Broadband Dielectric Spectroscopy Research Articles

Insight into the relationship between interfacial structure and thermal conductivity in epoxy resin/Al2O3 composite

AbstractThe relationship between interfacial structure and thermal conductivity in epoxy resin/Al2O3 composite is important but is still elusive. Here, we illuminate this issue by investigating the role of four silane coupling agents in thermal conductivity for epoxy resin/Al2O3 composites. The results show that 3‐glycidoxypropyltrimethoxysilane (KH560) is the best silane coupling agent to modify Al2O3, resulting in the lowest viscosity and the highest thermal conductivity of the epoxy resin/Al2O3‐KH560 composite compared with alternative systems. This is attributed to the strong interfacial interaction between Al2O3‐KH560 and epoxy resin, as confirmed by the Fowkes model and broadband dielectric spectroscopy. The interfacial thermal resistance between the epoxy resin and Al2O3 is also quantitatively measured via a photothermal radiometry technique. The epoxy resin/Al2O3‐KH560 also has the lowest interfacial thermal resistance (1.50 ± 0.02 × 10−6 m2 K W−1), compared with the other three systems. This study advances the understanding of the relationship between interfacial structure and thermal conductivity in polymer composites.Highlights The relationship between interfacial structure and thermal conductivity in epoxy resin/Al2O3 composite is understood. The interfacial structure is characterized by broadband dielectric spectroscopy. The interfacial thermal resistance is quantitatively measured via a photothermal radiometry technique.

The Influence of PVP Polymer Topology on the Liquid Crystalline Order of Itraconazole in Binary Systems.

This study presents a novel approach by utilizing poly(vinylpyrrolidone)s (PVPs) with various topologies as potential matrices for the liquid crystalline (LC) active pharmaceutical ingredient itraconazole (ITZ). We examined amorphous solid dispersions (ASDs) composed of ITZ and (i) self-synthesized linear PVP, (ii) self-synthesized star-shaped PVP, and (iii) commercial linear PVP K30. Differential scanning calorimetry, X-ray diffraction, and broad-band dielectric spectroscopy were employed to get a comprehensive insight into the thermal and structural properties, as well as global and local molecular dynamics of ITZ-PVP systems. The primary objective was to assess the influence of PVPs' topology and the composition of ASD on the LC ordering, changes in the temperature of transitions between mesophases, the rate of their restoration, and finally the solubility of ITZ in the prepared ASDs. Our research clearly showed that regardless of the PVP type, both LC transitions, from smectic (Sm) to nematic (N) and from N to isotropic (I) phases, are effectively suppressed. Moreover, a significant difference in the miscibility of different PVPs with the investigated API was found. This phenomenon also affected the solubility of API, which was the greatest, up to 100 μg/mL in the case of starPVP 85:15 w/w mixture in comparison to neat crystalline API (5 μg/mL). Obtained data emphasize the crucial role of the polymer's topology in designing new pharmaceutical formulations.

Segmental Mobility in Linear Polylactides of Various Molecular Weights

In this work we synthesized and studied a series of linear polylactides (PLA) with a fixed L/D -lactide stereoisomer ratio, covering together the wide range of molecular weights, Mn, from ∼6 to ∼80 kg/mol. The basic aims are the segmental mobility mapping, in terms of time scale and fragility, the recording of the Mn dependences of the glass transition temperature and fragility, and, finally, the comparison with corresponding findings from the literature. Obviously, the direct Mn effects on Tg are assessed by preserving PLA at the amorphous state, i.e., via melting and relatively fast cooling. The segmental mobility is studied by conventional and temperature modulated calorimetry, whereas the molecular dynamics is assessed employing the technique of broadband dielectric spectroscopy in combination with proper critical analysis and evaluation routes. The calorimetric and dielectric Tg(Mn) trends were found consistent between each other as well as with previous findings in similar systems, both with respect to experimental findings and theoretic predictions for PLAs of similar L/D ratio. The Mn threshold of entanglements seems to lay between 13 and 28 kg/mol. The fragility index of the α relaxation, i.e., the dielectric analogue of the glass transition, exhibits a similar dependence to that of Tg(Mn). When decreasing Mn to very low values, the high fragility (∼150) drops down to 0 (Mn∼6 kg/mol), denoting the vanishing of the polymer chains’ cooperativity. Qualitatively interesting alternations are recorded in the dielectric strength of the α relaxation and the relaxation times width. The effects are discussed in terms of the various parameters co-affecting the polymer dynamics, namely, chain lengths, free volume and dynamical heterogeneities.

Normal-to-Supercooled Liquid Transition in Molecular Glass-Formers: A Hidden Structural Transformation Fuelled by Conformational Interconversion.

Molecular dynamics and transport coefficients change significantly around the so-called Arrhenius crossover in glass-forming systems. In this article, we revisit the dynamic processes occurring in a glass-forming macrocyclic crown thiaether MeBzS2O above its glass transition, revealing two crossover temperatures: TB at 309 and TA at 333 K. We identify the second one as the Arrhenius crossover that is closely related to the normal-to-supercooled liquid transition in this compound. We show that the transformation occurring at this point goes far beyond molecular dynamics (where the temperature dependence of structural relaxation times changes its character from activation-like to super-Arrhenius), being reflected also in the internal structure and diffraction pattern. In this respect, we found a twofold local organization of the nearest-neighbor molecules via weak van der Waals forces, without the formation of any medium-range order or mesophases. The nearest surrounding of each molecule evolves structurally in time due to the ongoing fast conformational changes. We identify several conformers of MeBzS2O, demonstrating that its lowest-energy conformation is preferred mainly at lower temperatures, i.e., in the supercooled liquid state. Its increased prevalence modifies locally the short-range intermolecular order and promotes vitrification. Consequently, we indicate that the Arrhenius transition is fuelled rather by conformational changes in this glass-forming macrocyclic crown thiaether, which is a different scenario from the so-far existing concepts. Our studies combine broadband dielectric spectroscopy (BDS), X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations.

Exploring relaxation behaviors in Hydrogen-Bond networks within binary mixtures of propylene carbonate and primary alcohols through broadband dielectric spectroscopy and molecular dynamic simulation

This work investigates the dielectric properties and the relaxation behaviors of propylene carbonate (PC) and its binary mixtures with methanol and ethanol. A coaxial-circular cutoff waveguide junction was employed to characterize broadband dielectric spectra ranging from 0.1 GHz to 18 GHz, and a two-group Debye model was used to extract relaxation parameters. Excess thermodynamic parameters, Kirkwood orientational correlation factors, and Bruggeman factors for PC-alcohol mixtures were also analyzed and presented. These data provide insights into microscopic dynamic processes as the PC concentration in the mixtures gradually increases. These processes include the continuous disassociation of the alcohols’ hydrogen-bond (H-bond) networks, the formation of new PC-alcohol H-bond networks, the parallel alignment of heterogeneous dipoles, and a significant enhancement of the dielectric effect. Furthermore, a series of molecular dynamics (MD) simulations using the TraPPE-UA forcefield were conducted, and the obtained thermodynamic and electromagnetic parameters demonstrated good agreement with experimental results. Based on radial distribution functions, dipole–dipole spatial orientational correlations, and H-bond life times extracted from the MD simulations, we observed the gradual formation of PC-cage and alcohol-cluster microstructures in PC-rich mixtures. The data and physical insights provided in this work are expected to have practical implications in high-energy battery applications and the pharmaceutical industry.

Revealing Dynamic Behavior in High Dielectric Poly(thiourethane)-Based Vitrimer-like Materials.

Here, we have explored covalent adaptable networks (CANs) comprising poly(thiourethane)-based systems (PTUs). The PTUs were synthesized through the combination of thiol and isocyanate monomers in stoichiometric proportions, in the presence of dibutyltin dilaurate (DBTDL) as catalyst. Dynamic mechanical analysis (DMA) provided detailed insight into the vitrimeric behavior. Through these investigations, we evaluated the viscoelastic, thermomechanical, and vitrimeric properties. Additionally, broadband dielectric spectroscopy (BDS) revealed the various relaxation processes inherent in such vitrimer-like materials. We methodically examined the evolution of each relaxation in every prepared sample to comprehend the operational mechanisms in these vitrimer-like systems. Our findings underscore that depending on the PTU formulation, the glass transition temperature (Tg) and the topology freezing transition temperature (Tv) can be effectively distinguished and studied. Considering the high dipole moment of the dynamic bonds present in these systems, there is potential for utilizing them as dielectric materials working under the concept of dipolar glass polymers. Furthermore, the reversibility exhibited by their inner chemical structures positions them as promising candidates for active layers in capacitor devices, particularly for energy-related applications, with the ability to be recyclable while maintaining almost invariant both their mechanical and dielectric properties, thus promoting the extension of the lifespan of electronic devices.

Advanced humidity sensing properties of CuO ceramics

This research explores the capacitive humidity sensing properties of CuO ceramic, selected for its simplicity as an oxide and ease of fabrication, in addition to its remarkable dielectric properties. The CuO sample was fabricated by sintering at 980 °C for 5 h. A microstructure with a relative density of 88.9% was obtained. X-ray diffraction confirmed the formation of a pure CuO phase. Broadband dielectric spectroscopy revealed that the observed giant dielectric properties at room temperature (RT) were attributed to extrinsic effects, including the internal barrier layer capacitor and sample-electrode contact effects. A key focus of this study was to examine the giant dielectric properties of CuO ceramic as a function of relative humidity (RH) at RT and frequencies of 102 and 103 Hz. It was observed that the capacitance of CuO continuously increased with rising RH levels, ranging from 30 to 95%. Notably, the maximum hysteresis errors were constrained to 2.3 and 3.3% at 102 and 103 Hz, respectively. Additionally, the CuO ceramic demonstrated very fast response and recovery times, approximately 2.8 and 0.95 min, respectively. The repeatability of the humidity response of the capacitance was also established. Overall, this research highlights the high potential of CuO as a giant dielectric material for application in humidity sensors.

Effects of the Charge Density of Nanopapers Based on Carboxymethylated Cellulose Nanofibrils Investigated by Complementary Techniques.

Cellulose nanofibrils (CNFs) with different charge densities were prepared and investigated by a combination of different complementary techniques sensitive to the structure and molecular dynamics of the system. The morphology of the materials was investigated by scanning electron microscopy (SEM) and X-ray scattering (SAXS/WAXS). The latter measurements were quantitatively analyzed yielding to molecular parameters in dependence of the charge density like the diameter of the fibrils, the distance between the fibrils, and the dimension of bundles of nanofibrils, including pores. The influence of water on the properties and the charge density is studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and broadband dielectric spectroscopy. The TGA measurements reveal two mass loss processes. The one at lower temperatures was related to the loss of water, and the second process at higher temperatures was related to the chemical decomposition. The resulting char yield could be correlated to the distance between the microfibrils. The DSC investigation for hydrated CNFs revealed three glass transitions due to the cellulose segments surrounded by water molecules in different states. In the second heating scan, only one broad glass transition is observed. The dielectric spectra reveal two relaxation processes. At low temperatures or higher frequencies, the β-relaxation is observed, which is assigned to localized fluctuation of the glycosidic linkage. At higher temperatures and lower frequencies, the α-relaxation takes places. This relaxation is due to cooperative fluctuations in the cellulose segments. Both processes were quantitatively analyzed. The obtained parameters such as the relaxation rates were related to both the morphological data, the charge density, and the content of water for the first time.

Tuning the Physical State of Aripiprazole by Mesoporous Silica.

The main purpose of our studies is to demonstrate that commercially available mesoporous silica (MS) can be used to control the physical state of aripiprazole (ARP). The investigations performed utilizing differential scanning calorimetry and broadband dielectric spectroscopy reveal that silica can play different roles depending on its concentration in the system with amorphous ARP. At low MS content, it activates recrystallization of the active pharmaceutical ingredient and supports forming the III polymorphic form of ARP. At intermediate MS content (between ca. 27 and 65 wt %), MS works as a recrystallization inhibitor of ARP. At these concentrations, the formation of III polymorphic form is no longer favorable; therefore, it is possible to use this additive to obtain ARP in either IV or X polymorphic form. At the same time, employing MS in concentrations >65 wt % amorphous form of ARP with high physical stability can be obtained. Finally, regardless of the polymorphic form it crystallizes into, each composite is characterized by the same temperature dependence of relaxation times in the supercooled and glassy states.

Effects of Atmospheric Pressure Plasma Jet on 3D-Printed Acrylonitrile Butadiene Styrene (ABS).

Polymers are essential in several sectors, yet some applications necessitate surface modification. One practical and eco-friendly option is non-thermal plasma exposure. The present research endeavors to examine the impacts of dielectric barrier discharge atmospheric pressure plasma on the chemical composition and wettability properties of acrylonitrile butadiene styrene surfaces subject to the action of additive manufacturing. The plasma source was produced by igniting either helium or argon and then adjusted to maximize the operational conditions for exposing polymers. The drop in contact angle and the improvement in wettability after plasma exposure can be due to the increased oxygen-containing groups onto the surface, together with a reduction in carbon content. The research findings indicated that plasma treatment significantly improved the wettability of the polymer surface, with an increase of up to 60% for both working gases, while the polar index increased from 0.01 up to 0.99 after plasma treatment. XPS measurements showed an increase of up to 10% in oxygen groups at the surface of He-plasma-treated samples and up to 13% after Ar-plasma treatment. Significant modifications were observed in the structure that led to a reduction of its roughness by 50% and also caused a leveling effect after plasma treatment. A slight decrease in the glass and melting temperature after plasma treatment was pointed out by differential scanning calorimetry and broadband dielectric spectroscopy. Up to a 15% crystallinity index was determined after plasma treatment, and the 3D printing process was measured through X-ray diffraction. The empirical findings encourage the implementation of atmospheric pressure plasma-based techniques for the environmentally sustainable manipulation of polymers for applications necessitating higher levels of adhesion and specific prerequisites.

Scalable and Highly Porous Membrane Adsorbents for Direct Air Capture of CO2.

Direct air capture (DAC) of CO2 is a carbon-negative technology to mitigate carbon emissions, and it requires low-cost sorbents with high CO2 sorption capacity that can be easily manufactured on a large scale. In this work, we develop highly porous membrane adsorbents comprising branched polyethylenimine (PEI) impregnated in low-cost, porous Solupor supports. The effect of the PEI molecular mass and loading on the physical properties of the adsorbents is evaluated, including porosity, degradation temperature, glass transition temperature, and CO2 permeance. CO2 capture from simulated air containing 400 ppm of CO2 in these sorbents is thoroughly investigated as a function of temperature and relative humidity (RH). Polymer dynamics was examined using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS), showing that CO2 sorption is limited by its diffusion in these PEI-based sorbents. A membrane adsorbent containing 48 mass% PEI (800 Da) with a porosity of 72% exhibits a CO2 sorption capacity of 1.2 mmol/g at 25 °C and RH of 30%, comparable to the state-of-the-art adsorbents. Multicycles of sorption and desorption were performed to determine their regenerability, stability, and potential for practical applications.

Downscaling grain size toward the nanometre range – A key-factor for tuning the crystalline structure, phase transitions, dielectric and ferroelectric behaviour in Ba0·8Sr0·2TiO3 ceramics

The present study aims to describe the role of the grain size on the properties of submicron- and nano-structured Ba0·8Sr0·2TiO3 (BST) ceramics. Dense (1 − 2% porosity) ceramics with average grain sizes in the range of (77 − 234) nm were consolidated under different spark plasma sintering conditions starting from nanopowders with a mean particle size of 70 nm, synthesized via the acetate variant of the sol-gel method. The structural analysis based on XRD data revealed a mixture of cubic and tetragonal modifications at room temperature for the precursor powders and for all the investigated ceramics. The structural heterogeneity of the individual ceramic grains with coexistence of cubic and tetragonal polymorphs was confirmed by HR-TEM investigations. Accordingly, a “brick-wall" model with cubic grain boundary regions and tetragonal grain cores is proposed. By increasing the grain size, from 77 to 234 nm, a decrease of the phase transitions diffuseness accompanied by an increase of the permittivity maxima (from 650 to 4500) and dielectric losses (from 5 to 7.5%, at 100 Hz), was detected by broadband dielectric spectroscopy. No variation of the Curie temperature in the investigated Ba0·8Sr0·2TiO3 ceramics was detected, unlike typically reported for BaTiO3 ceramics with similar grain sizes. The Curie-Weiss temperature and the Curie constant decrease when grain size is diminished, indicating an overall reduction of the ferroelectric active volume, as a scaling effect. The ferroelectric switching was demonstrated for all the selected fine-grained BST ceramics, either at nanoscale or macroscopically, with an increased ferroelectric character for the coarser submicron-structured ceramics, with respect to the nanocrystalline one. The observed properties of the fine-grained Ba0·8Sr0·2TiO3 ceramics are explained in the frame of multiphase coexistence and ferroelectricity “dilution” due to the increasing role of non-ferroelectric grain boundaries when reducing grain size and complete the knowledge on the scale-dependent properties of dense fine-grained BaTiO3-based ceramics.

Fabrication of Poly(ethylene furanoate)/Silver and Titanium Dioxide Nanocomposites with Improved Thermal and Antimicrobial Properties.

Poly(ethylene furanoate) (PEF)-based nanocomposites were fabricated with silver (Ag) and titanium dioxide (TiO2) nanoparticles by the in-situ polymerization method. The importance of this research work is to extend the usage of PEF-based nanocomposites with improved material properties. The PEF-Ag and PEF-TiO2 nanocomposites showed a significant improvement in color concentration, as determined by the color colorimeter. Scanning electron microscopy (SEM) photographs revealed the appearance of small aggregates on the surface of nanocomposites. According to crystallinity investigations, neat PEF and nanocomposites exhibit crystalline fraction between 0-6%, whereas annealed samples showed a degree of crystallinity value above 25%. Combining the structural and molecular dynamics observations from broadband dielectric spectroscopy (BDS) measurements found strong interactions between polymer chains and nanoparticles. Contact angle results exhibited a decrease in the wetting angle of nanocomposites compared to neat PEF. Finally, antimicrobial studies have been conducted, reporting a significant rise in inhibition of over 15% for both nanocomposite films against gram-positive and gram-negative bacteria. From the overall results, the synthesized PEF-based nanocomposites with enhanced thermal and antimicrobial properties may be optimized and utilized for the secondary packaging (unintended food-contact) materials.

Dynamics of polylactic acid under ultrafine nanoconfinement: The collective interface effect and the spatial gradient.

Polymers under nanoconfinement can exhibit large alterations in dynamics from their bulk values due to an interface effect. However, understanding the interface effect remains a challenge, especially in the ultrafine nanoconfinement region. In this work, we prepare new geometries with ultrafine nanoconfinement ∼10nm through controlled distributions of the crystalline phases and the amorphous phases of a model semi-crystalline polymer, i.e., the polylactic acid. The broadband dielectric spectroscopy measurements show that ultrafine nanoconfinement leads to a large elevation in the glass transition temperature and a strong increment in the polymer fragility index. Moreover, new relaxation time profile analyses demonstrate a spatial gradient that can be well described by either a single-exponential decay or a double-exponential decay functional form near the middle of the film with a collective interface effect. However, the dynamics at the 1-2nm vicinity of the interface exhibit a power-law decay that is different from the single-exponential decay or double-exponential decay functional forms as predicted by theories. Thus, these results call for further investigations of the interface effect on polymer dynamics, especially for interfaces with perturbed chain packing.

Thermogravimetric and temperature-dependent electrical investigations of Pr3+ doped multi-component silicate glasses for microelectronic technology

Multi-component silicate glasses doped with 0, 0.5, 1, and 1.5 mol% of praseodymium (Pr3+) were synthesized by the sol–gel method. Thermal analysis of the glasses, evinced a high working temperature of 351 °C and Hruby coefficient, K H = 1.415 in the highly doped system, corroborating the effective role of Pr3+ ions in endowing superior thermal stability to the glass. Broadband dielectric spectroscopy was applied to study the temperature-dependent electrical behavior of the glasses for their suitability as electrodes and solid electrolyte materials in batteries. A high dielectric constant of 4797 was evidenced at 1 kHz when recorded at 473 K. The AC conductivity of the glass doped with 1 mol% was observed to be the highest with 94.8 × 10−5 S cm−1 at 10 MHz and 473 K. Jonscher’s power law exponent decreased with temperature, attributing the conducting mechanism to the Correlated Barrier Hopping (CBH) model. The Nyquist impedance spectra demonstrated a depressed semicircle with a spur at the low-frequency end, validating the non-Debye relaxation in the glasses. The equivalent circuitry of the plot predicted parallel combinations of resistor and constant phase elements which reflects a Warburg diffusion and capacitive approach. Bode’s phasor diagram confirmed the capacitive nature by a phase angle of −90° in all the glasses. While a uniform increase in dielectric constant and conductivity was observed up to 1 mol% of Pr3+, a sharp decline in the electrical phenomenon was observed with 1.5 mol% of Pr3+, due to the possible blockade of the hopping of charge carriers by the largely quantified dopant ions. Extracting a high dielectric constant, and ionic conductivity at high frequencies, with an optimal dopant concentration of 1 mol% Pr3+, the composite glasses could be considered for their potential use in integrated microcomponent storage devices as cathode and solid electrolyte materials.

Fragility and Tendency to Crystallization for Structurally Related Compounds.

The present study was designed to investigate the physical stability of three organic materials with similar chemical structures. The examined compounds revealed completely different crystallization tendencies in their supercooled liquid states and were classified into three distinct classes based on their tendency to crystallize. (S)-4-Benzyl-2-oxazolidinone easily crystallizes during cooling from the melt; (S)-4-Benzylthiazolidine-2-thione does not crystallize during cooling from the melt, but crystallizes easily during subsequent reheating above Tg; and (S)-4-Benzyloxazolidine-2-thione does not crystallize either during cooling from the melt or during reheating. Such different tendencies to crystallize are observed despite the very similar chemical structures of the compounds, which only differ in oxide or sulfur atoms in one of their rings. We also studied the isothermal crystallization kinetics of the materials that were shown to transform into a crystalline state. Molecular dynamics and thermal properties were thoroughly investigated using broadband dielectric spectroscopy, as well as conventional and temperature-modulated differential scanning calorimetry in the wide temperature range. It was found that all three glass formers have the same dynamic fragility (m = 93), calculated directly from dielectric structural relaxation times. This result verifies that dynamic fragility is not related to the tendency to crystallize. In addition, thermodynamic fragility predictions were also made using calorimetric data. It was found that the thermodynamic fragility evaluated based on the width of the glass transition, observed in the temperature dependence of heat capacity, correlates best with the tendency to crystallize.

Modeling the Interface Between Phases in Dense Polymer‐Carbon Black Nanoparticle Composites by Dielectric Spectroscopy: Where Are We Now and What are the Opportunities?

AbstractThe macroscopic properties of polymer nanocomposites (PNC) rely largely on the interphase between the polymer chains and the filler particles. One significant difficulty to solve this issue is to quantitatively model the structure‐property correlations due to the interfacial region in these complex materials. While dielectric spectroscopy (DS) measurements are routinely used to characterize the effective permittivity of filled polymers, fitting standard effective medium models and mixing equations to these data remains notoriously difficult to interpret. This is due to the absence of explicit reference to internal length scales characterizing the interfaces in the PNC. As an illustrative example, a two‐level homogenization framework is proposed which enables the extraction of useful information on the impact of a thin interphase confined on a nanometer length scale based on broadband DS data. This model leads to new ways of tuning the interphase so as to optimize the material's response to electric field, a situation relevant for electromagnetic shielding. This approach provides guidance on how to observe directly and experimentally the actual properties of the interface between the phases (as opposed to model‐based inference). Aside from its secure physical foundation in the theory of effective medium, a significant advantage of this approach is that a genetic algorithm (GA) technique applied to this physics‐based model enables the uniqueness of the fit parameters to be considered, as the GA method is robust in terms of finding globally optimum solutions, therefore placing confidence in non‐universal values of the percolation exponents. Recent work in physics‐informed machine learning indicates that the effective dielectric properties of PNC with many degrees of freedom due to their complex morphology can be described by considering only a few degrees of freedom describing the interface features between the phases in these composites.

High damping, soft and reprocessable thermal interface materials inspired by the microstructure of skin tissue

Thermal interface materials (TIMs) play a critical role in heat dissipation of modern electronics with their development towards integration and miniaturization. Shock-induced vibration damage is another extremely harmful to electronics, which requires TIMs remarkable damping properties, reprocessability, softness, and high thermal conductivity. However, current TIMs still lack the ability to dampen vibrations and reprocessability, because integrating these properties into one TIM still remains a dilemma. Inspired by the synergistic effects of fiber networks and adipocytes in skin tissue, here, we report on a high damping, soft and reprocessable TIM, by infusing viscous polymer into polybutadiene bottlebrush polymer network meanwhile combing with aluminum nitride filler. The obtained TIMs show damping factor as high as 0.95–1.0 in the frequency range of daily life (1–300 Hz), excellent reprocessability efficiency (92 %), low Young's modulus (55.8 kPa) and thermal conductivity of 2.25 W m−1 K−1. Analyzing the polymer structure and chain dynamics by rheology, broadband dielectric spectroscopy, solid state nuclear magnetic resonance, and FTIR spectroscopy, we confirm that these excellent properties is attributed to the enhanced migration ability of polymer chains, thermally conductive aluminum nitride fillers, and hierarchical hydrogen bonds in the bottlebrush polymer network. We further demonstrate the potential application of the TIMs in heat dissipation of an electronic device under vibrations. The current work provides a unique structure design approach towards high-performance TIMs in shock-resistant electronics.

Multifunctional HDPE/Cu biocidal nanocomposites for MEX additive manufactured parts: Perspectives for the defense industry

In this study, we investigated the performance improvement caused by the addition of copper (Cu) nanoparticles to high-density polyethylene (HDPE) matrix material. Composite materials, with filler percentages of 0.0, 2.0, 4.0, 6.0, 8.0, and 10.0 wt% were synthesized through the material extrusion (MEX) 3D printing technique. The synthesized nanocomposite filaments were utilized for the manufacturing of specimens suitable for the experimental procedure that followed. Hence, we were able to systematically investigate their tensile, flexural, impact, and microhardness properties through various mechanical tests that were conducted according to the corresponding standards. Broadband Dielectric Spectroscopy was used to investigate the electrical/dielectric properties of the composites. Moreover, by employing means of Raman spectroscopy and thermogravimetric analysis (TGA) we were also able to further investigate their vibrational, structural, and thermal properties. Concomitantly, means of scanning electron microscopy (SEM), as well as atomic force microscopy (AFM), were used for the examination of the morphological and structural characteristics of the synthesized specimens, while energy-dispersive X-ray spectroscopy (EDS) was also performed in order to receive a more detailed picture on the structural characteristics of the various synthesized composites. The corresponding nanomaterials were also assessed for their antibacterial properties regarding Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) with the assistance of a method named screening agar well diffusion. The results showed that the mechanical properties of HDPE benefited from the utilization of Cu as a filler, as they showed a notable improvement. The specimen of HDPE/Cu 4.0 wt% was the one that presented the highest levels of reinforcement in four out of the seven tested mechanical properties (for example, it exhibited a 36.7% improvement in the flexural strength, compared to the pure matrix). At the same time, the nanocomposites were efficient against the S. aureus bacterium and less efficient against the E. coli bacterium. The use of such multi-functional, robust nanocomposites in MEX 3D printing is positively impacting applications in various fields, most notably in the defense and security sectors. The latter becomes increasingly important if one takes into account that most firearms encompass various polymeric parts that require robustness and improved mechanical properties, while at the same time keeping the risk of spreading various infectious microorganisms at a bare minimum.

Control of Dielectric Parameters of Micro- and Nanomodified Epoxy Resin Using Electrophoresis

This work presents the results of research on submicro- and nanocomposites with gradient properties, produced in a planned electrophoretic process. Epoxy-resin-based samples were filled with TiO2 particles of three different sizes (13 nm, 38 nm, and <1 µm) at four different values of average electric field Eav (0.0 Vmm−1, 125 Vmm−1, 250 Vmm−1, and 500 Vmm−1) for 1 h each. Changes in selected dielectric parameters (dielectric constant εr and dielectric loss factor tanδ) of the composites were analyzed using broadband dielectric spectroscopy (10−1 Hz to 105 Hz). The influence of the Eav and the current i(t) flowing through the sample material and the Joule heat generated in it on the resin curing process and the final gradient of dielectric parameters were investigated. The results show that the degree of modification of the εr gradient increases with increasing Eav and is more pronounced in the case of TiO2 nanoparticles. The largest modifications in the εr and tanδ were obtained for nanoparticles with a diameter of 13 nm at Eav = 500 Vmm−1, while the lowest for particles < 1 µm at Eav = 125 Vmm−1. The effect of electrophoresis on the dielectric parameters is significant, especially near the anode region. Increasing the concentration of TiO2 particles at the anode occurs at the expense of reducing their concentration in the remaining volume of the sample. The test results clearly demonstrate the importance of particle dimension and electric field strength for the gradient modification of the properties of the epoxy composite using electrophoresis. Numerical simulations of electric field stresses in the epoxy resin during the electrophoresis process, performed in the COMSOL program, revealed a significant increase in the E field strength in the areas close to the anode and cathode.