Abstract
Experimental and theoretical investigations of the twist-bend nematic (NTB) phase in liquid crystals have recently received significant attention for reasons of unusual and interesting characteristics of the NTB. Though NTB was predicted to exist for rigid bent-core LCs by Ivan Dozov in 2001, nevertheless it has unambiguously been proven to exist only in a few bent-core systems so far. Here the characteristics of both the nematic (N) and NTB phases are investigated for a hockey shaped achiral rigid bent-core LC, called BCI, in planar and homeotropic aligned cells, using polarizing optical microscopy, electro-optics, pyroelectricity and wide band dielectric spectroscopy. Spontaneous polarization measured for a bias field of 2.2 V/μm of a chiral domain using the pyroelectric effect is found to be only ∼5 nC/cm2. A measurement of the spontaneous polarization using pyroelectricity in general is unaffected by the sample’s dc conductivity except when its conductance dominates the admittance. As helix of NTB is partially unwound by the field, results lead to the identification of NTB as polar and chiral. The first and the second harmonics of the applied field are observed of higher amplitudes in NTB and N phases, respectively. The hockey-shaped bent-core system exhibits a large negative dielectric anisotropy over a wide range of temperatures than observed for any other compound so far. The large negative dielectric anisotropy is the highly desirable characteristic parameter of the NTB for exploring the field induced phases at relatively lower field strengths. The orientational order parameter characteristically jumps at the N-NTB transition temperature. The complex dielectric permittivity is measured as a function of frequency in the range 1 Hz to 10 MHz, the analysis of results reveales two collective modes in the dielectric spectra. Amplitude of the lower frequency mode is much higher than of higher frequency, former is assigned to the hydrodynamic mode q=qzz^ with z-dependent rotation of the heloconical director n^(r), this in turn involves compression and dilation of the pseudo-layers. The higher frequency mode corresponds to fluctuations of the tilt director and closer to the transition temperature, it exhibits a typical soft mode characteristic feature.
Highlights
In the last decade, significant attention of the liquid crystals community is focused on to investigating systems that exhibit nematic-nematic phase transitions, such as N-NTB and N to Ferroelectric Nematic (NF)
We report results obtained from experimental investigations of N and NTB phases of a rigid bent-core hockey shaped liquid crystal (LC) using planar and homeotropically aligned cells
If the elastic constant for the molecular tilt Kt is assumed constant close to the N-NTB transition temperature, we find from calculations of h that the helicoid is spontaneously formed in the N phase close to the transition temperature
Summary
Significant attention of the liquid crystals community is focused on to investigating systems that exhibit nematic-nematic phase transitions, such as N-NTB and N to Ferroelectric Nematic (NF). Prior to these observations, the phase transitions in nematic phase had rarely been observed. Lower temperature enantiomorphic phase though proven nematic was found to have different characteristics and it was initially denoted as the Nx phase [1] This phase is identified as the ‘twist bend nematic’ NTB [2,3], this was the first example of the chiral symmetry breaking in a fluid state, where achiral molecules lead to the spontaneous formation of chiral domains of opposite handedness.
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