Liquid crystal alignment behavior on transparent cellulose films

Abstract

The liquid crystal (LC) alignment properties of LC cells fabricated with plant-based cellulose films were investigated. These carefully prepared polymer films exhibited good optical transparency in the visible light region (400–700 nm). For example, the transmittance value (91%) of the cellulose film on the glass substrate at 550 nm is better than that (89%) of the polyimide film, the most commonly used LC alignment layer. The LC cells made from rubbed cellulose films showed homogeneous planar LC alignment in the parallel direction with respect to the rubbing direction. The electro-optical characteristics of the LC cells fabricated with the cellulose films such as applied voltage and response time were as good as those fabricated from rubbed polyimide films. We also found that the thermal stability of the LC cell made from the cellulose film is better than that of the polyimide film.

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Introduction

Polyimide (PI)¹ and polystyrene (PS)^2–6 have been primarily studied as polymeric liquid crystal (LC) alignment materials. Mechanical rubbing of polyimide (PI) surfaces, widely used in the liquid crystal display (LCD) industry, can produce homogeneous planar LC alignment layers where the LCs are aligned parallel with respect to the rubbing direction on these surfaces.⁶ This planar LC alignment on polyimide film has superior thermal stability as well as a strong azimuthal anchoring energy (>10⁻⁵ J m⁻²). However, polyimide has too high curing temperature to make from precursor to polyimide on the plastic substrates to apply the LC alignment layers for flexible displays. Mechanical rubbing of polystyrene (PS) surfaces can also produce homogeneous planar LC alignment layers where the LCs are aligned perpendicular with respect to the rubbing direction on these surfaces.^2–6 However, this perpendicular LC alignment on polystyrene film produces very low pretilt angle^3,6 as well as a weak azimuthal anchoring energy (<10⁻⁷ J m⁻²), which is too low to produce a stable LC cell for practical applications in LCD devices. Recently, planar and vertical LC alignment layers using polystyrene derivatives via simple polymer reaction have been developed for LC display including flexible one due to their advantages such as low temperature processability.^7–10

Cellulose is one of the most abundant polymer on earth and polysaccharide consisting of a linear chain of several hundred to many thousands of β-linked D-glucose units.¹¹ These polysaccharide-based polymers have been studied for food, textile, biomedical, energy, and electronic applications due to their attractive characteristics such as hydrophilicity, biocompatibility, biodegradability, and chemically modifiable properties.^12–16 Furthermore the processability and transparency of the cellulose-based polymers has been improved by chemical functionalization reaction attached to the pure celluloses having non-soluble characteristics in general solvent system¹⁷ and using nanocellulose fiber,¹⁸ respectively. For example, cellulose derivatives-based films prepared from hydroxypropyl- and ethyl-functionalized cellulose have been studied as the LC alignment layers using rubbing free method.^17,19,20

In this paper, the LC alignment behavior of the LC cells produced using the plant-based, abundant and renewable cellulose films as an alignment layers was studied (Fig. 1). Homogeneous planar LC alignment layers were produced from the LC cell fabricated with cellulose film through a rubbing process. The optical transparency of the carefully treated cellulose film and thermal stability of the LC cell is better than that of the widely used polyimide as a LC alignment layer, respectively. The electro-optical (E-O) characteristics of the LC cells fabricated with the polymer films are also included. To the best of our knowledge, it is the first time to report the LC alignment behavior on the cellulose films for eco-friendly display applications including flexible ones systematically.

Fig. 1 Chemical structure of α-cellulose.

Experimental

Film preparation and LC cell assembly

α-Cellulose (Sigma-Aldrich) as shown in Fig. 1 was used. To fully dissolve the dry α-cellulose, we first activate the α-cellulose by dispersing in a water bath at 50 °C for 12 h. And then, the activated α-cellulose was dissolved in N,N′-dimethylacetamide (DMAc)/LiCl at 90 °C under Ar atmosphere and was refluxed with mechanical stirring for 3 h, resulting in the transparent cellulose solution (1 wt%) in DMAc/LiCl.²⁰ These solutions were filtered through a polytetrafluoroethylene (PTFE) membrane with a pore size of 0.45 μm. Thin films of the cellulose were prepared by spin-coating (4000 rpm, 60 s) them onto 2 × 2 cm² indium tin oxide (ITO) coated glass substrates, followed by the drying at 80 °C for 1 h. It was rinsed by the deionized water; removed the used solvent and then dried at 80 °C for 1 h, sequentially repeated for 3 times, resulting forming a transparent cellulose film. Polyimide (PI, Nissan Chemical SE-7492K) alignment agents were spin coated (3000 rpm, 40 s) 2 × 2 cm² ITO coated glass substrates. The PI films were prebaked at 80 °C for 15 min and then were fully baked at 220 °C for 45 min. These polymer films were rubbed using a rubbing machine (RU-AS01, SHINDO Eng.); number of rubbing and rubbing depth were 2 and 0.5 mm, respectively. Twisted nematic (TN) and antiparallel LC cells were fabricated using the rubbed polymer films onto the ITO coated glass slides. The TN and antiparallel LC cells were made by assembling the films together orthogonally and antiparallel with respect to the rubbing direction for the rubbed polymer films using spacers with a thickness of 4.75 μm, respectively. The fabricated LC cells were filled with a nematic LC, 4-n-pentyl-4′-cyanobiphenyl (5CB, n_e = 1.7360, n_o = 1.5442, and Δε = 14.5, where n_e, n_o, and Δε represent extraordinary refractive indexes, ordinary refractive indexes, and dielectric anisotropy, respectively), in the isotropic state in order to avoid creating flow alignment by capillary action. The manufactured LC cells were sealed with epoxy.

Instrumentation

The optical transmittance of the polymer films was obtained using UV-Vis spectroscopy (Perkin Elmer Lambda 20 spectrometer). For the UV-Vis spectroscopy of polymer, the polymer films were prepared by spin-coating cellulose in DMAc/LiCl onto ITO coated glass substrates at 4000 rpm for 60 s. The polarized FTIR measurements of the rubbed polymer films were carried out using a Perkin Elmer Spectrum 2000 spectrometer in transmission mode equipped with a polarizer. The polymer films for the FTIR measurement were prepared by spin coating a polymer solution onto a silicon wafer. The surface morphology of the rubbed polymer films in an area of 20 × 20 μm² was examined using atomic force microscopy (AFM, SPA 400, Seiko) in contact mode (spring constant of the cantilever: 42 N m⁻¹, scan rate: 1 μm s⁻¹). The LC alignment direction of the antiparallel LC cells made from polymer films was investigated by measuring the angular dependence of the absorbance of a nitrile (C≡N) group in 5CB as a function of the rotation angle of the polarizer using polarized FTIR measurement. The cell gap was measured before filling the LCs using a spectrophotometer (Ocean optics Inc., S 2000). The polarized optical microscopy (POM) images of the LC cells were observed using an optical microscopy (Nikon, ECLIPSE E600 POL) equipped with a polarizer and digital camera (Nikon, COOLPIX995). The response time and voltage–transmittance (V–T) were measured from the LC cells using the same method as that reported by others.^21,22 The threshold voltage (V_th) and driving voltage (V_on) in the V–T curve are defined as the voltages at which the transmittance was decreased to 90% and 10% of the initial transmittance value, respectively. The rising (T_r) and falling (T_f) response times for the white-to-black and black-to-white changes, respectively, are defined as the time to transition from 10% to 90% transmittance and vice versa.^21,22 The voltage holding ratio (VHR) was measured using a VHR measurement system (autronic-MELCHERS, VHRM 105). The pulse width, frame frequency, and data voltages were 64 μs, 60 Hz, and 5.0 V, respectively. The measurement temperatures were 25 and 60 °C.

Results and discussion

Quantitative analysis of transparency of cellulose films was evaluated using UV-Vis spectra to investigate the possibility for the surface coating applications (Fig. 2). At first, the transmittance value of the coated cellulose film onto glass substrate is about 87% at 550 nm. This value is lower than that of the bare glass substrate, even that (89%) of the widely used polyimide film as a LC alignment layer. Since the mixture of N,N′-dimethylacetamide (DMAc) and LiCl in the cellulose film was remained, resulting in inferior optical transparency. However, the transmittance value of the carefully treated cellulose film using water was increased to 91% at 550 nm, which is similar with that of bare glass substrate. Conclusively, the optical transparency of the cellulose film in the visible light region is good enough to be used as optical materials for display devices.

Fig. 2 UV-Vis transmittance spectra of rubbed cellulose and polyimide alignment layers onto quartz substrates.

The LC alignment behavior of the LC cells fabricated with the rubbed cellulose and polyimide films was investigated using polarized optical microscopy (POM) and polarized FT-IR, respectively. The POM images of the LC cells made from rubbed cellulose ((a) dark and (b) bright) and polyimide ((c) dark and (d) bright) films clearly show homogeneous planar LC alignment behavior (Fig. 3). We found that the aligning ability of the LC cells made from the rubbed cellulose films exhibited similar LC alignment behavior compared with the LC cells fabricated with rubbed polyimide films by POM image as well as LC cell image by naked eyes under crossed polarizers.

Fig. 3 POM images of the antiparallel LC cells made from rubbed cellulose ((a) dark and (b) bright) and polyimide ((c) dark and (d) bright) films.

Angular dependence of the IR source transmittance through antiparallel LC cells made from rubbed polymer films was also observed by polarized FTIR in order to investigate LC alignment direction accurately. The antiparallel LC cells made from rubbed cellulose films showed maximum intensities along the 0° ↔ 180° direction by monitoring the polar diagrams of the intensity of the nitrile (C≡N) stretching band of the 5CB in the LC cells as a function of rotation angle of polarizer, indicating that the LC molecules on cellulose films are oriented parallel with respect to the rubbing direction (Fig. 4(a)), as in the case of the LC cell made from rubbed polyimide film as shown in Fig. 4(b). It has been known that the LC alignment direction depends on the molecular orientation and surface morphology on the polymer films after the rubbing process.²³ The change of the molecular orientation on the cellulose film caused by the rubbing process will be expected by the FTIR dichroic spectra and atomic force microscopy (AFM) study, respectively.

Fig. 4 FTIR dichroic spectra of the LC cells made from rubbed (a) cellulose and (c) polyimide films. Solid and dotted line indicate the results obtained from the IR source parallel and perpendicular, respectively, with respect to the rubbing direction. Polar diagram of specific vibrational IR peaks of nitrile (C≡N) of 5CB in the LC cells fabricated with rubbed (b) cellulose and (d) polyimide films measured as a function of rotation angle of polarizer.

The molecular orientation of the cellulose surfaces with respect to the rubbing direction was qualitatively verified from polarized FTIR studies. The IR spectrum, not shown here, of unrubbed cellulose film did not have such a dichroic aspect for polarized IR light. The IR spectrum in Fig. 5 of rubbed cellulose on Si wafer with the IR monitoring light parallel and perpendicular with respect to the rubbing direction shows a dichroic aspect. Such an anisotropy of IR absorption intensity has also been observed from other aligned thin films having in-plane orientation.^5,24 The peak intensity of C–O–C stretching bands (1156 cm⁻¹) and C–O stretching bands in cellulose (1068 cm⁻¹) were measured as a function of the rotation angle of the polarizer. The similar peak intensity of C–O–C on rubbed cellulose film was observed, regardless of the polarization direction of IR light, indicating that skeletal ether (C–O–C) groups including bridge structure of the glucose unit on rubbed cellulose film did not show any preferred parallel and perpendicular orientation with respect to the rubbing direction, although cellulose has linear configuration from the linkage stabilization due to the hydrogen bonding between hydroxyl group and oxygen of the adjoining ring molecules.²⁵ The peak of C–O stretching bands on the rubbed cellulose film is more intense when the polarization of the incident beam is perpendicular with respect to the rubbing direction, indicating that the C–O groups are reoriented perpendicular with respect to the rubbing direction on the rubbed polymer film. However, this expectation in previous paragraph, molecular orientation on cellulose film deduced from the FTIR dichroic spectra, does not perfectly determine the LC alignment direction (parallel LC alignment with respect to the rubbing direction).

Fig. 5 FTIR dichroic spectra of rubbed cellulose and polyimide on Si-wafer films. Solid and dotted line indicate the results obtained from the IR source parallel and perpendicular, respectively, with respect to the rubbing direction.

The surface morphology of the polymer films was obtained using atomic force microscopy (AFM) (Fig. 6). The surface morphology of the rubbed polyimide film having R_a (average roughness) of about 1.65 nm showed submicro- and/or subnano-scale groove-like structures formed in the parallel direction with respect to the rubbing direction. These groove-like structures on polyimide films can affect the parallel LC alignment direction with respect to the rubbing direction via the rubbing-induced groove effect mechanism.^26–29 However, it is difficult to observe the submicro- and/or subnano-scale groove-like structures on the rubbed cellulose film due to the high roughness having R_a of about 7.37 nm. We tried to observe the surface morphology change of the cellulose film fabricated with different coating conditions. As the concentration of cellulose solution was increased from 1 to 10 wt%, R_a value on coated cellulose film was increased from 7.43 to 12.98 nm as shown in the ESI.† However, we could not observe distinct changes for R_a of cellulose films according to the coating speed and time. Therefore, the cellulose film was fabricated using 1 wt% solution by spin-coating (4000 rpm, 60 s). We believe the rough surface of the cellulose makes it difficult to distinguish any difference in the groove-like structure, which might be responsible for the similarity and/or difference in the LC alignment properties between cellulose and polyimide films.

Fig. 6 AFM images of the rubbed polyimide ((a) 2D and (b) 3D) and cellulose ((c) 2D and (d) 3D) film on Si-wafer.

The electro-optical performance of the LC cells having the same cell gap of about 4.75 μm was determined by measuring the voltage–transmittance (V–T) and response time values using the same conditions. A stable V–T curve was observed for the TN LC cell fabricated with the rubbed cellulose and polyimide film (Fig. 7). The V–T curves were almost identical for the two alignment films when 10 V was applied to each cell.

Fig. 7 Voltage–transmittance curves of the LC cells fabricated with rubbed (a) cellulose and (b) polyimide films.

The response time characteristics of the LC cells made from rubbed cellulose and polyimide films are shown in Fig. 8. The V_th, V₅₀, and response time of the rubbed cellulose film were 1.13 V, 1.34 V, and 70.63 ms, respectively, which are close to those of rubbed polyimide in the LCD industry, 1.13 V, 1.20 V, and 126.98 ms, respectively (Table 1). The LC cell exhibited a voltage holding ratio (VHR) of above 97% at 25 °C, which is comparable that of commercial polyimides (about 97.10%) and this value was maintained at 60 °C. It is sufficiently high for practical applications as the LC alignment layer in TFT addressed LCD.

Fig. 8 Response time for TN LC cells made from rubbed cellulose ((a) rising and (b) falling time) and polyimide ((c) rising and (d) falling time) films.

Table 1 Voltage–transmittance and response time value of the LC cells made from polymer films

Sample	Voltage–transmittance (V)			Response time (ms)
	Vth	V50	Von	Tr	Tf	Tt
Cellulose	1.13	1.34	5.10	0.43	70.20	70.63
Polyimide	1.13	1.20	8.20	1.18	125.80	126.98

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UV-Vis transmittance of the cellulose and polyimide film onto quartz substrates after thermal treatment at 300 °C for 2 h was measured to investigate the optical stability of the polymer film. Fig. 9(a) shows the UV-Vis transmittance spectra of the cellulose film after heating for a sufficient time (2 h) at 300 °C. When the temperature increased to 300 °C, a decrease of the optical transmittance was observed. For example, the transmittance of the cellulose and polyimide film at 550 nm decreases from 91 to 89% and from 89 to 86% as the temperature is increased from room temperature to 300 °C, respectively, indicating that the thermal stability of the cellulose film is better than that of the polyimide film. We also investigated the thermal stability of the LC cells made from the rubbed cellulose and polyimide films when they were heated to various temperatures for 2 h and then cooled down to room temperature as shown in Fig. 9(b). The LC aligning ability of the LC cells made from the rubbed cellulose and polyimide film was sustained at temperatures below 250 °C, while LC alignment defects were observed when the temperature was higher than 260 °C. The LC cell prepared from cellulose film heated to 300 °C shows the planar LC alignment behavior with partial defect, whereas the polyimide LC cells show totally random LC orientation in the POM images; schlieren textures for nematic LCs were observed. Both the optical transparency of the rubbed cellulose film and LC aligning ability of the LC cells made from it can be maintained up to high temperature due to the inter- and intra-hydrogen hydrogen bonds between the adjacent chains in the cellulose.³⁰ Therefore, the thermal stability of the LC cell made from cellulose film is better than that of the polyimide film.

Fig. 9 (a) UV-Vis transmittance of the cellulose and polyimide films onto quartz substrates after thermal treatment at 300 °C for 2 h, respectively. (b) POM images in dark and bright states of the antiparallel LC cells made rubbed cellulose and polyimide films after thermal treatment at 260, 280, and 300 °C for 2 h, respectively.

Conclusions

The LC cell prepared using the plant-based and renewable cellulose film having good optical transparency as the alignment layer showed the homogeneous and planar LC alignment behavior. The enhanced thermal stability of the LC cell made from cellulose film compared with polyimide film, the most widely used in LCD industry, was ascribed to the stiff structures from the multiple hydrogen bondings between hydroxyl group on the glucose and oxygen atoms on the same glucose and/or neighbor glucose. Good electro-optical (E-O) properties were observed for the LC cells made from the cellulose film. For example, V_th, V₅₀, and response time of the rubbed cellulose film were 1.13 V, 1.34 V, and 70.63 ms, respectively, which are close to those of rubbed polyimide film, 1.13 V, 1.20 V, and 126.98 ms, respectively, indicating that these LC cells can be used for practical LC display applications, because they also have a low processing temperature. Therefore it was firstly found that the renewable and cellulose-based materials can be applied into a LC alignment system. These results can also give the basic idea for the fabrication of optical films such as LC alignment layers based on carefully treated cellulose films.

Acknowledgements

This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (2012H1B8A2025557) and by Industrial Strategic technology development program (10035613) funded by the Ministry of Trade, Industry & Energy (MOTIE, KOREA) and Dong-A University Research Fund.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03713a

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