Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene: Topological Characteristics and Mechanical Behavior

A Review of Polyolefin-Insulation Materials in High Voltage Transmission; From Electronic Structures to Final Products

This review focuses on the use of polyolefins in high-voltage direct-current (HVDC) cables and capacitors. A short description of the latest evolution and current use of HVDC cables and capacitors is first provided, followed by the basics of electric insulation and capacitor functions. Methods to determine dielectric properties are described, including charge transport, space charges, resistivity, dielectric loss, and breakdown strength. The semicrystalline structure of polyethylene and isotactic polypropylene is described, and the way it relates to the dielectric properties is discussed. A significant part of the review is devoted to describing the state of art of the modeling and prediction of electric or dielectric properties of polyolefins with consideration of both atomistic and continuum approaches. Furthermore, the effects of the purity of the materials and the presence of nanoparticles are presented, and the review ends with the sustainability aspects of these materials. In summary, the effective use of modeling in combination with experimental work is described as an important route toward understanding and designing the next generations of materials for electrical insulation in high-voltage transmission.

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.

Investigation of Crystallization and Relaxation Effects in Coarse-Grained Polyethylene Systems after Uniaxial Stretching

In this study, we investigate the thermo-mechanical relaxation and crystallization behavior of polyethylene using mesoscale molecular dynamics simulations. Our models specifically mimic constraints that occur in real-life polymer processing: After strong uniaxial stretching of the melt, we quench and release the polymer chains at different loading conditions. These conditions allow for free or hindered shrinkage, respectively. We present the shrinkage and swelling behavior as well as the crystallization kinetics over up to 600 ns simulation time. We are able to precisely evaluate how the interplay of chain length, temperature, local entanglements and orientation of chain segments influences crystallization and relaxation behavior. From our models, we determine the temperature dependent crystallization rate of polyethylene, including crystallization onset temperature.

Influence of Oligopeptide Length and Distribution on Polyisoprene Properties

The tuning of binding modes of polar groups is the key step to mimicking the structure and properties of natural rubber through the molecular design of synthetic polyisoprenes. Herein, the ordering and binding distances of oligopeptides could be altered systematically by changing their lengths and distribution along the polyisoprene chain, which impose huge impacts on the mechanical properties and chain dynamics of green rubber. In detail, a series of peptide-functionalized polyisoprenes with terminal blocks (B-2A-PIP, B-3A-PIP) or random sequences (R-2A-PIP, R-3A-PIP) are fabricated by using dipeptides (2A) or tripeptides (3A) as crosslinkers to explore the mechanism of terminal interaction on mechanism properties and chain dynamics. B-4A-PIP and R-4A-PIP served as control samples. It is found that the increased oligopeptide length and the block distribution improves the mechanical properties and confine the chain movement by elevate the contents of ordered and compact microstructures, which is indicated by XRD, broadband dielectric spectroscopy (BDS) and consistent with the result of molecular dynamics simulation. New relaxation signals belonging to oligopeptide aggregates are found which showed elevated dielectric strengths upon temperatures increase. Additionally, it also reveals that the binding modes of oligopeptide do not significantly influence the entanglements of polyisoprene.

Molecular dynamics simulation of extension-induced crystallization of branched bimodal HDPE: Unraveling the effects of short-chain branches

Bimodal HDPE models were designed for extension-induced crystallization imitating the architecture of industrial bimodal HDPE copolymerized with ethylene and 1-butene, 1-hexene, or 1-octene. Crystallites of bimodal HDPE experienced the emergence of precursors, shish nuclei, and lamellae. The compact conformation of branched polymers impeded the rolling-over, deposition, and folding of chains on the substrate, and thus the formation of nuclei and lamella. Moreover, this retardation was intensified with the rising branch density and length, causing a depression of crystallinity and an increment of tie-chains concentration. Besides, when branches were all located on long chains, the compact conformation enlarged the resistance to the disentanglement of main chains, thus relatively fewer branched long chains were involved in the precursors or nuclei, resulting in the attenuation of lamella formation. Furthermore, for ethyl branched polymers, the coexistent orthorhombic and monoclinic crystallites were built up, and a few expanded monoclinic cells occurred for butyl branches because of the larger butyl reeling into lamella, while hexagonal crystals were created for ethyl/1-hexyl copolymers because of cocrystallization. Additionally, relative to ethyl, larger butyl and hexyl were preferential to be repelled outside crystals to form tie-chains, and hexyl branched polymers acquired relatively fewer tie-chains because of hexagonal eutectoid.

Understanding thermal and rheological behaviors of bimodal polymethyl methacrylate (BPMMA) fabricated via solution blending

In this work, a series of bimodal polymethyl methacrylate (BPMMA) was fabricated via solution-blending two neat PMMA resins. Rheology, DMTA, thermal infrared imager measurements were used in an attempt to probe the internal structure of the as-prepared BPMMA. It was demonstrated that the thermorheological behavior of the BPMMA was heavily dependent on shear rate, temperature as well as blending ratio. In addition, a typical “V-shaped” response, namely, a dip in storage modulus (G′) followed by an upturn in the plot of G′ versus measuring temperature for D4 (with lower weight-average molecular weight) was observed, characteristic of occurrence of thermorheological complexity. Our experimental results of physical–mechanical testings suggested that the BPMMA had better comprehensive properties than those of their neat PMMA counterparts.

Atomistic Investigation on the Wetting Behavior and Interfacial Joining of Polymer-Metal Interface

Polymer-metal hybrid structures can reduce the weight of components while ensuring the structural strength, which in turn save cost and subsequently fuel consumption. The interface strength of polymer-metal hybrid structure is mainly determined by the synergistic effects of interfacial interaction and mechanical interlocking. In this study, the wetting behavior of polypropylene (PP) melt on metal surface was studied by molecular dynamics simulation. Atomistic models with smooth surface and nano-column arrays on Al substrate were constructed. Influences of melt temperature, surface roughness and metal material on the wetting behavior and interfacial joining were analyzed. Afterwards the separation process of injection-molded PP-metal hybrid structure was simulated to analyze joining strength. Results show that the initially sphere-like PP model gradually collapses in the wetting simulation. With a higher temperature, it is easier for molecule chains to spread along the surface. For substrate with rough surface, high density is observed at the bottom or on the upper surface of the column. The contact state is transitioning from Wenzel state to Cassie–Baxter state with the decrease of void fraction. The inner force of injection-molded PP-Fe hybrid structure during the separation process is obviously higher, demonstrating a greater joining strength.

Property Decoupling across the Embryonic Nucleus-Melt Interface during Polymer Crystal Nucleation

Spatial distributions are presented that quantitatively capture how polymer properties (e.g., segment alignment, density, and potential energy) vary with distance from nascent polymer crystals (nuclei) in prototypical polyethylene melts. It is revealed that the spatial extent of nuclei and their interfaces is metric-dependent as is the extent to which nucleus interiors are solid-like. As distance from a nucleus increases, some properties, such as density, decay to melt-like behavior more rapidly than polymer segment alignment, indicating that a polymer nucleus resides in a nematic-like droplet. This nematic-like droplet region coincides with enhanced formation of ordered polymer segments that are not part of the nucleus. It is more favorable to find nonconstituent ordered polymer segments near a nucleus than in the surrounding metastable melt, pointing to the possibility of one nucleus inducing the formation of other nuclei. In this vein, there is also a second region of enhanced ordering that lies along the nematic director of a nucleus, but beyond its nematic droplet and fold regions. These results indicate that crystal stacking, a key characteristic of lamellae in semicrystalline polymeric materials, begins to emerge during the earliest stages of polymer crystallization (i.e., crystal nucleation). More generally, the findings of this study provide a conceptual bridge between polymer crystal nucleation under nonflow and flow conditions and are used to rationalize previous results.

Monodisperse Polymer Melts Crystallize via Structurally Polydisperse Nanoscale Clusters: Insights from Polyethylene

This study demonstrates that monodisperse entangled polymer melts crystallize via the formation of nanoscale nascent polymer crystals (i.e., nuclei) that exhibit substantial variability in terms of their constituent crystalline polymer chain segments (stems). More specifically, large-scale coarse-grain molecular simulations are used to quantify the evolution of stem length distributions and their properties during the formation of polymer nuclei in supercooled prototypical polyethylene melts. Stems can adopt a range of lengths within an individual nucleus (e.g., ∼1-10 nm) while two nuclei of comparable size can have markedly different stem distributions. As such, the attainment of chemically monodisperse polymer specimens is not sufficient to achieve physical uniformity and consistency. Furthermore, stem length distributions and their evolution indicate that polymer crystal nucleation (i.e., the initial emergence of a nascent crystal) is phenomenologically distinct from crystal growth. These results highlight that the tailoring of polymeric materials requires strategies for controlling polymer crystal nucleation and growth at the nanoscale.

Lamellae-controlled electrical properties of polyethylene - morphology, oxidation and effects of antioxidant on the DC conductivity

Destruction of the spherulite structure in low-density polyethylene (LDPE) is shown to result in a more insulating material at low temperatures, while the reverse effect is observed at high temperatures. On average, the change in morphology reduced the conductivity by a factor of 4, but this morphology-related decrease in conductivity was relatively small compared with the conductivity drop of more than 2 decades that was observed after slight oxidation of the LDPE (at 25 °C and 30 kV mm^-1). The conductivity of LDPE was measured at different temperatures (25-60 °C) and at different electrical field strengths (3.3-30 kV mm^-1) for multiple samples with a total crystalline content of 51 wt%. The transformation from a 5 μm coherent structure of spherulites in the LDPE to an evenly dispersed random lamellar phase (with retained crystallinity) was achieved by extrusion melt processing. The addition of 50 ppm commercial phenolic antioxidant to the LDPE matrix (e.g. for the long-term use of polyethylene in high voltage direct current (HVDC) cables) gave a conductivity ca. 3 times higher than that of the same material without antioxidants at 60 °C (the operating temperature for the cables). For larger amounts of antioxidant up to 1000 ppm, the DC conductivity remained stable at ca. 1 × 10^-14 S m^-1. Finite element modeling (FEM) simulations were carried out to model the phenomena observed, and the results suggested that the higher conductivity of the spherulite-containing LDPE stems from the displacement and increased presence of polymeric irregularities (formed during crystallization) in the border regions of the spherulite structures.

Disentangling and Lamellar Thickening of Linear Polymers during Crystallization: Simulation of Bimodal and Unimodal Molecular Weight Distribution Systems

We have performed coarse-grained molecular dynamics simulations to study the isothermal crystallization of bimodal and unimodal molecular weight distribution (MWD) polymers with equivalent average molecular weight (M_w). By using primitive path analysis, we can monitor the entanglement evolution during the process of crystallization. We have discovered a quantitative correlation between the degree of disentanglement and crystallinity, indicating that chain disentanglement permits the process of crystallization. In addition, the crystalline stem length also displays a linear relation with the degree of disentanglement at different temperatures. Based on the observation in our simulations, we can build a scenario of the whole process of chain disentangling and lamellar thickening on the basis of chain sliding diffusion. Furthermore, we have enough evidence to infer that the temperature dependence of crystalline stem length is basically a result of temperature dependence of chain sliding diffusion. Our observations are also in agreement with Hikosaka's sliding diffusion theory. Compared to the unimodal system, the disentanglement degree of the bimodal system is more delayed than its crystallinity due to the slower chain sliding of the long-chain component; the bimodal system reaches a larger crystalline stem length at all temperatures due to the promotion of higher chain sliding mobility of the short-chain component.

The Mechanical Properties of Poly (Urea-Formaldehyde) Incorporated with Nano-SiO2 by Molecular Dynamics Simulation

Self-healing materials can promote the sustainable reuse of resources. Poly (urea-formaldehyde) (PUF) microcapsules can be incorporated into dielectric materials for self-healing. However, the mechanical properties of PUF microcapsules need to be improved due to insufficient hardness. In this paper, PUF models incorporated with nano-SiO₂ of different filler concentrations (0, 2.6, 3.7, 5.3, 6.7, 7.9 wt.%) were designed. The density, the fractional free volume, and the mechanical properties of the PUF-SiO₂ models were analyzed at an atomic level based on molecular dynamics simulation. The interfacial interaction model of PUF on the SiO₂ surface was also constructed to further investigate the interaction mechanisms. The results showed that the incorporation of nano-SiO₂ had a significant effect on the mechanical properties of PUF. Density increased, fractional free volume decreased, and mechanical properties of the PUF materials were gradually enhanced with the increase of nano-SiO₂ concentration. This trend was also confirmed by experimental tests. By analyzing the internal mechanism of the PUF–SiO₂ interfacial interaction, it was found that hydrogen bonds play a major role in the interaction between PUF and nano-SiO₂. Moreover, hydrogen bonds can be formed between the polar atoms of the PUF chain and the hydroxyl groups (–OH) as well as O atoms on the surface of SiO₂. Hydrogen bonds interactions are involved in adsorption of PUF chains on the SiO₂ surface, reducing the distance between PUF chains and making the system denser, thus enhancing the mechanical properties of PUF materials.

Molecular Dynamics Simulation of Improving the Physical Properties of Polytetrafluoroethylene Cable Insulation Materials by Boron Nitride Nanoparticle under Moisture-Temperature-Electric Fields Conditions

The physical properties in amorphous regions are important for the insulation aging assessment of polytetrafluoroethylene (PTFE) cable insulation materials. In order to study the effect of boron nitride (BN) nanoparticles on the physical properties of PTFE materials under moisture, temperature, and electric fields conditions at the molecular level, the amorphous region models of PTFE, BN/PTFE, water/PTFE, and water/BN/PTFE were respectively constructed by molecular dynamics (MD) simulation. The mechanical properties including Young's modulus, Poisson's ratio, bulk modulus, and shear modulus, along with glass transition temperature, thermal conductivity, relative dielectric constant, and breakdown strength of the four models have been simulated and calculated. The results show that the mechanical properties and the glass transition temperature of PTFE are reduced by the injection of water molecules, whereas the same, along with the thermal conductivity, are improved by incorporating BN nanoparticles. Moreover, thermal conductivity is further improved by the surface grafting of BN nanoparticles. With the increase of temperature, the mechanical properties and the breakdown strength of PTFE decrease gradually, whereas the thermal conductivity increases linearly. The injection of water molecules increases the water content in the PTFE materials, which causes a gradual increase in its relative dielectric constant. This work has shown that this effect is significantly reduced by incorporation of BN nanoparticles. The variation of physical properties for PTFE and its composites under the action of moisture, temperature, and electric fields is of great significance to the study of wet, thermal, and electrical aging tests as well as the life prediction of PTFE cable insulation materials.