Uniaxial Alignment of Discotic Liquid Crystal for Heat Dissipating Materials with High Thermal Conductivity

For the development thermal management system in the electronic, display, and automobile industries, high heat transfer property is required. In this research, triphenylene-based liquid crystal reactive discogen (LCRD) monomers containing the vinyl and thiol functional groups were newly synthesized for the heat transfer materials with enhanced thermal conductivity. The LCRD mixtures exhibits columnar liquid crystal phase by self-assembly of doscogen, and those phases were fixed by photopolymerization. Based on POM and DSC, the phase of mixture was measured according to temperature and composition of vinyl monomers. The mixture had various phase under each condition. The 50vinyl-50thiol mixture (1:1 molar ratio) at 80 °C show the isotropic structure and tilled hexagonal columnar structure coexist. Under this condition, the LCRDs were polymer-stabilized by irradiating 365 nm UV light for 1 h. After polymerization, the network morphology of the fabricated LCRD films was identified. Through POM, it was confirmed that the dark area and the birefringent area coexist in the film. This means that the isotropic domains are percolated the liquid crystal matrix. In-plane thermal conductivity of the LCDR films produced by curing at different temperatures was measured and compared at room temperature by the transient plane source technique. The LCDR film in which the isotropic structure and hexagonal columnar structure coexist show thermal conductivity up to 1.09 W/mK. This value is higher than conventional heat dissipating organic materials, high density polyethylene, calamitic liquid crystal organic materials. The high thermal conductivity of the LCRD film due to the ordered structure by self-assembly and the hexagonal lattice structure of triphenylene core. The alignment of the columnar structure in the film was controlled by magnetic field for higher thermal conductivity than the previously prepared film. The LCRD films were fabricated using a static magnetic field and a rotating magnetic field, respectively. The columnar axes oriented using a static magnetic field were perpendicular to the direction of magnetic field but not aligned in the one direction. To solve this problem, a rotating magnetic field was used. It was realized through 2D WAXD that the columnar axes of LCRDs were aligned perpendicular to the direction of magnetic field and one direction. The uniaxially oriented LCRD film exhibits enhanced thermal conductivity normal to the columnar axis. The oriented film along the transverse direction of the film plane shows higher thermal conductivity than the aligned film along the longitudinal direction of the film plane and the film without magnetic and field up to 3.00 W/mK. High thermal conducting properties as well as excellent mechanical and chemical stabilities of LCRD films make them possible to be effectively available for heat management organic materials. This work was mainly supported by BK21 FOUR program, Mid-Career Researcher Program (2021R1A2C2009423) and Basic Research Laboratory Program (2020R1A4A1018259).