Atomistic-scale insights into the crosslinking of polyethylene induced by peroxides

Management of selected waste generated during cable production

The subject of the research was the recovery of raw materials from waste generated in the production of cable insulation and the management of aluminum sludge. It was found that 49% (w/w) acetophenone, 6.8% (w/w) α-methylstyrene, and 17.2% (w/w) cumyl alcohol can be recovered from waste with a loss on ignition of 95% and used in various industries. A gas chromatograph equipped with a mass spectrometry detector was used to identify the recovered compounds. A waste distillation process was proposed to remove the water layer and obtain a concentrated acetophenone fraction. A method of neutralizing the water fraction and distillation residues is presented. The proposed waste management method is an alternative method to the currently used thermal transformation method. In turn, aluminum sludge was used to produce aluminum sulfate, which was used in the plant's sewage treatment plant as a coagulant. The effect of this action was a reduction of 67% in the content of total iron, 60% of trivalent iron, and 32% of chemical oxygen demand. The above-mentioned examples of waste management are part of a closed-loop waste management strategy.

Model Simulation and Rheological Research on Crosslinking Behavior of Polyethylene Resin

The crosslinking behavior of polyethylene (PE) determines its exceptional performance and application. In this study, we investigated the crosslinking behaviors of different PE resins through model simulation and rheological methods. Specifically, the mathematical equation of "S" model was established for PE resin. According to this equation, the optimal maximum gel content for high-density polyethylene (HDPE) was found to be around 85%. Moreover, the maximum crosslinking degrees for different PE resins depended largely on their density and molecular weight. The melt viscosities before crosslinking in PE resins were highly influenced by their melt index. The higher melt indexes resulted in the lower storage moduli, improving melt processability during processing. In addition, the crosslinking rates of PE resins were strongly influenced by peroxide concentration, independent of PE resin structures. For high molecular weight and low-density PE resins, they exhibited decreased ti values, increased A0 values, and decreased k6 values. However, there were no noticeable variations in the values of k2 and phi among different PE resins. All simulated modeling outcomes showed remarkable consistency with the experimental rheological data. These findings are of strong significance in the industrial manufacture of PE resin.

Hydroformylation of pyrolysis oils to aldehydes and alcohols from polyolefin waste

Waste plastics are an abundant feedstock for the production of renewable chemicals. Pyrolysis of waste plastics produces pyrolysis oils with high concentrations of olefins (>50 weight %). The traditional petrochemical industry uses several energy-intensive steps to produce olefins from fossil feedstocks such as naphtha, natural gas, and crude oil. In this work, we demonstrate that pyrolysis oil can be used to produce aldehydes through hydroformylation, taking advantage of the olefin functionality. These aldehydes can then be reduced to mono- and dialcohols, oxidized to mono- and dicarboxylic acids, or aminated to mono- and diamines by using homogeneous and heterogeneous catalysis. This route produces high-value oxygenated chemicals from low-value postconsumer recycled polyethylene. We project that the chemicals produced by this route could lower greenhouse gas emissions ~60% compared with their production through petroleum feedstocks.

Investigation of the Rheological Properties and Storage Stability of Waste Polyethylene/Ethylene-Vinyl Acetate-Modified Asphalt with Crosslinking and a Silicone Coupling Agent

As the market for polyethylene consumption continues to expand, the amount of waste polyethylene is also increasing. Modifying asphalt with waste polyethylene (PE) is economical and environmentally friendly. The low-temperature performance and storage stability of PE-modified asphalt has long been an insurmountable problem. The high vinyl acetate (VA) content of ethylene-vinyl acetate (EVA) and PE blended into asphalt can improve the compatibility of PE and asphalt. It compensates for the high VA content of EVA brought about by the lack of high-temperature resistance to permanent deformation but is still not conducive to the stable storage of PE at high temperatures. The effect of furfural extraction oil, a crosslinking (DCP) agent, a silicone coupling agent (KH-570), and calcium carbonate (CaCO₃) on the rheological properties and compatibility of PE/EVA-modified asphalt was investigated in this study. The conventional physical properties of PE/EVA-modified asphalt were tested after introducing furfural extraction oil, DCP, KH570, and CaCO₃ to determine the correlations of these materials. In addition, frequency sweep, multiple stress creep and recovery (MSCR), and linear amplitude sweep (LAS) were utilized to characterize the rheological properties and fatigue behavior. The results reveal that the addition of suitable ratios of furfural extract oil, DCP, KH-570, and CaCO₃ to PE/EVA-modified asphalt produces a remarkable improvement in the viscoelastic characteristics and viscosity compared with PE/EVA-modified asphalt. Furthermore, fluorescence microscopy (FM) was utilized to evaluate the modification mechanism, which shows that PE/EVA undergoes significant crosslinking in asphalt, forming a three-dimensional network structure that dissolves in the asphalt. The storage stability of the PE-modified bitumen was fully determined, and its high-temperature rheology was substantially improved.

Improving the DC Dielectric Properties of XLPE with Appropriate Content of Dicumyl Peroxide for HVDC Cables Insulation

In this research, crosslinked polyethylene (XLPE) is developed with selective content of dicumyl peroxide (DCP), and the influence of microstructural properties and chemical composition on the mechanical and direct current (DC) dielectric properties are investigated. The measurements for the microstructural analysis are taken by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), gel content test and Fourier transform infrared (FTIR). The mechanical properties of XLPE are evaluated by hot-set test. The results of microstructural and chemical composition show that the increase in DCP content increases the crosslinking degree from 74.3% to 81.6%, reduces the crystallinity/lamella thickness from 36.8% to 35.5%/7.6-7.1 nm, reduces the average molecular weight between two crosslinks by 0.01 kg/mol and reduces the oxidation level/carbonyl index. The increase in DCP in XLPE samples decreases the permanent elongation from 2.2% to 0% and elongation rate from 300% to 80% of the cable insulation. The rise in DCP content increases the crosslinking degree due to which the DC resistivity and activation energy is increased. The DC breakdown strength at 30-90 °C is increased due to the increase in crosslinking degree and reduction in carbonyl index/oxidation level. The space charge accumulation is measured at 30 °C under 20-60 kV/mm, resulting in less homo-charges and hetero-charges with the increase in DCP. It is proven that the role of appropriate DCP content is vital in increasing the DC dielectric performance, internal material characteristics and mechanical performance of XLPE.

Using C-DFT to develop an e-ReaxFF force field for acetophenone radical anion

Increased electricity usage over the past several decades has accelerated the need for efficient high-voltage power transmission with reliable insulating materials. Cross-linked polyethylene (XLPE) prepared via dicumyl peroxide (DCP) cross-linking has emerged as the insulator of choice for modern power cables. Although DCP cross-linking generates the desired XLPE product in high yield, other by-products are also produced. One such by-product, acetophenone, is particularly intriguing due to its aromaticity and positive electron affinity. In this work, constrained density functional theory (C-DFT) was utilized to develop an e-ReaxFF force field suitable for describing the acetophenone radical anion. Initial parameters were taken from the 2021 Akbarian e-ReaxFF force field, which was developed to describe XLPE chemistry. Then, C-DFT geometry optimizations were performed wherein an excess electron was constrained to each atom of acetophenone. The resulting C-DFT energy values for the various electronic positions were added to the e-ReaxFF training set. Next, an analogous set of structures was energy-minimized using e-ReaxFF, and equilibrium mixture compositions for the two methods were compared at multiple temperatures. Iterative fitting against C-DFT energy data was performed until satisfactory agreement was achieved. To test force field performance, molecular dynamics simulations were performed in e-ReaxFF and the resulting electronic distributions were qualitatively compared to unconstrained-DFT spin density data. By expanding our e-ReaxFF force field for XLPE, namely, adding the capability to describe acetophenone and its interactions with an excess electron, we move one step closer to a comprehensive molecular understanding of XLPE chemistry in a high-voltage power cable.

Machine Learning-Assisted Hybrid ReaxFF Simulations

We have developed a machine learning (ML)-assisted Hybrid ReaxFF simulation method ("Hybrid/Reax"), which alternates reactive and non-reactive molecular dynamics simulations with the assistance of ML models to simulate phenomena that require longer time scales and/or larger systems than are typically accessible to ReaxFF. Hybrid/Reax uses a specialized tracking tool during the reactive simulations to further accelerate chemical reactions. Non-reactive simulations are used to equilibrate the system after the reactive simulation stage. ML models are used between reactive and non-reactive stages to predict non-reactive force field parameters of the system based on the updated bond topology. Hybrid/Reax simulation cycles can be continued until the desired chemical reactions are observed. As a case study, this method was used to study the cross-linking of a polyethylene (PE) matrix analogue (decane) with the cross-linking agent dicumyl peroxide (DCP). We were able to run relatively long simulations [>20 million molecular dynamics (MD) steps] on a small test system (4660 atoms) to simulate cross-linking reactions of PE in the presence of DCP. Starting with 80 PE molecules, more than half of them cross-linked by the end of the Hybrid/Reax cycles on a single Xeon processor in under 48 h. This simulation would take approximately 1 month if run with pure ReaxFF MD on the same machine.

DFT Functionals for Modeling of Polyethylene Chains Cross-Linked by Metal Atoms. DLPNO-CCSD(T) Benchmark Calculations

Density functional theory (DFT) functionals for calculations of binding energies (BEs) of the polyethylene (PE) chains cross-linked by selected metal atoms (M) are benchmarked against DLPNO-CCSD(T) and DLPNO-CCSD(T1) data. PEX-M-PEX complexes as compared with plain parallel PEX···PEX chains with X = 3-9 carbon atoms are model species characterized by a cooperative effect of covalent C-M-C bonds and interchain dispersion interactions. The accuracy of DLPNO-CC methods was assessed by a comparison of BEs with the canonical CCSD(T) results for small PE3-M-PE3 complexes. Functionals for PEX···PEX and closed-shell PEX-M-PEX complexes (M = Be, Mg, Zn) were benchmarked against DLPNO-CCSD(T) BEs; open-shell complexes (M = Li, Ag, Au) were benchmarked against the DLPNO-CCSD(T1) method with iterative triples. Three dispersion corrections were combined with 25 DFT functionals for calculations of BEs with respect to PEX-M and PEX fragments employing def2-TZVPP and def2-QZVPP basis sets. Accuracy to within 5% for the closed-shell PEX-M-PEX complexes was achieved with five functionals. Less accurate are functionals for the open-shell PEX-M-PEX complexes; only two functionals deviate by less than 15% from DLPNO-CCSD(T1). Particularly problematic were PEX-Li-PEX complexes. A reasonable overall performance across all complexes in terms of the mean absolute percentage error is found for the range-separated hybrid functionals ωB97X-D3 and CAM-B3LYP/D3(BJ)-ABC.

Numerical Analysis of a Continuous Vulcanization Line to Enhance CH4 Reduction in XLPE-Insulated Cables

Herein, we apply a computational diffusion model based on Fick’s law to study the manner in which a cable production line and its operating conditions can be enhanced to effectively reduce the CH4 concentration in cables insulated with cross-linked polyethylene (XLPE). Thus, we quantitatively analyze the effect of the conductor temperature, curing tube temperature distribution, transition zone length, and online relaxation on CH4 generation and transport during the production of 132 kV cables with an insulation thickness of 16.3 mm. Results show that the conductor temperature, which is initially controlled by a preheater, and the curing tube temperature distribution considerably affect the CH4 concentration in the cable because of their direct impact on the insulation temperature. The simulation results show 2.7% less CH4 remaining in the cable when the preheater is set at 160 °C compared with that when no preheater is used. To study the curing tube temperature distribution, we consider three distribution patterns across the curing tube: constant temperature and linear incremental and decremental temperature. The amount of CH4 remaining in the cable when the temperature was linearly increased from 300 to 400 °C was 1.6% and 3.7% lower than in the cases with a constant temperature at 350 °C and a linear temperature decrease from 400 to 300 °C, respectively. In addition, simulations demonstrate that the amount of CH4 removal from the cable can be increased up to 9.7% by applying an elongated and insulated transition zone, which extends the residence time for CH4 removal and decelerates the decrease in cable temperature. Finally, simulations show that the addition of the online relaxation section can reduce the CH4 concentration in the cable because the high cable temperature in this section facilitates CH4 removal up to 2.2%, and this effect becomes greater at low production speeds.

Water distillation modeling by disjoint CNT-based channels under the influence of external electric fields

Using molecular dynamics method, the ion rejection and water flow inside flexible disjoint carbon-based channels were examined in the presence of electric fields. The effects of the carbon nanotube diameters and field magnitude on the nano-channel efficiency were investigated. It was observed that water flow through the filter was modified by increasing the radius of nanotubes, while the salt rejection was reduced. The particles' behaviors inside the channel were described in view of Van der Waals interactions between the water molecules, ions, and carbon atoms. Furthermore, the results indicated that the ion rejection and water flow were increased under the application of proper magnitude of electric fields. Graphical abstract Using MD simulation method, a disjoint CNT-based filter was designed to produce freshwater from a NaCl solution by the aid of external electric field. It was observed that the filter operation was significantly affected by channel structural parameters and amount of applied electric fields.

Full-Scale Ab Initio Simulation of Magic-Angle-Spinning Dynamic Nuclear Polarization

Theoretical models aimed at describing magic-angle-spinning (MAS) dynamic nuclear polarization (DNP) NMR have great potential in facilitating the in silico design of DNP polarizing agents and formulations. These models must typically face a trade-off between the accuracy of a strict quantum mechanical description and the need for using realistically large spin systems, for instance, using phenomenological models. Here, we show that the use of aggressive state-space restrictions and an optimization strategy allows full-scale ab initio MAS-DNP simulations of spin systems containing thousands of nuclei. Our simulations are shown to reproduce experimental DNP enhancements quantitatively, including their MAS rate dependence, for both frozen solutions and solid materials. They also reveal the importance of a previously unrecognized structural feature found in some polarizing agents that helps minimize the sensitivity losses imposed by the spin diffusion barrier.

Simulations of the Biodegradation of Citrate-Based Polymers for Artificial Scaffolds Using Accelerated Reactive Molecular Dynamics

In this study, we investigate the reactivity and mechanical properties of poly(1,6-hexanediol-co-citric acid) via ReaxFF molecular dynamics simulations. We implement an accelerated scheme within the ReaxFF framework to study the hydrolysis reaction of the polymer which is provided with a sufficient amount of energy known as the restrain energy after a suitable pretransition-state configuration is obtained to overcome the activation energy barrier and the desired product is obtained. The validity of the ReaxFF force field is established by comparing the ReaxFF energy barriers of ester and ether hydrolysis with benchmark DFT values in the literature. We perform chemical and mechanical degradation of polymer chain bundles at 300 K. We find that ester hydrolyzes faster than ether because of the lower activation energy barrier of the reaction. The selectivity of the bond-boost scheme has been demonstrated by lowering the boost parameters of the accelerated simulation, which almost stops the ether hydrolysis. Mechanical degradation of prehydrolyzed and intermittent hydrolyzed polymer bundles is performed along the longitudinal direction at two different strain rates. We find that the tensile modulus of the polymers increases with increase in strain rates, which shows that polymers show a strain-dependent behavior. The tensile modulus of the polyester-ether is higher than polyester but reaches yield stress faster than polyester. This makes polyester more ductile than polyester-ether.

Atomistic investigation on effect of Ca doping ratio on mechanical behaviors of nanocrystalline Mg-Ca alloys

The effects of doping ratio of calcium (Ca) on mechanical behaviors are investigated using molecular dynamics (MD) and the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism for nanocrystalline (NC) Mg-Ca alloys system. Research results indicate that mechanical behaviors of Mg-Ca alloys are independent of lower strain rate (under 1.0 × 10⁹ s^-1). In addition, we observe that Ca doping can affect the mechanical properties of the Mg-Ca alloys, and the optimal 2.0 at% of Ca atoms, which has excellent plasticity, is revealed. When the doping ratio is lower than critical atomic percent (CAT) of Mg₂Ca, Young's modulus and yield stress decrease increasing at% of substitutional Ca. The pyramidal <c + a > dislocations are observed frequently at more active grain boundary (GB) with higher Ca doping ratios. In contrast, with doping ratio above CAT, Mg₂Ca reinforcement dominates brittleness Mg/Mg₂Ca nanocomposites to obtain high strength. By calculating, a significant increase of strength is discovered when at% of Mg₂Ca is above 18.85 (5.34 at% Ca). Intergranular fractures are more likely to nucleate and propagate along weaker Mg/Mg₂Ca interfaces. These results are instrumental in design and improving the mechanical properties of Mg-Ca alloys.

Understanding the anticorrosive mechanism of a cross-linked supramolecular polymer for mild steel in the condensate water: comprehensive experimental, molecular docking, and molecular dynamics investigations

A supramolecular polymer (CDP-DA) was prepared through the crosslinking reaction among the assembled complexes (CDDA) based on β-cyclodextrin (β-CyD) and octadecylamine (ODA). The structural properties of CDP-DA were clarified by experimental techniques such as Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, and thermal analysis. Based on the results of molecular docking, the crosslinking unit, CDDA, in the CDP-DA structure favors to exhibit the configuration that narrow rims of two host compounds (β-CyD) are opposite to each other leaving the amino group of ODA outside the host cavity. The corrosion inhibition performance of CDP-DA for mild steel in industrial condensate water was evaluated by electrochemical measurements and surface analyses, the mechanism of which was disclosed by molecular dynamics simulations in the aspects of adsorption equilibrium and ions diffusion models. The results of electrochemical tests indicate that CDP-DA effectively retards the anodic and cathodic reactions and improves the interfacial charge transfer resistance of mild steel in condensate water, which can be categorized as the mixed-type inhibitor. Surface analyses reveal that CDP-DA adsorbs on the steel surface in the integral form showing a monolayer nature, which is consolidated by molecular dynamics simulations. The diffusion behavior of in situ ions in the adsorbed layer is prominently suppressed as compared with those in bulk solution. The robust barrier layer and the mitigated diffusion of ions may contribute to the effective inhibition for CDP-DA against steel deterioration in the condensate water. Anticorrosive mechanism of a cross-linked supramolecular polymer for mild steel in the condensate water.

Molecular dynamics simulation of reversible electroporation with Martini force field

Background

After the discovery of membrane-reversible electroporation decades ago, the procedure has been used extensively in biology, biotechnology and medicine. The research on the basic mechanism has increasingly attracted attention. Although most research has focused on models that consider all atomic and molecular interactions and much atomic-level information can be obtained, the huge computational demand limits the models to simulations of only a few nanometers on the spatial scale and a few nanoseconds on the time scale. In order to more comprehensively study the reversible electroporation mechanism of phospholipid membrane on the nanoscale and at longer time intervals of up to 100 ns, we developed a dipalmitoylphosphatidylcholine (DPPC) phospholipid membrane model with the coarse-grained Martini force field. The model was tested by separately examining the morphology of the phospholipid membrane, the hydrophilic channel size, the distribution of the voltage potential on both sides of the membrane, and the movement of water molecules and ions during electroporation.

Results

The results showed that the process went through several stages: (1) the formation of the pore with defects originating on the surface. (2) The maintenance of the pore. The defects expanded to large pores and the size remains unchanged for several nanoseconds. (3) Pore healing stage due to self-assembly. Phospholipid membrane shrunk and the pore size decreased until completely closed. The pores were not circular in cross-section for most of the time and the potential difference across the membrane decreased dramatically after the pores formed, with almost no restoration of membrane integrity even when the pores started to close.

Conclusions

The mechanism of the reversible electroporation process on the nanoscale level, including defects, expansion, stability, and pore closing stages on a longer time scale of up to 100 ns was demonstrated more comprehensively with the coarse-grained Martini force field, which took both the necessary molecular information and the calculation efficiency into account.