Research at CMMPE — Materials — Polymer dispersed liquid crystals (PDLCs)
Polymer dispersed liquid crystals (PDLCs) are formed by making immiscible mixtures (emulsions) of liquid crystal and polymer. The resulting combination has the electro-optical performance of liquid crystals, together with structural and mechanical advantages associated with polymers. Furthermore, by careful control of mixing ratios and fabrication methods, we can introduce new optical properties, unique to these hybrid materials.
In order for an emulsion to be formed, the liquid crystal must be mixed with the polymer whilst both are in liquid form. For this purpose, optically curing adhesives (eg: poly-acrylates and poly-thiolenes), which can be later cured under UV illumination, are often chosen, although thermally curing adhesives (including PVA) are also sometimes preferred. As the polymer is cured, phase separation occurs between the liquid crystal and the immiscible polymer. The rate and temperature at which polymer curing occurs will determine the size and shape of liquid crystal and polymer domains within the structure.
The most common variety of PDLC is the optically scattering form. The concentration of polymer within the liquid crystal is within the approximate range of 30% to 50%. The polymer is cured within the LC/polymer emulsion, such that droplets of liquid crystal separate out within the polymer structure. These droplets are typically of the micron size scale. Liquid crystal molecules within each droplet have localised order, however each droplet can be randomly aligned relative to others. The combination of droplet size and isotropic orientation of droplets leads to a highly optically scattering state, and giving the cell a milky appearance. When the same material is then subjected to an electric field, electro-optic reorientation of the liquid crystal droplets occurs. This then reduces the degree of optical scattering through the cell, giving rise to a transparent state.
A scattering PDLC in the 'off' (scattering) and 'on' (non-scattering) state.
Scattering PDLCs using micron-sized droplets such as this have many applications. Switchable privacy screens/windows (switchable between a transparent/clear state and an opaque/scattering state) are already commercially available, which utilise PDLC technology. Chemical dyes can also be added to the PDLC mixtures, so that they may preferentially scatter red, green or blue light respectively. Many researchers are attempting to then use these materials as RGB pixels in flexible displays. Furthermore, the high droplet surface area (between the liquid crystal and the polymer) gives rise to strong anchoring energies, and therefore rapid switching times, which is of further benefit to the display industry.
The trade-off with PDLC performance is that higher voltages are required to switch PDLCs (approx. 2-10 V/micron), compared to conventional LCs such as nematics (approx. 1-5 V/micron). This leads to higher power consumption, and more expensive driving electronics. Furthermore, due to the random alignment of the 'off' state, the full liquid crystal birefringence cannot be utilised, giving rise to devices with reduced stroke. However, this can be partially compensated by using thicker devices, thanks to their switching time being largely independent of cell thickness.
If one increases the polymer concentration within a PDLC to values in the region of approximately 60% to 80%, and then cures the polymer very quickly (using high intensity UV light sources), then it is possible to form a PDLC with very small droplet size. Once droplet size reduces to approximately the nano size scale, transmitted light through the mixture is no longer scattered at optical wavelengths. The resulting nano-PDLC mixture will still switch between randomly aligned and vertically aligned states when an electric field is applied, but no change in apparent scattering occurs. Instead, the optical phase of the transmitted light is modulated only, determined by the average orientation (and average refractive index) of the LC within the PDLC.
A nano-PDLC in the (non-scattering) 'off' and 'on' states, modulating the phase of transmitted light.
The resulting nano-PDLC phase-modulation devices have wide-ranging applications. The random nature of LC alignment means that nano-PDLC devices are polarisation insensitive (a major drawback of nematic and smectic competitors). Furthermore, by extension of the reasons outlined above for micron-sized scattering PDLCs, they switch at very fast speeds (10s of microseconds), making them potentially suitable for wavefront correction devices in adaptive optics (used in astronomy, line-of-sight communications and ophthalmics). Nano-PDLCs are often referred to as "Holographic PDLCS" (H-PDLCs). This comes from a particular application of using their phase modulation properties as a method of producing reconfigurable holograms.
For similar reasons as outlined for scattering PDLCs, nano-PDLCs suffer from significantly increased driving voltages and reduced stroke. However, it should be noted that for most phase modulation applications, a phase shift of 2x pi radians is usually sufficient.
At the opposite extreme of polymer concentrations, in the region of approximately 1% to 10%, the resulting PDLC mixture mostly consists of liquid crystal. Under the correct curing conditions, a diffuse network of polymer chains can penetrate throughout the volume. The resulting material is the consistency of a viscous liquid or gel. It's electro-optical behaviour is almost identical to that of the LC on it's own, but with improvements in switching times.
A PNLC cell, illustrating the network of polymer stabilising the liquid crystal structure.
Further improvements in switching times can also be achieved using sheared PNLCs. In this system, the cell is subjected to a shearing force, parallel to the glass substrates, which tends to orientate the polymer chains within the PNLC in the direction of the shearing movement. The resulting sheared PNLC devices have been quoted to have switching speeds of 10s of microseconds, comparable to those of nano-PDLCs but with far greater stroke and lower voltage requirements. Like conventional LC devices, PNLCs however are still polarisation sensitive devices and require alignment layers to be deposited on the internal surfaces of the cell. The presence of a polymer network can also sometimes have a detrimental effect to the quality of liquid crystal textures.