1,720,991 research outputs found
The cybotactic nematic phase of bent-core mesogens: state of the art and future developments
No full textThe molecular clustering observed in the fluid nematic phase of nonlinear liquid crystal molecules underlies exaggerated field effects that portend unique technological advances in next-generation liquid crystal displays. However, the detailed nature of the molecular organization within the clusters and the temporal and spatial persistence of the organization remain unclear. Herein we review the evolution of structural studies of this unique nematic phase. The mounting experimental evidence points to a converging picture of the microscopic nature of this relatively new class of liquid crystals
Extraordinary Magnetic Field Effect in Bent-Core Liquid Crystals
No full textA bent-core mesogen that forms a cybotactic nematic phase exhibits a giant magnetic field-induced shift of its nematic-isotropic and smectic-C-nematic transition temperatures: Delta T(H) = 4 K for H = 10 kOe. In contrast with molecular nematics, in cybotactic nematics the field couples with the anisotropic susceptibility of clusters containing several hundred partially ordered molecules. X-ray diffraction data corroborate a quantitative estimate of inferred cluster size (similar to 300 molecules). The results represent an unequivocal demonstration of the cluster picture of the nematic phase of this class of nonlinear liquid crystals
Extraordinary field sensitivity of bent-core cybotactic nematics
No full textSeveral experimental and theoretical studies conclude that the unconventional properties of bent-core nematics derive from cybotactic clusters, that is, very short-range correlation fluctuations of positional and orientational order. We show how X-ray diffraction from oxadiazole-based compounds strongly supports this model. Moreover, we recently reported an extraordinary magnetic-field-induced shift of both nematic-isotropic and smectic-C-nematic phase transition temperatures (ΔT = 4 K for B = 1 T). We review this effect, which is orders of magnitude larger than previously reported data, in the framework of the cybotactic model of bent-core nematics
Insights into biaxial ordering of bent-core nematics: X-ray diffraction evidence
No full textRecently we have reported the first X-ray diffraction evidence of biaxial order in the fluid cybotactic nematic phase of a pair of trimethylated bent-core mesogens. This evidence is based on the splitting of the wide-angle diffuse scattering in the plane normal to the nematic director. The additional experimental data presented here provide further insights into this unprecedented behavior observed over the entire nematic phase. Although we could not determine the spatial extent of biaxial order-intracluster or intercluster-our observations reveal an enhanced biaxial orientational correlation in the transverse molecular packing, possibly reflecting stronger anisotropic interactions between nearest-neighbor mesogens
Molecular engineering room-temperature bent-core nematics
No full textWe report the transition temperatures for several series of structural variations on bent-core mesogens (BCMs) derived from the asymmetric oxazole biphenol mesogenic core. Trimethyl-substituted mesogens exhibit nematic phases that can be supercooled to room temperature. Preliminary X-ray diffraction (XRD) results confirm a cybotactic supramolecular organisation in these nematics; moreover, the splitting of the diffuse wide-angle XRD feature observed in trimethylated mesogens points to an enhanced biaxial orientational correlation in the transverse molecular packing compared to other BCMs. Finally, one of the trimethyl-substituted mesogens shows the formation of a room temperature metastable fibre-like crystalline order when supercooled in a magnetic field
Cybotaxis dominates the nematic phase of bent-core mesogens: a small-angle diffuse x-ray diffraction study
No full textNew temperature dependent X-ray diffraction (XRD) data on the bent-core mesogens based on the nonlinear 2,5-bis(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) unit enable a consistent interpretation of the supramolecular structure in this class of liquid crystals. Strong evidence for cybotaxis in the high temperature phase explains the small-angle four-spot pattern and calls into question prior XRD interpretations. We find that the data can be satisfactorily explained by skewed cybotaxis, a stratified arrangement of tilted, bent-core mesogens (BCMs). We also observe the temperature-induced evolution of skewed cybotaxis to normal cybotaxis-strata wherein the long axes of the BCMs are normal to the layer fluctuations. Our XRD interpretation is compatible with the NMR data that exhibit biaxiality in the nematic phase of ODBP mesogens
Electric field effect on the phase diagram of a bent-core liquid crystal
No full textWe have investigated the effects of a low frequency electric field on the thermotropic behavior of a nematogenic bent-core liquid crystal by means of X-ray diffraction. We have determined for the first time the 2D phase diagram of the bent-core mesogen over an extended temperature-field (T, E) space. The results show that the electric field strongly affects both the mesophase sequence and the transition temperatures of the sample. An extraordinary field-induced shift of the nematic-isotropic phase transition is found, which has neither any experimental counterpart in conventional calamitic LCs, nor can be explained by the presently available theories. The phase diagram points out the existence of exotic phase transitions driven by either electric field or temperature along specific paths in the (T, E) space. In particular, the isothermal field-induced transition from the NCybC to the N CybA phase represents a breakthrough in the interpretation of previous XRD experimental results. These experimental findings provide further strong support of the now widely accepted cybotactic cluster picture of the nematic phase of bent-core mesogens
Insights into the nanostructuring and phase behaviour of an all-aromatic prototypical nematic liquid crystal
No full textAll-aromatic calamitic liquid crystals are an unconventional family of rigid linear mesogens that represents the closest embodiment of the idealised rod-like molecule central to liquid crystal theories. 2,6-biphenyl naphthalene (PPNPP), a prototypical all-aromatic nematogen, has recently been the focus of scientific interest for both fundamental and technological purposes. While it provides a valuable benchmark for classical theories of nematic order, its experimental study presents challenges given the high temperature of the nematic phase. Herein, molecular dynamics (MD) simulations are contrasted with X-ray diffraction (XRD) data to resolve the thermotropic phase behaviour of PPNPP and the main structural features of its liquid crystal order. The observed trend of the molecular conformation with temperature points to an unexpected and significant distortion of the molecular framework in the nematic phase, which demonstrates the substantial non-linearity of the PPNPP molecule. The simulated mesomorphic behaviour and related thermodynamic parameters are in close agreement with the experimental data. The different phases are described in terms of their molecular organisation, pair distribution function, as well as the orientational and positional order parameters. We compare the classical and extended Maier-Saupe nematic theories with the computationally (MD) and experimentally (XRD) determined orientational order parameters. The observed deviation from the theoretical models indicates substantial inadequacies of the classical theories to describe the nematic-isotropic phase transition for this class of compounds. We suggest that modifications of theory should include the effects of short-range positional order, i.e., cybotaxis
Uniaxial to biaxial nematic phase transition in a bent-core thermotropic liquid crystal by polarising microscopy
No full textCover Journal Articl
Chain deformation for a polymer melt under shear
No full textWe used a Couette cell placed within an NMR microscopy apparatus to examine molecular alignment in polydimethylsiloxane melts under shear.
1
H
dipolar interactions are used to examine main chain ordering while
2
H
NMR quadrupole splittings are measured in a deuterated oligomer probe, in each case selectively observing different alignment orientations with respect to the flow field. While the main chain results are consistent with the alignment tensor as measured by birefringence, the oligomer senses a uniaxial environment whose director is normal to the velocity-vorticity plane. In the light of these results we propose supramolecular ordering involving polymer stratification
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