Synthesis and characterization of alkylthio-attached azobenzene-based liquid crystal polymers: Roles of the alkylthio bond and polymer chain in phase behavior and liquid crystal formation

Abstract

We demonstrate for the first time that the alkylthio (or thioether) bonds in the side-chains of liquid crystal (LC) polymers considerably influence the phase-transition behavior, mesomorphic properties, and nanostructures of the LC and crystal (Cr) phases. We synthesized three alkylthio-attached azobenzene-based polymethacrylates; namely, P–OS, P–SO, and P–SS, in which alkylthio bonds were introduced at an outer (terminal) 4-position, at an inner (polymer-chain side) 4-position, and at both the outer and inner 4-positions, respectively, of the azobenzene side chains. In marked contrast to the corresponding pre-monomers and methacrylate monomers, the P–OS, P–SO, and P–SS polymers form well-defined enantiotropic LC phases. Notably, while an outer terminal alkylthio bond renders azobenzene-based polymers (P–OS and P–SS) predominantly smectogenic, a sole inner alkylthio bond renders the P–SO polymer nematogenic. In addition, the outer and inner alkylthio bonds contribute to increasing and decreasing the phase-transition temperatures, respectively. The presence of the alkylthio bonds potentially provides LC polymers with crystallizability. Furthermore, X-ray diffractometry results reveal that the P–OS, P–SO, and P–SS polymers form deeply interdigitated bilayer structures in their SmA, highly ordered, and Cr phases. The specific packing mechanism is discussed in terms of intermolecular attractive interactions involving the sulfur atoms.