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.     

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.     

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.     

Liquid-crystalline epoxies dispersed liquid crystals

Dispersing liquid crystal droplets in a rigid polymer matrix results in an electrically controllable light scattering medium. A polymer with high glass transition temperature phase-separated with a low molecular weight liquid-crystalline material is a good binder for polymer dispersed liquid crystals (PDLC). Main-chain liquid-crystalline epoxies were synthesized for the phase separation with low molecular weight nematic liquid crystals E7 (mixture of 4-alkyl-4′-cyanobiphenyls) and studied for their mesomorphic properties and response to an applied electrical field.     

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.     

Phase separation in binary polymer/liquid crystal mixtures: network breaking and domain growth by coalescence-induced coalescence

A small-angle light scattering (SALS) technique together with optical microscopy observation are used to investigate phase separation kinetics in films of low molecular weight thermotropic liquid crystal (4-cyano-4'-n-octyl-biphenyl, 8CB) with flexible polymer (polystyrene, PS). The growth of domains is studied as a function of time, film thickness, and film composition. The light scattering results are correlated with the images obtained by optical microscopy observation. In this paper, we study the breaking of a bicontinuous network of polymer in liquid crystal into droplets and their further growth via the coalescence-induced coalescence mechanism. The appearance of droplets in the system leads to a strong scattering at small wave vectors, while the bicontinuous network gives a peak at a nonzero wave vector. Superposition of these scattering intensities leads to the appearance of a second peak in the full scattering intensity signal, when the bicontinuous network starts to break up into disjointed elongated domains. Finally, both peaks merge into a single peak, which moves quickly toward zero wave vectors, indicating a complete transformation of elongated domains into spherical droplets of variable size. We found that the separation process does not depend on the size of the system. Irrespective of the sample thickness, the network breaks into fragments always at the same time after temperature quench. On the basis of morphological analysis, we found that the average size of the droplets which formed from the network grows with time, t, as t(alpha), alpha = 0.9 +/- 0.1, in the isotropic phase and in the nematic phase.     

Size control of phase‐separated liquid crystal droplets in a polymer matrix based on the phase diagram

When a mixture of liquid crystal (LC) and photo reactive monomer is irradiated by UV light, polymerization occurs and LC droplets form through phase separation, producing polymer dispersed LCs (PDLCs). Although size control of LC droplets and reduced amounts of LC in PDLC films are important in applications, precise size control of LC droplets at a low LC fraction has not yet been accomplished. In this study, the phase diagrams of the LC/initial monomer and the LC/polymer during polymerization were used to control LC droplet size at various LC fractions. Both the relative position of the sample in the initial phase diagram and the shift of the phase separation line during polymerization were shown to be important in determining the size of LC droplets. Our results are expected to provide a new strategy for precise size control of LC droplets especially at a low LC fraction range, which would be a great help for PDLC applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

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.     

Stationary noise of the light scattered by a polymer-dispersed liquid crystal

Room temperature measurements of the stationary noise of the monochromatic light scattered by a polymer-dispersed liquid crystal (PDLC) have been performed in order to get information about the parameters affecting the fluctuation modes within the LC droplets, such as the effective viscosity constant of the anisotropic medium. The power spectra of the scattered light appear to be described in terms of lorentzian functions, whose intensity and cut-off frequency are strongly dependent on both the scattering angle and the electric field across the sample. Hints about the optical-axis configuration within the LC droplets are provided by a thorough analysis of the measured noise. The main discrepancies between the experimental results and the predictions for an infinite nematic LC are pointed out and discussed in terms of the finite size of the LC droplets.     

Striped pattern formation in polymer dispersed liquid crystal films

Composite films of polymer and liquid crystal (LC) have been prepared by a simple solution casting technique. The films obtained exhibit a polymer dispersed liquid crystal structure where LC droplets are dispersed in the polymer matrix. Casting the mixture of polymer solution and LC on a tilted substrate results in a specific texture in which arrays of LC droplets align parallel to each other, i.e. a striped pattern can be formed. The size of the droplets and the spacing between the lines are dependent on the substrate tilt angle and the preparation temperature. By using a dip-coating technique, a similar striped texture appears at a fast dipping velocity. From in situ observation of the stripe development, it is seen that the translation of the phase-separating region, accompanying the flow, plays a more important role in the formation of the striped patterns than the flow of the solution itself.     

Optical properties of stretched polymer dispersed liquid crystal films

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The 'anomalous' nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.     

Striped pattern formation in polymer dispersed liquid crystal films

Composite films of polymer and liquid crystal (LC) have been prepared by a simple solution casting technique. The films obtained exhibit a polymer dispersed liquid crystal structure where LC droplets are dispersed in the polymer matrix. Casting the mixture of polymer solution and LC on a tilted substrate results in a specific texture in which arrays of LC droplets align parallel to each other, i.e. a striped pattern can be formed. The size of the droplets and the spacing between the lines are dependent on the substrate tilt angle and the preparation temperature. By using a dip-coating technique, a similar striped texture appears at a fast dipping velocity. From in situ observation of the stripe development, it is seen that the translation of the phase-separating region, accompanying the flow, plays a more important role in the formation of the striped patterns than the flow of the solution itself.     

Model for coherent transmittance calculation for polymer dispersed liquid crystal films

This paper analyses the methods of calculating the coherent (direct) transmittance of polymer dispersed liquid crystal (PDLC) films which are polymer-based films with embedded liquid crystal droplets. By comparison with experimental data it is shown that at a high concentration of LC droplets, Beer's law, which is frequently used, leads to large errors in calculations of the transmittance of PDLC films. To calculate coherent transmittance, it is expedient to use the interference approximation which takes into account the interference of waves scattered by individual LC droplets and leads to much more accurate results.     

Complex study of reorientational dynamics of the liquid crystal in PDLC films

Morphological, electro-optical and switching properties of polyester resin/nematic liquid crystal composite films have been studied for varying composition (10-40 wt% of LC), temperature (20-50 C), film thickness (10-75 mum) and UV curing time of the matrix (0.25-12 min). The PDLC films were formed by LC separation in a UV polymerization process of the thin layer of oligoester resin (liquid crystal mixture) between ITO coated glass plates. The electro-optical and response behaviour based on the electric field controlled light scattering of the composite films was observed. The results were interpreted in terms of effective anchoring strength at the interface of the polymer and liquid crystal depending either on the area fraction of the interface in the composite film (dependent on the size and shape of the liquid crystal droplets) or the stiffness and resistivity of the polyester resin changing in the course of the crosslinking polymerization.     

Model to describe light scattering by polymer film containing droplets with inhomogeneous anchoring of liquid crystal molecules at the polymer–droplet interface: asymmetry effect in the angular distribution of light

A model to describe light scattering by polymer film containing of monolayer of liquid crystal droplets with inhomogeneous anchoring of liquid crystal molecules at the polymer-droplet interface is developed. It is based on the interference approximation of the wave scattering theory. The director field distribution in the droplet volume is determined by solving the free energy density minimization problem using the relaxation method. The spatial distribution of droplets in the layer is described by the hard disks model. The amplitude scattering matrices of individual droplets are found in the anomalous diffraction approximation. The algorithm for numerical analysis of the characteristics of light scattered in a polymer film containing droplets at homogeneous and inhomogeneous surface anchoring is described in terms of the partial filling factors of the monolayer film. Electrically controllable symmetry breaking effect of angular distribution of light scattered by films containing droplets with inhomogeneous anchoring at the polymer-droplet interface is described and experimentally confirmed.     

Model for coherent transmittance calculation for polymer dispersed liquid crystal films

This paper analyses the methods of calculating the coherent (direct) transmittance of polymer dispersed liquid crystal (PDLC) films which are polymer-based films with embedded liquid crystal droplets. By comparison with experimental data it is shown that at a high concentration of LC droplets, Beer's law, which is frequently used, leads to large errors in calculations of the transmittance of PDLC films. To calculate coherent transmittance, it is expedient to use the interference approximation which takes into account the interference of waves scattered by individual LC droplets and leads to much more accurate results.     

Thermoplastic polymer‐dispersed liquid crystals prepared from solvent‐induced phase separation with predictions using solubility parameters

The optical effects of liquid crystals can be realized when the mesogens are dispersed in a supporting and stabilizing polymer phase. Thermoplastics were chosen for their structural reversibility and ease of fabrication of polymer‐dispersed liquid crystals (PDLCs) from solution via solvent‐induced phase separation (SIPS). The component match and tuning in PDLCs was achieved in a common solvent through predictions of solubility parameters. The PDLCs were first prepared using SIPS and were then exposed to thermal treatments on a hot stage polarizing microscope or in a differential scanning calorimeter. At elevated temperatures the polymer and mesogen may become miscible, while upon cooling thermally induced phase separation (TIPS) should occur, preferably above the isotropic–nematic transition temperature. The nematic phase existed within disperse phase droplets that were stabilized and supported by the matrix polymer. The temperature range of the nematic phase was extended in the PDLC configuration. The droplet size was important for liquid crystalline optical behaviour. Polymer–mesogen interactions, identified through solubility parameters, were important in ensuring sufficient but not coarse phase separation.     

Polymer dispersed liquid crystal droplets: methods of calculation of optical characteristics

The extinction efficiency factor and the angular distribution of radiation for polymer dispersed liquid crystal (PDLC) droplets have been calculated in the new discrete-dipole approximation (DDA) and compared with the data obtained in the commonly used Rayleigh-Gans approximation (RGA) and in the anomalous-diffraction approximation (ADA). The errors of calculations by the above mentioned methods for droplets with a homogeneous and for droplets with a radial director configuration are considered. The results of calculations of the optical properties of droplets with a radial and with a bipolar director configuration in the DDA are presented. Based on the results obtained the potentialities of the above methods for describing the scattering by liquid crystal droplets are illustrated.     

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.