negative birefringent monomer unit is PFMB. The birefringence values and R450/R550 of solution cast films of several different copolymers, which were prepared by the copolymerization of various amounts of the monomers, are listed in Table 6. The wavelength dispersion curves of the films are shown in Figure 6.

Table 6 Birefringence and R450/R550 value of polyimide copolymers based on

6FDA/PFMB/ACME

Example 12

The overall birefringence values (An₆₃₃) and UV absorption λmax of some polyester homopolymers based on non-cardo bisphenols are listed in Table 7. The wavelength dispersion curves of these polymers are shown in Figures 8a and 8b.

Table 7. Birefringence and the UV absorption λmax of polyesters based on non-cardo bisphenols

The following examples will illustrate how to use bisphenol monomers to prepare negative birefringent films with flat or reversed birefringence wavelength dispersions.

Example 13 The polyester based on IPC and MePh (E6) has a λmax of 315-330 nm, which is longer than that of IPC/TMBP (256nm) polyester. According to our invention, they can be combined to form a copolymer which gives an overall negative birefringence (Δr>₆₃3) and a wavelength dispersion that can be adjusted by varying the molar ratio of the TMBP and MePh. Here the positive birefringent monomer unit is MePh and the negative birefringent monomer unit is TMBP. The birefringence values and R450/R550 of solution cast films of several different copolymers, which were prepared by the copolymerization of various amounts of the monomers, are listed in Table 8. The wavelength dispersion curves of the films are shown in Figure 9.

Table 8. Birefringence and R450/R550 values of polyester copolymers based on

IPC/TMBP/MePh

Example 14.

The polyester based on IPC and MeBz (E9) has a λmax of 314 nm, which is longer than that of IPC/HMBP (232nm) polyester. According to our invention, they can be combined to form a copolymer which gives an overall negative birefringence (Δri₆₃₃) and a wavelength dispersion that can be adjusted by varying the molar ratio of the HMBP and MeBz. Here the positive birefringent monomer unit is MeBz and the negative birefringent monomer unit is HMBP. The birefringence values and R450/R550 of solution cast films of several different copolymers, which were prepared by the copolymerization of various amounts of the monomers, are listed in Table 9. The wavelength dispersion curves of the films are shown in Figures 10a and 10b.

Table 9. Birefringence and R450/R550 values of polyester copolymers based on

IPC/TMBP/MePh

While the invention has been described with reference to an exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

What is claimed is:

1. A negative optical compensation film, which comprises a polymer selected from the group consisting of polyesters, polyamides, polycarbonates, polyimides, polyaryletherketones, and polysulfones, having a flat or reversed birefringence wavelength dispersion curve when formed by solution casting and without subjecting the film to a stretching process.

2. The film of claim 1 wherein the polymer is a copolymer comprised of positive birefringent unit monomers and negative birefringent unit monomers.

3. The film of claim 2 wherein the positive and negative birefringent unit monomers are selected based upon the UV absorption peaks of the monomers.

4. The film of claim 2 wherein the λmax of the UV absorption spectrum of the positive birefringent unit monomer is longer than that of the negative birefringent unit monomer.

5. The film of claim 2 wherein the overall out-of-plane negative birefringence is achieved by adjusting the relative amount of the positive birefringent monomer units and negative birefringent monomer unit monomers.

6. The film of claim 2 wherein the positive birefringent unit monomer contains a cardo group.

7. The film of claim 2 wherein the positive birefringent unit monomer contains a substituted cardo group represented by the following formula:

wherein Al, A2, Bl, and B2 can be the same or different and can be H, halogen, alkyl, phenyl, substituted phenyl, biphenyl, substituted biphenyl, naphthyl, phenyl ethynyl, or benzoyl, and R1-R4 can be the same or different and can be H, halogen, phenyl, or alkyl.

8. The film of claim 2 wherein the copolymer is polyimide, which is prepared by condensation polymerization of dianhydride and diamine monomers and at least one of the monomers includes a substituted cardo structure which is selected from the following structures:

C13 C14

9. The film of claim 2 wherein the copolymer is polyester, which is prepared by condensation polymerization of dicarboxylic dichloride and bisphenol monomers and at least one of the monomers includes a substituted cardo structure selected from the following structures:

E1 E2 E3

E4 E5

E6 E7

E8

E9

10. The film of claim 1 wherein the polymer is soluble in a solvent selected from the group consisting of dimethylformamide, tetrahydrofuran, chloroform, cyclopentanone, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, toluene, and mixtures thereof.

11. The film of claim 1 wherein the film can be solution cast on an inert substrates or on optical films.

12. The film of claim 1 wherein the film is further laminated with another optical film.

13. The film of claim wherein the film is cast on cellulose triacetate (TAC) film.

14. The film of claim 1 having an absolute Δn₆₃₃ value that is greater than 0.003.

15. The film of claim 1 having an absolute Δn₆₃₁ value that is greater than 0.005.

16. The film of claim 1 having an absolute An₆₃₃ value that is greater than 0.01.

17. The film of claim 1 having an absolute Δn₆₃₃ value that is greater than 0.02.

18. The film of claim 1 having a dispersion coefficient K of the film

_550πm = R450/R550) satisfies the relation K < 1.10.

19. The film of claim 1 wherein the dispersion coefficient K of the film (K=K_45On1I1, 55₀nm = R450/R550) satisfies the relation K < 1 .00.

20. The film of claim 1 wherein the dispersion coefficient K of the film (K=K_45OOm, 55_0nm = R450/R550) satisfies the relation K < 0.90.

21. A method of making a negative optical compensation film, which comprises a polymer selected from the group consisting of polyesters, polyamides, polycarbonates, polyimides, polyaryletherketones, and polysulfones, having a flat or reversed birefringence wavelength dispersion curve when formed by solution casting and without subjecting the film to a stretching process, comprising the steps of a. forming a polymer from a combination of positive birefringent unit monomers and negative birefringent unit monomers; b. selecting the positive and negative birefringent unit monomers are selected according to the UV absorption peaks of the monomers; c. selecting the monomers so that the UV absorption wavelength of the positive birefringent unit monomer is larger than the UV absorption wavelength of the negative birefringent unit monomers; d. adjusting the relative amount of the positive and negative unit monomers so that the overall out-of-plane negative birefringence is achieved; and e. solution casting the polymer to form a film having a flat or reversed birefringence wavelength dispersion curve.