A cellulose ester optical film having stable wavelength dispersion and a method for preparing the same, and an optical compensation film

Abstract

This invention provides an optical thin film, which is prepared from a graft copolymer and is a cellulose ester optical thin film with stable wavelength dispersion characteristics. The optical thin film provided by this invention comprises a cellulose ester polymer matrix with a total acyl content of less than 60.5% and aromatic polymer side chains covalently grafted onto its main chain. After uniaxial stretching and orientation, within a stretching ratio range of 1.1 to 1.6, the ratio of its out-of-plane birefringence at wavelengths of 450 nm to 550 nm does not change by more than 3% (i.e., the dispersion stability coefficient S ≤ 3%), exhibiting good wavelength dispersion stability.

Description

Technical Field

[0001] This invention belongs to the field of optical compensation film preparation technology, and relates to an optical thin film and its preparation method, an optical compensation film and a liquid crystal display device, and particularly to a cellulose ester optical film with stable wavelength dispersion and its preparation method, and an optical compensation film. Background Technology

[0002] In liquid crystal display technology, optical compensation films are used to correct phase differences in the liquid crystal layer, improve viewing angles, and enhance contrast and color consistency. Key performance indicators of optical compensation films include birefringence and wavelength dispersion characteristics. In particular, the shape of the wavelength dispersion curve of out-of-plane birefringence (Δnth) is crucial to the compensation effect and industrial controllability of the film.

[0003] Existing cellulose ester optical films (such as cellulose triacetate CTA) typically modulate their optical properties in the following ways:

[0004] Physical blending: Small molecule plasticizers or anisotropic compounds are added to control birefringence or dispersion. However, this method has the following main drawbacks:

[0005] (1) High processing sensitivity: The orientation of additives in the blend system is strongly dependent on process parameters such as stretching temperature, stretching rate and stretching ratio, which makes it difficult to accurately control the shape of the dispersion curve of the final optical film and results in poor batch consistency.

[0006] (2) Poor long-term stability: The small molecule additives and the polymer matrix only have physical interactions. Under heat, humidity or long-term stress, they are prone to migration, volatilization or phase separation, which leads to uncontrollable drift of optical performance over time, seriously affecting the reliability and service life of the display.

[0007] (3) Strong performance coupling: The birefringence value and the wavelength dispersion are interrelated, making it difficult to achieve independent and precise "programmable" design.

[0008] Even for unmodified pure triacetate cellulose optical films, the wavelength dispersion characteristics of their out-of-plane birefringence are highly dependent on the stretching process conditions, making it difficult to maintain a stable and consistent curve shape under different batches or different stretching parameters. This limits the consistency and designability of their performance in high-end display applications.

[0009] Copolymerization or chemical modification: Some literature has proposed chemical grafting of CTA. However, most of these methods focus on introducing functional groups or simply increasing the birefringence value, failing to address the core issue of the sensitivity of the out-of-plane birefringence wavelength dispersion curve shape to stretching process parameters from the perspective of molecular orientation structure design. The orientation behavior of the grafted side chains may still change unpredictably with stretching conditions, leading to unstable dispersion curve shape and failing to meet the stringent requirements of optical performance consistency and designability for high-end display applications.

[0010] Therefore, how to develop a more suitable optical thin film as an optical compensation film and solve the above-mentioned technical problems of existing optical compensation films has become one of the focuses of attention for many front-line researchers in the industry. Summary of the Invention

[0011] In view of this, the technical problem to be solved by the present invention is to provide an optical thin film and its preparation method, an optical compensation film, and a liquid crystal display device, particularly a cellulose ester optical film with stable wavelength dispersion. The present invention provides an optical thin film with high wavelength dispersion stability, insensitivity to processing, and designable performance. After uniaxial stretching, the shape of the out-of-plane birefringence wavelength dispersion curve of this film remains basically stable, without significant changes with the stretching ratio, thereby significantly improving the compensation accuracy and production consistency of the display device. Moreover, the preparation method is simple, with mild conditions and good controllability, making it more suitable for industrial production and promotion.

[0012] This invention provides an optical thin film, which is obtained by preparing a graft copolymer;

[0013] The graft copolymer comprises a matrix resin and aromatic polymer side chains grafted onto the matrix resin;

[0014] The matrix resin is a cellulose ester polymer;

[0015] The aromatic polymer side chain is covalently grafted onto the cellulose ester polymer backbone;

[0016] The dispersion stability coefficient of the optical thin film is S. Where R is the ratio of the out-of-plane birefringence of the optical thin film at wavelengths of 450 nm and 550 nm. DR1 and DR2 are any two different draw ratios;

[0017] The stretching ratio is 1.1 to 1.6.

[0018] Preferably, the cellulose ester polymer is a cellulose ester polymer with a total acyl group mass content of less than 60.5%;

[0019] The cellulose ester polymers include cellulose acetate or cellulose mixed esters;

[0020] The stretching ratio is specifically the stretching ratio of the optical thin film after unidirectional stretching and orientation.

[0021] Preferably, the total acyl group mass content of the cellulose ester polymer is 30%~55%;

[0022] The aromatic polymer is polystyrene or a substituted derivative of polystyrene;

[0023] The grafting rate of the aromatic polymer side chains is 5% to 20%.

[0024] Preferably, the grafting rate of the aromatic polymer side chains is 12% to 15%;

[0025] The cellulose ester polymers include cellulose acetate propionate;

[0026] The optical thin film is an optical thin film used for optical compensation film.

[0027] Preferably, the optical thin film has a transparency of 92% or greater at a wavelength of 550 nm;

[0028] The optical thin film has a haze of less than or equal to 0.5% at a wavelength of 550 nm.

[0029] This invention provides a method for preparing an optical thin film as described in any of the above technical solutions, comprising the following steps:

[0030] 1) A graft copolymer was obtained by mixing cellulose esters with a total acyl content of less than 60.5% with aromatic vinyl monomers and then subjecting the mixture to electron beam irradiation.

[0031] 2) After mixing the graft copolymer obtained in the above steps with the solvent, a graft copolymer solution is obtained. Then, the solution is cast onto the substrate to obtain a nascent film. After uniaxial stretching and cooling, an optical film is obtained.

[0032] Preferably, the mass ratio of the cellulose ester to the aromatic vinyl monomer is 1:(0.8~1.2).

[0033] The electron beam irradiation dose is 20~30 kGy;

[0034] The electron beam irradiation time is 2 to 8 hours;

[0035] The stretching ratio of the uniaxial stretching is 1.0~2.0.

[0036] The present invention provides an optical compensation film, wherein the optical compensation film comprises the optical thin film described in any one of the above technical solutions or the optical thin film prepared by the preparation method described in any one of the above technical solutions.

[0037] Preferably, the optical compensation film is an optical compensation film disposed in the polarizer assembly.

[0038] The present invention also provides a liquid crystal display device, characterized in that it includes an optical compensation film as described in any one of the above technical solutions.

[0039] This invention provides an optical thin film, which is obtained by preparing a graft copolymer; the graft copolymer includes a matrix resin and aromatic polymer side chains grafted onto the matrix resin; the matrix resin is a cellulose ester polymer; the aromatic polymer side chains are covalently grafted onto the cellulose ester polymer backbone; the dispersion stability coefficient of the optical thin film is S. Where R is the ratio of the out-of-plane birefringence of the optical thin film at wavelengths of 450 nm and 550 nm. DR1 and DR2 are any two different stretching ratios; the stretching ratio is 1.1~1.6. Compared with the prior art, the present invention has specially designed an optical thin film with a specific structure and composition, which is a cellulose ester optical thin film with stable wavelength dispersion characteristics.

[0040] The optical film provided by the present invention comprises a cellulose ester polymer matrix with a total acyl content of less than 60.5% and aromatic polymer side chains grafted onto its main chain via covalent bonds. After uniaxial stretching and orientation, the optical film exhibits good wavelength dispersion stability, with the ratio of its out-of-plane birefringence at wavelengths of 450 nm to 550 nm changing by no more than 3% within a stretching ratio range of 1.1-1.6.

[0041] This invention introduces aromatic side chains onto the cellulose ester backbone, achieving wavelength dispersion stability through molecular structure modulation. The prepared optical film exhibits excellent wavelength dispersion stability. This invention is the first to introduce the "dispersion stability coefficient S" into the optical film performance evaluation system. The "intramolecular orthogonal compensation" structure formed by covalently grafting aromatic side chains ensures that the shape of the out-of-plane birefringence wavelength dispersion curve of the film remains highly consistent under different stretching ratios. Compared with the existing technology that only qualitatively describes "almost no change with stretching ratio," this invention achieves quantitative control of wavelength dispersion stability for the first time, resulting in excellent consistency and predictability of the film in mass production, fundamentally solving the long-standing "processing sensitivity" technical problem of existing optical compensation films. This invention also enables independent and precise design of optical properties: the grafting rate mainly controls the shape of the dispersion curve; the stretching ratio mainly controls the absolute value of birefringence; the two are approximately decoupled, facilitating programmable design of optical properties. Moreover, it is processing-insensitive and has strong batch consistency: since the aromatic side chains are fixed to the backbone through covalent bonds, their orientation behavior is not affected by additive migration or phase separation. Furthermore, the optical thin film provided by this invention maintains excellent basic properties: while achieving the aforementioned advanced functions, the film still maintains optical-grade high transparency (>92%) and low haze (<0.5%), and its mechanical strength (storage modulus) is also improved. The preparation method provided by this invention features a green production process: electron beam irradiation-induced grafting eliminates the need for additional chemical initiators, resulting in a clean production process with no harmful residues, meeting both optical-grade and environmentally friendly production requirements.

[0042] Experimental results show that the S value of the unmodified cellulose acetate propionate film (Comparative Example 1) is approximately 2.6%; while the S value of Example 1 (grafting rate 12.5%) is <0.8%, and the S value of Example 3 (grafting rate 14.6%) can be as low as below 0.5%. This reduction of over 80% is not a simple performance optimization, but a qualitative leap achieved through molecular structure design. Detailed Implementation

[0043] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims.

[0044] There are no particular restrictions on the source of any raw materials used in this invention; they can be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0045] There are no particular restrictions on the purity of any of the raw materials used in this invention. However, this invention preferably uses analytical grade or conventional purity used in the field of optical compensation film preparation.

[0046] In this invention, the term "total acyl content" refers to the mass fraction of all acyl groups (including acetyl, propionyl, butyryl, etc.) in cellulose ester polymers, as determined by nuclear magnetic resonance (NMR) spectroscopy (H NMR). 1 Determined by ¹H NMR or chemical titration.

[0047] This invention provides an optical thin film, which is obtained by preparing a graft copolymer;

[0048] The graft copolymer comprises a matrix resin and aromatic polymer side chains grafted onto the matrix resin;

[0049] The matrix resin is a cellulose ester polymer;

[0050] The aromatic polymer side chain is covalently grafted onto the cellulose ester polymer backbone;

[0051] The dispersion stability coefficient of the optical thin film is S. Where R is the ratio of the out-of-plane birefringence of the optical thin film at wavelengths of 450 nm and 550 nm. DR1 and DR2 are any two different draw ratios;

[0052] The stretching ratio is 1.1 to 1.6.

[0053] In this invention, the stretching ratio is 1.1 to 1.6, and can be 1.2 to 1.5 or 1.3 to 1.4.

[0054] In this invention, the cellulose ester polymer is preferably a cellulose ester polymer with a total acyl mass content of less than 60.5%, which can be less than or equal to 55%, or less than or equal to 50%.

[0055] In this invention, the cellulose ester polymer preferably includes cellulose acetate or cellulose mixed ester.

[0056] In this invention, the stretch ratio is preferably the stretch ratio of the optical thin film after unidirectional stretching and orientation.

[0057] In this invention, the total acyl mass content of the cellulose ester polymer can be 30%~55%, or 35%~50%, or 40%~45%.

[0058] In this invention, the aromatic polymer is preferably polystyrene or a substituted derivative of polystyrene.

[0059] In this invention, the grafting rate of the aromatic polymer side chain can be 5%~20%, 8%~17%, or 11%~14%.

[0060] In this invention, the grafting rate of the aromatic polymer side chain can be 12%~15%, 12.5%~14.5%, or 13%~14%.

[0061] In this invention, the cellulose ester polymer preferably includes cellulose acetate propionate.

[0062] In this invention, the optical thin film is preferably an optical thin film used for optical compensation film.

[0063] In this invention, the transparency of the optical thin film at a wavelength of 550 nm can be greater than or equal to 92%, greater than or equal to 93%, or greater than or equal to 94%.

[0064] In this invention, the haze of the optical thin film at a wavelength of 550 nm can be less than or equal to 0.5%, less than or equal to 0.45%, or less than or equal to 0.4%.

[0065] This invention provides a method for preparing an optical thin film as described in any of the above technical solutions, comprising the following steps:

[0066] 1) A graft copolymer was obtained by mixing cellulose esters with a total acyl content of less than 60.5% with aromatic vinyl monomers and then subjecting the mixture to electron beam irradiation.

[0067] 2) After mixing the graft copolymer obtained in the above steps with the solvent, a graft copolymer solution is obtained. Then, the solution is cast onto the substrate to obtain a nascent film. After uniaxial stretching and cooling, an optical film is obtained.

[0068] The present invention first mixes cellulose esters with a total acyl content of less than 60.5% with aromatic vinyl monomers, and then irradiates them with an electron beam to obtain graft copolymers.

[0069] In this invention, the mass ratio of the cellulose ester to the aromatic vinyl monomer can be 1:(0.8~1.2) or 1:(0.9~1.1). Specifically, it can be 1:1.

[0070] In this invention, the dose of electron beam irradiation can be 20~30 kGy, 22~28 kGy, or 24~26 kGy.

[0071] In this invention, the electron beam irradiation time can be 2-8 hours, 3-7 hours, or 4-6 hours;

[0072] Finally, the graft copolymer obtained in the above steps is mixed with a solvent to obtain a graft copolymer solution, which is then cast onto a substrate by solution casting to obtain a nascent film. After uniaxial stretching and cooling, an optical film is obtained.

[0073] In this invention, the stretching ratio of the unidirectional stretching can be 1.0~2.0, 1.2~1.8, or 1.4~1.6.

[0074] The present invention provides an optical compensation film, wherein the optical compensation film comprises the optical thin film described in any one of the above technical solutions or the optical thin film prepared by the preparation method described in any one of the above technical solutions.

[0075] In this invention, the optical compensation film is preferably an optical compensation film disposed in a polarizer assembly.

[0076] The present invention provides a liquid crystal display device comprising an optical compensation film as described in any one of the above technical solutions.

[0077] This invention aims to complete and refine the overall technical solution, better ensure the composition and structure of optical thin films, and further improve the wavelength dispersion stability and overall performance of optical thin films. Specifically, the aforementioned cellulose ester optical film with stable wavelength dispersion, its preparation method, optical compensation film, and liquid crystal display device may include the following:

[0078] An optical thin film includes a matrix resin and aromatic polymer side chains grafted onto the matrix resin, wherein the matrix resin is a cellulose ester polymer; the aromatic polymer side chains are covalently grafted onto the cellulose ester polymer backbone; after uniaxial stretching and orientation, the optical thin film exhibits an out-of-plane birefringence ratio at 450 nm to 550 nm within a stretching ratio range of 1.1-1.6. The changes satisfy: , where DR1 and DR2 are any two different stretch ratios.

[0079] Specifically, the cellulose ester polymer is a cellulose ester with a total acyl content of less than 60.5%.

[0080] Specifically, the aromatic polymer side chain is polystyrene or a substituted derivative thereof.

[0081] Specifically, the grafting rate of the aromatic polymer side chains is 5% to 20%.

[0082] Specifically, the grafting rate is 12% to 15%.

[0083] Specifically, the transparency at a wavelength of 550 nm is greater than 92%, and the haze is less than 0.5%.

[0084] Furthermore, the aromatic vinyl monomer is styrene or a substitute thereof;

[0085] Furthermore, the grafting rate of the aromatic polymer side chains is 5% to 20%, preferably 12% to 15%.

[0086] Furthermore, the film exhibits high transparency (>92%) and low haze (<0.5%) at 550 nm.

[0087] The present invention provides a method for preparing an optical thin film as described in any of the above technical solutions, comprising the following steps: S1, mixing a cellulose ester with a total acyl content of less than 60.5% with an aromatic vinyl monomer, and then subjecting it to electron beam co-irradiation to graft the aromatic vinyl monomer onto the cellulose ester backbone to obtain a graft copolymer;

[0088] S2. Dissolve the graft copolymer in a solvent to obtain a graft copolymer solution;

[0089] S3. The graft copolymer solution is used to form a nascent film by solution casting.

[0090] S4. The nascent film is uniaxially stretched above its glass transition temperature, and then cooled and shaped to obtain the optical film.

[0091] Specifically, according to the method of claim 8, the electron beam irradiation dose in step S1 is 20 to 30 kGy.

[0092] Specifically, the uniaxial stretching ratio of the nascent film in step S4 is 1.0 to 2.0.

[0093] Specifically, when the stretch ratio is in the range of 1.1-1.6, the wavelength dispersion stability described in the above technical solution is preferably obtained.

[0094] Furthermore, the method for preparing the optical thin film provided by the present invention may include the following steps:

[0095] (1) Electron beam irradiation initiation: Cellulose esters with a total acyl content of less than 60.5% are mixed with aromatic vinyl monomers and then subjected to electron beam co-irradiation to graft the aromatic vinyl monomers onto the cellulose ester backbone, thereby obtaining a graft copolymer. The co-irradiation dose can be 20 to 30 kGy.

[0096] (2) Solution casting to form a film: The graft copolymer is dissolved in a solvent and cast into a nascent film, which is then dried in a controlled manner to obtain a transparent film.

[0097] (3) Uniaxial stretching and shaping: Uniaxial stretching is performed above the glass transition temperature (Tg) of the thin film, and the stretching ratio can be 1.0 to 2.0. The orientation structure is fixed by cooling and shaping (such as liquid nitrogen quenching).

[0098] The synergistic effect of the above steps in this invention enables the thin film to obtain a stable molecular orientation structure, thereby achieving shape stability of the wavelength dispersion curve.

[0099] The present invention provides an optical compensation film comprising an optical thin film as described in any of the above technical solutions.

[0100] Specifically, the optical compensation film is disposed in the polarizer assembly.

[0101] The present invention also provides a liquid crystal display device comprising the optical compensation film as described in the above technical solution.

[0102] The optical thin film with stable wavelength dispersion provided by the present invention is a graft copolymer of cellulose ester main chain with aromatic vinyl monomer side chains with a total acyl content of less than 60.5%.

[0103] The core of this invention lies in the following: a cellulose ester with a total acyl content of less than 60.5%; aromatic side chains containing conjugated structures such as benzene rings; during uniaxial stretching, the cellulose ester backbone is oriented along the stretching direction; the benzene ring planes of the aromatic side chains are statistically positioned substantially perpendicular to the stretching direction; this "intramolecular orthogonal" structure ensures that the shape of the out-of-plane birefringence wavelength dispersion curve remains highly consistent under different stretching ratios. This molecular orientation characteristic is achieved through the inherent covalent bond constraint of the grafted structure, distinguishing it from the unstable arrangement of physically blended systems.

[0104] This invention is the first to propose using the dispersion stability coefficient S as a quantitative index for evaluating the processing stability of optical thin films, which is defined as follows: ,in Experimental data show that the S value of the film of the present invention can be controlled within 3% (preferred to be as low as 0.5%), which is significantly better than that of unmodified cellulose ester film (S≈2.6%). Compared with the prior art, which only qualitatively describes that the wavelength dispersion stability is "almost unchanged with the stretch ratio", the present invention achieves quantitative control and significant improvement of wavelength dispersion stability.

[0105] The present invention provides a cellulose ester optical thin film with stable wavelength dispersion and its preparation method, an optical compensation film, and a liquid crystal display device. The optical thin film with a specific structure and composition designed in this invention comprises a cellulose ester polymer matrix with a total acyl content of less than 60.5% and aromatic polymer side chains covalently grafted onto its main chain. After uniaxial stretching and orientation, within a stretching ratio range of 1.1-1.6, the ratio of its out-of-plane birefringence at wavelengths of 450 nm to 550 nm does not change by more than 3% (i.e., the dispersion stability coefficient S≤3%), exhibiting excellent wavelength dispersion stability.

[0106] This invention introduces aromatic side chains onto the cellulose ester backbone, achieving wavelength dispersion stability through molecular structure modulation. The prepared optical film exhibits excellent wavelength dispersion stability. This invention is the first to introduce the "dispersion stability coefficient S" into the optical film performance evaluation system. The "intramolecular orthogonal compensation" structure formed by covalently grafting aromatic side chains ensures that the shape of the out-of-plane birefringence wavelength dispersion curve of the film remains highly consistent under different stretching ratios. Compared with the existing technology that only qualitatively describes "almost no change with stretching ratio," this invention achieves quantitative control of wavelength dispersion stability for the first time, resulting in excellent consistency and predictability of the film in mass production, fundamentally solving the long-standing "processing sensitivity" technical problem of existing optical compensation films. This invention also enables independent and precise design of optical properties: the grafting rate mainly controls the shape of the dispersion curve; the stretching ratio mainly controls the absolute value of birefringence; the two are approximately decoupled, facilitating programmable design of optical properties. Moreover, it is processing-insensitive and has strong batch consistency: since the aromatic side chains are fixed to the backbone through covalent bonds, their orientation behavior is not affected by additive migration or phase separation. Furthermore, the optical thin film provided by this invention maintains excellent basic properties: while achieving the aforementioned advanced functions, the film still maintains optical-grade high transparency (>92%) and low haze (<0.5%), and its mechanical strength (storage modulus) is also improved. The preparation method provided by this invention features a green production process: electron beam irradiation-induced grafting eliminates the need for additional chemical initiators, resulting in a clean production process with no harmful residues, meeting both optical-grade and environmentally friendly production requirements.

[0107] Experimental results show that the S value of the unmodified cellulose acetate propionate film (Comparative Example 1) is approximately 2.6%; while the S value of Example 1 (grafting rate 12.5%) is <0.8%, and the S value of Example 3 (grafting rate 14.6%) can be as low as below 0.5%. This reduction of over 80% is not a simple performance optimization, but a qualitative leap achieved through molecular structure design.

[0108] To further illustrate the present invention, the following detailed description of an optical thin film and its preparation method, an optical compensation film, and a liquid crystal display device provided by the present invention is provided in conjunction with embodiments. However, it should be understood that these embodiments are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and specific operation processes are given only to further illustrate the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention. The scope of protection of the present invention is not limited to the following embodiments.

[0109] Example 1

[0110] Cellulose acetate propionate (CAP, total acyl content approximately 48%) was mixed with purified styrene monomer at a mass ratio of 1:1 and then subjected to co-irradiation with an electron beam of 22 kGy to graft styrene onto the CAP backbone. Residual monomers were removed by washing with DMF, and the mixture was vacuum dried to constant weight. The calculated grafting percentage (G%) was 12.5%. The graft copolymer was dissolved in dichloromethane to prepare a 5 wt% solution, which was cast onto a glass plate and dried using a programmed temperature increase sequence of 25 °C / 40 min, 60 °C / 90 min, and 80 °C / 30 min to obtain the nascent film. The film had a transparency of 93.12% and a haze of 0.21%.

[0111] The film's Tg was determined to be approximately 198°C using DMA testing, and the stretching temperature (Ts) was set at 208°C. After cutting the film sample, it was preheated at 208°C for 10 min, then unidirectionally stretched at a rate of 0.1 mm / s to a draw ratio (DR) of 1.3, and subsequently quenched with liquid nitrogen.

[0112] Performance characterization: Out-of-plane birefringence (Δn) of the obtained thin film th The shape of the wavelength dispersion curve remains highly consistent under different stretching ratios. Specific values ​​are as follows: at a stretching ratio DR = 1.3, the out-of-plane birefringence Δn at a wavelength of 550 nm... th Δn is 0.00185 at 450 nm. th The value is 0.00242, i.e., R = Δn. th (450) / Δn th (550) = 1.31; when DR = 1.5, Δn th (550) increased to 0.00208, Δn th (450) is 0.00270, that is, R=1.30. Substituting into the formula, the dispersion stability coefficient (S) is calculated to be 0.76%, that is, the dispersion stability coefficient S is less than 0.8%.

[0113] The dispersion stability coefficient (S) is used to quantify the stability of the shape of the wavelength dispersion curve, and it is defined as follows: , where R=Δn th (450) / Δn th (550), DR1 and DR2 represent two different drawing ratios (DR1=1.3 and DR2=1.5 in this embodiment). The smaller the S value, the less the curve shape is affected by the drawing process.

[0114] Example 2

[0115] Cellulose acetate propionate (CAP, total acyl content approximately 48%) was mixed with purified styrene monomer at a mass ratio of 1:1 and then subjected to co-irradiation with an electron beam dose of 25 kGy. The grafting rate was measured to be 13.8%, the film transparency to be 93.05%, and the haze to be 0.25%. Under the same conditions, the film was stretched to DR=1.5 and DR=1.3.

[0116] Performance characterization: The thin film exhibits higher out-of-plane birefringence and better stability. At DR=1.5, Δn th (550) is 0.00221, Δn th (450) is 0.00285, R=1.29; when DR drops to 1.3, Δn th (550) is 0.00198, Δn th (450) is 0.00257, R=1.30. The dispersive stability coefficient S≈0.8% is calculated.

[0117] Compared with Comparative Example 1, the Δn of the thin film in this embodiment th The shape of the dispersion curve changes very little under different stretching ratios, indicating that its wavelength dispersion characteristics are highly decoupled from the stretching process.

[0118] Example 3

[0119] Cellulose acetate butyrate (CAB, total acyl content approximately 52%) was mixed with purified styrene monomer at a mass ratio of 1:1 and then subjected to co-irradiation with an electron beam dose of 28 kGy. The grafting rate was measured to be 14.6%, the film transparency to be 92.98%, and the haze to be 0.28%. Under the same conditions, the film was stretched to DR=1.5 and DR=1.3.

[0120] Performance Characterization: The thin film exhibits optimal out-of-plane birefringence stability and a high absolute value. At DR=1.5, Δn th (550) reaches 0.00235, Δn th (450) is 0.00301, R=1.28; when DR=1.3, Δn th (550) is 0.00211, Δn th (450) is 0.00270, R=1.28. The dispersion stability coefficient S is less than 0.5%. The stability of optical performance is more outstanding.

[0121] Comparative Example 1

[0122] Using unirradiated pure cellulose acetate propionate (CAP, total acyl content approximately 48%) as raw material, without the addition of styrene, the film was formed using the same solution casting process. The film transparency was 92.60%, and the haze was 0.22%. At the same temperature (based on its own T... g (Calculated at +10℃) Tension is performed.

[0123] Performance characterization: Δn of pure cellulose acetate propionate membrane th The shape of the wavelength dispersion curve changes significantly and irregularly with different stretching ratios. For example, when DR=1.3, Δn th (550) = 0.00092, Δn th (450) = 0.00107, Δn th (450) / Δn th (550) = 1.16; when DR = 1.5, Δn th (550) = 0.00118, Δn th (450) = 0.00133 but Δn th (450) / Δn th The ratio (550) becomes 1.13, and its dispersion stability coefficient S≈2.6% is calculated.

[0124] Comparative Example 2

[0125] Cellulose triacetate (CTA, total acyl content approximately 60.5%) was used instead of cellulose acetate propionate, under the same conditions as in Example 1: it was mixed with purified styrene monomer at a 1:1 mass ratio and then subjected to co-irradiation with an electron beam dose of 22 kGy, resulting in a grafting rate of 12.5%. The film had a transparency of 93.60% and a haze of 0.18%. Stretching was performed at the same temperature (calculated using its own Tg + 10°C). All other film preparation and stretching conditions were identical to those in Example 1.

[0126] Performance characterization: Testing showed that the film exhibited Δn at DR=1.3. th (550) = 0.00155, Δn th (450) = 0.00195, R = 1.26; when DR = 1.5, Δn th (550) = 0.00172, Δn th (450) = 0.00220, R = 1.28. The calculated S ≈ 1.6%, which is significantly higher than S < 0.8% in Example 1.

[0127] The results show that even with the same co-irradiation grafting process and the same grafting rate (12.5%), CTA with a total acyl content of 60.5% cannot achieve ultra-high stability with S < 0.8%, proving the necessity of "total acyl content below 60.5%" in this invention.

[0128] As can be seen from the above examples and comparative examples, the S-value of the pure CAP film in Comparative Example 1 is approximately 2.6%, while the S-value of Example 1 of the present invention (grafting rate 12.5%) has dropped to below 0.76%, a decrease of more than 70%. This non-linear performance leap indicates that grafting aromatic side chains is not a simple performance fine-tuning, but rather fundamentally changes the optical response behavior of the film through an "intramolecular orthogonal compensation" structure. Those skilled in the art could not foresee or deduce this quantitative and significantly improved stability effect from the qualitative descriptions of the prior art (such as "almost unaffected by DR"). As the grafting rate increases from 12.5% ​​to 14.6%, the S-value further decreases to below 0.5%, demonstrating a clear correlation between the grafting rate and dispersion stability, a pattern not revealed in the prior art.

[0129] A comprehensive analysis of the data from Examples 1-3 and Comparative Example 1 reveals the following: First, the grafted films prepared by the method of this invention (Examples 1-3) all exhibit significantly lower dispersion stability coefficients (S) than the pure CAP film (Comparative Example 1, S≈2.6%). Furthermore, as the grafting ratio increases, the S value further decreases from 0.76% to below 0.5%, confirming the beneficial effect of "a specific grafting ratio (12%~15%) primarily controlling the shape (stability) of the dispersion curve." Second, under the same grafting ratio, the out-of-plane birefringence value (Δn) is significantly lower. th The growth with increasing stretch ratio is regular and predictable (e.g., in Example 1, DR increases from 1.3 to 1.5, Δn). th (550) increased from 0.00185 to 0.00208), which confirms another effect of "stretch ratio mainly controlling the absolute value of birefringence". This invention successfully achieves the approximate decoupling and independent design of key optical performance parameters.

[0130] Molecular orientation and mechanism analysis

[0131] Infrared dichroism results show that after stretching, the cellulose ester backbone aligns along the stretching direction (x-axis). The grafted PS side chains tend to align perpendicular to the backbone direction, resulting in the PS benzene ring plane being perpendicular to the film plane (i.e., the xy-plane). The high polarizability axes within the benzene ring plane are therefore aligned along the y-axis, affecting the refractive index n. y This produces a strong positive contribution. This contribution compensates for the negative optical effects generated by the cellulose ester backbone and the acyl groups themselves, ultimately achieving Δn. th The size and wavelength dispersion are stabilized and regulated. This "intramolecular orthogonal compensation" mechanism is unique to covalently grafted structures.

[0132] In summary, this invention effectively alters the optical response behavior of pure cellulose ester films by constructing an "orthogonal compensation" structure at the molecular scale through covalent grafting, significantly improving wavelength dispersion stability, thereby solving the key problem in the background art where optical properties are sensitive to processing conditions.

[0133] Industrial Applications

[0134] The cellulose ester optical film with stable wavelength dispersion prepared by this invention can be directly used as an optical compensation film in the polarizer assembly of liquid crystal display panels, or combined with other optical films to form a composite compensation film. Its stable dispersion performance ensures the consistency of color and contrast of the display at different viewing angles, making it particularly suitable for high-end televisions, monitors, laptops, mobile phones, and other fields with stringent display quality requirements.

[0135] To more clearly and comprehensively demonstrate the technical effects of the embodiments of the present invention, key performance parameters are summarized in Table 1. Table 1 is a basic performance table of the thin films prepared in the embodiments and comparative examples of the present invention. Among them, out-of-plane birefringence (Δn) th ) and in-plane birefringence (Δn) in All values ​​refer to measurements taken at a wavelength of 550 nm; the calculation method for the dispersion stability coefficient (S) is as described above.

[0136] Table 1

[0137]

[0138] The foregoing has provided a detailed description of a cellulose ester optical film with stable wavelength dispersion, its preparation method, and an optical compensation film provided by the present invention. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are merely for the purpose of helping to understand the method and core ideas of the present invention, including the best mode, and also to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements that are not different from the textual description of the claims, or if they include equivalent structural elements that are not substantially different from the textual description of the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. An optical thin film, characterized in that, The optical thin film is obtained by preparing a graft copolymer; The graft copolymer comprises a matrix resin and aromatic polymer side chains grafted onto the matrix resin; The matrix resin is a cellulose ester polymer; The aromatic polymer side chain is covalently grafted onto the cellulose ester polymer backbone; The dispersion stability coefficient of the optical thin film is S. Where R is the ratio of the out-of-plane birefringence of the optical thin film at wavelengths of 450 nm and 550 nm. DR1 and DR2 are any two different draw ratios; The stretching ratio is 1.1 to 1.

6.

2. The optical thin film according to claim 1, characterized in that, The cellulose ester polymer is a cellulose ester polymer with a total acyl group mass content of less than 60.5%; The cellulose ester polymers include cellulose acetate or cellulose mixed esters; The stretching ratio is specifically the stretching ratio of the optical thin film after unidirectional stretching and orientation.

3. The optical thin film according to claim 1, characterized in that, The total acyl group mass content of the cellulose ester polymer is 30%~55%; The aromatic polymer is polystyrene or a substituted derivative of polystyrene; The grafting rate of the aromatic polymer side chains is 5% to 20%.

4. The optical thin film according to claim 1, characterized in that, The grafting rate of the aromatic polymer side chains is 12%~15%; The cellulose ester polymers include cellulose acetate propionate; The optical thin film is an optical thin film used for optical compensation film.

5. The optical thin film according to claim 1, characterized in that, The optical thin film has a transparency of 92% or greater at a wavelength of 550 nm. The optical thin film has a haze of less than or equal to 0.5% at a wavelength of 550 nm.

6. A method for preparing an optical thin film as described in any one of claims 1 to 5, characterized in that, Includes the following steps: 1) A graft copolymer was obtained by mixing cellulose esters with a total acyl content of less than 60.5% with aromatic vinyl monomers and then subjecting the mixture to electron beam irradiation. 2) After mixing the graft copolymer obtained in the above steps with the solvent, a graft copolymer solution is obtained. Then, the solution is cast onto the substrate to obtain a nascent film. After uniaxial stretching and cooling, an optical film is obtained.

7. The preparation method according to claim 6, characterized in that, The mass ratio of the cellulose ester to the aromatic vinyl monomer is 1:(0.8~1.2). The electron beam irradiation dose is 20~30 kGy; The electron beam irradiation time is 2 to 8 hours; The stretching ratio of the uniaxial stretching is 1.0~2.

0.

8. An optical compensation film, characterized in that, The optical compensation film comprises the optical thin film according to any one of claims 1 to 5 or the optical thin film prepared by the preparation method according to any one of claims 6 to 7.

9. The optical compensation film according to claim 8, characterized in that, The optical compensation film is an optical compensation film disposed in the polarizer assembly.

10. A liquid crystal display device, characterized in that, It includes the optical compensation film as described in any one of claims 8 to 9.

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