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
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.
RESUMO
Morphological properties of surfaces play a key role in natural and man-made objects. The development of robust methods to fabricate micro/nano surface structures has been a long pursuit. Herein, an approach based on molecular self-assembling of liquid crystal polymers (LCPs) is presented to directly translate 2D molecular director profiles obtained by a photoalignment procedure into 3D topographies, without involving further multi-step lithographic processes. The principle of surface deformation from a flat morphology into complex topographies is based on the coupling between electrostatic interactions and the anisotropic flow in LCPs. When activated by an electric field, the LCP melts and is driven by electrohydrodynamic instabilities to connect the electrode plates of a capacitor, inducing topographies governed by the director profile of the LCP. Upon switching off the electric field, the formed structures vitrify as the temperature decreases below the glass transition. When heated, the process is reversible as the formed topographies disappear. By pre-programming the molecular director a variety of structures could be made with increasing complexity. The height, pitch, and the aspect ratio of the textures are further regulated by the conditions of the applied electric field. The proposed approach will open new opportunities for optical and electrical applications.
RESUMO
Polymer stabilized liquid crystal (PSLC) devices can be used as smart privacy windows that switch between transparent and opaque states. The polyimide alignment layer of a PSLC device is usually obtained by the treatment of polyamide acid (PAA) with temperatures over 200 °C. This hinders the fabrication of PSLC devices on flexible substrates, which melt at these high temperatures. In this work, the fabrication of a PSLC alignment layer using a lower temperature that is compatible with most flexible substrates, is demonstrated. It was found that the treatment of PAA at 150 °C could generate the same alignment for liquid crystals. Based on this, a PSLC device was successfully fabricated on a flexible polyethylene terephthalate (PET) substrate, demonstrating excellent electro-optic performances.
