Abstract
Liquid crystal thin films obtained by spincoating and photopolymerization are widely used nowadays for geometric phase optical components. The liquid crystal-air interface during photopolymerization plays a crucial role as it should not disturb the photoalignment induced by the other interface. When photopolymerizing the liquid crystal layer in vacuum, processing parameters need to be optimized carefully to avoid alignment issues at the air interface. In this work we demonstrate that domains are formed due to a tilt angle which is induced at the air interface under certain conditions. A measurement method is developed to quantify the tilt angle at the interface. The method consists of optical analysis of the domains using crossed-polarizer transmission microscopy with an oblique sample holder. By careful analysis of the obtained tilt angles for different parameters, it is assumed that a non-negligible homeotropic anchoring strength at the air interface is responsible for the domain formation. The results are explained with a theoretical model that assumes a finite anchoring strength at both the photoaligned interface and the air interface.
Highlights
Liquid crystal polymers with well controlled orientation are attractive due to their extraordinary optical, electrical and mechanical properties [1,2,3]
We investigate the orientation of the liquid crystal (LC) at the LC-air interface using a polarization microscope and we interpret these results by combining them with optical simulations using Comsol
We demonstrate that the value of the LC surface tilt angle is linked to the surface anchoring energies at the alignment-LC and the LC-air interfaces
Summary
Liquid crystal polymers with well controlled orientation are attractive due to their extraordinary optical, electrical and mechanical properties [1,2,3]. Polymerization is carried out in a nitrogen environment, which generally requires the process to be carried out in a glove box This is because water and oxygen in the air can inhibit the polymerization reaction and affect the quality of the polymer film [2]. The LC alignment produced by rubbing is not completely homogeneous, especially at the microscopic level Photoaligment is another alignment technique in which linearly polarized UV or blue light is used to illuminate a photoresponsive material to set the alignment direction [5]. It is a high-quality non-contact method which offers the possibility of micro-patterning the liquid crystal alignment. We discuss the results and point out the importance of these findings for functional elements based on in-plane photoalignment and polymerization technology
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