1. Introduction

Cholesteric liquid crystal (ChLC) is a liquid crystalline phase in which the constituent molecules self-organize into a helical structure. The helical structure gives rise to a so-called selective reflection (SR) band, in which light with the same circular polarization handedness as the helix is Bragg reflected over the wavelength region n_o × p – n_e × p, where n_o, n_e, and p are the ordinary and extra-ordinary refractive indices and the helical pitch, respectively. Although the tunability of the SR band by external stimuli [1–5] makes ChLCs potentially useful for optical applications such as switches [6], lasers [7,8], and displays [9–12], their slow switching time has hindered their use in real-life applications. This originates from the field-induced deformation of the helical structure, which occurs on larger length and time-scales compared to the reorientation of the director in a nematic LC [13]. Polymer-stabilized ChLCs in which polymer networks are dispersed in ChLCs show an improved response time; however, the movement of the polymer network with the LC molecules limits the response time to few 10 ms [14]. Recently, our group has found that tuning of the SR band without deforming the macroscopic helical structure can lead to a tremendous improvement in the electro-optic response time [15,16]. This 'deformation-free' tuning mode is achieved by down-sizing the LC domains in a polymer/ChLC (PChLC) composite to several 10 nm such that the LC domains only contribute to the effective refractive index. Under an electric field, the LC molecules are reoriented along the field, but the polymer matrix is strongly fixed by the crosslinked polymer chains and is immobile; the result is that the effective extra-ordinary refractive index is decreased, while the ordinary refractive index and helical pitch remain constant. The motion of the non-reactive LC molecules confined in the nano-sized domains demonstrate a very short decay time of a few 10 μs, corresponding to a ~1000-fold improvement compared to that of conventional ChLCs.

The formation of nano-sized LC domains typically requires polymer concentrations of few to several 10 wt% in the sample, which because of the strong anchoring imposed, added with the small amount of mobile LCs, limits the tuning range. There is an urgent need to develop PChLC nanocomposites with reduced monomer concentrations, and hence larger tuning ranges (or lower driving voltages). In this study, we show that the 'deformation-free' switching behavior can be achieved in polymer-stabilized ChLCs with a monomer concentration of 6.6 wt%. By reducing the polymerization temperature, phase separation becomes suppressed in the composite, leading to the formation of smaller LC domains. As the LC domain sizes decreases from the hundred nanometer-range to ten nanometer-range, a qualitative change in the electro-optic response occurs, from 'polymer-stabilized' to 'deformation-free'. Not only is the response time improved by this change in electro-optic response, but the PChLC nanocomposite fabricated with a low monomer concentration shows an improved tuning range compared to the nanocomposite fabricated with a high monomer concentration, because of the increase in the number of LC molecules contributing to the refractive index tuning.

2. Experimental procedure

The PChLC precursor mixtures were prepared by mixing four materials, a photopolymerizable LC monomer (Merck, RM257), a nematic LC (Merck, MLC-6849-100), a chiral dopant (Merck, ZLI-4572) and a photoinitiator (Ciba, Irgacure 819). The detailed composition of each sample used in this study is listed in Table 1. The materials were dissolved in chloroform and left to evaporate for approximately 3 days. The mixture was injected into an indium-tin-oxide (ITO)-coated planar sandwich cell with a cell-gap of 10 μm (purchased from E. H. C Co.) in the isotropic phase. The sample was cooled to the cholesteric phase, and polymerized by irradiating UV light with a wavelength of 365 nm and a power of 200 mW/cm² for 1 hour. The helical axis of the ChLC was perpendicular to the glass substrate. The polymerization temperature of the low monomer concentration (6.6 wt%) samples was varied from 20 °C to −20 °C with 10 °C intervals. The others were polymerized at 20 °C. The reflection spectra were measured on a polarizing optical microscope using a fiber-optic spectrometer (Hamamatsu Photonics, PMA-11) with a spectral resolution of 2 nm and a 10 × objective lens, as a square wave electric field with a frequency of 1 kHz was applied along the helical axis on the cell. The response times were measured under the same conditions but by using a photomultiplier tube (Hamamatsu Photonics, H10722-20) and a digital oscilloscope (Tektronix, TDS 3012). The measured response times were the times required to change for the intensity from 10% to 90%. The polymer morphologies of the PChLC nanocomposites were observed by scanning electron microscopy (SEM) (Hitachi S-4300). For SEM observation, the cell was opened and rinsed by super-critical CO₂ (Rexxam Co. Ltd., SCRD401) after the polymerization process.

Table 1. Compositions of the samples used in this study

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3. Results and discussion

We first describe the change in electro-optic response of PChLCs with various monomer concentrations. Figure 1 shows the voltage-dependent reflectance of the PChLC nanocomposites with different monomer concentrations. The horizontal axis shows the wavelength, the vertical axis shows the applied electric field, and the color corresponds to the reflectance in each graph. The nanocomposite with the monomer concentration of 6.6 wt% showed a switching behavior typical of a polymer-stabilized ChLC; with increasing electric field, the reflection band blue-shifted and the peak reflectance decreased, eventually disappearing completely. This is explained by the tilting of the cholesteric helix before becoming unwound [17]. The 8.9 wt% sample showed a different electro-optic response in that the SR band narrowed without showing the blue-shift and the peak reflectance also decreased. As the concentration was increased further, the red-shift of the short band-edge wavelength became smaller, and ultimately became immobile at 13.5 wt%. Considering the theoretical band-width of the SR band (n_o × p – n_e × p), the response of the 13.5 wt% sample indicates a decrease in n_e with p and n_o remaining constant; i.e., a 'deformation-free' response is achieved, with the effective refractive index changing without distorting the helical structure.

 

Fig. 1 Electrical tuning of the SR band in the PChLC nanocomposites with different monomer concentrations (ϕ_mono) at polymerization temperature (T_p) of 20 °C.

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We will now show that a similar control over the electro-optic response can be achieved by controlling the degree of phase separation via polymerization temperature control. Figure 2 shows the voltage-dependent reflectance data of PChLC nanocomposites fabricated from a mixture containing 6.6 wt% of monomer, but with polymerization temperatures varying between 20 °C to −20 °C. The switching behavior of the PChLC nanocomposites gradually changes in a similar manner as for the monomer concentration. Similar to the sample with a high monomer concentration of 13.5 wt%, the nanocomposite at polymerization temperature of −20 °C also exhibited the 'deformation-free' switching behavior.

 

Fig. 2 Electrical tuning of the SR band in the PChLC nanocomposites with low monomer concentration of 6.6 wt% at different polymerization temperatures.

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To investigate the mechanism driving the change in electro-optic response, the morphology of the polymer network in each sample was observed by SEM after rinsing out the unpolymerized LC with super-critical CO₂. As shown in Fig. 3, the size of voids, which correspond to the LC domains, is smaller in the samples with lower polymerization temperatures. An analysis of the pores observed in the SEM image yields a size of ~297 nm (average over 20 voids) at 20 °C gradually decreasing to ~38 nm (average over 50 voids) at −20 °C. We therefore attribute the change in electro-optic response to the change in LC domain size formed in the composite. The driving force leading to the formation of smaller domain sizes is believed to be the increased viscosity at lower temperatures. LCs typically have viscosities approximately following an Arrhenius-type dependence on the temperature [18]. Therefore, phase separation and diffusion of the polymerized monomers becomes suppressed at low temperatures, leading to smaller LC domains. Low-temperature polymerization has proven useful in down-sizing the LC domains and improving the electro-optic response time in polymer/nematic LC composites [19]. Here we have shown that low-temperature polymerization in a PChLC composite can lead to a qualitative change in the electro-optic response from 'polymer-stabilized' to 'deformation-free'.

 

Fig. 3 Polymerization temperature dependence of the FE-SEM images in the PChLC nanocomposites with low monomer concentration of 6.6 wt%.

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Because the 'deformation-free' electro-optic response is caused by a change in the polymer morphology, the same effect can be expected in PChLC composites in general. However, the monomer concentration at which the 'deformation-free' response occurs would depend on the material parameters, such as viscosity, low temperature stability of the liquid crytal phase, reactivity of the polymer, and miscibility of the LC and polymer. In our material system, 6.6 wt% is close to the minimum concentration required to observe the effect, since the sample with 4.3 wt% monomer concentration did not show the 'deformation-free' response even at a polymerization temperature of −40 °C.

Figure 4(a) compares the electro-optic response of the PChLC nanocomposites fabricated from high (13.5 wt%, polymerized at 20 °C) and low (6.6 wt%, polymerized at −20 °C) monomer concentrations. The normalized SR band-width is the width of the SR band relative to the width at zero field and, because the short band-edge wavelength is immobile in both samples, corresponds to the efficiency in tuning the refractive index. Figure 4(b) depicts the 10-90% decay times for various applied field strengths and shows that comparable response times of about 20 μs are obtained for the two nanocomposites. The response time for the polymer-stabilized ChLC (6.6 wt%, polymerized at 20 °C) is also presented as a reference, and shows that the change in electro-optic response mode improves the response time by ~ × 1000. The following comments are in order regarding the electro-optic response. First, the two samples show similar thresholds around 15 V/µm regardless of the difference in the polymer concentration. This implies that the threshold voltage of the nanocomposite is determined not only by the concentration of the polymer but the size of LC domains in the composite. The fact that comparable response times were obtained also supports the presence of similar-sized pores in the two nanocomposites. Second, the slope gradient above threshold is larger for the low concentration sample, implying a more efficient tuning of the refractive index. This is likely attributed to the increased number of LC molecules that can contribute to the refractive index in the low monomer concentration sample. Since the driving voltage to tune the refractive index by a given amount is determined both by the threshold and slope gradient, low-temperature polymerization leads to PChLC nanocomposites with improved properties, i.e., comparable response times but with reduced driving voltage, making them more practical for real-life applications.

 

Fig. 4 Electro-optic characteristics of the PChLC nanocomposites with high monomer concentration of 13.5 wt% and low monomer concentration of 6.6 wt% at each polymerization temperatures. (a) Electric field dependence of the normalized SR band-width. (b) Electric field dependence of decay time.

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4. Conclusion

In this paper, we showed that PChLC nanocomposites showing the 'deformation-free' electro-optic response can be prepared in a sample containing only 6.6 wt% of monomer, by controlling the degree of polymerization-induced phase separation through polymerization temperature control. The appearance of the ‘deformation-free’ switching mode is determined by the size of the LC domains dispersed in an anisotropic polymer matrix. The low monomer concentration samples can reduce the driving voltage by increasing the number of the LC molecules in response to an external electric field. The low-temperature polymerization procedure is a step towards realizing PChLC composites with practical driving voltages.

Acknowledgments

This work was supported by MEXT KAKENHI Grants (#25630125 and #26820112) and MEXT Photonics Advanced Research Center Program (Osaka University).