Exploration Of The Molecular Design And Properties Of New Ferroelectric Nematic Liquid Crystal Materials

Liquid crystal,as a high-performance and anisotropic material,is widely used in the field of liquid crystal displays.However,traditional nematic（N）liquid crystals are generally non-polar fluids with low dielectric constants and lack polarization characteristics,which limits their performance in some key technical indicators such as dielectric constants and response speed to electric fields and the development of new optoelectronic functions.Therefore,researchers hope to introduce strong polarity or ferroelectricity to enhance the performance of liquid crystal materials,which is of great significance for flexible optoelectronic devices.Although Born predicted the existence of fluid ferroelectric materials in 1916,it was not until 1974 that the existence of fluid ferroelectricity was first discovered in layered phase liquid crystal（ferroelectric layered smectic phase,Sm C*）materials.However,such materials have weak polarity and ferroelectricity,and are in a semi-solid state with poor fluidity;secondly,they have too many defect structures and cannot be integrated with display applications and flexible devices.It was not until 2017 that researchers discovered a class of liquid,strongly polar low-temperature nematic phase,which was later confirmed to be the ferroelectric nematic（N_F）liquid crystal phase predicted by Born.In the intrinsic structure of the N_F liquid crystal material,the direction vector of the electric dipole is no longer distributed equally along n and-n,but is spontaneously oriented and arranged,producing ferroelectric stacking order and macroscopic polarization characteristics.Compared with traditional liquid crystals or soft materials,N_Fliquid crystals have various transformative properties,including ultra-high dielectric strength,strong non-linear optical response,low voltage driving,super-fast electric field response characteristics,and high fluidity,providing new possibilities for the development of advanced optical and electrical liquid crystal devices.Currently,the number of reported N_F liquid crystal materials is relatively small,and their phase transition temperature is high,their room temperature stability is poor,and their rotational viscosity is large,so there is still a long way to go before commercial applications can be achieved.To solve this problem,this article adopts a method combining material design and synthesis with machine learning to determine the common molecular characteristics that form the N_F phase,and greatly improves the success rate of N_F material design by guiding experimental design with theoretical models.At the same time,we successfully simulated the topological structure changes during the molecular phase transition of N_F liquid crystals,and qualitatively analyzed the relationship between the selection of the N_F phase transition path and molecular parameters.Finally,we successfully developed a series of N_F liquid crystal materials,including small molecules,oligomers,and polymers,which greatly expanded the types and application range of N_F liquid crystal materials.The specific research content and results are as follows:（1）The structures of existing N_F LC molecules RM734 and DIO were analyzed by introducing electron-withdrawing groups such as-NO₂,-CN,-F,-CF₃,and changing the types of bridging groups to modify the molecular dipole moment.The length and position of the substituents were adjusted to change the aspect ratio and tilt angle of the molecular dipole moment,leading to the synthesis of over 40 compounds with similar structures.A machine learning molecular library was constructed by combining these molecules with similar reported ones.The phase transition types of these molecules were determined through photoelectric testing and characterization.Then,the key molecular parameters of N_F liquid crystal formation were obtained using machine learning methods after calculating the basic parameters of the molecules using density functional theory（DFT）.Higher molecular dipole moments（μ>9 D）and the tilt angle between the dipole moment and the molecular axis are crucial for the formation of the N_F phase.All molecules entering the N_F phase exhibit super-high dielectric constants（ε>10⁴）and strong second harmonic generation（SHG）signals（about 2-100 times that of quartz）.Finally,the direction field model of the N_F ferroelectric domain boundary was determined to be the NéelⅠtype structure by combining polarizing microscope texture and optical simulation methods.（2）Generally,N_F materials undergo the Iso-N-N_F phase transition process（Iso denotes isotropic liquid）.However,some N_F molecules exhibit unique Iso-N_F phase transition behavior,skipping the non-polar nematic（N）phase.In order to systematically study the phase transition behavior of N_F materials and further expand the types and temperature ranges of small molecule N_F liquid crystals,this study adjusted the molecular parameters and measured the phase transition process in a wider range of molecular dipole moments and rod-shaped molecular shape anisotropy intervals.The relationship between molecular characteristics and the path to entering the N_F phase was studied using machine learning.The conclusion was drawn that a high polymer dipole density is a key driving factor for N_F phase formation,while a large shape anisotropy（such as molecular aspect ratio）is a necessary factor for inducing the formation of the N phase.Rod-shaped molecules with both high polymer dipole density and high shape anisotropy experience the Iso-N-N_F phase transition,while rod-shaped molecules with high polymer dipole density but low shape anisotropy undergo the Iso-N_F phase transition.In addition,this study also discovered a class of liquid crystal molecules that are stable at room temperature for a long period of time.（3）Furthermore,the main chain rod-shaped aromatic ester liquid crystal was extended from small molecules to oligomers/polymers（with a maximum of 12 repeating units）,and theμvalue was increased to approximately 30D.By gradually increasing the size and dipole moment value of the rod-shaped liquid crystal unit through precise synthesis,the interaction between the rod-shaped dipole elements was enhanced,and the driving force for N_F liquid crystal formation was improved.The experimental results show that the N_F phase appears in a nucleation mode and undergoes the Iso-N_F phase transition process,and can be observed in oligomers/polymers of all length ranges.In addition,this study proposes a new understanding of the basic physical structure of the traditional hydroxyl benzoate-based aromatic main chain liquid crystal polymer material,which has local polarized order.（4）In order to prepare polarized liquid crystal polymers,the main elements of the N_F liquid crystal molecules were introduced into the polymer backbone in different ways.Three series of comb-like liquid crystal polymers were designed and synthesized,namely the SP type with side-end linking,the MP type with middle linking,and the EP type with end linking.Detailed structural and polarity characterization was conducted on these polymers.Experimental results showed that SP1 had strong polarity,while SP2,SP3,and MP had weak polarity,and EP1 and EP2 had no polarity at all.These liquid crystal polymers exhibited significantly reduced SHG signals compared to small molecule N_F materials.We established stacking models for these liquid crystal polymers and explained the reasons for the strength of polarity in each polymer.