CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No. PCT/GB2019/053323, filed Nov. 26, 2019, which claims priority to GB 1819634.5, filed Nov. 30, 2018, which are entirely incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to polymer dispersed liquid crystals (PDLCs) and more specifically to PDLC films and droplets, and methods of manufacturing same.

BACKGROUND

Polymer dispersed liquid crystals are an important class of optical material that have found wide-ranging industrial application in devices such as smart windows, light shutters, holographic gratings and light valves. A PDLC film consists of droplets of liquid crystal (LC) that are suspended in a polymer matrix. Many of the electro-optical properties of a PDLC film are determined by the alignment of the LC molecules within the droplets. Typically, the independent LC droplets lack any preferential macroscopic orientation between LC domains within the droplet. However, the orientation of the LC molecules can be switched under the application of an external electric field. By matching the refractive index of the polymer matrix to one of the refractive indices of the LC, a scattering or transparent state can be obtained by reorienting the LC domains under an applied electric field.

In a known method for producing PDLC films, a homogenous mixture of LC and polymer is initially formed and then phase-separated either via thermal or photo-polymerization processes. Photopolymerization-induced phase separation has gained interest as it can be triggered upon irradiation of ultraviolet light, allowing for some control over the LC droplet size, orientation and shape. One of the many benefits of PDLCs is their suitability for large-scale manufacturing of large-area films. After the formation of a homogenous mixture of LC and polymer, a glass cell can be filled with the mixture or it can be deposited as a thin film (e.g. by bar-coating or roll-to-roll processes) and photopolymerized. Once cured, the film is fully encapsulated and ready to use.

A drawback with known processes for producing PDLC films is the inherent homogeneity that results from the mixing and coating process. This results in films consisting of a single, homogenous LC and polymer formulation. Because of the inherent homogeneity, typical PDLC smart window panels (e.g. panels of 30-120 cm in width/height), are switched from a transparent to an opaque state across the entire window pane and the sub-division or patterning of the transmission state and/or colour of the panel is not readily possible.

Accordingly, it is an object of the invention to provide alternative manufacturing processes for producing PDLC films which allow for greater control over the distribution of the LC within the polymer layer.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of manufacturing a polymer dispersed liquid crystal, PDLC, film, the method comprising:forming a region of liquid polymer on a substrate;depositing one or more liquid crystal, LC, formulations into the liquid polymer; andthereafter curing the liquid polymer to thereby induce phase separation between the polymer and the liquid crystal formulations.

According to the first aspect, the liquid polymer region, which may be layer or a droplet, is first deposited onto a substrate and then subsequently, one or more liquid crystal, LC, formulations are deposited into the liquid polymer region (e.g. so that the LC formulations are encapsulated by, or dissolved into, the liquid polymer region). This means that it is possible to produce a non-homogenous distribution of LC within the liquid polymer region. This enables a patterned PDLC film to be produced, as well as a PDLC film having PDLC regions made using different liquid crystal formulations. This allows for designs, motifs and additional information such as company logos or advertisements to be imprinted on the film which will be apparent when used in applications such as smart windows.

Curing and phase separation may be performed at least partially by photo-polymerization, or substantially entirely by photo-polymerization.

Curing and phase separation may be performed at least partially by thermal curing, or substantially entirely by thermal curing.

Photo-polymerization may be performed using an ultraviolet, UV, light source having an intensity in the range 10 mW/m2to 200 mw/m2at the surface of the liquid polymer.

Photo-polymerization may be performed for a duration in the range 1 minute to 10 minutes, preferably 4 minutes.

A time delay between depositing a liquid crystal formulation and performing photo-polymerization may be in the range 1 s to 60 s, preferably 5 s to 15 s.

Depositing one or more liquid crystal formulations may comprise inkjet printing liquid crystal droplets from a print head into the liquid polymer.

The inkjet printing process may be a drop-on-demand inkjet process, continuous inkjet printing process or other droplet-based inkjet process such as an electrohydrodynamic jet, e-jet, printing process.

The inkjet printing process may be performed using a print head having a nozzle diameter in the range 50 μm to 180 μm, preferably 120 μm.

The temperature of the print head may be held in the range 20° C. to 80° C., preferably 65° C., during depositing the one or more liquid crystal formulations. Optionally, the temperature of the print head is raised to a level such that the liquid crystal is printed when in the isotropic liquid phase.

An actuation voltage applied to the print head may be in the range 40V to 100V, preferably 80V.

The temperature of the substrate may be held in the range 20° C. to 80° C., preferably 65° C.

The region of liquid polymer may be a layer and may have a thickness in the range 40 μm to 100 μm, preferably 70 μm.

The liquid polymer may comprise or consist of a thiolene-based optical adhesive.

The refractive index of the liquid polymer may be in the range 1.50 to 1.54, preferably 1.52. The liquid crystal formulations may comprise nematic liquid crystals having an ordinary refractive index in the range 1.50 to 1.54, preferably 1.52, and an extraordinary refractive index in the range 1.78 to 1.82, preferably 1.80. In general, the refractive index of the liquid polymer may be selected to match a refractive index of the liquid crystal, e.g. either the ordinary or extraordinary refractive index of the liquid crystal, for a wavelength range over which the PDLC film is intended to be operated, e.g. visible light in the range of approximately 400 nm to 700 nm or a sub-band thereof.

A deposition velocity of the one or more liquid crystal formulations may be in the range 0.5 ms−1to 2.0 ms−1immediately before impact with the region of liquid polymer.

The method may further comprise monitoring the formation and deposition of the liquid crystal formulations using a video camera, for instance in a shadowgraphy configuration.

The method may further comprise performing thermal curing after photo-polymerization.

Thermal curing may be performed for a duration in the range 1 hour to 10 hours at a temperature in the range 60° C. to 80° C., preferably for 4 hours at a temperature of 70° C.

The method may further comprise applying an external electric field to the substrate during the step of photo-polymerization.

The method may further comprise positioning a top substrate over the polymer after the step of curing.

The method may further comprise, after the step of curing, coating the region of polymer with a conductive polymer and, optionally, affixing electrodes to the conductive polymer. The conductive polymer may be inkjet printed and may be a layer.

The region of liquid polymer may be deposited by a thin-film deposition technique or inkjet printing process.

The LC formulations may comprise one or more of the following: nematic liquid crystals; and/or chiral nematic liquid crystals; and/or smectic liquid crystals; and/or blue phase liquid crystals; and/or reactive mesogens.

The LC formulations may further comprise one or more of the following: colour dyes; and/or additional particles to increase light scattering properties; and/or ionic dopants; and/or surfactants to improve printability of the LC formulations.

The nematic liquid crystal may have either a positive or negative dielectric anisotropy.

Either in-plane or transverse electric fields can be used to switch a PDLC film transmission state.

According to a second aspect, there is provided a method of manufacturing a polymer dispersed liquid crystal, PDLC, film, the method comprising:inkjet printing a mixture comprising a liquid polymer and liquid crystal formulations onto a substrate; andthereafter curing the liquid polymer to thereby induce phase separation between the polymer and the liquid crystal formulations.

The mixture may be inkjet-printed to form a substantially even layer on the substrate.

The mixture may be printed at discrete locations on the substrate to form localized sessile droplets.

The step of curing may form PDLC droplets with micron-sized domains.

The step of curing may form PDLC droplets with nano-sized domains.

The step may form encapsulated LC droplets.

According to a third aspect, there is provided a polymer dispersed liquid crystal, PDLC, film, manufactured using the method of the first aspect (including any of the optional features disclosed above).

