Rheo-optics studies in the development of hybrid liquid crystalline based MWIR polarizers

We present here a rheo-optical study of the flow behavior of two cholesteric liquid crystals, one with a large pitch and one with a small pitch. The large pitch compound has been investigated as a possible fixed wavelength polarizer in the mid-wavelength infra-red region (3-5 micron). The investigation of these compounds is driven by their low melt viscosity and ability to vitrify order, and thus functionality, into films with a wide range of thickness. In our attempts to obtain consistent thin films with reproducible contrast ratios, we explored the defect textures of both compounds under a polarizing optical microscope. These materials were sheared at various strain rates and at various temperatures in an attempt to determine the best processing window for defect free films. The pitch lengths of the two materials investigated were 160 and 1330 nm. The flow behavior of the large pitch material resembles a pure nematic with defect refinement taking place under flow. The short pitch material exhibited the typical Grandjean oily streaks upon shearing followed by coarsening. Observations made during this rheological study were used to identify a processing technique for the large pitch materials. Upon application of a conventional buffed alignment layer, films with consistent quality were routinely made. The measured polarization contrast of >70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength regime.