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Hemang Shah
Advisor: Adam K. Fontecchio, Ph.D.
Abstract:
Liquid Crystals (LCs) are an exciting state of matter that exhibit unique electro-optic properties due to their chemical structure. Applications, which employ liquid crystals, are TV screens, wrist-watches, telecommunication switches, and imaging of biological cells. The key factor in each of these applications is the surface alignment of the LC molecules.
The substrate interface is most influential in providing alignment to the LC molecules. Since the interactions between the LC molecules are long-range in nature, the alignment imparted at the surface can be transmitted to the bulk molecules. This thesis explains three different approaches to control the behavior of the LC molecules at the sub-micron scale and at the macromolecular scale.
First, we have considered purely the effect of a patterned surface morphology on the LC alignment. Grid features were patterned on a polymer substrate using the MTS NanoIndenter. The features had the dimensions of 500nm x 500nm x $100nm. Polarized light microscopy revealed strong influence of the surface morphology in controlling the LC alignment. Our results are in agreement with predictions obtained through finite difference modeling of LC alignment on patterned substrates.
Our second approach involves the use of ferroelectric polymers to control LC alignment. Ferroelectric polymers are materials, which have a ready distribution of charge orientation. The coupling between the charge and the LC molecules is governed through dipolar interactions. Through control of the polymer processing, morphology, and composition, we have demonstrated a voltage- dependent visible wavelength progression, a phenomenon that can increase current display resolutions by 300%. These devices are applicable for LC display (LCD) technology as well as for use in optical communication as active wavelength filters. We have also demonstrated a proof-of-concept optical data storage device by writing charges on the ferroelectric polymers with the LC providing the optical readout. Finally, plasma processing was used to modify the surface chemical functionality of the polymer to improve LC switching and alignment for display applications.
Our third approach tries to understand the interactions of LCs with carbon nanotubes (CNTs). The wetting of LCs in confined geometries plays an important role in the working of polymer dispersed LC devices, commonly used as electrically switchable windows. However, imaging the LC molecules within active devices is difficult. Using the environmental mode of the scanning electron microscope (SEM), we have imaged LC molecules within open-ended CNTs. This study is an important step towards understanding the behavior of biological molecules mimicked by LCs within confined geometries like veins in our body. Our LC - CNT suspensions have also revealed a two-way alignment phenomenon in these materials. Their favorable interactions have resulted in the observation of well-aligned LC textures in sandwich cells without alignment layers. Finally, the addition of conductive tubes to LCs has resulted in the observation of a repeatable, phase transition from the nematic phase to the isotropic phase using an electric field only.
Friday, August 10th, 2007 at 10:30 a.m.
Bossone 303
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