Influence of structure and solubility of chain transfer agents on the RAFT control of dispersion polymerisation in scCO2

Reversible addition–fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO₂) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO₂ also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO₂. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situ two-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO₂.

RAFT dispersion polymerisation of methyl methacrylate is performed in scCO₂ with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight.     

Preparation of porous CaCO3/PAM composites by CO2 in water emulsion templating method

Emulsion templating is an effective method to prepare well-defined porous polymeric materials. In this paper, porous CaCO₃/polyacrylamide (PAM) composites were prepared by emulsion templating polymerization in supercritical CO₂(scCO₂) by using a commercial grade surfactant (FC4430), therefore, the amount of the fillers and the pore size distribution of the composites can be modulated based on the demands of those potential applications as biomaterials. Calcium carbonate crystals can be in situ synthesized in the porous PAM matrix, and the morphology of CaCO₃ varied with the conditions of the reaction, the results indicated that three kinds of crystals were observed in the porous matrix. The results of scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) showed that the macropores in PAM were interconnected and with narrow pore size distributions.     

Design of Bimodal PCL and PCL‐HA Nanocomposite Scaffolds by Two Step Depressurization During Solid‐state Supercritical CO2 Foaming

This communication reports the design and fabrication of porous scaffolds of poly(ε‐caprolactone) (PCL) and PCL loaded with hydroxyapatite (HA) nanoparticles with bimodal pore size distributions by a two step depressurization solid‐state supercritical CO₂ (scCO₂) foaming process. Results show that the pore structure features of the scaffolds are strongly affected by the thermal history of the starting polymeric materials and by the depressurization profile. In particular, PCL and PCL‐HA nanocomposite scaffolds with bimodal and uniform pore size distributions are fabricated by quenching molten samples in liquid N₂, solubilizing the scCO₂ at 37 °C and 20 MPa, and further releasing the blowing agent in two steps: (1) from 20 to 10 MPa at a slow depressurization rate, and (2) from 10 MPa to the ambient pressure at a fast depressurization rate. The biocompatibility of the bimodal scaffolds is finally evaluated by the in vitro culture of human mesenchymal stem cells (MSCs), in order to assess their potential for tissue engineering applications.

     

Thermoplastic starch and glutaraldehyde modified thermoplastic starch foams prepared using supercritical carbon dioxide fluid as a blowing agent

Supercritical carbon dioxide (scCO₂) processed thermoplastic starch (scCO₂^aTPS), cellulose nanofiber (CNF) modified scCO₂^aTPS (scCO₂^aTPS₁₀₀CNF_0.02) and glutaraldehyde (GA) modified scCO₂^aTPS₁₀₀CNF_0.02 (scCO₂^aTPS₁₀₀CNF_0.02GA_x) foams were prepared for the first time using scCO₂ as a blowing agent during their foaming processes. The expansion ratio, cell density, moisture resistance, and compressive strength (σ_c) retention properties of each foam series were considerably improved with increasing scCO₂ pressure during the foaming processes. The expansion ratios and cell densities of each scCO₂^aTPS₁₀₀CNF_0.02GA_x foam series were increased considerably to a maximum value, as the GA content approached an optimum value. The optimal scCO₂¹¹TPS₁₀₀CNF_0.02GA_1.6 foam material exhibited a high expansion ratio and cell density at approximately 50 and approximately 8 × 10⁸ cells/cm³, respectively. Compared with corresponding aged scCO₂^aTPS and scCO₂^aTPS₁₀₀CNF_0.02 foam specimens, considerably better moisture resistance and σ_c retention properties were observed for scCO₂^aTPS₁₀₀CNF_0.02GA_x foam specimens, when they were modified with the corresponding optimum GA content. The moisture resistance and σ_c retention for optimal prepared scCO₂⁷TPS₁₀₀CNF_0.02GA_0.4, scCO₂⁹TPS₁₀₀CNF_0.02GA_0.8 and scCO₂¹¹TPS₁₀₀CNF_0.02GA_1.6 foam materials improved further with increasing scCO₂ pressure. Possible reasons accounting for the highly expansion ratio, moisture resistance, and σ_c retention properties for scCO₂^aTPS₁₀₀CNF_0.02GA_x foams are presented.     

Nitrogen-Doped Porous Carbon Materials Derived from Graphene Oxide/Melamine Resin Composites for CO2 Adsorption

CO₂ adsorption in porous carbon materials has attracted great interests for alleviating emission of post-combustion CO₂. In this work, a novel nitrogen-doped porous carbon material was fabricated by carbonizing the precursor of melamine-resorcinol-formaldehyde resin/graphene oxide (MR/GO) composites with KOH as the activation agent. Detailed characterization results revealed that the fabricated MR(0.25)/GO-500 porous carbon (0.25 represented the amount of GO added in wt.% and 500 denoted activation temperature in °C) had well-defined pore size distribution, high specific surface area (1264 m²·g⁻¹) and high nitrogen content (6.92 wt.%), which was mainly composed of the pyridinic-N and pyrrolic-N species. Batch adsorption experiments demonstrated that the fabricated MR(0.25)/GO-500 porous carbon delivered excellent CO₂ adsorption ability of 5.21 mmol·g⁻¹ at 298.15 K and 500 kPa, and such porous carbon also exhibited fast adsorption kinetics, high selectivity of CO₂/N₂ and good recyclability. With the inherent microstructure features of high surface area and abundant N adsorption sites species, the MR/GO-derived porous carbon materials offer a potentially promising adsorbent for practical CO₂ capture.     

Is a polymer semiconductor having a “perfect” regular structure desirable for organic thin film transistors?

This study utilized high temperature NMR and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry to reveal that appreciable amounts of structural defects are present in the diketopyrrolopyrrole (DPP)–quaterthiophene copolymers (PDQT) synthesized by the Stille coupling polymerization with Pd(PPh₃)₂Cl₂, Pd₂(dba)₃/P(o-tol)₃, and Pd(PPh₃)₄ catalyst systems. It was proposed that these structural defects were produced via homocoupling side reactions of the C–Br bonds and the organostannane species. Model Stille coupling reactions further substantiated that the amount of structural defects are catalyst-dependent following the order of Pd(PPh₃)₂Cl₂ > Pd₂(dba)₃/P(o-tol)₃ > Pd(PPh₃)₄. To verify the structural assignments, “perfect” structurally regular PDQT polymers were prepared using Yamamoto coupling polymerization. When compared to the structurally regular polymers, the polymers containing defects exhibited notable redshifts in their absorption spectra. Surprisingly, the “perfect” structurally regular polymers showed poor molecular ordering in thin films and very low charge transport performance as channel semiconductors in organic thin film transistors (OTFTs). On the contrary, all the “defected” polymers exhibited much improved molecular ordering and significantly higher charge carrier mobility.     

Characterization of Hydrophobic SiO2 Powders Prepared by Surface Modification on Wet Gel

Hydrophobic porous silica has been prepared by surface modification of TEOS (tetraethylorthosilicate) wet gel with 6 and 12 vol.% of TMCS (trimethylchlorosilane). We characterized the products by using FT-IR, TGA, DTA, N₂ adsorption/desorption, contact angle and SEM. Surface silanol groups of the gel were widely replaced by–Si(CH₃)₃ to result in a hydrophobic SiO₂ powder as confirmed by contact angle measurements with H₂O, 1-butanol and ethanol. The modified dried gels had a surface area of 950–1000 m²/g (average pore size 120 Å), compared to the non-modified surface which had a surface area of 690 m²/g (average pore size 36 Å). The adsorption/desorption isotherm curves indicated they had similar pore characteristics as aerogels prepared by the supercritical drying process.     

Sol-Gel Preparation of Fast Proton-Conducting P2O5-SiO2 Glasses

We have investigated the proton conductivities of the sol-gel-derived P₂O₅-SiO₂ glass at –50 to 120°C. The obtained glass is porous, where the surface area, pore volume and pore diameter are 740 m²/g, 0.5 cm³/g and <5 nm, respectively. The freezing temperature of water molecules adsorbed in the pores was –20°C, which is much lower than that of free liquid water due to the quantum size effect of the water confined in the pores. The electrical conductivities followed the Arrhenius equation in the temperatures between –20 and 120°C. Below –20°C, the adsorbed-water molecules were frozen, resulting in a rapid decrease of the proton conductivity. Considering the high conductivity, chemical and thermal stability, this oxide glass membranes have potential for the fuel cell membrane.     

Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics

Intermolecular bonding attraction at π-bonded centers is often described as “electrostatically driven” and given quasi-classical rationalization in terms of a “pi hole” depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO⁻, CN⁻) with simple atomic anions (H⁻, F⁻) or with one another. Such “anti-electrostatic” anion–anion attractions are shown to lead to robust metastable binding wells (ranging up to 20–30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 Å width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) n_D-π*_AX interactions that are fully analogous to the n_D-σ*_AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi–Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that “deletion” of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi–Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency (“charge transfer”) rather than envisioned Coulombic properties of unperturbed monomers.     

Progress in the Foaming of Polymer-based Electromagnetic Interference Shielding Composites by Supercritical CO2

As a critical action plan formulated for peaking carbon dioxide emissions, polymeric electromagnetic interference (EMI) shielding materials based on CO₂ foaming technology have recently been attracting widespread attention in both research and industry, attributable to their efficient use of CO₂, high specific strength, corrosion resistance and low-cost characteristics. In the past decade, the emergence of novel design concepts and preparation techniques for CO₂ foaming technology has led to the development of new high-performance EMI shielding materials in this field. This review summarizes the research progress made to date on the fabrication of EMI shielding composite foams by supercritical carbon dioxide (scCO₂) foaming. We also explore the structure-activity relationships between the component/distribution and EMI shielding properties. Additionally, the application prospects and development challenges of new EMI shielding composite foams are described.     

When Attempting Chain Extension,Even Without Solvent,It Is Not Possible to Avoid Chojnowski Metathesis Giving D3

A simple, mild and efficient method to prepare HSi- or HOSi-telechelic, high-molecular-weight polydimethylsiloxane polymers (to 41,600 g·mol⁻¹) using the one-shot hydrolysis of M^HM^H is reported; titration of the water allowed for higher molecular weights (to 153,900 g·mol⁻¹). The “living” character of the chain extension processes was demonstrated by adding a small portion of M^HM^H and B(C₆F₅)₃ (BCF) to a first formed polymer, which led to a ~2-fold, second growth in molecular weight. The heterogeneous reaction reached completion in less than 30 min, much less in some cases, regardless of whether it was performed neat or 50 wt% in dry toluene; homogeneous reactions in toluene were much slower. The process does not involve traditional redistribution, as judged by the low quantities (<3%) of D₄ produced. However, it is not possible to avoid Chojnowski metathesis from M^HDDM^H giving D₃, which occurs competitively with chain extension.     

Application of CO2-Switchable Oleic-Acid-Based Surfactant for Reducing Viscosity of Heavy Oil

CO₂-switchable oligomeric surfactants have good viscosity-reducing properties; however, the complex synthesis of surfactants limits their application. In this study, a CO₂-switchable “pseudo”-tetrameric surfactant oleic acid (OA)/cyclic polyamine (cyclen) was prepared by simple mixing and subsequently used to reduce the viscosity of heavy oil. The surface activity of OA/cyclen was explored by a surface tensiometer and a potential for viscosity reduction was revealed. The CO₂ switchability of OA/cyclen was investigated by alternately introducing CO₂ and N₂, and OA/cyclen was confirmed to exhibit a reversible CO₂-switching performance. The emulsification and viscosity reduction analyses elucidated that a molar ratio of OA/cyclen of 4:1 formed the “pseudo”-tetrameric surfactants, and the emulsions of water and heavy oil with OA/cyclen have good stability and low viscosity and can be destabilized quickly by introducing CO₂. The findings reported in this study reveal that it is feasible to prepare CO₂-switchable pseudo-tetrameric surfactants with viscosity-reducing properties by simple mixing, thus providing a pathway for the emulsification and demulsification of heavy oil by using the CO₂-switchable “pseudo”-oligomeric surfactants.     

Preparation of a large Mesoporous CeO2 with crystalline walls using PMMA colloidal crystal templates

Inverse opals of crystalline CeO₂ were synthesized by using close-packed poly(methyl methacrylate) (PMMA) latex spheres of various sizes as templates, resulting in pore sizes, which could be scaled down even to the mesopore region (30–40 nm). The latex spheres were synthesized by emulsion polymerization, and the PMMA particle size could be substantially decreased by addition of sodium dodecyl sulfate (SDS) as surfactant. Owing to the larger pore wall thickness, the CeO₂ with large mesopores preserves an intact porosity to higher temperatures than previously reported mesoporous CeO₂ obtained from surfactant templates. The porosity and crystallinity were studied by microscopic techniques, wide angle X-ray diffraction (XRD), N₂ sorption, and Hg porosimetry. The evolution of crystallinity (crystallite size and lattice parameters) was determined for different annealing temperatures by means of Rietveld refinements of the XRD data. Thereby, our study allowed getting general insights into the crystallization behavior of sol–gel derived porous CeO₂ frameworks.     

傅立叶变换红外光谱技术在超临界CO2作用聚合物体系中的应用

超临界CO₂（scCO₂）作为一种物理化学性质优良、具有高扩散速率及优良溶解性能的溶剂,在科学研究及工业生产中广受青睐。将scCO₂应用于聚合物体系中,CO₂ 与聚合物间特殊的相互作用有利于CO₂分子在聚合物中的吸附与扩散。同时通过CO₂的吸附及其对聚合物的溶胀和塑化作用,聚合物所处微观化学环境以及整体结构性质会发生一定的变化。由于傅立叶变换红外光谱（FTIR）技术能够有效地考察化学环境变化对分子结构造成的影响,这一表征技术在超临界CO₂作用体系中广为应用。本文主要选取了近年来利用FTIR技术考察scCO₂作用于聚合物体系的一些实例,从CO₂-聚合物相互作用机理,scCO₂对聚合物或生物大分子的加工过程的影响两方面,阐述了利用红外光谱技术在scCO₂作用体系中的应用以及前景。     

Reinvestigation of Reactions of HgPh2 with Eu and Yb Metal and the Synthesis of a Europium(II) Bis(tetraphenylborate)

Europium bis(tetraphenylborate) [Eu(thf)₇][BPh₄]₂⋅thf containing a fully solvated [Eu(thf)₇]²⁺ cation, was synthesized by protolysis of “EuPh₂” (from Eu and HgPh₂) with Et₃NHBPh₄, and the structure was determined by single-crystal X-ray diffraction. Efforts to characterize the putative “Ph₂Ln” (Ln = Eu, Yb) reagents led to the synthesis of a mixed-valence complex, [(thf)₃Yb^II(μ-Ph)₃Yb^III(Ph)₂(thf)]⋅2thf, resulting from the reaction of Yb metal with HgPh₂ at a low temperature. This mixed-valence Yb^II/Yb^III compound was studied by ¹⁷¹Yb-NMR spectroscopy and single-crystal X-ray diffraction, and the oxidation states of the Yb atoms were assigned.     

Fabrication of open‐porous PCL/PLA tissue engineering scaffolds and the relationship of foaming process,morphology, and mechanical behavior

Tissue engineering scaffolds should provide a suitable porous structure and proper mechanical strength, which is beneficial for the delivery of growth factor and regulation of cells. In this study, the open‐porous polycaprolactone (PCL)/poly (lactic acid) (PLA) tissue engineering scaffolds with suitable porous scale were fabricated using different ratios of PCL/PLA blends. At the same time, the relationship of foaming process, morphology, and mechanical behavior in the optimized batch microcellular foaming process were studied based on the single‐factor experiment method. The porous structures and mechanical strength of the scaffolds were optimized by adjusting foaming parameters, including the temperature, pressure, and CO₂ dissolution time. The results indicated that the foaming parameters influence the cell morphology, further determine the mechanical behavior of PCL/PLA blends. When the PCL content is high, with the increase of temperature and time, the cell diameter and the elastic modulus increased, and the tensile strength and elastic modulus increased with the increase of the average cell size, and decreased as the increase of the cell density. While when the PLA content was high, the cell diameter showed the same trend, and the tensile strength and elastic modulus were higher, and the elongation at break was lower, and tensile strength and elastic modulus decreased with the increase of the average cell size and increased with the increase of cell density. This work successfully fabricated optimized porous PCL/PLA scaffolds with excellent suitable mechanical properties, pore sizes, and high interconnectivity, indicating the effectiveness of modulating the batch foaming process parameters.     

Preparation and Formation Mechanism of Porous TiO2 Films Using PEG and Alcohol Solvent as Double-Templates

Porous nanocrystalline TiO₂ anatase thin films have been synthesized on glass substrates via a sol-gel dip-coating method. The coating sol was obtained by suppressed hydrolysis of Ti(OC₄H₉)₄ through the addition of complexing molecules as stabilizers in an alcohol solution containing polyethylene glycol (PEG). Chemical changes taking place during the sol-gel process were discussed based on IR spectra analysis. A model concerning the pore formation was established to explain the role of PEG and solvent with core-shell configuration as double-templates. The structural characteristics of porous TiO₂ films were found to greatly depend on the concentration and molecular weight of PEG, the types of stabilizing agents and solvents. The pore size of the films was tunable in the range of 10–500 nm and their surface area varied from 51 to 72 m²·g^–1.     

Synthesis of Zn2+-Pre-Intercalated V2O5·nH2O/rGO Composite with Boosted Electrochemical Properties for Aqueous Zn-Ion Batteries

Layered vanadium-based materials are considered to be great potential electrode materials for aqueous Zn-ion batteries (AZIBs). The improvement of the electrochemical properties of vanadium-based materials is a hot research topic but still a challenge. Herein, a composite of Zn-ion pre-intercalated V₂O₅·nH₂O combined with reduced graphene oxide (ZnVOH/rGO) is synthesized by a facile hydrothermal method and it shows improved Zn-ion storage. ZnVOH/rGO delivers a capacity of 325 mAh·g⁻¹ at 0.1 A·g⁻¹, and this value can still reach 210 mAh·g⁻¹ after 100 cycles. Additionally, it exhibits 196 mAh·g⁻¹ and keeps 161 mAh·g⁻¹ after 1200 cycles at 4 A·g⁻¹. The achieved performances are much higher than that of ZnVOH and VOH. All results reveal that Zn²⁺ as “pillars” expands the interlayer distance of VOH and facilitates the fast kinetics, and rGO improves the electron flow. They both stabilize the structure and enhance efficient Zn²⁺ migration. All findings demonstrate ZnVOH/rGO’s potential as a perspective cathode material for AZIBs.     

Piroxicam Loading onto Mesoporous Silicas by Supercritical CO2 Impregnation

Piroxicam (PRX) is a commonly prescribed nonsteroidal anti-inflammatory drug. Its efficacy, however, is partially limited by its low water solubility. In recent years, different studies have tackled this problem and have suggested delivering PRX through solid dispersions. All these strategies, however, involve the use of potentially harmful solvents for the loading procedure. Since piroxicam is soluble in supercritical CO₂ (scCO₂), the present study aims, for the first time, to adsorb PRX onto mesoporous silica using scCO₂, which is known to be a safer and greener technique compared to the organic solvent-based ones. For comparison, PRX is also loaded by adsorption from solution and incipient wetness impregnation using ethanol as solvent. Two different commercial mesoporous silicas are used (SBA-15 and Grace Syloid^® XDP), which differ in porosity order and surface silanol population. Physico-chemical analyses show that the most promising results are obtained through scCO₂, which yields the amorphization of PRX, whereas some crystallization occurs in the case of adsorption from solution and IWI. The highest loading of PRX by scCO₂ is obtained in SBA-15 (15 wt.%), where molecule distribution appears homogeneous, with very limited pore blocking.     

Heterostructured FeNi hydroxide for effective electrocatalytic oxygen evolution

Hydrogen production technology by water splitting has been heralded as an effective means to alleviate the envisioned energy crisis. However, the overall efficiency of water splitting is limited by the effectiveness of the anodic oxygen evolution reaction (OER) due to the high energy barrier of the 4e⁻ process. The key to addressing this challenge is the development of high-performing catalysts. Transition-metal hydroxides with high intrinsic activity and stability have been widely studied for this purpose. Herein, we report a gelatin-induced structure-directing strategy for the preparation of a butterfly-like FeNi/Ni heterostructure (FeNi/Ni HS) with excellent catalytic performance. The electronic interactions between Ni²⁺ and Fe³⁺ are evident both in the mixed-metal “torso” region and at the “torso/wing” interface with increasing Ni³⁺ as a result of electron transfer from Ni²⁺ to Fe³⁺ mediated by the oxo bridge. The amount of Ni³⁺ also increases in the “wings”, which is believed to be a consequence of charge balancing between Ni and O ions due to the presence of Ni vacancies upon formation of the heterostructure. The high-valence Ni³⁺ with enhanced Lewis acidity helps strengthen the binding with OH⁻ to afford oxygen-containing intermediates, thus accelerating the OER process. Direct evidence of FeNi/Ni HS facilitating the formation of the Ni–OOH intermediate was provided by in situ Raman studies; the intermediate was produced at lower oxidation potentials than when Ni₂(CO₃)(OH)₂ was used as the reference. The Co congener (FeCo/Co HS), prepared in a similar fashion, also showed excellent catalytic performance.

A butterfly-like FeNi/Ni HS featuring a “torso” of Ni-doped FeOOH and two “wings” of Ni₂(CO₃)(OH)₂ showed excellent activity in electrocatalytic oxygen evolution reaction attributable to the increase of higher-valance Ni³⁺ in the heterostructure.