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We report a microsecond electro-optic response in an anisotropic-polymer/liquid-crystal composite, which forms a homogeneously mixed structure in the nanoscale range owing to the high miscibility between them. The nanocomposite was fabricated by photopolymerizing a nematic liquid crystal (NLC) mixture doped with a cross-linkable mesogenic monomer at a concentration of 30 wt%. Our system is inherently different from polymer-dispersed liquid crystals in that the LC molecules are almost miscible in the anisotropic polymer matrix and do not form observable domains. When an electric field is applied to such a nanocomposite, the molecular alignment of the polymer matrix is retained, while the non-polymerizable NLC reorients along the electric field, leading to a shift in the birefringence. Furthermore, the reorientation of the NLC molecules in a space sufficiently smaller than the wavelength of visible light results in scattering-free characteristics over the entire visible wavelength range and a short decay response time of 15 μs.
© 2014 Optical Society of America
1. Introduction
Nematic liquid crystals (NLCs) have been widely used for modern electro-optic applications such as displays, light shutters, and phase plates [1–3]. Owing to the simple molecular order in which the LC molecules align uniaxially, NLCs have many advantages for practical use, such as easy alignment control, a low operation voltage, and high thermal stability [4]. However, one of the weaknesses of NLCs is their slow response time; although the rise time can be improved by applying a large electric field, the decay response is a relaxation process which is limited by material parameters such as the elastic constant and viscosity, and consequently occurs in a timescale on the order of ~10 ms [5,6]. While new LC materials such as the cholesteric blue phase and the chiral smectic C phase are being developed, they suffer from other problems such as a narrow temperature range and difficulty of alignment control. Therefore, it is desirable to shorten the decay time of NLCs.
To date, two major approaches have been proposed to improve the electro-optic response of NLCs. One method is to change the material parameters by doping impurities into LCs [7,8]. For example, carbon nanotubes have been reported to decrease the viscosity while increasing the elastic constant of NLCs, leading to a decrease in the decay time. The second approach is to mix a small amount (< 15 wt%) of polymer into NLCs to prepare a polymer-stabilized LC (PSLC) [9]. In this system, both the polymer network and the LC reorient in the direction of the field, but because the polymer network imposes molecular anchoring on the surface, the restoration of the NLC alignment is promoted, resulting in a shorter decay response time [10]. However, the polymer networks also give rise to scattering in the visible range and are therefore suitable for applications in the near-infrared range [11].
Herein, we demonstrate that a miscible polymer/LC nanocomposite can achieve a scattering-free, microsecond response in the visible wavelength range. Our material is fabricated by photopolymerizing an NLC mixture containing a cross-linkable mesogenic monomer at a concentration of 30 wt% (note that this concentration is higher than the typical concentration used to prepare PSLCs). In the proposed nanocomposite, both the polymer matrix and the LC are uniaxially aligned, which gives rise to linear birefringence; however, the two components microscopically mix in the nanoscale range, forming a single polymer/LC mixing phase, unlike PSLCs and polymer-dispersed LCs (PDLCs) which undergo phase separation at a scale ranging from 100 nm to 10 μm [11–15]. When an electric field is applied to the nanocomposite, the molecular alignment of the cross-linked polymer matrix is retained but the non-polymerizable NLCs within the polymer matrix reorient along the field, resulting in a shift of the birefringence. Because the reorientation occurs at a scale significantly smaller than the wavelength of light, scattering-free switching is obtained along with a short decay response time of 15 μs. In the following, we report the electro-optic performance of the proposed nanocomposite evaluated by visible spectroscopy, and discuss its mechanism from the results of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). To emphasize the change of the electro-optic response induced by the formation of the single polymer/LC mixing phase, we compare the electro-optic property of the material prior to and after photopolymerization.
2. Sample preparation
An NLC mixture was prepared by mixing two LC materials obtained from Merck, RM-257 (crosslinkable mesogenic monomer) and BL-011 (NLC mixture with positive dielectric anisotropy and birefringence Δn = 0.27), at a weight ratio of 3:7. The NLC mixture was injected in a sandwich cell (4 μm) made from two polyimide-coated glass substrates subjected to planar rubbing treatment (purchased from E.H.C. Co.). The NLC cell was maintained at room temperature (25 °C) and was irradiated with UV light with a wavelength of 365 nm and a power of 300 mW/cm2 for 1 min.
3. Results and discussion
Figure 1 shows the electric-field dependence of the birefringence at λ = 633 nm, evaluated by measuring the transmittance of the samples before and after polymerization between crossed polarizers. Although the maximum birefringence decreased by ~0.02 in the polymerization process, the macroscopic molecular order with the uniaxial alignment was retained in polymer/LC composite. As the electric field was increased, the sample before polymerization (that is, “bulk NLC”) exhibited a decrease in birefringence of ~0.20 owing to the reorientation of both polymerizable and non-polymerizable NLC molecules along the applied field. On the other hand, birefringence in the polymerized sample linearly decreased above a threshold electric field (Ethreshold) of 20 V/μm, yielding a shift of 0.03 at 40 V/μm.
Fig. 1 Electric-field dependence of birefringence in the sample (a)before and (b)after polymerization.
The response characteristics were measured by plotting the transmission of a He-Ne laser (632.8 nm) from the LC cell between crossed polarizers, while applying a pulse electric field to the cell (Fig. 2). The rise and decay times were estimated from time positions corresponding to shifts of 10%-90% of the total shift in birefringence calculated from the transmission. The rise times both before and after polymerization decreased in proportion to (E2-E2threshold)−1, as reported for conventional NLC materials [16]. However, the driving electric field required to obtain the same speed of the response increased by a factor of 4. On the other hand, the decay times were almost constant regardless of the field intensity, but dramatically improved by a factor of 10000 after the polymerization process, reaching 15 μs.
Fig. 2 Field-intensity dependence of the (a) rise and (b) decay times in the samples before and after polymerization.
To investigate the mechanism of the electro-optic effect, we observed the polymer morphology by SEM (Hitachi S-4300) after rinsing the solidified film with super-critical CO2 (Rexxam Co. Ltd., SCRD401). Figure 3(a) shows a typical SEM image of the rinsed film. Despite the fact that the composite had a non-polymerizable LC component of 70 wt%, clear polymer-LC domain boundaries were not observed, indicating the formation of a single polymer/LC mixing phase that is similar to the situation before polymerization (where the mesogenic monomers and non-reactive LC molecules are mixed on the molecular level). We believe that the formation of a single mixing phase is attributed to the high miscibility between the NLC and the mesogenic monomer: in fact, when the mesogenic monomer is replaced by a non-mesogenic monomer (NOA65, Norland), strong polymerization-induced phase separation occurs, resulting in the formation of PDLCs with micrometer-sized pores (Fig. 3(b)) [13,14,17]. The difference in the polymer morphology strongly affects the transparency of the composite in the visible wavelength range (Fig. 4). The nanocomposite using the mesogenic monomer exhibited a high transmittance of almost 100% regardless of the applied electric field intensity, while the composite using the isotropic monomer showed strong scattering and switched to the transparent state upon applying a field of 3.0 V/μm. In addition, the decay response time for the switching between scattering and non-scattering states was ~40 ms, which was quite long compared to that of the electro-optic effect in the nanocomposite (15 μs). Although some studies using a PDLC with an unusually high polymer concentration (~80%) have reported the formation of nanosized LC droplets and fast response [15,18], the attainable value of the field-induced birefringence was very small (10−3-10−4), because of the low LC concentration. Our system is unique compared to previously-reported PDLCs in that the concentration of the monomer can be low owing to the high miscibility between the polymer and LC. Phase separation is suppressed and LC droplets are confined in nano-sized volumes, leading to a high transmittance over the entire visible wavelength range, a fast response, and a relatively large shift in birefringence. Our nanocomposite is therefore advantageous for applications to high-speed electro-optic devices in the visible range.
Fig. 3 The SEM image of the polymer structure using (a) cross-linkable mesogenic monomer of 30 wt% and (b) cross-linkable isotropic monomer of 30 wt%.
Fig. 4 The field-intensity dependence of transmission spectra for an unpolarized light in the polymer/LC composite materials using (a) the mesogenic monomer and (b) the isotropic monomer.
The nanostructured polymer morphology observed by SEM along with the fast response suggests that the change in birefringence is due to an effective medium effect in which the non-polymerizable LCs that are confined in nano-sized volumes are reoriented along the direction of field. Infrared absorption spectra of the samples before and after polymerization were investigated by FTIR spectrometry (JASCO, FT/IR-4200) to confirm the effective medium effect. Figure 5(a) shows the absorbance of pure BL011 and a BL011 + RM257 mixture before and after polymerization parallel and perpendicular to the rubbing direction (i.e., the long axis of the molecules). The absorption spectra showed three peaks at 1495, 1511, and 1606 cm−1 with strong absorption dichroism and one peak at 1730 cm−1 with little absorption dichroism, originating from the phenyl (C = C) group of the benzene skeleton and the ester (C = O), respectively [19]. The peaks at 1511 and 1730 cm−1 originate from the cross-linkable mesogenic monomer (RM257). Comparing the spectra before and after polymerization, when the rubbing direction was parallel to the polarization, the absorbance at peaks based on the phenyl (C = C) group slightly decreased, whereas when the rubbing direction was perpendicular to the polarization, the absorbance slightly increased, indicating that the order parameter of the molecules decreased in the polymerization process. From the absorbance at 1606 cm−1, where the peaks of both the polymer and LC molecules overlap, the order parameter (S) before and after polymerization was estimated to be 0.64 and 0.51, respectively, using the equation
where A|| and A⊥ are the absorbances parallel and perpendicular to the optic axis or nematic director, respectively [20]. We consider that the decrease in the order parameter caused the decrease in the maximum birefringence after polymerization shown in Fig. 1. Also, we believe that the decrease of the order parameter is attributed to the diffusion of free-radicals generated in the polymerization process and heat generated by UV irradiation. Next, we focused on the two peaks at 1495 and 1511 cm−1 with strong absorption dichroism to investigate the mechanism of the electro-optic effect. Figure 5(b) shows the field-intensity dependence of the absorbance before and after polymerization. When the electric field was applied to the samples, we found clear differences. In the sample before polymerization, the absorbance of both peaks decreased as a result of the molecular reorientation of both BL011 and RM257. On the other hand, in the sample after polymerization, the peak at 1495 cm−1 decreased while the peak at 1511 cm−1 remained constant. The field-intensity dependence of the absorption peaks, obtained by fitting the peaks to a Lorentzian, is shown in Fig. 5(c). The absorbance at 1511 cm−1 remained at 0.4 regardless of the field intensity, while the absorbance at 1495 cm−1 decreased from 0.72 to 0.60 with a threshold of ~20 V/μm. This indicates that in the polymerized sample, only BL011 is able to reorient along the field, while RM257 (the polymer matrix) retains the planar alignment.Fig. 5 (a) Absorption spectra in BL011 and BL011(70 wt%) + RM257(30 wt%) parallel and perpendicular to the rubbing direction, (b) field-intensity dependence of absorption spectra in the samples before and after polymerization, and (c) field-intensity dependence of absorbance at 1495 and 1511 cm−1.
From the results of SEM, FTIR, and visible spectroscopy, we propose the following model of the electro-optic effect in the nanocomposite (Fig. 6). In the nanocomposite, both the polymer matrix and the LC are uniaxially aligned and are miscible with each other. When an electric field is applied to the nanocomposite, the polymer matrix retains its alignment by the cross-linking of polymer chains, while the non-polymerizable LC molecules reorient along the electric field. The local reorientation in a space sufficiently smaller than the wavelength of visible light leads to the scattering-free characteristics over the entire visible wavelength range. The localization of the motion of LC molecules affects the response speed in the electro-optic effect.
The importance of confining LCs into small volumes to achieve fast switching can be understood by the following simple argument. Let us consider the Fredericks transition of the NLC molecules with a correlation length of ξ between two anchoring walls (Fig. 7) [16]. We assume that an electric field applied along the z-axis deforms the molecular distribution such that the angle between the easy axis and the director φ takes a maximum value of φmax at the center position (z = ξ/2). If φmax is small, the distribution of φ along the z-axis can be approximated by a sinusoidal profile given by φ(z) = φmax sin(πz/ξ). The elastic force is then given by
where K is the effective elastic constant. The above expression implies that as ξ becomes smaller, the elastic force increases with a quadratic dependence on ξ. Because the elastic force hinders the rise motion while promoting the molecules to return to its original state, the driving field increases while the decay time decreases. ξ corresponds to the cell gap (4 μm) in the sample before polymerization (bulk LCs), but in the nanocomposite, ξ is limited to the volume in which the LC molecules are confined, which is on the few 10 nm order. We believe this is the reason a dramatic improvement was achieved in the decay response time. The above model does not allow quantitative discussions, since the LC molecules in the nano-composite are confined by polymer walls and thus experience anchoring in three-dimensions. However, even from this simple model, we can understand that one of the main factors allowing a fast response is the scale size at which the reorientation of LC molecules occurs.4. Demonstration of an optical amplitude modulator
The fast response allows us to realize various electro-optic devices. Here, we demonstrate an optical amplitude modulator using the nanocomposite. Figure 8 shows transient response curves obtained by measuring the transmission of a He-Ne laser from samples before and after polymerization between crossed polarizers driven by a 10 kHz sine wave. In the sample before polymerization (Fig. 8(a)), the transmission was constant because the bulk NLC could not respond to the sine wave with a time period of 100 μs. On the other hand, the sample after polymerization showed an almost delay-free response for modulation frequencies of up to at least 20 kHz (Fig. 8(b)), note that the device operates at twice the frequency of the modulation voltage because of the apolar nature of the LC molecules). In this way, the composite material can potentially be used in low-cost and compact photonic components such as tunable phase plates and light shutters or in display applications.
Fig. 8 Demonstration of an optical amplitude modulator. Transient transmission curves of a He-Ne laser obtained from samples (a)before and (b)after polymerization between crossed polarizers being driven by a 10 kHz sine wave.
5. Conclusion
A nanocomposite, in which a polymer and LC molecules are miscible in the nanoscale range, was fabricated by photopolymerizing a NLC mixture containing a cross-linkable mesogenic monomer at a concentration of 30 wt%. SEM observation revealed that our system is inherently different from conventional PDLCs in that although the amount of polymer in the composite is similar, the high miscibility between the polymer and LC molecules inhibites phase separation, resulting in scattering-free characteristics in the entire visible wavelength range. FTIR mesurement revealed that when an electric field was applied to the nanocomposite, the polymer matrix was strongly fixed by cross-linking, while the non-polymerizable LC molecules reoriented along the field. The local reorientation of LC molecules enabled a fast electro-optic response with a short decay time of 15 μs.
Acknowledgments
This work was supported by a Grant-in-Aid for Japan Society for the Promotion of Science (JSPS) Fellows (13J01237), Grant-in-Aid for Exploratory Research (25630125), and the Photonics Advanced Research Center (PARC) at Osaka University. Y. I. acknowledges the JSPS Research Fellowship, and H. Y. acknowledges the PRESTO Program from the Japan Science and Technology Agency (JST). The authors thank Dr. Satoru Shoji and Prof. Satoshi Kawata of Osaka University for allowing us to use the supercritical rinser, and Mr. Shinji Bono, Prof. Yoichi Takanishi and Prof. Jun Yamamoto of Kyoto University for valuable discussions.
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