PHYSICAL REVIEW E 88, 042503 (2013) Phases and properties of nanocomposites of hydrogen-bonded liquid crystals and carbon nanotubes M. Petrov, 1,* B. Katranchev, 1 P. M. Rafailov, 1 H. Naradikian, 1 U. Dettlaff-Weglikowska, 2 E. Keskinova, 1 and T. Spassov 3 1 Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria 2 School of Electrical Engineering, Korea University, Anam-Dong, Sungbuk-Gu, 136-713 Seoul, Korea 3 Faculty of Chemistry and Pharmacy, University of Sofia “St. Kl. Ohridski,” 1 James Bourchier Boulevard, 1164 Sofia, Bulgaria (Received 20 June 2012; revised manuscript received 25 April 2013; published 11 October 2013) We investigated a series of nanocomposites, built of the hydrogen-bonded liquid crystal (LC) p-n-heptyloxybenzoic acid (7OBA) and single-walled carbon nanotubes (SWCNTs) by optical microtexture analysis and other complementary methods. The surface orientation strength of the LC cell and the bulk interaction of the dimeric LC molecules with the SWCNTs turn out to mainly govern the type (symmetry), thermal stability, and chirality of the LC states induced in these nanocomposites. As a result, a cascade of phase transitions and phases not typical for pristine 7OBA were observed and additionally confirmed by temperature-dependent Raman spectroscopy and differential scanning calorimetry. The most effective SWCNT concentrations in the LC matrix, ensuring both the necessary conformability between these materials and induction of liquid crystal phases with unique optical and electro-optical properties, were found to be in the range of 0.01–0.007 wt %. Reversal of smectic phases into reentrant nematic states as well as induction of chirality in all LC phases were observed in the SWCNT-7OBA nanocomposite, even though pure 7OBA is typically achiral. However, our most intriguing result is the detection below the reentrant nematic of a triclinic smectic-C G phase, which is chiral and biaxial, and exhibits bulk ferroelectricity. DOI: 10.1103/PhysRevE.88.042503 PACS number(s): 61.30.Eb, 61.30.Hn, 73.63.Fg I. INTRODUCTION Mixtures of liquid crystals (LCs) with some nonmesogenic nanostructured materials yield promising nanocomposites, able to create a new trend in the LC display techniques [1–16] The concentration of the nonmesogenic component, its homogeneity, and its compatibility with the liquid crystal are of particular significance for the structural, optical, and electrical properties of the nanocomposite. In this sense, single-walled carbon nanotubes (SWCNTs) are the subject of intense research due to their unique anisotropic mechanical, thermal, and electro-optical properties [1,14–20]. Dispersion of SWCNTs in nematic LCs (NLCs) as well as in ferroelectric LCs was applied to create a suitable nanocomposite for low-threshold voltage and faster switching time [14,17]. These improvements in the electro-optical behavior of such nanocomposites are due to trapping by the nanotubes of impurity ions [17], which decreases the dielectric losses [21], as well as to an enhancement of the dielectric anisotropy and the rotational viscosity, and to a conductivity decrease in the nanocomposite [11,14,17]. Furthermore, it has been shown that the self-organization phenomena in the LCs can orient the SWCNTs in the bulk as well as on the surface, especially if an orienting surface is applied in the LC cell [1,2]. The SWCNTs embedded in the LC matrix could also affect the structure of the constituent LC molecules. This is particularly important when these molecules are dimers, and especially dimers formed by linear hydrogen bonding (H bonding). In the latter case, the interaction with the nonmesogenic component in the nanocomposite could provoke a conformation of the LC molecules from closed dimers to opened dimers and monomers or bent dimers which may lead to new electrical and thermal properties, including new phases. Recently, a smectic-G phase * Corresponding author: mpetrov@issp.bas.bg was induced through intermolecular H bonding between meso- genic p-n-alkyloxybenzoic acids (nOBA, n being the homolog number) and nonmesogenic p-aminobenzonitrile [22]. The closed dimers can be viewed also as flexible bent- core-like molecules, showing similarities with phases found in large-size bananalike systems [23–25]. A close similarity was suggested between the smectic-X phase found in these substances and the C G phase or its analog B 7 phase in bent- core liquid crystals [26]. These phases are examples of the typical smectic states where left- and right-handed helices are induced [27,28] in an LC system whose constituent molecules are achiral. The smectic-C G phase was predicted long ago by de Gennes and Prost [29]. Being biaxial and fluid in the layers and having a macroscopic hand due to its structure, this phase is a natural candidate to demonstrate induced helices in a system composed of achiral molecules. Since the layer normal N defines a direction independent on the normal to the molecular plane and the molecular polar axis, a large variety of mesophases with different symmetries may result (see Ref. [30] for more details on bent-core molecules). The predicted C G is interesting because it is characterized by two tilt directions with respect to the layer normal: a tilt of the molecular plane (clinic) and a tilt of the molecular kink direction (leaning). As a result, each smectic layer has a polarization component normal to the layer and the symmetry of the structure is lowered to C 1 —a triclinic symmetry in a bulk smectic-LC system [27,28,30–34]. The classical LC p-n-alkyloxybenzoic acid is a basic component in forming supramolecular systems. Even with- out complementary nonmesogenic molecules it displays a supramolecular state [35] due to the dimeric form of its molecule, and its donor ability at contact with a nonmesogen substance predicts rich mesomorphic states. On the other hand, as an achiral LC system, its dimeric structure makes it possible to create an induced chirality, i.e., it is a hopeful candidate for 042503-1 1539-3755/2013/88(4)/042503(12) ©2013 American Physical Society