Paper TF-MoE2
Orientational Control of Polar Molecules in Macroporous Systems and their Dielectric Properties

Monday, December 12, 2016, 6:00 pm, Room Makai

Organic ferroelectric materials have been attracting much interest as a new family of memory materials that operate under lower electric field. However to date, most of the reported organic ferroelectric materials are solid and they show dielectric hysteresis only in the presence of high electric field. Accordingly, there is a compelling need for new dielectric materials working at a lower electric field and ambient temperature. In this light, we have focused on supramolecular gels as organic soft dielectric materials. Gelator molecules were dissolved in solvents with large dipole moment such as nitrobenzene by heating and succeeding cooling of the mixture afforded organogels. In the electron microscopes (SEM, TEM) observations, extended nanofibrous aggregates were abundantly observed for the dried gel. In this gel, polar solvent molecules are confined and their molecular motion are expected to be limited since they interact with fibrous gel networks which contain extended hydrogen bonding networks. Thus, the dielectric properties of these supramolecular gels were investigated systematically. The gels showed increase in the magnitude of polarization with distinctive polarization-electric field hysteresis loop, though such hysteresis was not observed for the polar solvents without gelators. In addition, in contrast to the gels formed with polar solvents, gels formed with non-polar solvents didn’t show the hysteresis loop. From these findings, confinement of polar solvent molecules in the suramolecular gel fiber networks play an important role to control mobility of solvent molecules, thus leading to orientational polarization.

To better understand this confinement effect and to generalize the approach, we employed porous polymer films with controlled pore-size as matrixes. Dielectric properties of polar molecules introduced into these porous polymer systems were investigated, which show similar hysteresis. We also found that polar molecules trapped in the different pore-sized polymer films showed distinct phase transition and dielectric behaviors. From these observations, it is suggested that polar molecules in the pores exert interactions with the pore surfaces and accordingly, their orientation is maintained even after cutting off the electric field. Furthermore, this system works at low electric fields. We believe that our approach would provide a new concept to design soft ferroelectrics.