Fast thermally-responsive azatriphenylene ionic discotic liquid crystalline polymers with shape-memory properties

Abstract

Ionic discotic liquid crystalline polymers (IDLCPs) are a versatile class of multifunctional polymers that exhibit electron and ion transport, luminescence, and thermal and mechanical responsiveness. In this study, we successfully synthesized IDLC homopolymers H-PTPNn-X and copolymers C-PTPNn-X with varying content ratios of cyclooctene and ionic monomer, r : m (10 : 1, 40 : 1) via ring-opening metathesis polymerization (ROMP). The polymers consisted of a quaternized azatriphenylene core surrounded by four flexible alkoxy chains (n-C₆H₁₃O, n-C₁₂H₂₅O) and inorganic anions of varying volume sizes (X = Br⁻, BF₄, PF₆⁻, and TFSI⁻). The number-average molecular weights (M_n) of C-PTPNn-X were 14–20 kDa, while those of H-PTPNn-X were 1.26 to 3.15 kDa, indicating that the latter were predominantly dimers or trimer oligomers. We found that all the COE-TPNn-X discotic monomers, except COE-TPNn-Br, displayed a hexagonal columnar mesophase (Col_h) readily recognized by characteristic optical textures. Furthermore, only the H-PTPNn-TFSI homopolymer and C-PTPNn-TFSI copolymer, bearing sizeable bis(trifluoromethylsulfonimide) anions, exhibited a hexagonal mesophase at room temperature. The electrostatic interactions between the anions and quaternized azatriphenylene core significantly affected the mesophase temperature range and thermostability of these compounds. Additionally, both monomers and polymers emitted yellow light photoluminescence in the range of 530 to 560 nm in solution and as films, and the absolute emission quantum yield in solution reached 33.8%. The C-TPN6-TFSI (10 : 1) copolymer exhibited a fracture strain of 937% at room temperature, excellent shape-memory properties, thermally driven contraction, and success in origami and weight-lifting experiments. Moreover, this copolymer is amenable to solution-spinning and exhibits an ionic conductivity of 2.8 × 10⁻⁷ S m⁻¹ at 85 °C.