Liquid Crystalline Polymers from Renewable Resources: Synthesis and Properties

Abstract

Materials from renewable sources are attracting immense attention due to the excellent properties and ecological advantages it can offer and due to the growing environmental concerns. Liquid crystalline polymers (LCP) from renewable sources have engrossed considerable attention during the past few decades. Nowadays, agro based renewable materials are the subject of a growing number of academic and industrial research projects, because of diminishing fossil resources combined with the increasing environmental concern of petroleum based polymers. Renewable resources such as cellulose, vegetable oils and other plant and animal originated products are ideal alternatives to provide base chemicals for various materials, as they are abundant throughout the world and contain several reactive chemical sites such as double bonds, allylic carbons, ester groups, and the alpha carbons of the ester groups, which can be used for polymerization. LCPs can be successfully synthesized from a good number of natural sources including cardanol, castor oil and can be derived from biopolymers such as cellulose, DNA, proteins etc. The study of liquid crystal phases of cellulosic esters and ethers become attractive owing to their properties such as toughness and processability. Cardanol obtained from cashew nut shell liquid possesses functional groups for creating polymers and has interesting structure for exhibiting liquid crystalline properties. Cardanol based LCP can form cross linked network polymers due to the unsaturation of side chains and possibly can freeze the liquid crystalline phase. On the other hand liquid crystal properties of cellulose in electro optical applications open new horizons for these traditional materials. A milestone in the development of nano cellulose science and technology is the discovery of cellulose nano crystals (CNC) from natural cellulose sources. CNC is made from cellulose which is a natural polymer of wide abundance and is an almost non-exhausting source. These nanofibrils under particular concentration can form ordered structures leading to lyotropic liquid crystals. It was noticed that rod like CNC can form a stable chiral nematic liquid crystalline phase. The chiral nematic (cholesteric) phase thus formed is characterized by long-range orientational order of the nanorods combined with a helical modulation of the direction in which they align. The intriguing ability of CNCs to self-organize into a cholesteric liquid crystal phase with a helical arrangement has attracted significant interest, as this arrangement gives dried CNC films a photonic band gap. Thus formed film has attractive optical properties, creating possibilities for use in applications such as security papers and mirrorless lasing. Controlling and understanding the mechanisms of liquid crystalline self-assembly of LCPs are not only of fundamental importance but are steps along the route to produce novel materials with desirable optical or mechanical properties. Nanostructured films with a photonic band gap phase of CNC suspensions arising from the spontaneous helix formation in the cholesteric liquid crystal have been the focus of several studies. The use of CNC suspension as a self-assembled template for the synthesis of inorganic materials offers a promising and versatile platform to fabricate multifunctional mesoporous materials with photonic crystal properties of very large surface areas. The combination of CNC and inorganic materials with higher refractive indices may lead towards photonic devices such as tunable mirrorless lasers, and CNC-templated materials with specific surface functionalities may pave way for the development of enantioselective sensors.