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

Effect of the Structure of Epoxy Monomers and Curing Agents: Toward Making Intrinsically Highly Thermally Conductive and Low-Dielectric Epoxy Resins

The low intrinsic thermal conduction and high dielectric properties of epoxy resins have significantly limited their applications in electrical and electronic devices with high integration, high frequency, high power, and miniaturization. Herein, a liquid crystalline epoxy (LCE) monomer with a biphenyl mesogenic unit was first synthesized through an efficient one-step reaction. Subsequently, bisphenol AF (BPAF) containing low-polarizable -CF3 groups and 4,4'-diaminodiphenylmethane (DDM) were applied to cure the LCE and commercial diglycidyl ether of bisphenol A-type epoxy (E-51), respectively, to afford four kinds of epoxy resins with various intrinsic thermal conductivity and dielectricity values. Owing to the dual effect of microscopically stacking of mesogens and the contribution of fluorine to the formation of liquid crystallinity, ordered microstructures of the nematic liquid crystal phase were formed within the cross-linking network of LCE as confirmed by polarized optical microscopy and X-ray diffraction. Consequently, phonon scattering was suppressed, and the intrinsic thermal conductivity was improved considerably to 0.38 W/(m·K), nearly twice as high as that of E-51 cured with DDM (0.20 W/(m·K)). Additionally, the ordered microstructure and ultralow polar -CF3 groups within LCE cured with BPAF enabled the epoxy resin to exhibit a remarkably lower and stable dielectric constant (ε) and dielectric loss tangent (tan δ) over both low and high frequencies compared to E-51 cured with DDM. The ε decreased from 3.40 to 2.72 while the tan δ decreased from 0.044 to 0.038 at 10 GHz. This work presents a scalable and facile strategy for breaking the bottleneck of making epoxy resins simultaneously with high inherent thermal conduction and low dielectric performance.

Highly Thermally Conductive and Structurally Ultra-Stable Graphitic Films with Seamless Heterointerfaces for Extreme Thermal Management

Highly thermally conductive graphitic film (GF) materials have become a competitive solution for the thermal management of high-power electronic devices. However, their catastrophic structural failure under extreme alternating thermal/cold shock poses a significant challenge to reliability and safety. Here, we present the first investigation into the structural failure mechanism of GF during cyclic liquid nitrogen shocks (LNS), which reveals a bubbling process characterized by "permeation-diffusion-deformation" phenomenon. To overcome this long-standing structural weakness, a novel metal-nanoarmor strategy is proposed to construct a Cu-modified graphitic film (GF@Cu) with seamless heterointerface. This well-designed interface ensures superior structural stability for GF@Cu after hundreds of LNS cycles from 77 to 300 K. Moreover, GF@Cu maintains high thermal conductivity up to 1088 W m-1 K-1 with degradation of less than 5% even after 150 LNS cycles, superior to that of pure GF (50% degradation). Our work not only offers an opportunity to improve the robustness of graphitic films by the rational structural design but also facilitates the applications of thermally conductive carbon-based materials for future extreme thermal management in complex aerospace electronics.

Construction of Marigold-like Poly(vinyl alcohol) Microspheres for Catalytic Microreactors

It has long been pursued to develop polymer microspheres with various special morphologies and structures for better results in applications such as catalysis, drug delivery, and bioscaffolds. However, it remains a challenge to develop a facile method to produce poly(vinyl alcohol) (PVA) microspheres with special morphologies. Herein, a micron-sized marigold-like poly(vinyl alcohol) (CE-PVATPA) microsphere was engineered and fabricated by a feasible strategy, that is, emulsification-cross-linking, freeze-drying, and secondary acetal reaction steps. The morphological evolution of microspheres was systematically investigated under different conditions, and the procedure of constructing PVA microspheres with stabilizing marigold-like structures was proposed. More importantly, a specially structured PVA microsphere microreactor synergistically loading palladium metal nanoparticles (CE-PVATPA@Pd) for the heterogeneous catalyst 4-nitrophenol (4-NP) could be further demonstrated, which indicated high catalytic activity and excellent recyclability. The resultant stabilized fabricating method is promising to provide valuable guidance for the design and fabrication of a high-performance PVA microsphere microreactor.

Electric-Field-Induced Alignment of Functionalized Carbon Nanotubes Inside Thermally Conductive Liquid Crystalline Polyimide Composite Films

The positive liquid crystals, 4'-heptyl-4-biphenylcarbonitrile (7CB), are used to functionalize carbon nanotubes (LC-CNT), which can be aligned in the liquid crystalline polyimide (LC-PI) matrix under an alternating electric field to fabricate the thermally conductive LC-CNT/LC-PI composite films. The efficient establishment of thermal conduction pathways in thermally conductive LC-CNT/LC-PI composite films with a low amount of LC-CNT is achieved through the oriented alignment of LC-CNT within the LC-PI matrix. When the mass fraction of LC-CNT is 15 wt %, the in-plane thermal conductivity coefficient (λ∥ ) and the through-plane thermal conductivity coefficient (λ⊥ ) of the LC-CNT/LC-PI composite films reach 4.02 W/(m ⋅ K) and 0.55 W/(m⋅K), which are 90.5 % and 71.9 % higher than those of the intrinsically thermally conductive LC-PI films respectively, also 28.8 % and 5.8 % higher than those of the CNT/LC-PI composite films respectively. Meanwhile, the thermally conductive LC-CNT/LC-PI composite films also possess excellent mechanical and heat resistance properties. The Young's modulus and the heat resistance index are 2.3 GPa and 297.7 °C, respectively, which are higher than the intrinsically thermally conductive LC-PI films and the thermally conductive CNT/LC-PI composite films under the same amount of CNT.

Liquid Crystal Materials for Biomedical Applications

Liquid crystal is a state of matter being intermediate between solid and liquid. Liquid crystal materials exhibit both orientational order and fluidity. While liquid crystals have long been highly recognized in the display industry, in recent decades, liquid crystals provide new opportunities into the cross-field of material science and biomedicine due to their biocompatibility, multifunctionality, and responsiveness. In this review, the latest achievements of liquid crystal materials applied in biomedical fields are summarized. The start is made by introducing the basic concepts of liquid crystals, and then shifting to the components of liquid crystals as well as functional materials derived therefrom. After that, the ongoing and foreseeable applications of liquid crystal materials in the biomedical field with emphasis put on several cutting-edge aspects, including drug delivery, bioimaging, tissue engineering, implantable devices, biosensing, and wearable devices are discussed. It is hoped that this review will stimulate ingenious ideas for the future generation of liquid crystal-based drug development, artificial implants, disease diagnosis, health status monitoring, and beyond.

Multifunctional Thermally Conductive Composite Films Based on Fungal Tree-like Heterostructured Silver Nanowires@Boron Nitride Nanosheets and Aramid Nanofibers

Thermal conduction for electronic equipment has grown in importance in light of the burgeoning of 5G communication. It is imperatively desired to design highly thermally conductive fillers and polymer composite films with prominent Joule heating characteristics and extensive mechanical properties. In this work, "solvothermal & in situ growth" method is carried out to prepare "Fungal tree"-like hetero-structured silver nanowires@boron nitride nanosheet (AgNWs@BNNS) thermally conductive fillers. The thermally conductive AgNWs@BNNS/ANF composite films are obtained by the method of "suction filtration self-assembly and hot-pressing". When the mass fraction of AgNWs@BNNS is 50 wt%, AgNWs@BNNS/ANF composite film presents the optimal thermal conductivity coefficient of 9.44 W/(m ⋅ K) and excellent tensile strength of 136.6 MPa, good temperature-voltage response characteristics, superior electrical stability and reliability, which promise a wide application potential in 5G electronic devices.

Enhancing intrinsic thermal conductivities of epoxy resins by introducing biphenyl mesogen-containing liquid crystalline co-curing agents

Epoxy resins were prepared with an enhanced intrinsic thermal conductivity of 0.42 W (m K)−1 using a biphenyl mesogen-containing co-curing agent.