Ceramic membrane characterization via the bubble point technique

Porous membranes having nanometer and smaller pores challenge existing characterization methods. An easy, fast and inexpensive method to determine the maximum pore size and the pore size distribution is offered by the bubble point technique. Several methods based on similar principles have been presented in the literature. Here the original bubble point test and a liquid-liquid variation are considered. Two types of ceramic membranes with very different morphologies have been characterized with these methods in our labs. Results obtained for Anodisc^® flat disc membranes with straight, non-intersecting pores using binary and ternary probing liquid systems are in very good agreement with each other and the manufacturers characteristics. On the other hand, analysis of tubular γ-alumina membranes, i.e., Membralox^®, resulted in two phase flow within the pores, and hence information about the pore size distribution could not be extracted from these tests.     

Visualisation of the fibrillar and pore morphology of cellulosic fibres applying transmission electron microscopy

Applying transmission electron microscopy (TEM) on ultra-thin cross-sections of fibres, the main characteristics of the internal morphology of cotton and the main man-made cellulosic fibres (modal, viscose and lyocell) could be visualised. To obtain an appropriate contrast for TEM, isoprene was polymerised into the swollen fibres after a stepwise solvent exchange from water to acetone. The included polymer is stainable with osmium tetraoxide. Significant differences in distribution of pore sizes and pore arrangements in the cellulosic fibres were seen. Cotton showed very small pores in the bulk of the fibre, but drying cracks and flat pores between the sheets of the secondary wall appear as larger pores. Lyocell contains only nanopores in the bulk of the fibre with a slight gradient in pore density, and a very porous skin layer. In viscose and modal, a very wide pore size distribution from nanometer to micrometer size can be seen.     

Evidence of mechanisms occurring in thermally induced phase separation of polymeric systems

Thermally induced phase separation is a fabrication technique for porous polymeric structures. By means of easy‐to‐tune processing parameters, such as system composition and demixing temperature, a vast latitude of average pore dimensions, pore size distributions, and morphologies can be obtained. The relation between demixing temperature and morphology was demonstrated via cloud point curve measurement and foams fabrication with controlled thermal protocols, for the model system poly‐l ‐lactide–dioxane–water. The morphologies obtained at a temperature lower than cloud point showed a closed‐pore architecture, suggesting a “nucleation‐and‐growth” separation mechanism, which produced larger pores at higher holding times. Conversely, the porous structures attained when holding the sample above the cloud point exhibited open pores with dimensions independent of time, denoting a phase separation occurring during sample freezing. Finally, the influence of the cooling rate on final morphology was investigated, showing a clear correlation with microstructure and pore size. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 979–983     

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.     

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     

High Tg polyimide nanofoams derived from pyromellitic dianhydride and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane

New routes for the synthesis of high T_g thermally stable polymer foams with pore sizes in the nanometer regime have been developed. Foams were prepared by casting well-defined microphase-separated block copolymers comprised of a thermally stable block and a thermally labile material. At properly designed volume fractions the morphology provides a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon thermal treatment, the thermally unstable block undergoes thermolysis generating pores, the size and shape of which are dictated by the initial copolymer morphology. Triblock copolymers comprised of a high T_g, amorphous polyimide matrix with poly(propylene oxide) as the thermally decomposable coblock, were prepared. The copolymer synthesis was conducted through the poly(amic acid) precursor and subsequent cyclodehydration to the polyimide by either thermal or chemical means. Dynamic mechanical analysis confirmed microphase separated morphologies for all copolymers, irrespective of the propylene oxide block lengths investigated. Upon decomposition of the thermally labile coblock, a 9–18% reduction in density was observed, consistent with the generation of a foam which was stable to 400°C. © 1996 John Wiley & Sons, Inc.     

Porous alginate-Ca hydrogels interpenetrated with PNIPAAm networks: Interrelationship between compressive stress and pore morphology

Thermo-sensitive porous hydrogels composed of interpenetrated networks (IPN) of alginate-Ca²⁺ and PNIPAAm have been obtained. The hydrogels were prepared by cross-linking alginate-Na⁺ with Ca²⁺ ions inside PNIPAAm networks. Compressive tests and scanning electron microscopy were used to evaluate gel strength and pore morphology, respectively. IPN hydrogels displayed two distinct pore morphologies under thermal stimuli. Below 30-35 °C, the LCST of PNIPAAm in water, IPN hydrogels were highly porous. The pore size of hydrogel heated above LCST became progressively smaller. Alginate-Ca²⁺ and PNIPAAm hydrogels, used as references, did not present such behaviour, indicating that the porous effect is due to IPN hydrogel. It was verified that higher strength is achieved when the hydrogel presents small pore size and the temperature is increased. It is suggested that at temperatures above LCST, the PNIPAAm chains shrink and pull the alginate-Ca²⁺ networks back. During shrinking, the polymer chains occupy the open spaces (pores from which water is expelled), and therefore, the hydrogel becomes less deformable when subjected to compressive stress. The results presented in this work indicate that the mechanical properties as well as the pore morphologies of these IPN hydrogels can be tailored by thermal stimulus.     

Nanostructures in a binary mixture confined in slit-like pores with walls decorated with tethered polymer brushes in the form of stripes: dissipative particle dynamics study

Using dissipative particle dynamics, we investigate the behavior of a binary mixture, exhibiting demixing in a bulk phase, confined in slit-like pores with walls modified by the stripes of tethered brush of chains. Our main interest is to determine possible morphologies that can be formed inside the pore, depending on the geometrical parameters characterizing the system (the size of the pore and the width of the stripes). In order to describe the observed morphologies we calculate several characteristics, as the density and local temperature profiles, the radii of gyration for the attached polymers, and the minimum polymer-polymer distances in the direction parallel and perpendicular to the pore walls. The summary of our findings is presented as a sketch of the diagram of morphologies.     

热致相分离技术制备聚氨酯多孔膜的条件控制

采用自制的模具 ,利用热致相分离 (TIPS)的原理制备了聚氨酯 (PU)多孔膜 ,并重点研究了聚合物浓度对多孔膜的表面形貌、孔度大小、孔隙率和透湿率的影响 .在不同的聚合物浓度条件下制备的PU多孔膜的共同特征是底面 (与成膜平台接触面 )光滑平整 ,孔洞尺寸较小 ,为纳米级 ;而表面 (与空气接触的自由面 )的形貌结构较为复杂 ,但都有明显的孔洞出现 ,且孔洞的尺度大于底面 ,在微米级以上 .聚氨酯 1,4 二氧六环 (DO)形成的是上临界共溶温度 (UCST)体系 ,在发生相分离后底面与表面粗化时间的不同是导致形貌结构差异的主要原因 .改变冷台温度或调整DO H2 O的比例也会对PU多孔膜的孔度大小和形貌结构产生明显的影响     

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.     

Organic-inorganic hybrid mesoporous silicas: functionalization, pore size, and morphology control

Topological design of mesoporous silica materials, pore architecture, pore size, and morphology are currently major issues in areas such as catalytic conversion of bulky molecules, adsorption, host-guest chemistry, etc. In this sense, we discuss the pore size-controlled mesostructure, framework functionalization, and morphology control of organic-inorganic hybrid mesoporous silicas by which we can improve the applicability of mesoporous materials. First, we explain that the sizes of hexagonal- and cubic-type pores in organic-inorganic hybrid mesoporous silicas are well controlled from 24.3 to 98.0 A by the direct micelle-control method using an organosilica precursor and surfactants with different alkyl chain lengths or triblock copolymers as templates and swelling agents incorporated in the formed micelles. Second, we describe that organic-inorganic hybrid mesoporous materials with various functional groups form various external morphologies such as rod, cauliflower, film, rope, spheroid, monolith, and fiber shapes. Third, we discuss that transition metals (Ti and Ru) and rare-earth ions (Eu(3+) and Tb(3+)) are used to modify organic-inorganic hybrid mesoporous silica materials. Such hybrid mesoporous silica materials are expected to be applied as excellent catalysts for organic reactions, photocatalysis, optical devices, etc.     

Nuclear magnetic resonance study of the vapor contribution to diffusion in silica glasses with micrometer pores partially filled with liquid cyclohexane or water

The contribution of the vapor phase to molecular diffusion in porous silica glass (Vitrapor#5; mean pore diameter 1 micrometer) partially filled with cyclohexane (nonpolar) or water (polar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane increased up to ten times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. A two-phase exchange theory is presented accounting well for all experimental diffusion features. The diffusion behavior in the samples with micrometer pores under investigation here is in contrast to previous findings for the same solvents in a material with nanometer pores (Vycor; mean pore diameter 4 nm) where the fast-exchange limit had to be assumed [Ardelean et al., J. Chem. Phys. 119, 10358 (2003)]. It is concluded that the pore size plays a crucial role for the relevance of molecular exchange limits relative to the experimental diffusion/exchange time.     

Fine-tuning in size and surface morphology of rod-shaped polypyrrole using porous silicon as template

We investigated electropolymerization of polypyrrole into porous silicon template. We used three types of porous silicon templates, i.e., porous silicon with ordered macro pores, medium-sized pores and meso pores. Polypyrrole was electropolymerized from pore bottoms to pore tops in all the porous silicon templates. After removal of the templates we obtained rod-shaped polypyrrole arrays. Changing the porous silicon templates easily controlled the size of the rod-shaped polypyrrole. We also used dendritic medium-sized pores for template, and then polypyrrole rod having a roughened surface was obtained. Here we demonstrated that the size and the surface morphology of polypyrrole rod were easily tuned using porous silicon.     

Controlling optical properties and interchain interactions in semiconducting polymers by encapsulation in periodic nanoporous silicas with different pore sizes

The photophysics of MEH-PPV incorporated into the pores of periodic silica hosts has been investigated in an effort to understand the role played by interchain aggregation and chain morphology in polaron production. In this work, guest/host interactions were used to incorporate MEH-PPV into the straight, homogeneous pores of hexagonal surfactant- or polymer-templated mesoporous silicas of varying pore diameters. Polarized photoluminescence and photoluminescence excitation spectroscopy were then used to investigate the polymers' environment within the silica pores. Experiments exploiting luminescence peak shifts and depolarization indicate that depending on the pore size and preparation conditions, the alignment and packing of the polymer chains within the pores could be controlled. Samples could be produced with isolated chains, interacting straight chains, and coiled interacting chains. The sub-bandgap absorption by polarons was then measured with photoinduced absorption as a function of pore size. Small-diameter pores that allowed single polymer chains to reside within the pore showed little evidence of interchain contact and had a low polaron yield. Increasing the number of polymer chains within the pore increased the polaron yield. Finally, when the pores were large enough that the chains could coil, strong polaron absorption was observed, indicative of a further increase in polaron yield or an increase in polaron lifetime. The polaron absorption spectra also sharpen and red shift with increasing pore diameter, suggesting that excitons may migrate to lower energy polymer segments in samples where polymer chains are both coiled and interacting.     

Structure–property relationships of low dielectric constant,nanoporous, thermally stable polyimides via grafting of poly(propylene glycol) oligomers

Synthesis of high temperature polyimide foams with pore sizes in the nanometer range was developed. Foams were prepared by casting graft copolymers comprising a thermally stable block as the matrix and a thermally labile material as the dispersed phase. The copolyimides as the matrix material were prepared via polycondensation reactions of pyromellitic dianhydride with three new diamines (4BAP, 3BAP, and BAN) through the poly(amic acid) precursors. Functionalized poly(propylene glycol) (PPGBr‐1000 and PPGBr‐2500) as the labile oligomer was prepared via reaction of poly(propylene glycol) monobutyl ether with 2‐bromoacetyl bromide. Graft copolymers were prepared by the reaction of the poly(amic acid)s with these thermally labile constituents. Upon thermal treatment the labile blocks were subsequently removed leaving pores with the size and shape of the original copolymer morphology. The polyimides and foamed polyimides were characterized by some conventional methods including FTIR, H‐NMR, DSC, TGA, SEM, TEM, and dielectric constant. The average pore size of the polyimide nanofoams was in the range of 5–20 nm. The structure–property relationships of the prepared nanofoams were investigated based on the diamine structures and also molecular weights of labile groups. Copyright © 2008 John Wiley & Sons, Ltd.     

Relating pore structure to activity at the subcrystal level for ZSM-5: an electron backscattering diffraction and fluorescence microscopy study

The catalytic performance of microporous materials such as zeolites is determined not only by the active sites' molecular architecture, but also by the organization of the surrounding pores with varying diameter, shape, and directionality. These pores control transport of reagents and products and induce shape selectivity. Rather than being ideal single crystals, zeolites often have complex 3-dimensional morphologies, comprising intergrowths and various defect types. Here, the underlying pore architecture of the hexagonal facet of an individual zeolite ZSM-5 crystal is successfully determined by electron beam scattering diffraction and is correlated with the initial reactivity toward the acid-catalyzed oligomerization of furfuryl alcohol using polarized fluorescence microscopy.     

A method for the determination of accessible surface area,pore volume,pore size and its volume distribution for homogeneous pores of different shapes

We develop a novel method to determine the accessible pore volume, the accessible pore size and its distribution for pores having homogeneous surfaces but taking an arbitrary shape. The accessible pore volume is essentially the volume space that is accessible to the centre of an adsorbate molecule, while the accessible pore size is defined by the largest sphere that can be accommodated in the accessible space. The size of this sphere depends on the point in the accessible volume that we select. The accessible pore size is therefore, a local variable and this means that even a geometrically simple pore can possess many sizes. Each local accessible pore size is associated with a local accessible pore volume and the relationship between this pore volume and pore size is called the accessible pore size distribution. In this paper, we illustrate this methodology with a number of model pores ranging from simple to complex geometry and present the analytical accessible pore size distribution.     

Nanoscale pore morphology and distribution of lacustrine shale reservoirs:Examples from the Upper Triassic Yanchang Formation,Ordos Basin

Pore structure plays an important role in the gas storage and flow capacity of shale gas reservoirs. Fieldemission environmental scanning electron microscopy(FE-SEM) in combination with low-pressure carbon dioxide gas adsorption(CO2GA),nitrogen gas adsorption(N2GA),and high-pressure mercury intrusion(HPMI) were used to study the nanostructure pore morphology and pore-size distributions(PSDs) of lacustrine shale from the Upper Triassic Yanchang Formation,Ordos Basin. Results show that the pores in the shale reservoirs are generally nanoscale and can be classified into four types: organic,interparticle,intraparticle,and microfracture. The interparticle pores between clay particles and organic-matter pores develop most often,l with pore sizes that vary from several to more than 100 nm. Mercury porosimetry analysis shows total porosities ranging between 1.93 and 7.68%,with a mean value of 5.27%. The BET surface areas as determined by N2 adsorption in the nine samples range from 10 to 20 m2/g and the CO2 equivalent surface areas(2 nm)vary from 18 to 71 m2/g. Together,the HPMI,N2 GA,and CO2 GA curves indicate that the pore volumes are mainly due to pores 100 nm in size. In contrast,however,most of the specific surface areas are provided by the micropores. The total organic carbon(TOC) and clay minerals are the primary controls of the structures of nanoscale pores(especially micropores and mesopores). Micropores are predominantly determined by the content of the TOC,and mesopores are possibly related to the content of clay minerals,particularly the illite-montmorillonite mixed-layer content.     

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.     

X-ray diffraction study of pore structure of pyrocarbon coatings obtained at various temperatures

X-ray diffraction (large- and small-angle scattering) has been used in an investigation of the pore structure and crystal structure of pyrocarbon coatings obtained by pyrolysis of a number of gaseous hydrocarbons at temperatures of 1300–2000°C. The coatings have a well-developed pore structure. The pore dimensions cover a range from fractions of a nanometer to hundreds of nano-meters. The pore volume and distribution depend primarily on the temperature of deposition. This relationship is extremal in nature. The volume of small pores (R=0.5-3.0 nm) is the greatest when T_d=1500°C; owing to the closed nature of the pores, this leads to an anomalous decrease in pycnometric density, the minimum density being observed at this same temperature. With T_d > 1500°C, growth of the crystals is observed, correlating with the decrease in volume of small pores.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya; No. 5, pp. 982–987, May, 1990.