Effects of side-chain flexibility on properties of azatriphenylene discotic liquid-crystalline ionomers

Abstract

To investigate the influence of side-chain flexibility on the properties of discotic liquid-crystalline ionomers (DLCIs), a series of side-chain azatriphenylene ionic copolymers, denoted as C-PTPN_m-X (where X  = Br⁻ or TFSI⁻ and m = 3, 5, 7, 15), were synthesised via ring-opening metathesis polymerisation. These copolymers comprise a polycyclooctene backbone connected to discotic mesogenic units via flexible spacers of different lengths. Monomers and ionomers containing bis(trifluoromethanesulfonyl)imide ions (TFSI⁻) displayed columnar liquid-crystalline phases, whereas those with bromide anions exhibited non-mesogenic properties. Both the monomers and copolymers emitted yellow fluorescence with wavelengths ranging from 515 to 561 nm in both the solution and film states. Notably, replacing bromide anions with TFSI⁻ significantly enhanced the absolute quantum yield in both monomers and copolymers. Additionally, the mechanical properties of the ionomers were dependent on the spacer length, with the stress and Young’s modulus of C-PTPN_m-TFSI being approximately 30 times lower than those of H-PCOE, a cycloolefin homopolymer, thereby facilitating solution spinning. All the DLCIs demonstrated rapid thermal responsiveness and excellent shape memory characteristics. Flexible spacers in ionic monomers and copolymers enhance their thermal stability and mechanical properties while also influencing their clearing points and mesogenic temperature ranges. The type of anion significantly improved the thermal stability and affected the luminescence properties, whereas the spacer length had less impact on these attributes. The ionic conductivity of the DLCIs was strongly temperature-dependent and only slightly affected by the spacer chain length.