The use of volume-averaging techniques to predict temperature transients due to water vapor sorption in hygroscopic porous polymer materials

Volume-averaging techniques developed for modeling drying processes in porous materials offer a convenient framework for analyzing vapor sorption in porous hygroscopic polymeric materials. Because of the large temperature changes associated with water vapor sorption in these materials (from 10° to 20°C), sorption/diffusion processes are best characterized through the coupled differential equations describing both the transport of energy and mass through the porous structure. Experimental and numerical results are compared for a variety of natural and man-made porous polymeric materials (textiles) using the volume-averaging technique. Boundary heat and mass transfer coefficients and assumptions about thermal radiative properties of the experimental apparatus are shown to influence results obtained with the numerical solution method. © 1997 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

J Appl Polym Sci 64: 493–505, 1997     

Determination of Both Mesopores and Macropores in Three-Dimensional Ordered Porous Materials by Nitrogen Adsorption

Three-dimensionally ordered silica structures containing both mesopores and macropores are created using polystyrene coacervate spheres with a diameter of ca. 146 nm. The close-packed polystyrene coacervate spheres are intercalated with tetraethyl orthosilicate. The spheres are removed by calcination leaving an inverse silica replica with a spherical macropore cavity diameter of 110 nm. Due to the nature of these porous structures, pores leading into the macropore cavity are in the mesopore regime, 40 nm in diameter. The nitrogen adsorption data described in the following paper gives a pore size for both the macropore cavity and the mesopore openings leading into the cavity. The pore sizes as determined by nitrogen sorption are in good agreement with the pore sizes observed by scanning electron microscopy. Mercury intrusion porosimetry results confirm the size of the mesopore openings leading into the macropore cavity, however due to destruction of the sample upon intrusion, extrusion results can not be obtained to determine main cavity diameters. As a result, nitrogen sorption may be a viable option for determining pore sizes with these three-dimensionally ordered materials containing both mesopores and macropores.     

Silica-Based, Cubic Mesostructures: Synthesis, Characterization and Relevance for Catalysis

We review the formation mechanism, synthetic history and catalytic applications of the cubic phases of mesoporous silica. MCM-48 and SBA-1, analogous to surfactant lyotropic liquid crystal phases, possess a three-dimensional pore array, a 15–30 pore aperture and a narrow pore size distribution. Evidence for intermediate range order within the silicate walls based on PAS-FTIR spectra will be discussed. Isomorphic heteroatom substitution of aluminum and a number of transition metals has been achieved by both hydrothermal and post-synthetic methods in order to modify the surface activity of these materials. Modified materials containing fully oxidized, early transition metals were found to be catalytically active for photocatalytic reduction of CO₂ with water as well as peroxidative oxidation and halogenation of large organic substrates.     

Controlling the textural parameters of mesoporous carbon materials

The mesoporous carbon materials prepared by inorganic templating technique using mesoporous silica, SBA-15 as a template and sucrose as a carbon source, have been systematically investigated as a function of sucrose to mesoporous silica composition, with a special focus on controlling the mesoporous structure, surface morphology and the textural parameters such as specific surface area, specific pore volume and pore size distribution. All the materials have been unambiguously characterized by XRD, N₂ adsorption–desorption isotherms, high-resolution transmission electron microscopy, high-resolution field emission scanning electron microscopy, and Raman spectroscopy. It has been found that the porous structure, morphology and the textural parameters of the mesoporous carbons materials, CMK-3-x where x represent the sucrose to silica weight ratio, can be easily controlled by the simple adjustment of concentration of sucrose molecules. It has also been found that the specific surface area of the mesoporous carbon materials systematically increases with decreasing the sucrose to silica weight ratio. Moreover, the specific pore volume of the materials increases from 0.57 to 1.31 cm³/g with decreasing the sucrose to silica weight ratio from 5 to 1.25 and then decreases to 1.23 cm³/g for CMK-3-0.8. HRTEM and HR-FESEM also show a highly ordered pore structure and better surface morphology for CMK-3-1.25 as compared to other materials prepared in this study. Thus, it can be concluded that the sucrose to silica weight ratio of 1.25 is the best condition to prepare well ordered mesoporous carbon materials with good textural parameters, pore structure and narrow pore size distribution.     

Modification of Porous Silica with Activated Carbon and its Application for Fixation of Yeasts

Mixtures of small silica particles and activated carbon were heated at 1250–1450°C in an inert atmosphere to make nano- and macro-sized porous silica for incorporating yeast in the porous strucrure. Without activated carbon, porous silica of 45–60% porosity and 15–30 m pore diameter was produced with a specific surface area below 1 m²/g. By the addition of 8 wt% of activated carbon granules, the surface area of porous silica increased to 100 m²/g at 1250°C. It was confirmed that there were micropores(1.2 nm) and mesopores(4.0 nm) due to activated carbon granules in porous silica when granule type activated carbon was used. However, in the case of activated carbon fiber, its micro- and mesoporous structure was destroyed in the firing process. The fixation of Z. rouxii yeasts was promoted on the porous silica with activated carbon.     

A green process for the synthesis of controllable mesoporous silica materials

The synthesis, characterization, and application of mesoporous silicas have attracted a lot of attention for over two decades, which stems from their fascinating structures, formation mechanisms and prospects of their applications. Various methods have been developed for the synthesis of these silicas with a tunable pore diameter and a narrow pore size distribution. In this paper, mesoporous silica materials with controllable pore diameters (3-9 nm), narrow pore size distributions, high surface area (>700 m² g⁻¹) and pore volume (>1 cm³ g⁻¹) were prepared by a green template, amphiphilic dendritic polyamidoamine. The resulting silica materials were characterized by ¹H, ¹³C NMR spectroscopy; thermogravimetic analysis; nitrogen adsorption; transmission electron microscope. It was shown that the template could be completely removed and recycled from the silica in an environmentally friendly way by means of a simple water extraction. Furthermore, it was shown that the pore diameter of these materials could be controlled by dendritic polyamidoamine with different generations and functional groups. Meanwhile, the porous framework showed strong thermal stability. Thus, a new environmentally friendly pathway for the controllable synthesis of this fascinating silicas has been proposed.     

Lightweight porous silica ceramics with ultra-low thermal conductivity and enhanced compressive strength

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m^?1 K^?1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint.     

Hierarchically porous monoliths of carbon and metal oxides with ordered mesopores

Hierarchically porous carbon and metal oxide materials offer great benefits in separations, catalysis and renewable energy. We have here used hierarchically porous silica monoliths with ordered mesopores as hard templates to produce nanocast carbon, Co₃O₄, and NiO monoliths with similar structures. Besides providing the materials with more well-defined physicochemical properties, the ordered mesopore structure also offers an excellent model system for investigating the nanocasting process in detail. The mesopores of the silica monoliths were first infiltrated with furfuryl alcohol or metal nitrate precursor solutions, which subsequently could be thermally converted to carbon or the corresponding metal oxides. After the silica scaffolds have been removed by etching in base solutions, the resulting replica monoliths display macroscopic morphology and macropore structure similar to the original silica template. However, while the carbon and Co₃O₄ materials both display a well-organized nanowire structure, giving rise to high surface area and narrow pore size distribution, the NiO monoliths exhibit a significantly lower surface area and less well-defined mesopore structure implying that only part of the silica mesopores has been replicated. We believe this apparent difference between the two metal oxides is a consequence of differences in mass transport.     

Analyses of (AD)-sorption for the estimation of pore-network dimensions and structure

The interpretation of pore dimensions based on physical ad-desorption analyses is central to the characterization of pore network structure. Several approaches have been proposed and are commonly employed in the analysis of physical adsorption and/or desorption to deduce the dimensions of the porous network. These approaches assume either theoretical (e.g., BET, the Halsey equation as interpreted by Pierce et al., or the more recent analyses of microporosity) or standard isotherms as model(s) for the sequential calculations required in estimating the pore network dimensions. Subsequent representation of the pore dimensions and the relationship between these distributions in dimension and other experimental parameters (such as catalytic activity, adsorptivity or transport); thus, depend explicitly on the model employed in the analyses. Each instrument currently available for the measurement of porous solid structure by sorption employs the same specific models for the relationship between the volume ad-desorbed and the dimensions of the porous network that is being characterized.This paper analyzes the interpretation of porous dimensions based on the sequential calculations required in the analyses. A new approach is proposed which is based on a modification to current practices reflecting Halsey's original theory for the thickness of the adsorbed layer (as a function of P/P ₀). Further, the calculations of the incremental changes in the exposed surface area are discussed as they relate to pore network structure. A method is proposed to infer the differences in pore shape. Sorption data are analyzed by these new approaches, and these analyses will be compared with those approaches currently employed. Analyses based on these modified approaches provide a dramatically more consistent interpretation of the sorption data and the corresponding pore network structures.     

Properties of standard materials for reflection

This article gives a short survey of some literature on the calibration, suitability, properties, and use of various materials which are used as standards of reflectance. The classical ultimate reference standard, smoked magnesium oxide, is discussed briefly, and information is given on the properties of materials presently used: BaSO₄, Russian opal glasses, ceramic tiles, and Halon.

1 Halon is a registered trade name of the Allied Chemical Corporation.

The suitability of these materials for use as transfer standards or working standards is discussed.     

Effect of Surface Modification on the Synthesis of Pore‐Filling Polymeric Monoliths in Microfiltration Membranes Made from Poly(propylene) and Poly(ethylene terephthalate)

The effect of pre‐modification on the interaction of macroporous substrates (membranes) with mainly micro‐ and mesoporous polymer monoliths has been studied. Bulk, porous polymer monoliths were synthesized to optimize the synthesis conditions and their pore morphology, and the data were used as benchmark for this study. Pre‐modification of the entire pore surface of PP microfiltration membranes and PET track‐etched membranes by UV‐initiated grafting with PEGMA was performed using well‐established methods, including coating with the photo‐initiator, benzophenone. Subsequently, these membranes were functionalized by filling the pores with porous polymer monoliths from MAA and EDMA and compared with membranes that had been functionalized without the pre‐modification step. The materials were characterized mainly by the degree of grafting, SEM and by the gas‐adsorption‐isotherm method. The DG values, after composite‐membrane preparation under identical conditions, were not influenced by the pre‐modification. However, it could be clearly seen from the SEM images that the pre‐modification step prevents the formation of voids at the monolith‐membrane pore interface. Larger specific surface area and pore volume values for composite membranes, prepared after pre‐modification, fully support the SEM results. Especially large differences in pore structure between the two different composite membranes were found in the mesopore range. The results of this study indicate that it is possible to prepare porous, composite membranes where the trans‐membrane transport is exclusively controlled by the pore and surface structure of a functional polymeric monolith, for example, made from a molecularly‐imprinted polymer.

     

Porous bead aliphatic-aromatic methacrylate copolymers,I. Effect of polymerization conditions on porous structure formation of methyl methacrylate — di(methacryloyloxymethyl)naphthalene copolymers

The influence of inert porogenic mixture and monomer mixture composition and concentration on specific surface areas, pore size distribution, volumes of pores capable of capillary condensation of nitrogen and true density of copolymers were investigated for a new type of methacrylate monomer system: methyl methacrylate and di(methacryloyloxymethyl)naphthalene. The obtained copolymers exhibit similar character as it is typical for PPS

1 PPS = Porous by Precipitant and Solvent.

styrene-divinylbenzene resins.     

Synthesis of superhydrophobic core–shell mesoporous silica nanoparticles

Mesoporous silica nanoparticles (MSNPs) have been used in variety of applications due to their morphology and porous structure. This work reports the one-pot synthesis of ultrahydrophobic MSNPs using N-cetyl-n,n,n trimethyl ammonium bromide as a cationic surfactant template and ethanol (EtOH) as a cosolvent to form mesopores in the MSNPs. The effects of EtOH on the size and the pore structure of the MSNPs were studied by scanning electron microscopy and transmission electron microscopy. The results show that an addition of EtOH led to an enlargement of the MSNPs and a change in pore structure from a lamellar structure to a radially oriented structure. Co-condensation with two different types of fluoroalkyl silanes; trimethyl(fluoromethyl)silane, and trichloro(1H,1H,2H,2H-perfluorooctyl)silane provided low surface energy MSNPs with a core–shell structure. An assembly on the surface of these F-MSNPs generated nanostructure surface roughness rendering an improvement in surface wettability with water contact angle of 158.6°, which is a characteristic of oleophobic and ultrahydrophobic material.     

Calixarene Derivatives as Novel Nanopore Generators for Templates of Nanoporous Thin Films

Summary: We report herein calixarene derivatives, which could adapt to various fields of application, as novel pore generators for making nanoporous materials. The pore structure of nanoporous materials exhibits disordered pores with small mesopore diameter (2–3 nm), which is similar to the micelle‐like assembled structure of the calixarene compounds. The electro‐optical properties such as dielectric constants and refractive indexes of these porous thin films can easily be manipulated. The calixarene‐templated nanoporous films could find a variety of potential applications, such as low‐dielectric constant (k) materials and high‐surface area materials for catalysis and biotechnology.

PM3‐optimized structures of CA[4] and CA[6].     

Pore structure modification of silica matrix infiltrated with paraffin as phase change material

Temperature-controlling solid–liquid phase change material (PCM) infiltrated in a porous matrix is becoming attractive for aerospace applications. The properties of the pore structure are crucial factors in the selection of porous materials for PCM infiltration. The main purpose of this study is to adjust the pore structure of porous silica matrices with different molar ratios of ethanol (EtOH) and tetraethoxysilane (TEOS) for PCM infiltration. Five compositional ratios of EtOH/TEOS were introduced to prepare the silica with different pore size through sol–gel processing, and the pore structures were analyzed by N₂ adsorption–desorption measurements and scanning electron microscopy. Results indicate that the pore size increases with a larger value of the EtOH/TEOS molar ratio. Open cell pore structure and pore size of silica were observed and calculated. For EtOH/TEOS ratios of 10 and 20, the synthesized silica matrices had average pore sizes of 53.1 nm and 56.0 nm, respectively, exhibiting better infiltration. Moreover, the maximum mass fraction of paraffin as PCM in the silica matrices reached up to 75 wt%.     

Preparation and Structure of Porous Silk Sericin Materials

Summary: Porous materials of Bombyx mori silk sericin were prepared by freeze-drying. The effects of concentration of the sericin solution and that of freezing temperature on the structure and properties of materials were investigated. The results indicated that porous sericin materials, with average pore radius between 60 and 90 μm and porosity between 75 and 85%, could be prepared by freeze-drying. Thicker solution and lower freezing temperature resulted in smaller pore radius and porosity and moreover, bigger pore density. The condensed structure of interior sericin was mainly amorphous, and contained some crystal structure. Compared to the non-crosslinked porous sericin materials, the content of the β-sheet structure of the sericin materials, cross-linked by poly(ethylene glycol) diglycidyl ether, increased to some extent.

Cross section SEM photograph of the porous sericin material.     

Preparation of porous material from talc by mechanochemical treatment and subsequent leaching

Synthesis of porous silica via mechanochemical treatment of talc and subsequent acid leaching was investigated by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and N₂ adsorption techniques. Raw talc was ground for different times and then leached with 4 M hydrochloric acid (HCl) at 80 °C. Grinding for 6 h and subsequent leaching for 2 h of raw talc produced the porous silica with a specific surface area of 133 m²/g and total pore volume of 0.22 ml/g. The increase in specific surface area (S_BET) of the porous silica reflected the formation of micropores of 1.21.8 nm and mesopores of 4.05.5 nm in diameter inside the porous structure. The number of micropores decreased with prolonged leaching time, which can be attributed to a condensation reaction. The characteristic of hysteresis loop indicated mainly slit-shaped pores. The apparent activation energy for the leaching process was calculated to be about 21.6 kJ/mol, indicating that the kinetic process of talc leaching was the diffusion-controlled reaction. Mechanochemical treatment may promote the amorphization of talc, being greatly favorable to the subsequent leaching.     

Alumina trihydrate as a flame retardant filler for flexible vinyl compounds

In those flexible vinyl compounds which can accept fillers, alumina trihydrate can significantly improve flame resistance as measured by oxygen index

1 It should be appreciated that degrees of flammability as measured by small-scale tests such as oxygen index may in no way reflect the relative hazard of these same materials when involved in a fully developed fire.

. Over a range of plasticizer concentrations, the lower the concentration of burnable plasticizer, the greater the contribution of hydrate to flame retardancy. Combinations of alumina trihydrate with antimony oxide are especially effective for improving the flame resistance of flexible vinyl compounds.     

Structuring of Macroporous Silicon for Applications as Photonic Crystals

Uniformity and high refractive index contrast make macroporous silicon an ideal two-dimensional photonic crystal, that can be tailored over a wide range of frequencies. For optical transmission measurements the porous silicon has to be structured further. Light has to be coupled in perpendicular to the pore axis and to traverse a well defined number of pore layers. For this purpose a lateral structuring technique has been developed that allows to remove the porous silicon with a precision of less than one pore lattice constant. Bars of macroporous silicon which are 100 m high, 2–200 m wide and several mm long have been prepared. These bars have been aligned with designed defect structures like linear or bent waveguides in the porous silicon. The achieved samples are well suited to investigate the optical properties of these defects with light traveling perpendicular to the pore axis.     

Studies of the entrapment of non-wetting fluid within nanoporous media using a synergistic combination of MRI and micro-computed X-ray tomography

Three-dimensional magnetic resonance imaging (MRI) and micro-computed X-ray tomography (micro-CXT) have been combined to study the entrapment of mercury within nanoporous silica materials following porosimetry. MR images have been used to construct structural models of particular porous media within which several simulations of mercury intrusion and retraction have been performed with variations in the mechanism for the ‘snap-off’ of the mercury menisci. The simulations gave rise to different predictions for the pattern of the macroscopic spatial distribution of entrapped mercury, depending on ‘snap-off’ mechanism, which were then compared with corresponding experimental data obtained from micro-CXT images of real pellets containing entrapped mercury. The information obtained from the micro-CXT images, and also from the porosimetry curves themselves, was then used to constrain a model for the microscopic mercury retraction mechanism. Additional predictions of the retraction model were then subsequently confirmed using scanning loop experiments. The simulations showed that the overall level of entrapment of mercury was determined by the close interaction between the pellet macroscopic structure (particularly pore size spatial correlation), and the microscopic mercury retraction mechanism. Hence, it was subsequently possible to explain fully why high mercury entrapment occurred within one particular type of sol-gel silica material, while only low entrapment occurred in another batch of superficially similar material.