Tunable electro-optical and dielectric characteristics of polyvinyl alcohol doped nematic liquid crystal composites

In recent years, the novel and effective materials whose properties have been studied within the scope of nanotechnology research are widely preferred in wide range electro-optical device applications due to their some considerable characteristic features [1, 2]. Liquid crystals (LCs) are one of these novel materials. Especially, nematic liquid crystals (NLCs), a fluid with long-range orientational order and dislocation, are the most widely used LC member in particularly display technologies. Having a simple phase structure, NLCs can easily align in various directions, and their rod-like molecules can orient themselves along the director axis in external field (such as electric field, magnetic field, and etc.) [3‐5]. Nowadays, new and novel approaches have been tried to recover the physical characteristics of NLCs. In this context, the incorporation of several low-dimensional materials (quantum dots, carbon nanotubes, graphene flakes, polymeric nanoparticles, and etc.) into NLC phase is an important method, and has been the issue of broad discussions and studies. The addition of low-dimensional materials into NLCs has several advantages, including the development of novel physical properties within the device. Therefore, depending on the properties of low-dimensional materials, new advancements can be achieved in NLC composites in many aspects, varying threshold voltage and response time, and also improved dielectric constants [6‐13].

Contemporary research on the inclusion of polymers into LCs has been studied by several scientists and changed in the physical characteristics were observed. Ghosh et al. investigated the effects of conducting polymer poly (3, 4-ethylenedioxythiophene) nanotubes on the electro-optical and dielectric properties of a NLC 4-n-pentyl-4′-cyanobiphenyl host [14]. Manda et al. studied possible enhancement of physical properties of NLCs by doping of conducting polymer nanofibres [15]. Pathak et al. researched several essential display features for the low birefringent NLC dispersed with polymer [16]. Pande et al. investigated dielectric and electro-optical properties of polymer-stabilized LC system [17]. Moreover, Tripathi et al. studied dielectric and electro-optical properties of polymer PiBMA dispersed in MBBA LC [18]. The outcomes of these studies are very important and have motivated the present study by arousing curiosity in investigating the changes that will occur in the physical features of the composites will be obtained by combining varied polymers with NLCs.

In the current research, some electro-optical and dielectric parameters of polyvinyl alcohol doped E7 NLC were investigated. As is known, in scientific and technological research, E7 NLC is preferred due to their tunable electro-optical and dielectric properties. The changes in the electro-optical and dielectric parameters of E7 NLC combined with polymer structures are also quite significant. In this context, it is also important to produce novel composite structures with polymers whose properties are compatible with E7 NLC. In the proposed study, it was considered necessary to use a different polymer structure for E7 NLC, and polyvinyl alcohol (PVA) was chosen. PVA has high transparent film formation, semi-crystalline form, water solubility, and biocompatibility properties. Moreover, it is preferable owing to its superiorities like wide operating temperature range, low cost, and structural flexibility [19]. These aforementioned properties of PVA are among the most important reasons for its selection for the current study. Scope of the study, two samples were analysed comparatively: E7 and E7/PVA. The experimental measurement results of dielectric constants (ε′ and ε′′), dielectric anisotropy (Δε′), threshold voltage (V_th), response time (τ_R), and optical band gap (E_g) parameters were reported. The outcomes of the study indicated that significant electro-optical, optical, and dielectric parameters of PVA doped E7 NLC caused variation compared to pure E7 NLC. In other words, it was observed that the electro-optical, optical, and dielectric parameters of the produced PVA doped E7 NLC composite structure is tunable. Thus, it was concluded that the E7/PVA composite structure, thanks to its tunable physical properties, will make significant contributions to both future research and electro-optical device technology.

The base material of the experimental research is E7 NLC which is consisted of 4-pentyl-4′-cyanobiphenyl (5CB) 51wt.%, 4-heptyl-4′-cyanobiphenyl (7CB) 25wt.%, 4-octyl-4′-cyanobiphenyl (8OCB) 16wt.%, and 4-pentyl-4′-cyanoterphenyl (5CT) 8wt.%. It was purchased from Instec, and ordinary refractive index (n_o) is 1.521 and extra-ordinary refractive index (n_e) is 1.746. The birefringence (\(\Delta n={n}_{e}-{n}_{o}\)) value of E7 is 0.225. Furthermore, the phase sequence of the E7 is crystal–(–20 °C)–nematic–(60.5 °C)–isotropic. The dopant material is polyvinyl alcohol (PVA). It was purchased from Sigma Aldrich. The linear formula for PVA is [-CH₂CHOH-]_n. Moreover, the molecular structures of E7 NLC and PVA are shown in Fig. 1a, b. Indium tin oxide (ITO) coated planar aligned LC cell has been used for the measurements. LC cells were provided from Instec. The ITO coated glass plates of these cells contain a layer of polyimide (PI) (KPI-300B), and also have a thickness of 18 µm spacer, and a surface resistance of 25 Ω/cm².

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Sample preparation and characterizations methods are given schematically in Fig. 2. Firstly, PVA was dissolved in distilled water (H₂O) by heating the solution at 100 °C, 1400 rpm under stirring for 2 h. After, PVA-solution was cooled to room temperature [20]. Two different samples were prepared: pure E7 NLC (E7) and E7 NLC and 2.0wt.% PVA-solution (E7/PVA). The samples were heat treated in an ultrasonic bath at 80 °C. The heat treated process was proceeded for 5 h to provide homogeneous composites. In the final stage, samples were filled into LC cells with the capillary method.

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Dielectric measurements were performed with a Dielectric/Impedance Analyzer (Novocontrol Alpha-A) for the frequency range of 10 Hz to 5 MHz with a test signal of 100 mV_rms in the 0–40 V. Absorbance values were obtained in the wavelength range of 200–500 nm with Ultraviolet/Visible (UV/VIS) spectrophotometer (PG Instruments T70 +). Moreover, textures of samples were analysed using a polarising optical microscope (POM) (Motic BA310E) equipped with crossed polarizers. POM images for E7 and E7/PVA were recorded for both planar (V = 0 V) and homeotropic (V = 40 V) states.

Electro-optical parameters of samples were determined with an electro-optical measurement set-up. This measurement set-up is schematically shown in Fig. 3 and consists of a function generator (AA Tech AWG-1020), Helium–Neon (He–Ne) laser with a wavelength of 632.8 nm, photodiode amplifier and photodetector (Thorlabs-PDA200C), and digital oscilloscope (Tektronix-TDS2012C). In the electro-optical measurements, the samples to be measured were placed in a crossed position between the polariser and the analyser making an angle of 45° with the optical axis of the incident light. Using a 1 kHz square wave in the 0–5 V_rms range, voltage-dependent and time dependent transmittance graphs were obtained based on the transmittance of the incident light by the LC cell. The output intensity (I) of the light transmitted from the LC cell is given as following formula [21]:where, I_o is the maximum value of transmitted light, Δϕ is the phase difference and calculated using following formulas:where, m is maximum peaks in the transmittance-voltage graphs, I_max and I_min are the maximum and minimum transmitted light intensity passing through the LC cell, d is the thickness of the LC cell, Δn is the birefringence, and λ is the wavelength of He–Ne laser. Especially, in the voltage-dependent transmittance measurements, Eq. (4) provides important insights into determining the number of maximum and minimum transmittance values.

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Electro-optical, dielectric, UV/VIS measurements and surface analysis with POM for E7 and E7/PVA samples were performed at room temperature.

In the presence of an electric field, molecular orientations of LCs within the LC cell are very important in determining the dielectric, electro-optical, and optical characteristics of LC and LC based composites. Because the orientation of the LC molecules, whose physical properties are investigated using an LC cell, will change depending on the electric field generated by the applied voltage, and changes will occur in the electro-optical, optical, and dielectric parameters. In particular, the results obtained from these measurements, performed in the presence of an electric field depending on the orientation of LCs, are considered extremely important as they are guiding for many optoelectronic device applications. Figure 4a, b demonstrates the planar and homeotropic states of NLC molecules with positive dielectric anisotropy in planar aligned LC cell. In the planar state, the orientations of LCs are parallel to the surface of the ITO coated electrodes of the LC cell. Besides, in the presence of voltage, LCs start to orient and homeotropic state occurs at high voltages. In the homeotropic state, the molecules of LC are oriented perpendicular to the surface of the ITO coated electrodes of the LC cell. The changes in dielectric, optical, and electro-optical parameters of LC materials in their planar and homeotropic states, depending on molecular orientations, are important because they will provide vision for many scientific studies.

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Dielectric characterisation provides information about the molecular dynamics of the materials with dielectric properties and the effect of ions on the LC matrix with the applied electric field. In general, it deals with the reply of the molecular mechanism that is directly concerned with the macroscopic polarisation of the material. Changes in dielectric features are produced by changes in the orientation of the LC molecules. In this study, dielectric characteristics of E7 and E7/PVA were examined with the complex dielectric constant using the formula given below [22]:where ɛ′ and ɛ′′ is the real and imaginary parts of the complex dielectric constant, respectively and described as following formulas [23]:where C is the capacitance and C_o is the capacitance of the free space. Furthermore, d is the thickness and A is the area of the LC cell. \({\upvarepsilon }_{\text{o}}\) is the dielectric constant of free space, \(\upomega\) is the angular frequency, and G is the conductance.

Figure 5a–d represents the variation of ɛ′ and ε′′ as a function of frequency for planar (V = 0 V) and homeotropic (V = 40 V) states. The frequency dependent dielectric constant is related to the molecular motion of LCs resulting from the reorientation of the dipole moment. It can be seen from Fig. 5a, b that ɛ′ of E7/PVA at 10 Hz are higher when compared to E7. The high ɛ′ value is thought to be due to the Maxwell–Wagner-Sillars (MWS) effect, resulting from charge carriers accumulating at the interfaces within the sample and at the interface between the sample and the electrodes. In other words, it can be said that the ɛ′ value increases in E7/PVA compared to E7 due to the increased MWS effect [24‐26]. In the frequency range of 100 Hz-100 kHz, it is seen that the values of ɛ′ ​​are very close for E7 and E7/PVA. Furthermore, the ε′ value for the homeotropic state has increased compared to the planar state because the LC molecules in the samples E7 and E7/PVA exhibit orientation in the presence of an electric field formed depending on the applied voltage, ensuring regular molecular orientation. On the other hand, ε′ parameter has decreased for both voltages at high frequencies owing to the polarizability of molecules, ionic conductivity, and interface orientation for E7 and E7/PVA. Figure 5c, d indicates the variation of ɛ′′ as a function of frequency at planar and homeotropic states. It has been observed that ɛ′′ reaches its highest value at 10 Hz. Moreover, this value has increased with PVA contribution, similar to ε′ for planar state. In addition, the relaxation frequency can be determined at the peak points of the frequency dependent ɛ′′ graph and it is seen from Fig. 5c, d that the relaxation frequency is greater than 1 MHz for V = 0 V and around 1 MHz for V = 40 V. The results indicate that both ɛ′ and ε′′ values ​​change with PVA contribution, especially at low frequencies. Therefore, it can be concluded that ɛ′ and ε′′, which are important dielectric parameters, are modifiable with PVA contribution to E7 NLC.

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The planar and homeotropic states of the samples were also analysed using POM images. Figure 6a, b shows POM images for E7 and E7/PVA at planar (V = 0 V) and homeotropic (V = 40 V) states. Both samples were observed to have a coloured surface at V = 0 V (Fig. 6a), resulting from the orientation of LC molecules parallel to the surface of the ITO coated electrodes of the LC cell. On the other hand, at V = 40 V (Fig. 6b), a dark surfaces were obtained as a result of the orientation of LC molecules perpendicular to the surface of the ITO coated electrodes of the LC cell.

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Figure 7 illustrates the real part of the complex dielectric constant (ε′) as a function of voltage for E7 and E7/PVA samples. The measurements were carried out at a frequency of 1 kHz and room temperature. The reasons for choosing the frequency as 1 kHz are the ionic effects observed in LC based composites at low frequencies and the inability of LC molecules to follow the electric field at high frequencies. It is seen that the ε′ enhances with voltage, depending on the molecular ordering of the LC molecules. Thus, the ε′ value has changed from the planar state to the homeotropic state with increasing voltage. As seen in the Fig. 7, ε′ reaches its minimum value at V = 0 V and is in the planar state. When the voltage is increased, ε′ continues to increase and remains almost constant at certain voltage values. At V = 40 V, the ε′ values of both samples are almost equal and are in the homeotropic state. This situation is also confirmed by the POM images given in Fig. 6a, b. Additionally, parallel (\(\varepsilon^{\prime}_{\parallel }\)) and perpendicular (\(\varepsilon^{\prime}_{ \bot }\)) components of ɛ′ parameters are also shown on the graph. These data are used to calculate the dielectric anisotropy parameter. Dielectric anisotropy (Δε') is identified as the difference of \(\varepsilon^{\prime}_{\parallel }\) and \(\varepsilon^{\prime}_{ \bot }\), and is given as following formula [27]:

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When \(\varepsilon^{\prime}_{\parallel }\) is higher than \(\varepsilon^{\prime}_{ \bot }\) the material has a positive Δε′. As seen in Fig. 7 that \(\varepsilon^{\prime}_{\parallel }\) of both samples is greater than \(\varepsilon^{\prime}_{ \bot }\). E7 NLC is known to have a positive Δε′ value. Δε′ values of E7 and E7/PVA have also calculated with Eq. (8) and given in Table 1. The results confirm that E7 NLC has a positive dielectric anisotropic structure. Moreover, a small decrement in Δε′ value of E7/PVA has seen compared to E7. This change can be explained by Meier and Maier’s theory. In accordance with the Meier and Maier’s theory, Δε′ parameters of dielectric materials are given by the following formula [18, 28]:where N is the number of density, S is the rotational order parameter, h and F are the internal field factor, kT is the thermal energy, \(\Delta \alpha\) is the isotropy of polarizability, µ is the dipole moment, \(\upbeta\) is the angle between the molecular net permanent dipole moment and the long axis of molecules. The decrease in Δε′ parameter of the E7/PVA composite is predicted to be concerned with the decrease in the S parameter owing to the deterioration of the molecular orientation.

Threshold voltage (V_th) of LC materials depends on Δεʹ and splay elastic constant (K₁₁), and is given as following formula [21, 29, 30]:

From the equation it can be seen that the square of V_th varies proportionally with the ratio of K₁₁/Δε′. V_th of E7 and E7/PVA samples were calculated with electro-optical measurement set-up by voltage-dependent transmittance graphs. This electro-optical measurement set-up, which is also mentioned in the materials and methods section, is schematically illustrated in Fig. 3.

Figure 8a, b illustrates the voltage-dependent transmittance variations of the E7 and E7/PVA. When the voltage is equal to zero, LC molecules orient in the plane of the LC cell substrate on account of the anchoring effects and this proceeds up to V_th with increasing voltage. V_th is the minimum voltage required to orient the LCs and corresponds to the voltage at which the transmittance increases by 10%. After V_th, transmittance increases and reaches a maximum when the molecules of NLC are oriented perpendicular to the ITO coated electrodes of the LC cell. When the voltage is increased, minimum and maximum transmittance are observed again. These maximum and minimum transmittance values correspond to the even or odd integers in Eq. (2) and Eq. (3). Moreover, the number of maximum or minimum transmittance peaks of the voltage-dependent graphs of the samples is approximately \(d\Delta n/\lambda\) according to the Eq. (4). In this research, d = 18 μm, Δn = 0.225 (for E7 NLC), λ = 632.8 nm, so \(d\Delta n/\lambda \approx 6.\) Therefore, it can be said that the maximum and minimum transmittance alter depending on the thickness of the LC cell, the birefringence value of the sample, and the wavelength of the laser used in the measurement [21, 31]. V_th of the samples are included in Table 1 and V_th has increased with PVA contribution. According to Eq. (10), this result shows that the K₁₁/Δε′ ratio increases with PVA. In addition, in the presence of voltage, the movement of impurity ions present in the composite is provided and some of these ions are adsorbed by the alignment layer of the LC cell. This adsorption forms an electric field that reduces the influence of the external field on the LC molecules and is defined as the screening effect. The alteration in V_th is also associated with the application of an electric field to the E7/PVA sample, with more ions accessing the electrode with an increased screening effect [2]. Furthermore, the increase in V_th is quite small, and accordingly, PVA doped E7 NLCs can be considered as alternative materials for low voltage electro-optical device applications. Moreover, it is noteworthy that V_th can be adjusted by changing the PVA concentration for future studies.

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The response time of LC based composites was determined by time dependent transmittance measurements using the electro-optical measurement set-up given in Fig. 3. In time dependent transmittance measurements, the rise time (τ_r) is the time required for the transmittance to rise from 10 to 90%, and fall time (τ_f) is the time required for the transmittance to fall from 90 to 10%. Moreover, the response time (τ_R) is equal to the sum of τ_r and τ_f. The τ_r, τ_f, and τ_R are given as following formulas [17, 18]:

Figure 9a, b indicates the time dependent transmittance variations of the samples. The τ_R was calculated using these graphs and given in Table 1. As seen from the table, the τ_R increased with the PVA contribution. The change in the τ_R parameter is known to be related to rotational viscosity [32, 33]. With reference to, it is thought that the τ_R increases due to the increase in rotational viscosity with the contribution of PVA to E7 NLC. It can also be interpreted that the increase in the τ_R parameter of the E7/PVA compared to E7 is on account of different interactions such as atomic, elastic, and viscous forces between the PVA and LC molecules in the composite.

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The optical band gap (E_g) of E7 and E7/PVA has been calculated using the following formula [34, 35]:where α is the absorption coefficient, hv is the photon energy, E_g is the optical band gap of the material, and n determines the nature of the transition. The different values of n (1/2, 2, 3/2, 3) represent the allowed direct transition, allowed indirect transition, forbidden direct transition, and forbidden indirect transitions, respectively [36‐38]. In this study, wavelength dependent absorbance data were used to determine E_g values. The wavelength dependent absorbance variations are shown as insets in the graphs given in Fig. 10a, b. Furthermore, Eq. (14) has been used in the calculations using absorbance data and the best linear correlation has obtained by plotting (αhν)² versus hν and it gives n = 1/2. According to this result, it can be said that E_g of E7 and E7/PVA samples are allowed direct transitions. In Fig. 10a, b, (αhν)²-hν variations are showed and E_g values were determined using these graphs. The calculated E_g values are included in Table 1 and E_g decreases with the PVA contribution. This variation in E_g is thought to contribute to increased conductivity [39]. Furthermore, the use of materials with lower band gaps in photovoltaic devices has become increasingly popular in recent years. In this respect, the results of the study are considered significant.

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In summary, some physical properties of E7 NLC and PVA doped E7 NLC have been investigated in the current study. It was found that the presence of PVA affected Δε′, V_th, τ_R, and E_g parameters of E7. The Δε′ and E_g of E7 decreased with PVA contribution. Moreover, V_th and τ_R increased with PVA contribution. Specifically, it is noteworthy that the V_th value increased from 0.805 V_rms to 0.872 V_rms with the addition of PVA to the E7 NLC because this increase is quite small. Therefore, it can be concluded that E7/PVA composite structures may also show a tendency towards lower voltage values and considered for electro-optical devices that can operate with low energy consumption. Furthermore, it is considered that reducing the E_g value from 3.58 eV to 3.51 eV with the addition of PVA to the E7 NLC is quite advantageous for the production of photovoltaic devices with low band gaps. Additionally, ε′ and ε′′ parameters increased with PVA contribution at low frequencies. These results provide valuable insight into the effect of PVA on the electro-optical and dielectric characteristics of E7 NLC and highlight the potential to tailor the properties of LC materials for applications of electro-optical devices with tunable optical, electro-optical, and dielectric properties.