Orientational order and physical properties in a dimer system with a nematic-nematic phase transition

H.CACHITAS; A. ALUCULESEI; C. CRUZ; J. FIGUERINHAS; F. VACA CHAVEZ; PEDRO J. SEBASTIÃO; M. TAMBA; A. KOHLMEIR; G. MEHL

Congreso; 9TH LIQUID MATTER CONFERENCE; 2014

A new nematic phase named either nematic X (NX) or twist bend nematic (NTB) that presents a spontaneous periodic modulation was recently discovered in liquid crystalline dimer systems [1]. Studies on this phase point for a significant bending of the molecules and a helicoidal molecular structure with a nanometric pitch [2]. The existence of periodically modulated nematics was predicted by different theoretical contributions when the elastic constants and the surface anchoring obey specific conditions [3]. The compound studied in this work named CB-C9-CB, exhibits the conventional nematic phase at higher temperature and a NX phase in between 377.6K and 356.6K [4]. Using proton NMR spectroscopy we address the orientational order in this system spanning both nematic phases. Data were collected both at high fields (7 Tesla) where the helicoidal axis is aligned by the field and lower fields (1.5 Tesla) where spectra characteristic of polydomain structures are observed. The analysis of the results including also deuterium NMR data from other authors in the same system [5], allowied for a detailed characterization of orientational order in both nematics. Electro-optical measurements in the two nematic phases under electric and magnetic fields in planar aligning cells were used to investigate the elastic, dielectric and optical properties. These results are discussed in terms of the current models proposed for the periodically modulated N phase.[1-3,5] Acknowledgments: This work is partially supported by the Portuguese Science and Technology Foundation through Project: PEst-OE/FIS /UI0261/2011 [[1]] V.P.Panov, M. Nagaraj, J.K. Vij, Yu. P. Panarin, A. Kohlmeier, M.G. Tamba, R.A. Lewis, G.H. Mehl, Phys. Rev. Lett., 105, 167801 (2010). [2] M. Cestari et al. Phys. Rev. E 84, 031704 (2011); V. Borshch et al. Ncomms 3635, 4:2635 (2013); D. Chen et al., PNAS, 110, 15931 (2013). [3] C. Meyer, G. R. Luckhurst and I. Dozov1 Phys. Rev. Lett., 111, 067801 (2013); R. Memmer, Liquid Crystals, 29, 483 (2002); V.M. Pergamenshchik, Phys. Rev. E, 61, 3936 (2000); I. Dozov, Europhys Lett., 56, 247 (2001). [4] C.S.P. Tripathi, P.L. Perez, C. Glorieux, A. Kohlmeier, M.G. Tamba, G.H. Mehl, J. Leys, Phys. Rev. E, 84, 041707 (2011). [5] A. Hoffmann, A.G. Vanakaras, A. Kohlmeier, G. H. Mehland D. J. Photinos, arXiv:1401.5445 [cond-mat.soft] (2014).