Amorphous Polyethylene Research Articles

Study by Molecular Dynamics and First-Principles Calculation on the Influence of Length of Molecular Chain and Entanglement of Molecular Chains on the Strength of Amorphous Polyethylene

Study by Molecular Dynamics and First-Principles Calculation on the Influence of Length of Molecular Chain and Entanglement of Molecular Chains on the Strength of Amorphous Polyethylene

Effect of Chain Configuration on Thermal Conductivity of Polyethylene—A Molecular Dynamic Simulation Study

Stretched polyethylene (PE) fibers are found to have super high thermal conductivity, while the bulk of polyethylene is usually thermal insulating even for those with high crystalline degree. A molecular dynamic simulation is deliberately carried out to examine the relationship between chain configuration and thermal conductivity of polyethylene. In this simulation study, independent and interacting PE chains being stretched are compared with the aim to find out the effect of stretching on thermal conductivity of PE. Various crystallization conditions for PE bulk are considered. It is found that heat transports predominately along the covalent chain rather than across chains in PE crystals. Our simulation study helps to understand experimental findings on thermal conductivity of PE at different states. We also predict that amorphous PE may be super thermally conductive if chains are strictly stretched along heat flux.

Recyclable mono materials for packaging of fresh chicken fillets: New design for recycling in circular economy

The focus on sustainability and circular economy is leading to a need for development of new food packaging concepts, including recyclable materials that ideally consist of a single material in a monolayer system. This research was focused on the possibility of replacing complex multilayered material [amorphous polyethylene terephthalate/polyethylene (APET/PE)] with simple recyclable mono material [high‐density polyethylene (HDPE)] for packaging of chicken fillets in modified atmosphere packaging (CO2/N2: 60%/40%). Bacterial growth measured as total viable count (TVC), lactic acid bacteria and Enterobacteriaceae, Brochothrix thermosphacta and Escherichia coli for chicken fillets packed in HDPE mono materials was compared with chicken fillets packed in APET/PE.TVC increased during the storage period (24 days) with high level of TVC count (7 log10 CFU/g) recorded at Days 19–20 of storage in both HDPE and APET/PE material. No significant differences were recorded in off‐odour between chicken stored in APET/PE compared with HDPE in CO2/N2 atmosphere during the storage period (samples were regarded as acceptable on the 24th day of storage). The drip loss increased in all samples during storage, and no significant differences between samples stored in different materials were recorded.Significant differences in bacterial growth were recorded between samples with different gas volume to product volume (G/P) ratio (Day 17), implying that higher G/P ratio is resulting in lower TVC count. The lowest G/P ratio caused the highest drip loss, whereas addition of CO2 emitter reduced the drip loss to some extent.This research is very encouraging as it provides new insight into the use of monolayer materials as well as the importance of design for recycling in circular economy.

Ageing behaviour of a polyethylene blend: influence of chemical defects and morphology on charge transport

The properties of novel cable insulation systems will rely critically upon the morphology of the material. Here, a blend of high and low density polyethylene was processed in order to generate three sets of samples with different morphologies. The influence of thermo-oxidative ageing at 120 °C was then considered. The resulting chemical changes included the introduction of unsaturation and oxygen-containing groups and was determined by antioxidant consumption and oxygen permeability. Such chemical defects were found to be concentrated in the fraction of each system that was molten at 120 °C and, consequently, served to inhibit recrystallisation following ageing. The resulting spatial distribution of charge trapping sites was therefore strongly dependent on morphology. The electrical conductivity of each system varied non-monotonically with ageing: short times reduced the conductivity; a rapid increase in conductivity over five orders of magnitude occurred beyond a critical ageing threshold. Despite the pronounced structural differences between the morphologically distinct sets of samples, all exhibited comparable conductivity values beyond this threshold, implying that while charge transport is strongly influence by chemical factors, crystallinity is relatively unimportant. This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous polyethylene.

Analysis of elastic deformation of amorphous polyethylene in uniaxial tensile test by using molecular dynamics simulation

Analysis of elastic deformation of amorphous polyethylene in uniaxial tensile test by using molecular dynamics simulation

Tailored morphology and highly enhanced phonon transport in polymer fibers: a multiscale computational framework

Although tremendous efforts have been devoted to enhance thermal conductivity in polymer fibers, correlation between the thermal-drawing conditions and the resulting chain alignment, crystallinity, and phonon transport properties have remained obscure. Using a carefully trained coarse-grained force field, we systematically interrogate the thermal-drawing conditions of bulk polyethylene samples using large-scale molecular dynamics simulations. An optimal combination of moderate drawing temperature and strain rate is found to achieve highest degrees of chain alignment, crystallinity, and the resulting thermal conductivity. Such combination is rationalized by competing effects in viscoelastic relaxation and condensed to the Deborah number, a predictive metric for the thermal-drawing protocols, showing a delicate balance between stress localizations and chain diffusions. Upon tensile deformation, the thermal conductivity of amorphous polyethylene is enhanced to 80% of the theoretical limit, that is, its pure crystalline counterpart. An effective-medium-theory model, based on the serial-parallel heat conducting nature of semicrystalline polymers, is developed here to predict the impacts from both chain alignment and crystallinity on thermal conductivity. The enhancement in thermal conductivity is mainly attributed to the increases in the intrinsic phonon mean free path and the longitudinal group velocity. This work provides fundamental insights into the polymer thermal-drawing process and establishes a complete process–structure–property relationship for enhanced phonon transport in all-organic electronic devices and efficiency of polymeric heat dissipaters.

Interface formation and bonding mechanisms of laser transmission welded composite structure of PET on austenitic steel via beam oscillation

Laser lap welding of composite structure of amorphous polyethylene terephthalate (PET) to 304 steel was carried out via beam oscillation. The effects of laser power (P), oscillating radius (r) and frequency (f) on weld macroscopic morphologies, tensile properties and bonding mechanisms were investigated. The defect of thermal degradation could be eliminated, but the lack of fusion occurred with the r approaching 3 mm. The welds were homogeneous with the pores discretely or network distributed when the r was between 2 and 2.5 mm. The bonding strength increased from the center to the edge with more retained base steel on the PET surface. The welding parameters were optimized as the P of 300 W, the r of 2 mm, the f of 300 Hz under given welding speed of 50 cm/min and defocused distance of 20 mm. The shear force was 26% higher than the sample without beam oscillation, because of the C-M and C-O formed by the reactions of the PET with the steel and the air. The weld homogeneity was illustrated on the basis of the proposed energy distribution model. The formation and inhibition mechanisms of the thermal degradation and lack of fusion were then discussed.

Thermomechanical Characterization and Modeling of Cold-Drawing of Poly(ethylene Terephthalate).

The tensile testing of amorphous polyethylene terephthalate is observed until failure by IR thermography and optical strain measurement. The deformation can be subdivided in six deformation phases: elastic deformation, neck formation with a localized sharp temperature rise, neck propagation, which is also known as cold-drawing, with heat generation in a transition zone, crack initialization with local heating, crack growth, and rupture. These deformation phases are showing different mechanical and thermal reactions to the deformation. The initial and drawn samples are studied with differential scanning calorimetry. Alongside heating due to the dissipation of mechanical energy, latent heat due to strain-induced crystallization was detected. While the material is cold-drawn, a high dependence on the crosshead speed is found for the heat generation as well as the draw ratio, mechanical response, and morphological changes due to orientation and crystallization. For cold-drawing, a thermomechanical model is introduced, which is based on the first law of thermodynamics and reproduces the temperature distribution along the sample.

Molecular dynamics simulation of amorphous polyethylene (PE) under cyclic tensile-compressive loading below the glass transition temperature

The intensive applications of polymer in many engineering composites have imposed an urgent need on the understanding of the mechanical behavior and deformation mechanism of the polymer under cyclic loading. This paper presents the results of a numerical study on the behavior of amorphous polyethylene (PE) subjected to cyclic tensile and compressive loads using molecular dynamics (MD) simulations, based on a united-atom approach. The effects of polymer chain length, the number of chains and strain rates are studied at first. Hysteresis loops, as well as visco-elastoplastic of PE under cyclic loading predicted by MD simulations are qualitatively in agreement with previous experiments. Three distinct hysteresis loops observed in successive loading-unloading reveal the contribution of elasticity, viscosity and plasticity under different loading strains, respectively. The rubber-like recovery behavior of PE at low temperature is attributed to that the mobility of molecular chains is constrained at low temperature. Energy analysis shows that the van der Waals energy and dihedral angle energy are considered to be the primary factors that affects the cyclic behavior of PE.

Uncertainty Quantification for Mechanical Properties of Polyethylene Based on Fully Atomistic Model.

This study is to assess the effect of temperature and strain rate on the mechanical properties of amorphous polyethylene (PE) based on fully atomistic model. A stochastic constitutive model using data obtained from molecular dynamics (MD) simulations for the material is constructed. Subsequently, a global sensitivity analysis approach is then employed to predict the essential parameters of the mechanical model. The sensitivity indices show that the key parameter affecting Young’s modulus and yield stress is the temperature followed by the strain rate.

Enhancing the interlayer tensile strength of 3D printed short carbon fiber reinforced PETG and PLA composites via annealing

Previous studies have shown that 3D printed composites exhibit an orthotropic nature with inherently lower interlayer mechanical properties. This research work is an attempt to improve the interlayer tensile strength of extrusion-based 3D printed composites. Annealing was identified as a suitable post-processing method and was the focus of this study. Two distinct thermoplastic polymers, which are common in 3D printing, were selected to study the enhancement of interlayer tensile strength of composites by additive manufacturing: a) an amorphous polyethylene terephthalate-glycol (PETG), and b) a semi-crystalline poly (lactic acid) (PLA). It was determined that short carbon fiber reinforced composites have lower interlayer tensile strength than the corresponding neat polymers in 3D printed parts. This reduction in mechanical performance was attributable to an increase in melt viscosity and the consequential slower interlayer diffusion bonding. However, the reduction in interlayer tensile strength could be recovered by post-processing when the annealing temperature was higher than the glass transition temperature of the amorphous polymer. In the case of the semi-crystalline polymer, the recovery of the interlayer tensile strength was only observed when the annealing temperature was higher than the glass transition temperature but lower than the cold-crystallization temperature. This study utilized rheological and thermal analysis of 3D printed composites to provide a better understanding of the interlayer strength response and, therefore, overcome a mechanical performance limitation of these materials.

A Comparative Life Cycle Assessment of Meat Trays Made of Various Packaging Materials

In light of the debate on the circular economy, the EU strategy for plastics, and several national regulations, such as the German Packaging Act, polymeric foam materials as well as hybrid packaging (multilayered plastic) are now in focus. To understand the environmental impacts of various tray solutions for meat packaging, a comparative environmental assessment was conducted. As an environmental assessment method, a life cycle assessment (LCA) was applied following the ISO standards 14040/44. The nine packaging solutions investigated were: PS-based trays (extruded polystyrene and extruded polystyrene with five-layered structure containing ethylene vinyl alcohol), PET-based trays (recycled polyethylene terephthalate, with and without polyethylene layer, and amorphous polyethylene terephthalate), polypropylene (PP) and polylactic acid (PLA). The scope of the LCA study included the production of the tray and the end-of-life stage. The production of meat, the filling of the tray with meat and the tray sealing were not taken into account. The results show that the PS-based trays, especially the mono material solutions made of extruded polystyrene (XPS), show the lowest environmental impact across all 12 impact categories except for resource depletion. Multilayer products exhibit higher environmental impacts. The LCA also shows that the end-of-life stage has an important influence on the environmental performance of trays. However, the production of the trays dominates the overall results. Furthermore, the sensitivity analysis illustrates that, even if higher recycling rates were realised in the future, XPS based solutions would still outperform the rest from an environmental perspective.

Atomistic modeling of flexoelectricity in amorphous polymers

In this paper, the amorphous PVDF (Polyvinylidene Fluoride) and PE (Polyethylene) models are established. The relationship between configuration density and temperature is calculated by molecular dynamics simulation. The rationality of model is determined by comparing with the experimentally determined glass transition temperature Tg. Secondly, the strain gradient is applied to the main chain carbon atoms of the amorphous PVDF and PE equilibrium configuration. Electric polarization occurs when the microstructure of the configuration changes. The transverse flexoelectric coefficients μ12 of amorphous PVDF and PE configurations are 1.24×10-9 C/m and −2.63×10-9 C/m, respectively, obtained by fitting the relationship between polarization and strain gradient. Finally, through the micromechanical analysis, the CF2 and CH2 dipoles rotate around the main chain along the inner side of the curved surface under the strain gradient, accompanied by some small structural adjustments. This leads to the electric polarization of the amorphous PVDF and PE configuration. Due to the difference in charge number and electronegativity between the amorphous PVDF and PE groups, the calculated transverse flexoelectric coefficients of the amorphous PVDF and PE are opposite, and the absolute value of the transverse flexoelectric coefficient of PE is greater than that of PVDF.

Free vibration analysis of polyethylene/CNT plates

In this work, we analyzed the free vibration of single-walled carbon nanotubes (SWCNTs)-reinforced composite plates with carbon nanotubes (CNTs) embedded in amorphous polyethylene. Here, the governing differential equations of simply supported and clamped boundary conditions were found using the generalized differential quadrature (GDQ) method. We used the rules of mixture according to different plate models including first-order shear deformation theory (FSDT), classical plate theory (CLPT), and higher-order shear deformation theory (HSDT) to find the fundamental frequencies of nanocomposite plates. The properties of the materials used in the fabrication of nanocomposite plates were investigated using the Multiscale Finite Element Method (FEM) simulation for both short (10, 10) and long (10, 10) SWCNTs composites. The results of FEM simulations were fitted using those of the rule of mixture to obtain optimum values of CNT efficiency parameters. A few selected numerical results have been provided to investigate the effects of the volume fractions of CNTs and the types of edge supports on the value of fundamental frequency of long- and short-CNTs reinforced composite plates.

Effects of Gas Adsorption on the Mechanical Properties of Amorphous Polymer.

This study investigates the properties of a polymer–gas mixture formed through diffusion, based on the changes in the partial pressure and observed changes in the impact and tensile strengths owing to the gas dissolution. The high-pressure gas dissolves into a solid-state polymer through diffusion based on the difference in the partial pressure. This dissolved gas is present in the amorphous region within the polymeric material temporarily, which results in the polymer exhibiting different mechanical properties, while the gas remains dissolved in the polymer. In this study, the mechanical properties of amorphous polyethylene terephthalate (APET) specimens prepared by dissolving CO2 using a high-pressure vessel were investigated, and the resulting impact and tensile strengths were measured. These experiments showed that the increase in sorption rate of CO2 caused an increase in the impact strength. At 2.9% CO2 absorption, the impact strength of APET increased 956% compared to that of the reference specimen. Furthermore, the tensile strength decreased by up to 71.7% at 5.48% CO2 sorption; the stress–strain curves varied with the gas sorption rate. This phenomenon can be associated with the change in the volume caused by CO2 dissolution. When the APET absorbed more than 2.0% CO2 gas, sample volume increased. A decrease in the network density can occur when the volume is increased while maintaining constant mass. The CO2 gas in the polymer acted as a cushion in impact tests which have sorption rates above 2%. In addition to the reduction in the network density in the polymer chain, Van Der Waals forces are decreased causing a decrease in tensile strength only while CO2 is present in the APET. These observations only occur prior to CO2 desorption from the polymer.

Macrophage in vitro Response on Hybrid Coatings Obtained by Matrix Assisted Pulsed Laser Evaporation

The improvement in the research area of the implant by surface functionalization when correlated with the biological response is of major interest in the biomedical field. Based on the fact that the inflammatory response is directly involved in the ultimate response of the implant within the body, it is essential to study the macrophage-material interactions. Within this context, we have investigated the composite material-macrophage cell interactions and the inflammatory response to these composites with amorphous hydroxyapatite (HA), Lactoferrin (Lf), and polyethylene glycol-polycaprolactone (PEG-PCL) copolymer. All materials are obtained by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique and characterized by Atomic Force Microscopy and Scanning Electron Microscopy. Macrophage-differentiated THP-1 cells proliferation and metabolic activity were assessed by qualitative and quantitative methods. The secretion of tumor necrosis factor alpha (TNF-α) and interleukin 10 (IL-10) cytokine, in the presence and absence of the inflammatory stimuli (bacterial endotoxin; lipopolysaccharide (LPS)), was measured using an ELISA assay. Our results revealed that the cellular response depended on the physical-chemical characteristics of the coatings. Copolymer-HA-Lf coatings led to low level of pro-inflammatory TNF-α, the increased level of anti-inflammatory IL-10, and the polarization of THP-1 cells towards an M2 pro-reparative phenotype in the presence of LPS. These findings could have important potential for the development of composite coatings in implant applications.

Blends of polycarbonate with polyethylene terephthalate glycol: compatibility of components, rheological and mechanical properties

The influence of the component ratio in the stabilized binary blends of amorphous polycarbonate (PC) and polyethylene terephthalate glycol (PET-G) on the compatibility of components, relaxation, rheological and mechanical properties was studied. The materials were prepared by reactive mixing of the components in the melt in a mixing rectifier based on a twin-screw extruder with unidirectional screw rotation. It is established that the components in the blends are mutually soluble over the entire range of their concentrations. The temperature of the а -process of relaxation (the glass transition temperature, Tg, in the mixtures) is in the interval between the values of Tg PC = 148.3 °C, Tg PET-G = 82.0 °C and obeys the Fox equation for single-phase blends. The mechanical characteristics of the mixtures change in a non-additive way when the concentration of the components is varied. In particular, in blends in which the PC content prevails, a significant increase in the upper yield limit is observed in comparison with the additive values, which, according to the relaxation spectrometry, is due to intense intermolecular interactions.

Free volume and internal structural evolution during creep in model amorphous polyethylene by Molecular Dynamics simulations

All-atom Molecular Dynamics (MD) simulations were employed to investigate the structural and free volume evolution (correlated with damage) during creep of model amorphous polyethylene (PE) at various applied stress states (tension, shear, compression), stress levels (10–200 MPa), and temperatures (175–325 K). The Modified Embedded-Atom Method for saturated hydrocarbons is applied to show that the phenomenological macroscale creep response of PE can be captured through MD simulations. The model adequately predicts the three classical stages of creep (primary, secondary, and tertiary) and provides detailed insight into the underlying molecular mechanisms. The calculated glass transition temperature (Tg) was found to be very close to the experimental Tg. Simulations were performed at temperatures below Tg (175 K) to above Tg (325 K) and demonstrate that the transition from glassy to rubbery state is reflected in the chain dynamics and damage evolution. Under all the stress states and temperatures simulated, the evolution of void volume, nucleation, growth, and coalescence are shown to directly correlate with specific stages of the creep response and the underlying chain dynamics within each stage. A correlation between the steady-state creep rate and steady-state void nucleation rate is found, suggesting that secondary creep is heavily driven by void nucleation, while tertiary creep is driven by void growth and coalescence.

Monitoring the Isothermal Crystallization Kinetics of PET-A Using THz-TDS

Using terahertz time-domain spectroscopy (THz-TDS), we monitor the isothermal crystallization kinetics of amorphous polyethylene terephthalate (PET-A). PET-A was tempered at different temperatures and for a varied amount of time to induce a isothermal crystallization. Afterwards the THz spectra were recorded and analyzed. An adapted Avrami equation was used to analyze the spectral data to monitor the isothermal crystallization. It was found that the adapted Avrami theory is a good approach to describe the kinetics of isothermal crystallization and allows to determine kinetic parameters as with classical technologies. Therefore, we conclude that THz-TDS offers a non-destructive method to characterize the kinetics of isothermal crystallization in polymers.

Multiscale modeling of charge transfer in polymers with flexible backbones.

In order to evaluate the carrier transfer properties in polymers with flexible backbones, we have proposed a simplified multi-scale modeling approach combining molecular dynamics simulations, first-principles calculations and kinetic Monte Carlo simulations. Hole transfer in amorphous polyethylene (PE) is studied as a model system. It is shown that the characteristic length scale of hole localized states in PE is comparable to the Kuhn length of PE, which is the characteristic length scale in terms of the geometric structure. Because a PE oligomer whose length is equivalent to the Kuhn length (C12H26) has a similar electronic structure to amorphous PE, C12H26 is considered as an approximate model of amorphous PE. In line with experimental findings, computed hole mobility in the amorphous PE oligomer is several orders of magnitude smaller than that in crystalline PE. We show that this difference originates from strong hole localization and large site energy variation in amorphous PE due to the conformational disorder of PE oligomer chains. The agreement between experimental and calculated hole mobilities strongly supports the rationale of our modeling approach. The modeling approach proposed in this paper is considered capable of predicting carrier mobilities in polymers with flexible backbones with reasonable computational load.