Synergistic Improvement in the Thermal Conductivity of Hybrid Boron Nitride Nanotube/Nanosheet Epoxy Composites

Epoxy composites with excellent thermal properties are highly promising for thermal management applications in modern electronic devices. In this work, we report the enhancement of the thermal conductivity of two different nanocomposites, using epoxy resins LY564 (epoxy 1) and LY5052 (epoxy 2), by incorporating multiwalled boron nitride nanotubes (BNNT) and boron nitride nanosheets (BNNS) as fillers. The synergistic interaction between the 1D BNNT and 2D BNNS allows for improved thermal conductivity via several different mechanisms. The highest thermal conductivity was measured at a loading of 1/30 wt % of BNNT/BNNS, resulting in values of 2.6 and 3.4 Wm-1 K-1, respectively, for each epoxy matrix. This improvement is attributed to the formation of a three-dimensional heat flow path formed through intercalation of the nanotubes between the BNNS. The thermal conductivity of the epoxy 1 and 2 nanocomposites improved by 940 and 1500%, respectively, making them suitable as thermal interface materials in electronic applications requiring electrical resistivity.

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.

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

State-of-the-Art, Opportunities, and Challenges in Bottom-up Synthesis of Polymers with High Thermal Conductivity

In contrast to metals, polymers are predominantly thermal and electrical insulators. With their unparalleled advantages such as light weight, turning polymer insulators into heat conductors with metal-like thermal conductivity is...

Advances in Liquid Crystalline Epoxy Resins for High Thermal Conductivity

Epoxy resin (EP) is one of the most famous thermoset materials. In general, because EP has a three-dimensional random network, it possesses thermal properties similar to those of a typical heat insulator. Recently, there has been substantial interest in controlling the network structure of EP to create new functionalities. Indeed, the modified EP, represented as liquid crystalline epoxy (LCE), is considered promising for producing novel functionalities, which cannot be obtained from conventional EPs, by replacing the random network structure with an oriented one. In this paper, we review the current progress in the field of LCEs and their application to highly thermally conductive composite materials.

Thermal Energy Harvest and Reutilization by the Combination of Thermal Conducting Reactive Mesogens and Heat-Storage Mesogens

Utilizing a newly programmed and synthesized heat storage mesogen (HSM) and reactive mesogen (RM), advanced heat managing polymer alloys that exhibit high thermal conductivity, high latent heat, and phase transition at high temperatures were developed for use as smart thermal energy harvesting and reutilization materials. The RM in the heat-managing RM-HSM polymer alloy was polymerized to form a robust polymeric network with high thermal conductivity. The phase-separated HSM domains between RM polymeric networks absorbed and released a lot of thermal energy in response to changes in the surrounding temperature. For the fabrication of smart heat-managing RM-HSM polymer alloys, the composition and polymerization temperature were optimized based on the constructed phase diagram and thermal energy managing properties of the RM-HSM mixture. From morphological investigation and thermal analysis, it was realized that the heat storage capacity of polymer alloys depends on the size of the phase-separated HSM domain. The structure-morphology-property relationship of the heat managing polymer alloys was built based on the combined techniques of thermal, scattering, and morphological analysis. The newly developed mesogen-based polymer alloys can be used as smart thermal energy-harvesting and reutilization materials.

Homeotropically Aligned Monodomain-like Smectic-A Structure in Liquid Crystalline Epoxy Films: Analysis of the Local Ordering Structure by Microbeam Small-Angle X-ray Scattering

For the development of functional thin films with high thermal conductivity, the local ordering structure of a cured liquid crystalline epoxy resin (LCER) droplet was investigated by using synchrotron radiation microbeam small-angle X-ray scattering. The cured LCER in the vicinity of a substrate with low surface free energy was revealed to form a polydomain smectic-A (SmA) structure in which the normal direction of the layers was random in each domain, although the alignment was planar near the air interface. On the other hand, the cured LCER on a substrate with high surface free energy formed a homeotropically aligned SmA structure in the region within 21 μm from the surface of the substrate. Therefore, a 20 μm thick LCER film was fabricated and found to form a homeotropically aligned monodomain-like SmA structure throughout the whole film with a high thermal conductivity (0.81-5.8 W m^-1 K^-1). This film with a high thermal conductivity is expected to be applicable for adhesion and precoating materials for electrical and electronic devices.

Epoxide and oxetane based liquid crystals for advanced functional materials

Liquid crystalline elastomers (LCEs) and liquid crystalline networks (LCNs) are classes of polymers very suitable for fabricating advanced functional materials. Two main pathways to obtain LCEs and LCNs have gained the most attention in the literature, namely the two-step crosslinking of LC side-chain polymers and the photoinitiated free-radical polymerisation of acrylate LC monomers. These liquid crystal polymers have demonstrated remarkable properties resulting from their anisotropic shapes, being used in soft robotics, responsive surfaces and as photonic materials. In this review, we will show that LCs with cyclic ethers as polymerisable groups can be an attractive alternative to the aforementioned reactive acrylate mesogens. These epoxide and oxetane based reactive mesogens could offer a number of advantages over their acrylate-based counterparts, including oxygen insensitivity, reduced polymerisation shrinkage, improved alignment, lower processing viscosity and potentially extended resistivity. In this review, we summarise the research on these materials from the past 30 years and offer a glimpse into the potential of these cyclic ether mesogens.

Interfacial Thermal Conductivity and Its Anisotropy

There is a significant effort in miniaturizing nanodevices, such as semi-conductors, currently underway. However, a major challenge that is a significant bottleneck is dissipating heat generated in these energy-intensive nanodevices. In addition to being a serious operational concern (high temperatures can interfere with their efficient operation), it is a serious safety concern, as has been documented in recent reports of explosions resulting from many such overheated devices. A significant barrier to heat dissipation is the interfacial films present in these nanodevices. These interfacial films generally are not an issue in macro-devices. The research presented in this paper was an attempt to understand these interfacial resistances at the molecular level, and present possibilities for enhancing the heat dissipation rates in interfaces. We demonstrated that the thermal resistances of these interfaces were strongly anisotropic; i.e., the resistance parallel to the interface was significantly smaller than the resistance perpendicular to the interface. While the latter is well-known—usually referred to as Kapitza resistance—the anisotropy and the parallel component have previously been investigated only for solid-solid interfaces. We used molecular dynamics simulations to investigate the density profiles at the interface as a function of temperature and temperature gradient, to reveal the underlying physics of the anisotropy of thermal conductivity at solid-liquid, liquid-liquid, and solid-solid interfaces.

Liquid Crystal Elastomer Waveguide Actuators

While most photomechanical materials developed to date have relied on free-space illumination to drive actuation, this strategy fails when direct line-of-site access is precluded. In this study, waveguided light is harnessed by liquid crystal elastomer (LCE) nanocomposites to drive actuation. Using photo-chemical reduction of gold salts to plasmonic nanoparticles, prescription of photoresponsive regions within fibers of mono-domain LCEs is demonstrated with control over both the location along the fiber axis, as well as in the azimuthal direction. Due to localized photothermal heating provided by plasmonic absorption of waveguided light and resulting inhomogeneous thermally induced deformation of the LCE, reversible bending along multiple axes is demonstrated.

Anisotropic Thermal Interface Materials: Directional Heat Transfer in Uniaxially Oriented Liquid Crystal Networks

For the development of anisotropic thermal interface materials (TIMs), a rod-shaped reactive monomer PNP-6MA is newly designed and successfully synthesized. PNP-6MA reveals a smectic A (SmA) mesophase between crystalline (K) and isotropic (I) phases. PNP-6MA can be oriented under a magnetic field ( B = 2 T), and its macroscopic orientation can be robustly stabilized by in situ polymerization. Even without macroscopic orientations, the fabricated thermal conducting liquid crystal (TCLC) films show the outstanding thermal conductivity of 1.21 W/m K, which is higher than conventional organic materials. The thermal conductivity of uniaxially and macroscopically oriented TCLC films can be 2.5 W/m K along the long axis of mesogenic core. The newly developed TCLC film can be used as a TIM between a high-power light-emitting diode and a heat sink.

Enhanced Thermal Conductivity of Liquid Crystalline Epoxy Resin using Controlled Linear Polymerization

A powerful strategy to enhance the thermal conductivity of liquid crystalline epoxy resin (LCER) by simply replacing the conventional amine cross-linker with a cationic initiator was developed. The cationic initiator linearly wove the epoxy groups tethered on the microscopically aligned liquid crystal mesogens, resulting in freezing of the ordered LC microstructures even after curing. Owing to the reduced phonon scattering during heat transport through the ordered LC structure, a dramatic improvement in the thermal conductivity of neat cation-cured LCER was achieved to give a value ∼141% (i.e., 0.48 W/mK) higher than that of the amorphous amine-cured LCER. In addition, at the same composite volume fraction in the presence of a 2-D boron nitride filler, an approximately 130% higher thermal conductivity (maximum ∼23 W/mK at 60 vol %) was observed. The nanoarchitecture effect of the ordered LCER on the thermal conductivity was then examined by a systematic investigation using differential scanning calorimetry, polarized optical microscopy, X-ray diffraction, and thermal conductivity measurements. The linear polymerization of LCER can therefore be considered a practical strategy to enable the cost-efficient mass production of heat-dissipating materials, due to its high efficiency and simple process without the requirement for complex equipment.

Highly Oriented Liquid Crystalline Epoxy Film: Robust High Thermal-Conductive Ability

The molecular orientation effect of a liquid crystalline (LC) epoxy resin (LCER) on thermal conductivity was investigated, with the thermal conductivity depending on the surface free energy of amorphous soda-lime-silica glass substrate surfaces modified using physical surface treatments. The LC epoxy monomer was revealed to form a smectic A (SmA) phase with homeotropic alignments on the surfaces of substrates that possess high surface free energies of 71.3 and 72.7 mN m^-1, but forming a planar alignment on the surface of a substrate that possesses a relatively low surface free energy of 46.3 mN m^-1. The optical microscopy observations and the X-ray analyses revealed that the LC epoxy monomer also induced a homeotropically aligned SmA structure due to cross-linking with a curing agent on the high-free-energy surface. The orientational order parameter of the resulting homeotropic SmA structure was calculated from the grazing incidence small-angle X-ray scattering patterns to be 0.73-0.75. The thermal conductivity of the cross-linked LCER forming a homeotropically aligned SmA structure was also estimated to be 2.0 and 5.8 W m^-1 K^-1 for the average and maximum in the direction of the Sm layer normal. The value of the thermal conductivity was remarkable among the thermosetting polymers and ceramic glass, and the LCER could be applied for high-thermal-conductive adhesives and packaging materials in electrical and electronic devices.

Functional Liquid Crystals towards the Next Generation of Materials

Since the discovery of the liquid-crystalline state in 1888, liquid crystal science has made great advances through fusion with various technologies and disciplines. Recently, new molecular design strategies and new self-assembled structures have been developed as a result of the progress made in synthetic procedures and characterization techniques. Since these liquid crystals exhibit new functions and properties derived from their nanostructures and alignment, a variety of new functions for liquid crystals, such as transport for energy applications, separation for environmental applications, chromism, sensing, electrooptical effects, actuation, and templating have been proposed. This Review presents recent advances of liquid crystals that should contribute to the next generation of materials.

Highly Anisotropic Conductors

Composite materials with ordered microstructures often lead to enhanced functionalities that a single material can hardly achieve. Many biomaterials with unusual microstructures can be found in nature; among them, many possess anisotropic and even directional physical and chemical properties. With inspiration from nature, artificial composite materials can be rationally designed to achieve this anisotropic behavior with desired properties. Here, a metallic wood with metal continuously filling the wood vessels is developed, which demonstrates excellent anisotropic electrical, thermal, and mechanical properties. The well-aligned metal rods are confined and separated by the wood vessels, which deliver directional electron transport parallel to the alignment direction. Thus, the novel metallic wood composite boasts an extraordinary anisotropic electrical conductivity (σ_|| /σ_⊥ ) in the order of 10¹¹ , and anisotropic thermal conductivity (κ_|| /κ_⊥ ) of 18. These values exceed the highest reported values in existing anisotropic composite materials. The anisotropic functionality of the metallic wood enables it to be used for thermal management applications, such as thermal insulation and thermal dissipation. The highly anisotropic metallic wood serves as an example for further anisotropic materials design; other composite materials with different biotemplates/hosts and fillers can achieve even higher anisotropic ratios, allowing them to be implemented in a variety of applications.

A homeotropic main-chain tolane-type liquid crystal elastomer film exhibiting high anisotropic thermal conductivity

The development of pure polymeric films with anisotropic thermal conductivities for electronic device packaging applications has attracted intense scientific attention. In order to enhance the polymeric film's normal-direction thermal conductivity, homeotropic alignment of macromolecular chains is the primary concern. One of the promising preparation strategies is to perform in situ photopolymerization of homeotropic-oriented liquid crystal monomers. In this work, we design and synthesize a novel tolane-core thiol-ene-tailed liquid crystal monomer. Benefitting from the conjugated and extended tolane π-system of the mesogenic core and length extension of the terminal aliphatic tails, the normal-to-plane thermal conductivity value and the thermal conductivity anisotropy value of the corresponding cross-linked main-chain end-on liquid crystal polymer (xMELCP) film reach 3.56 W m^-1 K^-1 and 15.0, respectively. Compared with the data of a previously reported ester-type thiol-ene xMELCP film, the two primary values of this novel tolane-type thiol-ene xMELCP material are increased dramatically by 46% and 29%, respectively.

Highly thermal conductive resins formed from wide-temperature-range eutectic mixtures of liquid crystalline epoxies bearing diglycidyl moieties at the side positions

Liquid crystalline epoxy resins with a wide temperature range exhibit a high thermal conductivity of 0.4 W m⁻¹ K⁻¹.

Highly anisotropic thermal conductivity of discotic nematic liquid crystalline films with homeotropic alignment

Homeotropic alignment and photo-crosslinking of discotic nematic liquid crystals yield free-standing films with high in-plane thermal conductivity and thermal conductivity anisotropy.

Heat Transfer Organic Materials: Robust Polymer Films with the Outstanding Thermal Conductivity Fabricated by the Photopolymerization of Uniaxially Oriented Reactive Discogens

For the development of advanced heat transfer organic materials (HTOMs) with excellent thermal conductivities, triphenylene-based reactive discogens, 2,3,6,7,10,11-hexakis(but-3-enyloxy)triphenylene (HABET) and 4,4',4″,4‴,4'''',4'''''-(triphenylene-2,3,6,7,10,11-hexaylhexakis(oxy))hexakis(butane-1-thiol) (THBT), were synthesized as discotic liquid crystal (DLC) monomers and cross-linkers, respectively. A temperature-composition phase diagram of HABET-THBT mixtures was first established based on their thermal and microscopic analyses. From the experimental results, it was realized that the thermal conductivity of DLC HTOM was strongly affected by the molecular organizations on a macroscopic length scale. Macroscopic orientation of self-assembled columns in DLC HTOMs was effectively achieved under the rotating magnetic fields and successfully stabilized by the photopolymerization. The DLC HTOM polymer-stabilized at the LC phase exhibited the remarkable thermal conductivity above 1 W/mK. When the DLC HTOM was macroscopically oriented, the thermal conductivity was estimated to be 3 W/mK along the in-plane direction of DLC molecule. The outstanding thermal conductivity of DLC HTOM should be originated not only from the high content of two-dimensional aromatic discogens but also from the macroscopically oriented and self-assembled DLC. The newly developed DLC HTOM with an outstanding thermal conductivity as well as with an excellent mechanical sustainability can be applied as directional heat dissipating materials in electronic and display devices.

Homeotropically-aligned main-chain and side-on liquid crystalline elastomer films with high anisotropic thermal conductivities

Homeotropically-aligned main-chain and side-on liquid3crystalline elastomer films are reported. Their thermal conductivities in the normal direction are both dramatically higher than those along the horizontal direction.

Biphenyl liquid crystalline epoxy resin as a low-shrinkage resin-based dental restorative nanocomposite

Low-shrinkage resin-based photocurable liquid crystalline epoxy nanocomposite has been investigated with regard to its application as a dental restoration material. The nanocomposite consists of an organic matrix and an inorganic reinforcing filler. The organic matrix is made of liquid crystalline biphenyl epoxy resin (BP), an epoxy resin consisting of cyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (ECH), the photoinitiator 4-octylphenyl phenyliodonium hexafluoroantimonate and the photosensitizer champhorquinone. The inorganic filler is silica nanoparticles (∼70-100 nm). The nanoparticles were modified by an epoxy silane of γ-glycidoxypropyltrimethoxysilane to be compatible with the organic matrix and to chemically bond with the organic matrix after photo curing. By incorporating the BP liquid crystalline (LC) epoxy resin into conventional ECH epoxy resin, the nanocomposite has improved hardness, flexural modulus, water absorption and coefficient of thermal expansion. Although the incorporation of silica filler may dilute the reinforcing effect of crystalline BP, a high silica filler content (∼42 vol.%) was found to increase the physical and chemical properties of the nanocomposite due to the formation of unique microstructures. The microstructure of nanoparticle embedded layers was observed in the nanocomposite using scanning and transmission electron microscopy. This unique microstructure indicates that the crystalline BP and nanoparticles support each other and result in outstanding mechanical properties. The crystalline BP in the LC epoxy resin-based nanocomposite was partially melted during exothermic photopolymerization, and the resin expanded via an order-to-disorder transition. Thus, the post-gelation shrinkage of the LC epoxy resin-based nanocomposite is greatly reduced, ∼50.6% less than in commercialized methacrylate resin-based composites. This LC epoxy nanocomposite demonstrates good physical and chemical properties and good biocompatibility, comparable to commercialized composites. The results indicate that this novel LC nanocomposite is worthy of development and has potential for further applications in clinical dentistry.