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Abstract
The vertical alignment of reactive mesogen molecules was induced on an oxygen plasma-treated glass surface without using a liquid crystal (LC) alignment layer. Optical retardation of the vertically aligned LC cell resulted in a critical limit that depends on the thickness of liquid crystals. However, the vertical alignment in the LC cell could not be maintained after UV curing if its thickness was greater than 5 μm or if the curing temperature was within the LC phase temperature. In this study, very thin films with vertical LC orientation were obtained by curing reactive mesogen in the supercooled nematic phase below the equilibrium melting temperature.
© 2017 Optical Society of America
1. Introduction
Liquid crystal displays have inherently poor viewing angle performance; as a result, the need to enhance viewing angle performance has become important. Currently, anisotropic birefringent films used in LCD industries are made by uniaxially stretched polycarbonate or polyvinyl alcohol films. Although the birefringent polymer films have been used to enhance the performance of LCDs [1], the use of reactive liquid crystals (often called reactive mesogen) as optical films has increased because polymerizable liquid crystals can provide a very thin film with a wide variety of optical properties [2–7]. Reactive mesogen is a low molecular weight liquid crystal that can be UV-cured into the network structures of a thin film. The anisotropic structure formed in the liquid crystal (LC) phase can be preserved in the UV-cured networks of thin films [8,9]. Because of the ability to produce a thin film that can control the alignment of molecules arbitrarily, some compensation films based on reactive mesogens are already being used for displays.
Reactive mesogens can be coated onto flexible plastic foils by roll to roll processes to form retardation films or used inside the display to form in-cell optical elements, either as patterned or monolithic retarders. For the use of a retardation film to compensate for the undesired birefringent effect of optical devices, it is important to understand the molecular orientations in the LC film that can be fabricated in an LC phase. The retardation profile of vertically aligned nematic liquid crystals shows that, on the axis, retardation is close to zero but becomes more positive at more acute viewing angles. A film with this retardation profile is termed a positive C plate. At non-normal angles of incidence, such films tend to convert elliptically polarized light to circularly polarized light, the extent of which is dependent on the film thickness and the angle of normal incidence.
Retardation is very dependent on the orientation of LC molecules in the film. In this study, as shown in Fig. 1, the vertical alignment of reactive LC molecules was induced on a glass surface and UV-cured to produce a positive C type film. The effects of film thickness on the vertical orientation of LC molecules were investigated by measuring the retardation profiles [10] of the LC film. Temperature control of liquid crystals is also critical because the precise temperature affects molecular ordering and thus the birefringence of the final LC film. A sophisticated molecular architecture can be permanently captured on a polymer film by the in-situ curing process although there can be a slight reduction in the ordering of LC molecules during the curing process. The changes in LC orientations were monitored by measuring the retardation profiles of LC cells and films.
2. Experimental
Nematic liquid crystal LC242 (BASF; Δε = −2, Δn = 0.14) was used in this study. As seen in Fig. 1, LC242 is a reactive liquid crystal with acrylate groups at both ends and a transverse dipole moment due to carbonyl groups [11,12].
The crystal to nematic LC (Tkn = 79 °C) and nematic LC to isotropic melt transition (Tni = 118 °C) temperatures of LC242 were determined by differential scanning calorimetry (DSC). Its DSC thermal data is shown in Fig. 2.
Glass substrates were sonicated with 5% NaOH solution at 60 °C and dried at 90 °C after flushing with deionized water. In order to introduce polarity on the glass substrate, the glass was treated with atmospheric RF plasma (13.56 MHz RF power). Argon (5 Lpm) and oxygen (20 sccm) gases were used as the carrier and reactive gases, respectively. The glass substrates were treated 10 times with oxygen plasma just before fabricating the LC cells. Sandwich-type LC cells were fabricated using glass substrates with various cell gaps (d), as shown in Fig. 3(a). Liquid crystals were introduced into the cell in the melt state by capillary action; then, LC cells were completely sealed with epoxy resin. To fabricate the LC films, 1 wt% of IG 907 (Ciba) was added to LC242 and mixed by stirring at the isotropic temperature. LC242 in the melt state was injected into the cells and cured with a 5 mW UV lamp for 10 sec.
Fig. 3 Schematic diagrams of (a) a vertically oriented LC cell and (b) retardation measurement system.
When polarized light is incident on a birefringent medium, the polarization phase is retarded depending on the optic axes of the medium. The optical retardation can be expressed by Δnd (Δn = ne - no, d = thickness of cell) and was measured from the ratio of the transmitted light intensities at crossed polarizers using the retardation measurement system shown in Fig. 3(b) and calculated using Eq. (1).
Here, is the angle between the analyzer and the projection of the optic axis. Figure 3(a) shows a simplified structure of a homeotropic (vertical) LC cell. When light with a wavelength (λ) is incident to the air/LC cell interface at an angle (), the effective refractive index (neff) of the LC can be determined depending on a refraction angle () as follows [13]:The angular dependent phase retardation (δ) of the LC cell is expressed as follows:The variation in retardation with incident angle creates a characteristic retardation profile, which is dependent on the macroscopic orientations of LC molecules. Optical retardations of the LC cells and LC films were measured using a RETS R&D system (Otsuka Electronics) [10].Liquid crystal textures were observed with a polarized optical microscope (Eclipse 500, Nikon), and homeotropic alignment was identified using a conoscope (Eclipse 500, Nikon). Attenuated total reflection (ATR) FTIR spectra were obtained for thin LC films using a Jasco FTIR-6100 spectrometer. The polarized ATR accessory (PIKE Tech.) was used to determine the orientation of the LC molecules. ZnSe ATR plate was used for the reflection FTIR experiments. IMV-4000 (Jasco) IR imaging system was used to obtain the mapping images of the LC242 films. When measuring temperature dependent data, a hot stage (Linkam PE120) was used.
3. Results and discussion
The vertical alignment of liquid crystals is normally induced on a substrate coated with an LC alignment layer with a low surface energy [14]. However, the vertical alignment of LC242 could be induced on a bare glass substrate without using an alignment layer. In our previous study [15], the effects of the surface polarity of a glass substrate on LC alignment were studied for a nematic liquid crystal possessing negative dielectric anisotropy. LC242 shows random planar alignment on a glass surface without any treatment and vertical alignment on the surface of oxygen plasma-treated glass. On the untreated bare glass surface, most nematic liquid crystals including LC242 exhibit a random planar alignment where the LC director is parallel to the surface but points in different directions.
In Fig. 4(a), the Schlieren texture of nematic liquid crystals due to the random planar alignment is shown in the polarized optical microscopic image of the LC242 cell made with bare glass substrates. In comparison to the planar alignment of liquid crystals, the vertical alignment of LC242 can be achieved on the oxygen plasma-treated glass. The LC242 cell with the vertical alignment exhibits a completely dark image in the polarized optical microscope image of Fig. 4(b) and a clear “Maltese cross” interference pattern under the convergent light in the conoscope image shown in the inset of Fig. 4(b).
Fig. 4 Polarized optical microscope images of LC242 cells made with (a) untreated bare glass and (b) O2 plasma-treated glass substrates (inset: conoscopic image of an LC242 cell).
Retardation of vertically aligned LC242 in the cell was investigated for different cell thicknesses. The variation of retardation with incident angle creates a characteristic retardation profile, which is dependent on the macroscopic orientation of liquid crystals, such as planar, vertical, and tilted alignments.
Figure 5 shows a typical retardation profile with a symmetrical shape for the vertical LC alignment. With a larger thickness and wider viewing angle, more light is delayed in the LC242 cell. Although retardation depends on cell thickness, the retardation value does not increase linearly with cell thickness. The retardation values of the 50 μm thick LC cell are smaller than those of the 40 μm thick cell. This result suggests that the retardation of the LC242 cell might have a critical limit that depends on the thickness of the liquid crystals. Our results indicate that the vertical alignment of LC242 molecules becomes worse as the cell thickness becomes greater than 40 μm.
The temperature dependence of retardation was evaluated for vertically oriented LC242 cells. Figures 6(a) and 6(b) show the effects of temperature on retardation of LC cells with thicknesses of 5 and 25 μm. Both LC cells exhibit a decrease of retardation with increasing temperature. This is interpreted to result from a decrease in birefringence of the cell due to enhanced movements of LC molecules with increasing temperature. Since molecular motions increase with temperature, the LC director cannot maintain a constant direction, resulting in poor birefringence of the cell. However, for the LC242 cell with a 25 μm thickness, the retardation profile is not symmetrical at higher temperatures although the LC cell is within or below the LC phase temperatures (79-118 °C). This result indicates that the vertical alignment of LC242 molecules in the cell with a 25 μm thickness cannot be maintained in the perfect order above room temperature. The molecular alignment in the LC cell will affect the final LC orientation of the cured film because the LC film is made by directly UV-curing liquid crystals in the LC cell.
Fig. 6 Retardation profiles of vertically oriented LC242 cells measured at different temperatures with cell thicknesses of (a) 5 μm and (b) 25 μm.
Since LC242 is a reactive liquid crystal with acrylate groups at both ends, a cured film with crosslinked structures can be obtained through a photoreaction of the acrylates. To determine if the molecular alignment formed in the LC phase of the cell can be maintained after curing, vertically aligned LC242 molecules in the cell were UV-cured, and the retardation of the cured film was measured.
As shown in Fig. 7, the retardation profile of the cured LC film with a 5 μm thickness is no longer symmetrical even though it exhibits a symmetrical shape before curing in the LC cell. The retardation profile of the cured LC film with a 5 μm thickness shows a linear change in the incidence angle of zero. This result indicates that LC242 molecules in the cured LC film with a thickness of 5 μm adopt a tilted orientation after photo-reaction of acrylate groups.
A typical retardation profile of vertically oriented liquid crystals was obtained from very thin photo-cured LC242 films (1.0 and 2.5 μm thick). These films were obtained by UV-curing LC cells at temperatures below the crystal to nematic transition temperature (79 °C). The vertical alignment of LC242 molecules in the cell cannot be maintained after photo-curing if the thickness of the LC cell is greater than 5 μm or the curing temperature is within the LC phase temperatures. Although LC242 molecules are easily aligned vertically in the LC phase, it is difficult to maintain the alignment of the molecules during the curing reaction. Since photo-reactive acrylate groups are attached to the end of flexible alkyl chains of LC242 (see Fig. 1), it is difficult to prevent LC molecules from moving during the photoreaction in the LC phase. This leads to disruption of the LC alignment after the photoreaction of the acrylates. The reason why vertically oriented LC films, as seen in Fig. 8, are obtained by photo-curing the LC cell at the temperature (40 °C) of the crystalline phase can be understood from the DSC data of LC242.
In the DSC heating experiment (Fig. 2), a large endothermic peak at 79 °C and a small endothermic peak at 118 °C were observed for LC242 powder. The endothermic process at 79 °C is a phenomenon in which LC242 changes phase from crystal to liquid crystal, and the small change at 118 °C is due to the phase transition from liquid crystal to isotropic melt phase. However, during the cooling experiment after LC242 is completely melted, a small exothermic peak appears near Tni, and no recrystallization peak is observed until −20 °C. These results indicate that LC242 molecules in the LC state do not change back into a crystalline phase upon cooling. LC242 is gradually solidified while maintaining the order of the LC phase beyond the Tkn transition temperature (79 °C). LC242, which has a larger molecular size compared to usual liquid crystal molecules, does not change rapidly into a crystalline form from the liquid crystal state. The recrystallization of LC242 molecules proceeds very slowly while maintaining the order of the LC phase at temperatures below the liquid crystal to crystal transition temperature. Thus, it might be possible for acrylate groups at the ends of the LC242 molecule to react with only adjacent molecules without disrupting the LC orientations. This is why vertically oriented LC films, as seen in Fig. 8, can be obtained by photo-curing the LC cell at the temperature (40 °C) of the crystalline phase.
Polarized ATR-FTIR spectroscopy was employed to investigate the degree of curing and the uniformity of LC films. By monitoring the decrease in peak intensity of the 1635 cm−1 band, which is due to the photo-reactive C = C bond in the acrylate group, it was found that more than 90% of LC242 reacted to form a cured network.
The FTIR image in Fig. 9(b) shows that it was possible to obtain a uniform pattern on LC films by employing the photo-reactive LC242. The FTIR imaging technique is an experimental method to obtain the IR spectra by dividing it into several micrometer-sized pixels and then comparing the intensity of a specific peak for each pixel to display the distribution of the sample. In order to easily visualize the change in intensity of a specific peak with a 2D image corresponding to each sample region, the peak intensity is displayed in proportion to the color of each pixel. The intensity of the parallel tendency band at 1605 cm−1 (stretching mode of the C = C phenyl ring) of each pixel is represented by color in the FTIR image data in Fig. 9(b), where the area with a vertical orientation of LC242 is displayed as a bright color, and the area with a planar LC orientation is denoted by a dark blue area. In the crossed polarized microscope image of the LC242 film shown in Fig. 9(a), the area of vertically oriented liquid crystals appears as a dark image. The FTIR image of the LC alignment is in good agreement with the crossed polarized microscope image.
4. Conclusions
The vertical alignment of LC242 molecules could be induced on an oxygen plasma-treated glass surface without using an LC alignment layer. Retardation of vertically aligned LC242 was investigated for LC cells with different cell thicknesses. The greater was the thickness, the greater was the light delay in the LC242 cell. However, the retardation of the LC242 cell might have a critical limit that depends on the thickness of liquid crystals. The vertical alignment of LC242 molecules in the cell cannot be maintained after photo-curing if the thickness of the LC cell is larger than 5 μm or the curing temperature is within the LC phase temperature range. Although LC242 molecules are easily aligned vertically in the LC phase, it is difficult to maintain the alignment of the LC molecules during the UV-curing reaction. Since photo-reactive acrylate groups are attached to the ends of flexible alkyl chains of LC242, it is difficult to prevent LC molecules from moving during the photoreaction in the LC phase. This leads to disruption of the LC orientation after the reaction of the acrylates. In this study, however, vertically oriented LC films can be obtained by photo-curing LC242 at the temperature of the crystalline phase. Since the recrystallization of LC242 molecules proceeds very slowly while maintaining the order of the LC phase at temperatures below the liquid crystal to crystal transition temperature, it is possible for acrylate groups at the ends of the LC242 molecule to react with only adjacent molecules without disrupting the LC orientations.
Funding
Industrial Strategic Technology Development Program (#10053627) funded by the Ministry of Trade, Industry & Energy of Korea.
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