Liquid Crystals ISSN: 0267-8292 (Print) 1366-5855 (Online) Journal homepage: http://www.tandfonline.com/loi/tlct20 Study on polyvinylidene fluoride as alignment layer in twist-nematic liquid crystal display Yi-Fei Wang, Yu-Qiang Guo, Ya-Xuan Ren, Ming-Zhu Fu, Ji-Liang Zhu & Yu-Bao Sun To cite this article: Yi-Fei Wang, Yu-Qiang Guo, Ya-Xuan Ren, Ming-Zhu Fu, Ji-Liang Zhu & YuBao Sun (2017): Study on polyvinylidene fluoride as alignment layer in twist-nematic liquid crystal display, Liquid Crystals, DOI: 10.1080/02678292.2017.1390791 To link to this article: http://dx.doi.org/10.1080/02678292.2017.1390791 Published online: 24 Oct 2017. Submit your article to this journal Article views: 2 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tlct20 Download by: [UAE University] Date: 25 October 2017, At: 23:59 LIQUID CRYSTALS, 2017 https://doi.org/10.1080/02678292.2017.1390791 Study on polyvinylidene fluoride as alignment layer in twist-nematic liquid crystal display Yi-Fei Wanga, Yu-Qiang Guoa,b, Ya-Xuan Rena, Ming-Zhu Fua, Ji-Liang Zhua and Yu-Bao Suna,b,c ABSTRACT ARTICLE HISTORY The material with high dielectric constant can significantly affect the distribution of the electric field, so this kind of material has great potential in liquid crystal display. In this paper, polyvinylidene fluoride (PVDF) as alignment layer in liquid crystal display was analysed. The optical property, mechanical property, thermal stability and electrical property of PVDF were measured. Experiments show that the absorbance of PVDF material is 0.2 (or less) in visible light, which is better than the conventional alignment material polyimide (PI). The alignment effect can be generated by mechanical friction and the liquid crystal molecules are ordered or aligned, and PVDF can maintain good thermal stability as temperature is lower than 400°C. Since the dielectric constant of PVDF is usually between 6.0 and 8.0, it has significant effect on the distribution of the electric field in the liquid crystal display, and its dielectric loss is also less than PI. The lower operating voltage and the faster response time are obtained from the experiment. It can be confirmed by the experiments that PVDF could be used in liquid crystal display (LCD) as the alignment layer to improve LCD’s characteristics. Received 3 July 2017 Accepted 8 October 2017 1.0 Polyvinylidene fluoride; polyimide; dielectric constant; operating voltage PVDF PI Transmittance 0.8 0.6 0.4 0.2 0.0 0.0 KEYWORDS 1.0 PVDF PI 0.8 Transmittance Downloaded by [UAE University] at 23:59 25 October 2017 a Department of Applied Physics, Hebei University of Technology, Tianjin, PR China; bSchool of Electronic and Information Engineering, Hebei University of Technology, Tianjin PR China; cTianjin Key Laboratory of Electronic Materials and Devices, Hebei University of Technology, Tianjin, PR China 0.6 0.4 0.2 (b) 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0 10 Voltage/V 1. Introduction Nowadays, liquid crystal displays (LCDs) have become a kind of common display device with different size in our daily life, and a number of researches have been done to optimise the LCDs over the years [1–4]. Many research groups have demonstrated that nanomaterials in nematic LCs improved the performance of LCDs [5– 8].The conductive carbon nanotubes were found that it could be used in TN-LCDs to decrease the driving CONTACT Yu-Bao Sun email@example.com © 2017 Informa UK Limited, trading as Taylor & Francis Group 20 30 40 500 510 520 530 540 550 Time/ms voltage by lowering the ion effect . Moreover, the incorporation of carbon nanotubes (CNTs) in the liquid crystal host was confirmed to decrease the optical response time of TN-LCDs . The research field of alignment layer, to achieve the desired results, has recently emerged as a promising new area of technological development [11–14]. The alignment layer with nanosised ferroelectric particles has much larger local electric fields, which can polarise the liquid crystal Y.-F. WANG ET AL. molecules and thus indirectly increase the intermolecular interaction, which will cause the lowering of switching voltage [15,16]. A super-faster response time can be achieved due to the single-wall carbon nanotube and PI composite alignment layer in TNLCDs . Moreover, Hong-Gyu Park et al.  produced homogeneously aligned LCs on ZnO using a sol–gel method and IB irradiation. The results indicate that ZnO alignment layers deposited by a sol–gel method have considerable potential in LCD device applications. Rajratan Basu et al.  have argued that graphene can act as the alignment layers and the transparent electrodes in an LC cell. When the dielectric constant of the alignment layer is increased, the potential drop in the alignment layer is lowered and the potential drop in the liquid crystal layer is increased, so that the operating voltage can be reduced. The potential distribution is computed by solving the Poisson equation: Ñ ðε ÑΦÞ ¼ 0 We had used polyimide/polyvinylidene fluoride/ polyaniline (PI/PVDF/PANI) composite as the alignment layer of TN-LCD to lower the operating voltage . When the thickness of the alignment layer is constant, the appropriately increase dielectric constant of the alignment layer lower the operating voltage of the liquid crystal display. However, the optical property, mechanical property, thermal stability and electrical property of the material are not investigated. Also, the transparency of the PI/PVDF/PANI composite film is not good because the conductive PANI is dark green powder, which affects the application of this composite material in LCD. In this paper, we use PVDF as the alignment layer to lower the operating voltage and reduce the response times. The dielectric constant of PVDF film is about 6.3. The influence of PVDF film as alignment layer in TN-LCD was studied by simulation and experiment, the optical property, mechanical property, mechanical property, thermal property and dielectric property of PVDF film were studied. 2. Simulation In order to evaluate the electro-optic characteristics of the proposed alignment layer material, we made a series of simulations by the commercial simulation software TechWiz 1D (Sanayi System Co., Korea). The parameters of the LC in both simulations and experiments are shown in Table 1. The rotational Table 1. Parameter of LCD. Item LC N (Refractive index) ne no 1.721 1.517 P/A ITO Glass 1.5 + 0.001929i 1.7 1.5 ε ε∥ 26.3 ε⊥ 6.8 / / 4.5 K (pN) K11 = 13.6 K22 = 6.6 K33 = 9.9 / / / d (μm) 2.8 230.0 0.1 500 ITO: indium tin oxides. viscosity of LC is 87.0 mPas, and the light wavelength is 550 nm. The electro-optical curves of TN-LCDs with different dielectric constant alignment layers are shown in Figure 1. The operating voltage is significantly reduced when the dielectric constant of the alignment layer is increased. From Figure 1, when PVDF is used as the alignment layer, which dielectric constant is larger than 6.0, instead of the PI which has a dielectric constant of 3.8, the operating voltage is remarkably reduced. When the dielectric constant is larger than 20, the operating voltage will be reduced more, but the reduction becomes inconspicuous. However, the alignment layer materials with the dielectric constant between 10 and 20 are scarce. The response process of the TN-LCDs was simulated and shown in Figure 2; the driven voltage is set as 3 V and 2.5 V respectively. From Figure 2, the rise time is reduced with the increasing dielectric constant because of the lower threshold voltage in the cell with higher dielectric alignment layer as shown in Figure 1, and the decay time is similar. 3. Experiments PVDF is a semi crystalline polymer with excellent piezoelectric and ferroelectric properties. There are 1.0 ε=3.8 ε=4.5 ε=5.5 ε=6.3 ε=10 ε=20 ε=50 ε=100 ε=200 0.8 Transmittance Downloaded by [UAE University] at 23:59 25 October 2017 2 0.6 0.4 d=0.1 0.2 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Voltage/V Figure 1. (Colour online) Voltage-dependent transmittance curves of different dielectric constants of alignment layer. LIQUID CRYSTALS 1.0 1.0 ε=6.3 ε=3.8 d=0.1 ε=6.3 ε=3.8 0.8 Transmittance Transmittance 0.8 0.6 0.4 d=0.1 0.6 0.4 0.2 0.2 (a) 0.0 3 (b) 0.0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time/ms Time/ms Downloaded by [UAE University] at 23:59 25 October 2017 Figure 2. (Colour online) Time dependent transmittance curves of different dielectric constant of alignment layer at (a) 3 V and (b) 2.5 V. five crystalline phases of PVDF called α, β, γ, δ and ε. The β-phase is the polar phase because of its special structure, in which the H and F are on the opposite side of the main backbone chain. The special structure cause non-zero dipole moment in PVDF and lead to high ferroelectric, piezoelectric and dielectric properties [21–25]. The structural formulas of PI and PVDF are shown in Figure 3. PI has the low dielectric constant because the symmetrical structure is perpendicular to the polymer’s chain. In PVDF, the asymmetrical structure shows the dipole moment effect, so it has a high dielectric constant. Moreover, the resistivity of PVDF is about 3 × 1012 Ω·cm, which is small lower than PI [26–28]. In addition, the PVDF material has been widely used because of its great dielectric property and filmforming property. PVDF films with different thicknesses (5 nm to 1 μm) can be obtained by spin coating . The results show that the content of β-phase in PVDF can be improved by annealing and hot pressing at high temperature, so that the dielectric properties of PVDF can be improved [30,31]. A certain PVDF powder (Shanghai 3F New Material Co., Ltd) is added into the N,N dimethylformamide (DMF, Tianjin Damao Chemical Reagent Co., Ltd), and PVDF was completely dissolved in DMF after magnetic stirring and ultrasonic dispersion. Three drops of PVDF– DMF solution or PI solution on (with the same solid state content) were dropped on the conductive surface of indium tin oxides (ITO) glass (2 cm × 2 cm), and the solution was evenly tiled on the ITO glass by spin coating process. The rotational speeds of spin coating process are v1 = 880 r/min (for 9 s) and v2 = 1450 r/min (for 30 s). These glasses were then placed into the oven and heated at 90°C for 30 min and then at 180°C for 2 h. 0.1-μm thickness alignment layers were obtained. After that we prepared TN-LCD cells through normal fabrication processes, and the cell gap was 3.1 μm which was measured by UV spectrophotometer. We prepared and tested LCD cells by series of instruments of Chengdu Century Branch Instrument Co., Ltd. The absorbance of PVDF and PI were measured by spectrophotometer (723CRT, Shanghai Youke Instrument Co., Ltd). The surface of the materials was characterised by a tapping mode atomic force microscope (Aglient 5500 AFM). Thermo-gravimetric analysis of the PVDF was carried out by synchronous thermal analyser (SDT-Q600). The dielectric property was measured by JK2828 LCR Meter. 4. Results and discussion Figure 4 is the light absorption spectrum of PVDF and PI solutions in the UV-visible light, and the inner picture on the upper right corner in the figure is the photo of two solutions in natural light. From the absorbance graph, the PI solution has a certain light absorption between 320 and 430 nm, and the absorbance reaches a maximum of 2.184 at 361 nm. Moreover, it can be seen that the PI solution shows yellow colour from the inner picture. This is due to the fact that there are carboxyl group on the imine ring and the phenyl group attached to the imine ring in the PI. Although the absorption ranges of these two groups are in the Figure 3. (Colour online) The structural formula of PI (left) and PVDF (right). 4 Y.-F. WANG ET AL. 3.0 2.5 100 PVDF PI 80 Weight / % Absorbancy 2.0 1.5 1.0 PVDF 60 40 0.5 20 0.0 400 500 600 700 100 800 200 300 ultraviolet region, the red shift of the absorption spectrum occurs when putting them together to form an aromatic polymer. PI material has a significant absorption peak in the shortwave range of visible light. When PI is used as the alignment layer, it cannot affect the display’s colour because of the thin thickness (less than 100 nm). While the absorption of PVDF material is relatively uniform, it is almost transparent in the natural light. As a result, PVDF can be fabricate a thicker layer for the better dielectric and insulation characteristics, because it is a transparent layer. Figure 5 shows the AFM plot of the surface of PI film and PVDF film, respectively. (a) is the morphology of the PI surface, and (b) is the morphology of the PVDF surface. Although it looks like that there are more grooves on PI, the grooves on PI are much shallower than that on PVDF. Figure 6 shows the thermo-gravimetric curve of the PVDF. It can be seen from the figure that the thermal decomposition temperature of PVDF is over 400°C. When heated to about 500°C, the thermal decomposition of the material mass fraction is about 70%. Its decomposition temperature is lower than that of conventional 500 600 700 800 Figure 6. (Colour online) TGA thermograph of pure PVDF. PI, but is far more than the maximum deal temperature (180°C) during the experiment and production. As a result, PVDF has good thermal stability. The electro-optical characteristic curve and switching behaviour of the liquid crystal display was tested with visible light. Figure 7 shows the electro-optical 1.0 PVDF PI 0.8 Transmittance Downloaded by [UAE University] at 23:59 25 October 2017 Figure 4. (Colour online) The absorbance of PI and PVDF. 400 Temperature / wavelength / nm 0.6 0.4 0.2 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Voltage/V Figure 7. (Colour online) Voltage-dependent transmittance for the different kinds of TN-LCD. Figure 5. (Colour online) AFM surface topography for (a) PI and (b) PVDF after rubbing process. LIQUID CRYSTALS North LCD Engineering R&D Center). Figure 9 shows the response processes of TN-LCD with alignment layer of PVDF and PI under two driven voltages ((a) is 3.0 V and (b) is 2.5 V). The voltage is applied at t = 0 ms and cut at t = 500 ms. From Figure 9(a), the rise and decay times (τ r /τ d ) of PVDF are (6.00 ms/15.91 ms) which is less than that of PI (6.00 ms/17.59 ms) for 3.0 V; in the decay process, there has a optic bounce in the two cell, the reason is the backflow effect in TN-LCD when the driven voltage is large enough. From Figure 9(b), the rise time of PVDF is 8.59 ms, and the decay time is 10.55 ms, and the response times of PI is 9.87 ms/ 11.56 ms. The response times of PVDF are less than that of PI because of the stronger electric field strength and the different pretilt angles (PVDF is characteristics of these two cells. The increasing dielectric constant of the alignment layer can reduce the potential drop in the alignment layer, so the horizontal electric field in liquid crystal layer is increased. As a result, the operating voltage is reduced. From Figure 7, the operating voltage (for the transmittance equal to 1.0) of the cell with PVDF as the alignment layer (1.67 V) is significantly lower than the operating voltage of the cell with PI as the alignment layer (1.87 V), which is consistent with the simulation results. Figure 8 shows the POM images of the dark and bright states of the PI and PVDF TN-LC cells, the difference of these two kinds of alignment layers is very little and has no effect on contrast ratio. The switching behaviours of two TN-LCDs were tested by LCD parameter testers (LCT-5016C. The Downloaded by [UAE University] at 23:59 25 October 2017 5 P 50µm A a b c d P 50µm A Figure 8. (Colour online) The polarised optical microscope (POM) pictures of the (a) (b) PI TN LC cell and (c) (d) PVDF TN LC cell at dark and bright states, respectively. 1.0 1.0 PVDF PI PVDF PI 0.8 Transmittance Transmittance 0.8 0.6 0.4 0.6 0.4 0.2 0.2 (a) 0.0 (b) 0.0 0 10 20 30 40 500 510 520 530 540 550 Time/ms 0 10 20 30 40 500 510 520 530 540 550 Time/ms Figure 9. (Colour online) Transmittance-dependent time for the TN-LCDs with two kinds of alignment layers and two different driven voltages: (a) 3.0 V and (b) 2.5 V. 6 Y.-F. WANG ET AL. property and film-forming property. It can be used to make the thicker film and form uneven and strong electric field in liquid crystal devices. Acknowledgment (a): PI 100 Hz (b): PI 1k Hz This work was supported by the National Natural Science Foundation of China [Grant Numbers 61475042, 11304074 and 11274088] and the National Natural Science Foundation of Hebei Province [Grant Numbers A2015202320 and GCC2014048]. Disclosure statement No potential conflict of interest was reported by the authors. Downloaded by [UAE University] at 23:59 25 October 2017 (c): PVDF 100 Hz (d): PVDF 1k Hz Figure 10. The capacitance and dielectric loss of PI and PVDF. 1.7° and PI is 3.0°). The anchoring energy of PI and PVDF is in the same scale because of the similar response time. Finally, in order to clear the dielectric characteristics of PVDF, we prepared two cell filled with PI or PVDF. The capacitance and dielectric loss of PI and PVDF are tested at two frequency (a) and (c) under 100 Hz, and (b) and (d) under 1000 Hz, and shown in Figure 10. The dielectric loss of PVDF is less than that of PI at the same frequency. The high dielectric constant of PVDF is due to the trans-conformation in the main chain. Hydrogen atoms (H) and fluorine atoms (F) are on the opposite side of the main backbone chain, resulting in polar polymorph β-phase, as a result, the ion accumulation issues can be negligible. PVDF’s dielectric property is better than PI’s. 5. Conclusion In this paper, we studied the PVDF’s characteristics as alignment layer. The experiments show that the absorbance of PVDF material has a better transparency than PI. PVDF material has good thermal stability, and its thermal weight loss curve remains stable at the annealing temperature of 180°C, and its dielectric loss is also less than PI. The operating voltage of TN-LCD with PVDF is reduced compared with that of PI, and the response times can be reduced as well. As a result, PVDF can be used as the alignment layer of liquid crystal display for improving the electric-optic performance. Furthermore, as we expect, PVDF can be used in liquid crystal lens because of its great dielectric Funding This work was supported by the National Natural Science Foundation of China [11274088,11304074,61475042]; National Natural Science Foundation of Hebei Province [A2015202320,GCC2014048], the Key Subject Construction Project of Hebei Province. References  Kim JK, Joo SH, Song JK. Reflective-emissive photoluminescent cholesteric liquid crystal display. Appl Optics. 2013;52:8280–8286.  Moreira MF, Carvalho ICS, Cao W. Cholesteric liquid crystal laser as an optic fiber-based temperature sensor. Appl Phys Lett. 2004;85:2691–2693.  Lee SH, Bhattacharyya SS, Jin HS, et al. Devices and materials for high performance mobile liquid crystal displays. J Mater Chem. 2012;22:11893–11903.  Lee SH, Kim SM, Wu ST. Emerging vertical-alignment liquid-crystal technology associated with surface modification using UV-curable monomer. J Soc Inf Disp. 2009;17:551–559.  Zhulai DS, Bugaychuk SA, Klimusheva GV, et al. Structural characteristics of different types of nanoparticles synthesised in mesomorphic metal alkanoates. Liq Cryst. 2017;44:1–8.  Prasad SK, Sandhya KL, Nair GG, et al. Electrical conductivity and dielectric constant measurements of liquid crystal gold nanoparticle composites. Liq Cryst. 2006;33:1121–1125.  Jung HY, Kim HJ, Yang S, et al. Enhanced electrooptical properties of Y2O3 (yttrium trioxide) nanoparticle-doped twisted nematic liquid crystal devices. Liq Cryst. 2012;39:789–793.  Yadav SP, Manohar R, Singh S. Effect of TiO2 nanoparticles dispersion on ionic behaviour in nematic liquid crystal. Liq Cryst. 2015;42:1–7.  Wei L, Wang CY, Shih YC. Effects of carbon nanosolids on the electro-optical properties of a twisted nematic liquid-crystal host. Appl Phys Lett. 2004;85:513–515. Downloaded by [UAE University] at 23:59 25 October 2017 LIQUID CRYSTALS  Kumar J, Manjuladevi V, R K G, et al. Fast response in TN liquid-crystal cells: effect of functionalised carbon nanotubes. Liq Cryst. 2016;43:488–496.  Rim LT, Kim JH, Jun MC, et al. Optimisation of alignment materials for minimising residual retardation in in-plane switching liquid crystal display. Liq Cryst. 2017;44:500–509.  Lee TR, Kim JH, Lee SH, et al. Investigation on newly designed low resistivity polyimide-type alignment layer for reducing DC image sticking of in-plane switching liquid crystal display. Liq Cryst. 2017;44:738–747.  Gong Q, Zheng XG, Gong SM, et al. Synthesis of novel soluble rubbing-resistant polyimides used as liquid crystal vertical alignment layers. Liq Cryst. 2016;43:131–141.  Sergan T, Schneider T, Kelly J, et al. Polarizing-alignment layers for twisted nematic cells. Liq Cryst. 2000;27:567–572.  Park HG, Lee JJ, Dong KY, et al. Homeotropic alignment of liquid crystals on a nano-patterned polyimide surface using nanoimprint lithography. Soft Matter. 2011;7:5610–5614.  Nimmy JV, Shiju E, Arun R, et al. Effect of ferroelectric nanoparticles in the alignment layer of twisted nematic liquid crystal display. Opt Mater. 2017;67:7–13.  Liu Y, Y J L, Kundu S, et al. Super-fast switching of twisted nematic liquid crystals with a single-wall-carbon-nanotube-doped alignment layer. J Korean Phys Soc. 2015;66:952–958.  Park HG, Han JJ, Seo DS. Liquid crystal alignment on solution derived zinc oxide films via ion beam irradiation. J Nanosci Nanotechno. 2016;16:2883–2886.  Basu R, Kinnamon D, Garvey A. Graphene and liquid crystal mediated interactions. Liq Cryst. 2016;43:2375– 2390.  Guo YQ, Wang YF, Sun YB, et al. Effect of thickness and dielectric constant of alignment layer on the TNLCD. Chin J Liq Crys Disp. 2016;31:1017–1022.  Kar E, Bose N, Das S, et al. Enhancement of electroactive β phase crystallization and dielectric constant of           7 PVDF by incorporating GeO2 and SiO2 nanoparticles. Phys Chem Chem Phys. 2015;17:22784–22798. Thakur P, Kool A, Bagchi B, et al. Improvement of electroactive β phase nucleation and dielectric properties of WO3·H2O nanoparticle loaded poly(vinylidene fluoride) thin films. Rsc Adv. 2015;5:62819–62827. Sajkiewicz P, Wasiak A, Gocłowski Z. Phase transitions during stretching of poly(vinylidene fluoride). Eur Polym J. 1999;35:423–429. Sharma M, Srinivas V, Madras G, et al. Outstanding dielectric constant and piezoelectric coefficient in electrospun nanofiber mats of PVDF containing silver decorated multiwall carbon nanotubes: assessing through piezo response force microscopy. Rsc Adv. 2016;6:6251–6258. Gregorio R, Ueno EM. Effect of crystalline phase, orientation and temperature on the dielectric properties of poly (vinylidene fluoride) (PVDF). J Mater Sci. 1999;34:4489–4500. A B D S, Marini J, Gelves G, et al. Synergic effect in electrical conductivity using a combination of two fillers in PVDF hybrids composites. Eur Polym J. 2013;49:3318–3327. F C C, Chen YJ. Evaluation of thermal, mechanical, and electrical properties of PVDF/GNP binary and PVDF/ PMMA/GNP ternary nanocomposites. Compos Part A. 2015;68:62–71. Zhu YL, Properties YY. Usage of polyvinylidene fluoride resin. Shanghai Plastics. 2005;4:35–37. Zhang L, Wang DD, Zhang CM, et al. Preparation and characterization of ultra-thin, continuous PVDF ferroelectric thin films. J Beijing Inst Graph Commun. 2015;23:65–68. Huang C, Zhang QM, Dielectric E. Electromechanical responses in high dielectric constant all-polymer percolative composites. Adv Mater. 2004;14:501–506. Jia N, Xing Q, Xia G, et al. Enhanced β-crystalline phase in poly (vinylidene fluoride) films by polydopaminecoated BaTiO3 nanoparticles. Mater Lett. 2015;139:212– 215.