A. V. Pastukhov

High‐resolution mapping of ecosystem carbon storage and potential effects of permafrost thaw in periglacial terrain, European Russian Arctic

This study describes detailed partitioning of phytomass carbon (C) and soil organic carbon (SOC) for four study areas in discontinuous permafrost terrain, Northeast European Russia. The mean aboveg ...

Hypercrosslinked polystyrene: A polymer in a non‐classical physical state

Deformation and relaxation properties of hypercrosslinked polystyrene networks have been studied by thermomechanical method at a uniaxial compression using individual spherical beads of the polymer. The networks examined were prepared by postcrosslinking of highly swollen beads of a styrene-0.3% DVB copolymer with 0.3–0.75 mole of monochlorodimethyl ether, which results in the introduction of 0.6–1.5 methylene bridges between each two polystyrene phenyl rings. The polymers obtained are shown to belong neither to typical glassy materials, nor to typical elastomers. Though no characteristic plateau of rubberlike elasticity was observed on the deformation curves of the beads, the polymers exhibit two fundamentally important features of the rubberlike state: The deformations are large (up to 30–40% of the initial diameter) and reversible. Relaxation of residual deformations, however, requires prolonged heating of the sample, or a cycle of swelling and drying. The deformation can start in the temperature range from −70 to +150°C depending on pressure applied. The crosslinking degree in the range from 40–100% and higher does not affect noticeably the behavior of the hypercrosslinked polystyrene. Nature of the high mobility of the hypercrosslinked network is discussed.

Unusual mobility of Hypercrosslinked polystyrene networks : Swelling and dilatometric studies

Hypercrosslinked polystyrene samples were prepared by an intensive post- crosslinking of highly swollen styrene- divinylbenzene copolymer beads to extremely high degrees of crosslinking that amounted to 100% or even higher. When in the dry state, the materials obtained represent transparent beads of reduced density. Despite the high degree of crosslinking, the materials manifest large increases in volume on swelling with any liquid media as well as large changes in volume on heating. The factors determining the unusual swelling ability of the hypercrosslinked polymers are briefly discussed. Thermodilatometric tests of the polymers with a moderate degree of crosslinking reveal a certain contraction of the beads at temperatures higher than 100 °C. When crosslinked far beyond 100%, the networks demonstrate unusual expansion in the 100 -220 °C range; this is followed by a sharp shrinking at higher temperatures. The former is caused by an increased intensity of the vibration movements of the network and a partial relaxation of strong inner stresses; the latter is due to partial chemical oxidation, degradation of network units, or both. This degradation, however, is not accompanied by any loss in weight of the polymer but only results in a transfor- mation into a more dense conventional nonporous material. The strained, rigid open- work structure of the homogeneous expanded hypercrosslinked polystyrene networks is responsible for their unusual mobility.

Hypercrosslinked Polystyrene: The First Nanoporous Polymeric Material

The principles of synthesis—the structural and distinctive features of hypercrosslinked polystyrene—are described. The material is obtained by intensely crosslinking the solvated polystyrene chains with rigid bridging spacers. The resulting single-phase open-worked elastic network can be considerably deformed upon swelling or the application of an external force. The huge free volume in the rigid hypercrosslinked network, which is essentially a porosity of a new type, is formed by interchain nanosized cavities (1.5–3 nm). The complete availability of the interior structure of hypercrosslinked polystyrene for low-molecular-weight compounds and the considerable uncompensated force field of polymer chains separated by spacers determines the excellent adsorption properties of the material. Several processes that are possible only due to the use of nanoporous hypercrosslinked polystyrene as a sorbent for chromatography or a matrix for preparing nanocomposites are briefly described in the paper.

Paradoxes of thermodynamics of swelling equilibria of polymers in liquids and vapors.

An automatic registration of the changing size of a single spherical microbead of a cross-linked polymer was applied for studying the swelling process of the bead by the sorption of vapors and/or liquids. Many representatives of all three basic types of polymeric networks, gel-type, hypercrosslinked, and macroporous, were examined. Only the first two display large volume changes and prove suitable for following the kinetics and extent of swelling by the above dilatometric technique. The results unambiguously prove that swelling of all polymeric networks in liquids is always higher than in corresponding saturated vapors (Schroeders paradox). The general nature of this phenomenon implies that the absolute activity of any sorbate in its liquid form is always larger than in the form of its saturated vapor. Surprisingly, gels with any solvent contents, which fall into the broad range between the vapor-equilibrated and liquid-equilibrated extreme contents, retain their volumes constant in the saturated vapor atmosphere. This paradox of a wide range of gels swollen to a different extent and, nevertheless, standing in equilibrium with saturated vapor is explained by the specificity of the network polymers, namely, that the energy of the solvent-polymer interactions is easily compensated by the energy of remaining between-chain interactions at any solvent content in the above range. Therefore, the strain-free swollen gels do not generate enhanced vapor pressure, but neither display the ability to take up more sorbate from its vapor.

Structure and properties of magnetic composite sorbents based on hypercrosslinked polystyrenes

Magnetic composite sorbents based on microporous and biporous hypercrosslinked polystyrenes (HCPs) with inclusions of iron oxide nanoparticles were studied by X-ray diffraction and differential thermal analysis. In microporous composites, the size of impregnated magnetite nanoparticles was less than ∼6 nm, the nanocomposites remaining optically transparent. Biporous HCPs (with micro- and macropores) had larger nanoparticles (∼16 nm). The sorption studies revealed that composite magnetic sorbents, as well as the starting hypercrosslinked polystyrenes, are effective adsorbents with high capacity for many compounds including toxic and physiologically active compounds.

Specifics of Positron Studies of Microheterogeneous Systems as Applied to New Polymeric Sorbents

The size distribution of free volume holes in mesoporous, microheterogeneous polymer sorbents based on linear rubber-modified divinylbenzene copolymers has been studied. Analysis of the results of the study and published data has shown that positron annihilation lifetime spectroscopy (PALS) applied to microheterogeneous systems, in contrast to homogeneous systems, is not necessarily sensitive to mesopores that occur at the boundaries of heterogeneities and determine the sorption or membrane properties of the modified materials. To obtain the complete information, additional procedures such as low-temperature (BET) gas adsorption and thermally stimulated luminescence (TSL) need to be used. This circumstance is due to a considerably long distance between mesopores in heterogeneous systems and limited mobility of positronium during its localization in a pore. In addition to the BET data, which largely give information on mesopores, PALS appears useful for studying micropores. The TLS technique makes it possible to judge on heterogeneity of test materials. The minimal concentration of mesopores detectable by the positron technique in the systems of interest has been determined.

Paradoxes of thermodynamics of aqua-vapor-polymer interface equilibrium

Schroeder’s paradox is the different equilibrium degree of swelling for a hydrophilic polymer in saturated aqueous vapor and in liquid water; it is experimentally verified by the registration of increasing volume of spherical polymer samples upon the processes of vapor and liquid water sorption. Specimens of three main classes of hydrophilic cross-linked polymers were tested. These had gel, hypercrosslinked, and macroporous structures that differed by the scale of the Schroeder effect. A similar effect is typical also for hydrophobic polymers upon swelling in liquid organic solvents and their saturated vapor. The paradox is explained by the lower activity of a sorbate in a saturated vapor compared to its activity in a liquid phase. The ability of many samples of cross-linked polymers with different degrees of saturation with a sorbate to be at equilibrium with the saturated vapor of a sorbate is explained by the differences in the inner structure of these samples, i.e., by differing in the swelling combinations and the intensity of the interchain interactions in a polymer network.

The adsorption of chlorobenzene on a carbon adsorbent obtained by the pyrolysis of hypercrosslinked polystyrene

The adsorption of chlorobenzene vapor on an experimental D4609 (Purolite Int.) adsorbent prepared by the pyrolysis of hypercrosslinked polystyrene was studied. The adsorbent was characterized by a large specific surface area, a narrow pore size distribution with the predominant fraction of pore diameters 0.9–3.4 nm, and a high absorption ability with respect to chlorobenzene. The adsorption isotherms were measured over the temperature range 100–140°C, and the thermodynamic characteristics of adsorption were determined. In spite of the correspondence to the formal requirements of the theory of volume filling of micropores, the mechanism of adsorption in narrow micropores (<1.2—1.5 nm) was different from filling with a condensed phase.

A novel hybrid material based on polytrimethylsilylpropyne and hypercrosslinked polystyrene for membrane gas separation and thermopervaporation

To improve the membrane permeability and separation properties in gas separation processes and thermopervaporative (TPV) recovery of butanol from model fermentation mixtures, hybrid membranes based on polymers with an extremely high free fractional volume—polytrimethylsilylpropyne (PTMSP) and hypercrosslinked polystyrene (HCL-PS)—have been first prepared and experimentally studied. The composite membranes have been fabricated using the commercial sorbent Purolite Macronet MN-200 exhibiting high sorption capacity for organic solvents. It has been found that in the hybrid membranes, HCL-PS sorbent particles are nonuniformly distributed throughout the volume: they are located in the surface layer of the membrane. It has been shown that the introduction of a small amount of a modifying component (0.5–1.0 wt %) into the PTMSP matrix improves the time stability of transport properties and increase by a factor of 1.5–2 the permeability coefficients of the material to light gases (N2, O2, CO2, CH4) and butane vapor. It has been found that hybrid PTMSP/HCL-PS membranes have higher separation factors than those of PTMSP membranes in the TPV separation of a butanol/water binary mixture.