Liquid crystal polymers and ionomers for membrane applications

Abstract

Transport phenomena through polymeric composite membranes depend on several complex factors, involving not only their chemical composition but also their microstructure. Due to the combination of mesomorphic and macromolecular behaviour, thermotropic liquid crystal polymers (LCP) may enhance the transport properties in membranes by improving parameters related to the order, orientation, connectivity and distribution of the different domains arising from phase separation. This paper provides an overview of some existing and potential applications of liquid crystals (LC) in polymeric membranes as effective structure-directing templates or as components providing a dynamic response to external stimuli. The first part is a description of the composition and phase behaviour of LCP containing ionic and ionogenic groups, the so-called liquid crystal ionomers (LCI). The second part is focused on the description of LCP and LCI used in transport applications, such as proton-conducting films in fuel cells, ionic polymer transducers and hydrogels. The paper highlights the relationships between the composition and the LC properties of the materials and their potential as membranes for ionic and solvent transport, with particular attention paid to covalently linked materials prepared by copolymerisation, due to their higher versatility and stability.