Nazarychev, Victor M

Nazarychev V.M.

Thermoplastic polyimides have garnered significant interest in the electronic and electrical industries owing to their performance characteristics. However, their relatively low thermal conductivity coefficients pose a challenge. To address this issue, this study focused on the properties of nanocomposites comprising two thermoplastic semicrystalline polyimides R-BAPB and BPDA-P3, one amorphous polyimide ULTEMTM, and hexagonal nanoparticles. Polyimide R-BAPB was synthesized based on 1,3-bis-(3′,4-dicarboxyphenoxy)benzene (dianhydride R) and 4,4′-bis-(4′-aminophenoxy)biphenyl (BAPB diamine); polyimide BPDA-P3 was synthesized based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and diamine 1,4-bis[4-(4-aminophenoxy)phenoxy]benzene (P3); and amorphous polyimide ULTEMTM was commercially produced by Sabic Innovative Plastics. Using microsecond-scale all-atom molecular dynamics simulations, the effects of incorporating hexagonal nanoparticles with enhanced thermal conductivity, such as graphene, graphene oxide, and boron nitride, on the structural and thermophysical characteristics of these materials were examined. The formation of stacked aggregates was found for graphene and hexagonal boron nitride nanoparticles. It was observed that graphene oxide nanoparticles exhibited a dispersion in polyimide binders that was higher than those in graphene and hexagonal boron nitride nanoparticles, leading to reduced translational mobility of polymer chains. Consequently, the decrease in polyimide chain mobility correlated with an increase in the glass transition temperature of the nanocomposites. Aggregates of nanoparticles formed a pathway for phonon transport, resulting in improved thermal conductivity in polyimide nanocomposites. An increase in the thermal conductivity coefficient of polyimide nanocomposites was observed when the concentration of graphene, graphene oxide, and hexagonal boron nitride nanofillers increased. The enhancement in thermal conductivity was found to be strongest when graphene nanoparticles were added.

Dobrovskiy A.Y., Nazarychev V.M., Larin S.V., Lyulin S.V.

In this study, we have conducted a comparative analysis of the structural ordering of short oligoetherimide chains (dimers) near the bounding surface, depending on the structure of that surface. In order to clarify the possibility of oligoetherimide ordering along the symmetry axes of graphene, two types of bounding surfaces were considered: graphene, with a regular discrete position of interaction centers (carbon atoms), and a smooth, structureless impermeable wall. The chemical structures of the considered dimers consist of two repeating units of BPDA-P3, ODPA-P3, or aBPDA-P3 thermoplastic polyetherimides. Using all-atom molecular dynamics simulations, the process of structural ordering of the dimers near the surface of the graphene or wall was established. The ODPA-P3 and BPDA-P3 dimers form an ordered state near the graphene surface, while the aBPDA-P3 dimers do not demonstrate structural ordering. The simulation results confirmed that the ordering direction of the BPDA-P3 and ODPA-P3 dimers near the graphene surface is chosen randomly. Comparison of the oligoetherimide structure formed near the attracting wall without a symmetrical location of the interaction centers shows the similarity of the ordering of dimers near the graphene surface and the wall. As in the case of the graphene surface, the ordering of oligoetherimide molecules near the structureless wall demonstrates one direction of ordering. Therefore, we confirmed that the key factor for the onset of ordering is the presence of a confining surface, rather than the symmetrical arrangement of interaction centers in the substrate structure.

Dobrovskiy A.Y., Nazarychev V.M., Volgin I.V., Lyulin S.V.

The authors wish to make a change to the published paper [...]

Glova A., Nazarychev V.M., Larin S.V., Gurtovenko A.A., Lyulin S.V.

Recent experiments and atomistic computer simulations have shown that asphaltene byproducts of oil refineries can serve as thermal conductivity enhancers for organic phase-change materials such as paraffin and therefore have...

Nazarychev V.M., Lyulin S.V.

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, ULTEMTM. 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 ULTEMTM 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.

Gurtovenko A.A., Nazarychev V.M., Glova A.D., Larin S.V., Lyulin S.V.

Asphaltenes represent a novel class of carbon nanofillers that are of potential interest for many applications, including polymer nanocomposites, solar cells, and domestic heat storage devices. In this work, we developed a realistic coarse-grained Martini model that was refined against the thermodynamic data extracted from atomistic simulations. This allowed us to explore the aggregation behavior of thousands of asphaltene molecules in liquid paraffin on a microsecond time scale. Our computational findings show that native asphaltenes with aliphatic side groups form small clusters that are uniformly distributed in paraffin. The chemical modification of asphaltenes via cutting off their aliphatic periphery changes their aggregation behavior: modified asphaltenes form extended stacks whose size increases with asphaltene concentration. At a certain large concentration (44 mol. %), the stacks of modified asphaltenes partly overlap, leading to the formation of large, disordered super-aggregates. Importantly, the size of such super-aggregates increases with the simulation box due to phase separation in the paraffin–asphaltene system. The mobility of native asphaltenes is systematically lower than that of their modified counterparts since the aliphatic side groups mix with paraffin chains, slowing down the diffusion of native asphaltenes. We also show that diffusion coefficients of asphaltenes are not very sensitive to the system size: enlarging the simulation box results in some increase in diffusion coefficients, with the effect being less pronounced at high asphaltene concentrations. Overall, our findings provide valuable insight into the aggregation behavior of asphaltenes on spatial and time scales that are normally beyond the scales accessible for atomistic simulations.

Tolmachev D., Nazarychev V., Fedotova V., Vorobiov V., Lukasheva N., Smirnov M., Karttunen M.

Deep eutectic solvents (DESs) are multi-component solvents appearing in a broad range of applications. The next necessary step for the development of new DESs is understanding the molecular mechanisms of DES formation and the interactions that determine its structure and properties. In this work, we use multiscale simulations supported by experiments to investigate the detailed structure and properties of polymerizable DESs based on choline chloride and acrylic acid as a basis for creating inks for 3D printing. Thermodynamic and structural analyses show the physical mechanisms of DES formation in these materials: due to the significant size difference between the acrylic acid and choline ions, and favorable interactions between acrylic acid and the Cl- ions, the acrylic acid molecules are able to incorporate into the free spaces of the first coordination shells of the Cl- ions. As a consequence, the mixture has less volume than its individual components and this excess volume determines the negative value of the enthalpy of mixing. Structurally, the mixture is a network with the Cl- ions as nodes connecting the other DES components. This was confirmed by both the FTIR experiments and the atomistic MD simulations. The calculations show the necessity of correct accounting of excess enthalpy and entropy for determining DESs structures and other properties.

Nazarychev V.M., Glova A.D., Larin S.V., Lyulin A.V., Lyulin S.V., Gurtovenko A.A.

A molecular-level insight into phase transformations is in great demand for many molecular systems. It can be gained through computer simulations in which cooling is applied to a system at a constant rate. However, the impact of the cooling rate on the crystallization process is largely unknown. To this end, here we performed atomic-scale molecular dynamics simulations of organic phase-change materials (paraffins), in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold (1.2 × 1011 K/min) in the values of cooling rates exists. When cooling is slower than the threshold, the simulations qualitatively reproduce an experimentally observed abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity of paraffins upon crystallization. Beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the supercooled liquid state. Thus, a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials, which are of great promise for use in domestic heat storage devices.

Volgin I.V., Batyr P.A., Matseevich A.V., Dobrovskiy A.Y., Andreeva M.V., Nazarychev V.M., Larin S.V., Goikhman M.Y., Vizilter Y.V., Askadskii A.A., Lyulin S.V.

Larin S.V., Makarova V.V., Gorbacheva S.N., Yakubov M.R., Antonov S.V., Borzdun N.I., Glova A.D., Nazarychev V.M., Gurtovenko A.A., Lyulin S.V.

Adding carbon nanoparticles into organic phase change materials (PCMs) such as paraffin is a common way to enhance their thermal conductivity and to improve the efficiency of heat storage devices. However, the sedimentation stability of such blends can be low due to aggregation of aromatic carbon nanoparticles in the aliphatic paraffin environment. In this paper, we explore whether this important issue can be resolved by the introduction of a polymer agent such as poly(3-hexylthiophene) (P3HT) into the paraffin–nanoparticle blends: P3HT could ensure the compatibility of aromatic carbon nanoparticles with aliphatic paraffin chains. We employed a combination of experimental and computational approaches to determine the impact of P3HT addition on the properties of organic PCMs composed of paraffin and carbon nanoparticles (asphaltenes). Our findings clearly show an increase in the sedimentation stability of paraffin–asphaltene blends, when P3HT is added, through a decrease in average size of asphaltene aggregates as well as in an increase of the blends’ viscosity. We also witness the appearance of the yield strength and gel-like behavior of the mixtures. At the same time, the presence of P3HT in the blends has almost no effect on their thermophysical properties. This implies that all properties of the blends, which are critical for heat storage applications, are well preserved. Thus, we demonstrated that adding polyalkylthiophenes to paraffin–asphaltene mixtures led to significant improvement in the performance characteristics of these systems. Therefore, the polymer additives can serve as promising compatibilizers for organic PCMs composed of paraffins and asphaltenes and other types of carbon nanoparticles.

Boomstra M.W., van Asseldonk M.W., Geurts B.J., Nazarychev V.M., Lyulin A.V.

• Increased branching of paraffin molecules leads to decreased thermal conductivity. • Polydispersity has a minor effect on thermal conductivity at 250 K, but none at 450 K. • For branched paraffin, there is a strong concomitance of crystallinity and thermal conductivity. • Increased thermal conductivity of polydisperse paraffin is explained purely by increasing average chain length. Paraffin waxes are promising phase change materials, abundantly available at very low cost. Having large latent heat, these materials can be used for thermal energy storage. However, when used in heat batteries, paraffin’s low thermal conductivity prevents fast charging and discharging. This calls for the design of tailored hybrid materials with improved properties, the present study concentrates on properties of pure paraffin wax. Using fully atomistic molecular-dynamics (MD) simulations, we study the effects of polydispersity and branching on the thermal conductivity of paraffin waxes, in molten (450 K) and solid (250 K) state. Both branching and polydispersity affect the density and especially the crystallinity of the solid. Branching has a pronounced effect on crystallisation caused by inhibited alignment of the polymer backbones while the effect of polydispersity is less pronounced. The thermal conductivity (TC) has been simulated using the reverse non-equilibrium molecular-dynamics method, as well as the equilibrium Green-Kubo approach. Increased branching, added to backbones comprised of twenty monomers, results in decreasing TC of up to 30%, polydispersity only has an effect in the semi-crystalline state. Comparison to available experiments shows good agreement which validates the model details, applied force field and the calculation methods. We show that at comparable computational costs, the reverse non-equilibrium MD approach produces more reliable results for TC, as compared to the equilibrium Green-Kubo method. The major contribution to TC by acoustic phonon transport along the backbone was shown by analysing extreme cases. The phonon density of states (PDOS) of samples with high branching or with small chain length displayed diminished peaks in the acoustic range as compared to the PDOS of samples with low branching or larger chain length, respectively. The suggested MD approach can definitely be used to investigate specific material modifications aimed at increasing the overall TC.

Dobrovskiy A.Y., Nazarychev V.M., Volgin I.V., Lyulin S.V.

The effect of polymer chain ordering on the transport properties of the polymer membrane was examined for the semi-crystalline heterocyclic polyetherimide (PEI) BPDA-P3 based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and diamine 1,4-bis [4-(4-aminophenoxy)phenoxy]benzene (P3). All-atom Molecular Dynamics (MD) simulations were used to investigate the gas diffusion process carried out through the pores of a free volume several nanometers in size. The long-term (~30 μs) MD simulations of BPDA-P3 were performed at T = 600 K, close to the experimental value of the melting temperature (Tm ≈ 577 K). It was found during the simulations that the transition of the PEI from an amorphous state to an ordered one occurred. We determined a decrease in solubility for both gases examined (CO2 and CH4), caused by the redistribution of free volume elements occurring during the structural ordering of the polymer chains in the glassy state (Tg ≈ 487 K). By analyzing the diffusion coefficients in the ordered state, the presence of gas diffusion anisotropy was found. However, the averaged values of the diffusion coefficients did not differ from each other in the amorphous and ordered states. Thus, permeability in the observed system is primarily determined by gas solubility, rather than by gas diffusion.

Nazarychev V.M., Vaganov G.V., Larin S.V., Didenko A.L., Elokhovskiy V.Y., Svetlichnyi V.M., Yudin V.E., Lyulin S.V.

Recently, a strong structural ordering of thermoplastic semi-crystalline polyimides near single-walled carbon nanotubes (SWCNTs) was found that can enhance their mechanical properties. In this study, a comparative analysis of the results of microsecond-scale all-atom computer simulations and experimental measurements of thermoplastic semi-crystalline polyimide R-BAPB synthesized on the basis of dianhydride R (1,3-bis-(3′,4-dicarboxyphenoxy) benzene) and diamine BAPB (4,4′-bis-(4″-aminophenoxy) biphenyl) near the SWCNTs on the rheological properties of nanocomposites was performed. We observe the viscosity increase in the SWCNT-filled R-BAPB in the melt state both in computer simulations and experiments. For the first time, it is proven by computer simulation that this viscosity change is related to the structural ordering of the R-BAPB in the vicinity of SWCNT but not to the formation of interchain linkage. Additionally, strong anisotropy of the rheological properties of the R-BAPB near the SWCNT surface was detected due to the polyimide chain orientation. The increase in the viscosity of the polymer in the viscous-flow state and an increase in the values of the mechanical characteristics (Young’s modulus and yield peak) of the SWCNT-R-BAPB nanocomposites in the glassy state are stronger in the directions along the ordering of polymer chains close to the carbon nanofiller surface. Thus, the new experimental data obtained on the R-BAPB-based nanocomposites filled with SWCNT, being extensively compared with simulation results, confirm the idea of the influence of macromolecular ordering near the carbon nanotube on the mechanical characteristics of the composite material.

Tolmachev D., Lukasheva N., Ramazanov R., Nazarychev V., Borzdun N., Volgin I., Andreeva M., Glova A., Melnikova S., Dobrovskiy A., Silber S.A., Larin S., de Souza R.M., Ribeiro M.C., Lyulin S., et. al.

Deep eutectic solvents (DESs) are one of the most rapidly evolving types of solvents, appearing in a broad range of applications, such as nanotechnology, electrochemistry, biomass transformation, pharmaceuticals, membrane technology, biocomposite development, modern 3D-printing, and many others. The range of their applicability continues to expand, which demands the development of new DESs with improved properties. To do so requires an understanding of the fundamental relationship between the structure and properties of DESs. Computer simulation and machine learning techniques provide a fruitful approach as they can predict and reveal physical mechanisms and readily be linked to experiments. This review is devoted to the computational research of DESs and describes technical features of DES simulations and the corresponding perspectives on various DES applications. The aim is to demonstrate the current frontiers of computational research of DESs and discuss future perspectives.

Glova A.D., Nazarychev V.M., Larin S.V., Lyulin A.V., Lyulin S.V., Gurtovenko A.A.

The practical use of paraffin and other organic phase-change materials for heat storage is largely limited by their low thermal conductivity. In this paper we employed 60 microsecond-long atomic-scale computer simulations to explore for the first time whether the asphaltenes, natural polycyclic aromatic hydrocarbons, can be used as thermal conductivity enhancers for paraffin. We focused on a simple model molecule of asphaltene (a polycyclic aromatic core decorated with the peripheral alkane chains) and showed that the asphaltenes of such molecular architecture are not able to improve the thermal conductivity of paraffin. This is most likely due to the steric constraints imposed by the peripheral alkane groups, which prevent formation of the extended ordered asphaltene aggregates. To overcome this, we proposed a possible chemical modification of the asphaltene molecules through removing the peripheral alkane groups from their aromatic cores; this could be achieved e.g. by thermal cracking (dealkylation) of asphaltenes. It turns out that such a chemical modification drastically changes the situation: the modified asphaltenes form extended columnar aggregates which can serve as thermal conduction paths, considerably enhancing the thermal conductivity of a liquid composite sample. This effect, however, vanishes upon cooling because the columnar extended stacks of chemically modified asphaltenes transform into the helical twisted structures, which reduces the overlap of adjacent asphaltenes in aggregates. Importantly, all the simulations have been carried out with two different all-atom force fields. We have demonstrated that both computational models give qualitatively similar results. Overall, our findings clearly show that chemically modified asphaltene molecules can be considered as promising carbon-based thermal conductivity enhancers for liquid paraffin; this result can be used for optimizing the paraffin-based thermal energy storage systems.

Zhang A., Lu Y.

Sun M., Lin L., Di H., Feng Y.

Muther T., Dahaghi A.K.

Boukezzoula F., Khaoua O., Chouha N., Bendaas R., Benbellat N., Menéndez J.C., Kirsch G., Messaoudi A.

Khan W.A., Arain M.B., Balal S., Mollahosseini A., Soylak M.

Ojeda G.A., Vallejos M.M., Samorì C., Galletti P.

Cao Y., Su J., Xiao Y., Ren J., Algadi H., Yeszhanov B., Sartayeva A., Huang J., Guo Z., Tynybekov B., Min Y.

Bide Y., Shahvelayati A.S., kherad E.M., Mohammadpour Z., Mahyari M.

Spera M.B., Darouich S., Pleiss J., Hansen N.

Li Z., Chen J., Qi H.

Heidari M., Labousse M., Leibler L.

The instant crystallization of semi-crystalline polymers has become possible following the recent advances in Fast Scanning Calorimetry (FSC) and enables us to make a bridge between the time scale available experimentally with those accessible with computer simulations. Although the FSC observations have provided new information on the crystallization kinetics and evolution of the crystals, the molecular details on the chain exchange events between the ordered and disordered domains of crystals have remained elusive. Using molecular dynamics simulations, we examined the detailed chain dynamics and thermodynamics of polyamide 6 (PA6) system under two heating treatments: (i) quenching PA6 melt deeply below the melting temperature Tm and (ii) annealing the resulting quenched system to a temperature close to Tm. We categorized the chains into mobile amorphous fraction (MAF) and rigid amorphous fraction (RAF), based on the length of consecutive chain’s bond angles in the trans state. In the deep quenched system close to the glass transition temperature Tg, the mobility of the MAF chains is strongly suppressed and they remain in the glassy state. However, upon rising the temperature close to melting temperature, the system undergoes recrystallization, leading to the coexistence of RAF and supercooled liquid MAF chains. The highly mobile unentangled MAF chains explore the interphase domains, and during the late-stage of crystallization, they are thermally translocated into the lamella by reducing the fold number of RAF chains. The chain mobility in the annealed system could potentially lead to improved biodegradation in semi-crystalline chains.

Wang M., Xu Z., He C., Cai L., Zheng H., Sun Z., Liu H.K., Ying H., Dou S.

Lado-Touriño I., Merodio-Perea R.G.

Polylactic acid (PLA) and poly(ethylene adipate) (PEA) are biodegradable, biobased polymers renowned for their versatility and environmental advantages. This study explores the potential of PLA-PEA blends as green binders in the metal injection molding (MIM) process, a crucial manufacturing technique for producing complex metal components. Substituting conventional, environmentally harmful binders with these blends offers a sustainable strategy to reduce the environmental footprint of MIM. Achieving compatibility between binder components is essential to ensure optimal molding performance in this application. To evaluate this compatibility, molecular dynamics (MD) simulations were employed to analyze the interaction and miscibility of both polymers. Simulations across various blend compositions and temperatures consistently yielded negative Flory–Huggins interaction parameters, demonstrating strong miscibility between PLA and PEA. Notably, blends with lower PEA content exhibited the most favorable compatibility. Radial distribution function analyses further confirmed these results, revealing enhanced miscibility with lower-molecular-weight PEA. This study underscores the potential of PLA-PEA blends as sustainable alternative binders in MIM, advancing the use of biobased materials in energy-efficient and eco-friendly industrial processes. By integrating PLA into MIM, this research contributes to the development of greener engineering practices and highlights the viability of sustainable material solutions for industrial applications.

Zhu C., Xiao S., Ding Y., Zhang Z., He J., Zhao J.

Ogbonna V., Popoola O., Popoola A.

Fiber-reinforced polyimides were observed to exhibit high brittleness and friction coefficient, hence limit their use for mechanical friction component applications. Thus, in the present study, glass fiber-reinforced polyimide composite containing various nano-TiO2 additives (0, 2, 4, and 6 wt%) for better mechanical and tribological performance properties produced by spark plasma sintering processes were investigated. The microstructure, mechanical (hardness and elastic modulus), and tribological (coefficient of friction and wear) properties of the produced nanocomposites were studied using the scanning electron microscopy (SEM), nanoindentation test at an applied load of 200 mN, and pin-on-disc tribometer analyzer, respectively. For the wear test, a load of 10 N and sliding speed of 150 r/min was applied under 15 min for each sample test. The SEM results revealed that the fillers were evenly distributed within the polyimide matrix composites. Glass fiber/polyimide composites with 0 wt% nano-TiO2 depicted 123 HV hardness and 11.40 GPa elastic modulus. Comparatively to the neat glass-fiber/polyimide composite, an improvement of 21.9% in hardness and 9% in modulus were recorded for the composite filled with 4 wt% nano-TiO2 particles. Furthermore, characterizing the tribological behaviour of the nanocomposites, results show that the coefficient of friction and wear rate of the nanocomposites filled with 4 wt% nano-TiO2 were reduced by 32.2% and 76.9%, respectively, compared to pure glass fiber/polyimide composite. However, the findings suggest the facile and cost-effective means of producing polyimide nanocomposites and their potential application in mechanical load-bearing and mechanical friction components.

Glova A., Nazarychev V.M., Larin S.V., Gurtovenko A.A., Lyulin S.V.

Recent experiments and atomistic computer simulations have shown that asphaltene byproducts of oil refineries can serve as thermal conductivity enhancers for organic phase-change materials such as paraffin and therefore have...

Chen S., Xu N., Gorbatikh L., Seveno D.

AbstractThe ultrahigh in‐plane thermal conductivity makes the graphene nanoplatelet a promising reinforcement filler for improving the thermal conductivity of polymer materials. Up to now, the highest thermal conductivity enhancement has been achieved by aligning the nanoplatelets along the heat flux direction. In this work, extensive molecular dynamics simulations are carried out to understand the thermal conductivity enhancement capabilities of different architectures of the graphene nanoplatelets within the polyamide‐6 matrix. Surprisingly, we find that the orthogonally arranged graphene nanoplatelets offer even better thermal conductivity enhancement than the simply aligned graphene nanoplatelets. An in‐depth investigation shows that the orthogonal structure can achieve a balance between the global percolation and the alignment of graphene nanoplatelets. Specifically, such an orthogonal structure can take advantage of both thermal percolation and graphene's ultrahigh in‐plane thermal conductivity. Moreover, we have systematically investigated the effects of the size and number density of the nanoplatelets on the thermal conductivity enhancement capability of the orthogonal configuration. Finally, by proposing a validated analytical model, we have identified the pathways to maximize the thermal conductivity of the orthogonally arranged graphene nanoplatelets. The conclusion of this work points out the possible way to develop the graphene‐polymer composite system with exceedingly high thermal conductivity.Highlights Different graphene configurations are constructed for polymer composite. Chemical reactions at the edge of graphene nanoplatelets are considered. High‐throughput molecular dynamics simulations are conducted to measure thermal conductivity. Competition between graphene alignment and thermal percolation is identified. A theoretical model is established for graphene‐polymer composite.

Roy R., Paul S.

Nazarychev V.M., Lyulin S.V.

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, ULTEMTM. 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 ULTEMTM 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.

Ghasemi A., Liao Y., Li Z., Xia W., Gao W.

Molecular dynamics simulations revealed distinctive crystallization and melting behaviors of confined polymer chains, influenced by polarity and surface chemistry, providing valuable insights for the design of graphene-based polymer heterostructures.

Gurtovenko A.A., Nazarychev V.M., Glova A.D., Larin S.V., Lyulin S.V.

Asphaltenes represent a novel class of carbon nanofillers that are of potential interest for many applications, including polymer nanocomposites, solar cells, and domestic heat storage devices. In this work, we developed a realistic coarse-grained Martini model that was refined against the thermodynamic data extracted from atomistic simulations. This allowed us to explore the aggregation behavior of thousands of asphaltene molecules in liquid paraffin on a microsecond time scale. Our computational findings show that native asphaltenes with aliphatic side groups form small clusters that are uniformly distributed in paraffin. The chemical modification of asphaltenes via cutting off their aliphatic periphery changes their aggregation behavior: modified asphaltenes form extended stacks whose size increases with asphaltene concentration. At a certain large concentration (44 mol. %), the stacks of modified asphaltenes partly overlap, leading to the formation of large, disordered super-aggregates. Importantly, the size of such super-aggregates increases with the simulation box due to phase separation in the paraffin–asphaltene system. The mobility of native asphaltenes is systematically lower than that of their modified counterparts since the aliphatic side groups mix with paraffin chains, slowing down the diffusion of native asphaltenes. We also show that diffusion coefficients of asphaltenes are not very sensitive to the system size: enlarging the simulation box results in some increase in diffusion coefficients, with the effect being less pronounced at high asphaltene concentrations. Overall, our findings provide valuable insight into the aggregation behavior of asphaltenes on spatial and time scales that are normally beyond the scales accessible for atomistic simulations.

Nataj Z.E., Xu Y., Wright D., Brown J.O., Garg J., Chen X., Kargar F., Balandin A.A.

AbstractThe development of cryogenic semiconductor electronics and superconducting quantum computing requires composite materials that can provide both thermal conduction and thermal insulation. We demonstrated that at cryogenic temperatures, the thermal conductivity of graphene composites can be both higher and lower than that of the reference pristine epoxy, depending on the graphene filler loading and temperature. There exists a well-defined cross-over temperature—above it, the thermal conductivity of composites increases with the addition of graphene; below it, the thermal conductivity decreases with the addition of graphene. The counter-intuitive trend was explained by the specificity of heat conduction at low temperatures: graphene fillers can serve as, both, the scattering centers for phonons in the matrix material and as the conduits of heat. We offer a physical model that explains the experimental trends by the increasing effect of the thermal boundary resistance at cryogenic temperatures and the anomalous thermal percolation threshold, which becomes temperature dependent. The obtained results suggest the possibility of using graphene composites for, both, removing the heat and thermally insulating components at cryogenic temperatures—a capability important for quantum computing and cryogenically cooled conventional electronics.

Yoon D., Lee H., Kim T., Song Y., Lee T., Lee J., Hun Seol J.

Due to their remarkable mechanical, chemical, and electrical properties, polymers are widely utilized in industry. However, the low thermal conductivity of polymers limits more extended applications of them that require high degrees of heat dissipation. Here, we demonstrate that the combination of increased chain orientation and powerful intermolecular interaction can boost the thermal conductivity of polyimide (PI). The thermal conductivity values of electrospun individual PI nanofibers (INF-PI) and hot-pressed nanofibrous PI films (HP-PI) formed with these nanofibers were as high as 0.44 and 0.98 W m−1 K−1, respectively, at room temperature. The former and the latter were measured by the suspended microdevice and hot-disk methods, respectively. These values are considerably higher than the solution-casting PI film (SC-PI) value of 0.1 W m−1 K−1. Furthermore, molecular-scale structural characterizations revealed that the electrospinning process improved chain orientation and that high pressure and thermal treatment during the hot-pressing process facilitated the formation of π–π interactions, resulting in an exceptionally high thermal conductivity in HP-PI.

Ouinten M., Szymczyk A., Ghoufi A.

Organic solvent nanofiltration (OSN) has recently proved to be a promising separation process thanks to the development of membrane materials with suitable resistance toward organic solvents. Among those materials, P84 polyimide membranes are currently the most used in OSN while PIM-1 membranes have recently attracted attention due to their high permeance in apolar solvents and alcohols. Both P84 and PIM-1 membranes have nanosized free volumes, and their separation performance is finely connected to polymer/solvent interactions. Consequently, modeling OSN membranes at the molecular scale is highly desirable in order to rationalize experimental observations and gain a deeper insight into the molecular mechanisms ruling solvent and solute permeation. A prerequisite for understanding solvent transport through OSN membranes is therefore to characterize the membrane/solvent interactions at the molecular level. For that purpose, we carried out molecular simulations of three different solvents, acetone, methanol, and toluene in contact with P84 and PIM-1 membranes. The solvent uptake by both membranes was found to be correlated to the degree of confinement of the solvent, the polymer swelling ability and polymer/solvent interactions. The translational dynamics of the solvent molecules in the PIM-1 membrane was found to be correlated with the solvent viscosity due to the relatively large pores of this membrane. That was not the case with the P84 membrane, which has a much denser structure than the PIM-1 membrane and for which it was observed that the translational dynamics of the confined solvent molecules was directly correlated to the affinity between the P84 polymer and the solvent.

Gorbacheva S.N., Borisova Y.Y., Makarova V.V., Antonov S.V., Borisov D.N., Yakubov M.R.

The low thermal conductivity of paraffin and other organic phase change materials limits their use in thermal energy storage devices. The introduction of components with a high thermal conductivity such as graphene into these materials leads to an increase in their thermal conductivity. In this work, we studied the use of inexpensive carbon fillers containing a polycyclic aromatic core, due to them having a structural similarity with graphene, to increase the thermal conductivity of paraffin. As such fillers, technogenic asphaltenes isolated from ethylene tar and their modified derivatives were used. It is shown that the optimal concentration of carbon fillers in the paraffin composite, which contributes to the formation of a structural framework and resistance to sedimentation, is 5 and 30 wt. %, while intermediate concentrations are ineffective, apparently due to the formation of large aggregates, the concentration of which is insufficient to form a strong framework. It has been found that the addition of asphaltenes modified with ammonium persulfate in acetic acid significantly increases the thermal conductivity of paraffin by up to 72%.

Dai B., Liu C., Liu S., Wang D., Wang Q., Zou T., Zhou X.

• CO 2 dual-temperature evaporation HTHP systems are proposed to recover waste heat. • Energy efficiency can be improved with ejector and dual-temperature evaporator. • New proposed HTHP shows superior life cycle carbon emissions and cost performances. • Ej-Evap2-C system has the shortest payback period compared with coal-fired boiler. Replacing fuel-fired boilers by using efficient heat pump plants to recover industrial waste heat is an effective solution to achieve the “dual carbon” target. Three novel transcritical CO 2 high-temperature heat pump systems (Ej-Evap2-A, Ej-Evap2-B, and Ej-Evap2-C) are proposed in this study, by introducing the technique of dual-temperature evaporation realized with an ejector for cascade heat absorption from the heat source. Considering the application in the scenario of industry requirement of hot water heating, the life cycle performances of the new proposed heat pump systems and fuel-fired boilers are comprehensively studied from the perspectives of energetic, emissions, and economic. A sensitivity analysis about the new configuration heat pump system is also conducted considering the variation in electricity and coal price. The results demonstrate there exists an optimum discharge pressure that maximizes the coefficient of performance (COP). Ej-Evap2-C shows a maximum COP of 4.85, which is 14.40% higher than the baseline CO 2 heat pump system (Base), and the exergy efficiency of Ej-Evap2-C is 7.86%∼15.19% higher than that of Base. Among the eight heating methods including coal-fired boilers (CFB), gas-fired boilers (GFB), electric heating boiler (EHB) and five kinds of CO 2 heat pump systems, Ej-Evap2-C shows the least pollutant emissions and life cycle cost. Furthermore, Ej-Evap2-C has the shortest payback period of fewer than 7 years compared with the CFB. The dual-temperature evaporation CO 2 high-temperature heat pump is promising to substitute traditional fuel-fired boilers to generate high-temperature fluid in the future.

Nazarychev V.M., Glova A.D., Larin S.V., Lyulin A.V., Lyulin S.V., Gurtovenko A.A.

A molecular-level insight into phase transformations is in great demand for many molecular systems. It can be gained through computer simulations in which cooling is applied to a system at a constant rate. However, the impact of the cooling rate on the crystallization process is largely unknown. To this end, here we performed atomic-scale molecular dynamics simulations of organic phase-change materials (paraffins), in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold (1.2 × 1011 K/min) in the values of cooling rates exists. When cooling is slower than the threshold, the simulations qualitatively reproduce an experimentally observed abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity of paraffins upon crystallization. Beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the supercooled liquid state. Thus, a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials, which are of great promise for use in domestic heat storage devices.

Volgin I.V., Batyr P.A., Matseevich A.V., Dobrovskiy A.Y., Andreeva M.V., Nazarychev V.M., Larin S.V., Goikhman M.Y., Vizilter Y.V., Askadskii A.A., Lyulin S.V.

Larin S.V., Makarova V.V., Gorbacheva S.N., Yakubov M.R., Antonov S.V., Borzdun N.I., Glova A.D., Nazarychev V.M., Gurtovenko A.A., Lyulin S.V.

Adding carbon nanoparticles into organic phase change materials (PCMs) such as paraffin is a common way to enhance their thermal conductivity and to improve the efficiency of heat storage devices. However, the sedimentation stability of such blends can be low due to aggregation of aromatic carbon nanoparticles in the aliphatic paraffin environment. In this paper, we explore whether this important issue can be resolved by the introduction of a polymer agent such as poly(3-hexylthiophene) (P3HT) into the paraffin–nanoparticle blends: P3HT could ensure the compatibility of aromatic carbon nanoparticles with aliphatic paraffin chains. We employed a combination of experimental and computational approaches to determine the impact of P3HT addition on the properties of organic PCMs composed of paraffin and carbon nanoparticles (asphaltenes). Our findings clearly show an increase in the sedimentation stability of paraffin–asphaltene blends, when P3HT is added, through a decrease in average size of asphaltene aggregates as well as in an increase of the blends’ viscosity. We also witness the appearance of the yield strength and gel-like behavior of the mixtures. At the same time, the presence of P3HT in the blends has almost no effect on their thermophysical properties. This implies that all properties of the blends, which are critical for heat storage applications, are well preserved. Thus, we demonstrated that adding polyalkylthiophenes to paraffin–asphaltene mixtures led to significant improvement in the performance characteristics of these systems. Therefore, the polymer additives can serve as promising compatibilizers for organic PCMs composed of paraffins and asphaltenes and other types of carbon nanoparticles.

Nazarychev V., Glova A., Larin S., Lyulin A., Lyulin S., Gurtovenko A.

A molecular-level insight into the phase transformations is of great demand for the molecular systems for which the phase transition is a key process from the point of view of practical applications, organic phase-change materials being one of the most prominent examples. Such a detailed insight can be gained through cooling-rate computer simulations, i.e. simulations in which cooling is applied to a system at a constant rate. However, the ability of the simulations to give an accurate description of the phase transition process is an open question, since the cooling rates accessible in atomic-scale computer simulations are many orders of magnitude larger than those in experiments. In this paper we present the first computational study that systematically explores as to how the cooling rate affects the crystallization of organic phase-change materials such as paraffins. To this end, we performed a series of atomistic molecular dynamics simulations in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold in the values of cooling rates exists. When cooling is performed with the rates smaller than the threshold, computer simulations are able to qualitatively reproduce the crystallization process in paraffins. This includes an abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity upon crystallization. The crystallization enthalpy for these cooling rates was found to agree quantitatively with experiment. However, beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the super-cooled liquid state. As a result, one can observe a systematic decrease in the fraction of crystalline domains in a paraffin sample with cooling rate, leading eventually to a complete lack of crystallization at low temperatures. Both the crystallization temperature and the crystallinity decrease with cooling rate in line with experimental data. All the above strongly supports the conclusion that a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials such as paraffins. Cooling with the rates that are too high can lead to a completely inaccurate description of the crystallization process, as well as the thermophysical and structural properties.