Photopolymerisable liquid crystals for additive manufacturing

Abstract

Liquid crystals are often regard as a fourth state phase of matter alongside solids, liquids and gases. It has been estimated that one out of every ten new synthetic compounds is a liquid crystal, mainly due to the fact that liquid crystals have been used in Liquid Crystalline Displays (LCDs) for over 50 years. Liquid crystals, often referred to as mesogens, exhibit an unique range of ordered mesophases with anisotropic physical properties, such as optical, magnetic and dielectric anisotropy, for example. These highly anisotropic properties and the ability of liquid crystals to self-organize on surfaces, has led to them being, are used in a wide range of non-LCD applications. These applications include photochromics, pressure sensitive paints, optical retarders and optical compensation films, etc. Highly crosslinked, anisotropic polymer networks, derived from photopolymerisable liquid crystals, so-called reactive mesogens (RMs), have enabled solution-fabricated organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic devices (OPVs) for plastic electronics. This review will be focus on the relatively recent application of liquid crystals, i.e., the 3D/4D printing of reactive mesogens and subsequent polymerization to form anisotropic polymer networks in advanced additive manufacturing applications. It will highlight the challenges of the synthesis and use of new liquid crystalline materials in 3D/4D printing for additive manufacturing applications. It will show how the study of the successful development of design strategies for the synthesis of novel liquid crystals for flat panel displays is inspiring the development of novel photopolymerisable liquid crystals for additive manufacturing. It is intended that this review will serve to inspire chemists, with little or no knowledge of liquid crystals, or liquid crystal chemists, to design and synthesize new liquid crystals for 3D/4D printing. Photopolymerisable liquid crystals (reactive mesogens) with advanced functional aromatic cores and two photoreactive endgroups are used to fabricate stimulus-responsive liquid crystalline elastomers (LCEs) using 3D/4D printing techniques, such as fused-deposition modeling (FDM), direct ink writing (DIW), digital light-processing (DLP) and two-photon polymerization (2PP). Reactive mesogens represent a highly promising class of advanced functional for new types of actuators, sensors, soft robotics, drug delivery systems, energy-absorbing and reinforced devices, etc., primarily due to their liquid crystalline properties, such as anisotropic swelling, high elastic modulus and sensitive actuation. • Stimulus-responsive photopolymerisable liquid crystals (PLCs) for additive manufacturing. • Structure-property discussion for PLCs with advanced aromatic cores and photoreactive endgroups. • 3D/4D printing PLCs for application in smart devices (actuators, sensors, soft robotics, etc). • Highly promising PLCs due to anisotropic swelling, high elastic modulus and sensitive actuation.