RESUMO
Cholesteric liquid crystals (CLCs) are chiral photonic materials with selective reflection in terms of wavelength and polarization. Helix engineering is often required in order to produce desired properties for CLC materials to be employed for beam steering, light diffraction, scattering, and adaptive or broadband reflection. Here, we demonstrate a novel photopolymerization-enforced stratification (PES)-based strategy to realize helix engineering in a chiral CLC system with initially one handedness of molecular rotation throughout the layer. PES plays a crucial role in driving the chiral dopant bundle consisting of two chiral dopants of opposite handedness to spontaneously phase separate and create a CLC bilayer structure that reflects left- and right-handed circularly polarized light (CPL). The initially hidden chiral information therefore becomes explicit, and hyper-reflectivity, i.e., reflecting both left- and right-handed CPL, successfully emerges from the designed CLC mixture. The PES mechanism can be applied to structure a wide range of liquid crystal (LC) and polymer materials. Moreover, the engineering strategy enables facile programming of the center wavelength of hyper-reflection, patterning, and incorporating stimuli-responsiveness in the optical device. Hence, the engineered hyper-reflective CLCs offer great promise for future applications, such as digital displays, lasing, optical storage, and smart windows.
RESUMO
Photo-embossing has been developed as a convenient and economical method for creating complex surface relief structures in polymer films. The pursuit for large aspect ratios of the photo-embossed structures has never stopped. Here, we demonstrate a simple strategy to obtain improved aspect ratios by adding a quick solvent developing step into the photo-embossing process. A good solvent for the monomer is used to remove unreacted monomers from the unexposed region, resulting in deepened valleys of the surface reliefs. In a polymer film as thin as 2.5 µm, the height of the surface reliefs can be increased by a factor of three to around 1.0 µm. This strategy is also shown to be compatible with other methods used to improve the aspect ratios of the photo-embossed structures. Lastly, we employ these surface relief structures in the fabrication of liquid crystal (LC) devices and investigate their performances for visible light regulation.
RESUMO
Alignment layers are vital to the function of numerous devices based on liquid crystal (LC) materials. The pursue of versatile, effective and even flexible alignment layers, preferably prepared by simple methods, is still actively ongoing. Herein, we propose a facile one-step method by mixing silanes into the starting LC mixtures, which in contact with a glass substrate secede and self-assemble in-situ to form a stable and highly effective homeotropic alignment layer at the interface. Tetradecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride (TDTA) is selected as the example to demonstrate the method, although a number of other silanes can produce similar results. With only 0.05 vol% of TDTA added to a mixture of liquid crystals and reactive mesogens, a uniform monolayer is chemically attached to the substrate, which automatically aligns the LCs homeotropically. Furthermore, by blending the TDTA with acrylate functionalized silanes like 3-(trimethoxysilyl)propyl methacrylate (A174), additional reactive functional groups can be easily introduced into the alignment layer, therefore offering opportunities to adjust the interface properties. An electro-responsive smart window based on the polymer stabilized liquid crystals (PSLCs) is successfully prepared using a one-step method, demonstrating excellent electro-optic performances and notably enhanced adhesion between the substrate and the in-situ formed polymer network. These findings are valuable especially for the development of flexible LC devices.