Nematic liquid crystals with fluid-like arrangements of molecules that pack parallel to each other are widely used in display and other applications because of the unique combination of orientational order and fluidity. In the uniaxial nematic (N) phase, rod-like molecules are on average parallel to the single director , but their centres of mass are arranged randomly, as in an isotropic fluid (Fig. 1a). The director is a nonpolar entity, , even if the molecules have dipole moments. Chiral molecules prefer to twist with respect to each other, forcing to follow a right-angle helicoid, either left-handed or right-handed (Fig. 1c). In 1973, Meyer1 predicted that polar molecular interactions that favour bend deformations might lead to a twist-bend nematic (Ntb) phase, in which the director draws an oblique helicoid, maintaining a constant oblique angle 0 U th, the optic axis realigns gradually and everywhere, as in the second-order transition. As the tilt direction is degenerate, it results in umbilics, that is, defects of winding numbers −1 and +1 (ref. 28). The +1 umbilics show an in-plane bend of , which is expected, as K 3 1,000 °C s−1, to avoid further phase transitions, and was quickly transferred into a freeze-fracture vacuum chamber (BalTec BAF060) where the assembly was kept at −140 °C. Inside the chamber, a built-in microtome was used to break the assembly and expose the fractured surface. Approximately 4-nm-thick Pt/C was then deposited onto the fractured surface at a 45° angle to create shadowing of the surface structure, followed by an ~20-nm-thick C deposition from the top to form a continuous supporting film. The samples were then warmed up and removed from the freeze-fracture machine. The liquid crystal material was dissolved in chloroform, whereas the replica film (often flakes) was picked up and placed onto carbon-coated TEM grid and observed using room temperature TEM (FEI Tecnai F20). Synchrotron XRD studies The material was filled into 1 mm diameter quartz tubes located inside a hot stage (Instec model HCS402). The cylindrical neodymium iron boron magnets were used to align the material in the magnetic field 1.5 T perpendicular to the incident X-ray beam. Small-angle X-ray scattering was recorded on a Princeton Instruments 2,084 × 2,084 pixel array charge-coupled device detector in the X6B beamline at the National Synchrotron Light Source. The beamline was configured for a collimated beam (0.2 × 0.3 mm2) at energy 16 keV (0.775 Å). In the N phase, there are two diffused peaks centred along the magnetic field (Figure 7b,c), with the wavenumber at the maximum intensity decreasing from q o =2.95 nm−1 near the clearing point to q o =2.77 nm−1 at 110 °C (inset in Fig. 7a), corresponding to periodicities from a=2.16 to 2.23 nm. At lower temperatures, a secondary peak at a doubled periodicity 4.46 nm is observed. These two length scales might correspond to (i) the length of one arm of the dimers and the length of the monomer, and (ii) to the length of the entire dimer, respectively. The full width at half maxima (Δq) is decreasing from Δq=1.5 nm−1 at 140 °C to Δq=0.8 nm−1 at 110 °C. This means that the correlation length ξ=2π/Δq is increasing from ξ≈4 to 8 nm. Such a behaviour is typical for an N phase with ‘cybotactic’ smectic clusters, that is, nanosized clusters of layers with correlation length ξ that increases on cooling. However, the macroscopic structure is still that one of a fluid N phase. Interestingly, the vertical lobes are nearly straight, indicating no rigid restriction on the layer spacing inside the clusters. In the Ntb phase, q o increases rapidly from q o =2.77 to 2.93 nm−1 (Fig. 7a), which corresponds to local periodicity decreasing from d=2.23 to 2.14 nm. This can be explained by a bend of dimers with arms tilted away from the straight configuration by about ~15–20° or by the increased mosaicity of the Ntb phase. The width of the peak at half maxima is fairly temperature independent, Δq=0.75 nm−1, which corresponds to ξ=8.4 nm. It is interesting to see that the lobes of the diffused peaks are much closer to the circular shape, showing that the tilt of the arms of the dimers are much more defined than that in the N phase. Such a macroscopically fluid N phase with almost constant smectic nanoclusters is typical of bent-core N materials6. Author contributions V.B. performed electro-optical studies and analysed nanostructures; M.G. performed the TEM study and result analysis with the assistance of Y.-K.K.; J.X. measured elastic constants, A.J. analysed the XRD data, Y.-K.K., V.P.P. and J.K.V. explored electro-optic and alignment properties, C.T.I. thermally characterized M1, M.G.T. prepared M1 and M2, G.H.M. directed the molecular design, O.D.L. directed the research, analysed the data and wrote the text with an input from all co-authors. Additional information How to cite this article: Borshch, V. et al. Nematic twist-bend phase with nanoscale modulation of molecular orientation. Nat. Commun. 4:2635 doi: 10.1038/ncomms3635 (2013). Supplementary Material Supplementary Information Supplementary Figures S1-S5 and Supplementary Methods
