SIYAMAK Shaha LIUDMILA Filippovih HORA A. Almoarrsiyh MASOOME Shikhi RAKESH Kumar

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Thermostable Broad Band Polarizing PVA-Film: Theoretical and Experimental Investigations①

SIYAMAK Shahaba,b,cLIUDMILA Filippovichb,cHORA A. AlmodarresiyehbMASOOME Sheikhid②RAKESH Kumare

a(220072)b(220141)c()d()e(144012())

In the present work, for the first time on the basis of poly (vinyl alcohol) (PVA), 2-(4-dimethylaminostyryl)-1-ethylquinolinium iodide(quinaldine red (QR)) and trisodium (4E)-5-oxo-1-(4-sulfonatophenyl)-4-[(4-sulfonatophenyl)hydrazono]-3 pyrazolecarboxylate(tartrazine (T)), thermostable polarizing film ina widerange of spectra (max= 394～511 nm) with polarization efficiency(PE) = 98%in absorption maximum and stretching degree(s) = 3.5 was developed.The basic spectral-polarization parameters (polarization efficiency and transmittance) of oriented colored PVA-films were measured and discussed. During the work it was found that oriented PVA-films are the phenomenon of anisotropy of thermal conductivity (||/^). It is a very important parameter for the development of thermostable PVA-polarizing films. For the first time quantum-chemical calculations using density functional theory (DFT) approach for structural analysis and electronic spectrum of the QR were carried outthe B3LYP/and TDB3LYP/methods. Interpretation of absorption strips in visible region of spectrum was also reported. The excitation energies, electronic transitions and oscillator strengths for the studied structures have also been calculated (B3LYP/). The NBO analysis and Mulliken atomic charges of the QR were carried out.

thermostable broad band polarizer film, Quinaldine Red, Tartrazine, electronic spectrum, anisotropy of thermal conductivity;

1 INTRODUCTION

Polarizer films are widely used in modern Indus-tries such as instrumentation consumer and industrial electronics, medical equipment and others. Espe-cially, these polarizers play an important role in the development of display technology[1-4]. The main types of them are designed for the visible spectrum and applied in liquid crystal display devices (LCD) intended for products and devices of a wide technical and domestic purposes (such as measuring devices, the latent image identifiers that protect trademarks and securities forgery panels with indicator light, personal computers (PCs), notebook, electronic watches and calculators)[1-14]. The biggest market for sheet polarizers is the flat-panel display industry. Many different types of polymeric sheet polarizers are available today; the most used in the practice of film polarizers are obtained on the basis of polyvinyl alcohol (PVA), colored with iodine or organic dyes[1-6]. Iodine polarizers are characterized by high optical parameters and polarizing efficiency in visible spectral region. However, they are not sufficiently resistant to high temperature (>60 ℃) and humidity of environment, which leads to a decrease in their polarizing ability for LCDs operating in harsh conditions. PVA-film with organic dyes, such as stilbene, anthraquinone, and azo dyes,are more resistant to high temperatureand humidity of the environment, but inferior to “iodine” polarizers on spectral polarization characteristics[7-15]. Therefore, the search for effective dichroic dyes and synthesis of new compounds remain an urgent task of research and development in the field of film polarizers. Different types of polarizers are made by several companies around the world. Most of them in Japan are by Nitto Denko Corporation of Osaka. Pola-Techno, Sumitomo Chemical, and Sanritz of Sanritsu Electrical Co., Ltd. Ace Digitech Co. of South Korea and Polaroid Co of the U.S., which determine the prices for the products on the world market. In recent years, China and Taiwan are active in the market polarizers. In the last 15 years in the joint Laboratory of Optical Anisotropic Films of the Institute of Chemistry of New Materials and the Institute of Physical Organic Chemistry of the National Academy of Sciences of Belarus conducted a study on the creation of film polarizers for various functional purposes. The goal is providing polarizing films from low cost material with excellent optical transparency and polarizing efficiency that have high light stability and anisotropy of thermal conductivity. Currently, a variety of electronic devices for indu- strial, special and household polarizers are required not only to improve the contrast, brightness and stability indicators, but also to work in a wide spectral range of spectrum. There are no samples with high polarization efficiency in a wide spectral range and with sufficient anisotropy of thermal conductivity. This study contains the results of a systematic research to obtain broadband PVA-polarizing films containing organic dichroic dyes, investigation their spectral and polarization pro- perties and anisotropy of thermal conductivity. The effects of dyes concentration and stretching degree on the optical properties of PVA-polarizing films were investigated. Because each dichroic dye is optically active in only one narrow range of wave-lengths, it is necessary to incorporate several dichroic dyes into one film to obtain polarized light in a wide spectral range. Mixture of selected dichroic dyes is capable to polarize light in a broad spectral range of spectrum[11]. Theoretical quantum chemistry methods based on Hartree Fock (HF) and density functional theory (DFT) are used for the calculation of optimi-zed geometry, absorption spectrum, UV, IR and NMR spectra of the organic dichroic molecules used in PVA-polarizing films[16-24]. In this work, on the basis of PVA and dichroic dyes QR and T, thermos-table polarizing films absorbing in a wide range of spectra (λ= 394～511 nm) were developed.

2 EXPERIMENTAL

2.1 Reagents and apparatus

All chemicals used were of analytical reagent grade.PVA used in this work “Mowiol 28-99” was purchased from Hoёchst Akiengesllschaft Co.(Germany). The QR (CAS Number 117-92-0) and T (CAS Number 1934-21-0) (Table 1) which were employed as dichroic agents were purchased from "Sigma-Aldrich Co." and used without further puri-fication. The experimental UV absorption spectra of the molecules and PVA-films were recorded on UV-Visible Spectrophotometer Cary 300 (Varian, USA). The optical transmission spectra were measured in polarized light with UV-NIR Spectrophotometer HR4000 (Ocean optics, USA). Thermal conductivity of films was measured on the complex equipment LC–201 (Alfa Laval Group, Sweden) using indicator method for the determination of thermal conductivity of polymer materials and thin films.

Table 1. Chemical Structures of the Investigated Dichroic Dyes in Water Solution with Their lmax(nm)

2.2 Preparation of the PVA-films

The PVA-films were prepared from 10% PVA solution, containing the 0.01～0.04 wt.% QR and T(0.10～0.40 wt.% in film), 0.01 wt.% boric acid (H3BO3), 6.0 wt.% ethyl alcohol (C2H5OH) and water. An initial composition was prepared by dissolving PVA in distilled water and ethyl alcohol. The composition was mixed at 85～90 ℃. The QR, T and additives were added after heating the PVA solution at an interval of 20 minutes for 3 hours. The mixture was heated for 3 hours. The hot solution was filtered through two layers of technical nylon. Deaeration was occurred during 12～15 hours. The composition was cast on the polished glasses and dried in the closed box at 20～22 ℃. Uniaxial orientation was done in the 4% boric acid solution at42～45 ℃. The washed film was dried for 30 minutes at 60～63 ℃. The value of Rswas determined as the ratio between the length offilms after and before (aft/bef) the uniaxial orientation. The thickness of the resulting films fall between 50 and 55 μm[1-4]measured with a micrometer with an accuracy of±5 μm (GS SSSR6507-90). Accuracy of measurements in this research limits the accuracy of the results[24]:

-Sample of the starting composition for prepara-tion of solutions and films for casting taken on an analytical balance accurate to 0.0005 g;

-Temperature measurement error was:±2.0 ℃in the preparation of polymer solutions with additives,±1.0 ℃in the chemical treatment of films,±3.0 ℃ at thermal fixation and drying films;- Relative error of spectrophotometric analysis did not exceed 1.0% and thermal conductivity 0.1%.

The standard deviation of parallel measurements determined by the formula:

whered-the result of a single measurement,- arithmetic mean- number of measurements

2.3 Optical properties of the PVA-films

The main optical properties of polarizing films such as transmittance (^,||, polarizing efficiency (PE) and dichroic ratio (d) were evaluated at the absorption maximum of the polarizing films according to Eqs. (2,3) [1]:

where,||,^,^,||transmittance and absorbance for linearly polarized light are parallel (||) and perpendicular (^) tothe direction of stretching of colored film.

Polarizing efficiency of colored oriented PVA-films depends on the concentration of injected dye and stretching degree (s= 3) of the film[2-4]. Therefore, the optimum concentration ofQR and T in PVA-film was obtained (Table 2). Changes in the concentration of QR and T from 0.10 to 0.18 wt.% in the colored oriented PVA-film show that with increasing the concentration ofdichroic agents, maximum light transmissions in parallel and perpendicular directions are reduced, suggesting that with increasing the concentration ofdichroic agents, optical density of films in parallel and perpendicular directionsis increased. At [QR,T] = 0.18 wt.%, PE = 96% (photo of the polarizing film containing 0.18 wt.%of the dichroic agents QR and tin parallel and perpendicular directions of stretching is presented in Fig. 1), with increasing the concentration of dichroic agents (0.22～0.32 wt.%), PE decreasesdue to the increasing intensity of the absorptionlight by the used dyes. The best optical parameters have PVA-film containing 0.18 wt.% of the compounds QR and T(^= 51.6%,||= 1.1%,^= 1.96% and||= 0.29%). The interaction of PVA and dyes (QR,T) is accompaniedwith hypsochromic shift from 403 to 394 nm and a bathochromic shift from 499 to 511 nm (Table 2).

Table 2. Optical Characteristics of PVA-Films at Different Concentration of the QR and T, Rs = 3.0

Dichroism (anisotropy in absorption) (Fig. 1(a),(b)) appears under uniaxial orientation of the colored PVA-film. Dichroic ratio (R= 1.96/0.29) of the PVA-polarizing film containing 0.18 wt.% of the compounds QR and Tis equalto 6.76.

Fig. 1. Broad band PVA-films containingparallel (a) and perpendicular (b) directions of stretching

Fig. 2. Polarizing efficiency of the PVA-film containing the mixture of two dyes (QR and) at concentration 0.18 wt.% and stretching degree 3

It is seen from Fig. 2 that the polarization effi-ciency of the film containing 0.18 wt.% of dyes (QR and) is 95～96% in a wide spectral range (max= 394～511 nm).

We also have studied the effect of Rson PE of the PVA-films (Table 3). Stretching (uniaxial orientation) of film containing 0.18 wt.% of the QR andfrom 2.5 to 3.5 times leads to the increase in itsfrom 49.1% to 54.7% and increase PE from 68% to 98%. Increase Rsto 4.0～4.5 times causes the decrease of PE in PVA-films. It is likely that the decrease in PE ats> 3.5 times occurs as a result of changing the orientation of the dye molecules in polymer matrix.

Table 3. Influence of Rs on the Optical Characteristics of Films Containing [QR,T] = 0.18 wt.%

Thus we found that a PVA-film containing 0.18wt.% ofQR and Tats= 3.5 has the best optical properties (PE = 98%,^= 54.7% and||= 0.6%) (Table 3).

2.4 Anisotropy of thermal conductivity of PVA-films

We have studied thermal conductivity of the systems: PVA-dyes for the development of thermos-table polarizing films. In the researches were used special paints able to sharply change the initial color at a critical temperature (T). The isothermal surface moves at a certain speed in the proportion to a local gradient of the temperature field. Thermo-physical characteristics of material can be judged by the speed of isotherm movement and the form of flowed surface. Thermal indicators of Ciba Com. (Switzer-land) with small sizes ofTrare used in order to avoid thermal destruction of polymer materials. Thermal indicator is put on a film by a thin layer at regular intervals by a draw plate. Then, after some drying periods its unpainted surface is resulted on some time (30 sec.) in dense contact to a dot source of heat (it is heated up approximately to 55 ℃ metal needles). For estimation of thermo-physical pro- perties of films, thermal conductivity of samples was determined in parallel (||) and perpendicular (^) directions of the stretching axis. Anisotropy of thermal conductivity in unstretched PVA-films is observed not appreciably (||= 0.875 W/(m×℃);^= 0.869 W/(m×℃)) as comparison to the stretched PVA-filmthat is observed very clearly. Anisotropy of thermal conductivity instretched PVA-film ats= 4 is 1.86 (Table 4).

Table 4. Dependence of Thermal Conductivity on Stretching Degree in Pure PVA-Films

During this work it was established that the oriented PVA-films with individual dyes (QR and T) and their mixtures are the phenomenon of anisotropy of thermal conductivity (||/^)[2]. Results of thermal conductivity measurements of PVA-films containing individual dichroic compounds QR and T depending on the stretching degree are given in Tables 5 and 6.

Table 5. Thermal Conductivity of PVA-Films Containing Dichroic Dye QR at Concentration 0.18 wt.%

Table 6. Thermal Conductivity of PVA-Films Containing Dichroic Dye T at Concentration 0.18 wt.%

It is clear from the results that the thermal con-ductivity in a direction (||) orientation is higher than the perpendicular orientation (^)[3]. On resulting anisotropy at a known degree of extension it is possible to judge the anisotropy ofchain structures. It is necessary to note that during thermal expansion and thermal conductivity, geometric parameters of molecule and intermolecular forces play a significant role. Thermal conductivity of PVA films changes after the injection of dye (Tables 5～7).Along an orientation axisit has increased, whereas it decreases in the perpendicular axis. Results of thermal conductivity measurements of PVA-films containing the mixture ofdichroic dyes compounds QR and T dependent on the stretching degree are given in Table 7.

Table 7. Thermal Conductivity of PVA-Films Containing the Mixture of QR and T at Concentration 0.18wt.%

When the PVA-films colored with the mixtureof dyes,practically the values of||were unchanged (||≈ 0.9 W/(m×℃)) and^reduced from 0.110 to 0.056 W/(m×℃) ats= 2 and 5,respectively. Consequently, under the joint action of dyes, the anisotropy of thermal conductivity is observed stronger than films with individual dyes.

3 COMPUTATIONAL DETAILS

In order to develop broad band polarizing PVA-films for visible spectral region, the QR and T which absorb at different wavelengths are used (Table 1). The selected dichroic components effectively po-larize the light in the visible region of 394～511 nm in PVA matrix. It has been known[5,6]that the absorption spectrum is related to molecular structure and a relationship between the absorption maximum and the structure is much desired. The HF and DFT methods have been used widely to describe optical and spectroscopic properties of organic dichroic molecules[5,6,16,17, 21-24].

3.1 Structural analysis of the molecule QR

The theoretical molecular structure ofQR in the ground state was optimized by the B3LYP/dgdzvp (density Gauss double-zeta with polarization functions) level of theory[18](Fig. 3). The optimized parameters (bond lengths, angles) of QR are presented in Table 8. To account the solvent effect,the IEFPCM (Integral Equation Formalism PCM) method coupled to UAKS radii was used. The Integral Equation Formalism PCMby Cances, Mennucci and Tomasiis the most popular PCM version. It employs a molecule shaped cavity com-posed of spheres centered on the nuclei, while the reaction field is modeled by placing charges on the cavity surface[11]. All density functional calculations were performed with the Gaussian 09W software package and Gauss view 05 visualization programs[19].

Fig. 3. Optimized structure of QR by the B3LYP/dgdzvp method

Table 8. Main Optimized Geometric Parameters of QR by the B3LYP/dgdzvp Method

3.2 Electronic structure of the molecule QR

For the first time, the theoretical absorption spectrum of QR optimized in a solvent (Water) was calculated using the TDB3LYP/dgdzvp method. The equations were solved for 20 excited states, where the computational studies were performed using the IEFPCM (Integral Equation Formalism PCM) method coupled to UAKS radii. The Time Depen- dent Density Functional Theory (TDDFT) method is able to detect accurate absorption wavelength atrelatively smallcomputing time, which corresponds to electronic transitions computed on the ground state geometry (Table 9). Here are presented transitions with≥ 0.02. The transitions with<0.02 are nearly forbidden by orbital symmetry considerations, so they are not discussed here.

Table 9. Electronic Absorption Spectrum of QR in theVisible Spectral Region Calculated by the TDB3LYP/dgdzvp Method

Calculations show the excited state at 500.84 nm described by a wave function corresponding to the imposition of the five configurations for single-electron excitations ((HOMO-4→LUMO), (HOMO-1→LUMO), (HOMO→LUMO),(HOMO→LUMO+1), (HOMO→LUMO+2)). Excitation of an electron from 62(HOMO) to 63(LUMO) gives the main contribution to the formation of the absorption band at 500.84 nm (Table 9, Fig. 4). It can be seen that the highest occupied MO (HOMO) 62 is localized on the benzene rings and C2H5groups whereas LUMO 63 is distributed on the benzene ring, =CH=CH= bond and -N(CH3)2groups. In visible region of spectrum there is also excitation at 399.27 nm. Excited state at 399.27 nm described by a wave function corresponds to the imposition of nine configurations for single-electron excitations ((HOMO-7→LUMO), (HOMO-5→LUMO+1), (HOMO-5→ LUMO+2), (HOMO-4→LUMO), (HOMO-4→LUMO+1), (HOMO-1→LUMO), (HOMO-1→LUMO+1), (HOMO→LUMO+1), (HOMO→LUMO+2)). Excitation of an electron from HOMO-5 to LUMO+1, HOMO-4 to LUMO and HOMO-1 to LUMO give the main contribution to the formation of absorption band at 399.27 nm. Excitation of electrons at 452.67, 390.44, 386.70 nm has little intense (= 0.03, 0.02 and 0.03) and doesn’t play roles in the formation of absorption spectrum ofQR.

Fig.4. Form of MO involved in the formation of absorption spectrum of QR

3.3 NBO analysis of the molecule QR

NBO analysis provides an efficient method for studying intra- and intermolecular bonding and interaction among bonds, and also gives a convenient basis for investigations of charge transfer or conju-gative interactions in molecular systems. Delocaliza-tion of electron density between occupied Lewis type NBO orbitals and formally unoccupied non Lewis NBO orbitals correspond to a stabling donor-acceptor interaction[21]. NBO analysis was performed on the molecule at the DFT/B3LYP/dgdzvp method in order to elucidate the intramolecular rehybridiza-tion and delocalization of electron density within the molecule (Table 10).

Table 10. Occupancy of NBOs and Hybrids of QR Calculated by the B3LYP/dgdzvp Method for C, N and I Atoms

The intramolecular hyperconjugative interactions of→* transitions have the most resonance energy compared with the→* transitions. The important intramolecular hyperconjugative interaction of→* transitions in the phenyl ring that lead to a strong delocalization are such as C(2)=C(3) → C(10)=C(9), C(8)=C(7) → N(11)=C(6) and C(5)=C(6) → C(4)=N(22) with the strong resonance energies of 20.33, 19.68 and 23.67 kcal/mol, respectively. The→* transitions have the highest resonance energy compared with other interactions of the title compound. The highest resonance energy of the title compound is observed for the C(13)=C(14)→C(15)=C(12) transition with resonance energy 187.59 kcal/mol respectively, that leads to the stability of the title compound.

3.4 Mulliken atomic charges of the molecule QR

The Mulliken charge is related to the vibrational properties of the molecule, and quantifies how molecule changes under atomic displacement. It is related directly to the chemical bonds in molecule. Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system[21]. The Mulliken population analysis inQR was carried out using B3LYP/dgdzvp method. The Mulliken atomic charges for the title molecule are listed in Table 11.

Table 11. Mulliken Atomic Charges of the Molecule QR

The charge distribution of the QR shows that all hydrogen atoms are positively charged. The Mulliken charge distribution shows the most negative charge observed at I1. The most positive charge is observed at C(6) for the title molecule. The calculations show that carbon atom (C(6)) with charge 0.24831 attached to the iodine atom (I1) with charge –0.99179 shows higher positive charge than the other carbon atoms (C(4) with charge 0.09626 and C(3) with charge 0.17409) and a chemical bond thus comes into being.

4 CONCLUSION

For the first time, on the basis of polyvinyl alcohol (PVA), Quinaldine Red (QR) and Tartrazine (T), thermostable broad band PVA-polarizing film absorbed atmax= 394～511 nm for display technologies and optoelectronic applications was developed. PVA-film containing 0.18wt.%of QR and Tats= 3.5has the bestoptical properties (PE = 98%,^= 54.7% and||= 0.6%). During the work the oriented colored PVA-films have anisotropy of thermal conductivity (||/^). In the PVA-films colored with the mixtureof dyes the||valuewas practically unchanged (||≈ 0.9 (W/m·℃)) and^reduced from 0.110 to 0.056 (W/(m·℃)) ats= 2 and 5,respectively. Consequently, under the joint action of dyes the anisotropy of thermal conductivity is observed stronger than films with individual dyes. The molecular structure and absorption spectrum of QR in solvent (water) were calculated by B3LYP/dgdzvpand TDB3LYP/methods. NBO analysis was performed on QR at the DFT/B3LYP/dgdzvp method in order to elucidate the intramolecular rehybridization and delocalization of electron density within the title molecule. Mulliken charge distribution shows that carbon atom (C(6)) with charge 0.24831 attached to the iodine atom (I1) with charge –0.99179 and a chemical bond thus comes into being.

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27 May 2017;

26 December 2017

10.14102/j.cnki.0254-5861.2011-1732

①This project was supported by the National Academy of Sciences of Belarus

②E-mail:m.sheikhi2@gmail.com(M. Sheikhi)