Liquid crystalline and ion-conducting properties of mesogenic dendron–coil-dendron copolymers: characterization of LC phases using normalized conductivity

Abstract

We synthesized three dendron–coil-dendron block copolymers consisting of ionophilic poly(ethylene oxide) (PEO) coils and mesogenic dendrons with four octadecyl peripheries via stepwise click reactions. The obtained polymers were doped with lithium triflate, whose concentration per ethylene oxide unit was 0.05. As characterized by optical polarized microscopy (POM) and X-ray scattering techniques, polymer 1 with the shortest PEO coil (M_n = 2000 g mol⁻¹) did not show the liquid crystalline (LC) phase, while its ionic sample (1-Li⁺) exhibited a hexagonal columnar LC phase. On the other hand, 2 and 3 (with PEOs of M_n = 4000 and 8000 g mol⁻¹, respectively) displayed identical LC morphologies to 2-Li⁺ and 3-Li⁺, respectively, although the phase transition temperatures increased upon salt doping. For 2 and 2-Li⁺, gyroid and lamellar LC phases were observed with increasing temperature, while 3 and 3-Li⁺ showed only a lamellar LC phase. The observed ion-transporting behavior was strongly dependent upon the connectivity of ion-conducting domain structures. The investigation of the morphology–conductivity correlation using a normalized conductivity (σ* = σ/f, where σ and f are the original conductivity and the PEO volume fraction) indicated that the 3-D gyroid LC phase showed the highest value, while the lowest conductivity was found in the 1-D columnar structure at identical temperatures. Additionally, the gyroid to lamellar phase transition temperature of 2-Li⁺ could be determined by the σ*, which was consistent with the X-ray data. Consequently, the results can be explained by the fact that the ionic pathway becomes complicated when going from higher to lower dimensional structures in polygrain samples.