Complete Tg–p profile for PMMA/CO2 system.
•The volume change ratio of PMMA at 12MPa has the most significant increment, instead of that at 20MPa.•Tg of PMMA decreased from 380.5 to 311.4K withCO2 ...pressure 0.1–12MPa, and increased from 311.4 to 323.1K with 12–22MPa.•When CO2 pressure is beyond 12MPa, hydrostatic pressure has a more significant effect on swelling ratio than swelling.•The Tsat−pCO2 curve has similar change with the Tg−pCO2 curve.
The glass transition temperature (Tg) reduction of polymer by plasticization is a paramount thermodynamic property in polymer/diluent systems, thus effecting microcellular foaming process. Experiment was conducted in a high-pressure cell furnished with an optical channel. In this research, the swelling ratios (ΔV/V0) of polymethyl methacrylate (PMMA) in carbon dioxide (CO2) with pressures ranging from 2 to 22MPa were measured, thus the Tg data of PMMA was determined in the same pressure range. It was found that the swelling ratio increases with temperature but reaches the maximum at 12MPa, instead of 22MPa. And the glass transition temperature decreases from 380.5 to 311.4K within the pressure range of 0.1–12MPa but increases gradually to 323.1K from 12 to 22MPa. Additionally, the change of maximum saturation temperature as a function of CO2 pressures agrees well with Tg of the polymer-CO2 system.
The role of solvents is pivotal for determining the primary and hierarchical structures of electrospun fibers. The preparation of beads-on-string structures or uniform poly methyl methacrylate (PMMA) ...fibers with circular and collapsed ribbon-like cross sections from six solvents with different properties was described in this work. The formation of a fiber cross section can be explained by buckling instability. Moreover, all of the resultant fiber surfaces are porous or wrinkled when electrospun with these solvents. It was concluded that vapor-induced phase separation (VIPS) by high relative humidity is the mechanism responsible for the formation of hierarchical structures. The nucleation growth (NG) mechanism during phase separation accounted for the round nanopores with radii of 60-150 nm and elliptical nanopores with long axis of 60-140 nm, short axis of 20-40 nm on the electrospun PMMA fibers from dichloromethane (DCM), chloroform, and ethyl acetate, whereas the formation of the wrinkled fiber surface of the PMMA/acetone, PMMA/tetrahydrofuran (THF), and PMMA/
N
,
N
-dimethyl formamide (DMF) systems resulted from spinodal decomposition (SD) mechanism. Furthermore, the fibers with round cross sections and highly porous interiors and surfaces were observed due to the VIPS and phase separation caused by the different evaporation rates of DCM and DMF.
PMMA fibers with porous surfaces and interiors were produced when electrospun with a binary solvent of DMF and DCM. The vapor-induced phase separation is responsible for the generation of pores on PMMA fibers.
•Dehydrogenation mechanism of H2O on Pd (100) was studied using periodic DFT calculations.•The optimized structures and adsorption energies were obtained. The results offered an explanation to use ...oxygen to modify Pd (100) surface. The energy barriers and reaction energies were calculated.
Based on density functional theory together with periodic slab models, the adsorption and the corresponding dehydrogenation reaction of H2O on clean and oxygen modified Pd (1 0 0) have been investigated. The preferential sites for H2O, OH, O, and H were obtained on the surfaces. According to the optimized structural and energetic properties, it was found that H2O prefers to adsorb on the top site with weak adsorption energy (physisorption), whereas O and H atoms are prone to adsorb on the hollow site and OH occupies the bridge site. In addition, this work identified the optimum configurations for the relevant co-adsorption groups. The results confirmed that co-adsorption tends to weaken the adsorbate-substrate interaction due to the existence of oxygen atom, and that the OH group, O and H atoms are less stable on oxygen-covered Pd (1 0 0) surface than on the clean surface. Finally, the transition states and related barrier energies were ascertained to analyze the dehydrogenation mechanism of H2O. Water decomposition was found favorable on O-covered Pd (1 0 0) surface (0.49eV), in agreement with the experimental observations. This result indicated that the joining of Oads could reduce the barrier energy and facilitate the decomposition of H2O. Besides, the distinct differences over Pd (1 1 1) and Pd (1 0 0) surface implied that water decomposition over Pd-based catalysts is a structure-sensitive reaction.
An integrated process of supercritical methanol (SCM) and sodium methoxide catalyst was developed to produce fatty acid methyl esters (FAMEs) via continuous esterification from crude biodiesel. The ...crude biodiesel with high free fatty acid (FFA) content must be refined to reduce the acid value (AV) for meeting the quality standards. The process parameters were studied by Box-Behnken design (BBD) of response surface methodology (RSM). The experimental results revealed that the AV of crude biodiesel decreased from 18.66 to 0.55 mg KOH g
−1
at the reaction conditions of 350 °C, 0.5 % amount of sodium methoxide catalyst, and 10 MPa. Temperature shows the most significant effect on the esterification, followed by pressure and amount of sodium methoxide catalyst. This integrated process proved to be a potential route to refine the crude biodiesel because of its continuity, high efficiency, and less energy consumption with relatively moderate reaction conditions compared with conventional methods.
•Interaction effects were investigated in PET supercritical methanolysis process.•RSM was used to develop mathematical model and analysis for process.•A modified square root transformation model was ...used in RSM for better prediction.•Terms including MeOH to PET mass ratio are uninfluential among experiment conditions.•The optimum DMT yield was much closer to the extent of the complete reaction.
Polyethylene terephthalate (PET) is one of the most widely used raw materials in chemical industry. Chemical recycling of the waste PET is a sustainable way not only considering the economic benefit but also the environmental goals. This paper focused on the effects among three foremost variables (methanolysis temperature, time, and mass ratio of methanol (MeOH) to PET), established a mathematical model depicted the complete effects of the parameters on the process, and sought the optimal solution of this PET methanolysis reaction. Response surface methodology (RSM) was used to model and analysis the recycling process of dimethyl terephthalate (DMT) from waste PET under supercritical methanol conditions. Methanolysis temperature and time could increase the DMT yield. Mass ratio of MeOH to PET hardly affected DMT yield when the methanol is excess. Different operation parameters including reaction temperature (240–320°C), reaction time (15–120min), and MeOH to PET mass ratio (6:1–10:1) were modeled and optimized using RSM. The quadratic equation with mathematical transformation (square root) modified was established for predicting the DMT yield and evaluated the validity on the basic of analysis of variance (ANOVA). The result indicated that the MeOH to PET mass ratio and relative term exhibit a less significant influence on the response surface in comparison to reaction temperature and time in the range of the experiment conditions. Prediction made by the developed model was in reasonably good agreement with the test runs. Under the optimal condition, the yield of DMT reached 99.79% which was much closer to the extent of the complete reaction.
Display omitted
•Anisotropic electrical conductivity was obtained in Ti3C2Tx MXene composite film.•MXene had aligned structure in film and random orientation in counterpart foam.•CO2-assisted foaming ...facilitated isotropic conduction and improved EMI shielding.•Attenuation of EM radiations exhibited absorption-dominated shielding mechanism.•High thermal and mechanical properties were obtained in both composite film and foam.
Integration of a foam structure into a shielding material not only reduces its mass density but also enhances its electromagnetic interference (EMI) shielding performance. In this study, poly(vinylidene fluoride) (PVDF) composite films and foams consisting of carbon nanotubes (CNTs) and bulk Ti3C2Tx MXenes were fabricated. Prominent anisotropy with high in-plane electrical conductivity (σ∥) and low through-plane electrical conductivity (σ⊥) was achieved (e.g., σ∥ = 17.02 S/m and σ⊥ = 0.42 S/m with 1 wt% MXenes) in the composite films due to the aligned structure of the MXenes in the PVDF matrix. With CO2-assisted foaming, the intercalation of PVDF chains and cell-induced biaxial stretch facilitated reorganized orientation and delamination of the MXenes. Consequently, σ∥ decreased and σ⊥ increased, and exhibiting an isotropic feature. Furthermore, the integration of the foam structure resulted in an increased σ⊥, which improved the EMI shielding effectiveness (EMI SE) to 65.1 dB at a MXene content of 12 wt%, owing to enhanced multi-reflections, dielectric loss and conduction loss. Moreover, the shielding effectiveness coming from reflection was reduced to lower than 3 dB due to the decrease in σ∥ and destructive interference in the composite foam, thus leading to an absorption-dominated mechanism. Additionally, the prepared composite foams exhibited outstanding thermal conductivity and mechanical strength, enabling them to be used as lightweight and robust EMI shielding materials that could transform EM wave energy into Joule heat.
Polymer composites receive attentions for protecting from electromagnetic (EM) pollution. However, their EM wave (EMW) attenuation mechanism primarily results from reflection rather than absorption. ...Herein, we prepared poly(vinylidene fluoride)/cobalt (Co)/MXene composite foams that exhibited applicable impedance matching, enhanced EMW absorption and high-performance thermal conduction properties. With CO2-assisted foaming, a uniform foam structure was integrated into the polymer composites, and meanwhile, the introduced MXenes were partially oxidized and transformed into TiO2 and amorphous carbon. The formed TiO2 not only provided extra heterogeneous interfaces and capacitor-like structures in favor of dielectric polarization but also reduced the excessive electrical conductivity of the pristine MXenes to favor impedance matching. Accordingly, the EMW absorbing performance of the composite foam was enhanced with a minimum reflection loss of −45.6 dB at 4 mm when the filler content was only 12 wt% (6 wt% MXene and 6 wt% Co). Additionally, the synergism between the foam structure and TiO2 nanocrystals resulted in improved thermal conductivity, ranging from 1.28 W/(m·K) to 1.36 W/(m·K), which were 2–6 times higher than that in the solid composite films. This study provided new insights into the simultaneously enhanced EMW absorption and dissipating heat ability in polymer composite foams with a low percolation threshold.
Display omitted
•TiO2/C is formed from oxidization of MXene in composite via CO2-assisted foaming.•Dielectric loss and impedance match are improved by formed TiO2 and foam structure.•Thermal conductivity in composite foam exhibit 2–6 times higher than that in film.•Synergism between TiO2 and conduction network result in high thermal conductivity.
Although transition metal carbides/carbonitrides (MXenes) exhibit immense potential for electromagnetic wave (EMW) absorption, their absorbing ability is hindered by facile stacking and high ...permittivity. Layer stacking and geometric structures are expected to significantly affect the conductivity and permittivity of MXenes. However, it is still a formidable task to simultaneously regulate layer stacking and microstructure of MXenes to realize high‐performance EMW absorption. Herein, a simple and viable strategy using electrostatic adsorption is developed to integrate 2D Ti3C2Tx MXene nanosheets into 3D hollow bowl‐like structures with tunable layer stacking thickness. Density functional theory calculations indicate an increase in the density of states of the d orbital from the Ti atom near the Fermi level and the generation of additional electrical dipoles in the MXene nanosheets constituting the bowl walls upon reducing the layer stacking thickness. The hollow MXene bowls exhibit a minimum reflection loss (RLmin) of −53.8 dB at 1.8 mm. The specific absorbing performance, defined as RLmin (dB)/thickness (mm)/filler loading (wt%), exceeds 598 dB mm−1, far surpassing that of the most current MXene and bowl‐like materials reported in the literature. This work can guide future exploration on designing high‐performance MXenes with “lightweight” and “thinness” characteristics for superior EMW absorption.
A simple and viable strategy using electrostatic adsorption is developed to integrate 2D Ti3C2Tx MXene nanosheets into 3D hollow bowl‐like structures with tunable layer stacking thickness. The layer stacking and geometric structures of the MXenes are simultaneously regulated to realize high‐performance electromagnetic wave absorption due to improved Ti vacancies, lattice distortions, degree of aggregation/stacking, and hollow structure.
•TEAs for two co-processing of VGO and bio-oil scenarios are conducted.•Minimum gasoline selling price of FPCP scenario is $3.497/gal.•Minimum gasoline selling price of HTLCP scenario is ...$2.910/gal.•Sensitivity analyses of the two scenarios are also carried out.
As bio-energy is the only carbon-containing renewable energy source, its development and utilization are of great significance to alleviate the energy crisis and the greenhouse effect. However, the production cost of bio-fuels is higher due to the higher investment cost of bio-refineries. To reduce the production cost of bio-fuels, considering the similar processing equipment of refineries and bio-refineries, bio-oil derived from both biomass and algae is co-processed with vacuum gas oil (VGO) in an existing catalytic cracking unit, and then gasoline and diesel products with renewable carbon can be obtained. To further understand whether the co-processing technology has the popularization and application value, two co-processing scenarios, VGO co-processing with bio-oils produced through fast pyrolysis or hydrothermal liquefaction (FPCP scenario and HTLCP scenario), are evaluated through techno-economic analysis (TEA). The TEA results showed that the minimum gasoline selling prices in FPCP scenario and HTLCP scenario were $3.497 and $2.910 per gallon, HTLCP scenario provided a clear economic advantage. Sensitivity analysis showed that gasoline prices were extremely sensitive to fluctuations of fuel yield and VGO price.
