Phase Behavior And Topological Structure Of Emerging Ferroelectric Liquid Crystals

Ferroelectrics are a class of materials exhibiting spontaneous polarization and the polarization direction can be modified by an applied electric field.The traditional nematic phase is thermodynamically equivalent in the head-to-tail arrangement direction.Although individual molecules have permanent dipole moments,the whole system is non-ferroelectric.A ferroelectric nematic liquid crystal can be obtained if the molecule’s dipole moments are aligned in the same direction.Born predicted this ferroelectric fluid in 1916,suggesting that as long as the molecule polarity is large enough to overcome the thermodynamic disturbances,it is possible to form a stable ferroelectric nematic phase.However,it was not until 2017 that ferroelectric nematic liquid crystals were independently discovered by Mandle and Nishikawa,respectively,and named RM734 and DIO.The discovery of ferroelectric nematic（N_F）has started a new chapter in liquid crystal.New liquid crystal molecules and new application scenarios must solve a series of related physical and chemical problems.The most critical ones are the phase behavior,the topological defect structures,new ferroelectric phases,and new physics under external fields such as electricity,magnetism,and light.At present,there are still many problems in this field.This paper will focus on the above problems,and the details are as follows:1.The influence of molecular configuration on the phase behavior of the ferroelectric phase.The formation of the ferroelectric nematic phase is very sensitive to the molecular dipole moment and molecular shape.Typically,molecules with a dipole moment greater than 9D and a pear-shaped structure can be formed.Two phase transition paths were found in regulating the molecular structure process.One is from the isotropic phase（Iso）to the nematic phase（N）and then entering to the ferroelectric nematic phase（N_F）,another one is directly from the Isotropic phase（Iso）to the ferroelectric nematic phase（N_F）.DIO,as a classic ferroelectric nematic molecule,has a rich phase diagram.In this chapter,two phase transition paths are obtained in one system by mixing different ratios of cis-DIO.By analyzing the sample viscoelasticity and polarity before and after entering the N_F phase,demonstrated that the second transition path to the N_F phase is dominated by polar interactions with a nonclassical anisotropy-driven force.2.Polarization topology analysis of novel ferroelectric liquid crystalsAfter entering the N_F phase,the n director will reorganize due to spontaneous polarization.It will cause new topological structures,which are quite different from the traditional liquid crystals.Since the polarization vector（P）has head-tail asymmetry property,it needs to be distinguished from the n director.So,it is necessary to establish a method to observe the polarization vector.In this chapter,we built second harmonic interference microscopy to observe the polarization topology structure of ferroelectric liquid crystals.3.Spontaneously polarization waves in helical ferroelectric liquid crystal（HN*）When chirality is introduced into the ferroelectric nematic phase,analogous to the spin topology in ferromagnetic materials,the topology structure formed by non-collinear polar interactions in ferroelectric materials remains to be explored.In this chapter,we observed a layered undulating structure of electric polarization waves spontaneously formed in the helical ferroelectric liquid crystal,and its three-dimensional structure was confirmed by second harmonic confocal microscopy.We also analyzed its formation conditions and considered that the polarization waves are caused by the polarization gradient energy which can enlarge the periodic helical pitch.And combined with the experimental results,it is numerically analyzed.4.The influence of surface polarization on the topological structure of ferroelectric smectic A phaseClark et al.found an antiferroelectric smectic phase structure termed Sm Z_A during the N and the N_F phases of DIO.Additionally,in the DIO derivatives,ferroelectric smectic A phase structure was also found,and termed Sm A_F.The Sm A_F phase is a one-dimensional layered fluid with C_∞v symmetry.The n director is parallel to the molecular long axis and perpendicular to the smectic layer.The polarization vector is consistent with the direction of the n director.In this chapter,we studied the influence of surface polarization on the topological structure of the ferroelectric smectic A phase.It is assumed that exist the transitional structure,distorted smectic layer structure,under the anti-rubbing surface polarization effect.In addition,we introduce the air to observe the bubble topology structure in the ferroelectric smectic A phase and test the electric drive deformation of the bubble.