In Pursuit of High-Performance Materials

A variation of liquid crystal technology, the holographic polymer-dispersed liquid crystal (H-PDLC), shows promise for many commercial applications, such as low-power wearable displays, video projectors, and fiber optic switches. The material is lightweight and creates bright, saturated colors that can be switched rapidly.

H-PDLCs are formed by directing laser beams on a mixture of liquid crystal and light-sensitive monomers, which are the building blocks of polymers. As the mixture gels and hardens, the two constituents separate into layers, forming a matrix of liquid crystal droplets and solid polymer planes that reflect different colors of light holographically. The reflection properties of H-PDLCs can be modulated because the orientation of the liquid crystal droplets changes with the intensity of an applied electric field.

In an early stage commercially, H-PDLCs still have a few obstacles to overcome. For example, before a display material is commercially acceptable, it must withstand many hours of continuous operation at high temperatures without significant degradation. Acrylate, an easily fabricated polymer that is commonly used in reflective H-PDLCs, has not met this benchmark. Its electro-optical properties degrade with both temperature and time.

SAIC's Lalgudi Natarajan and his colleagues have achieved a significant breakthrough in the creation of a commercially viable H-PDLC. Using a practical, yet elegant technique, the scientists directed a single ultraviolet (UV) laser beam through a prism onto the pre-polymer thiol-ene, which is a UV-curable optical device adhesive. The method yields an H-PDLC with superior electro-optical properties as well as thermal and long-term stability.

The material's superiority is the result of its polymer growth mechanism and internal structure. In acrylates, monomers combine to form a long polymer chain, which hardens the material in less than five seconds. The rapidity of the process prevents a complete conversion of monomers to polymers and traps stress in the system, which later equilibrates, changing the properties of the material. In thiol-ene, a slower, more complex growth mechanism is at work, creating large spherical droplets of liquid crystal that are uniformly dispersed through a highly converted thiol-ene polymer matrix. This ordered structure is responsible for the material's improved electro-optical characteristics, such as reduced switching voltage and scattering of light.

"We are getting there," states Natarajan. "Thiol-ene shows better separation between the liquid crystal and polymer, which improves all the properties of the material. This will have a significant impact on the display and telecommunications industries."

The new material offers additional advantages over its predecessors. Unlike acrylates, thiol-ene requires no complex additives, which undermine stability at high temperatures. Thiol-ene is not inhibited by oxygen as are acrylates, which makes production cheaper and more practical. Lastly, the new material consumes less power and is much brighter than previous H-PDLCs.

Device prototypes that incorporate SAIC's new H-PDLC technology have already been produced. The team's subsequent research will focus on creating an H-PDLC with visible laser light, which is readily available. SAIC has obtained eight H-PDLC patents and has five more pending. The paper that Natarajan and colleagues Christina Shepherd, Donna Brandelik, Richard Sutherland, Suresh Chandra, and Vincent Tondiglia wrote, "Switchable Holographic Polymer-Dispersed Liquid Crystal Reflection Gratings Based on Thiol-Ene Photopolymerization," was published in Chemistry of Materials. The research was sponsored by SAIC and the Air Force Materials Directorate.