Engineered Interfaces for Liquid Crystal Technology

Hemang J Shah

Drexel University

Liquid Crystals (LCs) are an exciting state of matter that exhibit unique electro-optic properties due to their chemical structure. They are widely used for manufacturing displays (LCDs), developing optical switches and for mimicking of biological interactions. An important factor that influences the image uniformity and switching characteristics in these applications is the surface alignment of the LC molecules. This thesis explains the control of LC molecules through surface morphology, surface polarization and doping the LC with carbon nanotubes (CNTs). The influence of surface morphology was studied by patterning a polymer with grid features having dimensions of 500nmx500nmx100nm. LC alignment was observed through polarized light microscopy. Through image analysis of the microscopy images, we improved the resolution of alignment variations by three times. 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 by dipolar interaction. 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. The structure of CNTs allows its use as a pipe for LC confinement and the electrical properties of CNTs influence LC switching. In this thesis, we have imaged LC confinement within open-ended CNTs using a scanning electron microscope. The effect of CNT conductivity on LCs was observed through electro-optic switching. Due to joule heating imparted by the CNTs, phase transition from the nematic phase to the isotropic phase was observed after application of electric field. This observation encourages the development of active materials that change phase in response to electric field.

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