Amorphous Polyethylene Research Articles

Investigation on the Coalescence of Filaments in Fused Filament Fabrication Process for Amorphous Polyether Ketone Ketone for Varying Nozzle Diameters and Chamber Temperatures.

The use of Fused Filament Fabrication (FFF) of high-performance polymers is becoming increasingly prevalent, leading to the exploration of new applications. The use of such materials in critical cases for aerospace applications necessitates the verification of industry standards, particularly with regard to the requirements for part porosity. The authors investigate the effect of nozzle diameter and cooling temperature printing parameters on the porosity of the part by using existing modelling methods based on the sintering of cylinders and spheres and comparing the results to microscope snapshots of sections of parts. The models are able to be used as limits for predicting the longitudinal neck growth of the part. The authors show through experiments that the value of the cooling temperature of the deposited filament has a minimal effect on the outcome, while nozzle diameter has a strong impact on the resulting porosity. The modelling results show that there is a significant impact of both the nozzle diameter and cooling temperature on the porosity of the part. This implies that further refinement of the models is needed for the resulting parts to be applied in critical structures.

Molecular investigation on physical and tensile properties of polyethylene (PE) under high-pressure hydrogen environments

Polymer liners are key components of high-pressure hydrogen storage tanks, and understanding their behaviour under such environments is crucial for safety. This study employs molecular dynamics (MD) simulation to investigate the physical and tensile properties of amorphous polyethylene (PE) under the effects of hydrogen content, pressure, temperature. It is found that hydrogen molecules decrease glass transition temperature by approximately 3%, while pressure have no noticeable impact. Meanwhile, hydrogen molecules enhance diffusivity, while pressure suppresses diffusivity and exhibits a more pronounced impact, resulting in an overall decrease of 44% in diffusivity under high-pressure hydrogen. Tensile stress also increases the diffusivity by 17%. Hydrogen molecules weaken the non-bonded molecular interactions of PE, which reduces the percentage of trans conformation and orientation parameters. Meanwhile, hydrogen also suppresses the internal energy change of PE induced by tensile deformation. Therefore, hydrogen molecules lead to a decrease in tensile properties. Comparatively, pressure causes PE to produce more trans conformations under tension, which improves orientation, entanglement parameters, and the internal energy change of PE, and finally enhances the tensile properties of PE. The effect of pressure surpasses that of hydrogen, which results in an approximately 40% increase in tensile properties under high-pressure hydrogen. This study provides molecular-level insights into the individual and coupled effects of hydrogen and pressure, forming a foundation for predicting polymer behaviour under high-pressure hydrogen conditions.

Unraveling the Mechanical Response of Short-Chain Branched Amorphous Polyethylene: Insights from Molecular Dynamics Simulations

The deformation behavior of short-chain branched, amorphous polyethylene (PE) was investigated using united atom molecular dynamics (MD) simulations under uniaxial stretching. Our research reported in this paper examined the internal mechanisms underlying the mechanical response of the PE by analyzing potential energies, dihedral angle distributions, chain orientation and entanglements. The findings indicated that the short-chain branching generally enhanced Young’s modulus, except for sample B60 with a critical ethyl branch content of 60/1000 (60 ethyl branches per 1000 backbone carbons). The Young’s modulus of B60 was the local minimum value in the various samples. Analysis of the evolution of dihedral angles shows that the short-chain branching hinders the transition from gauche to trans configurations, leading to a decrease in the trans population as the branch content increases. Regarding the sample B60 with critical branch content 60/1000, the orientation parameter and entanglement parameter of the molecular chains along the backbone were lowest in the initial structure, and the efficiency of chain disentanglement was at its lowest during deformation. The simulation results elucidated the deformation mechanism related to the conformational evolution of the molecular chains, which can provide atomic-scale insights into the mechanical properties and processability of branched polyethylene, thereby allowing for optimization in its design and application.

Heating Rate Sensitive Polyethylene Terephthalate.

Smart materials react to external triggers by changing size, color, mechanical properties, or permeability. The next generation of smart materials will be able to not only recognize and react to external triggers but also to their dynamics. The only existing example of such a material is heating rate-sensitive polymorphous cross-linked syndiotactic polypropylene. This study presents a new principle of a heating rate-sensitive material on the example of cross-linked and fully amorphous quenchable semi-crystalline polyethylene terephthalate (x-PET). The x-PET is stretched to high elongation above its melting temperature and constrained quenched to a fully amorphous state. Then the polymer is heated to 120-170°C with different heating rates. Due to its heating-rate sensitivity, x-PET shrinks to different stabilized lengths dependent on the heating rate. The new length can be used to read out the heating rate and to specifically answer to this by mechanically switching a process. Detailed analytics of this process reveal that amorphous stretched x-PET is starting the retraction above Tg and simultaneously stopping it by crystallization. The different rates of these processes result in the heating rate sensitivity of x-PET.

Structural-Mechanical Modification of Amorphous Polyethylene Terephthalate and Hybrid Nanocomposite Materials Based on It

Hybrid nanocomposite materials based on polyethylene terephthalate and inorganic flame-retardant diammonium phosphate were prepared according to the fundamental strategy of environmental crazing of polymers. Structural-mechanical modification of the initial amorphous polyethylene terephthalate and the resultant nanocomposite polyethylene terephthalate-based was carried out by rolling at room temperature. The effect of preliminary rolling on the deformation behavior of polyethylene terephthalate during subsequent stretching in air and in physically active liquid media was established.

Hydrolase-Catalyzed Depolymerization Mechanism toward Crystalline and Amorphous Polyethylene Terephthalate

Hydrolase-Catalyzed Depolymerization Mechanism toward Crystalline and Amorphous Polyethylene Terephthalate

Transport property of hydrogen sulfide in amorphous polyethylene using grand canonical Monte Carlo and molecular dynamics simulations

H2S corrosion poses a significant hurdle to safely transporting sulfur-containing natural gas through metal gathering pipes. Reinforced thermoplastic composite pipes have emerged as a solution, using the polyethylene inner lining layer that resists H2S corrosion. However, the permeation of H2S in the polyethylene may lead to the failure of reinforced thermoplastic composite pipes, resulting in natural gas leakage and environmental pollution. Therefore, it is crucial to investigate the gas transport properties of H2S in polyethylene materials to ensure the reliable functioning of gathering pipes. In this study, the dissolution behavior of H2S for pure H2S and H2S/CH4 mixture in amorphous PE is investigated using the grand canonical Monte Carlo method in conjunction with the NPT, with consideration given to the swelling effect. In addition, the diffusion characteristic is explored through the NVT on the amorphous PE adsorbing H2S. Subsequently, the permeability coefficient is calculated based on the obtained data. The findings reveal that with as the temperature increases, the solubility coefficient decreases, while both the diffusion and permeability coefficients increase. With increasing pressure, both the diffusion and permeability coefficients increase. Additionally, the above three coefficients all increase with increasing H2S mole fraction. The relationships between solubility, diffusion, and permeability coefficients & temperature all conform to the Arrhenius law. Finally, the diffusion mechanism of H2S in amorphous PE is revealed at the microscopic level as the “jumping” mechanism by tracking the diffusion pathway of H2S molecule.

Hop, Skip, and Jump: Hydrogen Molecular Transport through Amorphous Polyethylene Matrices Studied via Molecular Dynamics Simulations.

In the pursuit of advancing and diversifying energy technologies for a more sustainable future, the possibilities of hydrogen (H2) usage will broaden, as will our understanding of its containment materials. Polyethylene (PE) has a vast assortment of uses and applications, which are growing with demands for alternative energy possibilities. One use of PE liner is as a prime candidate for nonmetallic piping and pressurized type IV storage devices. Such applications require PE to effectively prevent H2 transport through containment systems. To study the molecular transport mechanism of hydrogen through polymeric barriers, a system containing hydrogen molecules absorbed within amorphous PE is modeled here using molecular dynamics simulations. The simulations are conducted within a range of temperatures that span the glass transition temperature of amorphous PE. The simulated PE displays bulk density, radius of gyration, and self-diffusion coefficient that are consistent with experimental data. The simulated trajectories are interrogated to study the movement of the guest gas molecules. The results show that the diffusion coefficients increase with temperature, as expected. However, the mobility of the PE chains is found to affect the mobility of absorbed H2 molecules to a much lower extent than it affects that of CH4 molecules because of the much smaller size of the former than of the latter guest. From a molecular perspective, a "hopping" mechanism is observed, according to which H2 molecules hop between one vacant free volume space to another within the polymer matrix, in combination with longer, straight, undisturbed "jumps" or "skips" along directions aligned with regions of ordered PE chains. This suggests that the orientation of the crystallites within the semicrystalline PE matrix affects the H2 containment. Implications of these findings toward PE usage as containment material are discussed.

Coarse-Grained Model for Prediction of Hole Mobility in Polyethylene.

Electrical conductivity measurements of polyethylene indicate that the semicrystalline structure and morphology influence the conductivity. To include this effect in atomistic charge transport simulations, models that explicitly or implicitly take morphology into account are required. In the literature, charge transport simulations of amorphous polyethylene have been successfully performed using short oligomers to represent the polymer. However, a more realistic representation of the polymer structure is desired, requiring the development of fast and efficient charge transport algorithms that can handle large molecular systems through coarse-graining. Here, such a model for charge transport simulations in polyethylene is presented. Quantum chemistry calculations were used to define six segmentation rules on how to divide a polymer chain into shorter segments representing localized molecular orbitals. Applying the rules to amorphous systems yields distributions of segments with mode and median segment lengths relatively close to the persistence length of polyethylene. In an initial test, the segments of an amorphous polyethylene were used as hopping sites in kinetic Monte Carlo (KMC) simulations, which yielded simulated hole mobilities that were within the experimental range. The activation energy of the simulated system was lower compared to the experimental values reported in the literature. A conclusion may be that the experimental result can only be explained by a model containing chemical defects that generate deep traps.

Direct observation of radiation-induced graft polymerization on a polyethylene film

Radiation-induced graft polymerization is widely used in the synthesis of polymeric materials with various functionalities; however, the details of the reaction mechanism are still unresolved. Herein, the initial stage of the graft polymerization reaction was investigated through atomic force microscopy (AFM). We used glycidyl methacrylate (GMA) in methanol (monomer solution) and a high-density polyethylene (HDPE) film with a well-ordered lamellar structure as the substrate. Using polymer film surfaces with a maximum grafting degree of 0.5%, AFM can distinguish between crystalline and amorphous polyethylene (PE) phases as well as a grafted polymer GMA (PGMA) phase. The graft polymerization reaction started from the interface between the crystalline and amorphous PE phases. As the reaction proceeded, the PGMA covered the crystalline PE phase: i.e., the entire surface of the PE film were covered by the PGMA. Further, the crystalline lamellae of the PE film eventually became a scaffold for the formation of aggregates comprising grafted polymer chains alone. The graft polymerization reaction mechanism was explained by comparing the solubility parameters of the monomer solution, substrate, and grafted polymer chains. Understanding the reaction mechanism at a low grafting degree obtained in this study may contribute to enhancing the efficiency of the graft polymerization reaction on any polymeric material.

Evolution of the microstructure of amorphous polyethylene under friction-induced plastic flows: A reactive molecular investigation.

The plastic flow of ultra-high molecular weight polyethylene (UHMWPE) at a frictional interface, which is critical to the wear behavior, was investigated by reactive molecular dynamics simulations. The UHMWPE substrate was found to experience various deformations during the friction process. First, some polyethylene (PE) chains could detach from the substrate because of their rapid movement. Second, the frequent motion of PE chains also resulted in the intermittent formation and breaking of cavities between intermolecular PE chains. These deformations were more obvious on a surface with a convex protrusion, where the plowing effect exacerbated the cavitation and elastic deformation of PE chains. Correspondingly, the plastic flow in turn reconstructed the convex protrusion by displacing the surface atoms on the Fe slab. The plastic flow of PE chains broke the C-C bonds, and the carbon moieties were then chemically bonded onto the metal surface. A rapid change of atomic charge, hence, happened when the bonds broke. Meanwhile, PE chains release short alkyl radicals gradually after bond breakage, indicating gradual wear of the substrate during friction. This work provides molecular insight into the evolution of interfacial microstructure under plastic flow on a UHMWPE substrate.

Carbon Capture Membranes Based on Amorphous Polyether Nanofilms Enabled by Thickness Confinement and Interfacial Engineering.

Thin-film composite membranes are a leading technology for post-combustion carbon capture, and the key challenge is to fabricate defect-free selective nanofilms as thin as possible (100 nm or below) with superior CO2/N2 separation performance. Herein, we developed high-performance membranes based on an unusual choice of semi-crystalline blends of amorphous poly(ethylene oxide) (aPEO) and 18-crown-6 (C6) using two nanoengineering strategies. First, the crystallinity of the nanofilms decreases with decreasing thickness and completely disappears at 500 nm or below because of the thickness confinement. Second, polydimethylsiloxane is chosen as the gutter layer between the porous support and selective layer, and its surface is modified with bio-adhesive polydopamine (<10 nm) with an affinity toward aPEO, enabling the formation of the thin, defect-free, amorphous aPEO/C6 layer. For example, a 110 nm film containing 40 mass % C6 in aPEO exhibits CO2 permeability of 900 Barrer (much higher than a thick film with 420 Barrer), rendering a membrane with a CO2 permeance of 2200 GPU and CO2/N2 selectivity of 27 at 35 °C, surpassing Robeson's upper bound. This work shows that engineering at the nanoscale plays an important role in designing high-performance membranes for practical separations.

Fabrication and characterization of a bifunctional zinc oxide/multiwalled carbon nanotube/ poly(3,4-ethylenedioxythiophene): Polystyrene sulfonate composite thin film

Hybrid thin film sensors based on semiconductor oxides and carbon nanostructures are promising materials to improve the sensing, thermomechanical and optical properties. In this study, a thin film based on a composite from ZnO nanoparticles, milled multiwalled carbon nanotubes, and conducting polymer was fabricated. The layer was spray-coated to the surface of amorphous polyethylene terephthalate substrate. The hybrid layer and components have been systematically characterized by transmission and scanning electron microscopy, dynamic light scattering and the optical absorption spectrum was observed by ultraviolet-visible spectroscopy. The change in glass transition temperature was measured by dynamic mechanical analysis. Gas sensing tests reveal that the ZnO/multiwalled carbon nanotube/ poly(3,4-ethylenedioxythiophene): polystyrene sulfonate film on amorphous polyethylene terephthalate has remarkably enhanced performance compared to the same layer on indium tin oxide /polyethylene terephthalate. It could detect ethanol vapor in a wide concentration range with good response, fast recovery, and repeatability at room temperature. The layer was also tested as an ultraviolet photodetector, showing a promising signal. These features indicate the composite usability in further sensor applications.

Structure and function of the metagenomic plastic-degrading polyester hydrolase PHL7 bound to its product

The recently discovered metagenomic-derived polyester hydrolase PHL7 is able to efficiently degrade amorphous polyethylene terephthalate (PET) in post-consumer plastic waste. We present the cocrystal structure of this hydrolase with its hydrolysis product terephthalic acid and elucidate the influence of 17 single mutations on the PET-hydrolytic activity and thermal stability of PHL7. The substrate-binding mode of terephthalic acid is similar to that of the thermophilic polyester hydrolase LCC and deviates from the mesophilic IsPETase. The subsite I modifications L93F and Q95Y, derived from LCC, increased the thermal stability, while exchange of H185S, derived from IsPETase, reduced the stability of PHL7. The subsite II residue H130 is suggested to represent an adaptation for high thermal stability, whereas L210 emerged as the main contributor to the observed high PET-hydrolytic activity. Variant L210T showed significantly higher activity, achieving a degradation rate of 20 µm h−1 with amorphous PET films.

Dynamics of Phonon-Assisted Holes Trapping and Transport over Chemical Defects in Polyethylene.

Charge trapping and transport over chemical defects in polyethylene have significant impacts on its electrical and dielectric properties. However, the dynamics of this phenomenon and its underlying mechanisms remain unclear. To understand this fundamental aspect, we conducted a time-domain ab initio nonadiabatic molecular dynamics study of phonon-assisted holes dynamics in polyethylene over C═O and C-OH defect states. Our results suggest that the hole transfer and energy fluctuations substantially depend on temperature and local morphology. When the temperature decreases from 300 to 100 K, the hole transfer efficiency and the energy fluctuations are severely suppressed due to the weakened interactions between holes and phonons. Furthermore, amorphous polyethylene exhibits a severe suppression of the hole transfer process compared to crystalline polyethylene. An explanation for the influence of morphology on the hole transfer process can be found in the differences in the hole-phonon coupling and the electronic coupling between two chemical defect states in crystalline and amorphous polyethylene. Advancing the fundamental understanding of the dynamics of hole transfer over chemical effects in polymers is a key to improving their insulating properties for the next-generation high-voltage cables.

Colossal barocaloric effect achieved by exploiting the amorphous high entropy of solidified polyethylene glycol

The barocaloric effect (BCE) has emerged as an intense research topic in regard to efficient and clean solid-state refrigeration. Materials with solid-liquid phase transitions (SL-PTs) usually show huge melting entropies but cannot work in full solid-state refrigeration. Here, we report a colossal barocaloric effect realized by exploiting high entropy inherited from huge disorder of liquid phase in amorphous polyethylene glycol (PEG), which is solidified by introducing 5 wt.% polyethylene terephthalate (PET). Transmission electron microscopy (TEM) combined with X-ray diffraction (XRD) demonstrates the amorphous nature of the high-temperature phase after fixation by PET. Although PEG loses its –OH end mobility in amorphous solid, high entropy still retains owing to the retained high degrees of freedom of its molecular chains. The remaining entropy of amorphous PEG is up to 83% of that of liquid PEG in PEG10000/PET15000, and the barocaloric entropy change reaches ΔSp ∼ 416 J·kg−1·K−1 under a low pressure of 0.1 GPa, which exceeds the performance of most other BCE materials. Infrared spectra combined with density function theory (DFT) calculations disclose conformational change from the liquid to amorphous state, which explains the origin of the large entropy retained and hence the colossal BCE of the solidified PEG. This research opens a new avenue for exploring full solid-state barocaloric materials by utilizing genetic high entropy from huge disordering of liquid phases in various materials with SL-PTs.

Molecular dynamics investigations into the hydrogen permeation mechanism of polyethylene pipeline material

To avoid hydrogen embrittlement and other hydrogen damage caused during hydrogen transportation using metal pipelines, it is feasible to transport hydrogen through nonmetallic polyethylene (PE) pipelines. However, gas permeation and leakage are more obvious for hydrogen delivery using PE pipelines than those using metal pipelines. To reveal the permeation and diffusion mechanisms of hydrogen in PE pipelines, the Grand Canonical Monte Carlo simulation and molecular dynamics are conducted. The solubility and diffusion characteristics of hydrogen in amorphous PE are investigated at temperatures of 270–310 K and pressures of 0.1–0.7 MPa. The influences of temperature and pressure on hydrogen permeation are also analyzed. The results show that the solubility, diffusion, and permeability coefficients of hydrogen in amorphous PE increase with increasing temperature, and their relationships with temperature are consistent with the Arrhenius law. Pressure has a negligible effect on the permeability coefficient of hydrogen in amorphous PE, whereas temperature appreciably affects the permeability coefficient. The diffusion of hydrogen in PE conforms to the “hopping” mechanism. The hydrogen molecule vibrates in the free volume pore for a long time and then quickly hops to the adjacent pores to complete diffusion. The hydrogen molecule continues to vibrate and hop, eventually moving further away from its initial position and permeating through PE materials.

Molecular dynamics simulation of H2 in amorphous polyethylene system: H2 diffusion in various PE matrices and bubbling during rapid depressurization

Polyethylene (PE) is a candidate liner material for Type IV storage devices. In this case, all-atom molecular dynamics simulations are employed to study the properties of Polyethylene with the presence of H2, including tensile properties, glass transition, diffusion in different PE and bubbling during rapid depressurization. The presence of H2 deteriorates the polyethylene matrix's tensile performance and decreases the glass transition temperature. The branch, side chain and small molecules promote the diffusion of H2 in the amorphous region by introducing more free volume below Tg. With a sufficient length, the length of polymer chain has minor effect on the diffusion of H2. Graphene, as a 2D reinforcement, could decrease the diffusion of H2 but suffers from poor interfacial bonding. Finally, H2 bubble(s) formed from the over-saturated H2 molecule and were observed in both the exclusive and free volume and stabilised at low pressure during rapid depressurization. According to the result obtained in this work, branchless HDPE is expected to give superior performance while the viscosity, which is important during processing, could be tailored by molecular weight. Processing technique leads to orientation is preferred, such as injection moulding.

Microbial enzymes will offer limited solutions to the global plastic pollution crisis.

Global economies depend on the use of fossil-fuel-based polymers with 360-400 million metric tons of synthetic polymers being produced per year. Unfortunately, an estimated 60% of the global production is disposed into the environment. Within this framework, microbiologists have tried to identify plastic-active enzymes over the past decade. Until now, this research has largely failed to deliver functional biocatalysts acting on the commodity polymers such as polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), ether-based polyurethane (PUR), polyamide (PA), polystyrene (PS) and synthetic rubber (SR). However, few enzymes are known to act on low-density and low-crystalline (amorphous) polyethylene terephthalate (PET) and ester-based PUR. These above-mentioned polymers represent >95% of all synthetic plastics produced. Therefore, the main challenge microbiologists are currently facing is in finding polymer-active enzymes targeting the majority of fossil-fuel-based plastics. However, identifying plastic-active enzymes either to implement them in biotechnological processes or to understand their potential role in nature is an emerging research field. The application of these enzymes is still in its infancy. Here, we summarize the current knowledge on microbial plastic-active enzymes, their global distribution and potential impact on plastic degradation in industrial processes and nature. We further outline major challenges in finding novel plastic-active enzymes, optimizing known ones by synthetic approaches and problems arising through falsely annotated and unfiltered use of database entries. Finally, we highlight potential biotechnological applications and possible re- and upcycling concepts using microorganisms.

Uniaxial stretching and shape memory behavior of branched amorphous polyethylene

The current work investigates the effects of branching and temperature on the mechanical properties and shape memory properties of branched polyethylene. The results show that the mechanical properties of low density polyethylene are strongly influenced by the length, number and temperature of the branched chains. The lower the temperature, the lower the modulus of elasticity. When the length is longer and the number is smaller, the stress in the hardening phase is higher. For shape memory properties, we found that the fewer and shorter the branched chains, the better the shape memory properties, and the shape memory behavior can promote the repolymerization and crystallization of the structure, which will cause the enhancement of stress hardening.