Highly anisotropic thermal conductivity of mesogenic epoxy resin film through orientation control

Higher-Order Structural Analysis of a Transparent and Flexible High Thermal Conductive Liquid Crystalline Elastomer Sheet and Its Composite

Transparency, flexibility, and high thermal conductivity are trade-offs. Specifically, we have investigated a cross-linked acrylic liquid crystal elastomer (LCE) that exhibits both transparency and flexibility while maintaining a high level of thermal conductivity. The transparent monodomain LCE sheet was achieved through a process of stretching an initially opaque polydomain sheet to 80% elongation and subsequently subjecting it to photocuring. The thermal conductivity in the stretching direction (x) of the monodomain LCE sheet was found to be 1.8 times higher than that of the prestretched polydomain sheet, consistent with findings from previous studies. However, in the orthogonal direction (y) to the stretching (x) direction, the thermal conductivity exhibited an even higher value, being 1.7 times greater than in the x-direction, with a value of 3.0 W/(m·K). This unique observation prompted us to conduct further investigation through higher-order structural analysis of these LCE sheets using 2D wide-angle X-ray scattering (WAXS) analysis. In the transparent sheet, the LCE molecules were aligned in the sheet in the stretching x-direction (monodomain structure) for the out-of-plane direction. However, in the in-plane x-direction, the molecular plane spacing exhibited random orientation at a period of 0.45 nm. In contrast, within the y-direction of the inner layer, the molecular plane spacing exhibited a uniaxial horizontal orientation at the same period length as in the x-direction. The heat energy entering into the y-direction once spreads to the x-direction, but it was considered that the reason for the higher thermal conductivity to the y-direction would be forming covalent bonds that function as new heat transmission paths, in the direction intersecting to the x-direction during photocuring. Therefore, we concluded that the synergistic effect of the high level of the ordered inner structure and covalent bonding structure due to cross-linking in the y-direction contributes to its higher thermal conductivity compared to that in the x-direction, which exhibits a random in-plane structure. Additionally, we have fabricated an LCE composite sheet filled with 75 vol % of alumina particles using a polydomain-type LCE as the base material. The composite sheet exhibits remarkable thermal conductivity in the thickness direction, measuring at 9.8 W/(m·K), while maintaining a flexibility characterized by an elastic modulus of 70 MPa. This thermal conductivity surpasses that of a nonmesogenic acrylic composite sheet with identical alumina particle filling, which measured at 3.9 W/(m·K), more than twice as much. The presence of the mesogen skeleton has been demonstrated to enhance heat transfer, even within soft composites, by facilitating the formation of an ordered structure.

Forming a Homeotropic SmA Structure of Liquid Crystalline Epoxy Resin on an Amine-Modified Surface

The molecular orientation of a liquid crystalline (LC) epoxy resin (LCER) on silane coupling surfaces of amorphous soda-lime-silica glass substrates was investigated. The LC epoxy monomer on the silane coupling surfaces of the substrates was revealed to form a smectic A (SmA) phase with planar alignments because of the relatively low surface free energy. An LCER with a curing agent, however, formed a homeotropically aligned SmA structure by curing on a substrate surface modified using a silane coupling agent with amino groups. This formation of homeotropic alignment was considered due to the attribution of the reaction between the amino group on the surface of the substrate and the epoxy group of the LCER. The homeotropic alignment had a relatively high orientation parameter of 0.95. Therefore, it is expected to possess high thermal conductivity and be applied as high-thermal-conductivity adhesives or packaging materials for electrical and electronic devices.

Enhanced Thermal Conductivities of Liquid Crystal Polyesters from Controlled Structure of Molecular Chains by Introducing Different Dicarboxylic Acid Monomers

Enhancing thermal conductivity coefficient (λ) of liquid crystal polyesters would further widen their application in electronics and electricals. In this work, a kind of biphenyl-based dihydroxy monomer is synthesized using 4, 4'-biphenyl (BP) and triethylene glycol (TEG) as raw material, which further reacts with three different dicarboxylic acids (succinic acid, p-phenylenediacetic acid, and terephthalic acid, respectively) by melt polycondensation to prepare intrinsically highly thermally conductive poly 4', 4”'-[1, 2-ethanediyl-bis(oxy-2, 1-ethanediyloxy)]-bis(p-hydroxybiphenyl) succinate (PEOS), poly 4', 4”'-[1, 2-ethanediyl-bis(oxy-2, 1-ethanediyloxy)]-bis(p-hydroxybiphenyl) p-phenyldiacetate (PEOP) and poly 4', 4”'-[1, 2-ethanediyl-bis(oxy-2, 1-ethanediyloxy)]-bis(p-hydroxybiphenyl) terephthalate (PEOT), collectively called biphenyl-based liquid crystal polyesters (B-LCPE). The results show that B-LCPE possess the desired molecular structure, exhibit smectic phase in liquid crystal range and semicrystalline polymers at room temperature, and possess excellent intrinsic thermal conductivities, thermal stabilities, and mechanical properties. λ of PEOT is 0.51 W/(m·K), significantly exceeds that of polyethylene terephthalate (0.15 W/(m·K)) which has similar molecular structure with PEOT, and also higher than that of PEOS (0.32 W/(m·K)) and PEOP (0.38 W/(m·K)). The corresponding heat resistance index (T_HRI), elasticity modulus, and hardness of PEOT are 174.6°C, 3.6 GPa, and 154.5 MPa, respectively, and also higher than those of PEOS (162.2°C, 1.8 GPa, and 83.4 MPa) and PEOP (171.8°C, 2.3 GPa, and 149.6 MPa).