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Abstract
We developed polymer-dispersed liquid crystals (PDLCs) that effectively scatter light both in the visible and near-infrared ranges simultaneously. Such PDLCs are characterized by an optimal size distribution of nematic liquid crystal droplets within 0.4 − 3 µm, which is achieved due to the specially selected copolymer, the elaborated liquid crystal material as well as the proper cooling mode from the isotropic phase. These PDLC films provide electrically controlled light scattering modulation in the spectral range 300 − 2300 nm with the response time around 10 ms.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Polymer-dispersed liquid crystals (PDLCs) is a dispersion of liquid crystal (LC) droplets in a polymer matrix. The whole system is opaque since scatters light strongly. A remarkable property of this material is the ability to “clear up” under the influence of an electric field, becoming transparent. PDLCs attract scientists and technologists attention as a class of materials suitable for light shutters and displays [1–5], they have been used for smart windows [6], sensors applications [7], tunable microlenses [8], lasers [9], and biomedical devices [10–12].
Electrical control of the PDLC light scattering [13,14] is based on ordering NLC director orientation in the droplets along the applied electric field.
To obtain high-quality PDLC films, the following requirements must be taken into account: first, a liquid crystal (LC) has not to be dissoluble in the polymer, i.e. phase separation has to take place; second, one of the refractive indices of LC, usually $n_{\mathrm {o}}$ (in case of the positive dielectric anisotropy $\varepsilon _{\mathrm {a}} > 0$), should match the refractive index of the polymer: $n_{\mathrm {o}} \approx n_{\mathrm {p}}$.
There are five basic methods for producing PDLC films: the first one consists in filling the polymer’s micropores with LC [13], the second one involves the formation of PDLC films from an aqueous emulsion [1]. The third method is a solvent induced phase separation (SIPS) [15]; the fourth way is polymerization induced phase separation (PIPS) [3,4]. The PIPS method is initiated by either the use of UV radiation or heat treatment. The fifth method, thermally induced phase separation (TIPS) [16], is applicable when the polymer matrix has a melting point below its decomposition temperature.
Alternatively, control of the configuration and size of droplets may be provided by the introduction of a dichroic azo dye into PDLC [17]. A detailed study of the morphological and electro-optical properties of such PDLCs with various dye concentrations has been carried out and it has been shown that the use of an azo dye can improve the electro-optical and thermo-optical characteristics of PDLC films [18–21].
The effects of light scattering in PDLCs are usually studied in the visible spectral range. The goal of this work is to create PDLC that provides the electrically controlled scattering both in the visible and near-infrared spectral range simultaneously.
To achieve this goal, we obtained PDLC films, a distinctive feature of which is the presence of several subsystems of liquid crystal droplets of different sizes (from 0.5 µm and up to 3.0 µm). Theoretical and experimental studies of the effect of droplet size on the transmittance of PDLC films were presented in the work [22], where theoretical analysis was based on the Rayleigh-Hans approximation in the single scattering mode. The averaged refractive index of the NLC was taken as the refractive index of the droplet. Due to this reason the presented theoretical analysis did not allow the authors [22] to consider the change in the PDLC scattering under the action of an electric field. Nevertheless, the dependence of the PDLC transmittance on the wavelength was obtained for various droplet sizes. In this work, we have succeeded to create PDLC films, where the droplet size distribution provides efficient light scattering in the visible and infrared spectral regions. In addition, the created PDLCs can change the light scattering intensity in the visible and infrared regions under an electrical field.
2. Experimental
2.1 Components and PDLC creation method
A ternary copolymer based on methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA) and alkyl methacrylates (AMA-n), where n is the number of carbon atoms in the alkyl chain (n = 6, 8 or 10) [23], was chosen as the polymer matrix for the NLC composite, see Fig. 1.
The average molecular weight of the copolymer is $M \approx (2 \div 5) \times 10^5$, the glass transition temperature is $80 \div 90$ °C, the refractive index $n_{\mathrm {p}} = 1.492$. A series of liquid crystal materials (LCM) have been developed to produce the requested NLC. The most suitable among them was the LCM-91 mixture, its composition is shown in Fig. 2. The sequence of LCM-91 phase transitions during heating up from the solid crystalline phase (Cr) is as follows: Cr $\rightarrow$ $^{-12.7 °C}$ $\rightarrow$ N $\rightarrow$ $^{+75.2 °C}$ $\rightarrow$ Is (where N is a nematic phase, Is – an isotropic phase), so the nematic phase of LCM-91 exists in a wide temperature range. The developed NLC has a large positive dielectric anisotropy $\varepsilon _{\mathrm {a}} = 22$.
As it was noted earlier, for the electro-optical properties of PDLC, the refractive indices of the polymer and liquid crystal are of great importance. Using the well-known Abbe refractometry method, tuned for achromatic measurements [24,25], and polarized light, we measured the dependences of the refractive indices of both the NLC ($n_{\mathrm {o}}$ and $n_{\mathrm {e}}$) and the polymer ($n_{\mathrm {p}}$) on the light wavelength, which are shown in Fig. 3. The indices difference between $n_{\mathrm {o}}$ of LCM-91 and $n_{\mathrm {p}}$ of the polymer does not exceed 1.4% across the spectrum.
Fig. 3. Dispersions of refractive indices $n_{\mathrm {o}}$ and $n_{\mathrm {e}}$ of LCM-91 and the copolymer $n_{\mathrm {p}}$.
For the sample preparation we used a SIPS method to create PDLC material, for this we made a solution of copolymer and LCM-91 in chloroform in 1:1 weight ratio. Then, the solvent was evaporated slowly overnight, and after that material was heated up to an isotropic phase and kept for one hour under a vacuum to evaporate the chloroform traces. As a result, we obtained a PDLC material suitable for film formation using the SIPS method. For electro-optical measurements, we used two types of cells ($2\times 2$ cm or $10\times 10$ cm), each with two gaps: 25 µm and 42 µm. Cells were formed by two glass plates with transparent Indium-Tin-Oxide (ITO) electrodes on the inner side of both plates. The PDLC material fills the cell ($2\times 2$ cm) with capillary forces at 110 °C. Subsequent cooling resulted in the formation of a PDLC film inside the cell, in this case by the TIPS method.
For the fabrication of large-area cells, we used the thermoplastic properties of the PDLC material. We put a small PDLC drop at temperature 100 °C on the surface of one of the glass plates and then squeezed by the second plate to the required gap value, controlled by spacers. Thus, it was possible to produce cells with an area of $10\times 10$ cm with a variation in the film thickness less than 2 µm. In the last case, the PDLC films were formed upon cooling by the TIPS method.
NLC concentration in the copolymer strongly influences on the light transmittance of PDLCs. Figure 4 presents the transmittance versus voltage applied to PDLCs with various concentrations of NLC in the copolymer. The highest transmittance (about 75%) is achieved at a sufficiently low driving voltage amplitude of about 70 V for a 25 µm thick sample containing 50 wt% of NLC (see Fig. 4). Further, we dealt only with PDLCs, in which the weight ratio of NLC and copolymer was 1:1. Moreover, we found that the cooling rate dramatically affects the droplet size distribution, rendering the possibility of the created PDLC films to scatter the light in different spectral ranges.
Fig. 4. Transmittance ($\lambda = 632.8$ nm) versus the voltage applied to the PDLC at various concentrations of NLC in the copolymer: 1 – 50 wt%; 2 – 65 wt%; 3 – 45 wt%. The PDLC layer thickness is 25 µm.
2.2 Influence of cooling process on the size distribution of nematic droplets in PDLC
To develop PDLCs, which are able to drive light scattering in both the visible and infrared regions of the spectrum, it was necessary to ensure the optimal size distribution of NLC droplets.
In the work [26] an emulsion method for a PDLC fabrication utilizing a membrane filter for accurate control of the distribution of LC droplets was reported. PDLC cells were created with different NLC droplet size distributions: 1.0 µm, 1.9 µm, and 3.5 µm, as well as mixtures of two or three different NLC droplet sizes. The effect of droplet size on the light scattering was shown, but only for one wavelength.
Using a Zeiss Axiolab Pol polarizing microscope and a Linkam heating table, we investigated the PDLC texture changing during heating and cooling process. It was found that during heating NLC droplets gradually dissolve in the copolymer, and a transparent isotropic medium appears at a temperature of 73.5 °C. Upon cooling, phase separation occurs, and NLC droplets reappear. We have shown that by adjusting the cooling mode, it is possible to control the droplet size distribution: the PDLC cells were heated to 110 °C , then they were placed in heat-insulating layers and put in an air thermostat at a temperature of 20 °C , so natural cooling took place. As an insulator, we used tissue paper, changing the number of layers, and controlling the cooling rate thereby. The example of the nucleation and growth of NLC droplets during the cooling process shown in Fig. 5.
Fig. 5. Nucleation and growth of NLC droplets during cooling mode with $\tau = 62$ s. Microphotos are made in polarizing microscope at temperatures: (a) $T_{\mathrm {cool}} = 55$ °C; (b) $T_{\mathrm {cool}} = 50$ °C. The bar size is 100 µm.
The time ($t$) dependence of the cell temperature ($T_{\mathrm {cool}}$) at the cooling process is described by the known formula:
where: $T_{\mathrm {e}} = 20$ °C is the ambient temperature, $T_0 = 110$ °C is the sample temperature at $t = 0$, $\tau$ is the cooling time constant of the system. This constant depends on the size and heat capacity of the cell as well as on the heat transfer coefficient (and thus, the numbers of the heat-insulating layers). The $\tau$ values were calculated from the measured dependencies $T_{\mathrm {cool}}(t)$ at different cooling modes using formula (1).2.3 Electro-optical measurements techniques
The most significant works on the study of PDLC are presented in the books [27–29] where it can be found optical characteristics of PDLC films: light transmittance ($T$), switching-on ($\tau _{\mathrm {on}}$) and switching-off ($\tau _{\mathrm {off}}$) times. The typical values for PDLCs containing NLCs would be: $T \approx 70 \div 80~\%$, $\tau _{\mathrm {on}}$, $\tau _{\mathrm {off}} \approx 1 \div 5$ ms [23].
To measure the transmittance spectrum versus driving voltage we used the set-up shown in Fig. 6.
Note that when the cell is illuminated, only part of the incident light energy is scattered by the PDLC layer, the other one is reflected by the air-glass interface and by transparent electrode layers, and some part is absorbed by all these layers.
It is convenient to characterize light transmittance for the case when only a small region of the cell is illuminated.
For this purpose, the light beam was collimated so that the wavefront remains flat before hitting the cell. In our experimental setup, we used a tungsten-halogen incandescent lamp LS-1 from Ocean Optics Ink., as a broadband light source. In order to collimate the light, we used an optical fiber with a core diameter of 400 µm and an achromatic lens with a small numerical aperture ($\approx 0.1$). A small area was illuminated behind a circular diaphragm 1 mm in diameter placed in the proximity of the sample. A second similar diaphragm (1 mm in diameter) was placed at a distance of two meters from the sample. Thus, the solid angle in which the radiation reached the photodetector did not exceed $2 \times 10^{-7}$ steradians. As a broadband photodetector ($200 - 1100$ nm) Avaspec-2048-USB2-UA multichannel spectrometer from Avantes was used. Measurements in the near-infrared range ($1000 - 2500$ nm) were carried out using the AvaSpec-NIR256-2.5-HSC-EVO fiber optic spectrometer.
Measurements of electro-optical parameters such as a transmittance dynamics $T(t)$, $\tau _{\mathrm {on}}$ and $\tau _{\mathrm {off}}$ times were carried with He-Ne laser ($\lambda = 632.8$ nm) and two light emitting diodes ($\lambda = 394$ nm and $\lambda = 837$ nm with full width at half maximum of 13 nm and 25 nm, respectively). A silicon photodiode was used as a photodetector. For this purpose an optical setup similar to one shown in Fig. 6 was used but the light source and the detector were changed.
A high-frequency ($f = 10$ kHz) AC voltage was applied to the cells for the $15 \div 100$ ms. The signal from the photodiode was recorded by an 8-bit ADC with the driving voltage simultaneously. In such manner we measured the intensity of the light transmitted through the prepared PDLC samples, as well as the time of switching on and off, depending on the applied voltage.
3. Results and discussion
3.1 Cooling modes of the PDLC cells
To select the optimal cooling mode for PDLC cells, satisfying requirements for light scattering both in the visible and infrared ranges, we carried out the following investigations.
As a quantitative criterion for the light scattering of PDLC samples obtained with different cooling modes, we used the optical density $D$ written as
where $\left \langle I_0\right \rangle = \int _{380}^{1100}{I_0(\lambda )\mathrm {d}\lambda }$ is the total luminous flux in the range from 380 to 1100 nm reaching the photodetector in the absence of a PDLC cell, $I_0(\lambda )$ represents the incident light spectrum, and $\left \langle I(0)\right \rangle$ is the total flux which passes through the PDLC cell in the absence of a driving voltage. Here $\left \langle I(0)\right \rangle = \int _{380}^{1100}{I_{\mathrm {p}}(\lambda )\mathrm {d}\lambda }$, where $I_{\mathrm {p}}(\lambda )$ is the transmitted spectrum.In addition, in order to estimate the proportion of scattered light as a function of wavelength, we use the optical transmittance:
where $I(\lambda ,U)$ is the light intensity reaching the photodetector in the presence of a PDLC cell under different driving voltages $U$.The prepared PDLC cells were made at different cooling modes (different $\tau$). Using formula (2), the value of $D$ was evaluated dependently on $\tau$, that is shown in Fig. 7. From the figure, it is evident that with $\tau \approx 62$ s, the parameter $D$ reaches the highest value ($D \approx 4.17$), which corresponds to the optimal cooling mode. For this mode the PDLC texture and droplet size distribution are shown in Fig. 8(a).
Fig. 8. Size (diameter) distribution of liquid crystal droplets depending on the cooling modes: (a) for the optimal mode with $\tau \approx 62$ s (the inset shows a photograph of the droplets made in polarizing microscope at temperature of 22 °C); (b) distribution curves for three different cooling modes.
Cooling mode strongly affects the spectral dependence of $T(\lambda ,U)$. Figure 9 represents the dependencies of the transmittance $T(\lambda ,0)$ on the light wavelength for three samples with different cooling time constants $\tau$. It is evident from this figure that the transmittance of the sample at $\tau \approx 32$ s increases in the infrared region of the spectrum, while at $\tau \approx 242$ s the maximum is in the visible range of the spectrum.
Fig. 9. Transmittance $T(\lambda , 0)$ of the PDLC samples for three different cooling modes (different $\tau$). The upper curves correspond to the PDLC layer thickness of 25 µm, while the lower curves correspond to the thickness of 42 µm.
Different kinds of $T(\lambda ,0)$ dependencies of the PDLC cells are associated with the variation in the droplet size distribution, which are defined by cooling modes. The size distribution of LC droplets for the three cooling modes is presented in Fig. 8(b). With rapid cooling ($\tau \approx 32$ s), the droplets do not have time to increase in size during cooling and therefore an almost monodisperse distribution with a maximum of about 0.5 µm is obtained. In the case of prolonged cooling ($\tau \approx 242$ s), the maximum shifts to longer wavelengths and relates to a diameter of droplets about 4.5 µm. In this case the infrared radiation scatters well (Fig. 8(b)), but the transmittance $T(\lambda ,0)$ in the visible range is higher than in the previously considered (Fig. 9, left upper curve). For a cooling mode with $\tau \approx 62$ s, the maximum of droplet size is in the region around 1.0 µm with distribution up to 3 µm (Fig. 8(a)). Thus, we conclude that the cooling mode with $\tau \approx 62$ s forms the optimal droplet distribution for the purpose of light scattering in the visible and near infrared spectral regions.
3.2 Electro-optical measurements
Transmittance spectra of the PDLC cells prepared at an optimal cooling mode with $\tau \approx 62$ s according to the technique described in Section 2.3 were measured at zero and various applied voltages ($U = 44$ V and $U = 70$ V). These spectra are shown in Fig. 10.
It is clear from Fig. 10, that the developed PDLC material makes it possible to drive the intensity of the luminous flux not only in the visible, but also in the near infrared region of the spectrum. Note that at $U = 70$ V, the transmittance in the red and near infrared regions of the spectrum is about 70%, and in two spectral intervals it exceeds the transmittance of an empty cell. This phenomenon can be explained by a combination of two factors: good optical quality of the PDLC layer and immersion at the interface of the copolymer and NLC.
Fig. 10. Transmittance spectra of the 25 µm thick PDLC layer measured at various applied voltages and the temperature of 24 °C. PDLC was prepared with the optimal cooling mode ($\tau \approx 62$ s).
The PDLC electro-optical response dynamics was investigated as it is described in section 2.3. Figure 11 shows the typical time dependence of the light transmittance $T(t)$, through the PDLC layer, when a high-frequency ($f = 10$ kHz) AC voltage is applied to it for 40 ms. As can be seen from this figure the switching-off time $\tau _{\mathrm {off}}$ is about 10 ms at three different wavelengths: 394 nm, 632.8 nm, 837 nm. This experimental result seems to suggest that the PDLS polydisperse droplet structure behaves as a collective structure, and not as a set of independent droplets of different sizes.
Fig. 11. Time dependences $T(t)$ of light transmittance ($\lambda = 394$ nm, 632.8 nm, 837 nm) through a PDLC layer 25 µm thick, after applying for 40 ms AC voltage with an amplitude of 70 V at a frequency of 10 kHz, at temperature of 22 °C.
Since the electro-optical switching-on time $\tau _{\mathrm {on}}$ decreases with an increase of the applied voltage in proportion to $1/(\varepsilon _{\mathrm {a}} U^2)$, it is always negligible in comparison with $\tau _{\mathrm {off}}$ in our experiments. The longest electro-optical response time is $\tau _{\mathrm {off}}$, which is determined by the properties of the NLC (mainly by a rotational viscosity and elastic modulus). It is not so easy to reduce them. Therefore, the PDLC created by us can be characterized by the maximum free relaxation time $\tau _{\mathrm {off}} = 10$ ms.
The dependence of the transmittance on the applied voltage is calculated by varying the voltage amplitude applied to the PDLC using relation (3).
As an illustration of the developed PDLC operation in the near infrared region, Fig. 12 shows photographs taken in the wavelength range from 780 to 1100 nm. There is the PDLC layer between the person (Professor Alfredo Strigazzi, Politecnico di Torino, Italy) and the IR camera. When voltage is applied to this layer, light scattering is reduced and the image becomes clear.
Fig. 12. Photos taken in the near-infrared spectral range in the wavelength interval of $780 - 1100$ nm (at maximum sensitivity of the camera matrix about 850 nm) at various voltages applied to the 25 µm thick PDLC layer (left to right): 80 V; 45 V; 0 V.
4. Conclusion
The PDLC electro-optical material has been elaborated to modulate the transmittance in the visible and near-infrared regions of the spectrum ($380 - 2300$ nm), covering almost all the range of thermal solar radiation. This result is achieved since the developed PDLC material has an optimal size distribution of NLC droplets due to the specially selected copolymer, the elaborated liquid crystal, as well as a proper cooling mode. The optimal size distribution operates effectively in the broad spectral range because in addition to droplets, the sizes of which are comparable to the wavelength of visible light, there are a sufficient number of droplets with a size of 1 µm or larger to provide effective near infrared light scattering.
It can be concluded that the developed PDLC material can be used in those areas of optics, where electrically controlled modulation of light scattering is required both in the visible and near-infrared ranges.
Funding
Russian Foundation for Basic Research (19-52-06005 MNTI_a, 20-02-00746 A).
Acknowledgments
The authors are grateful to Professor Alfredo Strigazzi for fruitful discussions.
Disclosures
The authors declare no conflicts of interest.
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