In situ formation of nanocomposites based on polyethylene and silica nanospheres

Polyethylene nanocomposites containing silica nanospheres, synthesized by the sol–gel method, were produced via in situ polymerization. The silica nanospheres were added together with the catalytic system [metallocene catalyst and methylaluminoxane (MAO) as cocatalyst] directly to the reactor and used for the polymerization of ethylene. The polymerization activity increased slightly in the presence of 1 wt % silica nanospheres in comparison to the homogeneous polymerization sans filler. The Young's modulus of the nanocomposites increased 19–25% without a significant decrease in the elongation at break with respect to the neat polyethylene. The polymer particle morphology was also significantly improved with the incorporation of silica nanospheres. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Formation,morphology and control of high-performance biomedical polyurethane porous membranes by water micro-droplet induced phase inversion

Biomedical polyurethane (BPU) porous membranes with controlled morphology and excellent permeability and mechanical properties were prepared via a method involving a phase inversion induced by water micro-droplets, which were generated by an ultrasonic atomizer. The cross-section morphology, air permeability and mechanical properties of the porous membranes were investigated. The SEM images demonstrated that the adjacent pores were connected by a micro-hole, serving as a “backdoor” for the pore. An interconnected porous structure was obtained, improving the air permeability of the BPU membrane relative to the membrane produced by immersion precipitation. Our studies indicated that the diameter of the pores in the membrane depended on the solution viscosity, allowing porous membranes with a desired morphology to be obtained by adjusting the polymer concentration and solution viscosity. The application of micro-droplets of water during membrane preparation reduced the exchange rate between the solvent and nonsolvent, resulting in the microphase separation of polymer molecules and the formation of a uniform porous structure in the membrane, which improved the air permeability and mechanical properties of the BPU porous membranes. This is a simple and effective preparation method for high-performance porous membranes with potential applications in tissue engineering scaffolds, controlled-release drug delivery and vascular grafts.     

溶胶凝胶法多孔炭材料的制备及其表征

以十六烷基三甲基溴化胺(CTAB)稳定过的商业硅溶胶为模板硅源、蔗糖为炭前体、运用溶胶凝胶法制备了多孔炭材料。并采用低温N2等温吸脱附、X射线衍射等对材料的结构进行了测试与表征。结果表明:CTAB的加入使所得的多孔炭孔径分布更加集中,由于炭化温度较低,所得的炭材料仍为无定形结构。     

Flexible bi-continuous mesostructured inorganic/polymer composite membranes

Flexible bi-continuous mesostructured polymer/inorganic composite membranes have been synthesized by undertaking co-assemblies of surfactants and inorganic sol-gel processes inside pores of a preformed porous polymer membrane such as PP porous membrane Celgard^® 2400. The pores are interconnected across the membranes, therefore the continuity of the inorganic mesopores across the membranes is guaranteed in principle. The solvent ethanol used for silica sol is conducive to the synthesis of inorganic materials such as silica within the pores although the PP membrane is hydrophobic. The composite membranes show new properties such as transparency, flexibility, uniform nanosized pores and enhanced permeability.     

The effect of nanospheres on the permeability of PA6/SiO2 nanocomposites

The permeation of O₂, N₂ and H₂O through polyamide‐6 nanocomposites, prepared with silica nanospheres of different diameters, is reported in this contribution. The mechanical and thermal properties were also evaluated. The nanocomposites were produced by melt mixing the polyamide‐6 with silica nanospheres 12–150 nm in diameter, prepared using standard sol‐gel methods; it was found that the addition of 3 wt% of silica nanospheres to the polyamide‐6 matrix results in an increase in the O₂ and N₂ permeability. This increase was inversely correlated with the nanosphere diameter but directly correlated with the number of spheres added to the matrix, using the same percentage by weight. The same behaviour was found for the water vapour transmission rate. The increase in permeability is probably due to the formation of voids around the particles, which increases the free volume of the polymer. The addition of the nanospheres, regardless of the sphere diameter, generated an increase of 6% in the Young's modulus, a decrease of 65% in the elongation at break and a 24% decrease in the percentage of crystallinity. Copyright © 2011 Society of Chemical Industry     

Direct fabrication of mesoporous carbons using in-situ polymerized silica gel networks as a template

Mesoporous carbons were synthesized by in-situ polymerized silica gel networks as a template. The co-condensation of carbon precursor (sucrose) and silica precursor (sodium silicate) followed by heat treatment generated a carbon/silica nanocomposite. After etching the silica template, mesoporous carbons were obtained. Under optimum synthesis conditions a mesoporous carbon with a high surface area of >800 m²/g and a narrow pore size distribution centered at 3 nm was produced. The three-dimensionally interconnected silica structures effectively functioned as the template for the porous carbon materials.     

Growth of high-density parallel arrays of ultralong carbon nanotubes with catalysts pinned by silica nanospheres

High-density parallel arrays of ultralong carbon nanotubes (CNTs) were prepared by utilizing catalyst nanoparticles anchored by silica nanospheres through chemical vapor deposition (CVD). Silica nanospheres and catalyst solution were sequentially spin-coated onto the substrates for the growth of ultralong CNTs, followed by annealing to remove the polymer residues. Catalyst nanoparticles can be anchored on the top of or around the silica nanospheres. Then, ultralong CNTs were synthesized with methane as the carbon source at 1010 °C under ambient pressure. Our results show that the areal density of the ultralong CNTs produced by this nanosphere-assisted process was obviously improved compared with that by the typical process without nanospheres.     

Bicontinuous porosity in ceramics utilizing polymer spinodal phase separation

This paper demonstrates how the combination of inorganic and organic polymers can be used to form bicontinuous porosity in ceramics with pore sizes larger than 5 μm. Spinodal phase separation of pseudo-binary polymer mixtures allows to form larger bicontinuous pore structures than spinodal phase separation of inorganic glasses. Addition of salts allows even more complex compositions of ceramics and glasses to be formed. Here, bioactive glasses are presented that were produced via sol–gel processing of a pseudo-binary mixture of an inorganic and an organic polymer. Due to the addition of an organic polymer to the gelling sol and the spinodal phase separation at a specific equilibrium temperature, both an inorganic polymer ceramic phase and organic polymer-rich phase are formed. The evaporation of the solvent and the burnout of the organic polymer produce a microstructure of interconnected and nearly uniform porosity, which can be controlled by several processing parameters. The dependency of pore size and connectivity is best predicted by polymer phase separation rather than glass melt separation. Results suggest that polymer spinodal phase separation could be useful for the manufacture of a variety of porous ceramics.     

Foaming of PCL/clay nanocomposites with supercritical CO2 mixtures: The effect of nanocomposite fabrication route on the clay dispersion and the final porous structure

Supercritical fluids have been established as alternative foaming agents in various polymers as well as nanocomposite systems. Most recently, supercritical carbon dioxide (scCO₂) has also been used in some studies as a medium of clay dispersion in the polymer matrix providing a solvent-free fabrication route for nanocomposites. In this work, this latter route was followed for the development of porous poly(ɿ-caprolactone) (PCL)/clay nanocomposites after pressure quench. Similarly, PCL/clay nanocomposites were also prepared using the solvent casting and melt blending methods and were then processed with scCO₂ with the batch foaming technique (isothermal pressure quench) to produce their porous counterparts. Poor clay dispersion and non-uniform porous structures were observed when pure CO₂ was used as a dispersion medium for nanocomposite preparation and as a blowing agent, respectively. On the contrary, polymer intercalation and more uniform cell structures were produced when CO₂⿿ethanol mixtures were used as blowing agents.     

Membranes with through-thickness porosity prepared by unidirectional freezing

Directional freezing is a simple and environmentally friendly method for producing aligned porous materials. Porous structures of uniaxially aligned nanoparticles can be produced by unidirectional freezing a concentrated nanoparticle dispersion and subsequent freeze-drying. However, mechanically strong through-thickness membranes have seldom been reported. The film prepared by directional freezing and subsequent freeze-drying is usually too weak to be a free-standing membrane. By using precise control of freezing rate and direction, we successfully produced free-standing films (with 20-200 μm thickness and 40-60 vol% through-thickness porosity) from inorganic particles/polymer (polytetrafluoroethylene and polyvinylidene fluoride) composites. The pore size could be conveniently controlled by freezing rate and dispersion concentration. The use of composite materials, emulsion states, and post-annealing processes facilitated the preparation of free-standing two-dimensional particulate networks due to enhanced interparticulate coherence. This method could provide novel porous networks with controlled morphology, reduced tortuosity, and enhanced mechanical properties, which have broad applications such as separation/purification, fuel cells, nanocomposites, catalysts, tissue engineering, controlled delivery, and other medical applications.     

Preparation and testing of polyvinyl alcohol composite membranes for reverse osmosis

Thin film composite membranes were prepared by coating porous polysulfone membranes with a polyvinyl alcohol layer and further cross-linking its surface. The thin layer of cross-linked polyvinyl alcohol served as a selective membrane. The membranes were prepared under various conditions and tested for sodium chloride separation. A high sodium chloride separation was achieved but the permeation rate was low compared with commercially available thin film composite membranes. Resistance against the flow of solvent water and sodium chloride solute were determined for individual component barrier layers.     

Preparation of macroporous carbons from phase-inversion membranes

Preparation of porous carbons from phase-inversion membranes was investigated as a control method of pore structure in carbon materials. The structure in carbon films was estimated by means of electron microscopy, mercury porosimetry, and gas-adsorption methods. When phase-inversion membranes of Kapton-type polyimide were carbonized, they maintained the film form and gave macroporous carbon films having high porosity. However, micro- and mesopore structures in the carbon films were not influenced by phase inversion in the polymer stage, and, thus, the macroporous carbons had a molecular sieve property similar to that of carbons prepared from nonporous polyimide films. A macroporous structure in cellulose membranes was similarly maintained through the carbonization step, but some of these were fractured or deformed owing to the large shrinkage. Polymer membranes have a capability as porous carbon precursors if they satisfy two requirements: solid-state carbonization and relatively high carbon yield. A composite membrane of a macroporous carbon with a dense carbon having an impervious ability was readily produced by shaping at the precursor stage. © 1995 John Wiley & Sons, Inc.     

Preparation of porous carbon derived from mixtures of furfuryl resin and glycol with controlled pore size distribution

This paper describes the preparation and properties of porous carbon by a technique which consists of mixing a carbon precursor (furfuryl resin and furfural alcohol), a pore-forming agent and a solvent (glycol), polymerizing the resin mixture, and pyrolyzing the hybrid of resin and glycol. The properties of porous carbons have been systematically investigated as a function of composition and heat treatment, with emphasis on understanding and controlling their morphology and pore size distribution. The results seem to indicate that by varying the ratios of the constituents in the polymer system, porous carbons with a wide variation in pore size distribution and morphology can be obtained. Three types of morphologies were observed: interconnected carbon with secondary spherical pores, discrete carbon particulates, and a crosslinked carbon network. Porous carbons with a very narrow pore size distribution have been obtained and the average pore size was controlled between 5 and 0.008 μm. The microstructure of porous carbon formed as a result of phase separation of resin-rich phase and glycol-rich phase, rather than a result of the pyrolysis process. Heat treatment had little effect on the properties of the porous carbons.     

The effect of silica template structure on the pore structure of mesoporous carbons

We have synthesized two kinds of mesoporous carbons using a spherical silica sol (SMC1 carbon) and an elongated silica sol (SMC3 carbon) as templates. Nitrogen isotherms and electrochemical experiments were performed to investigate the effect of the silica template structure on the pore structure of the resulting mesoporous carbons. When carbons produced using the same silica to resorcinol molar ratio were compared, both nitrogen isotherms and electrochemical studies revealed that the SMC3 carbons exhibit simpler pore connectivity than SMC1 carbons.     

A statistical study of poly(ε‐caprolactone) crystallinity in poly(ε‐caprolactone)–silica sol–gel materials and their in vitro calcium phosphate‐forming ability

Composite poly(ε‐caprolactone) (PCL)–silica materials for potential use in orthopaedic applications have been prepared by a sol–gel method using an experimental design approach to investigate the effect of synthesis variables, separately and together, on the physical form of the organic polymer. A combination of differential scanning calorimetry, X‐ray diffraction and Fourier‐transform infrared methods were used to obtain information on the arrangement of the organic polymer in the hybrid material. As our studies investigated the effect of synthesis variables simultaneously, it was possible to establish that the increase of tetraethyl orthosilicate (TEOS)/PCL and HCl/TEOS molar ratios decreased the poly(ε‐caprolactone) crystallinity and provided for a better mixing of the two phases. At a mechanistic level it was possible to show that increase in catalyst content affected the condensation of silicon containing species. In vitro calcium phosphate‐forming ability tests using the static biomimetic method have been carried out on selected PCL–silica sol–gels. In vitro bioactivity was only observed for PCL–silica sol–gel composites with high silica content (30% weight). Changes in catalyst levels had a smaller but still significant effect. Calcium phosphate formation on largely non‐porous surfaces is proposed to occur via the formation of a silica sol–gel layer, and is influenced by the topography and the chemistry of the materials surface. Copyright © 2003 Society of Chemical Industry     

Separation of CO2/CH4 through carbon molecular sieve membranes derived from P84 polyimide

The separation of CO₂/CH₄ separation is industrially important especially for natural gas processing. In the past decades, polymeric membranes separation technology has been widely adopted for CO₂/CH₄ separation. However, polymeric membranes are suffering from plasticization by condensable CO₂ molecules. Thus, carbon molecular sieve membranes (CMSMs) with excellent separation performance and stability appear to be a promising candidate for CO₂/CH₄ separation. A commercially available polyimide, P84 has been chosen as a precursor in preparing carbon membranes for this study. P84 displays a very high selectivity among the polyimides. The carbonization process was carried out at 550–800 °C under vacuum environment. WAXD and density measurements were performed to characterize the morphology of carbon membranes. The permeation properties of single and equimolar binary gas mixture through carbon membranes were measured and analyzed. The highest selectivity was attained by carbon membranes pyrolyzed at 800 °C, where the pyrolysis temperatures significantly affected the permeation properties of carbon membranes. A comparison of permeation properties among carbon membranes derived from four commercially available polyimides showed that the P84 carbon membranes exhibited the highest separation efficiency for CO₂/CH₄ separation. The pure gas measurement underestimated the separation efficiency of carbon membranes, due to the restricted diffusion of non-adsorbable gas by adsorbable component in binary mixture.     

Assembly of methylated hollow silica nanospheres toward humidity-resistant antireflective porous films with ultralow refractive indices

Humidity-resistant antireflective (AR) porous films with ultralow refractive indices were prepared via an one-step sol–gel process by assembly of methylated hollow silica nanospheres (HSNs). The hydrophobicity and refractive indices of silica-based AR porous films could be varied between 66.0° and 115.0°, and between 1.08 and 1.14, respectively, by tuning the MTES/TEOS molar ratios. A possible assembly mechanism was proposed according to the microstructure of methylated HSNs and porous films. The various properties of silica-based AR porous films were investigated and the AR porous film could demonstrate a good AR capability of 97.6% and excellent humidity resistance.     

Effects of silica sol content on the properties of poly(acrylamide)/silica composite hydrogel

Silica sol was first used as an inorganic component to form poly(acrylamide) (PAM)/silica composite hydrogels via in situ free-radical polymerization. The gelation reaction of the composite materials was monitored on a rheometer, indicating that the gelling induction time becomes longer with the increasing content of silica sol. Compression strength and elastic modulus of the composite hydrogels were significantly improved by adding silica sol compared with pure PAM hydrogels. Silica particles formed by silica sol were dispersed on the surface of PAM polymer network in nanosize, promoting high degree of attachment to the polymer chain and enhancing the interfacial interaction between these two components. TGA analysis showed that the silica stiffens the hydrogel network by creating additional physical cross-linking point, but the breakable nature of the bonds and a broad distribution of distances between crosslinks may creates most likely a wide dissipative zone.     

Structuring adsorbents and catalysts by processing of porous powders

Microporous materials such as zeolites, metal organic frameworks, activated carbons and aluminum phosphates are suitable for catalysis and separation applications. These high surface area materials are invariably produced in particulate forms and need to be transformed into hierarchically porous structures for high performance adsorbents or catalysts. Structuring of porous powders enables an optimized structure with high mass transfer, low pressure drop, good heat management, and high mechanical and chemical stability. The requirements and important properties of hierarchically porous structures are reviewed with a focus on applications in gas separation and catalysis. Versatile powder processing routes to process porous powders into hierarchically porous structures like extrusion, coatings of scaffolds and honeycombs, colloidal processing and direct casting, and sacrificial approaches are presented and discussed. The use and limitations of the use of inorganic binders for increasing the mechanical strength is reviewed, and the most important binder systems, e.g. clays and silica, are described in detail. Recent advances to produce binder-free and complex shaped hierarchically porous monoliths are described and their performance is compared with traditional binder-containing structured adsorbents. Needs related to better thermal management and improved kinetics and volume efficiency are discussed and an outlook on future research is also given.     

Synthesis of porous carbon and silica spheres using PEO-PF polymer blends

In this paper, we performed a physical mixture of PEO and PF polymers (i.e. a polymer blend) as an organic template for synthesizing PF-PEO-silica homogeneous composites in a dilute silicate solution at pH = 4.0–5.0. The PF-PEO-silica composites exhibit spherical morphology, in micrometer dimension, and the sphere size is dependent on the pH value of the solution. After undergoing calcination to remove the organic part of the PF-PEO-silica composites with and without the hydrothermal treatment, porous silica spheres of different pore sizes were obtained. Due to the existence of the carbonizing PF polymer in the PF-PEO-silica composite, porous carbon spheres can be conveniently obtained from pyrolysis of the PF-PEO-silica composites under a N₂ atmosphere and HF-etching procedures. TEM images demonstrate that the mesostructures of the mesoporous silica and porous carbons are disordered.