Thermal control of transmittance/diffraction states of holographic structures composed of polymer and liquid crystal phases

A microperiodic structure composed of polymer and liquid crystal (LC) phases, called holographic polymer dispersed liquid crystal (HPDLC), was fabricated by a photo-induced phase-separation technique. It is anticipated that the HPDLC can be used as a thermo-driven light controller, in particular, for an autonomous solar-ray control window, which utilizes the nematic-to-isotropic (N-I) phase transition phenomenon in the LC phase of the periodic structure. In the present study, the transmittance/diffraction properties of samples with different thermal behaviors were examined, and it was found that 15% of the solar radiation was switched between the transmittance and diffraction states as a function of temperature at around 30 degrees C. Furthermore, a more practical solution for using the HPDLC for a solar-ray control window was demonstrated. From the viewpoint of structural analysis, scanning electron microscopy reveals that LC droplets, a few tens of nanometers in size are, distributed and coalesce to form periodic LC layers. By carrying out polarizing microscopy and performing transmittance/diffraction measurements as a function of temperature, it is observed that the LC molecules in the droplets are strongly oriented along the grating vector and their orientations decline with a rise in temperature. These observed structures are of importance for the generation of a large change in the transmittance/diffraction properties due to N-I transition. (C) 2010 Elsevier B.V. All rights reserved.