Evidence of Tilted Dimeric Mesophase for Terminally Polar Polyphilic Mesogens

We report an X-ray diffraction and infra-red dichroism study of the structure and phase transitions for a new homologous series of cyanoterminated polyphilic liquid crystals. Analysis of the X-ray scattering profiles in the direction parallel and perpendicular to the smectic layers provides detailed data on lamellar ordering in smectic phases of polyphilics. The infra-red dichroism measurements allow molecular conformations and the tilt of various molecular moieties in smectic phases to be determined. In addition to the partly bilayer smectic A$_{\rm d}$ phase composed of antiparallel dimers, its tilted analogue-smectic C$_{\rm d}$ phase has been detected. The temperature variations of the interlayer spacing in the smectic C$_{\rm d}$ phase are very small as compared to the classical smectic A–smectic C transitions. This unusual behaviour results from the fact that both the tilt angle for different molecular moieties and the relative displacement of molecules in dimers vary with temperature. The enhanced stability of dimeric smectic C$_{\rm d}$ phase and the features of smectic A$_{\rm d}$–smectic C$_{\rm d}$ phase transition are discussed in terms of sterical coupling of molecules with differently tilted fragments

Structure and frustration in liquid crystalline polyacrylates II. Thin-film properties

We report on the thin-film behaviour of a polyacrylate with phenyl benzoate mesogenic side groups and a narrow polydispersity. Depending on the degree of polymerisation, these polymers show a nematic, a smectic-A$_{{\rm d} }$, a re-entrant nematic and a $\text{\~C}$ phase with a two-dimensional monoclinic lattice. X-ray reflectivity and atomic-force microscopy have been used to characterize the structure and surface morphology. The system exhibits two stable side-chain packing configurations with incommensurate spacings that can be both stabilized at a free surface. Thin films in the nematic phase show a structural dewetting induced by the growth of surface domains of the $\text{\~C}$ phase. Additionally, surface-induced ripples with a nanoscale lateral period form at the air-film interface. We attribute these patterns to a coupling between the local liquid crystalline ordering of the mesogenic side groups and the surface curvature energy

Layer-by-layer crystallization and the role of fluctuations in free standing smectic films

Specular x-ray reflectivity has been used to study the changes in the thermal fluctuation behavior of eight layer freely suspended N-(4-n-butoxybenzilidene)-4-n-octylaniline (4O.8) films during the process of successive layer-by-layer crystallizations. Each of these steps is preceded by the formation of an intermediate layer structure with in-plane hexatic ordering. We find an unusually large reduction of the fluctuations after the emergence of only the first hexatic toplayer. The fluctuation profiles over the film are in all cases quenched at the surfaces though less so after each crystallization step. This behavior supports theoretical arguments that hexatic order is reduced by out-of-plane smectic layer fluctuations

Structure and frustration in liquid crystalline polyacrylates I. Bulk behaviour

The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crystalline (LC) polymer, a polyacrylate with phenylbenzoate mesogenic side groups and a narrow polydispersity. At a high degree of polymerisation the LC polymers show a nematic, a smectic-A$_{\rm d}$, a re-entrant nematic and a $\text{\~C}$ phase, for shorter chains only a nematic and a $\text{\~C}$ phase. This constitutes a new example of nematic re-entrance for which the driving field is the length of the polymer chain. The smectic-A$_{\rm d}$ layers consist of partially overlapped side groups while in the $\text{\~C}$ phase the side chains are rearranged into chevron-like blocks of bilayers. We propose an explanation of the frustrated phase behaviour in terms of these two different competing length scales and their coupling to the backbone conformations

Ultra short helix pitch and spiral ordering in cholesteric liquid crystal revealed by resonant soft X ray scattering

The spontaneous formation of chiral structures offers a variety of liquid crystals LC phases that could be further tailored for practical applications. In our work, the characteristic features of spiral ordering in the cholesteric phase of EZL10 10 LC were evaluated. To disclose resonant reflections related to a nanoscale helix pitch, resonant soft X ray scattering at the carbon K edge was employed. The angular positions of the observed element specific scattering peaks reveal a half pitch of the spiral ordering p 2 amp; 8776; 52 nm indicating the full pitch of about 104 nm at room temperature. The broadening of the peaks points to a presence of coherently scattering finite size domains formed by cholesteric spirals with lengths of about five pitches. No scattering peaks were detectable in the EZL10 10 isotropic phase at higher temperatures. The characteristic lengths extracted from the resonant soft X ray scattering experiment agree well with the periodicity of the surface fingerprint pattern observed in the EZL10 10 cholesteric phase by means of atomic force microscopy. The stability of LC molecules under the incident beam was proven by X ray absorption spectroscopy in transmission geometr