Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K1, twist K2, and bend K3 elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K1 and K3 are comparable to each other and are an order of magnitude higher than K2. At higher concentrations and lower temperatures, K1 and the ratios K1/K3 and K1/K2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.

The surface alignment of lyotropic chromonic liquid crystals (LCLCs) can be not only planar (tangential) but also homeotropic, with self-assembled aggregates perpendicular to the substrate, as demonstrated by mapping optical retardation and by three-dimensional imaging of the director field. With time, the homeotropic nematic undergoes a transition into a tangential state. The anchoring transition is discontinuous and can be described by a double-well anchoring potential with two minima corresponding to tangential and homeotropic orientation.PACS numbers: 42.70.Df Spatial bounding of a liquid crystal (LC) lifts degeneracy of molecular orientation specified by the director n and sets an "easy axis" n 0 at the surface. Deviation of n from n 0 requires some work thus establishing a phenomenon of "surface anchoring" that has been explored extensively for thermotropic LCs [1][2][3][4][5][6][7][8][9][10][11]. For lyotropic LCs, such as water solutions of polyelectrolytes, surfactants, dyes, etc., the studies of anchoring are scarce. The view is that the surface alignment of lyotropic LCs is determined by an excluded volume effect, which favors the longest dimension of building units to be parallel to a substrate [12][13][14][15]. We study surface phenomena in nematic lyotropic chromonic LCs (LCLCs), a distinct class of self-assembled LCs formed by water solutions of planklike molecules with polyaromatic cores and ionic peripheral groups [16]. Reversible chromonic assembly and mesomorphism are displayed broadly by dyes, drugs and nucleotides [16]. In water, the LCLC molecules stack face-to-face, forming elongated aggregates. The aggregates are not fixed by covalent bonds, being polydisperse with an average length l ∝ √ φ ln (E/k B T ) that depends on temperature T , volume fraction φ, and stacking energy E ∼ (4 − 10) k B T [17]. We demonstrate that in LCLCs, n 0 can be either parallel to a substrate (planar or tangential alignment, denoted "P") or perpendicular (homeotropic, or H alignment), with discontinuous transitions between the two, thus suggesting that both entropy and anisotropic molecular interactions control the surface phenomena.We study disodium cromoglycate (DSCG) [16], C 23 H 14 O 11 N a 2 (Spectrum Inc, purity 98%), dissolved in water at 15 wt % (mixture A) and 12.5wt% doped with 1.5wt% of Na 2 SO 4 (mixture B). The H alignment was achieved by treating glass plates with 1% water solution of N,N-dimethyl-N-octadecyl-3-aminopropyl trimethoxysilyl chloride (DMOAP) [2]. The two plates are separated by Mylar strips; the cell thickness d was measured by light interference technique. The cells were filled at T N I + 10 K, sealed with a UV-cured Norland epoxy glue, and cooled down to T = 298 K with a rate 5 K/ min in a thermal stage HS-1 (Instec, Inc.). We used an LC PolScope for in-plane mapping of opticale sin 2 θ −1/2 , θ is the angle between n and the normal z to the cell, n o and n e are the ordinary and extraordinary refractive indices, respectively. At 546 nm and T = 298 K, we determined n o = 1.37 ± ...

We propose lyotropic chromonic liquid crystals (LCLCs) as a distinct class of materials for organic electronics. In water, the chromonic molecules stack on top of each other into elongated aggregates that form orientationally ordered phases. The aligned aggregated structure is preserved when the material is deposited onto a substrate and dried. The dried LCLC films show a strongly anisotropic electric conductivity of semiconductor type. The field-effect carrier mobility measured along the molecular aggregates in un-optimized films of LCLC V20 is 0.03 cm 2 V −1 s −1 . Easy processibility, low cost and high mobility demonstrate the potential of LCLCs for microelectronic applications.

No abstract