Novel Polyethylene Fibers of Very High Thermal Conductivity Enabled by Amorphous Restructuring

Reversible two-way tuning of thermal conductivity in an end-linked star-shaped thermoset

Polymeric thermal switches that can reversibly tune and significantly enhance their thermal conductivities are desirable for diverse applications in electronics, aerospace, automotives, and medicine; however, they are rarely achieved. Here, we report a polymer-based thermal switch consisting of an end-linked star-shaped thermoset with two independent thermal conductivity tuning mechanisms-strain and temperature modulation-that rapidly, reversibly, and cyclically modulate thermal conductivity. The end-linked star-shaped thermoset exhibits a strain-modulated thermal conductivity enhancement up to 11.5 at a fixed temperature of 60 °C (increasing from 0.15 to 2.1 W m-1 K-1). Additionally, it demonstrates a temperature-modulated thermal conductivity tuning ratio up to 2.3 at a fixed stretch of 2.5 (increasing from 0.17 to 0.39 W m-1 K-1). When combined, these two effects collectively enable the end-linked star-shaped thermoset to achieve a thermal conductivity tuning ratio up to 14.2. Moreover, the end-linked star-shaped thermoset demonstrates reversible tuning for over 1000 cycles. The reversible two-way tuning of thermal conductivity is attributed to the synergy of aligned amorphous chains, oriented crystalline domains, and increased crystallinity by elastically deforming the end-linked star-shaped thermoset.

Dressing Paraffin Wax/Boron Nitride Phase Change Composite with a Polyethylene "Underwear" for the Reliable Battery Safety Management

Phase change material (PCM) can provide a battery system with a buffer platform to respond to thermal failure problems. However, current PCMs through compositing inorganics still suffer from insufficient thermal-transport behavior and safety reliability against external force. Herein, a best-of-both-worlds method is reported to allow the PCM out of this predicament. It is conducted by combining a traditional PCM (i.e., paraffin wax/boron nitride) with a spirally weaved polyethylene fiber fabric, just like the traditional PCM is wearing functional underwear. On the one hand, the spirally continuous thermal pathways of polyethylene fibers in the fabric collaborate with the boron nitride network in the PCM, enhancing the through-plane and in-plane thermal conductivity to 10.05 and 7.92 W m-1 K, respectively. On the other, strong polyethylene fibers allow the PCM to withstand a high puncture strength of 47.13 N and tensile strength of 18.45 MPa although above the phase transition temperature. After this typical PCM packs a triple Li-ion battery system, the battery can be promised reliable safety management against both thermal and mechanical abuse. An obvious temperature drop of >10 °C is observed in the battery electrode during the cycling charging and discharging process.

Advanced Cooling Textiles: Mechanisms, Applications, and Perspectives

High-temperature environments pose significant risks to human health and safety. The body's natural ability to regulate temperature becomes overwhelmed under extreme heat, leading to heat stroke, dehydration, and even death. Therefore, the development of effective personal thermal-moisture management systems is crucial for maintaining human well-being. In recent years, significant advancements have been witnessed in the field of textile-based cooling systems, which utilize innovative materials and strategies to achieve effective cooling under different environments. This review aims to provide an overview of the current progress in textile-based personal cooling systems, mainly focusing on the classification, mechanisms, and fabrication techniques. Furthermore, the challenges and potential application scenarios are highlighted, providing valuable insights for further advancements and the eventual industrialization of personal cooling textiles.

Processing and Characterization of UV Irradiated HDPE/POSS Fibers

High-performance polyethylene fibers, renowned for their superior attributes encompassing a high strength, modulus, and lightness, are conventionally manufactured through the gel spinning method. However, this method is encumbered by several drawbacks, including the requisite application of a separate process to eliminate solvents from the fibers and the utilization of chemicals deleterious to both the environment and human health. Alternatively, the adoption of the melt spinning method presents a cleaner and environmentally friendly approach to attain high-performance fibers. In the present investigation, high-density polyethylene (HDPE) fibers were produced employing the melt spinning method. After the spinning process, strategic orientation procedures were implemented to enhance the crystallinity of the spun fibers. As a concluding step, seeking to elevate the overall performance of the oriented spun HDPE fibers, a cross-linking treatment was applied via UV irradiation. Notably, this study pioneers the incorporation of polyhedral oligomeric silsesquioxane (POSS) hybrid nanoparticles into HDPE during melt spinning, presenting a novel advancement aimed at further enhancing the mechanical properties of oriented HDPE fibers during UV irradiation. For this purpose, two distinct types of POSS, namely octavinyl POSS (OVPOSS) and methacryl POSS (MACPOSS), both having unsaturated double bonds capable of participating in the network structure of oriented HDPE spun during UV cross-linking, were used. The thermal, morphological, and mechanical properties, as well as the crystal structure of samples with and without POSS molecules, were investigated. The mechanical properties of the fibers exhibited higher values in the presence of OVPOSS. The incorporation of OVPOSS and MACPOSS resulted in a noteworthy improvement in the material's tensile strength, exhibiting a marked increase of 12.5 and 70.8%, respectively. This improvement can be attributed to the more homogeneous dispersion of OVPOSS in HDPE, actively participating in the three-dimensional network structure. After orientation and UV irradiation, the tensile strength of HDPE fibers incorporating OVPOSS increased to 293 MPa, accompanied by a concurrent increase in the modulus to 2.8 GPa. The addition of POSS nanoparticles thus yielded a substantial improvement in the overall performance of HDPE fibers.

Study into the Fire and Explosion Characteristics of Polymer Powders Used in Engineering Production Technologies

Polymers and their processing by engineering production technologies (injection, molding or additive manufacturing) are increasingly being used. Polymers used in engineering production technologies are constantly being developed and their properties are being improved. Granulometry, X-ray, FTIR and TGA were used to characterize polymer samples. Determination of the fire parameters of powder samples of polyamide (PA) 12, polypropylene, and ultra-high molecular weight (UHMW) polyethylene is the subject of the current article. An explosive atmosphere can be created by the powder form of these polymer materials, and introduction of preventive safeguards to ensure safety is required for their use. Although the fire parameters of these basic types of polymers are available in databases (e.g., GESTIS-DustEx), our results showed that one of the samples used (polypropylene) was not flammable and thus is safe for use in terms of explosiveness. Two samples were flammable and explosive. The lower explosive limit was 30 g·m-3 (PA12) and 60 g·m-3 (UHMW polyethylene). The maximum explosion pressure of the samples was 6.47 (UHMW polyethylene) and 6.76 bar (PA12). The explosion constant, Kst, of the samples was 116.6 bar·m·s-1 (PA12) and 97.1 bar·m·s-1 (UHMW polyethylene). Therefore, when using polymers in production technologies, it is necessary to know their fire parameters, and to design effective explosion prevention (e.g., ventilation, explosive-proof material, etc.) measures for flammable and explosive polymers.

The Effect of Mechanical Elongation on the Thermal Conductivity of Amorphous and Semicrystalline Thermoplastic Polyimides: Atomistic Simulations

Over the past few decades, the enhancement of polymer thermal conductivity has attracted considerable attention in the scientific community due to its potential for the development of new thermal interface materials (TIM) for both electronic and electrical devices. The mechanical elongation of polymers may be considered as an appropriate tool for the improvement of heat transport through polymers without the necessary addition of nanofillers. Polyimides (PIs) in particular have some of the best thermal, dielectric, and mechanical properties, as well as radiation and chemical resistance. They can therefore be used as polymer binders in TIM without compromising their dielectric properties. In the present study, the effects of uniaxial deformation on the thermal conductivity of thermoplastic PIs were examined for the first time using atomistic computer simulations. We believe that this approach will be important for the development of thermal interface materials based on thermoplastic PIs with improved thermal conductivity properties. Current research has focused on the analysis of three thermoplastic PIs: two semicrystalline, namely BPDA-P3 and R-BAPB; and one amorphous, ULTEM^TM. To evaluate the impact of uniaxial deformation on the thermal conductivity, samples of these PIs were deformed up to 200% at a temperature of 600 K, slightly above the melting temperatures of BPDA-P3 and R-BAPB. The thermal conductivity coefficients of these PIs increased in the glassy state and above the glass transition point. Notably, some improvement in the thermal conductivity of the amorphous polyimide ULTEM^TM was achieved. Our study demonstrates that the thermal conductivity coefficient is anisotropic in different directions with respect to the deformation axis and shows a significant increase in both semicrystalline and amorphous PIs in the direction parallel to the deformation. Both types of structural ordering (self-ordering of semicrystalline PI and mechanical elongation) led to the same significant increase in thermal conductivity coefficient.

Adhesion Properties and Stability of Non-Polar Polymers Treated by Air Atmospheric-Pressure Plasma

Atmospheric-pressure plasma (APP) has advantages for enhancing the adhesion of polymers and has to provide uniform, efficient treatment, which also limits the recovery effect of treated surfaces. This study investigates the effects of APP treatment on polymers that have no oxygen bonded in their structure and varying crystallinity, aiming to assess the maximum level of modification and the post-treatment stability of non-polar polymers based on their initial structure parameters, including the crystalline-amorphous structure. An APP reactor simulating continuous processing operating in air is employed, and the polymers are analyzed using contact angle measurement, XPS, AFM, and XRD. APP treatment significantly enhances the hydrophilic character of the polymers, with semicrystalline polymers exhibiting adhesion work values of approximately 105 mJ/m² and 110 mJ/m² for 0.5 s and 1.0 s exposure, respectively, while amorphous polymers reach approximately 128 mJ/m². The maximum average oxygen uptake is around 30%. Short treatment times induce the roughening of the semicrystalline polymer surfaces, while the amorphous polymer surfaces become smoother. The polymers exhibit a limit to their modification level, with 0.5 s exposure being optimal for significant surface property changes. The treated surfaces remain remarkably stable, with the contact angle only reverting by a few degrees toward that of the untreated state.

Research on In Situ Thermophysical Properties Measurement during Heating Processes

Biomass pyrolysis is an important way to produce biofuel. It is a chemical reaction process significantly involving heat, in which the heating rate will affect the yield and composition (or quality) of the generated biofuel. Therefore, the heat transfer inside the biomass pellets is important for determining the rate of temperature rise in the pellets. The accurate knowledge of the thermophysical properties of biomass pellets is required to clarify the process and mechanism of heat transfer in the particles and in the reactor. In this work, based on the transient thermoelectric technology, a continuous in situ thermal characterization method for a dynamic heating process is proposed. Multiple thermophysical properties, including thermal conductivity and volumetric heat capacity for corn leaves, are measured simultaneously within a heating process. In temperatures lower than 100 °C, the volumetric heat capacity slightly increases while the thermal conductivity decreases gradually due to the evaporation of water molecules. When the temperature is higher than 100 °C, the organic components in the corn leaves are cracked and carbonized, leading to the increase in the thermal conductivity and the decrease in the volumetric heat capacity against temperature.

Enhanced High Thermal Conductivity Cellulose Filaments via Hydrodynamic Focusing

Nanocellulose is regarded as a green and renewable nanomaterial that has attracted increased attention. In this study, we demonstrate that nanocellulose materials can exhibit high thermal conductivity when their nanofibrils are highly aligned and bonded in the form of filaments. The thermal conductivity of individual filaments, consisting of highly aligned cellulose nanofibrils, fabricated by the flow-focusing method is measured in dried condition using a T-type measurement technique. The maximum thermal conductivity of the nanocellulose filaments obtained is 14.5 W/m-K, which is approximately five times higher than those of cellulose nanopaper and cellulose nanocrystals. Structural investigations suggest that the crystallinity of the filament remarkably influence their thermal conductivity. Smaller diameter filaments with higher crystallinity, that is, more internanofibril hydrogen bonds and less intrananofibril disorder, tend to have higher thermal conductivity. Temperature-dependence measurements also reveal that the filaments exhibit phonon transport at effective dimension between 2D and 3D.

Progress of Polymer-Based Thermally Conductive Materials by Fused Filament Fabrication: A Comprehensive Review

With the miniaturization and integration of electronic products, the heat dissipation efficiency of electronic equipment needs to be further improved. Notably, polymer materials are a choice for electronic equipment matrices because of their advantages of low cost and wide application availability. However, the thermal conductivity of polymers is insufficient to meet heat dissipation requirements, and their improvements remain challenging. For decades, as an efficient manufacturing technology, additive manufacturing has gradually attracted public attention, and researchers have also used this technology to produce new thermally conductive polymer materials. Here, we review the recent research progress of different 3D printing technologies in heat conduction and the thermal conduction mechanism of polymer matrix composites. Based on the classification of fillers, the research progress of thermally conductive materials prepared by fused filament fabrication (FFF) is discussed. It analyzes the internal relationship between FFF process parameters and the thermal conductivity of polymer matrix composites. Finally, this study summarizes the application and future development direction of thermally conductive composites by FFF.

Recent advances in thermally conductive polymer composites

Polymer matrix composites (PMCs) with high thermal conductivity (TC) play an important role in improving the heat dissipation capacity of a new generation of electronic devices, particularly for 5G and aviation applications. Over the last few decades, considerable efforts have been made in the fabrication of highly thermally conductive PMCs. Advances in the thermal conduction mechanism of polymer composites are induced to, and then commonly used thermally conductive fillers are presented. In the following, the factors affecting the TC of polymer composites are discussed in detail, including fillers, interfaces, polymer matrices and processing technologies. Special attention is paid to the thermally conductive fillers. Then, some application areas of thermally conductive polymer composites are introduced. Finally, the deficiencies and future development trends in this research field are put forward. It is expected that this review will provide some beneficial inspiration in improving the TC of PMCs.

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...

Phonon-engineered extreme thermal conductivity materials

Materials with ultrahigh or low thermal conductivity are desirable for many technological applications, such as thermal management of electronic and photonic devices, heat exchangers, energy converters and thermal insulation. Recent advances in simulation tools (first principles, the atomistic Green's function and molecular dynamics) and experimental techniques (pump-probe techniques and microfabricated platforms) have led to new insights on phonon transport and scattering in materials and the discovery of new thermal materials, and are enabling the engineering of phonons towards desired thermal properties. We review recent discoveries of both inorganic and organic materials with ultrahigh and low thermal conductivity, highlighting heat-conduction physics, strategies used to change thermal conductivity, and future directions to achieve extreme thermal conductivities in solid-state materials.

On the Structural Peculiarities of Self-Reinforced Composite Materials Based on UHMWPE Fibers

The structure of self-reinforced composites (SRCs) based on ultra-high molecular weight polyethylene (UHMWPE) was studied by means of Wide-Angle X-ray Scattering (WAXS), X-ray tomography, Raman spectroscopy, Scanning Electron Microscopy (SEM) and in situ tensile testing in combination with advanced processing tools to determine the correlation between the processing conditions, on one hand, and the molecular structure and mechanical properties, on the other. SRCs were fabricated by hot compaction of UHMWPE fibers at different pressure and temperature combinations without addition of polymer matrix or softener. It was found by WAXS that higher compaction temperatures led to more extensive melting of fibers with the corresponding reduction of the Herman's factor reflecting the degree of molecular orientation, while the increase of hot compaction pressure suppressed the melting of fibers within SRCs at a given temperature. X-ray tomography proved the absence of porosity while polarized light Raman spectroscopy measurements for both longitudinal and perpendicular fiber orientations showed qualitatively the anisotropy of SRC samples. SEM revealed that the matrix was formed by interlayers of molten polymer entrapped between fibers in SRCs. Moreover, in situ tensile tests demonstrated the increase of Young's modulus and tensile strength with increasing temperature.

Thermal and mechanical characterization of high performance polymer fabrics for applications in wearable devices

With advances in flexible and wearable device technology, thermal regulation will become increasingly important. Fabrics and substrates used for such applications will be required to effectively spread any heat generated in the devices to ensure user comfort and safety, while also preventing overheating of the electronic components. Commercial fabrics consisting of ultra-high molecular weight polyethylene (UHMW-PE) fibers are currently used in personal body armor and sports gear owing to their high strength, durability, and abrasion resistance. In addition to superior mechanical properties, UHMW-PE fibers exhibit very high axial thermal conductivity due to a high degree of polymer chain orientation. However, these materials have not been widely explored for thermal management applications in flexible and wearable devices. Assessment of their suitability for such applications requires characterization of the thermal and mechanical properties of UHMW-PE in the fabric form that will ultimately be used to construct heat spreading materials. Here, we use advanced techniques to characterize the thermal and mechanical properties of UHMW-PE fabrics, as well as other conventional flexible materials and fabrics. An infrared microscopy-based approach measures the effective in-plane thermal conductivity, while an ASTM-based bend testing method quantifies the bending stiffness. We also characterize the effective thermal behavior of fabrics when subjected to creasing and thermal annealing to assess their reliability for relevant practical engineering applications. Fabrics consisting of UHMW-PE fibers have significantly higher thermal conductivities than the benchmark conventional materials while possessing good mechanical flexibility, thereby showcasing great potential as substrates for flexible and wearable heat spreading application.

The Role of Polyethylene Wax on the Thermal Conductivity of Transparent Ultradrawn Polyethylene Films

Transparency and thermal conductivity of ultradrawn, ultrahigh-molecular-weight polyethylene films containing different contents of low-molecular-weight polyethylene wax (PEwax) are explored from experimental and theoretical viewpoints. It is shown that the addition of PEwax decreases light scattering in all directions, resulting from a reduction of defects while having little effect on crystallinity or chain orientation of ultradrawn films. In general, upon the addition of PEwax, the thermal conductivity of ultradrawn films increases with the highest conductivity being 47 (W m-1 K-1) and subsequently decreases at higher concentrations. The thermal conductivity also depends on draw ratio and number-average molecular weight (Mn) of the films. A model is presented which correlates the thermal conductivity of the films with the draw ratio and Mn, enabling explanation of the experimental results. Hence, the thermal conductivity of ultradrawn polyethylene films can be predicted as a function of Mn and draw ratio.

Characterization of Anisotropic Properties of Hot Compacted Self-Reinforced Composites (SRCs) via Thermal Diffusivity Measurement

The mechanical properties of self-reinforced composites (SRCs) produced in a hot compaction process significantly depend on the process parameters. Only a little deviation of the process temperature or pressure causes the component to act differently under mechanical load. This is a chance and a challenge at the same time, since this process is difficult to handle but by properly controlling the process parameters, the mechanical properties can be adjusted, even locally for one component. In this research SRC are manufactured in a hot compaction process. A correlation between process parameters and density is found. Density increased from 0,8 to 0,91 g/cm³ by increasing temperature and pressure in the hot compaction process. The different thermal properties in the direction of orientation (IP) and transverse to orientation (TP) are measured with a laser flash device. It was found that, due to a change in density and molecular orientation, diffusivity and conductivity are influenced in different degrees in IP and TP directions. For interpretation of thermal measurement results, microstructures are analysed with a confocal laser scanning microscope after preparing the specimen with a permanganate etching. A schematic model of conductive path is worked out and discussed. With measurement data the anisotropy of IP and TP diffusivity is calculated, and a model is built to describe relative density related to anisotropy. The highest anisotropy between IP and TP diffusivity was calculated with a ratio of 6 at a relative density of approximately 0,82 g/cm³. Since mechanical properties in correlation to process parameters have already been investigated, results of this investigation, in combination with previous research on mechanical properties, will enable the development of a non-destructive testing method for SRCs by measuring the thermal diffusivity.

Nanostructured polymer films with metal-like thermal conductivity

Due to their unique properties, polymers - typically thermal insulators - can open up opportunities for advanced thermal management when they are transformed into thermal conductors. Recent studies have shown polymers can achieve high thermal conductivity, but the transport mechanisms have yet to be elucidated. Here we report polyethylene films with a high thermal conductivity of 62 Wm-1 K-1, over two orders-of-magnitude greater than that of typical polymers (~0.1 Wm-1 K-1) and exceeding that of many metals and ceramics. Structural studies and thermal modeling reveal that the film consists of nanofibers with crystalline and amorphous regions, and the amorphous region has a remarkably high thermal conductivity, over ~16 Wm-1 K-1. This work lays the foundation for rational design and synthesis of thermally conductive polymers for thermal management, particularly when flexible, lightweight, chemically inert, and electrically insulating thermal conductors are required.

Unusual strain-dependent thermal conductivity modulation of silver nanoflower-polyurethane fibers

Thermal management of stretchable and wearable electronic devices is an important issue in enhancing performance, reliability, and human thermal comfort. Here, we constructed a unique experimental setup which investigated the strain-dependent thermal conductivity. The thermal conductivity of flower-shaped silver nanoparticle (silver nanoflower)-polyurethane (Ag-PU) composite fibers was systematically investigated as a function of strain. The strain-dependent temperature distribution of the Joule-heated fiber was measured using an infrared camera, and the thermal conductivity was obtained from the 1-dimensional Fourier's conduction model. There was a monotonic decrease in both lattice and electronic thermal conductivity with stretching at 25 °C. However, there was an initial increase in lattice and total thermal conductivity in the low strain region (<10%), when the fiber was stretched at 45 °C, although the electronic thermal conductivity decreased monotonically. The softening of the polymer at increased temperatures enhanced Poisson's ratio. Resultantly, the fiber cross-sectional area and radial-direction inter-particle distance between silver nanoflowers decreased. This could increase the thermal transport in conductive fibers by modulating the interfaces between silver nanoflowers and polyurethane. A further stretching decreased the lattice thermal conductivity due to the significantly increased axial distance between silver nanoflowers and the decreased filler fraction. The weft-knitted fabric also demonstrated an increased thermal conductance in the low strain region (≤30%) at 45 °C.

Crystalline polymer nanofibers with ultra-high strength and thermal conductivity

Polymers are widely used in daily life, but exhibit low strength and low thermal conductivity as compared to most structural materials. In this work, we develop crystalline polymer nanofibers that exhibit a superb combination of ultra-high strength (11 GPa) and thermal conductivity, exceeding any existing soft materials. Specifically, we demonstrate unique low-dimensionality phonon physics for thermal transport in the nanofibers by measuring their thermal conductivity in a broad temperature range from 20 to 320 K, where the thermal conductivity increases with increasing temperature following an unusual ~T¹ trend below 100 K and eventually peaks around 130–150 K reaching a metal-like value of 90 W m⁻¹ K⁻¹, and then decays as 1/T. The polymer nanofibers are purely electrically insulating and bio-compatible. Combined with their remarkable lightweight-thermal-mechanical concurrent functionality, unique applications in electronics and biology emerge.

Polymers compared to structural materials usually have low strength and thermal conductivity. Here the authors show a fabrication method to form bio-compatible crystalline polyethylene nanofibers that exhibit ultra-high strength, thermal conductivity and electrical insulation.