PDLC films for control of light transmission

Abstract

We have investigated the light switching properties of polymer dispersed liquid crystal (PDLC) films, which can be characterized by the spectral change in transmittance and reflectance due to switching by application of a suitable electrical voltage. The PDLC samples were prepared using the standard liquid crystal mixture E7 in a UV-curable matrix material. Using these samples, a transmission reduction of up to 30% was achieved in the solar spectral range. A strong dependence of the light switching behaviour on the mass ratio of liquid crystal and matrix material as well as on the curing rate of the matrix material was found, whereas the film thickness has only a small influence. Light scattering measurements allowed us to determine the average size of the scattering centres. In the samples investigated the liquid crystal droplets have radii below 2 mu m. Additional measurements of the electric properties of the PDLC films showed that the capacity and the resistivity of the PDLC films depend on both the applied AC voltage and its frequency. The power consumption of the above PDLC films is in the range of 5 W m-2, if powered with a line frequency of 50 Hz; at lower frequencies (e.g. 20 Hz) the power consumption can be reduced to about 2.5 W m-2 without lowering the change in transmittance significantly.

Thiol–enes: Chemistry of the past with promise for the future

“…To date, the liquid crystals are mixtures of several cyanobiphenyl molecules, and the crosslinked networks are formed by the copolymerization of a thiol–ene mixture (Norland 65 or a related thiol–ene system, or another similar thiol–ene mixture from Norland Optical Adhesives, Inc.) and a critical concentration of the nonreactive small‐molecule liquid crystal. A very large number of publications (several hundred) have appeared since 1991 dealing with every aspect of the phase‐separation process, morphology, and optical switching performance of many types of PDLCs based on multifunctional thiol–enes, virtually all from Norland Optical Products, and we only list a few of the references in this review to provide lead references to the interested reader 140–211. In traditional PDLCs, the photocurable resin, initially optically clear because of the mixing of the liquid crystal with the thiol–ene, which yields a homogeneous solution, becomes opaque as the polymerization proceeds upon illumination with a nontemporally and nonspatially coherent light source, eventually resulting in a crosslinked network with phase‐separated liquid‐crystal‐rich regions and a crosslinked network matrix that has a much smaller impurity concentration of liquid‐crystal molecules in the network.…”

Comparative study on the electrooptical properties of polymer‐dispersed liquid crystal films with different mixtures of monomers and liquid crystals

“…In the PDLCs, T off depended on the polymer refractive index ( n p ) mismatch with the ordinary refractive index ( n o ) of the LCs. As this mismatch increased between the refractive indices of the two, the transmittance of light increased, and a more opaque state appeared at zero voltage 23, 24…”

“…As this mismatch increased between the refractive indices of the two, the transmittance of light increased, and a more opaque state appeared at zero voltage. 23,24 We evaluated T off for different monomer mixtures, and it is plotted in Figure 4 against the variation in the refractive indices of the LCs with the monomers [n o(LC) À n p ]. To better understand, we put a linear trend line over the data; this showed that T off decreased with increasing difference [n o(LC) À n p ].…”

Enhanced Adhesion and Transmittance Uniformity in Laminated Polymer-Dispersed Liquid Crystal Films

“…When a PDLC film is made with an LC having an extraordinary refractive index (ne) different from the refractive index (np) of the polymer matrix, this refractive index mismatch condition and the randomly distributed optical axes of the droplets leave the PDLC film opaque in its field-off state [5][6][7]. However, when the ordinary refractive index (no) of the LC is equal to np, the initially opaque PDLC layer can be made transparent by applying a voltage to reorient the LCs in the polymer matrix and ensure that the refractive indexes match for normally incident light [5][6][7]. Unlike other LC-based devices, PDLCbased devices are easily implemented as LC alignment layers and polarizers are not required.…”