Periodic Electro-Optical Characteristics of PDLC Film Driven by a Low-Frequency Square Wave Voltage
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Polymer-dispersed liquid crystal (PDLC) is a composite material consisting of LC droplets randomly dispersed into an isotropic polymer matrix and represents a significant new class of electro-optical materials [1,2,3,4]. The electro-optical properties of PDLCs are dependent on the electrically controlled light scattering from the dispersed LC droplets. An alternating current (AC) voltage (frequency range of 101–103 Hz) is typically used to prevent conductivity effects and typically applied to voltages from 20 to 100 Vrms to keep the devices transparent [5,6]. LC droplets in PDLCs can respond to an applied electrical field, aligning to maximize the dielectric constant. For LC with positive dielectric anisotropy, LC droplets can be in the direction of the applied electric field so that PDLC films can be switched from an opaque off-state to a highly transparent on-state [7,8]. The competition between the applied electric field and LC elastic and viscous torques governs the response time and switching voltage of PDLCs [9]. The response time of LC droplets is in the order of 10−3 s. Compared with conventional LCs and other heterogeneous systems, PDLCs have a new operating principle and dynamic characteristics [10].
In general, uncontrolled ions may occur in the LC during production [11,12,13], and almost all LCs contain free ions. For example, the ion concentration in LC E7 was estimated to be 1015/cm3 [14,15]. The free ions could decrease the dielectric constant and refractive index in LC droplets [16] and induce an internal polarization electric field (Eint) to affect the hysteresis and memory state in the PDLC films [8,17,18]. The voltage drop across the LC droplets is just a fraction of the voltage applied in a PDLC film because of a high voltage drop through the polymer matrix [17,19,20]. The electric field applied to LC droplets can be called the local electric field (Eloc) [20]. With a local electric field, the redistributed ions can accumulate on the surface of LC droplets to form Eint, which opposes Eloc [12,21]. Eint could play an essential role in the electro-optical response for PDLC devices. For example, Eint can screen the applied electric field (the charge screening effect) [12,14]. The PDLC films can be switched from transparent to scattering at a low frequency [14]. For frequencies below 100 Hz, the total electric field, i.e., the effective electric field (Eeff) in LC droplets, could be severely affected by the accumulated charge at the droplet/polymer interface [22]. Eint was investigated by observing the electro-optical response of a PDLC film to direct current (DC) and low-frequency sinusoidal wave-driving voltages [23]. It was revealed that Eint could align the LC droplets to produce electro-optical effects [23].
When investigating and applying the electro-optical effect of PDLCs, the AC square-wave voltage is usually used. In a period, the applied voltage changes polarity rather than amplitude, while it can be regarded as the voltage in a half-period as a DC voltage. However, the relationship between Eint and the frequency of the square wave voltage has not been investigated. In the work represented, we investigated the evolving curve of transmittance with time (T-t) with a square-wave voltage of 0–50 Hz. Moreover, depending on the characteristics of the T-t curves in different frequency ranges, the dynamic behavior and the electro-optical effect based on Eint were investigated. It was revealed that Eint can increase Eeff when the polarity of the AC square wave voltage is reversed.
The LC used in our experiments was nematic LC E7 (Shijiazhuang Slichen Display Materials Co. Ltd., Shijiazhuang, China). The prepolymer in our experiments was composed of an adhesive (NOA65, Norland Products Inc., Cranbury, NJ, USA) and a photoinitiator (Irgacure 907, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) with a weight ratio of 98 to 2. The length, width, and gap of the LC cells (Hebei Yiya Electronics Co. Ltd., Shijiazhuang, China) were 3.0 cm, 2.0 cm, and 20 μm, respectively.
The γ-Fe2O3 nanoparticles coated with oleic acid (called MNPs, the same as below) were prepared by a chemically induced transition method, and MNPs were about spherical with an average size of about 10 nm [24]. The LC colloids containing MNPs were prepared as follows. First, the MNPs were added in hexane to form the pre-colloid. Then, the pre-colloid was added to the LC to form a mixture. Second, the mixture was heated to 60 °C to volatilize the hexane. Finally, the LC colloid was obtained with the MNPs’ mass fractions ϕm = 0.11%.
The PDLC films were prepared as follows. First, the LC E7 and LC colloids were mixed separately with the prepolymer at a weight ratio of 1 to 1 to form the PDLC mixtures. Second, the mixtures were heated to 85 °C, cooled to room temperature. Then, they were left for 24 h to form the two homogenous mixtures. Third, the heated mixtures (85 °C) were filled separately in LC cells by capillarity and cured with ultraviolet light (36.2 mW/cm2 @ 325 nm) for 10 min at a 30-cm exposure distance. Consequently, the PDLC films undoped and doped MNPs were obtained, designated as the PDLC and M-PDLC samples, respectively.
The surface morphologies of the two samples were characterized by a scanning electron microscope (SEM, JSM-7100F, JEOL Ltd., Tokyo, Japan). Before the SEM analysis, the PDLC cells were punctured linearly and soaked in hexane for 48 h to extract the LC droplets. Then, the films were dried in a drying oven at 70 °C and plated with Au. The change in resistance with the frequency was measured using the precision LCR (E4980A, Agilent Technologies Inc., Beijing, China) (see Appendix A).
Figure 1 shows the schematic diagram of the electro-optical measurement. A light beam emitted from a He-Ne laser (1) (λ = 632.8 nm, Melles Griot Inc., Carlsbad, CA, USA) passed through an attenuator (2) (Zolix Inc., Beijing, China), a polarizer (3) (Thorlabs Inc., Newton, NJ, USA), a λ/4 wave plate (4) (λ = 633 nm, Thorlabs, Newton, NJ, USA), another polarizer (5) (Thorlabs Inc., Newton, NJ, USA), and a diaphragm (6) (Zolix Inc., Beijing, China), in turn, which provided a normal incident on a sample (7). The voltage applied to the sample (7) was a square-wave voltage with an adjustable frequency (0–50 Hz) controlled by a voltage amplifier (9) (Nantong Long Yi Electronic Technology Co., Ltd., Nantong, China). The distance between the sample (7) and a detector (8) (7ID230, SOFN Instruments Co., Ltd., Beijing, China) was 20 cm. The converted digital signals from a data acquisition card (10) (DAC, 6216, NI) were connected to the detector (8) and processed by a computer (11) in the LabVIEW environment. The applied square-wave voltage was 30 Vrms. Transmittance is defined as T=It/Ii, where Ii and It are the incident light and transmitted light intensities, respectively.
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