Construction method and evaluation method for evaluating and regulating optical performance of cellulose acetate film

Abstract

The invention discloses a construction method and an evaluation method for evaluating and regulating the optical performance of a cellulose acetate film. The construction method comprises the following steps: S1, molecular modeling: carrying out molecular modeling on an acetyl substituted monomer of cellulose acetate; s2, obtaining components alpha xx, alpha yy and alpha zz of polarizability in different coordinate directions corresponding to different wavelengths; s3, constructing a function relationship between the intrinsic birefringence delta n0 (monomer) of a certain monomer of the cellulose acetate monomer and the intrinsic birefringence delta n0i of a certain conformational isomer of the monomer; s4, respectively solving the intrinsic birefringence contributions of 2, 3 and 6 acetyl groups; s5, obtaining substitution degree distribution of 2, 3 and 6 bits; and S6, constructing a CA birefringence function relationship, and solving theoretical birefringence. According to the method, a logical relationship is established between the substitution degree of hydroxyl and acetyl in each monomer conformation, the conformation ratio and the birefringence, a theoretical value that the birefringence of the cellulose acetate is closer to an actual value is obtained, and an effective method is provided for accurate prediction and regulation of optical performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of bio-based optical materials, in particular to a method for constructing and evaluating the optical performance of cellulose acetate film and an evaluation method. BACKGROUND

[0002] Cellulose acetate (CA) film is often used as a protective film for polarizing plates or an optical compensation (retardation) film due to its high smoothness, excellent optical stability and good dimensional stability. In optical applications, the control of birefringence and its dispersion is a key and difficult point in material design. For example, a protective film for polarizing plates is expected to exhibit zero-zero birefringence in any direction or elastic deformation, while an optical retardation film is expected to provide a wideband birefringence dispersion, i.e., the absolute value of birefringence increases with the increase of the wavelength of light in the visible light range. However, the industrial CA is usually obtained by heterogeneous esterification of cellulose and acetic anhydride in the presence of sulfuric acid and further hydrolysis, and is a mixture of various substitution products. The relationship between the birefringence and the product composition, molecular structure is relatively complex. Therefore, it is of great theoretical significance and practical value to clarify the relationship between the composition and structure of cellulose ester and the optical properties for designing and synthesizing optical films with ideal birefringence and birefringence dispersion.

[0003] Birefringence is a manifestation of the anisotropy of the refractive index of a polymer film, which is derived from the orientation of the molecular chain. In addition, birefringence also exhibits wavelength dependence, i.e., the absolute value of birefringence increases (wideband birefringence dispersion) or decreases (ordinary birefringence dispersion) with the increase of the wavelength of incident light. When a polymer film is subjected to uniaxial stretching, the polymer molecular chains are arranged along the stretching direction, resulting in a difference in refractive index between the stretching direction and the vertical direction, which is called orientation birefringence. The orientation birefringence can be defined by the following formula:

[0004] Δn = f · Δn 0

[0005] In the formula, Δn 0 and f represent intrinsic birefringence and orientation function, respectively, the former is related to the chemical structure of the repeating unit, and the latter depends on the orientation state of the polymer chain.

[0006] Previous studies have shown that the degree of substitution, the distribution of substituents and the composition of substituted monomers are key factors affecting the birefringence characteristics of cellulose ester. However, these studies emphasize the important role of acetyl group in the birefringence characteristics of cellulose acetate, but lack a systematic evaluation method for the relationship between the acetyl group conformation, acetyl substitution and the birefringence and its dispersion. SUMMARY

[0007] To solve the above problems, the application aims to provide a construction method and evaluation method for evaluating and regulating the optical performance of cellulose acetate film, which can accurately evaluate the birefringence and birefringence dispersion characteristics of CA film, and provide a new approach and means for the design and processing of CA film with ideal optical performance. Cellulose acetate is a mixture of various substitution products, and the substitution and conformation are extremely complex. The accurate prediction and regulation of the optical birefringence and birefringence dispersion are the key points and difficulties in material design and application. The application establishes a logical relationship between the hydroxyl and acetyl substitution degree, conformation proportion and birefringence in each monomer conformation through the construction of key steps after molecular modeling, so as to obtain the theoretical value of the birefringence of cellulose acetate which is close to the actual value, and provide an effective method for accurate prediction and regulation of the optical performance of the material.

[0008] The application is realized by the following technical scheme:

[0009] A construction method for evaluating and regulating the optical performance of cellulose acetate film, comprising the following steps:

[0010] S1, molecular modeling: molecular modeling is performed on the acetyl substitution monomer of cellulose acetate;

[0011] S2, based on the molecular modeling, the stable conformation structure is obtained, the polarizability calculation of the stable conformation structure is performed, and the components of the polarizability in different coordinate directions under different wavelengths are obtained xx 、 yy and zz ;

[0012] S3, the function relationship between the intrinsic birefringence Δn 0 (monomer) of a certain monomer of cellulose acetate monomer and the intrinsic birefringence Δn 0 of a certain conformation isomer of the monomer is constructed i ;

[0013] Wherein F i is the distribution ratio of the stable conformation of the monomer to the total conformation of the monomer, ΔE i is the energy difference between the i conformation and the lowest energy conformation, R is the molar gas constant, T is the thermodynamic temperature, and e is the natural constant;

[0014] S4, assuming that the intrinsic birefringence of cellulose mainly comes from the 2, 3 and 6 hydroxyl groups, and the intrinsic birefringence contribution of the 2, 3 and 6 hydroxyl groups is equal, then the intrinsic birefringence contribution Δn 0(OH) can be regarded as the average value of the intrinsic birefringence of crystalline cellulose; the acetyl intrinsic birefringence of the substitution points at 2, 3 and 6 of the monomer is unique respectively, and the relationship between the intrinsic birefringence of the CDA of the disubstituted monomer and the contribution of the hydroxyl intrinsic birefringence and the contribution of the side chain substituent group, i.e. the acetyl intrinsic birefringence, is as follows:

[0015] Δn 0 (CDA23) = Δn 0 (2OAc) + Δn 0 (3OAc) + Δn 0 (OH) # (6)

[0016] Δn 0 (CDA26) = Δn 0 (2OAc) + Δn 0 (6OAc) + Δn 0 (OH) # (7)

[0017] Δn 0 (CDA36) = Δn 0 (3OAc) + Δn 0 (6OAc) + Δn 0 (OH) # (8)

[0018] Solving the equations 6-8, the intrinsic birefringence contributions Δn 0 (2OAc), Δn 0 (3OAc) and Δn 0 (6OAc) of the acetyl groups at 2, 3 and 6 are obtained.

[0019] S5, the nuclear magnetic resonance scanning of the cellulose acetate is performed, and the total substitution degree DS of the cellulose acetate is obtained by analyzing the obtained nuclear magnetic resonance graph, then after peak fitting and peak area normalization processing, the CA monomer molar number is obtained:

[0020] m2 = P O＝C2-2 × DS # (9)

[0021] m3 = P O＝C3-3 × DS # (10)

[0022] m6 = P O＝C6-6 × DS # (11)

[0023] m 23 = (P O＝C2-23 + P O＝C3-23 ) / 2 × DS # (12)

[0024] m 26 = (P O＝C2-26 + P O＝C6-26 ) / 2 × DS # (13)

[0025] m 36 =(P O＝C3-36 +P O＝C6-36 ) / 2×DS#(14)

[0026] m 236 =(P O＝C2-236 +P O＝C3-236 +P O＝C6-236 ) / 3×DS#(15)

[0027] In formulas 9-15, P represents the area ratio of different carbonyl peaks, and subscripts O＝C2-2 , O＝C2-23 , O＝C2-236 , etc. refer to the types of carbonyl peaks, such as P O＝C2-23 represents the area ratio of the carbonyl peak at the C2 position in the 2, 3-substituted CDA23 monomer; m 23 represents the molar number of the 2, 3-substituted CDA23 monomer, and DS is the total degree of substitution; based on this, the degrees of substitution at positions 2, 3, and 6 are obtained by formulas 16-18, respectively:

[0028] DS2=m2+m 26 +m 23 +m 236 #(16)

[0029] DS3=m3+m 23 +m 36 +m 236 #(17)

[0030] DS6=m6+m 26 +m 36 +m 236 #(18) S6, constructing a CA birefringence function relationship: substituting the intrinsic birefringence contribution values of the acetyl groups at positions 2, 3, and 6 and the degrees of substitution of the acetyl groups at different sites at positions 2, 3, and 6 into the formulas, we obtain:

[0031] Δn(theoretical)=fΔn 0 (theoretical)=f·[Δn 0 (OAc)+Δn 0 (OH)]=f·[DS2·Δn 0 (2OAc)+DS3·Δn 0 (3OAc)+DS6·Δn 0 (6OAc)+(3-DS)·Δn 0 (OH)]#(19)

[0032] Assuming that the CA film is a completely oriented film, i.e., the orientation function f = 1, then Δn(theoretical)=Δn 0 (theoretical).

[0033] In the S1 molecular modeling, the conformational isomers formed by the rotation of C5-C6 bond are represented by uv, vu and uu respectively, and the corresponding torsion angle χ5 ranges from 10-50°, 130-170° and 250-290° respectively, and the conformational isomers formed by the rotation of C6-O6 bond are represented by U+, V and U- respectively, and the corresponding torsion angle χ6 ranges from 10-50°, 130-170° and 250-290° respectively. In the present application, the selection of the torsion angle has an important influence on the theoretical birefringence value of cellulose acetate, and it is not possible to obtain accurate theoretical values by randomly selecting the torsion angle, and the selection of the torsion angle has an important influence on the polarizability value, combined with the assumption of the characteristic birefringence of p-hydroxybenzoic acid in the present technology, the conformational proportion, and the logical construction of the characteristic birefringence of acetic acid and the proportion of degree of substitution, the present application can obtain accurate theoretical birefringence value.

[0034] In S2 and S3, the intrinsic birefringence Δn of a certain conformational isomer of CA monomer is obtained by Lorentz-Lorenz equation (1) and formula (3) i 0 , and then the birefringence Δn of a certain conformational isomer of CA monomer can be obtained by formula (2) i :

[0035]

[0036] Δα=α || -α ⊥ =α zz -(α xx +α yy ) / 2#(3)

[0037] In formulas 1, 2 and 3, ρ is the density, the densities of CTA, CDA and CMA are 1.354 g / cm 3 , 1.156 g / cm 3 , 0.959 g / cm 3 , N A is the Avogadro constant, and M is the molecular weight of the monomer, <n>is the average refractive index (obtained by formula 1), is the average polarizability; in formula (2), is the orientation function, describing the orientation state of the chain in the film, f = 1 for a fully oriented film; a xx , a yy and a zz are the components of the polarizability in different coordinate directions, a || and a || are the polarizabilities parallel and perpendicular to the chain axis, respectively, a ┴ The difference between a and a, that is, the polarizability anisotropy, is represented by Δa.

[0038] An evaluation method for the optical performance of cellulose acetate film, the cellulose acetate film to be evaluated is subjected to the steps as described above to calculate the intrinsic birefringence of cellulose acetate, and Δn (theoretical) is calculated; the cellulose acetate film to be evaluated is measured by an ellipsometer in transmission mode to measure the phase difference delta of the film, and then the measured birefringence Δn (measured) of the film is calculated according to the following formula:

[0039]

[0040] Where delta is the phase difference of the film, λ is the wavelength of the incident light, and d is the thickness of the film; comparison of Δn (theoretical) and Δn (measured) can be evaluated.

[0041] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0042] 1. Reasonable molecular modeling setting and density functional theory (DFT) calculation have important influence on the consistency of the theoretical and measured values of birefringence.

[0043] 2. The selection and reasonable assumption of the twist angle to obtain the birefringence contribution of hydroxyl group play an important role in solving the intrinsic birefringence contribution of acetyl groups at positions 2, 3 and 6, and lay a foundation for accurate prediction of the optical performance of CA film.

[0044] 3. The calculation of the intrinsic birefringence contribution of acetyl groups at different substitution sites and the substitution degree distribution is the key to obtain the theoretical value of birefringence and the birefringence dispersion curve, which provides a new way and means for the design and regulation of the optical performance of CA film. BRIEF DESCRIPTION OF DRAWINGS

[0045] The drawings described herein are used to provide further understanding of the embodiments of the present application, constitute a part of the present application, and do not constitute a limitation on the embodiments of the present application. In the drawings:

[0046] Figure 1 is a structural model diagram of seven kinds of substitution monomers of cellulose acetate.

[0047] Figure 2 Figure 1 is a molecular structure model of cellulose acetate and the torsion angle diagram.

[0048] Figure 3 Figure 2 is a diagram of nine conformational isomers of cellulose acetate. DETAILED DESCRIPTION

[0049] In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description of the present application is given below in combination with examples and drawings, the illustrative embodiments of the present application and the description thereof are only used to explain the present application, and do not limit the present application.

[0050] (1) Substituted monomer molecule modeling

[0051] Cellulose acetate (CA) is a product in which the hydroxyl groups at positions 2, 3 and 6 of a dehydrated glucose unit in a cellulose molecule are substituted by acetyl groups. There are 8 substitution results for the 3 free hydroxyl groups. In addition to zero substitution, i.e. cellulose itself, there are 2-monosubstitution (CMA2), 3-monosubstitution (CMA3), 6-monosubstitution (CMA6), 2,3-disubstitution (CDA23), 2,6-disubstitution (CDA26), 3,6-disubstitution (CDA36) and 2,3,6-trisubstitution (CTA236) in total 7 substitution products. In the modeling process, hydrogen atoms (C1-H) and methoxyl groups (C4-OCH3) are used to saturate the terminal carbon atoms of the dehydrated glucose unit, respectively. The structures of the 7 substituted monomers of CA are shown in Figure 1. Figure 1

[0052] In addition to acetyl substitution, the rotation angle of the bond in the ester group also affects the intrinsic birefringence. The torsion angles χ and θ are defined as Figure 2 ​The stable conformations of C2 and C3 acetyl groups in CA monomers are consistent with the stable conformation of CTAI crystal structure, so there is only one stable conformation for each of CMA2, CMA3 and CDA23. Here, the effect of the acetyl group conformation change at C6 position on the intrinsic birefringence of CA monomers is mainly considered. According to the report of Sikorski et al. on the CTAI crystal structure, the torsion angles are set as χ2(H2-C2-O2-CA2) = 27.6°, θ2(C2-O2-CA2-CM2) = 176.4°, χ3(H3-C3-O3-CA3) = 19.6° and θ3(C3-O3-CA3-CM3) = 170.4°, respectively. The θ6(C6-O6-CA6-CM6) remains 180° in the stable conformation, and χ5(O5-C5-C6-O6) and χ6(C5-C6-O6-CA6) are the key factors for the conformation change. Taking CTA236 as an example, the conformation isomers formed by the rotation of C5-C6 bond are denoted as uv, vu and uu, respectively, and the corresponding torsion angles χ5(O5-C5-C6-O6) are 30°, 150° and 270° (the value range: 10-50°, 130-170° and 250-290°), respectively. Similarly, the conformation isomers formed by the rotation of C6-O6 bond are denoted as U+, V and U-, respectively, and the corresponding torsion angles χ6(C5-C6-O6-CA6) are 30°, 150° and 270° (the value range: 10-50°, 130-170° and 250-290°), respectively. Therefore, according to the different combinations of χ5 and χ6, each CA monomer containing C6 acetyl group (CMA6, CDA26, CDA36 and CTA236) can have 9 kinds of conformation isomers, i.e., uvU+, uvV, uvU-, vuU+, vuV, vuU-, uuU+, uuV and uuU-. Figure 3 ), and here there are 36 kinds of conformation isomers in total, plus the three stable conformations of CMA2, CMA3 and CDA23 and the zero-substitution (cellulose) conformation, so there are 40 possible conformations in total.

[0053] (2) Density functional theory (DFT) calculation

[0054] First, model parameters were input using GaussView 6.0 software to create a calculation input file, ensuring that the initial coordinates of all monomer input structures remained at the same position and level. Using the unit cell structure of CTAI crystal as a reference, the substitution of acetyl groups at positions 2, 3, and 6, as well as the torsion angle, were changed according to the type of substituted monomer to create CA monomer and cellulose structures with different conformations. Second, structure optimization was performed using Gaussian 16W software at the B3LYP / 6-31++G(d,p) level. During the optimization process, unstable conformations were transformed into relatively stable conformations. Then, frequency-dependent polarizability calculations were performed on the stable conformations at the same level (B3LYP / 6-31++G(d,p)). Wavelengths could be set to 300nm, 400nm, 500nm, 589nm, 600nm, 700nm, 800nm, and 900nm, etc., to obtain the polarizability components α in different coordinate directions at different wavelengths. xx α yy and α zz .

[0055] (3) Calculation of intrinsic birefringence

[0056] The relationship between polarizability α and refractive index n can be described by the Lorentz-Lorenz equation:

[0057]

[0058] CA monomer conformational isomer birefringence Δn i The calculation formula is as follows:

[0059]

[0060] Δα=α || -α ⊥ =α zz -(α xx +α yy ) / 2#(3)

[0061] In equations 1, 2, and 3, ρ represents density, and the densities of CTA, CDA, and CMA are 1.354 g / cm³. 3 1.156 g / cm 3 0.959g / cm 3 N A This is Avogadro's constant, and M is the molecular weight of the monomer. <n>is the average refractive index (obtained by formula 1), is the average polarizability. In the birefringence calculation formula, is the orientation function, describing the orientation state of the chain in the film, f = 1 for a fully oriented film, is the intrinsic birefringence of a certain conformational isomer of a certain monomer of CA. a xx , a yy and a zz are the components of the polarizability in different coordinate directions, obtained by DFT calculation, a || and a ┴ are the polarizabilities parallel and perpendicular to the chain axis, respectively, a || and a ┴ are the difference, i.e. the polarizability anisotropy, represented by Δa.

[0062] The electronic energy of the monomer structure is superimposed on the thermal equilibrium quantity (T = 298.15 K) to obtain the free energy E, and the distribution ratio of the stable conformation of the monomer (F i ) is calculated according to the Boltzmann distribution function:

[0063]

[0064] In formula 4, ΔE i is a relative value, representing the energy difference between the i conformation and the lowest energy conformation, R is the molar gas constant, T is the thermodynamic temperature, and e is the natural constant.

[0065] According to the proportion of the stable conformation of the monomer (F i ) and the contribution of the monomer conformation to the intrinsic birefringence (Δn i 0 ), the intrinsic birefringence Δn 0 of the monomer (monomer) is obtained by the following formula 5:

[0066]

[0067] (4) Birefringence contribution of acetyl groups at different sites

[0068] The birefringence of CA comes from the contribution of two parts, which are hydroxyl and acetyl groups respectively, and the birefringence provided by the main chain is too small to be considered. Similarly, it can be assumed that the intrinsic birefringence of cellulose mainly comes from the 2, 3 and 6 hydroxyl groups, and the intrinsic birefringence contribution of the 3 hydroxyl groups is equal, then the intrinsic birefringence contribution Δn 0 (OH) of the hydroxyl group can be obtained by averaging the intrinsic birefringence of crystalline cellulose. In addition, the intrinsic birefringence of the acetyl group at the 2, 3 and 6 substitution sites of the monomer is unique, so the relationship between the intrinsic birefringence of the monomer CDA and the birefringence contribution of the side chain substituent group is given by the following formulas 6-8:

[0069] Δn 0 (CDA23) = An 0 (2OAc) + An 0 (3OAc) + An 0 (OH) # (6)

[0070] An 0 (CDA26) = An 0 (2OAc) + An 0 (6OAc) + An 0 (OH) # (7)

[0071] An 0 (CDA36) = An 0 (3OAc) + An 0 (6OAc) + An 0 (OH) # (8)

[0072] By simultaneously solving equations 6-8, the intrinsic birefringence contributions of the acetyl groups at positions 2, 3, and 6, An(2OAc), An(3OAc), and An(6OAc), respectively, can be obtained. 0 (2OAc), An 0 (3OAc), and An 0 (6OAc).

[0073] (5) Substituted monomer ratio by nuclear magnetic resonance (NMR) analysis

[0074] Dissolve 30 mg of cellulose acetate in 0.6 mL of deuterated dimethyl sulfoxide (DMSO-d6), then transfer to a nuclear magnetic tube, and measure in a nuclear magnetic resonance spectrometer (Bruker AVIII HD 400 MHz, Germany), with a scan number set to 2048 times to obtain 13 C NMR nuclear magnetic resonance spectrum. Analyze the 13 C NMR spectrum using TopSpin software, and according to the ratio of the area of the C=O peak (~168-170 ppm) to the area of the C1 peak (~103-98 ppm), obtain the total substitution degree DS of CA, then after peak fitting, peak area normalization, and using the following formula to calculate the number of moles of CA monomers (m s ):

[0075] m2= P O＝C2-2 x DS # (9)

[0076] m3= P O＝C3-3 x DS # (10)

[0077] m6= P O＝C6-6 x DS # (11)

[0078] m 23 = (P O＝C2-23 + P O＝C3-23 ) / 2 x DS#(12)

[0079] m 26 =(P O＝C2-26 +P O＝C6-26 ) / 2 x DS#(13)

[0080] m 36 =(P O＝C3-36 +P O＝C6-36 ) / 2 x DS#(14)

[0081] m 236 =(P O＝C2-236 +P O＝C3-236 +P O＝C6-236 ) / 3 x DS#(15)

[0082] In formulas 9-15, P represents the area ratio of different carbonyl peaks, and subscripts O＝C2-2 , O＝C2-23 , O＝C2-236 , etc. refer to the types of carbonyl peaks, such as P O＝C2-23 represents the area ratio of the carbonyl peak at the C2 position in the 2, 3-substituted CDA23 monomer; m s is the number of moles of different substituted monomers, and s represents different substitution positions, such as m 23 represents the number of moles of the 2, 3-substituted CDA23 monomer, and DS is the total degree of substitution. Based on this, the degrees of substitution at positions 2, 3, and 6 are obtained by formulas 16-18, respectively:

[0083] DS2=m2+m 26 +m 23 +m 236 #(16)

[0084] DS3=m3+m 23 +m 36 +m 236 #(17)

[0085] DS6=m6+m 26 +m 36 +m 236 #(18)

[0086] (6) Theoretical birefringence value

[0087] The acetyl birefringence contribution of different substitution sites obtained in step 4 and the degree of substitution distribution of different sites obtained in step 5 can be used to calculate the theoretical birefringence value of CA and its dispersion curve by the following formula:

[0088] Δn(theoretical)=fΔn 0 (theoretical)=f·[Δn 0 (OAc)+Δn 0 (OH)]

[0089] = f - [DS2- An 0 (2OAc) + DS3- An 0 (3OAc) + DS6- An 0 (6OAc) + (3-DS) - An 0 (OH)] (19)

[0090] Assuming the CA film is a fully oriented film, i.e. the orientation function f = 1, then An (theoretical) = An 0 (theoretical).

[0091] (7) Measured birefringence value

[0092] The phase difference (delta) of the thin film was measured by an ellipsometer (SENpro, senech, Germany) in transmission mode, and then the birefringence (An) of the thin film was calculated according to the following formula:

[0093]

[0094] Where delta (°) is the phase difference of the thin film, lambda is the wavelength of the incident light, and d is the thickness of the thin film.

[0095] Example 1

[0096] 1) First, as described in steps 1-4, the twist angles χ5, χ6were all taken as 10°, 130° and 250° respectively, the wavelength was set as 400 nm, and molecular modeling, DFT calculation and Boltzmann distribution statistics were performed. The intrinsic birefringence contribution of the hydroxyl group was obtained by averaging the intrinsic birefringence of crystalline cellulose, which was 0.01468. By solving the simultaneous equations 6-8, the birefringence contributions of the acetyl groups at positions 2, 3 and 6 were -0.00081, 0.00574 and -0.01100, respectively.

[0097] (2) Second, as described in step 5, the commercial CA was analyzed by nuclear magnetic resonance, and the total degree of substitution was 2.87, and the degrees of substitution of the acetyl groups at positions 2, 3 and 6 were 0.93, 0.96 and 0.98, respectively.

[0098] (3) Then, as described in step 6, the theoretical birefringence value of the commercial CA film was calculated to be -0.00408 by formula 19 using the above obtained birefringence contributions of the acetyl groups and their degree of substitution distribution.

[0099] (4) Finally, as described in step 7, the instrument measured the birefringence value of the commercial CA film at a wavelength of 400 nm to be -0.00410.

[0100] By choosing proper torsion angles, and through reasonable molecular modeling and DFT calculation, the birefringence contribution of hydroxyl and different substituents can be understood in detail, and the accurate distribution of substituents can be obtained by NMR analysis. The theoretical birefringence value can be obtained by combining the birefringence contribution and the distribution of the degree of substitution, which is consistent with the measured value.

[0101] Example 2

[0102] (1) First, as described in steps 1-4, the torsion angles χ5, χ6 were taken as 20°, 140° and 260° respectively, and the wavelength was set as 500 nm. Molecular modeling, DFT calculation and Boltzmann distribution statistics were performed. The intrinsic birefringence contribution of hydroxyl was obtained by averaging the intrinsic birefringence of crystalline cellulose, which was 0.01441. By solving equations 6-8, the birefringence contributions of acetyl groups at positions 2, 3 and 6 were -0.00051, 0.00567 and -0.01008 respectively.

[0103] (2) Second, as described in step 5, the commercial CA was analyzed by nuclear magnetic resonance. The total degree of substitution was 2.87, and the degrees of substitution of acetyl groups at positions 2, 3 and 6 were 0.93, 0.96 and 0.98 respectively.

[0104] (3) Then, as described in step 6, the theoretical birefringence value of the commercial CA film was calculated to be -0.00301 by equation 19, using the birefringence contributions of acetyl groups and their degree of substitution distribution.

[0105] (4) Finally, as described in step 7, the birefringence value of the commercial CA film at a wavelength of 500 nm was measured to be -0.00304.

[0106] By changing the torsion angles within a certain range, and through reasonable molecular modeling and DFT calculation, the birefringence contribution of hydroxyl and different substituents can be understood in detail, and the accurate distribution of substituents can be obtained by NMR analysis. The theoretical birefringence value can be obtained by combining the birefringence contribution and the distribution of the degree of substitution, which is consistent with the measured value.

[0107] Example 3

[0108] (1) First, as described in steps 1-4, the torsion angles χ5, χ6 were taken as 30°, 150° and 270° respectively, and the wavelength was set as 589 nm. Molecular modeling, DFT calculation and Boltzmann distribution statistics were performed. The intrinsic birefringence contribution of hydroxyl was obtained by averaging the intrinsic birefringence of crystalline cellulose, which was 0.01428. By solving equations 6-8, the birefringence contributions of acetyl groups at positions 2, 3 and 6 were -0.00039, 0.00563 and -0.00968 respectively.

[0109] (2) Secondly, as described in step 5, the total degree of substitution of the commercial CA is 2.87, and the degrees of substitution of the acetyl groups at the 2, 3, and 6 positions are 0.93, 0.96, and 0.98, respectively.

[0110] (3) Then, as described in step 6, the theoretical birefringence value of the commercial CA film is calculated to be -0.00255 using formula 19 based on the above obtained birefringence contributions of the acetyl groups and their degree of substitution distribution.

[0111] (4) Finally, as described in step 7, the birefringence value of the commercial CA film at a wavelength of 589 nm is measured by the instrument to be -0.00256.

[0112] By changing the wavelength of the incident light, the theoretical birefringence values at different wavelengths can be obtained, which are consistent with the measured values.

[0113] Example 4

[0114] (1) First, as described in steps 1-4, the twist angles χ5, χ6 are taken as 50°, 170°, and 290°, respectively, and the wavelength is set to 700 nm. Molecular modeling, DFT calculation, and Boltzmann distribution statistics are performed. The intrinsic birefringence contribution of the hydroxyl group is obtained by averaging the intrinsic birefringence of crystalline cellulose, which is 0.01418. By solving equations 6-8, the birefringence contributions of the acetyl groups at the 2, 3, and 6 positions are -0.00030, 0.00560, and -0.00939, respectively.

[0115] (2) Secondly, as described in step 5, the total degree of substitution of the commercial CA is 2.87, and the degrees of substitution of the acetyl groups at the 2, 3, and 6 positions are 0.93, 0.96, and 0.98, respectively.

[0116] (3) Then, as described in step 6, the theoretical birefringence value of the commercial CA film is calculated to be -0.00223 using formula 19 based on the above obtained birefringence contributions of the acetyl groups and their degree of substitution distribution.

[0117] (4) Finally, as described in step 7, the birefringence value of the commercial CA film at a wavelength of 700 nm is measured by the instrument to be -0.00220.

[0118] By changing the wavelength of the incident light, the theoretical birefringence values at different wavelengths can be obtained, which are consistent with the measured values.

[0119] Example 5

[0120] (1) As described in step 5, the total degree of substitution of the commercial CA is 2.45, and the degrees of substitution of the acetyl groups at the 2, 3, and 6 positions are 0.81, 0.80, and 0.84, respectively.

[0121] (2) Using the known data in Example 3: the twist angles χ5, χ6are both taken as 30°, 150° and 270°, respectively, and the birefringence contributions of the acetyl groups at positions 2, 3, 6 and the remaining hydroxyl groups at 589 nm are -0.00039, 0.00563, -0.00968 and 0.01428, respectively.

[0122] (3) As described in Step 6, the theoretical birefringence value of the commercial CA film with a total degree of substitution of 2.45 is calculated to be 0.00391 using Formula 19 based on the above obtained birefringence contributions of the acetyl groups and their distribution of degrees of substitution.

[0123] (4) As described in Step 7, the birefringence value of the commercial CA film at 589 nm is measured to be 0.00390 by the instrument.

[0124] According to the known conditions and the modeling calculation data, the theoretical birefringence values of the CA film at different degrees of substitution can be directly given, and they are consistent with the measured values.

[0125] Example 6

[0126] (1) As described in Step 5, the total degree of substitution of the commercial CA is 1.93, and the degrees of substitution of the acetyl groups at positions 2, 3, 6 are 0.58, 0.64 and 0.71, respectively, by nuclear magnetic resonance analysis.

[0127] (2) Using the known data in Example 4: the twist angles χ5, χ6are both taken as 50°, 170° and 290°, respectively, and the birefringence contributions of the acetyl groups at positions 2, 3, 6 and the remaining hydroxyl groups at 700 nm are -0.00030, 0.00560, -0.00939 and 0.01418, respectively.

[0128] (3) As described in Step 6, the theoretical birefringence value of the commercial CA film with a total degree of substitution of 1.93 is calculated to be 0.01201 using Formula 19 based on the above obtained birefringence contributions of the acetyl groups and their distribution of degrees of substitution.

[0129] (4) As described in Step 7, the birefringence value of the commercial CA film at 700 nm is measured to be 0.01204 by the instrument.

[0130] According to the known conditions and the modeling calculation data, the theoretical birefringence values of the CA film at different degrees of substitution and different wavelengths can be directly given, and they are consistent with the measured values.

[0131] Example 7

[0132] (1) First, as described in Step 1-4, the twist angles χ5, χ6 are respectively taken as 30°, 150° and 270°, and the wavelength is set as 300 nm, and the molecular modeling, DFT calculation and Boltzmann distribution statistics are performed. The intrinsic birefringence contribution of hydroxyl group is obtained by averaging the intrinsic birefringence of crystalline cellulose, which is 0.01522. The birefringence contributions of acetyl groups at positions 2, 3 and 6 are respectively -0.00179, 0.00585 and -0.01361 by solving equations 6-8.

[0133] (2) Second, as described in Step 5, the commercial CA is analyzed by nuclear magnetic resonance, and the total degree of substitution is 2.87, and the degrees of substitution of acetyl groups at positions 2, 3 and 6 are respectively 0.93, 0.96 and 0.98.

[0134] (3) Then, as described in Step 6, the theoretical birefringence value of the commercial CA film is calculated as -0.00736 by equation 19 using the above obtained birefringence contributions of acetyl groups and their degree of substitution distribution.

[0135] (4) Finally, as described in Step 7, the birefringence value of the commercial CA film is measured by the instrument, and there is no measurement value because it is out of the set range of visible light wavelength.

[0136] The method of the present application can predict the theoretical birefringence value at any wavelength, which is not limited by the measurement range of the instrument.

[0137] Comparative Example 1

[0138] (1) First, as described in Step 1-4, the twist angles χ5, χ6 are respectively taken as 0°, 120° and 240°, and the wavelength is set as 589 nm, and the molecular modeling, DFT calculation and Boltzmann distribution statistics are performed. The intrinsic birefringence contribution of hydroxyl group is obtained by averaging the intrinsic birefringence of crystalline cellulose, which is 0.01397. The birefringence contributions of acetyl groups at positions 2, 3 and 6 are respectively -0.00031, 0.00559 and -0.00923 by solving equations 6-8.

[0139] (2) Second, as described in Step 5, the commercial CA is analyzed by nuclear magnetic resonance, and the total degree of substitution is 2.87, and the degrees of substitution of acetyl groups at positions 2, 3 and 6 are respectively 0.93, 0.96 and 0.98.

[0140] (3) Then, as described in Step 6, the theoretical birefringence value of the commercial CA film is calculated as -0.00211 by equation 19 using the above obtained birefringence contributions of acetyl groups and their degree of substitution distribution.

[0141] (4) Finally, as described in Step 7, the birefringence value of the commercial CA film at 589 nm wavelength is measured by the instrument, which is -0.00256.

[0142] The birefringence theoretical value and the measured value have a certain deviation due to the improper selection of the twist angle.

[0143] Comparative Example 2

[0144] (1) First, as described in Step 1-4, the twist angles χ5, χ6 were respectively taken as 60°, 180° and 300°, and the wavelength was set as 589 nm, and the molecular modeling, DFT calculation and Boltzmann distribution statistics were performed. The contribution of the intrinsic birefringence of the hydroxyl group was obtained by averaging the intrinsic birefringence of the crystalline cellulose, which was 0.01432. By simultaneously solving equations 6-8, the birefringence contributions of the acetyl groups at positions 2, 3 and 6 were -0.00047, 0.00594 and -0.01079, respectively.

[0145] (2) Second, as described in Step 5, the commercial CA was analyzed by nuclear magnetic resonance, and the total degree of substitution was 2.87, and the degrees of substitution of the acetyl groups at positions 2, 3 and 6 were 0.93, 0.96 and 0.98, respectively.

[0146] (3) Then, as described in Step 6, the birefringence theoretical value of the commercial CA film was calculated to be -0.00341 by formula 19 using the above obtained birefringence contributions of the acetyl groups and the degree of substitution distribution.

[0147] (4) Finally, as described in Step 7, the birefringence value of the commercial CA film at a wavelength of 589 nm was measured to be -0.00256.

[0148] The birefringence theoretical value and the measured value have a certain deviation due to the improper selection of the twist angle.

[0149] Comparative Example 3

[0150] (1) First, as described in Step 1-4, the twist angles χ5, χ6 were respectively taken as 90°, 210° and 330°, and the wavelength was set as 589 nm, and the molecular modeling, DFT calculation and Boltzmann distribution statistics were performed. The contribution of the intrinsic birefringence of the hydroxyl group was obtained by averaging the intrinsic birefringence of the crystalline cellulose, which was 0.01455. By simultaneously solving equations 6-8, the birefringence contributions of the acetyl groups at positions 2, 3 and 6 were -0.00053, 0.00611 and -0.01156, respectively.

[0151] (2) Second, as described in Step 5, the commercial CA was analyzed by nuclear magnetic resonance, and the total degree of substitution was 2.87, and the degrees of substitution of the acetyl groups at positions 2, 3 and 6 were 0.93, 0.96 and 0.98, respectively.

[0152] (3) Then, as described in Step 6, the birefringence theoretical value of the commercial CA film was calculated to be -0.00403 by formula 19 using the above obtained birefringence contributions of the acetyl groups and the degree of substitution distribution.

[0153] (4) Finally, the instrument measured the birefringence value of the commercial CA film at a wavelength of 589 nm to be -0.00256, as described in step 7.

[0154] The twist angle is not properly selected, and the deviation between the theoretically calculated birefringence value and the measured value is large.

[0155] Comparative Example 4

[0156] (1) First, as described in steps 1-4, the twist angles χ5, χ6 were all taken as 30°, 150° and 270°, respectively, and the wavelength was set as 589 nm, and molecular modeling, DFT calculation and Boltzmann distribution statistics were performed. The intrinsic birefringence contribution of the hydroxyl group was obtained by averaging the intrinsic birefringence of crystalline cellulose, which was 0.01428. By solving equations 6-8, the birefringence contributions of the acetyl groups at positions 2, 3 and 6 were -0.00039, 0.00563 and -0.00968, respectively.

[0157] (2) Second, the degree of substitution was determined by back titration. A certain amount of CA sample was dissolved in dimethyl sulfoxide (DMSO), and then a certain amount of sodium hydroxide solution was used to hydrolyze the sample. Then a certain amount of sulfuric acid was used to neutralize the excess sodium hydroxide. Then, a certain amount of sodium hydroxide was used to titrate the excess sulfuric acid. The amount of acetic acid contained in the sample was calculated from the amount of sodium hydroxide required for hydrolysis, and the total degree of substitution of the commercial CA was 2.67. However, the detailed substitution situation at positions 2, 3 and 6 cannot be known, and therefore the theoretical birefringence value cannot be given.

[0158] Without accurate substitution degree distribution, the theoretical birefringence value cannot be calculated.

[0159] Comparative Example 5

[0160] (1) The total degree of substitution of the commercial CA was determined by nuclear magnetic resonance analysis, which was 2.65, and the degrees of substitution of the acetyl groups at positions 2, 3 and 6 were 0.86, 0.88 and 0.91, respectively. However, the specific birefringence contributions of the acetyl groups at positions 2, 3 and 6 and the remaining hydroxyl groups cannot be known, and therefore the birefringence value of the CA film cannot be predicted.

[0161] Without the specific birefringence contribution values of the substituents, the theoretical birefringence value cannot be calculated.

[0162]

[0163] As can be seen from Table 1, the present application provides a construction method and evaluation method for accurately evaluating and regulating the birefringence and dispersion of cellulose acetate film, and the birefringence contribution of acetyl groups at different substitution sites (C2, C3, C6) and its change with wavelength are determined, and the substitution distribution of acetyl groups at different sites is analyzed. The birefringence optical properties of cellulose acetate film are accurately predicted by combining the birefringence contribution and the substitution degree distribution.

[0164] From the examples 1-6, it can be seen that by selecting a proper twist angle in step 1, reasonably assuming the birefringence contribution of hydroxyl group in step 4, and then using equations (6)-(8) to solve C2, C3, C6 simultaneously, the birefringence contribution of acetyl group at C2, C3, C6 is obtained, the distribution of substitution degree is obtained by combining with the nuclear magnetic resonance analysis, the theoretical value of birefringence and the birefringence dispersion curve are obtained by equation (19), and the theoretical value is consistent with the measured value.

[0165] Example 7 illustrates that the theoretical prediction value of the method is not limited by the measurement range of the experimental equipment.

[0166] From the comparative examples 1-3, it can be seen that improper twist angle value will cause deviation between the theoretical value and the measured value of birefringence, and sometimes the deviation is relatively large. Comparative examples 4-5 illustrate that only DFT calculation or NMR analysis cannot predict the birefringence characteristics of CA film alone.

[0167] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above description is only a specific embodiment of the present application and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application should be included in the protection scope of the present application.< / n> < / n>

Claims

1. A method for constructing an evaluation and regulation of optical properties of a cellulose acetate film, characterized by, The method comprises the following steps: S1, molecular modeling: molecular modeling is performed on acetyl-substituted monomers of cellulose acetate CA; S2, based on molecular modeling, obtain stable conformation structure, carry out frequency-dependent polarizability calculation on the stable conformation structure, obtain the components of polarizability in different coordinate directions corresponding to different wavelengths xx , α yy , and α zz ; S3, the function relationship between the intrinsic birefringence Δn of a certain monomer of cellulose acetate monomer and the intrinsic birefringence Δn of a certain conformational isomer of monomer 0 (monomer) and the intrinsic birefringence Δn of a certain conformational isomer of monomer 0 i between them: where F i is the distribution ratio of the monomer stable conformation over the total conformation of the monomer, ΔE i is the energy difference between the i conformation and the lowest energy conformation, R is the molar gas constant, T is the thermodynamic temperature, and e is the natural constant; S4, assuming that the intrinsic birefringence of cellulose is mainly from the 2, 3, 6 three hydroxyl groups, and the intrinsic birefringence contribution of the 2, 3, 6 three hydroxyl groups is equal, then the intrinsic birefringence contribution Δn of the hydroxyl group 0 (OH) can be regarded as the average value of the intrinsic birefringence of crystalline cellulose; the intrinsic birefringence of the acetyl group at the 2, 3, 6 substitution points of the monomer is unique, so the relationship between the intrinsic birefringence of the disubstituted monomer CDA and the intrinsic birefringence contribution of the hydroxyl group and the side chain substituent group, that is, the intrinsic birefringence contribution of the acetyl group, can be constructed as follows: Δn 0 (CDA23) = Δn 0 (2OAc) + Δn 0 (3OAc) + Δn 0 (OH) # (6) Δn 0 (CDA26) = Δn 0 (2OAc) + Δn 0 (6OAc) + Δn 0 (OH) # (7) Δn 0 (CDA36) = Δn 0 (3OAc) + Δn 0 (6OAc) + Δn 0 (OH) # (8) The intrinsic birefringence contribution Δn of the acetyl groups in positions 2, 3, 6 is obtained by simultaneous solution of equations 6-8 0 (2OAc), Δn 0 (3OAc), and Δn 0 (6OAc); S5, nuclear magnetic resonance scanning is performed on the cellulose acetate, and the obtained nuclear magnetic resonance diagram is analyzed to obtain the total degree of substitution DS of the cellulose acetate, then after peak fitting and peak area normalization processing, the molar number of CA monomers is obtained: m2 = P O＝C2-2 x DS# (9) m3 = P O＝C3-3 x DS# (10) m6 = P O＝C6-6 x DS# (11) m 23 = (P O＝C2-23 + P O＝C3-23 ) / 2 x DS# (12) m 26 = (P O＝C2-26 + P O＝C6-26 ) / 2 x DS# (13) m 36 = (P O＝C3-36 + P O＝C6-36 ) / 2 x DS# (14) m 236 = (P O＝C2-236 + P O＝C3-236 + P O＝C6-236 ) / 3 x DS# (15) In formulas 9-15, P represents the area ratio of different carbonyl peaks, and the subscript O＝C2-2、O＝C2-23、O＝C2-236 and the like refer to the type of carbonyl peak, such as P O＝C2-23 represents the area ratio of the carbonyl peak at the C2 position in the 2, 3-substituted CDA23 monomer; m 23 represents the number of moles of the 2, 3-substituted CDA23 monomer, and DS is the total degree of substitution; based on this, the degrees of substitution at positions 2, 3, and 6 are obtained from the following formulas 16-18, respectively: DS2 = m2+ m 26 + m 23 + m 236 (16) DS3 = m3 + m 23 + m 36 + m 236 #(17) DS6 = m6 + m 26 + m 36 + m 236 #(18) S6, constructing a CA birefringence function relationship: the intrinsic birefringence contribution values of the acetyl groups at positions 2, 3 and 6 and the degrees of substitution of the acetyl groups at different positions are substituted into the formula to obtain: Δn (theoretical) = f Δn 0 (2OAc) = f - [DS2- Δn 0 (OH)] = f - [DS2- Δn 0 (OH)] = f - [DS2- Δn 0 (2OAc) = f - [DS2- Δn 0 (3OAc) = f - [DS6- Δn 0 (6OAc) = f - [(3-DS) - Δn 0 (OH)]#(19) Assuming the CA film is a fully oriented film, i.e. the orientation function f = 1, then Δn(theoretical) = Δn 0 (theoretical).

2. The construction method of claim 1, wherein, In S1 molecular modeling, the torsion angle needs to be properly selected, for example, the conformational isomers formed by the rotation of the C5-C6 bond are represented by uv, vu and uu respectively, and the corresponding torsion angle χ5 is in the range of 10-50°, 130-170° and 250-290° respectively; the conformational isomers formed by the rotation of the C6-O6 bond are represented by U+, V and U- respectively, and the corresponding torsion angle χ6 is in the range of 10-50°, 130-170° and 250-290° respectively.

3. The construction method of claim 1, wherein, In S3, the intrinsic birefringence Δn of a certain conformational isomer of CA monomer is obtained by the CA monomer i 0 , the intrinsic birefringence Δn of a certain conformational isomer of CA monomer is obtained i : Δα = α || -α ⊥ = α zz -(α xx +α yy ) / 2#(3) In formulas 2 and 3, p is the density, the densities of CTA, CDA and CMA are 1.354 g / cm 3 , 1.156 g / cm 3 , 0.959 g / cm 3 , N A is the Avogadro constant, and M is the molecular weight of the monomer, <n>is the average refractive index, is the average polarizability; in equation (2), is the orientation function, describing the orientation state of the chains in the film, a perfectly oriented film f = 1; a xx , a yy , and a zz are the components of the polarizability in different coordinate directions, a || , and a ┴ are the polarizabilities parallel and perpendicular to the chain axis, respectively, a || , and a ┴ the difference between a and a, i.e. the polarizability anisotropy, is denoted by Δa.< / n> 4. A method for evaluating optical properties of a cellulose acetate film, characterized by, The cellulose acetate film to be evaluated is subjected to the steps in claim 1 to calculate the intrinsic birefringence of the cellulose acetate, and Δn(theoretical) is calculated; the phase difference delta of the cellulose acetate film to be evaluated is measured by an ellipsometer in transmission mode, and then the measured birefringence Δn(measured) of the film is calculated according to the following formula: Wherein delta is the phase difference of the film, λ is the wavelength of the incident light, and d is the thickness of the film; the evaluation can be performed by comparing Δn(theoretical) with Δn(measured).

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