Phase behavior and ionic conductivity of dendron–coil–dendron block copolymer/ionic liquid electrolytes

Abstract

In this paper, we report the thermal, morphological, lower critical solution temperature (LCST), and ion-conducting properties of electrolytes based on a dendron–coil–dendron block copolymer (BCP) and ionic liquids (ILs). The BCP consists of ionophilic poly(ethylene oxide) middle and ionophobic dendron ends with crystalline octadecyl peripheries. The two ILs that were employed, 1,2-dimethyl-3-propylimidazolium tetrafluoroborate ([PMMIM][BF₄]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), had identical molecular weights but different molecular structures in the imidazolyl cation. Electrolyte (1) with [PMMIM][BF₄] exhibited crystalline lamellar and disordered micellar phases as a function of temperature. On the other hand, electrolyte (2) with [BMIM][BF₄] showed an additional ordered hexagonal columnar liquid crystalline (LC) phase in between the crystalline lamellar and the disordered micellar phases. Infrared absorption studies suggest that the induction of the ordered LC phase is attributed to the strong hydrogen-bonding between PEO and [BMIM]. Furthermore, it was found that electrolyte (2) suddenly started to become turbid near the order-to-disorder transition temperature (TODT). This indicates that the LCST of 2 is identical to the TODT, above which liquid–liquid separation occurred gradually. The ionic conductivity curve of 1 increased continuously as the temperature increased. In contrast, 2 displayed a sudden drop in conductivity upon changing from the crystalline lamellar to the columnar phase, which was due to the variation of the ionophilic domain dimensionality from two to one. The calculated activation energy for ionic conduction in each phase was revealed to be affected by the translational motion and polarity of the ion-carrying medium.