Effects of dye doping on electro-optical, thermo-electro-optical and dielectric properties of polymer dispersed liquid crystal films
Introduction
Polymer dispersed liquid crystals (PDLCs) are thin composite films made from a high molecular weight polymer and low molecular weight thermotropic liquid crystal (LC) using the phase separation method. The LC droplets formed by the phase separation method are mechanically and structurally stabilized by the polymer matrix. The most common applications of PDLC films include flat panel flexible displays with large areas, switchable windows, light valves, high definition spatial light modulators, optical and thermal sensors, strain gauges, color projectors, bistable reflective displays, and optical shutters that operate in normal or reverse modes [[1], [2], [3], [4]].
The birefringence property of LCs is important for the operation of composite films. LCs are optically anisotropic materials with two distinct refractive indexes (RIs). In the absence of an electric field, directors present in micron–sized LC droplets are randomly oriented in the polymer matrix. When light falls on the film, it is scattered and refracted multiple times on the polymer–LC interfaces and the film appears opaque as a result. After the application of suitable electric field, the LC directors are aligned along the direction of the external electric field. Under these conditions, for an LC with positive birefringence , the ordinary RI () of the LC matches with the RI of the polymer matrix (), whereas for an LC with negative birefringence , the extra–ordinary RI () of the LC matches with , thereby leading to the transparent appearance of the film [[5], [6], [7]]. In addition to the optical anisotropy of LC, the excellent performance of PDLC composite films is influenced by many factors, such as the dielectric anisotropy of the LC, polymer-LC interactions, the size, shape, and structure of the LC droplets, and the match/mismatch of the RI between the LC and its host polymer, as well as between different LC droplets. The main role of the polymer matrix is to support the LC droplets and provide flexibility and mechanical strength to the device, but physical interactions with the polymer can affect the formation and configuration of meso-phases. In some cases, the spherical LC droplets may be deformed because of the polymer matrix. Parameters such as the physical properties of the constituents (e.g., conductivity, RI, and viscosity), their proportions, solubility and diffusion of LC in the monomer/pre-polymer, phase separation method employed, dopants and temperature determine the morphology of the droplets and electro-optical characteristics of PDLC films [[8], [9], [10], [11], [12], [13], [14], [15]]. The dopant and temperature affect the morphology of the LC droplets and thus the contrast ratio (CR) and transmittance of the composite films [16,17]. Therefore, in the present study, we used a dichroic dye to dope a nematic LC and form polymer-LC composite films, and the properties of the composite systems were determined.
Dichroic dye molecules are long and rigid molecules that exhibit a strong transition moment along one molecular axis, thereby leading to the anisotropic absorption of light, and they tend to align along the director when dissolved in an LC solvent. This phenomenon where guest dye molecules align with the host LC is known as the guest–host interaction. The color intensity of the system can be manipulated by only controlling the LC rotation because dye molecules tend to align with the LC molecules. The two types of dichroic dyes according to the form of dichroism are pleochroic or positive and negative dichroic dyes [[18], [19], [20], [21], [22], [23], [24]]. Pleochroic or positive dichroic dyes are generally used with LCs because they exhibit higher order parameters and dichroic ratios compared with their counterpart negative dichroic dyes. Dyes typically have a narrow absorption spectrum and the wavelength corresponding to the peak is denoted as λmax. The visible color of a dye is attributable to the light reflected by the dye or the complementary color.
The effects of doping an LC material with dye on the morphological, electro-optical, thermo-electro-optical, and dielectric properties of the resulting polymer–LC composite films were investigated in the present study. The responses of the composite films to the different applied electric fields and temperatures were also studied using theoretical and modeling techniques.
Section snippets
Materials and methods
The performance of any device depends on its properties, and thus the methods and materials which employed for its production. Selecting appropriate materials is very important for the optimum performance of display devices. Determining the different parameters for the constituent materials helps to select appropriate materials for a particular device. Materials are selected based on their individual chemical and physical properties, as well as their compatibility with the other component
Morphological analysis
POM and SEM instruments were employed to conduct morphological analyses of O00 N DPDLC composite films.
Conclusions
In this study, we characterized O00 N DPDLC composite films synthesized using the photopolymerization technique with morphological, electro-optical, thermo-electro-optical, absorbance, and DRS methods. Morphological analyses showed that the droplet size increased as the dye concentration increased. The application of an electric field significantly affected the configuration of the dye–doped LC droplets which was responsible for the electro-optical properties of the composite films.
Funding
This research was supported by the Department of Science and Technology (DST-WOS-A), India through Grant No. SR/WOS/A/PM-91/2018(G).
Authors' contributions
Dr. Anuja Katariya-Jain: Methodology, Conducting research, Data curation, Investigation, Writing-draft preparation.
Prof, R. R. Deshmukh: Conceptualization, Visualization, Supervision, Project administration, Writing- Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no conflicts of interest that might have influenced the work reported in this paper.
Acknowledgments
Dr. Anuja Katariya-Jain would like to thank the Department of Science & Technology (DST-WOS-A), India through Grant No. SR/WOS/A/PM-91/2018(G).
References (83)
- et al.
Opt. Mater.
(2019) - et al.
Opt. Mater.
(2020) - et al.
Displays
(1985) Dyes Pigments
(1982)- et al.
Phys. B Condens. Matter
(2010) - et al.
Curr. Appl. Phys.
(2007) - et al.
J. Mol. Liq.
(2019) - et al.
J. Phys. Chem. Solid.
(2013) - et al.
Thin Solid Films
(2011) - et al.
Opt. Mater.
(2004)