By: Pedro Almeida
At: Instituto de Investigação Interdisciplinar, Anfiteatro
In the last decade, Liquid Crystals (LC) have emerged as a new important class of chemical, electrical, mechanical and optical materials used for the production of all sorts of displays and sensors. One very useful application of liquid crystals is the so called Polymer dispersed liquid crystal (PDLC). PDLC films were conceived by Ferguson in 1984, which made the film by emulsifying a polymer together with water and a LC mixture. PDLCâ€™s have as main advantages, a fast electro-optic response, no leakage, no need for the use of polarizer and simple fabrication. In a PDLC film, a large number of very small LC droplets, in the size range of micro or submicron, with dielectric anisotropy and birefringence, are embedded in a polymer matrix. These composite systems emerged from the search for new LC based optical devices that meet several demands, such as high transmission in the on-state, wide viewing angle, and less restraining manufacturing requisites, allowing for inexpensive deformable large-area devices to be built. The electrically controlled light scattering behavior enables the PDLC film to act as a light shutter and, as such, finds potential use not only as a display technology, but also in other applications. The most common applications are the so-called vision products, i.e. windows and internal partitions, but other applications such as projection screens and lampshades have also been suggested. In some cases, dyes have been incorporated and simple reflective displays have been demonstrated.
Cellulose derivatives used as the polymeric matrix for electro-optical application were first introduced in 1982 by Craighead, when he characterized an optical cell formed by a transparent solid matrix of mixed esters of cellulose with micrometer-sized pores filled with a nematic LC. However, this cell needed unreasonably high fields to switch from one state to the other and the ON-state gave poor transmission values. We have produced PDLC type devices that present the same macroscopic effect of the traditional PDLCs, although a different constitution and working mechanism. We use a cellulose derivative (HydroxypropylCellulose â€“ HPC) as the polymer part of the PDLC, and the liquid crystals E7 and 5CB.We have produced devices where the polymer is used in the shape of a flat thin solid film and devices where the polymer is used as fibers. The characterization and optimization of these devices will be presented. This new type of devices present very high transmission values in the ON state, very low transmission values in the OFF state and low working voltages. Some very recent and interesting results about the interaction of the liquid crystal with the fibers will also be presented.