Polymer Dispersed Liquid Crystal Materials Research Articles

Ultrafast Switchable Passive Radiative Cooling Smart Windows with Synergistic Optical Modulation

AbstractSwitchable passive radiative cooling (PRC) smart windows can modulate sunlight transmission and spontaneously emit heat to outer space through atmospheric transparent window, presenting great potential in building energy conservation. However, realizing stable and on‐demand control of the cooling efficiency for PRC materials is still challenging. Herein, an electro‐controlled polymer‐dispersed liquid crystal (PDLC) smart window showing PRC property is designed and prepared by adding mid‐infrared emitting reactive monomers into the conventional PDLC matrix. It is found that not only the electro‐optical properties but also the PRC efficiency of PRC PDLC film are tunable by regulating the content of the mid‐infrared emitting components, film thickness, and micromorphology. This advanced PRC PDLC material achieves a near/sub‐ambient temperature when the solar irradiance is below 400 W m−2 and can dynamically manage daytime cooling efficiency. Importantly, its PRC efficiency is capable of being tuned in an on‐demand and ultrafast millisecond‐scale way, whose controllable transparency enables multistage heat regulation. This study is hoped to provide new inspiration in the preparation of advanced optical devices and energy‐efficient equipment.

Stepwise driving and multi-transparency of polymer dispersion liquid crystals

ABSTRACT Polymer dispersed liquid crystals (PDLCs) are frequently used as a display material owing to their simple design and easily driven characteristics, where the stepwise display effect can be shown through the same electrical switch. In this paper, single-layer PDLCs are laminated into a double-layer structure with an ultra-thin transparent film sandwiched in between, in contrast to single-layer PDLCs. The flat plateau area is clearly exhibited in the photoelectric curve graph, while the step-by-step display is demonstrated experimentally. Electro-optical properties of the composite film as well as its upper and lower layers as a single-layer, respectively, are characterised, by an insignificant increase in threshold voltage (Vth) for the former relative to the latter. In addition, the single-layer film morphology is additionally photographed using a scanning electron microscope. Furthermore, by choosing an ultra-film as a spacer for two PDLC materials, different transmittance regions of the film were obtained, and this result provides a new idea for the realisation of multi-display smart windows at the same voltage.

Doping white carbon black particles to adjust the electro-opticical properties of PDLC

ABSTRACT Polymer-dispersed liquid crystal (PDLC) is a possible active matrix transparent display technology due to its high transparency, good visibility, and low-power consumption. This paper examined the morphology of polymer-dispersed liquid-crystal materials (PDLCs), which show great potential for display applications. The materials were blends of an eutectic nematic liquid-crystal mixture and a UV-cross-linkable polymeric matrix. White carbon black nanomaterials with high surface adsorbability, high purity, and good stability are ideal materials for the driver circuit of the PDLC display. The influence of the ratio between polymerised monomers and between polymerised monomers and liquid crystals on the electro-optical properties of PDLC system had been studied. We explored the doping of white carbon black nanomaterials in the liquid crystal polymer systems. To achieve this, we have taken advantage of the liquid crystal electro-optical effect experimental instrument to study a series of electro-optical parameters of PDLC, using also additional information on the nanodroplet size and shape distribution provided by SEM images of the PDLC cross-section.

Smart-Glass Glazing using Arduino and Android Application

<p>The parts of buildings most susceptible to light and heat are the windows, which are necessary for basic cooling and heating. A huge amount of electrical energy is used to power air conditioners to adjust the inside temperature of residential buildings, while appropriate glazing techniques and materials can be used to reduce energy consumption. There are classic techniques used for tinting glass in buildings and cars, but they all continue to use fixed techniques with one degree of glazing. Users cannot adjust the glazing of these glasses in response to the intensity of sunlight either automatically or by hand. In this research, a modern technique is proposed to allow users to automatically control a glass’s glazing through sensors and Android application with a microcontroller. Arduino microcontroller is used along with a special film polymer dispersed liquid crystals (PDLC) material coating the glasses to enable tinting. The results show adjustable levels of tinting with PDLC film being enabled.</p>

Analysis of the Imaging Characteristics of Holographic Waveguides Recorded in Photopolymers.

In this work, we study the imaging characteristics of an optical see-through display based on a holographic waveguide. To fabricate this device, two transmission holograms are recorded on a photopolymer material attached to a glass substrate. The role of the holograms is to couple the incident light between air and the glass substrate, accomplishing total internal reflection. The role of noise reflection gratings and shrinkage on the imaging characteristics of the device will be also explored. The holograms (slanted transmission gratings with a spatial frequency of 1690 lines/mm) were recorded on a polyvinyl alcohol acrylamide holographic polymer dispersed liquid crystal (HPDLC) material. We will show that sufficient refractive index modulation is achieved in the material, in order to obtain high diffraction efficiencies. We will demonstrate that the final device acts as an image formation system.

Electro-optic properties of aligned and non-aligned polymer dispersed liquid crystals driven by an amplitude-modulated electric signal

We present an experimental study of the effect of field strength and modulating frequency of the applied amplitude modulated (AM) electric signal on the electro-optic (EO) properties of aligned and non-aligned polymer dispersed liquid crystals (PDLCs). A low modulating frequency is used to control the pulse duration of the driving AM electric signal. Two PDLC films of thickness 5 μm were prepared from a standard mixture of UV curable polymer/liquid crystal (NOA65/E7) with and without alignment layers. The EO properties are significantly enhanced in aligned PDLC thin film compared to non-aligned. A small increase of the modulating frequency decreases considerably the total response time at relatively low applied electric field strength. However, the increase of the applied field strength from 0.63 V/ μm to 2.05 V/μm reduces significantly the total response time by a factor of two. When proper combination of the applied field strength and modulating frequency is achieved the EO properties of PDLC is enhanced and tuned effectively at relatively low applied voltage. This study shows how relatively fast response and large dynamic range optical tuning of PDLC material can be achieved at low frequency. This is important findings to be implied in PDLC-based economic photonic devices.

Digital Photography Using a High-Transmittance Electro-Optical Iris

In this paper, a tunable electro-optical iris with high transmittance, based on polymer dispersed liquid crystal (PDLC) material, and a relevant image enhancement method are combined for digital photography. Our device is controllable by adjusting the voltage and no mechanical and moving components are required so as to improve the stability. The experiments show that transmittance of light throughput is tunable from nearly 0% to 77% by gradually increasing driving voltage. The digital camera system using our PDLC iris can capture the high-contrast image in real-scene photography. To further improve the image quality, we develop an iterative image enhancement method that is able to improve the signal-to-noise ratio by ~10 times and increase the image brightness simultaneously. The combination of the PDLC electro-optical device and a variant of the enhancement algorithm can also have the potential to be applied in optical imaging and sensing.

Photo-differential Scanning Calorimetry Study on Photopolymerization of Polyacrylate/E7 Liquid Crystal Blends

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics of a photo-initiated polymerization of tripropylene glycol diacrylate (TPGDA) monomers mixed with a eutectic liquid crystal mixture, E7, chosen due to its significant optical properties. The thermal photo-polymerization reactions were performed in isothermal mode in order to identify and evaluate the different thermal effects occurring during the photo-polymerization reaction under UV light. The results obtained help to better understand the curing mechanisms as well as the kinetics of the curing reactions. The isothermal photo-DSC provided complex thermograms showing the presence of two thermal effects: an exothermic heat (positive activation energy) generated during the conversion of monomers and an endothermic heat produced by the UV lamp. The latter phenomenon was observed in our previous work and was associated to the heating of the polymer dispersed liquid crystals (PDLC) material by the UV irradiation lamp. The dependence of photopolymerization on the time and intensities of the curing UV light were investigated. The results showed that the high intensity UV light heats the material and slows the process of phase separation which, in turn, affects the electro-optical properties of the material under study. The photo DSC results also revealed that the heating of the system was insignificant and can, therefore, be neglected, for materials exposed to UV radiation with an irradiation time of 5 seconds and a low intensity of 13.80 mW/cm2.

Regularities of Formation of Polymer-Dispersed Liquid Crystal Materials Doped with Carbon Nanotubes

Regularities of Formation of Polymer-Dispersed Liquid Crystal Materials Doped with Carbon Nanotubes

Design of an Electrically Tunable Micro-Lens Based on Graded Photonic Crystal

A micro-lens with an adjustable focal length (FL) is designed by using Graded Photonic Crystal (GPC) structures and a Polymer Dispersed Liquid Crystal (PDLC) material. The GPCs are formed by gradually changing the radius of the polymer rods in the Photonic Crystal (PC) with square lattices of polymer rods in the background of Liquid Crystals (LCs). The electrically tunable focusing characteristics of the micro-lens are investigated by loading a continuous voltage source to change the LC rotation angle. The sensitivity of the focal shift in terms of LCs tilting angle is 0.152 λ(nm/deg). Moreover, the effect of the defects and deviations on the focusing characteristics are also analyzed. This research is crucial for future applications of the proposed device in the integrated photonics and adaptive optics.

Effect of titanium dioxide nanoparticles on polymer network formation

ABSTRACTThis work presents a study of the effect of nanoparticles on polymer composites to develop a powerful polymer dispersed liquid crystal materials. Tri propylene glycol diarcrylate/titanium dioxide nanocomposites were produced at various titanium dioxide fractions ranging between 0 and 1 wt%, through ultraviolet curing technique during 30 min. Different technics such as polarized optical microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermal gravimetric analysis were used to characterize the samples. A dynamic swelling of polymer network was also investigated. The evolution of the acrylic double-bond conversion shows a decrease in the absorption intensity at 1637 cm−1. It is noted that the conversion rate decreases from 0.1 to 1 wt% of titanium dioxide nanoparticles. An increase in glass transition temperature is observed by differential scanning calorimetry. The thermogravimetric analysis results reveal a highly improved thermal stability upon the addition of the reinforcing phase. The follow of kinetics swelling of polymer network shows a decrease of the swell ratio with the inclusion of titanium dioxide nanoparticles.

The regulation of polymer structures and electro-optical properties of epoxy-mercaptan-based phase separated liquid crystals / polymer composites

Morphologies of polymer networks are playing a key role in affecting the electro-optical (E-O) properties of a polymer dispersed liquid crystals (PDLCs) material. In this paper, the relationships between the polymer morphologies and the E-O properties of PDLCs are investigated in an unexplored thermally curing system based on epoxy/low-molar-mass-mercaptan. The polymer morphologies in this curing system can be regulated from polymer beads to porous polymer matrix by varying the liquid crystals (LCs) contents, chemical structures of epoxy monomers, and functionality of thiol hardeners. Interestingly, great improvements of E-O properties occur as the polymer structures change from polymer beads to porous frameworks, even though with an abnormal low LCs contents. Eventually, the mechanism for the formation of different polymer structures are illustrated and the relationships between the E-O properties and polymer structures are established. This work provides new insights in morphology control and optimizing the E-O properties in thermally cured PDLCs films.

Polarization-Independent Electrically Tunable Holographic Polymer Dispersed Liquid Crystals Grating Doped with Chiral Molecules

This study proposes a holographic grating made of polymer dispersed liquid crystal (PDLC), with a small amount of chiral molecules doped into PDLC material. The major advantage of this grating is that it is independent of light polarization. This characteristic was verified by applying the interference beam intensity of a He-Cd laser at 150 mW/cm 2 , with an incidence angle between the two interference beams of 24°, for an irradiation curing duration of 120 s. The observed periodic structure of the grating is consistent with the theoretical value. As chiral molecules are doped, nematic-LC experiences a phase-change in the grating. However, the electro-optical features are only slightly affected. This proposed grating has greatly potential in 3D imaging because of its polarization-independent feature. DOI: http://dx.doi.org/10.5755/j01.ms.23.2.16312

Holographic Photopolymer Linear Variable Filter with Enhanced Blue Reflection

A single beam one-step holographic interferometry method was developed to fabricate porous polymer structures with controllable pore size and location to produce compact graded photonic bandgap structures for linear variable optical filters. This technology is based on holographic polymer dispersed liquid crystal materials. By introducing a forced internal reflection, the optical reflection throughout the visible spectral region, from blue to red, is high and uniform. In addition, the control of the bandwidth of the reflection resonance, related to the light intensity and spatial porosity distributions, was investigated to optimize the optical performance. The development of portable and inexpensive personal health-care and environmental multispectral sensing/imaging devices will be possible using these filters.

Structure transitions in oblate nematic droplets

We consider the structure transitions in oblate supramicrometer nematic droplets related to reorientation of the line defect in the electric field. These transitions can be used in optical devices based on polymer dispersed liquid crystal materials with high contrast ratio. We suggest a simple method for determination of director distribution in nematic droplets of an arbitrary shape with surface interaction and in the presence of constant electric field. Point and linear defects are taken into account. This method does not require any presuppositions about symmetry of the director distribution. The elasticity continuum theory is treated with Monte Carlo annealing on a simple lattice. A special triangulation-based technique is applied for accurate representation of the droplet boundaries. The method is tested on 5CB material.

Design of Functional Materials based on Liquid Crystalline Droplets.

This brief perspective focuses on recent advances in the design of functional soft materials that are based on confinement of low molecular weight liquid crystals (LCs) within micrometer-sized droplets. While the ordering of LCs within micrometer-sized domains has been explored extensively in polymer-dispersed LC materials, recent studies performed with LC domains with precisely defined size and interfacial chemistry have unmasked observations of confinement-induced ordering of LCs that do not follow previously reported theoretical predictions. These new findings, which are enabled in part by advances in the preparation of LCs encapsulated in polymeric shells, are opening up new opportunities for the design of soft responsive materials based on surface-induced ordering transitions. These materials are also providing new insights into the self-assembly of biomolecular and colloidal species at defects formed by LCs confined to micrometer-sized domains. The studies presented in this perspective serve additionally to highlight gaps in knowledge regarding the ordering of LCs in confined systems.

Full Color Image Splitter Based on Holographic Optical Elements for Stereogram Application

A color holographic beam splitter (CHBS) applied to an autostereoscopic display is proposed in this study. With high-stability polymer-dispersed-liquid-crystals (PDLC) being the holographic materials, holographic grating is utilized as a full-color beam splitter to split left and right field of views (FOVs) of a stereoscopic display. When CHBS is attached, the experimental results show that the correct left/right FOVs images could be successfully obtained at the preset observing distance of 25 cm. Moreover, the diffraction efficiency of the red, green and blue (RGB) left/right FOVs of the PDLC materials remained at 30.0% on average. Apparently, the brightness efficiency of the proposed CHBS stereo-displays is better than the traditional stereo-displays with a parallax barrier (22.4%). In addition, a crosstalk contrast with 95.0% indicates the crosstalk problem is almost avoided in the proposed technique.

Dichroic Dye Induced Nonlinearity in Polymer Dispersed Liquid Crystal Materials for Display Devices

The electro-optical characteristics of dye-doped polymer dispersed liquid crystals (PDLCs) have been investigated for display applications. The PDLC samples were obtained by polymerization induced phase separation of the nematic liquid crystal (LC)-dye-prepolymer mixtures under UV illumination with a constant intensity. The optimum conditions for the scattering characteristics of the dye-doped PDLC films as function of the dye concentration have been examined. It was seen that the phase separation and segregation of LC droplets is dependent on the amount of dye used. LC droplets in dye-doped PDLC films exhibit various configurations at lower and higher applied electric field when observed in situ under polarizing optical microscope. The effects of morphology on the electro-optical properties were examined. Experimental results indicate that the driving voltage and contrast ratio were affected considerably by the amount of dye. UV-VIS spectroscopy results showed that the molecular orientation of dye in LC droplets can be controlled to induce nonlinearity in these materials. The results showed that the dye concentration can be optimized to obtain promising electronic materials with minimum threshold and high contrast for display applications.

Development of passive and active microprism arrays to change the radiation pattern of solid-state lighting

This study implements a compact solid-state lighting chip with changeable illumination map as well as radiation pattern. The lighting chip consists of a microprism array, light-emitting diode (LED) chip and Si carrier. The polydimethylsiloxane (PDMS) and polymer-dispersed liquid crystal (PDLC) layers are respectively employed to implement the passive and active microprism arrays. The specific radiation pattern can be defined by the shape of the passive PDMS-microprism. Moreover, by using the scattering and transmitting modes of the PDLC layer, the PDLC-microprism enables the changing of light shaping by applying voltage. Thus, the radiation pattern can be changed by the driving voltage on the PDLC layer, and the deformable and movable micro optical components are not required. This study has established the low-temperature fabrication and packaging processes to realize the lighting chip, and the damage of the PDMS and PDLC material is prevented. Typical dimensions of the PDMS lighting chip are 5 mm wide, 6 mm long and 1 mm thick, and The PDLC lighting chip is 550 µm thick. The measurement results show that the PDMS-microprism array can change the radiation pattern from a 70° half-maximum viewing angle to 52° and 40° half-maximum viewing angles on two orthogonal axes. In addition, the PDLC-microprism array can change the radiation pattern from 51° and 43° half-maximum viewing angles at 0 V (i.e. scattering mode) to 48° and 33° half-maximum viewing angles at 100 V (i.e. transmitting mode).

Electrically switchable two-dimensional photonic crystals made of polymer-dispersed liquid crystals based on the Talbot self-imaging effect

Electrically switchable two-dimensional photonic crystals were demonstrated using polymer-dispersed liquid crystal materials based on the Talbot self-imaging effect of a single photomask. With the photomask subjected to a collimated Ar+ laser beam operating at 488 nm, a three-dimensional spatial light intensity pattern was created due to the Talbot self-imaging effect. The spatial light intensity pattern was then recorded inside a cell filled with the liquid crystal/prepolymer mixture to create photonic crystal structures. The surface morphology of the photonic crystals was examined by an atomic force microscopy. It showed square structures with a lattice constant of ∼0.9 µm. The diffraction and electro-optical properties were also presented. This approach shows a simple and fast fabrication.