We investigated the effect of the relative ratio between the diacrylate (DA) and the monoacrylate (MA) reactive mesogen (RM) molecules on the transmission spectrum of polymer-stabilized cholesteric liquid crystal (PSCLC). The reflection color from the top substrate where the UV was exposed was shifted from red to green with increasing the fraction of DA. It was also found that the PSCLC sample with the fraction of DA over 5 wt% formed 2-dimensional poly-grain textures on the bottom substrate. The periodicity of the grains was about 1-2 μm with the consequence of a light-scattering optical texture of the PSCLC sample. By optimizing the relative fraction between MA and DA, we could obtain a vivid broadband PSCLC sample without a scattering of light.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Cholesteric liquid crystal (CLC) was first discovered in cholesterol esters in 1888 [1–3] and has drawn a considerable attention for the electrooptical components [4–19]. The CLC has various interesting physical properties such as the selective reflection [4–13], flexoelectric effect [14-15], and memory effect [16-17]. The selective reflection property can be used as an switchable mirror  or lasing components [12-13]. The flexoelectric effect of CLC can be used for the fast switching phase modulation components [14-15]. In addition, the encapsulated CLC can be fabricated as flexible film and can also be used for the flexible display components [16–18] and optical sensors [19-20].
The CLC molecules form a helical structure and reflect a circularly-polarized light with the same handedness of helix by Bragg reflection . The reflection wavelength λ is given by nop<λ<nep, where ne and no are the extraordinary and ordinary refractive indices of the liquid crystal (LC), respectively, and p is the pitch of the helix. For the application such as a switchable mirror , Δλ should be broader enough to reflect the whole visible light. The early CLC materials of cholesterol esters has small Δn≡ne-no and was difficult to obtain a broad reflection bandwidth Δλ. In these days, various kinds of nematic LC (NLC) material with high Δn over 0.2 has been developed and the Δλ can be increased by doping small amount of chiral dopant. Nevertheless, Δλ is still limited less than 100 nm even using CLC medium with Δn about 0.3.
There have been many efforts to broaden Δλ [4–10]. First, analogous to the beetle plusiotis boucardi which reflects both green and red light , the method of stacking CLC layers with different p was studied . Second, the method of embedding p gradient by polymer-stabilization has been studied [6–11]. The p gradient of CLC can be imposed by exposing a UV light from one side of the sample which contains a mixture of reactive monomer and CLC [Fig. 1(a)] [6–9]. Because the UV intensity is decreasing via penetrating into the sample, denser polymer networks are formed near the UV-exposed surface than the bulk [Fig. 1(b)]. Near the polymer-rich surface, p becomes longer and reflects longer λ than the opposite surface provided achiral monomers were used [21-22]. It was claimed that this situation was solely due to the concentration gradient between an achiral nematic compound and a chiral compound. It has been shown that the reflection wavelength from the UV-exposed can be shortened as a consequence of the mechanical action of the polymer network built inside the LC . In addition, p gradient can be also obtained by adding small amount of UV-absorbing quencher [7-8] or by intrinsically UV-absorbing LC .
Because the polymer network morphology of the polymer-stabilized CLC (PSCLC) is affected by the reaction rate and the diffusion rate of the system, the reflection spectrum of PSCLC strongly depends on the constituent reactive molecules as well as the UV irradiation conditions [24-25, 30]. Nevertheless, the exact relation between the chemical structure of the constituent molecules of the network-forming material and the reflection property of the PSCLC has not been fully identified.
In this paper, we experimentally investigated the effect of the relative ratio between the diacrylate (DA) and the monoacrylate (MA) reactive mesogen (RM) molecules on the transmission spectrum of PSCLC. It was found that the reflection color from the top substrate where the UV light was exposed was shifted from red to green with increasing the fraction of DA. It was also found that the PSCLC sample with the fraction of DA over 5 wt% forms a 2-dimensionally ordered poly-grain structure near the bottom substrate. The weight fraction of each constituent molecules means its concentration with respect to the total amount of the CLC + RM mixtures, here in after. The periodicity of the grains was about 1-2 μm with the consequence of a light-scattering optical texture of the PSCLC sample.
2. Experimental procedure
A commercial CLC mixture (ZSM5089XX, JNC) was mixed a RM mixture at a weight ratio of 4:1. The RM mixture is composed of MA (JRM-CU1, JNC), DA (JRM-CU2, JNC) RM, UV-quencher (2-benzotriazol, Aldrich), and photoinitiator (Irgacure651, Ciba Chem). Δn of the ZSM5089XX, JRM-CU1, and JRM-CU2 at λ = 550 nm were measured with an Abbe refractometer (Kruess), and the results were 0.2, 0.05, and 0.07, respectively. The MA and the DA RM molecules were achiral. The relative ratio between the MA and DA molecules were varied, but the total fraction of them was kept to be 17.8 wt% in all samples. The UV-quencher and the photoinitiator was maintained to be 2.0 and 0.2 wt% to the total weight of the CLC-RM mixture. For the fabrication of the PSCLC sample, indium-tin-oxide (ITO)-deposited glass substrates were coated with a planar alignment polyimide (PI) PIA-X189-KU1 (JNC) and then baked at 230 °C for 1 h. The substrates were rubbed with a cotton cloth and assembled in an antiparallel fashion. The cell gap d was maintained to be 10 μm using bead spacers. The CLC-RM mixtures were injected into the empty cell at 25 °C by capillary force. Then, the UV light with an intensity of 0.3 mW/cm2 was exposed to the samples for 60 min at 25 °C.
The optical transmittance (TR) was measured using a UV-visible spectrometer SV2100R (KMAC). The scanning electron microscopy (SEM) image of the substrate surface was obtained using S3500N (Hitachi). For the SEM observation, the PSCLC sample was disassembled and diluted with hexane for 10 min, and then was deposited with gold nanoparticles.
3. Results and discussion
Figure 2 shows TR of the PSCLC samples with various fraction of DA vs. λ. Before UV exposure, the center wavelength λc of the stopband was about 650 nm and the full width at half maximum (FWHM) of Δλ was about 70 nm. The transmission spectra of the samples with various fraction of DA were similar before UV exposure. After UV exposure, Δλ of the PSCLC samples was broadened depending on the relative fraction of DA and MA. We need to mention that the total fraction of the DA and the MA molecules were same through the samples. The 0.4 wt% DA-mixed sample showed the stopband from 450 to 720 nm and Δλ was 270 nm [Fig. 2(a)]. With increasing the fraction of DA, the stopband was blue-shifted and Δλ became narrow. In addition, the PSCLC sample which contains DA over 5.2 wt% showed a significant decrease of TR due to scattering of light [Fig. 2(c)-3(e)].
In order to understand the physical reason of the different transmission spectra of the PSCLC samples, we first examined the reflected image of the samples [Fig. 3]. Figure 3(a)-3(e) were obtained via illuminating white light to the top substrate where the UV light was exposed, while Fig. 3(f)-3(j) were obtained via illuminating the bottom substrate. The reflected color from the top substrate was shifted from red to green with increasing the fraction of DA [Fig. 3(a)-3(e)]. The reflected color from the top and the bottom substrates became more similar with increasing the fraction of DA. On the other hand, the reflected color from the bottom substrate was not vivid as much as the top substrate and showed scattering when the fraction of DA was over 5.2 wt% [Fig. 3(h)-3(j)].
Comparing these pictures with the spectra data in Fig. 2, it is concluded that the broadband stopband of the PSCLC sample with a small fraction of DA was induced by p gradient across the sample. Near the top substrate, the RM molecules were predominantly polymerized and the p gets longer, resulting in red-shift of reflecting color [Fig. 3(a)]. Because the achiral RM molecules were diffused toward the top substrate, p near the bottom substrate gets shorter, resulting in blue-shift of reflecting color. The broad reflection color from the bottom substrate is also related to the reduction of the RM concentration diffused to the top substrate. Δn of the host LC is 0.2 greater than that of the RM 0.05-0.07. Thus, the relatively neutral reflection color from the bottom substrate where the host LC is rich can be wider than that near the top substrate. Nevertheless, the exact role of the pitch gradient and the birefringence gradient across the sample should be investigated in detail and we are planning to examine it using the total internal reflection technique.
We need to mention that Relaix et al  and Agez et al.  reported reverse distribution of p across the sample. Their results lie on direct reflection spectra added to investigations of the morphology of the polymer network by SEM and transmission electron microscopy (TEM). In their report, p gets shorter near the top substrate and gets longer near the bottom substrate. It was shown that the mechanical stress gradient imposed by the shrinkage of the polymers resulted in the shorter p approaching the top substrate . Meanwhile, our group recently reported that p could get longer near the top substrate when the MA RM possessing nematic phase at room temperature was used for the polymer-stabilization . We explained that the shrinkage effect of MA RM could be smaller than that of the DA RM. We think that the results in Fig. 2-3 can be interpreted with similar physical model. As shown in the reflected image of the PSLC samples from the top substrate [Fig. 3(a)-3(e)], the reflection color shifted to the shorter wavelength region with greater fraction of DA, and vice versa. Thus, p near the top substrate gets shorter with increasing the fraction of DA, while it gets longer with increasing the fraction of MA.
We also investigated the TR spectrum of the PSCLC sample with 0.4 wt% DA vs. UV exposure time [Fig. 4(a)]. The stopband was broadened to the longer wavelength region as well as to the short wavelength region after exposing the UV light for 20 min. The TR intensity at λ<580 nm was further decreased with the longer UV exposure and gradually the TR spectrum was saturated. Figure 4(b) shows the reflective POM image of the same PSCLC sample vs. UV exposure time. The reflection color of the top substrate was shifted to the longer wavelength region with increasing the UV exposure time, while that of the bottom substrate was shifted to the shorter wavelength region. Thus, it is more confirmed that p gradually gets longer near the top substrate and it gets shorter near the bottom substrate with longer UV exposure. It is also noticeable that the stopband is broadened to the shorter wavelength region more gradually with UV exposure time. This is presumably due to the slower photo-polymerization near the bottom substrate than the top substrate. Consequently, the stopband is gradually expanded to the short wavelength region with UV exposure time.
We also examined the reflective polarizing optical microscopy (POM) texture of the PSCLC samples [Fig. 5]. The reflective POM texture showed similar color as the results in Fig. 3. With greater fraction of DA, the reflected color from the top substrate was shifted from red to the green. The PSCLC sample with 0.4 and 2.0 wt% DA showed typical planar texture with some oily streak boundaries [Fig. 5(a)-5(b), 5(f)-5(g)]. On the other hand, the reflected textures of the samples with DA over 5.2 wt% showed different textures. The PSCLC with DA over 5.2 wt% showed 2-dimensional poly-grain structure [Fig. 5(c)-5(e), 5(h)-5(j)]. The grain structure were more clearly observed on the bottom substrate. The periodicity of the grains was about 1-2 μm similar to λ of visible light. Thus, the strong scattering from the bottom of the PSCLC sample with DA over 5.2 wt% in Fig. 3(h)-3(j) is due to the micron-sized grains. The reflective POM texture from the bottom substrate became darker with more fraction of DA, implying the increase of light scattering.
As an aside, the 2-dimensional grain structure is very similar to the 2-dimensional grid textures in the previous literatures [26–29]. The 2-dimensional pattern was observed when the planar-aligned CLC molecules were untwisted by external field or mechanical stress or temperature gradient. Thus, such a pattern was physically shown to reduce the excessive elastic energy of the CLC molecules. We think that our results can also be interpreted with a similar physical model. The PSCLC molecules Fig. 5 was also located between planar alignment layers. The UV-polymerization of the RM molecules and their diffuse toward the top substrate probably gives similar strains to the CLC molecules as the external field or temperature gradient. Although the grain texture in the previous literatures was transient and disappeared with time, the grains in Fig. 5 was retained with time by the polymer-stabilization effect. Further study for understanding the exact physical reason of the grain formation is beyond the scope of this paper and remained as a future work.
Figure 6 shows the SEM image of the PSCLC samples surface after washing off the CLC molecules with hexane. Figure 6(a)-6(e) and 6(f)-6(j) were obtained using the top and the bottom substrates, respectively. We should mention that the SEM image in Fig. 6 is not the cross-section image like in the previous literature [23, 25]. Thus, we could not firmly conclude that the polymer morphology in Fig. 6(a)-6(e) correspond to the one adjacent top substrate, vice versa. Nevertheless, the polymer network on the top substrate was denser than the bottom substrate. This results reminds the previous literature by Mitov et al.  wherein the polymer morphology was observed with the RM and the LC drops contacted. In that paper, the polymer network near the RM drop was denser compared to the network near the LC drop. The polymer morphology of the top substrates in Fig. 6(a)-6(e) was similar with the morphology of the RM rich region in the literature . Although we could not obtain the cross-section SEM image, the result in Fig. 6 may imply the predominant polymerization of the RM molecules near the top substrate.
The surface of the 0.4 wt% DA-mixed sample showed a relatively smooth morphology [Fig. 6(a) and 6(f)], while that of the 12.8 wt% DA-mixed sample showed a coarse morphology with larger voids [Fig. 6(e) and 6(j)]. Thus, the polymer network morphology became rough with increasing the fraction of DA. In the previous literatures [23–25], it was reported that a strong UV intensity or the RM with a poor miscibility with host LC resulted in the coarse morphology with large voids. The polymer morphology in Fig. 6 can be interpreted with a similar model. The greater fraction of DA also increases the reaction rate of the system, resulting in the coarse polymer morphology. In addition, the MA-RM which was used in this study has nematic phase at room temperature, while the DA-RM has nematic phase at 81-110 °C. Thus, the DA-RM molecules have poorer miscibility than the MA-RM molecules, resulting in the rough morphology. Meanwhile, the 2-dimensional poly-grain structures was clearly observed in Fig. 6(j). Because the size of the voids is similar to the wavelength of light, a strong light scattering is shown [Fig. 2(c)-2(e) and Fig. 3(h)-3(j)].
Figure 7 shows the photograph of the reflected image of the PSCLC sample with 0.4 wt% DA. A white light source was incident at an incident angle of 45°. The sample showed vivid color and specular reflection with least scattering. The background color looks yellowish and this is due to the shift of the stopband to the shorter wavelength range by the pitch decreasing effect at large incident angle. We think the suggested results can be helpful for developing electrooptical components with a broad stopband.
To summarize, we studied the effect of the relative ratio between DA and MA molecules on the transmission spectrum of PSCLC. The reflection color from the top substrate where the UV was exposed was shifted from red to green with increasing the fraction of DA. In addition, the reflection color from the top and the bottom substrates became similar with increasing DA. The PSCLC sample with DA over 5 wt% formed 2-dimensionally ordered grain structure with a periodicity about 1-2 μm near the bottom substrate. By optimizing the relative fraction between MA and DA, we could obtain a vivid broadband PSCLC sample with a stopband of 450 nm<λ<720 nm.
LG Display; National Research Foundation of Korea (NRF) (2016R1A2B4010361).
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