Effects of dye doping on electro-optical, thermo-electro-optical and dielectric properties of polymer dispersed liquid crystal films

Highlights

• Alignment of guest dye molecules with host liquid crystal (LC) to control electro-optic properties.

• Properties of polymer dispersed LC (PDLC) films enhanced by doping with dye molecules.

• Cole–Cole plots demonstrated Debye type relaxation in doped PDLC films.

Abstract

In this study, polymer dispersed liquid crystal (PDLC) films comprising a phase separated low molecular weight thermotropic liquid crystal (LC) and high molecular weight polymer were, prepared using the polymer–induced phase separation method. In order to enhance the optical efficiency of the PDLC films, LC was doped with dichroic azo dye. Various characterization techniques were employed to determine the properties of the components and, the operating principles of the dye–doped PDLC (DPDLC) films. Morphological studies using polarizing optical microscopy and scanning electron microscopy indicated that the LC droplet configuration changed with various dye concentrations and voltages. Electro-optical analysis showed that the performance of the DPDLC film with a low dye concentration (0.007%) was excellent, with a high contrast ratio (241) and transmittance difference (ΔT = 98.57%), low threshold (V_TH = 0.85 V/μm) and saturation voltages (V_SAT = 1.45 V/μm), and a minimum hysteresis effect. The effects of temperature on the electro-optical properties of the DPDLC films were studied. Absorbance analyses were conducted using ultraviolet–visible spectrophotometry to complement the transmittance analyses. Dielectric relaxation spectroscopy was employed to measure relaxation frequency/time of DPDLC films. Debye and Cole – Cole modeling were conducted to understand nature of the relaxation process. The zero values of the distribution parameter (α) for all of the DPDLC composite films confirmed their Debye type relaxation process.

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 $(\Delta n>0)$, the ordinary RI ($n_o$) of the LC matches with the RI of the polymer matrix ($n_p$), whereas for an LC with negative birefringence $(\Delta n<0)$, the extra–ordinary RI ($n_e$) of the LC matches with $n_p$, 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 λ_max. 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.