A Theoretical Investigation of the Polyaddition of an AB2+A2+B4 Monomer Mixture.

Hyperbranched polymers (HBPs) are widely applied nowadays as functional materials for biomedicine needs, nonlinear optics, organic semiconductors, etc. One of the effective and promising ways to synthesize HBPs is a polyaddition of AB2+A2+B4 monomers that is generated in the A2+CB2, AA'+B3, A2+B'B2, and A2+C2+B3 systems or using other approaches. It is clear that all the foundational features of HBPs that are manufactured by a polyaddition reaction are defined by the component composition of the monomer mixture. For this reason, we have designed a structural kinetic model of AB2+A2+B4 monomer mixture polyaddition which makes it possible to predict the impact of the monomer mixture's composition on the molecular weight characteristics of hyperbranched polymers (number average (DPn) and weight average (DPw) degree of polymerization), as well as the degree of branching (DB) and gel point (pg). The suggested model also considers the possibility of a positive or negative substitution effect during polyaddition. The change in the macromolecule parameters of HBPs formed by polyaddition of AB2+A2+B4 monomers is described as an infinite system of kinetic equations. The solution for the equation system was found using the method of generating functions. The impact of both the component's composition and the substitution effect during the polyaddition of AB2+A2+B4 monomers on structural and molecular weight HBP characteristics was investigated. The suggested model is fairly versatile; it makes it possible to describe every possible case of polyaddition with various monomer combinations, such as A2+AB2, AB2+B4, AB2, or A2+B4. The influence of each monomer type on the main characteristics of hyperbranched polymers that are obtained by the polyaddition of AB2+A2+B4 monomers has been investigated. Based on the results obtained, an empirical formula was proposed to estimate the pg = pA during the polyaddition of an AB2+A2+B4 monomer mixture: pg = pA = (-0.53([B]0/[A]0)1/2 + 0.78)υAB2 + (1/3)1/2([B]0/[A]0)1/2, where (1/3)1/2([B]0/[A]0)1/2 is the Flory equation for the A2+B4 polyaddition, [A]0 and [B]0 are the A and B group concentration from A2 and B4, respectively, and υAB2 is the mole fraction of the AB2 monomer in the mixture. The equation obtained allows us to accurately predict the pg value, with an AB2 monomer content of up to 80%.

Swelling of Ti3 C2 Tx MXene in Water and Methanol at Extreme Pressure Conditions.

Pressure-induced swelling has been reported earlier for several hydrophilic layered materials. MXene Ti3C2Tx is also a hydrophilic layered material composed by 2D sheets but so far pressure-induced swelling is reported for this material only under conditions of shear stress at MPa pressures. Here, high-pressure experiments are performed with MXenes prepared by two methods known to provide "clay-like" materials. MXene synthesized by etching MAX phase with HCl+LiF demonstrates the effect of pressure-induced swelling at 0.2 GPa with the insertion of additional water layer. The transition is incomplete with two swollen phases (ambient with d(001) = 16.7Å and pressure-induced with d(001) = 19.2Å at 0.2 GPa) co-existing up to the pressure point of water solidification. Therefore, the swelling transition corresponds to change from two-layer water intercalation (2L-phase) to a never previously observed three-layer water intercalation (3L-phase) of MXene. Experiments with MXene prepared by LiCl+HF etching have not revealed pressure-induced swelling in liquid water. Both MXenes also show no anomalous compressibility in liquid methanol. The presence of pressure-induced swelling only in one of the MXenes indicates that the HCl+LiF synthesis method is likely to result in higher abundance of hydrophilic functional groups terminating 2D titanium carbide.

Activated carbons with extremely high surface area produced from cones, bark and wood using the same procedure.

Activated carbons have been previously produced from a huge variety of biomaterials often reporting advantages of using certain precursors. Here we used pine cones, spruce cones, larch cones and a pine bark/wood chip mixture to produce activated carbons in order to verify the influence of the precursor on properties of the final materials. The biochars were converted into activated carbons with extremely high BET surface area up to ∼3500 m2 g-1 (among the highest reported) using identical carbonization and KOH activation procedures. The activated carbons produced from all precursors demonstrated similar specific surface area (SSA), pore size distribution and performance to electrodes in supercapacitors. Activated carbons produced from wood waste appeared to be also very similar to "activated graphene" prepared by the same KOH procedure. Hydrogen sorption of AC follows expected uptake vs. SSA trends and energy storage parameters of supercapacitor electrodes prepared from AC are very similar for all tested precursors. It can be concluded that the type of precursor (biomaterial or reduced graphene oxide) has smaller importance for producing high surface area activated carbons compared to details of carbonization and activation. Nearly all kinds of wood waste provided by the forest industry can possibly be converted into high quality AC suitable for preparation of electrode materials.

High-surface-area activated carbon from pine cones for semi-industrial spray deposition of supercapacitor electrodes.

High surface area carbons are so far the best materials for industrial manufacturing of supercapacitor electrodes. Here we demonstrate that pine cones, an abundant bio-precursor currently considered as a waste in the wood industry, can be used to prepare activated carbons with a BET surface area exceeding 3000 m2 g-1. It is found that the same KOH activation procedure applied to reduced graphene oxide (rGO) and pine cone derived biochars results in carbon materials with a similar surface area, pore size distribution and performance in supercapacitor (SC) electrodes. It can be argued that "activated graphene" and activated carbon are essentially the same kind of material with a porous 3D structure. It is demonstrated that the pine cone derived activated carbon (PC-AC) can be used as a main part of aqueous dispersions stabilized by graphene oxide for spray deposition of electrodes. The PC-AC based electrodes prepared using a semi-industrial spray gun machine and laboratory scale blade deposition of these dispersions were compared to pellet electrodes.

One-Pot Synthesis of Hyperbranched Polyurethane-Triazoles with Controlled Structural, Molecular Weight and Hydrodynamic Characteristics.

We report a simple and convenient approach to the one-pot synthesis of hyperbranched polyurethane-triazoles with desirable properties. This method is based on in situ generation of an AB2 + A2 + B4 azide-acetylene monomer mixture of known composition, due to quantitative reactions of urethane formation between isophorone diisocyanate (IPDI), 1,3-diazidopropanol-2 (DAPOL) (in the first stage) and propargyl alcohol (in the second stage). The obtained monomer mixture can be involved in step-growth polymerization by azide-alkyne cycloaddition without additional purification (in the third stage). The properties of the resulting polymers should depend on the composition of the monomer mixture. Therefore, first the model revealing the correlation between the monomer composition and the ratio and reactivity of the IPDI and DAPOL active groups is developed and proven. In addition, the newly developed structural kinetic model considering the substitution effect at polyaddition of the complex mixture of monomers allows the prediction of the degree of branching of the target polymer. Based on our calculations, the hyperbranched polyurethane-triazoles were synthesized under found conditions. All products were characterized by 1H NMR, FTIR, SEC, DLS, DSC, TGA and viscometry methods. It was shown that the degree of branching, molecular weight, intrinsic viscosity, and hydrodynamic radius of the final hyperbranched polymers can be specified at the first stage of one-pot synthesis. The obtained hyperbranched polyurethane-triazoles showed a degree of branching from 0.21 to 0.44 (calculated DB-0.25 and 0.45, respectively).

Temperature-dependent swelling transitions in MXene Ti3C2Tx.

Swelling is a property of hydrophilic layered materials, which enables the penetration of polar solvents into an interlayer space with expansion of the lattice. Here we report an irreversible swelling transition, which occurs in MXenes immersed in excess dimethyl sulfoxide (DMSO) upon heating at 362-370 K with an increase in the interlayer distance by 4.2 Å. The temperature dependence of MXene Ti3C2Tx swelling in several polar solvents was studied using synchrotron radiation X-ray diffraction. MXenes immersed in excess DMSO showed a step-like increase in the interlayer distance from 17.73 Å at 280 K to 22.34 Å above ∼362 K. The phase transformation corresponds to a transition from the MXene structure with one intercalated DMSO layer into a two-layer solvate phase. The transformation is irreversible and the expanded phase remains after cooling back to room temperature. A similar phase transformation was observed also for MXene immersed in a 2 : 1 H2O : DMSO solvent ratio but at a lower temperature. The structure of MXene in the mixed solvent below 328 K was affected by the interstratification of differently hydrated (H2O)/solvated (DMSO) layers. Above the temperature of the transformation, the water was expelled from MXene interlayers and the formation of a pure two-layer DMSO-MXene phase was found. No changes in the swelling state were observed for MXenes immersed in DMSO or methanol at temperatures below ambient down to 173 K. Notably, MXenes do not swell in 1-alcohols larger than ethanol at ambient temperature. Changing the interlayer distance of MXenes by simple temperature cycling can be useful in membrane applications, e.g. when a larger interlayer distance is required for the penetration of ions and molecules into membranes. Swelling is also very important in electrode materials since it allows penetration of the electrolyte ions into the interlayers of the MXene structure.

Carboxyl groups do not play the major role in binding metal cations by graphene oxide.

In this study, we investigate the chemical interactions of Mn2+ ions with graphene oxides, prepared by Hummers' (HGO) and Brodie's (BGO) methods in aqueous solutions by means of NMR relaxation. Carboxyl groups, which are always present in HGO in significant quantities, are often considered as the main binding sites for metal ions. Here we demonstrate that metal ions are bound efficiently by BGO, containing a negligibly small quantity of carboxyl groups. The difference in the shape of the relaxation curves is due mostly to the difference in the solubility and exfoliation degree of the two GO samples in aqueous media. HGO binds Mn2+ in the broad pH range, including highly acidic solutions, while BGO binds only at pH > 6, since it is not dispersible in water at lower pH values. The ability of BGO to chemically bind Mn2+ despite lacking sulfate and carboxyl groups, coupled with our earlier published findings, strongly suggests that carboxyl groups do not play the main role in binding metal ions by GO, as is commonly believed. We propose that metal ions initiate a significant transformation in the GO structure to attain the most efficient coordination of metal ions. This reorganization might involve the metal cation induced C-C bond cleavage with the formation of enols at the newly formed edges.

Intercalation of Dyes in Graphene Oxide Thin Films and Membranes.

Intercalation of dyes into thin multilayered graphene oxide (GO) films was studied by neutron reflectivity and X-ray diffraction. Methylene blue (MB) penetrates the interlayer space of GO in ethanol solution and remains intercalated after the solvent evaporation, as revealed by the expansion of the interlayer lattice and change in chemical composition. The sorption of MB by thin GO films is found to be significantly stronger compared to the sorption of Crystal violet (CV) and Rose bengal (RB). This effect is attributed to the difference in the geometrical shape of planar MB and essentially nonflat CV and RB molecules. Graphite oxides and restacked GO films are found to exhibit different methylene blue (MB) sorptions. MB sorption by precursor graphite oxide and thin spin-coated films of GO is significantly stronger compared to freestanding micrometer-thick membranes prepared by vacuum filtration. Nevertheless, the sorption capacity of GO membranes is sufficient to remove a significant part of the MB from diluted solutions tested for permeation in several earlier studies. High sorption capacity results in strong modification of the GO structure, which is likely to affect permeation properties of GO membranes. Therefore, MB is not suitable for testing size exclusion effects in the permeation of GO membranes. It is not only hydration or solvation diameter but also the exact geometrical shape of molecules that needs to be taken into account considering size effects for penetration of molecules between GO layers in membrane applications.

Facile fabrication of graphene-based high-performance microsupercapacitors operating at a high temperature of 150 °C.

Many industry applications require electronic circuits and systems to operate at high temperatures over 150 °C. Although planar microsupercapacitors (MSCs) have great potential for miniaturized on-chip integrated energy storage components, most of the present devices can only operate at low temperatures (<100 °C). In this work, we have demonstrated a facile process to fabricate activated graphene-based MSCs that can work at temperatures as high as 150 °C with high areal capacitance over 10 mF cm-2 and good cycling performance. Remarkably, the devices exhibit no capacitance degradation during temperature cycling between 25 °C and 150 °C, thanks to the thermal stability of the active components.

Swelling properties of graphite oxides and graphene oxide multilayered materials.

Graphite oxide (GtO) and graphene oxide (GO) multilayered laminates are hydrophilic materials easily intercalated by water and other polar solvents. By definition, an increase in the volume of a material connected to the uptake of a liquid or vapour is named swelling. Swelling is a property which defines graphite oxides and graphene oxides. Less oxidized materials not capable of swelling should be named oxidized graphene. The infinite swelling of graphite oxide yields graphene oxide in aqueous dispersions. Graphene oxide sheets dispersed in a polar solvent can be re-assembled into multilayered structures and named depending on applications as films, papers or membranes. The multilayered GO materials exhibit swelling properties which are mostly similar to those of graphite oxides but not identical and in some cases surprisingly different. Swelling is a key property of GO materials in all applications which involve the sorption of water/solvents from vapours, immersion of GO into liquid water/solvents and solution based chemical reactions. These applications include sensors, sorption/removal of pollutants from waste waters, separation of liquid and gas mixtures, nanofiltration, water desalination, water-permeable protective coatings, etc. Swelling defines the distance between graphene oxide sheets in solution-immersed GO materials and the possibility for penetration of ions and molecules inside of interlayers. A high sorption capacity of GO towards many molecules and cations is defined by swelling which makes the very high surface area of GO accessible. GtO and GO swelling is a surprisingly complex phenomenon which is manifested in a variety of different ways. Swelling is strongly different for materials produced using the most common Brodie and Hummers oxidation procedures; it depends on the degree of oxidation, ad temperature and pressure conditions. The value of the GO interlayer distance is especially important in membrane applications. Diffusion of solvent molecules and ions is defined by the size of "permeation channels" provided by the swelled GO structure. According to extensive studies performed over the last decade the exact value of the inter-layer distance in swelled GO depends on the nature of solvent, temperature and pressure conditions, and the pH and concentration of solutions and exhibits pronounced aging effects. This review provides insight into the fundamental swelling properties of multilayered GO and demonstrates links to advanced applications of these materials.

Acetylation of graphite oxide.

Unlike many methods of chemical modification of Graphite Oxide (GO) reported during 1930-1960 and re-studied in much detail over the last decade, acetylation somehow escaped attention and remained almost completely unexplored. Acetylated Graphite Oxide (AcGO) was prepared using a reaction with acetic anhydride. Successful acetylation is evidenced by an increase in the average interlayer distance from d(001) = 7.8 Å in the precursor GO to 10 Å in AcGO. The amount of oxygen in AcGO significantly decreased compared to the precursor GO (C/O = 2.2), reflecting partial reduction of GO in the process of acetylation and resulting in a scarcely functionalized material with C/O = 6.2. A theoretical model of the complete acetylation of GO results in a non-porous close packed molecular structure with an interlayer distance of ∼10 Å, in good agreement with experiment. Remarkably, AcGO shows significant swelling despite the oxidation degree being comparable to that of reduced GO, which does not swell in polar solvents. Moreover, AcGO shows swelling in acetonitrile similar to that of the precursor GO but not in water, thus providing an example of selectivity in the sorption of common polar solvents. The low oxidation degree combined with selective swelling properties makes AcGO a promising material for membrane applications.

Enhanced Sorption of Radionuclides by Defect-Rich Graphene Oxide.

Extremely defect graphene oxide (dGO) is proposed as an advanced sorbent for treatment of radioactive waste and contaminated natural waters. dGO prepared using a modified Hummers oxidation procedure, starting from reduced graphene oxide (rGO) as a precursor, shows significantly higher sorption of U(VI), Am(III), and Eu(III) than standard graphene oxides (GOs). Earlier studies revealed the mechanism of radionuclide sorption related to defects in GO sheets. Therefore, explosive thermal exfoliation of graphite oxide was used to prepare rGO with a large number of defects and holes. Defects and holes are additionally introduced by Hummers oxidation of rGO, thus providing an extremely defect-rich material. Analysis of characterization by XPS, TGA, and FTIR shows that dGO oxygen functionalization is predominantly related to defects, such as flake edges and edge atoms of holes, whereas standard GO exhibits oxygen functional groups mostly on the planar surface. The high abundance of defects in dGO results in a 15-fold increase in sorption capacity of U(VI) compared to that in standard Hummers GO. The improved sorption capacity of dGO is related to abundant carboxylic group attached hole edge atoms of GO flakes as revealed by synchrotron-based extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy.

Thermally reduced pillared GO with precisely defined slit pore size.

Graphene oxide (GO) pillared with tetrakis(4-aminophenyl)methane (TKAM) molecules shows a narrow distribution of pore size, relatively high specific surface area, but it is hydrophilic and electrically not conductive. Analysis of XRD, N2 sorption, XPS, TGA and FTIR data proved that the pillared structure and relatively high surface area (∼350 m2 g-1) are preserved even after thermal reduction of GO pillared with TKAM molecules. Unlike many other organic pillaring molecules, TKAM is stable at temperatures above the point of GO thermal reduction, as demonstrated by TGA. Therefore, gentle annealing results in the formation of reduced graphene oxide (rGO) pillared with TKAM molecules. The TKAM pillared reduced graphene oxide (PrGO/TKAM) is less hydrophilic as found using dynamic vapor sorption (DVS) and more electrically conductive compared to pillared GO, but preserves an increased interlayer-distance of about 12 Å (compared to ∼7.5 Å in pristine GO). Thus we provide one of the first examples of porous rGO pillared with organic molecules and well-defined size of hydrophobic slit pores. Analysis of pore size distribution using nitrogen sorption isotherms demonstrates a single peak for pore size of ∼7 Å, which makes PrGO/TKAM rather promising for membrane and molecular sieve applications.

Covalent Organic Framework (COF-1) under High Pressure.

COF-1 has a structure with rigid 2D layers composed of benzene and B3 O3 rings and weak van der Waals bonding between the layers. The as-synthesized COF-1 structure contains pores occupied by solvent molecules. A high surface area empty-pore structure is obtained after vacuum annealing. High-pressure XRD and Raman experiments with mesitylene-filled (COF-1-M) and empty-pore COF-1 demonstrate partial amorphization and collapse of the framework structure above 12-15 GPa. The ambient pressure structure of COF-1-M can be reversibly recovered after compression up to 10-15 GPa. Remarkable stability of highly porous COF-1 structure at pressures at least up to 10 GPa is found even for the empty-pore structure. The bulk modulus of the COF-1 structure (11.2(5) GPa) and linear incompressibilities (k[100] =111(5) GPa, k[001] =15.0(5) GPa) were evaluated from the analysis of XRD data and cross-checked against first-principles calculations.

Activated graphene as a material for supercapacitor electrodes: effects of surface area, pore size distribution and hydrophilicity.

Activated reduced graphene oxide (a-rGO) is a material with a rigid 3D porous structure and high specific surface area (SSA). Using variation of activation parameters and post-synthesis mechanical treatment we prepared two sets of materials with a broad range of BET (N2) SSA ∼1000-3000 m2 g-1, and significant differences in pore size distribution and oxygen content. The performance of activated graphene as an electrode in a supercapacitor with KOH electrolyte was correlated with the structural parameters of the materials and water sorption properties. a-rGO is a hydrophobic material as evidenced by the negligibly small BET (H2O) SSA determined using analysis of water vapor sorption isotherms. However, the total pore volume determined using water vapor sorption and sorption of liquid water is almost the same as the one found by analysis of nitrogen sorption isotherms. Ball milling is found to provide an improved bulk density of activated graphene and collapse of all pores except the smallest ones (<2 nm). A decrease in the activation temperature from 850 °C to 550 °C is found to result in materials with a narrow micropore size distribution and increased oxygen content. Elimination of mesopores using ball milling or a lower activation temperature provided materials with better specific capacitance despite a significant decrease (by ∼30%) of the BET (N2) SSA. The best gravimetric and volumetric capacitances in KOH electrolyte were achieved not for samples with the highest value of the BET (N2) SSA but for materials with 80-90% of the total pore volume in micropores and an increased BET (H2O) SSA. Comparing the performance of electrodes prepared using rGO and a-rGO shows that a more hydrophilic surface is favorable for charge storage in supercapacitors with KOH electrolyte.

Exactly matched pore size for the intercalation of electrolyte ions determined using the tunable swelling of graphite oxide in supercapacitor electrodes.

The intercalation of solvent molecules and ions into sub-nanometer-sized pores is one of the most disputed subjects in the electrochemical energy storage applications of porous materials. Here, we demonstrate that the temperature- and concentration-dependent swelling of graphite oxide (GO) can be used to determine the smallest pore size required for the intercalation of electrolyte ions into hydrophilic pores. The structure of Brodie graphite oxide (BGO) in acetonitrile can be temperature-switched between the ambient one-layer solvate with an interlayer distance of ∼8.9 Å and the two-layer solvate (∼12.5 Å) at low temperature, thus providing slit pores of approximately 2.5 and 6 Å. Using in situ synchrotron radiation X-ray diffraction (XRD) and the temperature dependence of capacitance in supercapacitor devices, we found that solvated tetraethylammonium tetrafluoroborate (TEA-BF4) ions do not penetrate into both the 2.5 and 6 Å slit pores formed by BGO interlayers. However, increasing the electrolyte concentration results in the formation of a new phase at low temperature. This phase shows a distinct interlayer distance of ∼15-16.6 Å, which corresponds to the insertion of partly desolvated TEA-BF4 ions. Therefore, the remarkable ability of the GO structure to adopt variable interlayer distances allows for the determination of pore sizes that are optimal for solvated TEA-BF4 ions (about 9-10 Å). The intercalation of TEA-BF4 ions into the BGO structure is also detected as an anomaly in the temperature dependence of supercapacitor performance. The BGO structure remains to be expanded, even after the removal of acetonitrile, adopting an interlayer distance of ∼10 Å.

Gravimetric tank method to evaluate material-enhanced hydrogen storage by physisorbing materials.

The most common methods to evaluate hydrogen sorption (volumetric and gravimetric) require significant experience and expensive equipment for providing reproducible results. Both methods allow one to measure excess uptake values which are used to calculate the total amount of hydrogen stored inside of a tank as required for applications. Here we propose an easy to use and inexpensive alternative approach which allows one to evaluate directly the weight of hydrogen inside a material-filled test tank. The weight of the same tank filled with compressed hydrogen in the absence of loaded material is used as a reference. We argue that the only parameter which is of importance for hydrogen storage applications is by how much the material improves the total weight of hydrogen inside of the given volume compared to compressed gas. This parameter which we propose to name Gain includes both volumetric and gravimetric characterization of the material; it can be determined directly without knowing the skeletal volume of the material or excess sorption. The feasibility of the Gravimetric Tank (GT) method was tested using several common carbon and Metal Organic Framework (MOF) materials. The best Gain value of ∼12% was found for the Cu-BTC MOF which means that the tank completely filled with this material stores a 12% higher amount of hydrogen compared to H2 gas at the same P-T conditions. The advantages of the GT method are its inexpensive design, extremely simple procedures and direct results in terms of tank capacity as required for industrial applications. The GT method could be proposed as a standard check for verification of the high hydrogen storage capacity of new materials. The GT method is expected to provide even better accuracy for evaluation of a material's performance for storage of denser gases like e.g. CO2 and CH4.