Anomalous Increase in Nematic-Isotropic Transition Temperature in Dimer Molecules Induced by a Magnetic Field

Abstract

We have determined the nematic-isotropic transition temperature as a function of applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15K when subjected to 22T magnetic field. The increase is conjectured to be caused by a magnetic field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers.Nematic liquid crystals (NLC) are anisotropic fluids that only exhibit uniaxial, apolar orientational order. In most liquid crystals, this order is temperature dependent, spontaneously arising at temperatures below the nematic-isotropic phase transition temperature (T NI );; above this temperature the material exhibits no order (i.e. is isotropic). In many nematics, orientational order is particularly responsive to external influences, for example electric and/or magnetic fields, mechanical strains, etc. [1-7].This response is the primary reason that nematic liquid crystals are extensively used in information display applications like liquid crystal displays (LCD's).

Mesophase structure and behaviour in bulk and restricted geometry of a dimeric compound exhibiting a nematic–nematic transition

“…As the temperature lowers, reaching the N x phase, twisted molecular conformations persist for longer time and a chiral separation takes place. We note that these conformations can be unwound by strong external magnetic fields 52 . The mesogens in the clusters adopt a tilted anticlinic configuration with neighbouring packs of mesogens shifted by half of the molecular length and exhibiting a helical arrangment 21 .…”

The Dependency of Nematic and Twist-bend Mesophase Formation on Bend Angle

“…16) exhibit an increase in the nematic to isotropic transition temperature when subjected to a large external magnetic field (22 T)44. This field induced increase is reported to be due to an increase in the inter-aromatic angle ( i.e.…”

Nematic twist–bend phase in an external field

“…A more comprehensible, field-induced modification of cholesterics involves reorientation of helical axis [11], or changing the pitch [53]. Similar effects can be expected for N TB , although recent magnetic field experiments [54,55] show only depletion of the N − N T B transition temperature, without a noticeable distortion of the structure. So far, theoretical attempts to characterize the interaction of N TB with field have been made on the basis of the Frank elastic theory [56][57][58].…”