High thermal conductivity through simultaneously aligned polyethylene lamellae and graphene nanoplatelets

Graphene/carbohydrate polymer composites as emerging hybrid materials in tumor therapy and diagnosis

Despite the introduction of various types of treatments for cancer control, cancer therapy faces several challenges such as aggressive behavior, heterogeneous characteristics, and the development of resistance. In contrast, the methods have depended on the creation and formulation of nanoparticles to impede tumor growth. Carbon nanoparticles have attracted considerable attention for cancer therapy, with graphene nanoparticles emerging as promising vehicles for delivering drugs and genes. Moreover, graphene composites can enhance immunotherapy, phototherapy, and combination therapies. Nonetheless, the biocompatibility and toxicity of graphene composites present difficulties. Consequently, this manuscript assesses the alteration of graphene nanocomposites using carbohydrate polymers. Altering graphene composites with carbohydrate polymers such as chitosan, hyaluronic acid, cellulose, and starch can enhance their efficacy in cancer treatment. Furthermore, graphene composites functionalized with carbohydrate polymers for tumor ablation induced by phototherapy. Graphene oxide and graphene quantum dots have been modified with carbohydrate polymers to enhance their therapeutic and diagnostic uses. These nanoparticles can transport gene therapy techniques like siRNA in the treatment of cancer. Despite the breakdown of these nanoparticles within the body, they maintain excellent biosafety and biocompatibility.

Superior enhancement in thermal conductivity of epoxy/graphene nanocomposites through use of dimethylformamide (DMF) relative to acetone as solvent

This method article describes the fabrication of graphene-epoxy nanocomposites using two different solvents, dimethylformamide (DMF) and acetone, and validates the resulting thermal conductivity improvements. The study compared the two solvents at a filler composition of 7 wt% and found that DMF resulted in more uniform dispersion of graphene nanoparticles in the epoxy matrix, leading to a 44% improvement in thermal conductivity compared to acetone. Laser scanning confocal microscopy (LSCM) imaging showed that DMF-based composites had more evenly dispersed graphene nanoplatelets than acetone-based composites, which exhibited larger graphene agglomerations. Effective medium theory calculations showed that DMF led to almost 35% lower interface thermal resistance between graphene and epoxy compared to acetone. The validated fabrication method and findings provide new possibilities for developing high thermal conductivity graphene-epoxy nanocomposites for various thermal management applications.•This article describes methods for fabricating graphene-epoxy composites using acetone and DMF as solvents, and validates that DMF is better for achieving higher thermal conductivity in the composite.

Enhanced Electromagnetic Shielding and Thermal Conductive Properties of Polyolefin Composites with a Ti3C2Tx MXene/Graphene Framework Connected by a Hydrogen-Bonded Interface

The rapid increase of operation speed, transmission efficiency, and power density of miniaturized devices leads to a rising demand for electromagnetic interference (EMI) shielding and thermal management materials in the semiconductor industry. Therefore, it is essential to improve both the EMI shielding and thermal conductive properties of commonly used polyolefin components (such as polyethylene (PE)) in electronic systems. Currently, melt compounding is the most common method to fabricate polyolefin composites, but the difficulty of filler dispersion and high resistance at the filler/filler or filler/matrix interface limits their properties. Here, a fold fabrication strategy was proposed to prepare PE composites by incorporation of a well-aligned, seamless graphene framework premodified with MXene nanosheets into the matrix. We demonstrate that the physical properties of the composites can be further improved at the same filler loading by nanoscale interface engineering: the formation of hydrogen bonds at the graphene/MXene interface and the development of a seamlessly interconnected graphene framework. The obtained PE composites exhibit an EMI shielding property of ∼61.0 dB and a thermal conductivity of 9.26 W m^-1 K^-1 at a low filler content (∼3 wt %, including ∼0.4 wt % MXene). Moreover, other thermoplastic composites with the same results can also be produced based on our method. Our study provides an idea toward rational design of the filler interface to prepare high-performance polymer composites for use in microelectronics and microsystems.

The superior effect of edge functionalization relative to basal plane functionalization of graphene in enhancing the thermal conductivity of polymer-graphene nanocomposites - a combined molecular dynamics and Green's functions study

To achieve polymer-graphene nanocomposites with high thermal conductivity (k), it is critically important to achieve efficient thermal coupling between graphene and the surrounding polymer matrix through effective functionalization schemes. In this work, we demonstrate that edge-functionalization of graphene nanoplatelets (GnPs) can enable a larger enhancement of effective thermal conductivity in polymer-graphene nanocomposites relative to basal plane functionalization. Effective thermal conductivity for the edge case is predicted, through molecular dynamics simulations, to be up to 48% higher relative to basal plane bonding for 35 wt% graphene loading with 10 layer thick nanoplatelets. The beneficial effect of edge bonding is related to the anisotropy of thermal transport in graphene, involving very high in-plane thermal conductivity (∼2000 W m^-1 K^-1) compared to the low out-of-plane thermal conductivity (∼10 W m^-1 K^-1). Likewise, in multilayer graphene nanoplatelets (GnPs), the thermal conductivity across the layers is even lower due to the weak van der Waals bonding between each pair of layers. Edge functionalization couples the polymer chains to the high in-plane thermal conduction pathway of graphene, thus leading to overall high thermal conductivity of the composite. Basal-plane functionalization, however, lowers the thermal resistance between the polymer and the surface graphene sheets of the nanoplatelet only, causing the heat conduction through inner layers to be less efficient, thus resulting in the basal plane scheme to be outperformed by the edge scheme. The present study enables fundamentally novel pathways for achieving high thermal conductivity polymer nanocomposites.

Large Enhancement in Thermal Conductivity of Solvent-Cast Expanded Graphite/Polyetherimide Composites

We demonstrate in this work that expanded graphite (EG) can lead to a very large enhancement in thermal conductivity of polyetherimide-graphene and epoxy-graphene nanocomposites prepared via solvent casting technique. A k value of 6.6 W⋅m-1⋅K-1 is achieved for 10 wt% composition sample, representing an enhancement of ~2770% over pristine polyetherimide (k~0.23 W⋅m-1⋅K-1). This extraordinary enhancement in thermal conductivity is shown to be due to a network of continuous graphene sheets over long-length scales, resulting in low thermal contact resistance at bends/turns due to the graphene sheets being covalently bonded at such junctions. Solvent casting offers the advantage of preserving the porous structure of expanded graphite in the composite, resulting in the above highly thermally conductive interpenetrating network of graphene and polymer. Solvent casting also does not break down the expanded graphite particles due to minimal forces involved, allowing for efficient heat transfer over long-length scales, further enhancing overall composite thermal conductivity. Comparisons with a recently introduced effective medium model show a very high value of predicted particle-particle interfacial conductance, providing evidence for efficient interfacial thermal transport in expanded graphite composites. Field emission environmental scanning electron microscopy (FE-ESEM) is used to provide a detailed understanding of the interpenetrating graphene-polymer structure in the expanded graphite composite. These results open up novel avenues for achieving high thermal conductivity polymer composites.

Chemically Edge-Carboxylated Graphene Enhances the Thermal Conductivity of Polyetherimide-Graphene Nanocomposites

In this work, we demonstrate that edge oxidation of graphene can enable larger enhancement in thermal conductivity (k) of graphene nanoplatelet (GnP)/polyetherimide (PEI) composites relative to oxidation of the basal plane of graphene. Edge oxidation offers the advantage of leaving the basal plane of graphene intact, preserving its high in-plane thermal conductivity (k_in > 2000 W m^-1 K^-1), while, simultaneously, the oxygen groups introduced on the graphene edge enhance interfacial thermal conductance through hydrogen bonding with oxygen groups of PEI, enhancing the overall polymer composite thermal conductivity. Edge oxidation is achieved in this work by oxidizing graphene in the presence of sodium chlorate and hydrogen peroxide, thereby introducing an excess of carboxyl groups on the edge of graphene. Basal plane oxidation of graphene, on the other hand, is achieved through the Hummers method, which distorts the sp² carbon-carbon network of graphene, dramatically lowering its intrinsic thermal conductivity, causing the BGO/PEI (BGO = basal-plane oxidized graphene or basal-plane-functionalized graphene oxide) composite's k value to be even lower than pristine GnP/PEI composite's k value. The resulting thermal conductivity of the EGO/PEI (EGO = edge-oxidized graphene or edge-functionalized graphene oxide) composite is found to be enhanced by 18%, whereas that of the BGO/PEI composite is diminished by 57%, with respect to the pristine GnP/PEI composite with 10 wt % GnP content. Two-dimensional Raman mapping of GnPs is used to confirm and distinguish the location of oxygen functional groups on graphene. The superior effect of edge bonding presented in this work can lead to fundamentally novel pathways for achieving high thermal conductivity polymer composites.

Enhanced Thermal Conductivity in Oriented Polyvinyl Alcohol/Graphene Oxide Composites

Polymer composites have attracted increasing interest as thermal management materials for use in devices owing to their ease of processing and potential lower costs. However, most polymer composites have only modest thermal conductivities, even at high concentrations of additives, resulting in high costs and reduced mechanical properties, which limit their applications. To achieve high thermally conductive polymer materials with a low concentration of additives, anisotropic, solid-state drawn composite films were prepared using water-soluble polyvinyl alcohol (PVA) and dispersible graphene oxide (GO). A co-additive (sodium dodecyl benzenesulfonate) was used to improve both the dispersion of GO and consequently the thermal conductivity. The hydrogen bonding between GO and PVA and the simultaneous alignment of GO and PVA in drawn composite films contribute to an improved thermal conductivity (∼25 W m^-1 K^-1), which is higher than most reported polymer composites and an approximately 50-fold enhancement over isotropic PVA (0.3-0.5 W m^-1 K^-1). This work provides a new method for preparing water-processable, drawn polymer composite films with high thermal conductivity, which may be useful for thermal management applications.

Hypergravity-Induced Accumulation: A New, Efficient, and Simple Strategy to Improve the Thermal Conductivity of Boron Nitride Filled Polymer Composites

Thermal conductive polymer composites (filled type) consisting of thermal conductive fillers and a polymer matrix have been widely used in a range of areas. More than 10 strategies have been developed to improve the thermal conductivity of polymer composites. Here we report a new "hypergravity accumulation" strategy. Raw material mixtures of boron nitride/silicone rubber composites were treated in hypergravity fields (800-20,000 g, relative gravity acceleration) before heat-curing. A series of comparison studies were made. It was found that hypergravity treatments could efficiently improve the microstructures and thermal conductivity of the composites. When the hypergravity was about 20,000 g (relative gravity acceleration), the obtained spherical boron nitride/silicone rubber composites had highly compacted microstructures and high and isotropic thermal conductivity. The highest thermal conductivity reached 4.0 W/mK. Thermal interface application study showed that the composites could help to decrease the temperature on a light-emitting diode (LED) chip by 5 °C. The mechanism of the improved microstructure increased thermal conductivity, and the high viscosity problem in the preparation of boron nitride/silicone rubber composites, and the advantages and disadvantages of the hypergravity accumulation strategy, were discussed. Overall, this work has provided a new, efficient, and simple strategy to improve the thermal conductivity of boron nitride/silicone rubber and other polymer composites (filled type).

Thermally enhanced polyolefin composites: fundamentals, progress, challenges, and prospects

The low thermal conductivity of polymers is a barrier to their use in applications requiring high thermal conductivity such as electronic packaging, heat exchangers, and thermal management devices. Polyolefins represent about 55% of global thermoplastic production, and therefore improving their thermal conductivity is essential for many applications. This review analyzes the advances in enhancing the thermal conductivity of polyolefin composites. First, the mechanisms of thermal transport in polyolefin composites and the key parameters that govern conductive heat transfer through the interface between the matrix and the filler are discussed. Then, the advantage and limitations of the current methods for measuring thermal conductivity are analyzed. Moreover, the progress in predicting the thermal conductivity of polymer composites using modeling and simulation is discussed. Furthermore, polyolefin composites and nanocomposites with different thermally conductive fillers are reviewed and analyzed. Finally, the key challenges and future directions for developing thermally enhanced polyolefin composites are outlined.

Lightweight Porous Polystyrene with High Thermal Conductivity by Constructing 3D Interconnected Network of Boron Nitride Nanosheets

A composite foam consisting of foamed cross-linking polystyrene (c-PS) and boron nitride nanosheets (BNNSs) was synthesized, which shows a higher thermal conductivity (TC) than the corresponding solid counterparts. The BNNS fillers are found to be aligned along the cell wall as a result of the biaxial stress field from cell expansion during the formation of three-dimensional interconnectivity in the foams, resulting in an enhanced TC of 1.28 W/m K, nearly two and four times those of its solid counterpart and pure c-PS, respectively. It is found that the foaming-assisted formation of the filler network is an efficient strategy to improve the TC at low filler loadings in the composites. Furthermore, the composite foams exhibit low density, rather low dielectric constants and dissipation factors at wide frequency and temperature ranges. The present work provides a novel approach to design and prepare lightweight heat conductive polymers with low filler loadings as low-density heat management materials for potential applications in aeronautics and aerospace components.

Strong Reinforcement Effects in 2D Cellulose Nanofibril-Graphene Oxide (CNF-GO) Nanocomposites due to GO-Induced CNF Ordering

Nanocomposites from native cellulose with low 2D nanoplatelet content are of interest as sustainable materials combining functional and structural performance. Cellulose nanofibril-graphene oxide (CNF-GO) nanocomposite films are prepared by a physical mixing-drying method, with focus on low GO content, the use of very large GO platelets (2-45μm) and nanostructural characterization using synchrotron x-ray source for WAXS and SAXS. These nanocomposites can be used as transparent coatings, strong films or membranes, as gas barriers or in laminated form. CNF nanofibrils with random in-plane orientation, form a continuous non-porous matrix with GO platelets oriented in-plane. GO reinforcement mechanisms in CNF are investigated, and relationships between nanostructure and suspension rheology, mechanical properties, optical transmittance and oxygen barrier properties are investigated as a function of GO content. A much higher modulus reinforcement efficency is observed than in previous polymer-GO studies. The absolute values for modulus and ultimate strength are as high as 17 GPa and 250 MPa at a GO content as small as 0.07 vol%. The remarkable reinforcement efficiency is due to improved organization of the CNF matrix; and this GO-induced mechanism is of general interest for nanostructural tailoring of CNF-2D nanoplatelet composites.

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.

Effect of Alignment on Enhancement of Thermal Conductivity of Polyethylene-Graphene Nanocomposites and Comparison with Effective Medium Theory

Thermal conductivity (k) of polymers is usually limited to low values of ~0.5 Wm⁻¹K⁻¹ in comparison to metals (>20 Wm⁻¹K⁻¹). (100)_T3//(926)_T4 The goal of this work is to enhance thermal conductivity (k) of polyethylene–graphene nanocomposites through simultaneous alignment of polyethylene (PE) lamellae and graphene nanoplatelets (GnP). Alignment is achieved through the application of strain. Measured values are compared with predictions from effective medium theory. A twin conical screw micro compounder is used to prepare polyethylene–graphene nanoplatelet (PE-GnP) composites. Enhancement in k value is studied for two different compositions with GnP content of 9 wt% and 13 wt% and for applied strains ranging from 0% to 300%. Aligned PE-GnP composites with 13 wt% GnP displays ~1000% enhancement in k at an applied strain of 300%, relative to k of pristine unstrained polymer. Laser Scanning Confocal Microscopy (LSCM) is used to quantitatively characterize the alignment of GnP flakes in strained composites; this measured orientation is used as an input for effective medium predictions. These results have important implications for thermal management applications.

High-Performance Thermally Conductive Phase Change Composites by Large-Size Oriented Graphite Sheets for Scalable Thermal Energy Harvesting

Efficient thermal energy harvesting using phase-change materials (PCMs) has great potential for cost-effective thermal management and energy storage applications. However, the low thermal conductivity of PCMs (K_PCM ) is a long-standing bottleneck for high-power-density energy harvesting. Although PCM-based nanocomposites with an enhanced thermal conductivity can address this issue, achieving a higher K (>10 W m^-1 K^-1 ) at filler loadings below 50 wt% remains challenging. A strategy for synthesizing highly thermally conductive phase-change composites (PCCs) by compression-induced construction of large aligned graphite sheets inside PCCs is demonstrated. The millimeter-sized graphite sheet consists of lateral van-der-Waals-bonded and oriented graphite nanoplatelets at the micro/nanoscale, which together with a thin PCM layer between the sheets synergistically enhance K_PCM in the range of 4.4-35.0 W m^-1 K^-1 at graphite loadings below 40.0 wt%. The resulting PCCs also demonstrate homogeneity, no leakage, and superior phase change behavior, which can be easily engineered into devices for efficient thermal energy harvesting by coordinating the sheet orientation with the thermal transport direction. This method offers a promising route to high-power-density and low-cost applications of PCMs in large-scale thermal energy storage, thermal management of electronics, etc.

Bionic Fish-Scale Surface Structures Fabricated via Air/Water Interface for Flexible and Ultrasensitive Pressure Sensors

In recent years, wearable and flexible sensors have attracted considerable research interest and effort owing to their broad application prospects in wearable devices, robotics, health monitoring, and so on. High-sensitivity and low-cost pressure sensors are the primary requirement in practical application. Herein, a convenient and low-cost process to fabricate a bionic fish-scale structure poly(dimethylsiloxane) (PDMS) film via air/water interfacial formation technique is presented. High-sensitivity flexible pressure sensors can be constructed by assembling conductive films of graphene nanosheets into a microstructured film. Thanks to the unique fish-scale structures of PDMS films, the prepared pressure sensor shows excellent performance with high sensitivity (-70.86% kPa^-1). In addition, our pressure sensors can detect weak signals, such as wrist pulses, respiration, and voice vibrations. Moreover, the whole process of pressure sensor preparation is cost-effective, eco-friendly, and controllable. The results indicate that the prepared pressure sensor has a profitable and efficient advantage in future applications for monitoring human physiological signals and sensing subtle touch, which may broaden its potential applications in wearable devices.