Controlling the Orientation of Liquid Crystal for Thermal Managing Materials

With the integration and miniaturization of electronics, the demand for thermal managing materials (TMM) that effectively dissipate the problematic heat has continuously increased. Much research has been conducted for composite materials consisting of high thermal conductive additives. But the limitation of processability with using large amounts of fillers makes it difficult to develop the advanced TMMs due to the intrinsic low thermal conductivity of organic polymer matrix. Therefore, improving the thermal conductivity of organic polymers can be an effective strategy for developing thermal conducting materials with a small amount of filler. In this study, a liquid crystal monomer (LCM) was newly synthesized for the fabrication of thermal conducting polymer (TCP) films which can be applied to the advanced TMMs. Based on thermal and structural analyses, it was realized that the synthesized LCM exhibited a smectic A mesophase. By using temperature-dependent properties of LCM, TCP films were fabricated at both liquid crystal and isotropic phases using photopolymerization process. Due to the mesogenic core of LCM which has a highly ordered lattice structure providing high phonon transfer pathway, photopolymerized TCP films show outstanding thermal conductivity over 1 W/mK regardless of the molecular self-assembly structure. Moreover, the heat transfer performance of TCP films can be further enhanced in a certain direction by controlling the orientation of LCM molecules in liquid crystal network. The uniaxially oriented TCP film can be fabricated by polymerizing under the magnetic field at Liquid crystal state. Due to the high intermolecular interaction of the LCM which forms the liquid crystalline domain at liquid crystal state, the magnetic coupling energy can overcome the thermal motion that disordering the molecular alignment. The uniaxially oriented TCP film exhibited outstanding thermal conducting property which was estimated to be 2.5 W/mK along the liquid crystal director. To compare the thermal conductivity of the polymeric films according to the molecular orientation, the TCP films were monitored using an infrared (IR) camera. Thermal imaging demonstrated that polymeric films with highly ordered lattice structure show improved heat-conducting performance. In addition, the anisotropic liquid crystal network in magnetically aligned TCP film can remarkably dissipate heat along the long axis of the mesogenic core. Therefore, TCP films with high thermal conducting properties can be applied as advanced thermal management materials. This work was supported by the BK21 FOUR, Mid-Career Researcher Program (2021R1A2C2009423) and Basic Research Laboratory Program (2020R1A4A1018259).