A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films

We present results from molecular-dynamics simulations of a generic bead-spring model of copolymer chains confined between solid walls and report on the glass-transition temperature and segmental dynamics as a function of film thickness and mesh size (the end-to-end distance of the subchains in the crosslinked polymer networks). Apparently, the glass-transition temperature displayed a steep increase for mesh-size values much smaller than the radius of gyration of the bulk chains, otherwise it remained invariant to mesh-size variations. The rise in the glass-transition temperature with decreasing mesh size and film thickness was accompanied by a monotonic slowing-down of segmental dynamics on all studied length scales. This observation is attributed to the correspondingly decreased width of the bulk density layer that was obtained in films whose thickness was larger than the end-to-end distance of the bulk polymer chains. To test this hypothesis, additional simulations were performed in which the crystalline walls were replaced with amorphous or rough walls. In the amorphous case, the high polymer density close to the walls vanished, but the dynamic response of the film was not affected. The rough walls, on the other hand, only slightly decreased the density close to the walls and led to a minor slowing-down in the dynamics at large length-scales

The Rare Earth Elements: demand, global resources, and challenges for resourcing future generations

The rare earth elements (REE) have attracted much attention in recent years, being viewed as critical metals because of China’s domination of their supply chain. This is despite the fact that REE enrichments are known to exist in a wide range of settings, and have been the subject of much recent exploration. Although the REE are often referred to as a single group, in practice each individual element has a specific set of end-uses, and so demand varies between them. Future demand growth to 2026 is likely to be mainly linked to the use of NdFeB magnets, particularly in hybrid and electric vehicles and wind turbines, and in erbium-doped glass fiber for communications. Supply of lanthanum and cerium is forecast to exceed demand. There are several different types of natural (primary) REE resources, including those formed by high-temperature geological processes (carbonatites, alkaline rocks, vein and skarn deposits) and those formed by low-temperature processes (placers, laterites, bauxites and ion-adsorption clays). In this paper, we consider the balance of the individual REE in each deposit type and how that matches demand, and look at some of the issues associated with developing these deposits. This assessment and overview indicate that while each type of REE deposit has different advantages and disadvantages, light rare earth-enriched ion adsorption types appear to have the best match to future REE needs. Production of REE as by-products from, for example, bauxite or phosphate, is potentially the most rapid way to produce additional REE. There are still significant technical and economic challenges to be overcome to create substantial REE supply chains outside China

Coarse-grained molecular-dynamics simulations of capped crosslinked polymer films: equilibrium structure and glass-transition temperature

We present our recent results from constant temperature-pressure (NPT) molecular dynamics (MD) simulations of a bead-spring copolymer model, in which the polymer is confined between two crystalline substrates. Our goal was to study the combined effect of the polymer crosslinking density and the degree of confinement on the glass-transition temperature and the equilibrium structure of the films. In the direction perpendicular to the substrates, the polymer chains are ordered in layers of increasing density towards the substrates, for all crosslinking densities and the degrees of confinement. In the direction parallel to the substrates, the polymer films display an amorphous structure, just like in the bulk. The glass-transition temperature increases with confinement and crosslinking density, with the former having a large effect compared to the later

A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films

We present results from molecular-dynamics simulations of a generic bead-spring model of copolymer chains confined between solid walls and report on the glass-transition temperature and segmental dynamics as a function of film thickness and mesh size (the end-to-end distance of the subchains in the crosslinked polymer networks). Apparently, the glass-transition temperature displayed a steep increase for mesh-size values much smaller than the radius of gyration of the bulk chains, otherwise it remained invariant to mesh-size variations. The rise in the glass-transition temperature with decreasing mesh size and film thickness was accompanied by a monotonic slowing-down of segmental dynamics on all studied length scales. This observation is attributed to the correspondingly decreased width of the bulk density layer that was obtained in films whose thickness was larger than the end-to-end distance of the bulk polymer chains. To test this hypothesis, additional simulations were performed in which the crystalline walls were replaced with amorphous or rough walls. In the amorphous case, the high polymer density close to the walls vanished, but the dynamic response of the film was not affected. The rough walls, on the other hand, only slightly decreased the density close to the walls and led to a minor slowing-down in the dynamics at large length-scales

A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films

We present results from molecular-dynamics simulations of a generic bead-spring model of copolymer chains confined between solid walls and report on the glass-transition temperature and segmental dynamics as a function of film thickness and mesh size (the end-to-end distance of the subchains in the crosslinked polymer networks). Apparently, the glass-transition temperature displayed a steep increase for mesh-size values much smaller than the radius of gyration of the bulk chains, otherwise it remained invariant to mesh-size variations. The rise in the glass-transition temperature with decreasing mesh size and film thickness was accompanied by a monotonic slowing-down of segmental dynamics on all studied length scales. This observation is attributed to the correspondingly decreased width of the bulk density layer that was obtained in films whose thickness was larger than the end-to-end distance of the bulk polymer chains. To test this hypothesis, additional simulations were performed in which the crystalline walls were replaced with amorphous or rough walls. In the amorphous case, the high polymer density close to the walls vanished, but the dynamic response of the film was not affected. The rough walls, on the other hand, only slightly decreased the density close to the walls and led to a minor slowing-down in the dynamics at large length-scales

Molecular-dynamics simulations of crosslinking and confinement effects on structure, segmental mobility and mechanics of filled elastomers

The significant drop of the storage modulus under uniaxial deformation (Payne effect) restrains the performance of the elastomer-based composites and the development of possible new applications. In this paper molecular-dynamics (MD) computer simulations using LAMMPS MD package have been performed to study the mechanical properties of a coarse-grained model of this family of nanocomposite materials. Our goal is to provide simulational insights into the viscoelastic properties of filled elastomers, and try to connect the macroscopic mechanics with composite microstructure, the strength of the polymer-filler interactions and the polymer mobility at different scales. To this end we simulate random copolymer films capped between two infinite solid (filler aggregate) walls. We systematically vary the strength of the polymer-substrate adhesion interactions, degree of polymer confinement (film thickness), polymer crosslinking density, and study their influence on the equilibrium and non-equilibrium structure, segmental dynamics, and the mechanical properties of the simulated systems. The glass-transition temperature increases once the mesh size became smaller than the chain radius of gyration; otherwise it remained invariant to mesh-size variations. This increase in the glass-transition temperature was accompanied by a monotonic slowing-down of segmental dynamics on all studied length scales. This observation is attributed to the correspondingly decreased width of the bulk density layer that was obtained in films whose thickness was larger than the end-to-end distance of the bulk polymer chains. To test this hypothesis additional simulations were performed in which the crystalline walls were replaced with amorphous or rough walls

Filler Size Effects on Reinforcement in Elastomer-Based Nanocomposites: Experimental and Simulational Insights into Physical Mechanisms

Nanocomposites consisting of polymers reinforced with filler particles are important for a wide variety of industries and processes, but although they exhibit unique viscoelastic properties and as such are widely applied in e.g. tires, the precise mechanism of their reinforcement is at best incompletely understood at present. In order to understand it at a fundamental level, and ultimately control it in practice, it is essential to determine the impact of interactions between filler particles and polymer matrix on the nanocomposite microstructure and its macroscopic dynamic mechanical properties. To this end, we performed experiments on two model systems as well as molecular dynamics simulations, aiming to determine to what extent widely used shear-distortion models of the reinforcement are applicable as well as the role played by molecular interactions on the enhancement of the mechanical properties. In both experiments and simulations a linear dependence of the reinforcement on the inverse radius of the nanoparticles was obtained. Deformation simulations of a linearly increasing strain showed an overall increase of 50% in the linear modulus when fillers were added to the polymer matrix, regardless of the use of direct interactions among the nanoparticles. Furthermore, the use of attractive nanoparticle interactions resulted in a higher matrix densification at the interfaces and to a sharp increase in the reinforcement

Filler size effects on reinforcement in Elastomer-based nanocomposites: experimental and simulational insights into physical mechanisms

Nanocomposites consisting of polymers reinforced with filler particles are important for a wide variety of industries and processes, but although they exhibit unique viscoelastic properties and as such are widely applied in e.g. tires, the precise mechanism of their reinforcement is at best incompletely understood at present. In order to understand it at a fundamental level, and ultimately control it in practice, it is essential to determine the impact of interactions between filler particles and polymer matrix on the nanocomposite microstructure and its macroscopic dynamic mechanical properties. To this end, we performed experiments on two model systems as well as molecular dynamics simulations, aiming to determine to what extent widely used shear-distortion models of the reinforcement are applicable as well as the role played by molecular interactions on the enhancement of the mechanical properties. In both experiments and simulations a linear dependence of the reinforcement on the inverse radius of the nanoparticles was obtained. Deformation simulations of a linearly increasing strain showed an overall increase of 50% in the linear modulus when fillers were added to the polymer matrix, regardless of the use of direct interactions among the nanoparticles. Furthermore, the use of attractive nanoparticle interactions resulted in a higher matrix densification at the interfaces and to a sharp increase in the reinforcement

Simulational insights into the mechanical response of prestretched double network filled elastomers

This paper deals with molecular-dynamics simulations of the mechanical properties of prestretched double network filled elastomers. To this end, we firstly validated the accuracy of this method, and affirmed that the produced stress-strain characteristics were qualitatively consistent with Lesser's experimental results on the prestretched tri-block copolymers with a competitive double network. Secondly, we investigated the effect of the crosslinking network ratio on the mechanical properties of the prestretched double network homopolymers under uniaxial tension. We found that the prestretched double network contributes greatly to the enhanced tensile stress and ultimate strength at fracture, as well as to the lower permanent set (the residual strain) and dynamic hysteresis loss, both parallel and perpendicular to the prestretching direction. Notably, though, an anisotropic behavior was observed: in the parallel direction, both the first and the second crosslinked networks bore the external force; whereas in the perpendicular direction, only the second crosslinked network was relevantly effective. Finally, the polymer nanocomposites with a prestretched double network exhibited tensile mechanical properties similar to those of the studied homopolymers with prestretched double networks. Summing up the results, it can be concluded that the incorporation of prestretched double networks with a specified crosslinking network ratio seems to be a promising method for manipulating the mechanical properties of elastomers and their nanocomposites, as well as for introducing anisotropy in their mechanical response