Computer simulation of elongated bipolar nematic droplets 1. External field aligned parallel to the droplet axis of symmetry

The magnetically-induced transient nematic director reorientation dynamics, confined in elongated bipolar droplets, is studied in this paper. Numerical results are obtained by solving the Leslie-Ericksen continuum theory in ellipses. The aspect ratio is varied to determine the effect of droplet shape on director reorientation dynamics. The magnetic field is restricted to the droplet axis of symmetry direction, which has not yet been studied but is fundamentally important in polymer dispersed liquid crystal (PDLC) film operation. The numerical results replicate frequently-reported experimental observations on the performance of PDLC films. These observations include the familiar exponential increases followed by saturation in light transmittance as the external applied field increases and the exponential increase (decrease) followed by saturation as time increases in the on- (off-) state. In addition, the experimental observation that switching field strength increases while decay time decreases as the droplet becomes more elongated, are also exhibited by the numerical results.     

Computer simulation of elongated bipolar nematic droplets 1. External field aligned parallel to the droplet axis of symmetry

The magnetically-induced transient nematic director reorientation dynamics, confined in elongated bipolar droplets, is studied in this paper. Numerical results are obtained by solving the Leslie-Ericksen continuum theory in ellipses. The aspect ratio is varied to determine the effect of droplet shape on director reorientation dynamics. The magnetic field is restricted to the droplet axis of symmetry direction, which has not yet been studied but is fundamentally important in polymer dispersed liquid crystal (PDLC) film operation. The numerical results replicate frequently-reported experimental observations on the performance of PDLC films. These observations include the familiar exponential increases followed by saturation in light transmittance as the external applied field increases and the exponential increase (decrease) followed by saturation as time increases in the on- (off-) state. In addition, the experimental observation that switching field strength increases while decay time decreases as the droplet becomes more elongated, are also exhibited by the numerical results.     

Computer simulation of elongated bipolar nematic droplets II. External field aligned normal to the droplet axis of symmetry

Numerical results from the modelling and computer simulation of the magnetic-induced director reorientation dynamics in elongated bipolar nematic droplets are presented in this paper. The magnetic field is applied normally to the droplet axis-of-symmetry direction, which is one possible scenario found in applications of polymer dispersed liquid crystal (PDLC) films. This case has not yet been studied numerically, and its understanding is far from complete. The model is composed of the Leslie-Ericksen and Frank continuum theories and is solved in two dimensions since bipolar nematic droplets exhibit mirror symmetry in certain planes. The numerical results replicate frequently reported experimental observations on the performance of PDLC films. These observations include the ubiquitous exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off)-state. Furthermore, in contrast to current understanding for both the on- and off-states, the model predicts that the directors in the centre (surface) region of the droplet exhibit a dead time (no dead time) before reorientation. The numerical results presented in this paper provide a better understanding of the director reorientation dynamics in elongated bipolar nematic droplets; this can be used to optimize the design and performance of devices using PDLC films.     

Computer simulation of elongated bipolar nematic droplets II. External field aligned normal to the droplet axis of symmetry

Numerical results from the modelling and computer simulation of the magnetic-induced director reorientation dynamics in elongated bipolar nematic droplets are presented in this paper. The magnetic field is applied normally to the droplet axis-of-symmetry direction, which is one possible scenario found in applications of polymer dispersed liquid crystal (PDLC) films. This case has not yet been studied numerically, and its understanding is far from complete. The model is composed of the Leslie-Ericksen and Frank continuum theories and is solved in two dimensions since bipolar nematic droplets exhibit mirror symmetry in certain planes. The numerical results replicate frequently reported experimental observations on the performance of PDLC films. These observations include the ubiquitous exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off)-state. Furthermore, in contrast to current understanding for both the on- and off-states, the model predicts that the directors in the centre (surface) region of the droplet exhibit a dead time (no dead time) before reorientation. The numerical results presented in this paper provide a better understanding of the director reorientation dynamics in elongated bipolar nematic droplets; this can be used to optimize the design and performance of devices using PDLC films.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

Light transmission, linear dichroism and birefringence of nematic/polymer dispersions

A theoretical study of light transmission, linear dichroism and birefringence of partially ordered dispersions of bipolar nematic droplets in a polymer matrix is presented. The treatment rests on the single scattering approach to an ensemble of uncorrelated and noninteracting anisotropic particles. Theoretical evaluations of the extinction cross sections and phase functions are performed in the anomalous diffraction approximation. Four basic model systems are analysed: PDLC and NCAP films in an external electric field, and PDLC and NCAP films under uniaxial mechanical deformation. The calculated dependences of the generalized parameters characterizing the light transmission, dichroism and birefringence on the angle of incidence of the probing light, droplet size and shape, refractive indices of the LC and polymer, and parameters of the external field are presented graphically and discussed in detail.     

Control of liquid crystal droplet configuration in polymer dispersed liquid crystal with macro-iniferter polystyrene

A polystyrene macro-iniferter was applied to control the alignment of liquid crystal molecules at the droplet wall of polymer dispersed liquid crystal (PDLC) films. The aspects of the alignment were monitored by observing the droplet in the PDLC film. With increasing the macro-iniferter polystyrene in the composition, the configuration of LC droplets changes from bipolar to radial. This is because the high concentration of the macro-iniferter polystyrene results in a small surface interaction between the LC and the polymer matrix, which favours the formation of radial configuration. The radial configuration was stable under our conditions. However, increasing the LC and the initiator concentrations resulted in the change from radial to bipolar.     

Novel techniques of PDLC film preparation furnishing manifold properties in a single device

Three novel techniques of polymer dispersed liquid crystal (PDLC) film preparation have been proposed to obtain/induce systematically varying manifold properties in a single device. These three techniques were used to prepare ‘wedge-shaped’, ‘multi-channelled’ and ‘grating type’ PDLC films. Arrangement and configuration of liquid crystal (LC) microstructures inside these PDLC films, which were conveniently divided into different zones, have been investigated using a polarising optical microscope (POM) and scanning electron microscope (SEM). POM images indicate a predominant bipolar structure in all zones of different types of PDLC films but with varying size and density. Further, the electro-optical (EO) properties of PDLC films for different zones have different morphological characteristics as indicated (observed) in POM and SEM images and were dependent on LC droplet shape, size and distribution. Also different zones show different absorbance/transmittance characteristics in the visible range. Thus, our study proposes a single device with manifold properties. Also, the desired properties can be obtained by selecting the suitable zone from the PDLC composite film.     

The fabrication of novel optical diffusers based on UV-cured polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) with different sizes of the LC droplets are prepared based on the ultraviolet (UV) light curable acrylate monomers/LCs composites to fabricate the optical diffuser films. To acquire light diffusers with high optical performance, the effects of the monomer structure and the UV light intensity on the micro-structure of the PDLC films are studied. Results show that the PDLC films could exhibit a strong light scattering at the premise of maintaining high transmittance in the visible region. As the LC droplets are spherically dispersed in the polymer networks, when the size of LC droplets is about 3.0 μm, the haze can reach 88.5% and the transmittance is nearly 90.0%, which can be used as a bottom diffuser film. While when the size of LC droplets is about 10.0 μm, the haze and transmittance are 39.2% and 90.2%, respectively; hence, it can be a good choice for a top diffuser film. With the advantages of simple preparation, roll-to-roll industrial production and tunable optical properties, it is supported that the films based on UV-cured PDLC films can be applied as outstanding optical diffuser films in the liquid crystal display industry.     

Director configurations in nematic droplets with tilted surface anchoring

The polymer dispersed nematic liquid crystal (LC) with the tilted surface anchoring has been studied. The droplet orientational structures with two point surface defects – boojums and the surface ring defect – are formed within the films. The director tilt angle α = 40° ± 4° at the droplet interface and LC surface anchoring strength W_s ~ 10^–6 (J m^?2) have been estimated. The bipolar axes within the studied droplets of oblate ellipsoidal form can be randomly oriented are oriented randomly relatively to the ellipsoid axes as opposed to the droplets with homeotropic and tangential anchoring.     

Studies on the electro-optical and the light-scattering properties of PDLC films with the size gradient of the LC droplets

Polymer dispersed liquid crystal (PDLC) films with the size gradient of the LC droplets were prepared based on the epoxy/acrylate hybrid polymer matrix. The ultraviolet (UV) intensity gradient was induced by the UV-absorbing dye over the thickness of the samples. Taking advantage of the difference between the epoxy monomers and acrylate monomers in polymerisation rates and the UV intensity gradient, the gradient distribution of the LC droplet size was formed in PDLC films. The effect of the size gradient of the LC droplets on the electro-optical and the light-scattering properties of PDLC films was investigated. The results showed that due to the size gradient distribution of the LC droplets, PDLC films could exhibit the strong light scattering in the UV-visible-near infrared (VIS-NIR) region. Consequently, it provides a potential approach for modulating NIR light transmittance.     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

大分子引发剂分子量对接枝聚合物基体的PDLC电光性能的影响

以可逆加成-断裂链转移(RAFT)、引发转移终止(iniferter)活性自由基聚合相结合的方法,用一步法制备了不同分子量的大分子引发剂RAFT-PS-co-PCMSI(MI),并通过紫外光聚合诱导相分离法制备了以接枝聚合物为基体的聚合物分散液晶(PDLC)膜.研究了不同分子量的MI对PDLC的微观形貌,关闭状态透光率,阈值电压,饱和电压以及记忆效应等方面的影响.研究表明,降低PDLC中MI的分子量,会使得液晶微滴粒径增大,阈值电压(Vth)、饱和电压(Vsat)减小,记忆效应、关闭状态透光率升高.     

Study on the electro-optical properties of polyimide-based polymer-dispersed liquid crystal films

The reflectivity control device, initially developed for attitude control, is utilised to control the solar sail orbit by switching the states between absorption and specular reflection. Actually, the major parts of the device are the polymer-dispersed liquid crystal (PDLC) films. Here, PDLC films based on polyimide (PI) as polymer matrix and a low molecular weight LC can be prepared by the thermally induced phase separation (TIPS) method. The influences of cooling rate and the content of LC on the size and uniformity of LC droplets dispersed in a polymer matrix by a TIPS process were investigated. It was found that a fast cooling rate gave smaller droplet sizes and hence a more uniform distribution as compared to the ones produced under a slow cooling rate. If the LC content was increased, the droplet size would be increased. Furthermore, the effect of LC droplet size on the electro-optical properties of the PI-based PDLC films was discussed, such as transmittance, threshold voltage, driving voltage and contrast ratio (CR).     

Propriétés optiques et thermophysiques de composites polyméres-cristaux liquides (PDLC)

Polymer dispersed liquid crystal (PDLC) films can be switched electrically from a light scattering off-state to a highly transparent on-state. Thin films were prepared via a polymerization-induced phase separation process using electron beam radiation. The liquid crystal (LC)/polymer materials were obtained from blends of a eutectic nematic mixture E7 and a polyester acrylate based polymer precursor. The optical and electrooptical properties of the obtained PDLC films strongly depend on the LC concentration. The LC solubility limit in the polymer matrix and the fractional amount of LC contained in the droplets were determined by calorimetric measurements.     

Effects of off-state alignment in polymer dispersed liquid crystals

Partial off-state alignment of the liquid crystal in polymer dispersed liquid crystal (PDLC) droplets was obtained by the application of electric or magnetic fields during their formation. Photopolymerization was used to induce phase separation of the liquid droplets from monomer/liquid crystal solutions. Substantial director directionality was retained in these PDLC films after removal of the fields used during their formation. This alignment affected both the off-state and the on-state electro-optic properties of the films. Transverse electrical fields (5 to 60 V across a 15 μm thickness) applied during PDLC formation from a solution of E7 (BDH Ltd) in a monomer resulted in PDLC films with progressively lower off-state scattering and lower threshold voltage. Strong longitudinal magnetic fields (9 to 14 T) applied during PDLC formation with these materials resulted in strong polarization effects in the light scattering off-state. In the infrared region, where there is less light scattering than in the visible region, the longitudinally aligned films shows tunable birefringent electro-optic effects while retaining the fast time response characteristics of PDLC films with small droplet sizes.