Colored PDLC Films with Wide Gamut Range

Abstract

Due to the discoloration properties under different applied voltages, dye-doped polymer-dispersed liquid crystal (PDLC) films are widely used as camouflage nets and invisibility cloaks. However, the range of the discoloration has an intuitive effect on their applications. In this work, we studied the gamut range of PDLC film doped with dyes of red, green, blue, and yellow, with the concentration corresponding to the minimum haze of these dyes. The influence of the applied voltage on the color range of single-layer and double-layer films with different backgrounds was studied. The relationship of the voltage with the color was set from 0 V to 60 V at steps of 5 V, to characterize the discoloration of the PDLC films. The results showed that the films could cover 42.48% of the sRGB gamut and even exceed the range.

1. Introduction

Currently, the demand for environmental protection and energy-conserving devices is impacting all aspects of our daily lives [1]. In this context, more people are concentrating on the study and application of eco-friendly and intelligent devices. For instance, the optical properties of electrochromic materials (such as reflectivity, transmittance, absorption, and so on) make it possible to implement steady and reversible discoloration with the influence of the applied voltage. Due to the excellent properties such as the stability, flexibility, and mechanical strength [2,3,4,5], the research related to polymer-dispersed liquid crystal systems manifests increasing significance.

Polymer-dispersed liquid crystals (PDLCs) are composite materials in which liquid crystals (LCs) are dispersed in a polymer matrix with micron-sized droplets [6]. As one kind of electrochromic material, PDLCs are able to implement flexible discoloration between the permeable state and non-permeable state. Besides, due to the eco-friendliness, processability, low consumption, and high efficiency, PDLCs have a wide application in optical and electronic fields such as smart windows, electronic devices, and multifunctional films [7,8,9,10,11,12,13,14].

Great improvement has been made in the mechanical properties and electro-optical performances in previous research by the addition of hydroxyproply methacrylate (HPMA) [15] and fluorine-containing liquid crystal molecules [16]. It has been found that the structure of the monomer could be optimized by the adjustment of the chain length [17], grafting degree [18], and terminal groups [5,19,20], thus improving the mechanical properties of PDLCs [21,22,23,24,25,26].

PDLC films have attracted the attention of many researchers due their excellent properties such as their flexible modulability, long-term stability, durability, and large-scale processibility [7,27,28,29,30,31,32,33,34,35].

Based on various improvements, colored PDLC films have been manufactured by doping dyes into the film. The following work aimed to explore the influence of different dyes with different concentrations on the properties of PDLC films. Besides the mechanical properties and electro-optical performances, the gamut range of the PDLC film also needs to be improved for practical applications.

In this work, an exploration of the representation of the color of the PDLC films with excellent mechanical properties and electro-optical performances was made. Based on the previous research, four kinds of colored PDLC films were selected, which included red, yellow, blue, and green films. Theoretically, when studying the color gamut, different combination modes result in differences, for instance a single-layer film with a background layer of different colors, double-layer films, and so on. These could present a different color gamut. Moreover, the color will be influenced by the applied voltage on different layers and the color of the background layer. The color gamut of single-layer and double-layer PDLC films and the properties of the different applied voltages and colors of the films were investigated in detail.

2. Materials and Methods

2.1. Materials

The chemical structures of the components used are shown in Figure 1. The nematic liquid crystal (LC) E8 (T_NI = 345.2 K, n_o = 1.527, n_e = 1.774) was supplied by Jiangsu Hecheng Display Technology Co., Ltd., Jiangsu, China and Tokyo Chemical Industry, Shanghai, Chian, respectively. The dyestuff (Solvent Red 111, Solvent Green 28, Solvent Blue 104, Solvent Yellow 93) was supplied by Dongguan Muyang Plastic Pigment Co., Ltd., Dongguan, China. Cyclohexyl methacrylate (CHMA) (Shanghai Meryer Chemical Technology Co, Ltd., Shanghai, China) was used as the photopolymerizable monomer. Hydroxyproply methacrylate (HPMA, 95%, Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China) and 2,2,2-trifluoroethyl methacrylate (TFEMA) (Beijing Kaibaite Technology Co., Ltd., Beijing, China) were used to provide the hydroxyl groups and fluoride groups, respectively. The crosslinking agent 1,4-butanediol diacrylate (BDDA) was purchased from Sartomer. The content of the photoinitiator Irgacure 651 (Guangzhou Evergreen Trading Co., Ltd., Guangzhou, China) was 2.0 wt%. There were also anthraquinone dyes used, Solvent Red 111, Solvent Yellow 93, Solvent Green 28, and Solvent Blue 104. All these materials were used as received without any further purification.

2.2. Preparation of the PDLC Films

Before the preparation of the samples, the proportion of the non-liquid crystalline polymerizable monomers (NLCMs) and anthraquinone dye needed to be determined. In this work, the proportion decided the mechanical properties and electronic properties. Excellent properties were found with the following proportion [36]. The proportion of each dye was tested before, and the dye proportion corresponding to the best electro-optical performance of the film was selected to prepare the samples. Above all, the information of the formulation was shown in

Table 1. The component content of the samples.

Sample Number	Component Content (wt%)
	CHMA/IBMA/HPMA/TFEMA/BDDA/PEGDA600/Dye	E8
A	18.0/6.0/6.4/1.6/1.6/6.4/0	60
B (red)	18.0/6.0/6.4/1.6/1.6/6.4/0.8	60
C (blue)	18.0/6.0/6.4/1.6/1.6/6.4/2.0	60
D (green)	18.0/6.0/6.4/1.6/1.6/6.4/0.3	60
E (yellow)	18.0/6.0/6.4/1.6/1.6/0.4/0.5	60

.

According to a certain proportion, the liquid crystal, the monomer, the dye, the initiator, and the glass microbeads were prepared and uniformly mixed. Then, the mixture was filtered and stirred for ten minutes. The mixed solution was poured into the middle of the conductive film between the two rollers of a film-laminating machine. After, the finished films were polymerized in an ultraviolet cure box and irradiated with ultraviolet light (365 nm) at 6 mW/cm² for 7 min.

2.3. Measurements

After the curing of the PDLC films, the samples were cut into strips and the strips were dipped in the cyclohexane solvent for ten days so as to separate the polymer network form the liquid crystal monomers. After that, the strips were dried in an oven at a temperature of 333.15 K for 24 h to remove the solvent entirely. The microscopic structure was observed by scanning electron microscopy (SEM, Hitachi, Beijing, China S-4800) after being sputtered with gold.

The transmittance was measured by an ultraviolet–visible–infrared (UV–Vis–IR) spectrophotometer (Lambda 35, Perkin Elmer, Waktham, MA, USA). The wavelength was controlled from 300 nm to 800 nm.

3. Results and Discussion

3.1. Analysis of the Polymer Morphologies of PDLC Films

Figure 2 shows the SEM micrographs of the pure PDLC film sample and the PDLC film samples doped with different dyes. Different from the uniform structure and complete morphology of the pure PDLC film shown in Figure 2A, the different dyes of the doped PDLC films had an effect on the morphologies. The films with dyes had a larger mesh size, as shown in Figure 2B–D. This was because, compared to the pure PDLC films, a part of the UV light for polymerization may be absorbed by the dye molecules, which results in a decrease of the polymerization rate [37]. Therefore, the liquid crystal molecules have plenty of time to diffuse and gather, so the size of the liquid crystals and the mesh size of the polymer matrix become larger.

3.2. Analysis of the Transmittance of PDLC Films

The transmittance of the dye-doped PDLC films is shown in Figure 3. It was obvious that the transmittance had a trend of increasing as the voltage increased. The dimpled area of each image indicates absorption or reflection, which caused a decline of transmittance [37]. The relationship between peak transmittance and voltage was characterized to express the transmittance of the visible light region.

As shown in Figure 4, the peak transmittance of the dye-doped PDLC films was selected. The region with wavelengths above 760 nm is the infrared region, so the points were selected from other areas. Finally, the wavelengths of the dye-dopped PDLC films were selected at 669 nm, 754 nm, 526 nm, and 560 nm, respectively, due to the differences of the light wavelengths. It can be seen that the transmittance had a trend of increasing as the voltage increased, and the dye-doped PDLC films had peak transmittances at voltages of 25 V, 2 0 V, 10 V, and 10 V, respectively.

3.3. Analysis of the Voltage Effect on the Lab Value of Single-Layer Colored PDLC Film

The regularity of the L^*a^*b^* value of the single-layer PDLC composite dimming film with the voltage was studied before the calculation of the color.

Figure 5 shows the color change of Sample B at different applied voltages, indicating that the color brightness of the dye-incorporated sample changed as the voltage increased. Generally speaking, as the sample gradually changed from scattered opaque to transparent, the color of the film changed from dark to light. It can be seen that the brightness changed significantly after the introduction of the dye into the colored PDLC film, and the color of the dimming film will be characterized in the next part.

Figure 6 shows the switching state of the four dyes of red, green, blue, and yellow. It is a switching state change diagram under the background of black and white. The applied voltage was set to 0~60 V, and the camera took the pictures under the standard light source D65 with a voltage gradient of 5 V.

The color was composed of black, white, and other achromatic colors. The ideal pure white object was fully reflective, with a light reflectance of 1, and is often used as a calibration or blank control plate for fiberoptic spectrometers and reflective UV–Vis spectrophotometers. The ideal pure black object did not reflect and had a light reflectivity of 0. Since black and white objects do not selectively reflect each band of the spectrum, black and white were selected as the background for color characterization in the experiment.

As shown in Figure 6, the International Commission on Illumination (CIE) has established the tristimulus XYZ color mixing system to form the internationally accepted “CIE1931”, which is also known as the color chromaticity diagram. As for the calculation of the color, we are supposed to determine the ambient light source where the extracted color is located first, and then, CIE-65 will be defined to represent the average daylight as the standard. In this article, the D65 standard light source was selected, and the observation angle was 2°. The three major attributes of color are the hue, lightness, and darkness. The hue is divided by wavelength, which is used to distinguish colors, such as red and green. The lightness refers to the brightness of the color; white is the brightest one, while black is the darkest one. Purity refers to the vividness of the color. In general, the mixture of white with a pure color will make the color purity lower and the brightness higher, while the mixture of black will have the opposite effect.

The chromaticity diagram contains the full gamut of the visible light band, and the three major attributes of the color can be visually identified by the position of a color point in the chromaticity diagram. It can also be used to compare the color ranges for monitors, printing, etc.

The coordinates of the color chromaticity diagram are represented by x and y, respectively, and each point (x, y) on the diagram represents a color point. The relationship between coordinates (x, y) and tristimulus values X, Y, Z is shown in Equation (1):

$$\left\{\begin{array}{c}x=\frac{X}{X+Y+Z}\\ y=\frac{Y}{X+Y+Z}\\ z=\frac{Z}{X+Y+Z}\end{array}\right.$$

(1)

The Lab color space is the color space with the widest color gamut, which represents the colors of real objects independent of computer monitors, software, or hardware. Its principle of color recognition is by the human eye, so it is the closest color space to the real object observed by the human eye. CIE was established in 1931 and revised in 1976 as CIE Lab, resulting in a digital representation of the color system recognized by the human eye, where the L^* value is used to represent lightness, and the range from black to white is [0, 100]. The a^* and b^* values both represent chroma, and the a^* value range is [127, −128], which means from red to green, while the b^* value range is [127, −128], which means yellow to blue.

The CIE1976 uniform color space L^*a^*b^* value is calculated by Equation (2):

$$\left\{\begin{array}{c}{L}^{*}=116{(\frac{Y}{Y_0})}^{\frac{1}{3}}-16\\ a^{*}=500[{(\frac{X}{X_0})}^{\frac{1}{3}}-{(\frac{Y}{Y_0})}^{\frac{1}{3}}]\\ b^{*}=200[{(\frac{Y}{Y_0})}^{\frac{1}{3}}-{(\frac{Z}{Z_0})}^{\frac{1}{3}}]\end{array}\right.$$

(2)

where X₀, Y₀, and Z₀ are the tristimulus values of the standard light source D65, respectively. X₀ = 95.045; Y₀ = 100; Z₀ = 108.255. It can be seen that the tristimulus value is converted to the L^*a^*b^* value. At the same time, conversely, the tristimulus value can also be calculated with the known L^*a^*b^* value by Equations (3) and (4):

$$\left\{\begin{array}{c}Y=Y_0{f}^{-1}\left[\frac{1}{116}(L^{*}+16)\right]\\ X=X_0{f}^{-1}\left[\frac{1}{116}(L^{*}+16)+\frac{1}{500}{a}^{*}\right]\\ Z=Z_0{f}^{-1}[\frac{1}{116}(L^{*}+16)-\frac{1}{200}{b}^{*}]\end{array}\right.$$

(3)

$$f^{-1}(t)=\left\{\begin{array}{ll}{t}^3& t>\frac{6}{29}\\ 3{(\frac{6}{29})}^2(t-\frac{4}{29})& t\le \frac{6}{29}\end{array}\right.$$

(4)

where t stands for $(\frac{X}{X_0})$, ${(\frac{Y}{Y_0})}^{\frac{1}{3}}$, or $(\frac{Z}{Z_0})$.

Based on the equations above, the data of the dimming films can be expressed intuitively by the CIE1931 color space.

Figure 7, Figure 8, Figure 9 and Figure 10 show the relationship between the color brightness and voltage of the PDLC films. The chroma factor a^* value represents the change from red to green in the Lab color space, and the L^* value and a^* value or the L^* value and b^* value are charactered by each point.

Figure 7a shows that, as the voltage increases, the L^* value has a tendency to increase, while the a^* value has a tendency to decrease when on a white background, because, under the applied voltage, the PDLC film changes from a scattering state to a transparent state and its color changes from dark to light. Brightness increases gradually, while purity decreases, which results in the increase of the L^* value and the decrease of the a^* value. When the liquid crystals rotate with the electric field and tend to the parallel electric field direction gradually within the voltage range from 0 V to 60 V, the dye dissolved in the liquid crystal droplets also rotates with the rotation of the liquid crystals, so that the color shows a sensitivity to voltage, which leads to the discoloration from dark to light. However, when the voltage reaches more than 60 V, most of the dye has been oriented because most of the liquid crystals are oriented along the electric field, which makes it less sensitive to the change of voltage. Since there are some dyes in the polymer matrix, the color does not disappear completely.

Figure 7b shows the properties when the red PDLC film is on a black background. It can be seen that both the brightness value L^* and the a^* value have a tendency to decrease as the voltage increases. This can be explained by the fact that, with the increase of the voltage, the color of the red PDLC film becomes lighter gradually, so that the background color can be seen. Compared to the white background, the film with the black background has a more obvious discoloration, and the L^* value and a^* value have a greater change. This can be explained by the fact that both the brightness and purity of the black background are low, which makes it easier to show the color of the background when superimposed. It can be concluded that both the backgrounds and applied voltage have an impact on the color of the film, which is manifested in the brightness changes.

Figure 7c shows that both the luminance factor L^* and the chromaticity factor b^* value increase as the voltage increases, and the color gradually becomes lighter. It can be seen from Figure 7d that the brightness factor L^* decreases, while the value of the chromaticity factor b^* increases, as the voltage increases, and the color gradually becomes darker.

Figure 8 shows the regularity of the La^*b^* value of the green PDLC film and blue PDLC film on the black and white backgrounds. As shown in Figure 8a, on the white background, both the L^* value and a^* value have a trend of increasing with the increase of the voltage. Differing from the red dyes, the green dyes have a higher intrinsic brightness, related to the greater L^* value, while the L^* value and the a^* value have different trends as the voltage increases when on the black background in Figure 8b. It is worth noting that the L^* value and the a^* value have obvious changes when the applied voltage reaches around 10 V. This is because 20 V is the threshold voltage and saturation voltage transition stage of the PDLC film, at which the transmittance of the film changes greatly, which has a more-obvious impact on the discoloration.

As shown in Figure 8c,d, it can be seen clearly that the luminance factor L^* and chromaticity factor b^* values increase as the voltage increases, and the color lightens gradually when on the black background. On the contrary, the luminance factor L^* value decreases, while the chromaticity factor b^* value increases as the voltage increases, and the color deepens gradually.

3.4. Character of Gamut Range of the Color-Changing PDLC Film

After the study of the L^*a^*b^* value of the monochromic PDLC film with the voltage in the previous part, the color gamut range of the above monochromic film was characterized using “MATLAB”, in which the CIE1931 color space can be cited in the application directly, differing from Python and Origin.

In the images, CIE1931 with sRGB needs to be cited first. The task and the figure can be completed by the codes shown in Code S1.

After the addition of the background, a file was created to store the parameters so as to invoke the data. The range can be expressed on the background in this way.

Finishing the preparation, points at the edge were selected, and then, the points were connected in turn to calculate the area of the region, as shown in Code S2:

After the calculation given in the Supporting Information, the result was “A = 0. 0476”, while the sRGB part covered the area of 0.11205. This accounts for 42.48% of the sRGB area and partially exceeds the sRGB color gamut range, which means it has a wide color gamut. The data support and method guidance are provided for the liquid crystal dye-doped color-changing light modulation film, and the liquid crystal dye-doped color-changing light modulation film is expected to be applied to the field of military camouflage to prepare camouflage products with specific colors.

In order to show the effect of different voltages on the color more clearly, the L^*a^*b^* value was converted to a tristimulus value and, finally, converted to the chromaticity coordinates to display it macroscopically on the CIE1931 chromaticity diagram. As shown in Figure 9a, the dots in the figures stands for the data of the color measured. It can be seen that the color changes from dark to light. The color of the single-layer colored dimming film covers part of the sRGB color gamut and even exceeds the sRGB coverage range, so the coverage is wide.

Figure 9b shows the color assembly gamut. In addition to the longitudinal brightness change, the lateral hue change is more obvious. Therefore, it can be shown that the effect of the superimposed color is relatively obvious, and the coverage range of the superimposed color is relatively wide. The photochromic film with a relatively wide color gamut coverage range was prepared, and part of the color gamut exceeds the sRGB color gamut.

Finally, in order to cover the total color gamut of the film used, a more-intuitive display is provided. The range of the measured data was limited by the blue line. As shown in Figure 10, the effect of the superimposed color is relatively wide. The photochromic film with a relatively wide color gamut coverage range was prepared, and a part of the color gamut exceeds the sRGB color gamut.

4. Conclusions

In this work, colored PDLC films with an electrochromism property were prepared and investigated. Anthraquinone dyes were introduced into the acrylate system with NLCM, which enriched the discoloration range. The color gamut properties of the PDLC films doped with different dyes at the concentration corresponding to the lowest haze were studied. The effects of different driving voltages, background colors, and assembly methods on the L^*a^*b^* chromaticity space were investigated. As the voltage rose, the brightness L^* value of the monochromatic film gradually increased under the white background; the a^* value of the green PDLC dimming films increased, while that of the red PDLC dimming films decreased regardless of the background; the b^* value of the blue PDLC dimming films increased, while that of the yellow PDLC dimming films decreased. Besides, the database of the color–voltage was set, and it is worth mentioning that about 42.48% of the sRGB gamut can be characterized by the system, which may be used in the future research of color-changing PDLC film. The electrochromic PDLC films with high mechanical properties and low driving voltages will have a wide range of applications in camouflage, invisibility cloaks, smart windows, and so on.