Polyethylene Nanofibers Research Articles

Ambient Catalytic Spinning of Polyethylene Nanofibers.

A novel single-atom Ni(II) catalyst (Ni-OH) is covalently immobilized onto the nano-channels of mesoporous Santa Barbara Amorphous (SBA)-15 particles and isotropic Anodized Aluminum Oxide (AAO) membrane for confined-space ethylene extrusion polymerization. The presence of surface-tethered Ni complexes (Ni@SBA-15 and Ni@AAO) is confirmed by the inductively coupled plasma-optical emission spectrometry (ICP-OES) and X-ray photoelectron spectroscopy (XPS). In the catalytic spinning process, the produced PE materials exhibit very homogeneous fibrous morphology at nanoscale (diameter: ~50 nm). The synthesized PE nanofibers extrude in a highly oriented manner from the nano-reactors at ambient temperature. Remarkably high Mw (1.62×106 g mol-1 ), melting point (124 °C), and crystallinity (41.8 %) are observed among PE samples thanks to the confined-space polymerization. The chain-walking behavior of surface tethered Ni catalysts is greatly limited by the confinement inside the nano-channels, leading to the formation of very low-branched PE materials (13.6/1000 C). Due to fixed supported catalytic topology and room temperature, the filaments are expected to be free of entanglement. This work signifies an important step towards the realization of a continuous mild catalytic-spinning (CATSPIN) process, where the polymer is directly synthesized into fiber shape at negligible chain branching and elegantly avoiding common limitations like thermal degradation or molecular entanglement.

Ambient Catalytic Spinning of Polyethylene Nanofibers

AbstractA novel single‐atom Ni(II) catalyst (Ni‐OH) is covalently immobilized onto the nano‐channels of mesoporous Santa Barbara Amorphous (SBA)‐15 particles and isotropic Anodized Aluminum Oxide (AAO) membrane for confined‐space ethylene extrusion polymerization. The presence of surface‐tethered Ni complexes (Ni@SBA‐15 and Ni@AAO) is confirmed by the inductively coupled plasma‐optical emission spectrometry (ICP‐OES) and X‐ray photoelectron spectroscopy (XPS). In the catalytic spinning process, the produced PE materials exhibit very homogeneous fibrous morphology at nanoscale (diameter: ~50 nm). The synthesized PE nanofibers extrude in a highly oriented manner from the nano‐reactors at ambient temperature. Remarkably high Mw (1.62×106 g mol−1), melting point (124 °C), and crystallinity (41.8 %) are observed among PE samples thanks to the confined‐space polymerization. The chain‐walking behavior of surface tethered Ni catalysts is greatly limited by the confinement inside the nano‐channels, leading to the formation of very low‐branched PE materials (13.6/1000 C). Due to fixed supported catalytic topology and room temperature, the filaments are expected to be free of entanglement. This work signifies an important step towards the realization of a continuous mild catalytic‐spinning (CATSPIN) process, where the polymer is directly synthesized into fiber shape at negligible chain branching and elegantly avoiding common limitations like thermal degradation or molecular entanglement.

A Neural Network Approach to Reducing the Costs of Parameter-Setting in the Production of Polyethylene Oxide Nanofibers.

Nanofibers, which are formed by the electrospinning process, are used in a variety of applications. For this purpose, a specific diameter suited for each application is required, which is achieved by varying a set of parameters. This parameter adjustment process is empirical and works by trial and error, causing high input costs and wasting time and financial resources. In this work, an artificial neural network model is presented to predict the diameter of polyethylene nanofibers, based on the adjustment of 15 parameters. The model was trained from 105 records from data obtained from the literature and was then validated with nine nanofibers that were obtained and measured in the laboratory. The average error between the actual results was 2.29%. This result differs from those taken in an evaluation of the dataset. Therefore, the importance of increasing the dataset and the validation using independent data is highlighted.

Dual-Driven Functional Fabric with High Electrothermal and Photothermal Conversion Efficiency Modified by CuS Nanoparticles

As an ideal substrate for active material integration, fabrics of nanofiber bundles have held great promise due to their outstanding surface area, unique mechanical flexibility, and textile structure. Herein, a polyethylene nanofiber bundle (PNB) fabric was prepared via melt blending extrusion followed by chemical deposition of a copper sulfide (CuS) active layer. Benefiting from the high conductivity and the outstanding light absorption characteristics of CuS, CuS/PNB fabrics exhibited excellent photo and electric dual-driven energy conversion performance. Specifically, the CuS/PNB fabrics showed the highest electrical conductivity (277 S m–1), exceptional Joule heating (up to 85.7 °C at a voltage of 0.8 V), a low working voltage (0.2–0.8 V), and long-term stability. Furthermore, the photo-absorption efficiency and surface temperature reached 94.3% and 91.2 °C under the irradiation of 1 sun, respectively. Meanwhile, the CuS/PNB fabrics exhibited a high specific evaporation rate of up to 1.81 kg m–2 h–1, as well as an energy conversion efficiency of 95.1% under one solar irradiation. We believe that such a functional fabric offers a strategy for energy harvesting and conversion fields and displays promising applications in warmth-keeping and solar-driven water evaporation in the future.

Ultrarobust Photothermal Materials via Dynamic Crosslinking for Solar Harvesting.

Highly efficient and mechanically durable photothermal materials are urgently needed for solar harvesting, but their development still remains challenging. Here, inspired by the hierarchically oriented architecture of natural spider silk, an ultrarobust liquid metals (LMs)/polymer composite is presented via dynamic crosslinking based on the unique mechanical deformable characteristic of LMs. Dynamically cross-linked core-shell structured LMs droplets can be squeezed along with the orientational crystallization of polymer chains during drawing, thus enabling LMs nanoparticles to be uniformly programmed in the rigid polyethylene nanofiber skeleton. The resultant composite exhibits an unprecedented combination of strong broad-band light absorption (96.9-99.3%), excellent photothermal conversion ability, remarkable mechanical property (tensile strength of 283.7MPa, which can lift 200000 times its own weight), and long-term structural reliability (bearing 100000 bending cycles). A powerful and durable solar thermoelectric generator system for real-environmental solar-heat-electricity conversion is further demonstrated, providing a valuable guidance for the design and fabrication of high-performance solar-harvesting materials.

Heterogeneous irradiated-pristine polyethylene nanofiber junction as a high-performance solid-state thermal diode

In this work, we demonstrate two types of heterogeneous irradiated-pristine polyethylene nanofiber junctions, ‘heavily-irradiated-pristine’ (HI-P) and ‘lightly-irradiated-pristine’ (LI-P) junctions, as high-performance solid-state thermal diodes. The HI-P junction rectifies heat flux in a single direction, while the LI-P junction shows dual-directional rectification under different working temperatures. We accurately model the phase transition of polyethylene nanofibers with a finite temperature range rather than a step function. The finite-temperature-range model suggests that the rectification factor increases with temperature bias and there is a minimum threshold of temperature bias for notable rectification. Besides, the finite-temperature-range model shows better prediction for the heat flow data from experiments, while the step function model tends to overestimate the rectification performance around the optimal length fraction of irradiation. Although both the models show that an optimal rectification occurs when the interface temperatures in the forward and the reverse biases are equal, the optimized rectification factor is determined by the temperature bias and the temperature range of phase transition. This work elucidates the influence of both the temperature bias and the temperature range of phase transition on thermal rectification performance, which could incredibly benefit the evaluation and design of thermal diodes.

Controlled individual and partial polyethylene nanofibers with high Tm2 prepared by PPM-supported Cp2TiCl2 catalysts

Controlled individual and partial nanofibrous polyethylenes with high second melting point (Tm2) were obtained through ethylene-confined polymerization using a porous polymer microsphere (PPM)-supported Cp2TiCl2 catalyst.

Effects of varying melt flow rates on laser melt‐electrospinning of low‐density polyethylene nanofibers

AbstractThe ultimate functionality and applicability of polymeric nanofibers are mainly to subject on its diameter. This study explores the influence of melt flow rates (MFRs) of low‐density polyethylene (LDPE) on the diameter of laser melt electrospun nanofibers. Ethylene‐vinyl alcohol (EVOH) copolymer was added to the nonpolar LDPE as a spinning aid. After electrospinning, the EVOH was removed from LDPE/EVOH blend fiber by treating with isopropanol/water solution and LDPE nanofiber was obtained with a diameter of only 190 ± 85 nm for the highest MFR. A linear diameter reduction was observed for pure LDPE and EVOH removed LDPE fiber with the increase of MFR. However, a slight diameter increment was reported for the LDPE/EVOH blend fiber with higher MFR due to the improved melt viscosity of the component. A massive diameter decrement was found after EVOH removal from the blended fiber, resulting in the renovation of microfiber to a stable nanoscale dimension.

High Melting Point of Linear, Spiral Polyethylene Nanofibers and Polyethylene Microspheres Obtained Through Confined Polymerization by a PPM‐Supported Ziegler‐Natta Catalyst

In this work, different types of polyethylene (linear, spiral nanofibers and microspheres) were obtained via confined polymerization by a PPM‐supported Ziegler‐Natta catalyst. Firstly, the Ziegler‐Natta catalyst was chemical bonded inside the porous polymer microspheres (PPMs) supports with different pore diameter and supports size through chemical reaction. Then slightly and highly confined polymerization occurred in the PPM‐supported Ziegler‐Natta catalysts. SEM results illustrated that the slightly confined polymerization was easy to obtain linear and spiral nanofibers, and the nanofibers were observed in polyethylene catalyzed by PPMs‐1#/cat and PPMs‐2#/cat with low pore diameter (about 23 nm). Furthermore, the highly confined polymerization produced polyethylene microspheres, which obtained through other PPM‐supported Ziegler‐Natta catalysts with high pore diameter. In addition, high second melting point (T m2: up to 143.3 °C) is a unique property of the polyethylene obtained by the PPM‐supported Ziegler‐Natta catalyst after removing the residue through physical treatment. The high T m2 was ascribed to low surface free energy (σ e), which was owing to the entanglement of polyethylene polymerized in the PPMs supports with interconnected multi‐modal pore structure.

Dual-mode solid-state thermal rectification

Thermal rectification is an exotic thermal transport phenomenon which allows heat to transfer in one direction but block the other. We demonstrate an unusual dual-mode solid-state thermal rectification effect using a heterogeneous “irradiated-pristine” polyethylene nanofiber junction as a nanoscale thermal diode, in which heat flow can be rectified in both directions by changing the working temperature. For the nanofiber samples measured here, we observe a maximum thermal rectification factor as large as ~50%, which only requires a small temperature bias of <10 K. The tunable nanoscale thermal diodes with large rectification and narrow temperature bias open up new possibilities for developing advanced thermal management, energy conversion and, potentially thermophononic technologies.

Melt-Electrospun Polyethylene Nanofiber Obtained from Polyethylene/Polyvinyl Butyral Blend Film.

We prepared low-density polyethylene (LDPE) nanofiber, a few hundred nanometers in diameter, using polyvinyl butyral (PVB) and a laser melt-electrospinning (M-ESP) device. We blended PVB with LDPE via an internal melt mixer, removed the PVB after M-ESP by ethanol treatment, and studied the influence of PVB on fiber diameter. A substantial diameter reduction with improved crystallinity of LDPE fiber was observed with increased PVB content in the blend. PVB inclusion also increased the polarity of the LDPE/PVB blend, resulting in better spinnability. The removal of PVB from LDPE/PVB blend fiber caused a massive drop in the LDPE fiber diameter, due to fiber splitting, particularly in PVB-rich samples. Fourier transform infrared (FTIR) spectroscopy of fibers confirmed that the prepared nanofiber was the same as pure LDPE fiber.

High-contrast and reversible polymer thermal regulator by structural phase transition.

In comparison with the advancement of switchable, nonlinear, and active components in electronics, solid-state thermal components for actively controlling heat flow have been extremely rare. We demonstrate a high-contrast and reversible polymer thermal regulator based on the structural phase transition in crystalline polyethylene nanofibers. This structural phase transition represents a dramatic change in morphology from a highly ordered all-trans conformation to a combined trans and gauche conformation with rotational disorder, leading to an abrupt change in phonon transport along the molecular chains. For five nanofiber samples measured here, we observe an average thermal switching ratio of ~8× and maximum switching ratio of ~10×, which occurs in a narrow temperature range of 10 K across the structural phase transition. To the best of our knowledge, the ~10× switching ratio exceeds any reported experimental values for solid-solid and solid-liquid phase transitions of materials. There is no thermal hysteresis observed upon heating/cooling cycles.

A processing method with high efficiency for low density polyethylene nanofibers reinforced by aligned carbon nanotubes via nanolayer coextrusion

A method for large-scale processing of low density polyethylene (LDPE) nanofibers reinforced by aligned multi-walled carbon nanotubes (MWCNTs) is achieved via nanolayer coextrusion. The morphology and nano-sized diameter of the nanofibers are observed on scanning electron microscope. The prioritized alignment and dispersion of MWCNTs along the fiber axis are determined by transmission electron microscopy, polarized Raman spectra, and high resolution optical microscopy. The dispersion of MWCNTs in nanofibers is better than that in homologous micron-fibers. In addition, MWCNTs can promote the crystallization of LDPE in varying degrees which depend on the fiber diameter and MWCNTs content. As a result of the alignment of MWCNTs, the nanofibers exhibit enhanced mechanical properties. The tensile strength of LDPE(1 wt% MWCNTs) nanofibers is more than twice of pure nanofibers (without MWCNTs). The volume resistivity of the LDPE(MWCNTs) nanofiber mats containing 9 wt% MWCNTs can be reduced to 100 Ω cm.

Isotope effect on quantum thermal transport in a polyethylene chain

both the theoretical and the experimental aspects. Bulk polyethylene is regarded as a thermal insulator because its thermal conductivity is typically on the order of 0.35 W·m-1·K-1. However, recent studies demonstrate that a polyethylene chain has an extremely high thermal conductivity and the reported thermal conductivity of ultra-drawn polyethylene nanofibers is as high as 104 W·m-1·K-1, about 300 times higher than that of bulk polyethylene. In order to cast off this dilemma, several simulation methods are used to detect the unusually high thermal conductivity of a polyethylene chain. Molecular dynamics (MD) simulation results are highly sensitive to the choice of empirical potential or simulation method. Even using the same potential (AIREBO potential), the obtained thermal conductivity of a polyethylene chain is different. By combining the Green-Kubo method with a modal decomposition approach, equilibrium molecular dynamics (EMD) indicates that the thermal conductivity is able to exceed 100 W·m-1·K-1 while the polyethylene chain is longer than 40 nm at room temperature. Compared with the simulation result obtained by equilibrium molecular dynamics, the simulation result provided by using the non-equilibrium molecular dynamics (NEMD) method is only 57 W m·m-1·K-1 for a 160-nm-long polyethylene chain at room temperature. We use the first-principles method to calculate the force constant tensor, and the characteristics of quantum thermal transport in a polyethylene chain can be revealed. In our algorithm, several shortcomings of molecular dynamics, i.e., different potential functions or simulation methods may lead to obviously different results for the same quantum thermal transport system, are overcome. Based on the density functional theory (DFT), the central insertion scheme (CIS) combined with nonequilibrium Green's function (NEGF) is used to evaluate the isotope effect on quantum thermal transport in a polyethylene chain, which includes 432 atoms in scattering region and has a length of 18.533 nm. It is found that the upper limit of thermal conductivity of a 100-nm-long pure 12C polyethylene chain reaches a high value of 314.1 W·m-1·K-1 at room temperature. Moreover, for the case of a pure polyethylene chain of 12C, with other conditions unchanged, the reduction of average thermal conductance caused by 14C impurity is more remarkable than that by 13C. The most outstanding isotope effect on quantum thermal transport can be detected in the polyethylene chain. When the doping concentration of 14C in 12C is 50% at room temperature, the average thermal conductance will be reduced by 51%. It is of great significance for studying the mechanism of isotope effect on thermal transport in polyethylene.

A rapid and simple method to draw polyethylene nanofibers with enhanced thermal conductivity

We report on a rapid and simple method to fabricate polyethylene (PE) nanofibers by one-step drawing from PE solution. The diameter of the fiber prepared with this method can be as small as 40 nm. The thermal conductivity of the drawn PE nanofiber was measured with suspended microdevices, and the highest value obtained is 8.8 W m−1 K−1, which is very close to that of electrospun PE nanofibers, and over 20 times higher than bulk value. Raman spectra of these drawn PE nanofibers indicate that molecular chains in these fibers can be as well aligned as that in electrospun fibers, which results in the enhanced thermal conductivity of the drawn PE nanofibers.

Giant Thermal Rectification from Polyethylene Nanofiber Thermal Diodes.

The realization of phononic computing is held hostage by the lack of high-performance thermal devices. Here, it is shown through theoretical analysis and molecular dynamics simulations that unprecedented thermal rectification factors (as large as 1.20) can be achieved utilizing the phase-dependent thermal conductivity of polyethylene nanofibers. More importantly, such high thermal rectifications only need very small temperature differences (<20 °C) across the device, which is a significant advantage over other thermal diodes which need temperature biases on the order of the operating temperature. Taking this into consideration, it is shown that the dimensionless temperature-scaled rectification factors of the polymer nanofiber diodes range from 12 to 25-much larger than those of other thermal diodes (<8). The polymer nanofiber thermal diode consists of a crystalline portion whose thermal conductivity is highly phase-sensitive and a cross-linked portion which has a stable phase. Nanoscale size effect can be utilized to tune the phase transition temperature of the crystalline portion, enabling thermal diodes capable of operating at different temperatures. This work will be instrumental to the design of high performance, inexpensive, and easily processible thermal devices, based on which thermal circuits can be built to ultimately enable phononic computing.

Confined polymerization: catalyzed synthesis of high Tm, nanofibrous polyethylene within porous polymer microspheres

Linear polyethylene nanofibers are formed through ethylene confined polymerization. Polyethylene nanofibers (&lt;70 nm) are aggregated to form intertwined thicker fibers (300 nm to 22 μm). The obtained PE has a high Mw and Tm2 (up to 142.4 °C).

Thermal conductivity of electrospun polyethylene nanofibers.

We report on the structure-thermal transport property relation of individual polyethylene nanofibers fabricated by electrospinning with different deposition parameters. Measurement results show that the nanofiber thermal conductivity depends on the electric field used in the electrospinning process, with a general trend of higher thermal conductivity for fibers prepared with stronger electric field. Nanofibers produced at a 45 kV electrospinning voltage and a 150 mm needle-collector distance could have a thermal conductivity of up to 9.3 W m(-1) K(-1), over 20 times higher than the typical bulk value. Micro-Raman characterization suggests that the enhanced thermal conductivity is due to the highly oriented polymer chains and enhanced crystallinity in the electrospun nanofibers.

Core/Shell and Hollow Ultra High Molecular Weight Polyethylene Nanofibers and Nanoporous Polyethylene Prepared by Mesoscopic Shape Replication Catalysis

Polyvinyl alcohol (PVA) nanofibers, produced by electrospinning, represent attractive high‐surface‐area supports for olefin polymerization catalysts. Tethered with metal alkyls, PVA nanofibers immobilize a great variety of transition metal compounds, thus producing highly active nanofiber‐supported Ziegler‐, metallocene‐, and post‐metallocene catalysts. Whereas most conventional heterogeneous polymerization catalysts form particles, PVA‐nanofiber‐supported catalysts enable polyolefin nanofiber and nanostructure formation by mesoscopic shape replication using electrospun nanofibers as templates. At low ethylene pressure, linear correlation between average PE/PVA core/shell fiber diameter and polymerization time are made. At elevated pressure, this control is lost, accounting for the formation of reactor blends consisting of PE granules and built‐in PE/PVA nanofibers. Whereas conventional catalysts produce micrometer‐sized particles of ultrahigh molecular weight PE (UHMWPE), PVA‐nanofiber supported chromium catalysts afford new families UHMWPE materials. They range from UHMWPE/PVA core/shell nanofibers and nonwovens to hollow UHMWPE fibers and nanoporous UHMWPE, obtained by removing the PVA component.

Crystalline Polyethylene Nanofibers with the Theoretical Limit of Young's Modulus

A highly crystalline polyethylene nanofiber is deflected by a cantilever under an atomic force microscope. The resulting approach and retract curves on both nanofiber and silicon substrate yield the Young's modulus of the nanofiber. The experiments show that the average Young's modulus of polyethylene nanofibers with diameters from 70 nm to 260 nm can be as high as 312 GPa.