Investigation of dielectric properties in polymer dispersed liquid crystal films doped with CuO nanorods

Journal of Molecular Liquids 295 (2019) 111667

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Investigation of dielectric properties in polymer dispersed liquid crystal films doped with CuO nanorods Zhipeng Jiang, Jihong Zheng ⁎, Yourong Liu, Qing Zhu Engineering Research Center of Optical Instrument and System, Ministry of Education, Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, Shanghai, PR China

a r t i c l e

i n f o

Article history: Received 16 May 2019 Received in revised form 28 August 2019 Accepted 31 August 2019 Available online 01 September 2019 Keywords: Polymers Liquid crystals Copper oxide nanorods Dielectric properties HN equation

a b s t r a c t Influence of CuO nanorods on the dielectric properties of polymer dispersed liquid crystal (PDLC) is studied in the 4 Hz–8 MHz frequency range. PDLC films doped with different concentrations of CuO nanorods were prepared by photopolymerization-induced phase separation. The scanning electron microscopy (SEM) images reveal that, the size of liquid crystal (LC) droplets in a polymer matrix is at the nanometer level. Besides, with the increasing of the concentration of CuO nanorods, the size of droplet can approach about 1 μm. The result indicates the doping of the CuO nanorods slows the polymerization rate of the PDLC and the size of the LC droplets becomes larger. Further analysis of the Cole-Cole plot fitted by the Havriliak-Negami (HN) equation shows that there are two relaxation processes in the low frequency and high frequency region of the PDLC doped with nanorods. With the nanorods concentration of 0.1 wt%, the electric conductivity of PDLC is increased N20 times and there is a saturation effect. © 2019 Published by Elsevier B.V.

1. Introduction Electric conductivity is one of the important LC parameters. In most publications it was shown [1–3] that the introducing of some nanoparticles leads to the decreasing of the electric conductivity. In many applications it is important to increase the value of electric conductivity, research shows the introducing of the carbon nanotubes leads to the increasing of the electric conductivity [4–7]. It is well-known that LC can be used not only as a homogeneous media, but also in the dispersed form in a polymer matrix made into PDLC [8]. PDLCs are composed of micron-size droplets of LCs dispersed in solid polymer binder and are formed by phase separation of a primarily homogeneous LC–polymer mixture. The methods for preparing PDLC are mainly polymerization induced phase separation (PIPS), solvent-induced phase separation (SIPS), thermally induced phase separation (TIPS) and microcapsule phase separation (MP). PIPS, is one of the methods commonly used for preparing PDLC due to its simple steps and fast preparation in speed. The composite film is sandwiched between two conductive Indium-Tin-Oxide (ITO) coated glass substrates with its conducting sides in contact with the composite film, it is very convenient to electronically control the LC molecules and study the dielectric properties of PDLC films. Various studies on phase behavior and physical properties of PDLC composites have been reported [9–12]. PDLCs are great significance because of their promising use in advanced optical device ⁎ Corresponding author. E-mail address: [email protected] (J. Zheng).

https://doi.org/10.1016/j.molliq.2019.111667 0167-7322/© 2019 Published by Elsevier B.V.

applications, such as switchable windows, holographic grating and gas sensors [13–15]. As a p-type semiconductor material, CuO has good chemical stability, sensing characteristics and electrochemical activity [16], and the dielectric properties of the PDLC will significantly affect the driving voltage, response time and other parameters. Therefore, this paper is proposed to investigate the effect of CuO nanorods doping on the dielectric properties of PDLC, with different concentrations on different frequencies, to expand its application in related fields.

2. Experimental 2.1. Materials The materials system for fabricating the PDLC films consisted of a photoinitiator (RB, Aldrich Inc.), a synergistic photoinitiator (NPG, Aldrich Inc.), a cross-linking agent (NVP, Aldrich Inc.), a surfactant (S271, Chemistry Inc.), an acrylic monomer (EB8301, np = 1.49, UCB Inc.), and nematic LCs (TEB50, no = 1.524, ne = 1.7136, Δn = 0.1896, Δε = 11.5, TNI = 63 °C, Beijing Tsinghua Yawang Liquid Crystal Material Co., Ltd.) with a mass ratio of 0.15:0.4:10:10:45:35 and total mass of 3 g. CuO nanorods (length 1–2 μm, diameter 40–60 nm, Suzhou Canfuo Nano Technology Co., Ltd.) in different quantities (3 and 6 mg) was added to the materials. Then the new hybrid material was heated by an ultrasonic emulsification instrument in dark conditions for about 2 h. Fabrication of the PDLC material with CuO nanorods doping was complete when it had been stable for 24–48 h in a darkroom.

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Z. Jiang et al. / Journal of Molecular Liquids 295 (2019) 111667

2.2. Sample preparation Fig. 1 presents the PDLC film preparation process. Spacers (diameter 20 μm) were deposited on a glass substrate coated with an ITO conductive film. Then cover another piece of ITO coated glass and vacuum-pack it to make a 20 μm interlayer. To form the PDLC film the material was heated to about 40 °C and entering between two pieces of ITO coated glass by capillary action. Then the samples were exposed in a 532 nm laser for about 120 s. The power exposed on the samples was 20mw/ cm2, and the ambient temperature was kept between 25 °C and 30 °C. Similar to the method in reference [17] the prepared sample was opened and immersed in ethanol for 12 h to extract LC droplets. Then morphology of the prepared films was studied by scanning electron microscope at a voltage that accelerates the electron beam to 1 keV. All the obtained images are recorded by a digital camera, were then analyzed the changes of surface microstructure, morphology of the PDLC film by the CuO nanorods doping. 2.3. Dielectric measurements The dielectric measurements were carried out using a LCR meter (IM3536, Hioki (Shanghai) Trading Co., Ltd.). The samples were investigated in temperature (26 °C) as a function of applied frequency (4 Hz to 8 MHz) and the amplitude of the sinusoidal driving voltage at 1 V. The area of the ITO coated glass electrode was 277.5mm2. The dielectric properties of the composite films were determined from the measurement of capacitance (Cp), dissipation factor (tan δ) and conductivity (S), and the relative dielectric constant was calculated using the following formula [18,19]: εr ¼

Cp Cp  D ¼ C 0 A  ε0

ð1Þ

where (εr) is the relative dielectric constant, (Cp) is the parallel capacitance, (D) is the thickness of composite film, (A) is the area of electrode, ε0 is the free space permittivity (8.85 × 10−12 F/m). 3. Results and discussion 3.1. Morphology of the films The Fig. 2 presents the SEM images for (a) PDLC, (b) PDLC+0.1 wt% CuO nanorods and (c) PDLC+0.2 wt% CuO nanorods separately. From the Fig. 2(a), it is not obvious to show the empty space occupied by LC droplets due to the LC droplet's size is too small to be observed. In Fig. 2(b) it can be observed that the size of the LC droplets is at the nanometer level. In Fig. 2(c) the size of LC droplets is close to 1 μm. The result

Fig. 2. SEM images for (a) PDLC, (b) PDLC with 0.1 wt% CuO nanorods and (c) PDLC with 0.2 wt% CuO nanorods.

Fig. 1. PDLC film preparation process.

Z. Jiang et al. / Journal of Molecular Liquids 295 (2019) 111667

indicates that the addition of CuO nanorods makes the polymerization phase separation process become slower. The similar report about the relationship between liquid crystal droplet size and polymerization rate can be found in the references [20]. However, CuO nanorods could not be observed directly in the SEM images because of the edges of nano-inclusions of LC were close to their diameter [21]. Similarly, according to the literature review or previous works [8], it is reported that after the phase separation in the process of PDLC samples preparation, clear boundary between LC and polymer can not be observed. Normally, there exists an 1 μm thickness intermediate layer composed of a mixture of polymer monomers and LC molecules. Thus the boundary is not so clear between two material [8].

3

and   ðωτÞα sinðαπ=2Þ θ ¼ arctan 1 þ ðωτÞα cosðαπ=2Þ

ð4Þ

where ε′(ω) is the real part of permittivity and ε″(ω) indicates the dielectric loss, εLF is the low-frequency permittivity and ε∞ is the permittivity in the HF limit and τ is the characteristic relaxation time of the medium, α and β are the distribution parameters satisfied that 0 ≤ α ≤ 1, 0 ≤ β ≤ 1 individually. Here, the α and β can be calculated as 0.985 and 0.99 separately using the method reported in the Ref. [20]. 105

3.2. Dielectric relaxation spectroscopy (DRS) 104

εHN ðωÞ ¼ ε0 ðωÞ‐iε00 ðωÞ ¼ ε ∞ þ

εLF −ε∞

103 102 Experimental data using HN function

ε"

Fig. 3. shows the frequency dependence of the real (ε′) and imaginary (ε″) parts of the complex dielectric permittivity for the PDLC with 0.1 wt% CuO nanorods. The analysis reveals the reduction in ε′, and ε″ on frequencies f b 100 Hz and the increment in ε″ on frequencies f N 105 Hz. It is obvious that, there are two relaxation processes in the low and high frequency regions separately. For the analysis of these two processes the dependences ε″ (ε′) were plotted and analyzed by Cole-Cole plot fitted using HN function. Fig. 4(a) shows the Cole-Cole plot of PDLC with 0.1 wt% CuO nanorods. According to the theory of relaxation processes, and using the HN model for fitting, the complex dielectric permittivity can be expressed by the equation [22].

100 10-1 10-2

ð2Þ


β 1 þ ðiωτÞα

101

10-3 100

101

103

104

ε'

Furthermore, the loss factor tan δ, that is described in Fig. 4(b), can be expressed by the formulation [20] as: tanδ ¼ ε 00 =ε0 ¼ ðεLF −ε∞ Þ sinð−βθÞ=  β=2 ðε ∞ 1 þ 2ðωτÞα cosðαπ=2Þ þ ðωτ Þ2α þ ðε LF −ε∞ Þ cosðβθÞÞ

102

(a) 102

ð3Þ

Experimental data using HN function

101

105 1 2

100

tan˄ ˅

104

10-1

103

ε‘ε"

10-2

102

10-3

101

10-4 100

102

104

106

108

f˄Hz˅ 10

0

100

102

104

106

108

f˄Hz˅ Fig. 3. Frequency dependences of the real ε′ (1) and imaginary ε″ (2) parts of the complex dielectric permittivity for PDLC with 0.1 wt% CuO nanorods.

(b) Fig. 4. Comparative presentations for PDLC with 0.1 wt% CuO nanorods: (a) the dependences ε″ (ε′) (open black circles) and HN fitting function (black lines); (b) loss factor (tan δ) versus frequency (open black circles) and HN fitting function (black lines).

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Z. Jiang et al. / Journal of Molecular Liquids 295 (2019) 111667

In the Fig. 4(a), the incomplete semicircle in the left side of the curve (deformed by a logarithmic scale) indicates the corresponding high frequency relaxation process, while the right side of the experimental curve fitted the NH simulation part quite well indicates the low frequency relaxation process. However, in the middle section of the curve, the experimental data looks depart away from the fitted curve, which is owing to the secondary relaxation process caused by the CuO nanorods and polymer matrix. Fig. 4(b) exhibits the variation of the loss factor (tan δ) versus frequency. It also demonstrates the above process. The curve shows that the main relaxation process happens around 100 Hz of frequency, where HN fitting curve just centered and matched quite well with each other. Then the secondary relaxation process happens with the frequency of voltage varying from 1 kHz to 100 kHz, because it is obviously

that the experimental data gradually departs from the fitting curve within this frequency range. After that, the third relaxation process occurs above 100kHz (105 Hz) where the experimental data bends again into a similar half circle. Furthermore, using the methods in the references [17,23–25], the data of Fig. 4(a) can be extrapolated. Fig. 5 is Cole-Cole plots of PDLC with 0.1 wt% CuO nanorods at lower frequency region (a) and at the higher frequency region (b) individually, which contains the experimental data ε″ (ε′) (open black circles) and HN fitting function (black lines). The fitting parameters of HN function are presented in Table 1. There are two explanations for low frequency relaxation mechanisms, which are superimposed in a certain range. One explanation is attributed to the dynamics of molecules present in the surface layer of the composite system. The structure of this layer can be considered to

(a)

(b) Fig. 5. Cole-Cole plots of PDLC with 0.1 wt% CuO nanorods in low frequency region (a) and high frequency region (b), the experimental data ε″ (ε′) (open black circles) and HN fitting function (black lines).

Z. Jiang et al. / Journal of Molecular Liquids 295 (2019) 111667

5

10-2

Table 1 The fitting parameters of HN function. τ, s

α

β

Low frequency High frequency

0.106 3.4 × 10−8

0.985 0.87

0.99 1

consist of the following two parts: 1. LC molecules directly connected to the surface by chemical bonds; 2. A certain number of molecules are not bound to the interface, and their dynamics are different from those of molecules with specific overall behavior, which hinders the close interaction of the adsorbed molecules. Therefore, the characteristic frequency of relaxation process in dielectric spectrum is less than that corresponding to the relaxation process in most LCs. This explanation is consistent with that given by Lippens [26], Cramer [27] and Bras [28]. The second explanation is due to the interface polarization, which is shown in the presence of a very low frequency of about 1 Hz, possibly due to the Maxwell-Wagner-Sillars effect [20,29–32]. The effect exists at the interface between two dielectrics with different dielectric constant and conductivity, which leads to the increase of dielectric constant and dielectric loss when the frequency decreases. 3.3. Influence of nanorods on the dielectric properties of PDLC Fig. 6 shows frequency dependence of the real part of the complex dielectric permittivity ε′ for PDLC with different concentration of CuO nanorods. It can be seen that the dielectric constant of PDLC is greatly increased by the doping of CuO nanorods, but the gap becomes smaller with the increase of frequency. It is speculated that the doping of CuO nanorods changes the polarization mechanism, and the contribution of CuO nanorods to dielectric constant decreases with the increase of frequency. It is noteworthy that the concentration of CuO nanorods from 0.1% to 0.2% has no obvious effect on the real part of the complex dielectric constant of PDLC. This may be due to the saturation of the doping concentration of CuO nanorods. Therefore, the increase in ε′ of PDLC is not obvious when the concentration of nanorods is increased. Fig. 7 is the frequency dependence graph, representing the electrical conductivity of PDLC with different concentration of CuO nanorods. The results show that the conductivity of PDLC is greatly improved by doping CuO nanorods at frequencies f b 104 Hz, but this is not obvious in the high-frequency region. This is due to the formation of conductive

Fig. 6. Frequency dependences of the real parts ε′ of the complex dielectric permittivity for: PDLC (1), PDLC with 0.1 wt% CuO nanorods (2), PDLC with 0.2 wt% CuO nanorods (3).

1 2 3

10-3

10-4

(S/m)

Frequency region

10-5

10-6

10-7 100

102

104

106

108

f˄Hz˅ Fig. 7. Frequency dependences of electrical conductivity for: PDLC (1), PDLC with 0.1 wt% CuO nanorods (2), PDLC with 0.2 wt% CuO nanorods (3).

network by CuO nanorods, which improves the conductive properties of materials. In addition, the concentration of CuO nanorod from 0.1% to 0.2% has no obvious effect on the conductivity of PDLC. It should be noted that the basic mechanism of charge transfer in these structures is through the ionic conductivity of LC and the electronic conductivity of polymer [33]. For ionic conductivity, the σ value is usually independent of frequency. For electronic conductivity, the disordered structure of polymer matrix is a typical exponential dependence [3]. The relationship between the conductivity and frequency of PDLC indicates that on frequency range f b 103 Hz, the conductivity is independent of frequency, the main contribution to the conductivity is caused by the ionic component. Such conductivity can be provided either by the presence of “channels” between the LC drops filled by the liquid crystal, or by the presence of a small amount of the LC drops whose diameter is close to the PDLC film thickness. For the frequencies f N 103 Hz the main contribution to the conductivity is caused by the electronic component. It should be noted that for the frequencies f N 104 Hz conductivity is independent of the concentration of nanorods. This is due to the electronic conductivity produced by the disordered structure of the polymer matrix, where electron transfer is produced by a jump-like mechanism [3,7]. For the frequencies 103 Hz b f b 104 Hz, it can be observed that conductivity is very sensitive to the frequency in the pure PDLC without nanorods (Fig. 7, curve 1), while it is not so sensitive of ionic component conductivity in the PDLC doped with CuO nanorods (Fig. 7, curve 2, 3). This is because the doping with CuO nanorods leads to an increase in the composition of the ionic conductivity. That is to say, the conductivity of PDLC increases with the increase of the concentration of CuO nanorods, because the effect of ionization on the nanorods dominates the increase of ionic composition of conductivity. Since the electronic component of the conductivity through the nanorods is also caused by jump-like electron transfer, its contribution must increase with the frequency increasing [3]. The conductivity values of PDCL with different concentrations of CuO nanorods at different frequencies can confirm this (Table 2). It is observed that with the frequency increasing the difference in the σ value between the samples with different nanorods concentration is increasing and for the frequencies f N 104 Hz the relationship between conductivity and CuO nanorods concentration becomes ambiguous. Thus, the ionic component of the conductivity of PDCL with CuO nanorods increases due to the ionization on the nanorods.

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Z. Jiang et al. / Journal of Molecular Liquids 295 (2019) 111667

Table 2 Dependence of the PDLC conductivity measured at various frequencies on the concentration of the CuO nanorods. Sample

σ(f = 4 Hz),S/m

σ(f = 10 Hz),S/m

σ(f = 100 Hz),S/m

3.75 × 10−7 4.71 × 10−6 5.91 × 10−6

3.96 × 10−7 5.19 × 10−6 6.67 × 10−6

4.55 × 10−7 5.51 × 10−6 7.11 × 10−6

PDLC PDLC+0.1 wt%CuO PDLC+0.2 wt%CuO

4. Conclusions It is shown by SEM images that the doping of the nanorods affects the polymerization rate of the PDLC resulting in a change in the size of the LC droplets. This was also confirmed by measuring the relationship between the real part of the complex permittivity and the frequency of PDLC with different concentrations of nanorods. Dielectric spectrum analysis of PDLC shows that there are two relaxation processes of high frequency and low frequency. There are two explanations for the low frequency relaxation mechanism, one is the dynamics of the molecules present in the surface layer of the composite system, and the other is the interfacial polarization, which is manifested at very low frequencies, of about 1 Hz, which may be attributed to the Maxwell-Wagner-Sillars effect. The results of dielectric studies on nano-doped PDLC indicate, when the CuO nanorods are doped into the PDLC, the conductivity of the PDLC is increased by N20 times, and the dielectric constant is increased by nearly 40 times. A small difference in conductivity and dielectric constant between samples at concentrations of 0.1 wt% and 0.2 wt% indicates that the saturation effect is achieved for the relationship between conductivity and dielectric constant and concentration. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 61975122].

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