Study of pore formation during etching of latent tracks of accelerated heavy ions in poly(ethylene terephthalate) by atomic force microscopy

The process of transformation of latent tacks of accelerated heavy ions of poly(ethylene terephthalate) into pores and the formation of a porous structure of track membranes was studied by atomic force microscopy. It was shown that on initial etching, 10-nm high knolls with an average diameter of 800–1000 Å are formed in place of tracks. Based on the knolls, through channels are formed, which emerge on the surface as conical cavities. It was shown that further etching gives first cylindrical channels of diameter 800–1000 Å, which then undergo radial etching.     

Preparation of novel porous carbon spheres from corn starch

Irregular porous carbon spheres were successfully prepared from Na₂SnO₃ coated corn porous starch by carbonization. The product was characterized with X-ray diffraction and scanning electron microscope (SEM). It is verified that the irregular porous carbon spheres are composed of disordered carbon, and the skeleton and pores of the corn porous starch was well preserved after carbonization. The pore size of the irregular porous carbon spheres is almost the same, which is similar to that of the porous starch. And the pore size decreases from about 0.91 μm to 0.53 μm measured from the SEM pictures. The texture of the irregular porous carbon spheres is mainly determined by that of porous starch.     

Nondestructive technique for the characterization of the pore size distribution of soft porous constructs for tissue engineering

Polymer scaffolds tailored for tissue engineering applications possessing the desired pore structure require reproducible fabrication techniques. Nondestructive, quantitative methods for pore characterization are required to determine the pore size and its distribution. In this study, a promising alternative to traditional pore size characterization techniques is presented. We introduce a quantitative, nondestructive and inexpensive method to determine the pore size distribution of large soft porous solids based on the on the displacement of a liquid, that spreads without limits though a porous medium, by nitrogen. The capillary pressure is measured and related to the pore sizes as well as the pore size distribution of the narrowest bottlenecks of the largest interconnected pores in a porous medium. The measured pore diameters correspond to the narrowest bottleneck of the largest pores connecting the bottom with the top surface of a given porous solid. The applicability and reproducibility of the breakthrough technique is demonstrated on two polyurethane foams, manufactured using the thermally induced phase separation (TIPS) process, with almost identical overall porosity (60-70%) but very different pore morphology. By selecting different quenching temperatures to induce polymer phase separation, the pore structure could be regulated while maintaining the overall porosity. Depending on the quenching temperature, the foams exhibited either longitudinally oriented tubular macropores interconnected with micropores or independent macropores connected to adjacent pores via openings in the pore walls. The pore size and its distribution obtained by the breakthrough test were in excellent agreement to conventional characterization techniques, such as scanning electron microscopy combined with image analysis, BET technique, and mercury intrusion porosimetry. This technique is suitable for the characterization of the micro- and macropore structure of soft porous solids intended for tissue engineering applications. The method is sensitive for the smallest bottlenecks of the largest continuous pores throughout the scaffold that contributes to fluid flow.     

Principles of hierarchical meso- and macropore architectures by liquid crystalline and polymer colloid templating

The generation of porous silica with hierarchically organized bimodal mesoporosity of adjustable size and well-defined shape was investigated by using surfactant mixtures and the nanocasting procedure (liquid crystalline templating). A systematic study of combinations of various block copolymers (Pluronics F127, KLE (poly(omega-hydroxypoly(ethylene-co-butylene)-co-poly(ethylene oxide))) and SE (PS-co-PEO)) with smaller surfactants (Pluronics P123, C16mimCl, and CTAB) revealed that hierarchical bimodal mesopore architectures could only be obtained by the usage of block copolymers with a strong hydrophilic-hydrophobic contrast, such as KLE and SE, giving rise to pores between 6 and 22 nm. Furthermore, the ionic liquid (IL) C16mimCl appeared to have advantageous templating properties, resulting in 2-3-nm pores being located between the block copolymer mesopores, whereas phase separation was observed for Pluronics and CTAB as small templates. Thereby, the study provided also general insights into the mixing and co-self-assembly behavior of block copolymers and ionic surfactants in water and confirmed the special templating properties of ILs, as recently proposed. In addition to the bimodal mesoporosity, additional tunable macroporosity was created by the presence of poly(styrene) or poly(methyl methacrylate) spheres, leading to well-defined trimodal hierarchical pore architectures with the small pores being located in the walls of the respective larger pores. As a major improvement, due to the pore hierarchy, these large-pore materials showed relatively large surface areas and pore volumes, and the size of densely packed macropores could even be decreased down to 90 nm. The materials were characterized by electron microscopy, small-angle X-ray scattering, and nitrogen sorption using a proper NLDFT (nonlocal density functional theory) approach for calculations of the pore size distribution in the entire range of micro- and mesopores.     

The influence of confinement and curvature on the morphology of block copolymers

In the bulk, at equilibrium, diblock copolymers microphase separated into nanoscopic morphologies ranging from body-centered cubic arrays of spheres to hexagonally packed cylinders to alternating lamellae, depending on the volume fraction of the components. However, when the block copolymers are forced into cylindrical pores, where the diameter of the pores are only several repeat periods of the copolymer morphology or less, then commensurability of the copolymer period and the pore diameter can impose a frustration on the microdomain morphology. In addition, due to the small pore diameter, a curvature is forced on the microdomain morphology. In combination with interfacial interactions between the blocks of the copolymer and the pore walls, the preferential segregation of one component to the walls, spatial confinement and forced curvature are shown to induce transitions in the fundamental morphology of the copolymers seen in the bulk. Lamellar morphologies transformed into torus-type morphologies, cylinders are forced into helices, and body-centered cubic arrays of spheres are force into helical arrays of spheres due to these restraints. The novel morphologies, not accesssible in the bulk, open a large array of nanoscopic structures that can be used as templates and scaffolds for the fabrication of inorganic nanostructured materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3377–3383, 2005     

Foam flow through porous media

There are two parts to the interaction of foam with porous media. How the foam interacts with the surface and the flow within the substrate, which is the focus of this review. Flow-through porous media has been investigated experimentally with the main focus in literature being on enhanced oil recovery and remediation. Recently, investigation of the flow of foam through a deformable substrate for dishwashing application has led to the development of mathematical models. It has been proposed that foam flow through pore channels is similar to the behaviour observed within microchannels. Meaning that to investigate the effects these properties have on foam flow it is best to observe them within a model channel then build up to a 3D structure of interlinking channels to resemble porous media. In this review, it is highlighted that a large amount of work is needed in understanding the interaction of foam and/or liquid within porous networks. Methods that can be applied to better represent foam and liquid flow in porous media are discussed within this review, including both using microchannels to simulate individual pores and using these systems to build up to a 3D structure of interlinking pores. In addition, more advanced imaging techniques to observe the flow through porous materials are discussed, including computed tomography scanning nuclear magnetic resentence and confocal microscopy. There is still more work required to fully understand the flow within porous media, including observing the affect of dead-end pores, closed loops and rough channel walls have on the flow.     

Preparation and performance of porous titania with a trimodal pore system as anode of lithium ion battery

A porous titania has been prepared by using polystyrene spheres and tri-block copolymer ((EO)₂₀–(PO)₇₀–(EO)₂₀, P123) as templates, and its structure, composition, and performance as anode of lithium ion battery are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, and galvanostatic charge/discharge test. The results from SEM and TEM indicate that the prepared porous titania has a trimodal pore system, in which the pores are in ordered arrangement and interconnected with the same pore diameter and uniform wall thickness. The charge/discharge tests show that the battery using the prepared porous titania as anode exhibits good rate capacity and cycle stability.     

Polymorph selectivity under nanoscopic confinement

Polymorph selectivity has been achieved during crystallization of anthranilic acid (AA) and 5-methyl-2-[(2-nitrophyenyl)amino]-3-thiophenecarbonitrile (ROY), both considered benchmarks of polymorphic behavior, within nanoporous glass beads and polymer monoliths. Whereas polymorph III of AA crystallizes from the melt on nonporous glass beads or within larger pores, the metastable polymorph II crystallizes in pores with diameters <23 nm, with the selectivity toward this form increasing with decreasing pore size. Of the six ROY polymorphs characterized by single-crystal X-ray diffraction, the yellow form (Y) crystallizes during evaporation of pyridine solutions imbibed by the 30-nm cylindrical pores of porous polycyclohexylethylene (p-PCHE) monoliths. Although both R and ON grow from the melt on the external surfaces of PCHE, only the red form (R) crystallizes in the pores. Amorphous ROY also forms in p-PCHE pores during evaporation from pyridine solutions, subsequently crystallizing to the R nanocrystals upon heating. Although heterogeneous nucleation on the pore walls may play a role, these observations suggest that nucleation and polymorph selectivity is governed by critical size constraints imposed by the ultrasmall pores. The ability to achieve polymorph selectivity in both glass and polymer matrices suggests wide-ranging compatibility with various organic crystalline solids, promising a new approach to controlling polymorphism and searching for unknown polymorphs.     

Synthesis of spherical polymer and titania photonic crystallites

The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 microm in diameter and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.     

Permanent microporosity and enantioselective sorption in a chiral open framework

A homochiral microporous material is presented. The phase has 47% permanently porous void volume and is shown to have >1 nm diameter pores with three-dimensional channels using probe molecule sorption. Enantioselective guest sorption is strongly dependent on guest size. The homochiral microporous phase was identified by reactive selection from a first-generation chiral but nonporous framework. Chiral permanent porosity is established by directional noncovalent interactions between framework-forming and nonframework forming components of the stable second-generation material, which become stronger upon loss of the guests from the pore system.     

Modelling of the pore flow in capillary electrochromatography

Pore flow in capillary electrochromatography (CEC) on porous silica particles has been investigated. To that end the migration behaviour of narrow polystyrene (PS) standards dissolved in di-methylformamide (DMF) with lithium chloride in 1 and 10 mmol/l concentration has been measured. These data have been compared to theoretical predictions. The latter were based on a model comprising cylindrical pores of varying diameter as measured experimentally by porosimetry, while the flow in each set of pores was calculated with the expression given by Rice and Whitehead. A reasonable to good agreement between experimental and predicted data was observed, provided it was assumed that pores of differing diameter occur in series. It was found that the flow in pores with a nominal size of 100 A can be considerable compared to the interstitial flow, especially at 10 mmol/l ionic strength. It is concluded that pore flow within porous particles in CEC, of great importance for improved efficiency in both interactive and exclusion type CEC, can be predicted fairly reliably by means of the Rice and Whitehead expression.     

三维有序多孔图案化SiC陶瓷的制备

结合毛细管微模塑技术、模板技术和先驱体转化技术, 以图案化聚二甲基硅氧烷(PDMS)弹性体为模具,以氧化硅凝胶小球为模板, 以液态聚碳硅烷(PCS)为先驱体, 经过氧化硅凝胶小球图案化模板的形成, 先驱体的渗入, 模板中先驱体的交联, 弹性模具的去除, 图案化先驱体的无机化和模板的去除, 制备了图案化多孔SiC 陶瓷.研究结果表明：所制备的图案化多孔陶瓷中, 图案的尺寸受图案化PDMS 弹性模具的控制, 球形孔的孔径可由氧化硅凝胶小球来调节. 图案化陶瓷中球形孔不仅三维有序排列, 而且由于模板中小球的相互接触形成的“窗 口”使球形孔三维贯通.     

Electrochemically assisted fabrication of size-exclusion films of organically modified silica and application to the voltammetry of phospholipids

Modification of electrodes with nanometer-scale organically modified silica films with pore diameters controlled at 10- and 50-nm is described. An oxidation catalyst, mixed-valence ruthenium oxide with cyano cross-links or gold nanoparticles protected by dirhodium-substituted phosphomolybdate (AuNP-Rh₂PMo₁₁), was immobilized in the pores. These systems comprise size-exclusion films at which the biological compounds, phosphatidylcholine and cardiolipin, were electrocatalytically oxidized without interference from surface-active concomitants such as bovine serum albumin. Ten-nanometer pores were obtained by adding generation-4 poly(amidoamine) dendrimer, G4-PAMAM, to a (CH₃)₃SiOCH₃ sol. Fifty-nanometer pores were obtained by modifying a glassy carbon electrode (GC) with a sub-monolayer film of aminopropyltriethoxylsilane, attaching 50-nm diameter poly(styrene sulfonate), PSS, spheres to the protonated amine, transferring this electrode to a (CH₃)₃SiOCH₃ sol, and electrochemically generating hydronium at uncoated GC sites, which catalyzed ormosil growth around the PSS. Voltammetry of Fe(CN)₆ ^3? and Ru(NH₃)₆ ³⁺ demonstrated the absence of residual charge after removal of the templating agents. With the 50-nm system, the pore structure was sufficiently defined to use layer-by-layer electrostatic assembly of AuNP-Rh₂PMo₁₁ therein. Flow injection amperometry of phosphatidylcholine and cardiolipin demonstrated analytical utility of these electrodes.     

Structure of the first silicate molecular sieve with 18-ring pore openings, ECR-34

The three-dimensional microporosity of zeolite frameworks have allowed their widespread use in industry as heterogeneous catalysts, absorbents, and ion-exchangers. While the phosphate analogues of zeolites having up to 24 tetrahedral atoms in the pore openings are known, silicate-based zeolites have, until now, been limited to 14-membered ring pore openings. We now disclose the structure and characterization of the synthetic zeolite ECR-34, which can be prepared from a mixed alkali metal reaction gel containing tetraethylammonium (TEA) cations. Its structure has been determined from powder diffraction data and shows ECR-34 to be hexagonal with the dimensions a, b = 21.030(1) A, c = 8.530(1) A, containing one-dimensional, 18-ring pores with 10 A diameter free openings. ECR-34 is stable to 800 degrees C and is able to absorb and ion-exchange large organic molecules. The existence of ECR-34 suggests the potential of preparing other thermally stable silicate molecular sieves with extra-large pores.     

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.     

Pore structural studies of monodisperse porous polymer particles

Monodisperse polystyrene latex particles with molecular weight on the order of 10⁶ were used as inert diluents for the preparation of monodisperse porous styrene-divinylbenzene copolymer particles via seeded emulsion polymerization techniques. Mercury porosimetry and nitrogen adsorption-desorption isotherms were used to assess pore structure and pore size distribution. Pore size distribution was very sensitive to the molecular weight of the polystyrene latex particles used as inert diluent. Qualitative evidence from the techniques used indicated that the monodisperse porous polymer particles were macroporous (average pore diameter > 500 Å) in nature. As the molecular weight of the linear polymer decreased, the porous structure of the polymer particles ranged in complexity across the spectrum of macro/mesopore structures. Scanning electron microscope results indicated the existence of voids between the microspheres and their agglomerates within the porous polymer particle, and nitrogen adsorption isotherms confirmed that the pores were due to interstices between these crosslinked microspheres and agglomerates.     

Packed uniform sphere model for solids: interstitial access opening sizes and pressure deficiencies for wetting liquids with comparison to reported experimental results

Mathematical relationships have been developed to describe the pressure deficiencies required for drainage and removal of wetting liquids through the access openings to the interstitial void space for a model comprised of uniform packed solid spheres. Access openings and associated pressure deficiencies are defined in terms of the packing and radius of the spheres, using a circular arc approximation for the liquid-vapor portion of the perimeter of the opening. This allows determination of equivalent particle radius rather than equivalent cylindrical pore radius within a porous solid sample by use of standard pressure, porosity and desorption data. For a known particle size and porosity, it allows comparison and prediction of drainage of wetting liquids and pressures required for removal of the liquid from compacted materials and collections of random packed spherical particles. Comparisons are made to experimental packing of spheres. Sorption isotherms for a volatile wetting liquid are presented, covering the access to the interstitial void space, the pendular liquid ring between adjacent touching spheres and the monolayer surface area. The larger size of the interstitial void space compared to the size of the access opening leads to lower imbibition pressures and hysteresis for both volatile and nonvolatile wetting liquids. The relationship to mercury porosimetry and the adjustment for contact angles other than 0 degrees and 180 degrees are discussed.     

Colloid-templated multisectional porous polymeric fibers

A fabrication method for porous polymeric fibers (PPFs) is reported. We show that a multisectional colloidal crystal can be assembled within a microcapillary by alternating dipping into colloidal solutions of varying size. Subsequent infiltration with curable polymer and washing with suitable solvents results in porous fibers with a cylindrical cross section. Along the length of the fiber, alternating sections of controlled length, pore size, and pore size distribution exist. These fibers present interesting materials for neural scaffolding, catalysis, and possibly photonics if produced with a high degree of crystallinity. The surface pores and bulk porosity of the fibers are characterized by variable-pressure scanning electron microscopy (vp-SEM). Careful analysis shows that the surface pores vary with the colloidal template diameter and polymer infiltration time.     

Pore network modelling: determination of the dynamic profiles of the pore diffusivity and its effect on column performance as the loading of the solute in the adsorbed phase varies with time

A three-dimensional pore network model for diffusion in porous adsorbent particles was employed in a dynamic adsorption model that simulates the adsorption of a solute in porous particles packed in a chromatographic column. The solution of the combined model yielded the dynamic profiles of the pore diffusion coefficient of beta-galactosidase along the radius of porous ion-exchange particles and along the length of the column as the loading of the adsorbate molecules on the surface of the pores occurred, and, the dynamic adsorptive capacity of the chromatographic column as a function of the design and operational parameters of the chromatographic system. The pore size distribution of the porous adsorbent particles and the chemistry of the adsorption sites were unchanged in the simulations. It was found that for a given column length the dynamic profiles of the pore diffusion coefficient were influenced by: (i) the superficial fluid velocity in the column, (ii) the diameter of the adsorbent particles and (iii) the pore connectivity of the porous structure of the adsorbent particles. The effect of the magnitude of the pore connectivity on the dynamic profiles of the pore diffusion coefficient increased as the diameter of the adsorbent particles and the superficial fluid velocity in the column increased. The dynamic adsorptive capacity of the column increased as: (a) the particle diameter and the superficial fluid velocity in the column decreased, and (b) the column length and the pore connectivity increased. In preparative chromatography, it is desirable to obtain high throughputs within acceptable pressure gradients, and this may require the employment of larger diameter adsorbent particles. In such a case, longer column lengths satisfying acceptable pressure gradients with adsorbent particles having higher pore connectivity values could provide high dynamic adsorptive capacities. An alternative chromatographic system could be comprised of a long column packed with large particles which have fractal pores (fractal particles) that have high pore connectivities and which allow high intraparticle diffusional and convective flow mass transfer rates providing high throughputs and high dynamic adsorptive capacities. If large scale monoliths could be made to be reproducible and operationally stable, they could also offer an alternative mode of operation that could provide high throughputs and high dynamic adsorptive capacities.