Polymer Dispersed Liquid Crystal Research Articles

Emerging tunable window technologies for active transparency tuning

Most modern high-rise buildings' facades use glasses for esthetics, daylight, and better environmental view through them. However, with the increasing use of a larger area of transparent glasses as walls, the visual privacy preservation of the indoors and heat energy loss through the windows are becoming a rising concern. Recent studies showed that nearly half of the energy consumed in a building goes to heating and air conditioning while approximately 40% of this energy is lost through windows. Windows with tunable optical properties that are generically termed “tunable windows or smart windows or switchable glass” are perceived as a potential solution for these problems. An optically tunable window can adjust the amount of daylighting passing through it, control the heat radiation, and/or change the transparency of the glasses for visual privacy preservation of indoors. Electrochromic glasses, polymer dispersed liquid crystal glasses, and suspended particle devices are available as commercial tunable windows but their high cost, limited optical performance, reliability, and operational complexity are hindering the widespread adaptation. Therefore, several other technologies for low-cost actively tunable windows capable of actively adjusting transparency are increasingly explored. However, most of such new technologies, working based on various optical principles, do not fulfill all the requirements of tunable windows. For instance, some can tune optical transmittance but do not affect energy transmission, and some can adjust heat radiation transmission but has a limited change in visual appearances. To fully take the advantage of the strengths as well as recognize the limitations of such emerging technologies, their optical principles need to be understood in-depth. Here, we review the recent developments in transmittance tunable windows by categorizing them based on the optics involved, namely, light absorption, reflection, and scattering. This in-depth review comprehensively discusses how the tunable window technologies compare to each other and offers insight into how their performance can be improved in the future.

Polymer-Dispersed Liquid Crystal (PDLC) smart switchable windows for less-energy hungry buildings and visual comfort in hot desert climate

Smart switchable glazing windows have become an alternative energy-saving solution for achieving visual comfort and reducing energy consumption. In Saudi Arabia, significant amounts of energy are lost through external envelope façades due to the integration of conventional glazing systems. Thus, in this work, experimental characterisation and numerical analysis were carried out on a scale model integrated with a Polymer-Dispersed Liquid Crystal (PDLC) smart window in a hot desert climate. In the first part of this paper, an outdoor scale model was used to evaluate the thermos-optical properties of a PDLC smart window. The experiments were conducted in the spring season in actual conditions, while the state of the window (transparent and opaque) was adjusted. The data were validated and employed to develop a numerical model for energy consumption and visual comfort analysis. A comprehensive parametric analysis of the window-to-wall ratio (WWR) in cardinal orientation was carried out using the EnergyPlus9.4 and Climate Studio packages. The results indicated that the PDLC glazing in the system was able to achieve the Leadership in Energy and Environmental Design (LEED) LEED4.1 standard partially due to the increased Annual Sunlight Exposure ASE>1000lx 250h, even with low transmittance and WWR in both states. The binary behaviour of the PDLC glazing was able to reduce the emission of CO2 from 32.5–89 kg CO2/m2 using the WWR fully for the transparent and opaque states, respectively. Overall, this study contributed to the knowledge on the integration of smart glazing in buildings with low energy consumption and indoor daylighting in a hot desert climate.

Application dependent stability of Ti3C2Tx MXene in PDLC-based smart-windows

Despite having many properties, Ti3C2Tx MXene lacks stability against oxidation. To exploit its applicability, the stability against oxidation needs to be improved. In this study, we have found an application of Ti3C2Tx MXene in polymer-dispersed liquid crystals (PDLC) based smart-windows which doesn't require additional treatment to MXene for preventing its oxidation. Before this application, the MXene surface passivation by a thin layer of UV-curable polymer NOA65 was also studied to estimate long-term stability against oxidation. In polymer passivated MXene, the sheet resistance increases by just 20% as compared to 800% in the case of non-passivated counterpart as studied up to 180 days. During this period, the transmittance of polymer passivated MXene film remains almost unchanged. These results encouraged the use of MXene-coated glass substrates as transparent conducting electrodes in PDLC-based smart-window applications. The NOA65 polymer used in smart-windows fabrication encapsulates the liquid crystals as well as it also preserves the MXene without any additional treatment to the MXene surface. The transparent conducting MXene electrodes and PDLC-based smart-windows maintain high transmittance of around 71% after 180 days as compared to 74% on day 1 at a voltage of 50 V.

Role of semiconductor nanoparticles on electro-optical performance of PDLC devices

ABSTRACT The performance of polymer dispersed liquid crystal (PDLC) can be adjusted according to the needs of the application by controlling its composition ratio and microstructure. The clatter-based PDLC film with polymerisation-induced phase separation was prepared by ultraviolet curing method, and semiconductor nanoparticles were doped in it. The effects of semiconductor nanoparticles doping on the morphology, electrical and optical properties of PDLC films were investigated. Research showed that the addition of semiconductor nanoparticles reduced the rise time, threshold voltage, saturation voltage, and enhances contrast. The development of PDLC doped with semiconductor nanoparticles’ film which has faster rise time, lower threshold voltage, lower saturation voltage and better contrast will significantly increase the potential of PDLC-based devices in smart windows, sensors and flexible equipment application. At the same time, semiconductor nanoparticles had the characteristics of high fluorescence efficiency, wide absorption, narrow emission and bright colours. When PDLC doped with semiconductor nanoparticles is used as smart glass, the optical properties of semiconductor nanoparticles can be used to absorb light in part of the wavelength band and convert it into light in another wavelength band. In addition, it can have variable colours to make the appearance more beautiful while expanding the scope of its use.

Electro-optical characteristics of polymer-dispersed liquid crystal containing copper (II) phthalocyanine as a function of UV irradiation time

We describe a method of dispersing copper (II) phthalocyanine that can expand the application range of polymer-dispersed liquid crystals (PDLCs) among smart-window technologies. Property changes were monitored during the polymerization-induced phase separation method as the ultraviolet (UV) irradiation time was increased from 70 to 175 s. With increasing UV irradiation time, the number of droplets in the polymer matrix within the PDLC increased, while the droplet size decreased, as observed by polarized optical and low vacuum-scanning electron microscopies. Voltage–transmittance and response time were measured to observe the effects on electro-optical properties; the threshold and saturation voltages increased with increasing UV-irradiation time. The falling time of the response decreased with increasing irradiation time; a fast switching time of < 20 ms was observed in all cases. This study provides the performance of the produced PDLC varies according to the UV irradiation time, and conditions and processes necessary for producing blue PDLC are presented.

Functionalized polyhedral oligomeric silsesquioxane-based polymer dispersed liquid crystal films with low driving voltage and high contrast ratio

In this study, an effective method was demonstrated to improve the performance of polymer dispersed liquid crystal (PDLC) films by incorporating nano-sized polyhedral oligomeric silsesquioxane (POSS) particles into the polymer matrix. The thiol-functionalized POSS dispersed uniformly in the polymer phase by participating in the crosslinking reaction of epoxy and thiol monomers. Consequently, low driving voltages within the safety range were achieved by introducing POSS in percentage of 6 wt%, due to the combined effects of regulating the polymer morphology and lowering the surface energy. Moreover, the contrast ratio was improved to be higher than 300. Stability and switchability were also verified in on-off cycles, indicating its lifetime of as least 50000 cycles. In addition, the epoxy and thiol material system contributed to the outstanding mechanical strength, which could help maintain the electro-optical properties even in flexible films. This work could provide an effective approach for the optimization of PDLC films and promote their potential applications in fields such as flexible displays and smart windows.

Optimization approach for the dilute magnetic polymer-dispersed liquid crystal

A method was investigated to improve the capacitance and electro-optical response of the dilute magnetic polymer-dispersed liquid crystals (PDLC). The proposed magnetic separation treatment further decreased the ion concentration of the liquid crystal colloid while preparing the diluted magnetic PDLC. The microstructure of the optimized diluted magnetic PDLC, including the shape, size, and distribution of the LC droplets, has not changed, but the capacitance and the electro-optical response of the PDLC have been further improved. Usually, increasing the concentration of the magnetic nanoparticles may reduce the ion concentration more, but the performance of PDLC has been a disadvantage. Our presented work shows an additional decrease in the ion concentration by doping the relatively high concentration of the magnetic nanoparticles and the magnetic separation treatment. However, the performance of the diluted magnetic PDLC has improved even more.

The role of nanomesh fibres loaded with fluorescent materials on the electro-optical performance of PDLC devices

ABSTRACT In this paper, polymer-dispersed liquid crystal (PDLC) films were prepared from a mixture of prepolymer, liquid crystal and phosphor-loaded reticular nanofibres films by a polymerisation-induced phase separation (PIPS) process. By comparing the microstructure and electro-optical curves of samples with different ratios of monomer types and liquid crystal contents, it was demonstrated that the cavity size of the polymer network had a significant effect on the electro-optical properties of the PDLC films. Reticular nanofibres films loaded with different phosphor contents were introduced into the liquid crystal mixture samples and the changes in the polymer network, electro-optical and fluorescence properties of these samples were analysed using SEM images, electro-optical curves and fluorescence curves. The electro-optical and fluorescence properties as well as the fluorescence properties of reticulated nanofibres films doped with PDLCs loaded with fluorescent materials were found to be improved and the anti-ageing properties are also greatly increased. Therefore, the development of meshed nanofiber membrane-doped PDLC films with faster response time, better optoelectronic properties, and better fluorescence properties will greatly improve the applications of PDLC-based devices in smart windows, sensors, energy efficiency and Potential technical applications in flexible devices.

Spectral, visual, thermal, energy and circadian assessment of PDLC glazing in warm and humid climate

This paper detailed the experimental investigation of the Polymer Dispersed Liquid Crystal (PDLC) glazing over visual comfort, thermal performance, circadian entrainment, and energy consumption of an office room in a warm and humid climate. In addition, an elaborated study was carried out on the PDLC glazing's spectral, visual, thermal, and circadian characteristics. The thermal evaluation was carried out with a test cell of size 0.3*0.4 m. It was observed that the internal glass surface temperature was comparatively higher than the external glass temperature. It indicates the high solar gain of PDLC glazing. Under opaque and transparent states, the Correlated Colour Temperature (CCT) and a Colour Rendering Index (CRI, Ra) were almost identical. For an ultraviolet (UV) lamp source, PDLC film reflects most of the radiation under an opaque condition. Circadian Stimulus is 10–20 % more under clear states than the opaque state. A real-time experiment was performed in a test room at Manipal, where the PDLC sheet was pasted on the west-oriented window. Colour rendering and circadian effectiveness were satisfactory. CS was always above 0.3, the minimum requirement for circadian entrainment. Due to the milky appearance of the opaque state, a low solar elevation angle induces more reflection on the workspace, and hence more daylight comes into the interior. Under these conditions, uniformity is less. It is observed that PDLC is most suitable in the south and the north orientation windows for the southern tropical climate. The energy consumption was observed as 0.5 kWh /day.

Design and Analysis of a PDLC-Based Reconfigurable Hilbert Fractal Antenna for Large and Fine THz Frequency Tuning.

The proposed reconfigurable radiating antenna design is based on the integration of a reconfigurable fractal antenna and electro-optic substrate material. This antenna can be adjusted to achieve either re-configurability or tunability in the desired frequency range for wireless systems. The electromagnetic characteristics of the fractal antenna are manipulated at both the level of fractal geometry, electrical length and dielectric substrate. The designed antenna features multiband responses, in which the geometry and length change create a large frequency shift and the dielectric change using polymer dispersed liquid crystal (PDLC) creates fine and/or continuous tuning. The far field and scattering properties of the antenna are analyzed using the Computer Simulation Technology (CST) Microwave Studio Suite. The proposed approach has successfully demonstrated reconfigurable switching for up to four frequency bands between 0.2 and 0.6 THz. The dielectric constant change in the PDLC substrate shows fine and continuous frequency tuning with an 8% maximum frequency shift when operating around 0.54 THz and a high directivity of 7.35 dBi at 0.54 THz and 8.43 dBi at 0.504 THz. The antenna can also realize a peak gain of 4.29 dBi at 0.504 THz in the extraordinary polarization state of PDLC. The designed antenna can be readily integrated in the current communication devices, such as satellites, smart phones, laptops, and other portable electronic devices, due to its compact geometry and IC compatible design. In satellite applications, the proposed antenna can play a significant role in terms of security. The antenna could be extremely useful for satellites that want to keep their information secret; by constantly switching their operating frequency, spy satellites can evade detection and data collection from enemy ears.

Effect of elastic constants on electrically induced transition in twisted radial cholesteric droplets

In this work, we investigated the behavior of cholesteric droplets with homeotropic boundary conditions experimentally and by computer simulations. Small droplets forming twisted radial structures were studied. We obtained two different paths of structural transformations under electric field in such droplets. The choice between these paths has probabilistic nature. The ratio between the two transition types was found to be sensitive to the elastic constants of LC forming the droplet. We suggest the principal approach for in situ estimation of ratios between elastic constants in cholesteric LCs deposited in polymer-dispersed LC material and discuss its strong and weak sides.

The regulation of electro-optical properties of the polymer dispersed liquid crystal films doped ZnO whiskers under applied pre-orientation voltage

ABSTRACT This study demonstrates a simple and effective method for the fabrication of PDLC films with low driving voltage by applying a pre-orientation voltage during the photoinitiated polymerisation process. The results indicated that the electro-optical performances of PDLC films have been regulated successfully with the pre-orientation voltage method, in which the voltage could pre-orient the LC molecules and change the interface between the LC molecules and the polymer matrix. By manipulating the proportion of liquid crystal and polymer monomers and introducing ZnO whiskers as the dopant, the driving voltage was dramatically reduced and the sample could be fully driven by an electric field with a voltage of 32 V below the safe voltage. The results indicated that the polymer meshes were broadened by the addition of ZnO whiskers due to their excellent UV-absorbing properties that result in a slower polymerisation rate, while the contrast ratio could maintain a good value of 31. It has been found that both application of pre-orientation voltage and doped ZnO whiskers can optimise the electro-optical performance of the PDLC and reduce the driving voltage. Therefore, this experiment is expected to provide certain research value to fabricate the low-driven PDLC films for practical applications.

22‐1: Invited Paper: Electrically Switchable Privacy Technology Suitable for Laptop PC

This paper introduces an e‐Privacy display, electrically switchable between wide viewing angle (share) mode and narrow viewing angle (privacy) mode, operated by louver film and polymer dispersed liquid crystal (PDLC), named as viewing angle control film (VCF). Especially, focusing on e‐Privacy suitable for laptop PC, reducing power consumption and maximizing privacy feature can be achieved through PDLC haze customization by input voltage control and three layers (louver, PDLC, and panel) lamination to eliminate air gap. As a result, the VCF technology is optimized for e‐Privacy laptop PC which can be used in various environmental conditions.

P‐107: Switchable Privacy Monitor Display Using Viewing‐Angle Control Film

We fabricated a privacy film that can be electrically switched between a wide viewing angle (Share) mode and a narrow viewing angle (Privacy) mode using Polymer Dispersed Liquid Crystal (PDLC) and louver. Both transmittance and privacy performance can be maximized through the optimization of the Back Light Unit (BLU) optical characteristics and the lamination of each PDLC and louver film. We named this technology as Viewing angle Control Film (VCF). Each share and privacy mode can perform its maximum feature independently through this VCF technology.

Effects of formulation composition and CeO2 nanoparticles doping on the morphologies of polymer spacer columns and electro-optical properties of PDLC

Polymer dispersed liquid crystal (PDLC) with polymer spacer columns were prepared by ultraviolet (UV) stepwise polymerization induced phase separation (PIPS) process. The influences of the formulation composition, such as the internal proportion of monomers and the liquid crystal (LC) content, on the morphologies of polymer spacer columns and electro-optical properties of PDLC films, were researched. Due to the difference in cross-linking density and polymerization rate, PDLC films exhibit different morphologies of polymer spacer columns and electro-optic properties. Based on the above, the CeO2 nanoparticles were doped into PDLC films. The influences of the doping content of CeO2 nanoparticles on the polymer network and electro-optical properties of PDLC were studied, and the EDS spectra were used to characterize the extent of dispersion of CeO2 nanoparticles in the polymer matrix. The results of the study showed that the contrast ratio (CR) of PDLC film increases from 10.9 to 27.3 at 0.8% CeO2 nanoparticles doping, while the saturation voltage (Vsat) decreases from 71.3 V to 47.9 V. Therefore, the work offers a new approach to the preparation of PDLC films with excellent comprehensive performance.

Dual-field responsive polymer-dispersed liquid crystal films with polymer spacer columns and fluorescent properties

ABSTRACT Polymer dispersed liquid crystal (PDLC) films with polymer spacer columns were prepared by photomask stepwise polymerisation method. The effects of crosslinker content and liquid crystal (LC) content on the morphologies of polymer spacer columns and electro-optical properties of PDLC films were investigated. Due to the difference of crosslink density and system viscosity, the polymer spacer columns exhibit different morphologies and the electro-optical properties of PDLC films are also altered. Based on the above, the PVA coatings loaded with rare earth phosphor were introduced into PDLC films. The influences of rare earth phosphor concentration on the fluorescent and electro-optical properties of PDLC films were studied. Also, it was found that the UVaging resistance of PDLC films was enhanced because of the loading of rare earth phosphor absorbing UV light. Therefore, the successful preparation of PDLC films with polymer spacer columns and reversible fluorescence properties opens new possibilities for expanding the flexible display applications of PDLC.

Multi-field driven thermochromic films and preparation of multi-color patterns

ABSTRACT In this paper, multi-field driven thermochromic films (TCF) were successfully prepared firstly. The upper layer is polymer-dispersed liquid crystal (PDLC) film prepared by polymerisation-induced phase separation (PIPS), and the lower layer is thermochromic liquid crystal microcapsule film prepared by centrifugal spin-coating method. The effects of diluent content, liquid crystal content and spin coating speed on the electro-optical properties and morphology of the thermochromic films were investigated by three sets of experiments secondly. Finally, multi-field driven multi-colour patterns were successfully prepared by film formation on conductive glass and flexible conductive films. Thermal ageing tests proved that the thermochromic films possess better service life, which provides a possibility for the wide application of liquid crystal films.

Characterization and driving of polymer-dispersed liquid-crystal sheet for smarthome applications

A commercial polymer-dispersed liquid-crystal (PDLC) sheet sample was characterized for its basic optical and electrical properties under DC and AC excitation. The results give driving voltage for 90% of transmittance for the PDLC of 10 V for DC excitation, and 9.5 Vrms for 50 Hz AC excitation. The measured contrast ratio of the PDLC is 2.55 and 2.47 under DC and AC excitation, respectively, which is suitable for smart window in a smarthome application. While it has rather similar optical properties under DC and AC excitation, it has significantly different electrical properties for DC and AC excitation. The PDLC exhibits a capacitive property which has capacitance per unit area of 18.5 μF/m2, indicating that DC excitation will give benefit compared to AC excitation which will scale with area of the PDLC. Based on these results, a DC driving circuit employing boost converter was designed to make a microcontroller-compatible driving circuit. The system was then integrated into a miniature smarthome demonstrator model exhibiting contrast ratio of 2.05 under switching from 0 to 100% of duty cycle of pulse width modulation (PWM) control signal from the microcontroller executed through voice command in an internet of things (IoT) platform.

Quantifying of Vision through Polymer Dispersed Liquid Crystal Double-Glazed Window

The visual linking of a building’s occupants with the outside views is a basic property of windows. However, vision through windows is not yet a metricized factor. The previous research employs a human survey methods to assess the vision through conventional windows. The recently fabricated smart films add a changeable visual transparency feature to the windows. The varied operating transparency challenges the evaluation of vision. Therefore, surveying human preferences is no longer a feasible approach for smart windows. This paper proposes an image-processing-based approach to quantify the vision quality through smart windows. The proposed method was experimentally applied to a polymer dispersed liquid crystal (PDLC) double-glazed window. The system instantaneously determines the available contrast band of the scenes seen through the window. The system adjusts the excitation of the PDLC film to maintain a desired vision level within the determined vision band. A preferred vision ratio (PVR) is proposed to meet the requirements of occupant comfort. The impact of the PVR on vision quality, solar heat gain, and daylight performance was investigated experimentally. The results show that the system can determine the available vision comfort band during daytime considering different occupant requirements.

A facile fabrication of ultrahigh haze optical diffuser using nematic liquid crystal encapsulated by cellulose acetate biopolymer

Polymer dispersed liquid crystals (PDLCs) are materials for next-generation optoelectronic devices. In this work, free-standing optical diffusers based on PDLC are formed by liquid crystals (LCs) and cellulose acetate polymer network. The micron and submicron-sized liquid crystal droplets embedded in the matrix of cellulose acetate biopolymer act as scattering particles, leading to an efficient diffused transmission of the incident laser beam. By optimizing the concentration of LC and polymer, the optical characterizations of the fabricated PDLC films reveal an ultrahigh haze value (99 %) and broad angular distribution of transmitted light with the full width of half maxima up to 114°. The film fabricated with 55 wt % LC results in a light transmitted profile similar to the ideal light diffusing surface with Lambertian distribution. Furthermore, the technique employed to develop the films offers a straightforward and inexpensive fabrication process of light-diffusing films for a high-performance foldable optoelectronic device.