Multiporous Materials From Thermoreversible Poly(vinyledene fluoride) Gels

Poly(vinylidene fluoride)(PVF₂) produces thermoreversible gels with alkyl diesters of general formula (CH₂)_n (COOEt)₂ as well as with camphor, a naturally occurring ketone. These gels containing polymer-solvent intercalates yield multiporous materials when subjected to controlled solvent removal techniques. The micro and meso pores are attributed to polymer-solvent complexation while the macro pores are formed as a result of removal of the solvent trapped in the fibrillar network. PVF₂ –diethyl azelate (DEAZ, n = 6) and PVF₂ -camphor gels produce porous polymer network when dried by cyclohexane leaching. FESEM images exhibit porous network structures with fibrillar morphology. Mercury intrusion porosimetry (MIP) shows presence of pores having diameter in the range 4 nm–400µm for both the systems. The BJH pore size distribution curves for both systems confirm the presence of mesoporosity. The HK pore size distribution plots indicate that micropores are also created and it also puts evidence of single molecule solvent intercalation between the PVF₂ strands. The hysteresis between the extrusion and the intrusion curves indicates the presence of channel type/ink-bottle type structure in these systems.     

Multiscale Porosity from Thermoreversible Poly(vinylidene fluoride) Gels in Diethyl Azelate

Poly(vinylidene fluoride) (PVF₂) produces thermoreversible gels in a series of diesters. The polymer-solvent complexation occurred for intermittent number of carbon atoms n ⩾ 2 and the enthalpy of complexation increased with increasing n. The gels were dried by replacing the diesters with low boiling solvent like cyclohexane (bp. 80 °C) and methylcyclohexane (bp. 99 °C). The porosity of the dried gels was measured using Poremaster-60. For PVF₂-DEAZ gel meso and macro porosity have been observed. The former pore dimensions have been attributed for polymer-solvent complexation while the macroporosity has been attributed for caging of solvent between the PVF₂ fibrils The porosity measured from nitrogen adsorption isotherms using BJH method indicate presence of minimum pore diameter of 3.8 nm for the 10% dried gel of PVF₂.     

Characterization of highly porous polymeric materials with pore diameters larger than 100 nm by mercury porosimetry and X-ray scattering methods

Highly porous polymeric materials with pore sizes ranging from 100 nm to 1 microm are a very challenging class of materials not only to prepare synthetically (due to the high capillary pressures generated upon solvent removal) but also to characterize structurally. Through the examples of three different types of porous compounds synthesized in our laboratory (i) high-density melamine-based "MF-hd" with monomodal pore diameters around 500-900 nm, (ii) low-density melamine-based "MF-ld" with bimodal pore size distribution and average diameters around 2.3 microm and 350 nm, (iii) highly porous polyurethane "PU" with monomodal pore sizes around 150 nm, we confirm the limitations of mercury porosimetry as a means to investigate the architecture of materials with very high porosity (>80 vol %) and low compressive strength. Instead, a combination of high-resolution scanning electron microscopy and small-angle and ultrasmall-angle X-ray scattering (SAXS and USAXS, respectively) studies of these three types of materials helps in determining both the network and the pore structures. This work elucidates the need and applicability of the SAXS/USAXS techniques in characterizing such porous materials. For instance, the polyurethane specimens can only be quantitatively characterized by scattering techniques, the results of which are corroborated by high-resolution scanning electron microscopy observations.     

Thermoporosimetry

The pore size distributions (PSDs) of microporous glass, which were controlled by acid leaching subsequent to phase separation of CaO-Al₂O₃-B₂O₃-SiO₂ glass, were determined via both mercury porosimetry and thermoporosimetry (thermal porosimetry). As a result, the pore radii, the cumulative pore volumes, and the surface areas determined via thermoporosimetry were in good agreement with those determined via mercury porosimetry. It was revealed that thermoporosimetry could be applied to pore structure analysis for porous materials having pore sizes at least up to 58 nm in radius.     

Nanostructured carbon—Ni(OH)2 composites

Deposition of Ni(OH)₂ from an aqueous solution of Ni(N₃)₂ onto highly porous carbon matrices of two types with different porous structure afforded high-purity nanostructured hydroxide—carbon composites with a regular spatial morphology, which are filled with Ni(OH)₂ nanocrystallites (up to 30.9 wt.%) and have high values of specific suface area (up to 1875 m² g^–1) and porosity (up to 2.65 cm³ g^–1). Largeand small-angle X-ray diffraction and low-temperature nitrogen absorption on composites showed that nanocrystallites with a brucite-type layered structure form as plates with a thickness of 2—4 nm and a size along the developed face (001) of 25—30 nm in mesopores and on the outer surface of matrices. The degree of mesopore filling with crystallites depends on the mesopore size and the composition of composites; the micropores remain mainly unfilled. The increase in the hydroxide content results in pore size redistribution: in general, the distribution curves shift in favor of smaller mesopore sizes; the portion of pores with sizes comparable with the thickness of filler nanoplates (3—6 nm), as well as the portion of mid-sized pores (20—30 nm) decrease significantly in favor of smaller pores (8—12 nm). Partial blocking (clogging) of pores with filler nanocrystallites was also observed.     

On the nanostructure of micrometer-sized cellulose beads

The analysis of the porosity of materials is an important and challenging field in analytical chemistry. The gas adsorption and mercury intrusion methods are the most established techniques for quantification of specific surface areas, but unfortunately, dry materials are mandatory for their applicability. All porous materials that contain water and other solvents in their functional state must be dried before analysis. In this process, care has to be taken since the removal of solvent bears the risk of an incalculable alteration of the pore structure, especially for soft materials. In the present paper, we report on the use of small-angle X-ray scattering (SAXS) as an alternative analysis method for the investigation of the micro and mesopores within cellulose beads in their native, i.e., water-swollen state; in this context, they represent a typical soft material. We show that even gentle removal of the bound water reduces the specific surface area dramatically from 161 to 109 m² g⁻¹ in cellulose bead sample type MT50 and from 417 to 220 m² g⁻¹ in MT100. Simulation of the SAXS curves with a bimodal pore size distribution model reveals that the smallest pores with radii up to 10 nm are greatly affected by drying, whereas pores with sizes in the range of 10 to 70 nm are barely affected. The SAXS results were compared with Brunauer–Emmett–Teller results from nitrogen sorption measurements and with mercury intrusion experiments.     

纳米孔莫来石陶瓷材料的制备

以正硅酸乙酯(TEOS)提供硅源、纳米氧化铝(d90=50 nm)提供铝源,通过溶胶-凝胶法与超临界干燥技术,制备了分散纳米氧化铝的SiO2气凝胶块体,所得复合气凝胶块体经1200℃、1300℃热处理后,得到了纳米孔莫来石陶瓷材料。XRD测试表明:凝胶体在1 200℃热处理后发生了莫来石化,1300℃莫来石化基本完成。压汞仪与场发射扫描电镜结果显示:凝胶块体经1 200、1 300℃热处理后,形成了具有纳米多孔结构的莫来石陶瓷材料,其骨架结构包含有200~400 nm的大孔,以及大量位于其孔壁上的6~30 nm的介孔。由于莫来石化的进行,热处理后的陶瓷材料的纳米孔结构具有更高的热稳定性。     

Evaluation of the porous structures of new polymer packing materials by inverse size-exclusion chromatography

A highly cross-linked porous polymer resin based on styrene-divinylbenzene matrix with pores created by the use of micellar imprinting technique was used as chromatographic packing material. Its performance as a column packing material in inverse size-exclusion chromatography was compared with a non-imprinted resin of the same polymer matrix. The porous structures (the pore size and the porosity) of the resins in the dry and wet states and their relationships with the elution volume of probe solutes (alkanes and polystyrene standards) were established. Characteristic properties of the resins such as specific pore volume, specific surface area and porosity are compared with results obtained by other methods of characterization such as mercury intrusion porosimetry, solvent regain and nitrogen sorption. The results show that the new porous resin can be used in the separation of small molecules. The separation is based on the size of the molecules, and the larger pores (meso- and macropores) in the porous resin can provide a much easier access to the smaller pores (micropores) which are useful in the chromatographic separations.     

Influence of lignin on cellulose-NaOH-water mixtures properties and on Aerocellulose morphology

Microcrystalline cellulose and organosolv lignin, both dissolved in 8%NaOH-water, were mixed with the objective to study the influence of lignin on the properties of cellulose solutions and on the morphology of dry porous materials. Mixture viscosity and gelation were investigated. Cellulose-lignin gels were regenerated in aqueous acid baths and dried under supercritical CO₂ to obtain Aerocellulose, an aerogel-like material. The presence of lignin in the mixture speeded up gelation. During regeneration part of lignin was washed out. This created large pores and channels in the dry materials. The overall results obtained showed that cellulose and lignin are not compatible in the solvent used.     

Adsorption porosimetry application in the study of a porous structure of thin layers

The method of adsorption porosimetry (AP) has been long known in physical chemistry of porous sorbents, but its practical application was limited to studies of massive samples of such materials as silica gels, zeolites, etc. We have developed a technique to study the porous structure of dielectric thin films (d ～ 100 nm). It is stated that it provides us with quite novel information which permits the prediction of properties of dielectric layers and makes it possible on this basis to solve optimization problems of technological processes. The use of ellipsometry has further extended the potential of this method, and currently it is a powerful nondestructive technique to measure porosity and the size distribution of pores in thin layers for the purposes of modern electronics.     

Bioactive sol–gel scaffolds with dual porosity for tissue engineering

Scaffolds containing dual porosity at the nano and macroscale appear to exhibit improved performance in terms of crystallization of hydroxycarbonate apatite plus cell adhesion and proliferation, as well as vascularization. The aim of the present work is to develop a novel, simple sol–gel process for the preparation of silica-based bioactive porous bone tissue scaffold, with a pore structure consisting of interconnected pores of both 100’s of micrometers and 10’s of nanometers in size, optimized for enhanced bone regeneration performance. SiO₂–CaO and SiO₂–CaO–P₂O₅ porous glass monoliths have been prepared with a dual pore structure including pores of both ~50–200 micrometers and a few to 10’s of nanometers in size, based on polymerization-induced phase separation together with the sol-gel transition, by adding a water soluble polymer to the precursor sol. The nanopore (~5–40 nm) structure of such macroporous gel skeletons was tailored by solvent exchange, followed by heat treatment at 600–700 °C. The overall pore structure has been studied by Scanning Electron Microscopy (SEM), N₂-adsorption (BET), Mercury intrusion porosimetry and Infrared spectroscopy. The scaffold bioactivity, tested in simulated body fluid, has been demonstrated by means of DRIFTS, SEM and X-ray diffraction measurements.     

Characterization of pore structure in a nanoporous low-dielectric-constant thin film by neutron porosimetry and X-ray porosimetry

A small-angle neutron scattering (SANS) porosimetry technique is presented for characterization of pore structure in nanoporous thin films. The technique is applied to characterize a spin-on organosilicate low dielectric constant (low-k) material with a random pore structure. Porosimetry experiments are conducted using a "contrast match" solvent (a mixture of toluene-d8 and toluene-h8) having the same neutron scattering length density as that of the nanoporous film matrix. The film is exposed to contrast match toluene vapor in a carrier gas (air), and pores fill with liquid by capillary condensation. The partial pressure of the solvent vapor is increased stepwise from 0 (pure air) to P0 (saturated solvent vapor) and then decreased stepwise to 0 (pure air). As the solvent partial pressure increases, pores fill with liquid solvent progressively from smallest to largest. SANS measurements quantify the average size of the empty pores (those not filled with contrast match solvent). Analogous porosimetry experiments using specular X-ray reflectivity (SXR) quantify the volume fraction of solvent adsorbed at each step. Combining SXR and SANS data yields information about the pore size distribution and illustrates the size dependence of the filling process. For comparison, the pore size distribution is also calculated by application of the classical Kelvin equation to the SXR data.     

Orienting the Pore Morphology of Core-Shell Magnetic Mesoporous Silica with the Sol-Gel Temperature. Influence on MRI and Magnetic Hyperthermia Properties

The controlled design of robust, well reproducible, and functional nanomaterials made according to simple processes is of key importance to envision future applications. In the field of porous materials, tuning nanoparticle features such as specific area, pore size and morphology by adjusting simple parameters such as pH, temperature or solvent is highly needed. In this work, we address the tunable control of the pore morphology of mesoporous silica (MS) nanoparticles (NPs) with the sol-gel reaction temperature (T_sg). We show that the pore morphology of MS NPs alone or of MS shell covering iron oxide nanoparticles (IO NPs) can be easily tailored with T_sg orienting either towards stellar (ST) morphology (large radial pore of around 10 nm) below 80 °C or towards a worm-like (WL) morphology (small randomly oriented pores channel network, of 3–4 nm pore size) above 80 °C. The relaxometric and magnetothermal features of IO@STMS or IO@WLMS core shell NPs having respectively stellar or worm-like morphologies are compared and discussed to understand the role of the pore structure for MRI and magnetic hyperthermia applications.     

A novel process to prepare porous membranes comprising SnO2 nanoparticles and P(MMA-AN) as polymer electrolyte

We have successfully developed a new process to prepare porous poly(methyl methacrylate-co-acrylonitrile) (P(MMA-AN)) copolymer based gel electrolyte. The porous structure in the polymer matrix is achieved by adding SnO₂ nanoparticles which are mostly used as gas sensor materials. The quasi-aromatic solvent, NMP, has an electron-repulsion effect with the space charge layer on the surface of SnO₂ nanoparticles and forms a special gas–liquid phase interface. Once the cast polymer solution is stored at an elevated temperature to evaporate the solvent, gas–liquid phase separation happens and spherical pores are obtained. The ionic conductivity at room temperature of the prepared gel polymer electrolyte based on the porous membrane is as high as 1.54 × 10⁻³ S cm⁻¹ with the electrochemical stability up to 5.10 V (vs. Li/Li⁺). This method presents another promising way to prepare porous polymer electrolyte for practical use.     

Poly(vinylidene fluoride) separators with dual-asymmetric structure for high-performance lithium ion batteries

Dual-asymmetric poly(vinylidene fluoride)(PVDF) separators have been fabricated by thermally induced phase separation with dimethyl sulfone(DMSO2) and glycerol as mixed diluents. The separators have a porous bulk with large interconnected pores(~1.0 μm) and two surfaces with small pores(~30 nm). This dual-asymmetric porous structure endows the separators with higher electrolyte uptake amount and rapider uptake rate, as well as better electrolyte retention ability than the commercialized Celgard 2400. The separators even maintain their dimensional stability up to 160 °C, at which temperature the surface pores close up, leading to a dramatic decrease of air permeability. The electrolyte filled separators also show high ion conductivity(1.72 m S?cm―1) at room temperature. Lithium iron phosphate(Li Fe PO4)/lithium(Li) cells using these separators display superior discharge capacity and better rate performance as compared with those from the commercialized ones. The results provide new insight into the design and development of separators for high-performance lithium ion batteries with enhanced safety.     

Low-Temperature Sol-Gel Synthesis of Single-Phase ZrTiO4 Nanoparticles

Ceramic ZrTiO₄ powders were prepared by a sol-gel method using zirconium oxychloride and titanium tetraisopropoxide. In situ high temperature X-ray diffraction results show that crystallization of the amorphous gel starts at 400 ^∘C. Single-phase ZrTiO₄ nanoparticles were obtained after heat treatment at 450 ^∘C for one hour. An average particle size of 46 nm has been determined by nitrogen adsorption analysis. After pressing these sinteractive powders, pellets with controlled pore size distribution were obtained by sintering at temperatures as low as 400 ^∘C. The analysis of pores by mercury porosimetry gives an average porosity of 45%. The electrical resistivity, determined by impedance spectroscopy measurements at 24 ^∘C under different humidity environments, shows the ability of these pellets to adsorb water vapor in the porous surfaces. Pellets fabricated with the nanosized powders prepared by the sol-gel technique are proposed as good candidates to be used in humidity sensing devices.     

Electrorheological effect of carbonaceous materials with hierarchical porous structures

Carbonaceous materials with different hierarchical porous structures for electrorheological (ER) dispersed phase have been synthesized by carbonization of as-prepared starch/silica precurser at different temperatures. The N₂ adsorption isotherms show that Cmeso-700 and Cmeso-500 particles have the BET surface areas of 1028 and 603 m² g⁻¹, respectively. They both have the mesoporous pores with size of about 4.6 nm and the microporous pores (1.1 and 1.5 nm, respectively). The BET surface areas and C/O atomic ratio of porous carbon materials can be increased with the carbonization temperatures. The rheological measurements indicate that the Cmeso-700 and Cmeso-500 ERF have the better ER effect resulted from their hierarchical porous structures. The shear stress of Cmeso-700 ERF is 900 Pa at 1000 s⁻¹ under 3 kV mm⁻¹, which is almost 4.5 times larger than that of Cmicro-350 ERF. The mesoporous carbon ERFs also show the better sedimentation stability than microporous carbon ERFs. The different ER effect of carbonaceous particles may derive from their different dielectric polarization property induced by the hierarchical porous structures.     

Formation and characterization of phosphate-modified silicate materials derived from sol–gel process

Phosphate-containing silicate materials prepared using sol–gel method from Si(OC₂H₅) were investigated at the variation of the amount of phosphate modifier from 5 to 50 wt% in term of P₂O₅. Chemical composition, textural and structural properties of these materials were characterized by FTIR-spectroscopy, TEM, X-ray diffraction and nitrogen adsorption. It was shown that the materials posse monomodal pore size distribution of 5–20 nm for the samples dried at 100 °C and 40–60 nm for the specimens calcined at 600 °C. The mean pore size and surface area depended on the amount of phosphoric acid. Before the stage of high temperature treatment phosphoric acid, introduced into the structure of the materials as a modifying agent, was uniformly distributed inside a porous space of the material and was not chemically bonded with silicate. After high temperature treatment both chemical interaction of silicate with phosphate, providing the formation of silicate-phosphate structures, as well as redistribution of free modifier from the bulk of granules to their surface took place. The polyphosphate layer is formed on the material surface closing the internal porous space. However, in this case a part of the phosphate modifier remains chemically unbound to SiO₂ structure.     

Formation of TiO2 ceramic foams from the integration of the sol–gel method with surfactants assembly and emulsion

A general and potentially easy method for synthesizing TiO₂ ceramic foams presenting hierarchical architecture of meso and macropores is presented here. The ceramics foaming method is based on the integration of the sol?Cgel process with sodium dodecyl sulfate (SDS) surfactant and oil droplets of isopropyl myristate (IPM) as dual pore templates. The main aim of this study was to evaluate the effect of ionic surfactant on the porous structure and specific surface area. The structural feature of these materials was characterized by analyzes of X-ray diffraction, nitrogen absorption/desorption isotherms, Hg porosimetry, He and Dried-Fluid^? picnometers. Mercury intrusion porosimetry shows that SDS and IPM induce the formation of hierarchical structure composed of two families of pores, namely macro and mesopores. The relative population of each family and the average size of macropores could be finely tuned by adjusting the SDS quantity. In the presence of this surfactant, a single anatase crystalline phase was observed for titania foams fired at 600?°C.     

Tailoring of nanoscale porosity in carbide-derived carbons for hydrogen storage

The poor performance of hydrogen storage materials continues to hinder development of fuel cell-powered automobiles. Nanoscale carbons, in particular (activated carbon, exfoliated graphite, fullerenes, nanotubes, nanofibers, and nanohorns), have not fulfilled their initial promise. Here we show that carbon materials can be rationally designed for H2 storage. Carbide-derived carbons (CDC), a largely unknown class of porous carbons, are produced by high-temperature chlorination of carbides. Metals and metalloids are removed as chlorides, leaving behind a collapsed noncrystalline carbon with up to 80% open pore volume. The detailed nature of the porosity-average size and size distribution, shape, and total specific surface area (SSA)-can be tuned with high sensitivity by selection of precursor carbide (composition, lattice type) and chlorination temperature. The optimum temperature is bounded from below by thermodynamics and kinetics of chlorination reactions and from above by graphitization, which decreases SSA and introduces H2-sorbing surfaces with binding energies too low to be useful. Intuitively, pores of different size and shape should not contribute equally to hydrogen storage. By correlating pore properties with 77 K H2 isotherms from a wide variety of CDCs, we experimentally confirm that gravimetric hydrogen storage capacity normalized to total pore volume is optimized in materials with primarily micropores ( approximately 1 nm) rather than mesopores. Thus, in agreement with theoretical predictions, a narrow size distribution of small pores is desirable for storing hydrogen, while large pores merely degrade the volumetric storage capacity.