447b Flexoelectric Networks from Bent-Core Nematic Liquid Crystal Polymers

Rafael Verduzco¹, Martin Chambers², Phillip F. Britt³, Antal Jákli⁴, Samuel Sprunt², and James T. Gleeson². (1) Center for Nanophase Materials Science, Oak Ridge National Laboratory, 1 Bethel Valley Road, PO Box 2008, MS 6494, Oak Ridge, TN 37831, (2) Department of Physics and Liquid Crystal Institute, Kent State University, Kent, OH 44242, (3) Center for Nanophase Materials Sciences and Chemical Science Division, Oak Ridge National Laboratory, Building 4500S, S-144, MS-6129, P.O. Box 2008, Oak Ridge, TN 37831-6197, (4) Liquid Crystal Institute, Kent State University, Kent, OH 44242

The flexoelectric effect is an electro-mechanical phenomenon that arises in nematic liquid crystals (LCs) whereby a macroscopic polarization develops in response to deformations of the liquid crystal alignment [1]. This effect has potential technological application in devices such as strain gauges, micropower generators, and nano-actuators that utilize an electro-mechanical response to operate. Recently, Harden and coworkers discovered that nematic bent core LCs exhibit a flexoelectric coefficient more than three orders of magnitude larger than in previously studied calamitic nematic LCs, paving the way for electro-mechanical devices that utilize the flexoelectric effect [2]. In order to develop practically viable flexoelectric materials, it is necessary to incorporate this type of nematic LC between flexible substrates or in an easily processible polymer matrix. Towards that goal, we synthesized reactive bent core LCs that can be used to prepare bent core polymers, elastomers, and gels. While previous studies have focused on reactive bent-core mesogens and bent-core LC networks that display smectic phases [3], we focus on nematic bent-core polymer networks. We prepared two different types of reactive bent core LCs with either an alkene or acrylate functionality. Monofunctional bent-core LCs with a reactive alkene group can be used to make aligned nematic elastomers using the well-known Finkelmann method [4], and these elastomers can either be studied in the bulk or swollen with bent core LC to investigate their flexoelectic behavior. Following a method demonstrated for calamitic LCs, monofunctional bent-core monomers with an acrylate functionality can also be dissolved in a nematic solvent and polymerized in the presence of a crosslinker to make bent-core LC networks and gels. Furthermore, monofunctional acrylate bent core LCs can be polymerized to make well-defined side-group liquid crystalline polymers. We present the synthesis and characterization of reactive, nematic bent-core LC mesogens.

[1] Meyer, R. B. Phys. Rev. Lett. 22, 918 (1969).

[2] Harden, J, Mbanga, J., Éber, N., Fodor-Csorba, K., Sprunt, S., Gleeson, J. T., and Jákli, A. Phys. Rev. Lett. 97, 157802 (2006).

[3] (a) Keum, C. D., Kanazawa, A. Ikeda, T. Adv Mater., 13, 321 (2001). (b) Sentman, A. C., Gin, D. L. Angew. Chem. Int. Ed.42, 1815 (2003). (c) Barberá, J. Gimeno, N., Monreal, L. Piñol, R. Blanca Ros, M., Serrano, J. L. J. Am. Chem. Soc., 126, 7190 (2004).

[4] (a) Küpfer J., Finkelmann H. Makromol. Chem., Rapid Commun. 12, 717 (1991). b) Küpfer, J., Finkelmann, H. Macromol. Chem. Phys., 195, 1353 (1994).

[5] (a) Urayama K., Arai Y. O., Takigawa, T. Macromolecules 38, 3469 (2005) (b) Urayama K., Okuno Y., (b) Kawamura T., Kohijiya S. Macromolecules 35, 4567 (2002).