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A mixture of nematic liquid crystals (NLCs) with a small amount of a polypropyleneimine-based liquid crystalline dendrimer (D-6PC5 and D-6PPCN) exhibited spontaneous homeotropic alignment between the native glass substrates. This dendrimer-induced homeotropic alignment required two conditions; 1) the dendrimer completely dissolves in the NLC, and 2) a substrate surface is hydrophilic with the surface free energy above approximately 65 mN·m−1. The interdigitated-electrode cells without any surface treatment of the substrates were fabricated by filling with the LC dendrimer (D-6PC5)/nematic LC mixtures. They exhibited an electro-optical behavior when applying AC, vertical-alignment drive occurred in the cells. Based on these results, we speculated that the dendrimer adsorbs on the substrate surface and acts as a vertical alignment layer.
© 2014 Optical Society of America
1. Introduction
Liquid crystal displays (LCDs) have a wide viewing angle, comparatively fast response time, high contrast ratio, and vivid color performance that have been utilized for televisions in the past decade. The LCDs have a variety of driving modes; twist nematic (TN), vertical alignment (VA), in-plane switching (IPS), optically compensated bend etc. In the LCDs, twist, vertical, parallel, and splay alignments of the LC molecules are formed in the off-state of the electric voltage. They require various alignment layers of polyimides having different chemical structures to realize such alignments. For example, main-chain type polyimides are used in the IPS-mode cell to form a homogeneous alignment [1–5], and side-chain type polyimides are used in the VA-mode cell to form a homeotropic alignment [6–11]. The preparation of the polyimide alignment layers usually requires large quantities of solvent, high temperature operation for the thermal imidization reaction, and a rubbing process. The rubbing process has a negative influence on the integrated circuits of the LCDs and cleanliness of the clean rooms.
Recently, it has been proposed that electro-optical behaviors can occur by adding fullerene particles to nematic LCs (NLCs) without using alignment layers [12]. The LC molecules were vertically aligned on the particle surface. Thus, optical isotropy was realized by dispersing the particles in the LCs of the cell, and the LCs became aligned when applying AC. Jeng et al. reported that adding nanoparticles of polyhedral oligomeric silsesquioxanes (POSS) into n-type LCs causes vertical alignment of the LCs without an alignment layer [13–15]. In this case, the electro-optical properties of the LC cell were similar to a conventional homeotropic LC cell with alignment layers. The POSS nanoparticle could be applied to a flexible homeotropic LCD using poly(ethylene terephthalate) substrates [16].
Dendrimers having peripheral mesogens exhibit a smectic LC nature [17–22], and the LC dendrimers vertically align on a bare glass substrate [18, 20–22]. We have prepared LC dendrimers based on polypropyleneimine dendrimers having a variety of mesogens to explore their vertical alignment properties [20–22]. When a small amount (typically below 1wt%) of LC dendrimers was dissolved in the NLC, the entire NLC spontaneously exhibited a vertical alignment property between bare glass substrates. We have already reported the development of an LC cell without the polyimide alignment layer using this dendrimer-induced vertical alignment [23]. The positive NLCs and a comb-like electrode were combined in the cell, in which the LC was vertically aligned by the LC dendrimer in the off-state and tilted in the on-state. Because the dendrimers completely dissolves in the NLCs, the homogeneous dendrimer system should have an advantage in reproducibility compared to the heterogeneous nanoparticle systems. However, the details of the dendrimer-induced vertical alignment and the electro-optical cell properties have not been reported. In this study, we investigated the alignment properties of the dendrimer (shown in Fig. 1)/NLC mixture. We then fabricated LC cells by filling them with the mixtures, and investigated the electro-optical behaviors of the LC cells and effects of the concentration of the LC dendrimer on the homeotropic alignment and electro-optical behavior.
2. Experimental
2.1. Materials
4′-Cyano-4-n-penthylbiphenyl (5CB) and NLC mixture (ZLI-4792) were used as the matrix NLC, which were produced by the Chisso Co. and Merck Ltd., respectively. Poly(propyleneimine) dendrimers having peripherally mesogenic derivatives, the 4′-cyanobiphenyl and trans-4-(4-pentylcyclohexyl)phenyl group, were prepared. They are abbreviated as D-6PPCN and D-6PC5, respectively. Their synthetic methods and characterization were described in previous papers [20, 22]. D-6PPCN (1wt%) was mixed with 5CB (99wt%) at 80 °C, which is above the nematic-isotropic phase transition temperature (TNI), using a magnetic stirring bar. No precipitation was observed after cooling to room temperature. D-6PC5 was also mixed with ZLI-4792 in the same manner.
2.2. LC cell fabrication
The LC cell used in this study was prepared with two 0.7 mm-thick bare glass plates which has 2.7 μm-thick polyester film spacers. The plates were overlapped and sealed except for one side by epoxy resin. The LC dendrimer/ NLC mixture was injected using a microsyringe into an empty LC cell through the unsealed side. Finally, the remaining side was sealed by epoxy resin. The obtained cell was heated above TNI of the mixture to remove any history of the nonuniform injection. The LC cell with electrodes was fabricated with a bare glass substrate and a glass substrate with interdigitated Cr electrodes, as shown in Fig. 2. The width between the electrodes was 10 μm, and the gaps in the LC cells were 3 μm and 7 μm, which were maintained by particle spacers. The cell was sealed, filled with the LC mixture and heated in the same way as the cell without an electrode.
Fig. 2 Schematic representation of LC cell used in this study. It was assembled with a bare glass substrate and a glass substrate with an interdigitated Cr electrode.
2.3. Measurements
The optical textures of the LC samples were examined using a polarizing optical microscope (POM: Olympus BX-50P) equipped with a heating stage (Linkam Co., TH-600RMS). The thermal properties of the samples were investigated using a differential scanning calorimeter (DSC: TA Instruments DSC Q200) under N2 purge with the heating and cooling rates of 10 °C·min−1. The response time of the cell was measured using our homemade apparatus [24]. The system recorded the input voltage and intensity of the transmitted light through the sample under crossed polarizers.
3. Results and discussion
3.1. Homeotropic alignment of nematic LC induced by LC dendrimer on glass substrate
The LC dendrimer D-6PPCN shows a liquid crystalline nature of smectic A (SmA); the SmA to isotropic phase transition (TSI) is 76 °C. It exhibited a spontaneous vertical alignment on bare glass substrates when it was slowly cooled at −1 °C·min−1 from the isotropic melt, as described in a previous paper [20]. D-6PC5 showed the SmA phase whose TSI was 73 °C, and it also exhibited a spontaneous vertical alignment upon slow cooling [22]. D-6PPCN was completely dissolved in 5CB at a 1wt% concentration. The mixed sample was heated to TNI (35 °C) at 10 °C·min−1, then cooled to room temperature at −1 °C·min−1 between the two bare glasses. The resulting mixture showed a complete dark field at room temperature by POM with crossed polarizers as shown in Fig. 3(a). Conoscopic observation of the sample (inset in Fig. 3(a)) showed a typical isogyre, which indicates homeotropic alignment of the entire LC mixture. When the sample was heated at 10 °C·min−1, the dark field was also observed at and above the TNI. But the isogyre did not appear at 45 °C due to the formation of an isotropic melt. Because the dark field was kept upon heating to above the TNI, the homeotropic alignment could be maintained close to TNI. Upon cooling the same sample at −1 °C·min−1, a typical schlieren texture, which appeared at around 35.4 °C, disappeared and became dark again at 35.2 °C, and then the dark field expanded over the entire area. Because 5CB exhibits no homeotropic alignment by itself between the bare glass substrates, the small amount of dissolved D-6PPCN induced the homeotropic alignment of 5CB.
Fig. 3 POM images of D-6PPCN/5CB (1/99, wt/wt) mixture during (a–c) heating at 10 °C·min−1, and (d–g) cooling at −1 °C·min−1.
Table 1 shows the solubility of the dendrimers in the NLCs. For the host LC, we selected 5CB as a typical LC having a CN group, and ZLI-4792 as an LC mixture containing fluorine atoms. D-6PC5 showed a good solubility in both of the host LCs at a 1 wt% concentration, whereas D-6PPCN did not dissolve in ZLI-4792. Thus the nature of mesogens significantly affects the solubility. Figure 4 shows the POM images of the mixture of the dendrimers in the host LCs between the bare glass substrates at room temperature after heating to TNI and cooling at −1 °C·min−1. The homogeneous mixtures, D-6PC5/5CB and D-6PC5/ZLI-4792, exhibited a dendrimer-induced homeotropic orientation of the host LCs with a dark field and isogyre during conoscopic observations. The heterogeneous mixture of D-PPCN/ZLI-4792 exhibited a thread-like texture due to the planar nematic orientation. These results indicated that the solubility of the dendrimer toward the host LCs is an important factor that cause the dendrimer-induced homeotropic orientation.
Table 1. Solubility of the dendrimers in the nematic host LCs a)
Fig. 4 POM images of the dendrimer/NLC (1/99, wt/wt) mixture between the bare glasses at room temperature after heating to TNI and then gradual cooling. The insets are conoscopic images.
3.2. Effect of the substrate surface on the induced homeotropic alignment
In order to estimate the effect of the substrate surface, except for the native glass, on the induced homeotropic alignment, we carried out POM observations of the homogeneous mixtures on the substrate consisting of organic polymers. The selected substrates are listed in Table 2. The organic polymers were spin-cast on the glass surface and the LC mixtures were sandwiched by them. To evaluate the nature of the surfaces, we determined the surface free energy (γs) according to the Owens-Wendt method (Eq. (1)) [25] by measuring the contact angle of water and glycerol on each surfaces.
where γl, , , and , respectively, indicate the surface free energy of the liquid, non-polar distributing component of the surface free energy of the liquid and solid, and polar hydrogen-bonding component of the surface free energy of the liquid and solid. We used the parameters for the liquids, γwater = 72.8, , , γglycerol = 64.0, , and [26]. The calculated and were combined to obtain γs, which are also listed in Table 2. The γs’s of the dendrimers cast on the glass substrate were calculated in a similar manner.Table 2. The substrates and their surface free energy (γs)
The POM images of the dendrimer/NLC mixture (1/99, wt/wt) on the selected surfaces are shown along the γs scale in Fig. 5. On PAAm, the dark field and isogyre were observed by POM regardless of the combination of the dendrimer and the host LC. Thus the hydrophilic surface of PAAm can cause the dendrimer-induced homeotropic orientation. On the other hand, no homeotropic orientation was observed on the hydrophobic surface having a γs value below 65 mN·m−1. On these surfaces, the schlieren or thread-like texture, which is typical of NLCs, was observed. Therefore, the hydrophilic surfaces work well and the critical γs can be estimated at around 65 mN·m−1.
Fig. 5 POM images of dendrimer/LC (1/99, wt/wt) mixtures on the organic polymer film cast on a glass plate. The scale indicates the surface free energy of the polymers. For abbreviation of the polymers, see Table 2.
3.3. Response of the mixture toward horizontal electric field
LC cells were fabricated by adding the dendrimer/NLC mixtures to the empty cells with the interdigitated electrodes shown in Fig. 2. The cell gap was maintained at 3 μm by particle spacers. Figure 6 shows the electro-optical behaviors of the LC cells filled with the D-6PPCN/5CB and D-6PC5/ZLI-4792 mixtures (99/1, wt/wt). The cells were observed under POM with crossed polarizers, in which the stripe of the electrodes was placed at a diagonal position (45°) to the crossed polarizers. They also showed a dark field at 0 V due to the homeotropic alignment similarly on the glass surface without electrode. When applying the AC voltage of 10 V, a bright field immediately appeared due to distortion of the homeotropic alignment by the horizontal electric field. The bright field instantaneously changed to a dark field again in the off-state, and this indicated that the distorted NLC went back to the homeotropic alignment.
Fig. 6 POM images of (a) D-6PPCN/5CB and (b) D-6PC5/ZLI-4792 mixture (1/99, wt/wt) applied a horizontal AC electric field of 10 V at room temperature. The cell gaps are 3 μm. The dark circles are due to the spacer particle and the orientation defect around them.
Figure 7 shows the change in intensity of the transmitted light from the cell with 1wt% D-6PC5/ZLI-4792 under an AC field of 10V. As described above, the light transmitted in the on-state and the cell became dark in the off-state. The dissolved dendrimer acts as a vertical alignment agent like an alignment layer in the VA mode drive. The time needed to change the intensity from 90% to 10% of the full response (τoff) was unchanged at 12–13 msec with repeated cycling. Thus, the response to the electric field could be repeatable at least several times and this should indicate the dendrimers have potential to apply to LC display devices.
Fig. 7 Change in transmittance of the LC cell versus time when applying an AC voltage (10V) at 150 msec intervals. Periods of on-state of the AC voltage are indicated by the arrows. The cell was filled with the D-6PC5/ZLI-4792 (1/99, wt/wt) mixture, and the cell gap was 7μm.
3.4. Speculation of the dendrimer-induced homeotropic alignment
In our previous paper, we showed that the layer spacing in the smectic A phase of D-6PPCN agrees with its length calculated by assuming a stretched structure [20]. Therefore, the den-drimers in the bulk should form a dumbbell-like structure with 8 mesogens existing on the both sides of the molecule. When we prepared a cell using the ITO-coated glass substrate, the TOF-SIMS analysis indicated that the dendrimer was adsorbed on the surface of the ITO substrate [23]. Based on this result, we assumed the mechanism of the induced homeotropic alignment as follows: the dissolved dendrimer moves from the bulk and adsorbs on the substrate surface, the dendrimer aligns vertically on the surface, and the adsorbed dendrimer act as an alignment layer to align the host LC according to the orientation determined by the den-drimer alignment. As described above, the dendrimers with the different mesogens exhibited a similar behavior to induce the hemeotropic alignment of NLCs on the different surfaces. The hydrophilicity of the dendrimers is significantly different depending on the mesogenic structure with the γs of 38 and 13 mN·m−1 for D-6PPCN and D-6PC5, respectively, as shown in Table 2. In spite of this different hydrophilicity, the alignment behaviors were similar for all the dendrimer/NLC combinations. Thus we speculate that the polyamine dendrimer core, which is the common structure of both dendrimers, adsorbs on the surface. This is illustrated in Fig. 8. The hydrophilic polyamine should prefer a hydrophilic surface and the homeotropic orientation could not be induced on the hydrophobic surfaces.
Fig. 8 Schematic representation of the dendrimer-induced vertical alignment. The filled and open bars indicate the mesogens in the denrimer and NLC, respectively. The lines represent the dendrimer core and the spacer chain.
In a previous paper, we reported the effect of the dendrimer generation on the homeotropic alignment of the dendrimer itself on the native glass [21]. As a result, the 5th generation den-drimer exhibited a smectic phase, but has a poor ability to homeotropically align whereas the 2nd–4th generation dendrimers oriented well. It is well known that the higher generation den-drimer forms a spherical structure and the core is surrounded by the peripheral groups. In our case, the core might be shielded by the peripheral mesogenic groups and thus the core could not approach the surface. This might indicate the importance of the adsorption of the core on the surface for the homeotropic alignment and suggests our proposed mechanism about the dendrimer-induced homeotropic alignment.
4. Conclusion
Homeotropic alignment of the NLCs between bare glass substrates was achieved by dissolving a small amount of the LC dendrimers. To achieve this dendrimer-induced homeotropic alignment, the solubility of the dendrimer into the NLC was important and the heterogeneous mixture did not exhibit the alignment. The substrate surface also has a strong influence and the hydrophobic substrate having the surface free energy above 65 mN·m−1 exhibited the alignment. The interdigitated-electrode cells filled with the LC dendrimer/NLC mixtures exhibited electro-optical behaviors at the off- and on-states. For the dendrimer-induced homeotropic orientation, we speculate that the cores of the dendrimer molecules are adsorbed on the surface and force the orientation of the NLCs.
Acknowledgments
This work was financially supported by JSPS KAKENHI Grant Number 23350108.
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