1. Introduction
Polymer dispersed liquid crystal (PDLC) is a new type of liquid crystal display device, since the LC molecules can change their orientation under the influence of an applied electrical field [
1], they can switch between a highly scattering opaque state and a transparent state [
2,
3,
4]. It has a simple preparation process, low price, and no need for polarizers, and can be used for large-area flexible displays [
5,
6,
7,
8]. In addition to the display, PDLC is now also used in electronically controlled smart glass, liquid crystal gratings, optical switches, and other fields [
9,
10,
11,
12,
13,
14,
15,
16,
17]. Its unique electro-optical performance has attracted widespread attention from domestic and foreign researchers.
Advances in the field of nanotechnology have led to nanostructured materials with adjustable optical, electronic, magnetic, and chemical properties [
18,
19,
20,
21]. These nanostructured materials are now very suitable for use as additives in PDLC systems to improve the characteristics of the latter [
22,
23]. In recent years, a large number of reports have witnessed the effectiveness of metal nanoparticles, inorganic oxide nanoparticles, and ferroelectric nanoparticles in changing the physical and electro-optical properties of PDLC [
24,
25,
26,
27,
28,
29]. For example, Martin U et al. [
30] studied the impact of functionalized gold nanoparticles on the impedance response of nematic nanoparticle/liquid crystal dispersions in the frequency range of 0.1 Hz~100 kHz. The result shows that nanoparticle do** does not alter the electro-optic response at frequencies above the occurrence of electrode polarization, while it strongly deteriorates the performance in the low-frequency regime. Zhang Y et al. [
31] prepared PDLC films doped with ITO nanoparticles by UV-induced polymerization based on a thiol-acrylate system. It is found that the film has a lower driving voltage (20.7 V), a relatively high contrast ratio (8.3), and a better performance with the lowest transmittance. John VN [
32] studied flexible BTO-doped polymer dispersed liquid crystal devices with ferroelectric nanoparticles, and found that due to the spontaneous polarization of ferroelectric nanoparticles, the threshold voltage (V
th) and saturation voltage (V
sat) of ferroelectric BTO nanoparticles were respectively added. The threshold voltage (V
th) and saturation voltage (V
sat) of PDLC were reduced by 85% and 41%.
At present, PDLC film still has problems such as high energy consumption, slow response, low contrast, etc., and there are still certain limitations in practical applications. MgO nanoparticles have the advantages of absorbing ultraviolet light, easy preparation, low price, etc. When they are doped into nematic liquid crystal systems, polymer dispersed liquid crystals with low driving voltage and high contrast can be obtained. In this study, it is found that the viscosity of UV64-5 is relatively large. When 2-EHA was added, the viscosity of the composite decreased. The more 2-EHA was added, the more the viscosity of the system decreased. Therefore, the number of liquid crystal droplets increases and the size decreases. When the LC content is 50%, the polymer mesh size matches the LC concentration in the mesh best. At this time, the CR is maximum, and the PDLC film has good electro-optical properties. Above all, in this work, we present experimental results on electro-optical properties of the SLC1717 nematic liquid crystal doped by MgO NPs. Scanning Electron Microscope and electro-optical measurements have been carried out to examine the effect of MgO NPs on the PDLC films. The results show that the addition of MgO NPs reduces the driving voltage of the PDLC films.
4. Conclusions
MgO nanoparticles were introduced into the PDLC material of UV64-5/2-EHA/SLC1717 formula to change its performance, and were observed through analysis and discussion. The results obtained from electro-optics show that the doped MgO NP system has higher performance, especially in terms of reducing the driving voltage (for example, after do** 0.8% MgO nanoparticles in the composite system, the Vth drops from 25.6 V to 7.5 V). This phenomenon can be explained by morphological SEM results, which show that the size of the LC droplets is different compared to the undoped system. When MgO nanoparticles are added to the composite material, the interaction force between MgO and LC molecules partially replaces the interaction force between the polymer matrix and the LC molecules, and the former is smaller than the latter, resulting in more complete phase separation. Therefore, adding MgO nanoparticles to the composite material will affect the anchoring force at the interface between the LC molecules and the polymer matrix, and reduce the voltage required to reorient the LC molecules.