Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. II. Comparative studies on the permeation characteristics of membranes prepared from N‐methylmorpholine‐N‐oxide and cuprammonium solutions

Two kinds of regenerated cellulose membranes for hemodialysis were prepared from casting solutions of N‐methylmorpholine‐N‐oxide (NMMO) and cuprammonium (denoted NMMO membranes and cuprammonium membranes, respectively). The concentration of cellulose in the casting solution investigated was 6–8 wt %. The permeation characteristics of both membrane series were compared in terms of the ultrafiltration rate (UFR) of pure water, the sieving coefficient (SC) of dextran, and the solute permeabilities of urea, creatinine, and vitamin B₁₂. The UFR and SC of the NMMO membranes were strongly affected by the cellulose concentration of the casting solution, and NMMO was a preferable solvent for the production of cellulose membranes with high performance; the cuprammonium solution gave low‐performance membranes. The pore structures of both types of membranes were estimated with the Hagen–Poiseuille law. The results showed that the NMMO membranes had larger pore radius and smaller pore numbers than the cuprammonium membranes. The differences in the membrane pore structures led to the differences in the performance between the two membrane series. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 333–339, 2003     

Permeability of carbon refractory materials used in a blast furnace hearth

Permeability of experimental and standard micropore carbon materials, before and after the heat treatment at 1550 °C, was investigated using gas permeability measurements (GP) and mercury intrusion porosimetry (MIP). Permeability of both materials increased after the annealing. Experimental micropore carbon material was less permeable than standard one, both before and after the heat treatment. Measured gas permeabilities were compared with permeabilities determined based on MIP measurements using two different models: Hagen?Poiseuille and Carman?Kozeny equation. Using the inverse problem approach, for the first time Carman?Kozeny constant was determined for micropore carbon materials. Carman?Kozeny model gave more accurate results (i.e. close to gas permeability measurements) than Hagen?Poiseuille model. Performed investigations showed that mercury intrusion porosimetry might be alternative method to investigate infiltration resistance of low permeable materials.     

Polyamide‐imide nanofiltration hollow fiber membranes with elongation‐induced nano‐pore evolution

The molecular design of nanoporous membranes with desired morphology and selectivity has attracted significant interest over the past few decades. A major problem in their applications is the trade‐off between sieving property and permeability. Here, we report the discovery of elongation‐induced nano‐pore evolution during the external stretching of a novel polyamide‐imide nanofiltration hollow fiber membrane in a dry‐jet wet‐spinning process that simultaneously leads to a decreased pore size but increased pure water permeability. The molecular weight cutoff, pore size, and pore size distribution were finely tuned using this approach. AFM and polarized FTIR verified the nano‐pore morphological evolution and an enhanced molecular orientation in the surface skin layer. The resultant nanofiltration membranes exhibit highly effective fractionation of the monovalent and divalent ions of NaCl/Na₂SO₄ binary salt solutions. More than 99.5% glutathione can be rejected by the nanofiltration membranes at neutral pH, offering the feasibility of recovering this tripeptide. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Hindered diffusion of flexible polymers through polyimide ultrafiltration membranes

The hindered diffusion of polystyrene in dilute solutions of ethyl acetate through polyimide ultrafiltration membranes has been investigated. The present system did not show specific membrane‐solute interactions; furthermore, polystyrene can be considered as a flexible polymer coil. It is shown that the hindered diffusive permeability for monodisperse dilute solutions for a series of molecular weights can be compared well with the diffusive permeability curve of one polydisperse dilute polystyrene solution. In the case of very dilute solutions, the polymer coils have no interaction with each other, and the whole range of molecular‐weight‐dependent permeabilities can be determined from only one measurement. The diffusion behavior of polydisperse solutions through various polyimide membranes has been investigated as well. It was found that the diffusive permeability curve is strongly dependent on the type of membrane, that is, on the pore size distribution. It was not possible to calculate a pore size distribution from diffusion experiments due to mathematical limitations. Nevertheless, it was shown that hindered diffusion measurements are useful to estimate a maximum pore size for each membrane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1180–1193, 2000     

pH‐responsive permeability of PE‐g‐PMAA membranes

Poly(methacrylic acid) (PMAA) grafted porous PE membranes (PE‐g‐PMAA) were studied. It was found that (1) a wide range of graft yields can be achieved by varying irradiation time (20–240 min) and monomer concentration (0.22M–0.66M), (2) the grafted membrane exhibits reversible permeability response, (3) the membrane shows a maximum permeability response at an intermediate permeant molecular weight due to size exclusion effect, and (4) depending on the graft yield, two types of permeability response can be obtained. These observations are consistent with our earlier study on poly(N‐isopropylacrylamide) (PNIPAAm)–grafted porous polyethylene membranes. In addition, it was observed that the solvent used during grafting may influence the graft location—presumably due to variations in pore wetting. Specifically, compared to water solvent, methanol can increase grafting inside membrane pores, an observation inferred from membrane swelling, thickness measurement, and SEM characterization. Moreover, preferential grafting inside the membrane pores, as affected by increasing methanol content in the grafting solvent, results in lower membrane permeability and a greater pore graft‐controlled type of permeability response. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 778–786, 2000     

Preparation of thermal‐responsive poly(propylene) membranes grafted with n‐isopropylacrylamide by plasma‐induced polymerization and their water permeation

Poly(propylene) (PP) membrane grafted with poly(N‐isopropylacrylamide) (PNIPAAm), which is known to have a lower critical solution temperature (LCST) at around 32°C, was prepared by the plasma‐induced graft polymerization technique. Graft polymerization of PNIPAAm onto a PP membrane was confirmed by microscopic attenuated total reflection/Fourier transform IR spectroscopy. The grafting yield of PNIPAAm increased with the concentration of N‐isopropylacrylamide monomer and the reaction time of graft polymerization. The average pore size of the PP membrane also affected the grafting yield. From the field emission scanning electron microscopy (FE‐SEM) measurement, we observed a morphological change in the PP‐g‐PNIPAAm membrane under wet conditions at 25°C below LCST. The permeability of water through the PP‐g‐PNIPAAm membrane was controlled by temperature. The PP‐g‐PNIPAAm membrane (PN05 and PN10) exhibited higher water permeability (L_p) than the original PP substrate membrane below LCST. As the temperature increased to above LCST, L_p gradually decreased. In addition, the graft yield of PNIPAAm and the average pore size of the PP substrate influenced water permeability. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1168–1177, 2002; DOI 10.1002/app.10410     

Wall friction and effective viscosity of a homogeneous dispersed liquid–liquid flow in a horizontal pipe

Homogenous oil in water dispersion has been investigated in a horizontal pipe. The mean droplet size is 25 μm. Experiments were carried out in a 7.5‐m‐long transparent pipe of 50‐mm internal diameter. The wall friction has been measured and modeled for a wide range of flow parameters, mixture velocities ranging from 0.28 to 1.2 m/s, and dispersed phase volume fractions up to 0.6, including turbulent, intermediate, and laminar regimes. Flow regimes have been identified from velocity profiles measured by particle image velocimetry in a matched refractive index medium. It is shown that the concept of effective viscosity is relevant to scale the friction at the wall of the dispersed flow. Based on mixture properties, the friction factor follows the Hagen‐Poiseuille and the Blasius' law in laminar and turbulent regimes, respectively. Interestingly, the transition toward turbulence is delayed as the dispersed phase fraction is increased. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Synthesis and characterization of dense and porous cellulose films

Whereas cellulose‐derived polymers are routinely used as membrane materials, the cellulose polymer itself is not directly used to synthesize dense/porous films for membrane applications. Recently, N‐methylmorpholine N‐oxide (NMMO) and dimethylacetamide (DMAc)/lithium chloride (LiCl) have been successfully employed for dissolving unmodified cellulose. This provides a strong rationale for reexamining the possibility of cellulose membrane fabrication using these solvents. By judiciously selecting solvents, casting conditions, and solvent exchange steps, we successfully synthesized dense/asymmetric‐porous cellulose films. The pore size and porosity of the porous films decreased systematically with increasing cellulose concentration. SEM analysis of the cross sections revealed an asymmetric skinned structure with monotonically increasing pore size away from the skin. The measured pore diameters were in the range 1.8–4.8 μm. Mechanical testing indicated that the dense films possessed tensile properties comparable to those of cellulose acetate (CA) films. Though nitrogen permeability values were comparable for cellulose and CA dense films, cellulose film permeability depended upon the type of drying protocol employed. Overall, these results demonstrate that processability need not be a constraint in the use of cellulose polymer for membrane fabrication. In selected applications, cellulose membranes could become a cost‐effective, environmentally friendly alternative to other more commonly employed membrane polymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

A new method to quantify parameters of membrane morphology from electron microscopy micrographs by texture recognition

A new method has been developed in order to automatically quantify parameters of membrane morphology from micrographs obtained through microscopy techniques. The parameters estimated by this algorithm are: pore size distribution, porosity, pore symmetry, regularity and tortuosity, as well as various statistical measures. These properties determine the performance of a membrane.The proposed method is based on texture recognition. It first identifies the pores present in the membrane from a cross-section micrograph of it, then labels them and finally makes the corresponding measurements. The main difference and advantage of this technique with respect to previous proposals is that the algorithm does not perform generic particle recognition, but direct scanning of typical pore structures and no user decisions are needed in all the steps of the process. Additionally, the proposed technique does not only determine typical parameters, such as pore size, but also particular characteristics of membrane topology, such as symmetry.The source information consists of cross-section membrane micrographs that can be typically obtained from electron microscopy (scanning or transmission), as well as from other types of microscopy, which are the most common acquisition techniques used by membranologists. The system provides quantitative, systematic and fast results, which represents a significant advance in the field of membrane analysis.     

膜蒸馏跨膜传质过程的新模型--TPKPT

以纯水为介质,用直接接触式膜蒸馏测定了材料或性能参数不同的三种孔疏水膜在不同温度下的渗透通量。根据测量结果计算出了各种膜在不同温度下的渗透系数,发现渗透系数均随着温度的升高而升高;这一结果说明Poiseuille流动在跨膜传质中起着非常重要的作用。据此提出了Knudsen扩散－Poiseuille流动两参数跨膜传质模型,即TPKPT模型。用此模型拟合实验数据,得到了三种实验用膜的模型参数。用这些模型参数计算膜在不同温度下的渗透系数,其值与实验测量值吻合较好,说明TPKPT模型能较好地描述膜蒸馏的跨膜传质过程。     

A new model for mass transfer in direct contact membrane distillation

The mass transfer process in direct contact membrane distillation (DCMD) for three kinds of membranes was measured. Water fluxes at different temperatures and the membrane distillation coefficients (MDC) for each membrane were obtained directly from experimental data. The fact that the MDC values of membranes with larger pore size increase with temperature indicates that Poiseuille flow plays an important role in the process of mass transfer through the membrane. Based on this conclusion, a three-parameter model, named the Knudsen diffusion-molecular diffusion-Poiseuille flow transition (KMPT) model, was developed to predict MDC and water flux for membrane distillation. The parameters of the KMPT model for each membrane employed in this study, by which MDC at various temperatures can be determined, were evaluated by a nonlinear regression. The values of MDC and water fluxes for each membrane predicted by KMPT model agree well with that obtained directly from the experiment results. A large contribution of Poiseuille flow to mass transfer was observed and can be attributed to the distribution of large pores in the membranes. The KMPT model also provides a method for estimation of the effect pore size using the ratio of the MDCs; the ratio of the Poiseuille flow to molecular diffusion MDC provides the best estimation.     

Pore‐size evaluation and gas transport behaviors of microporous membranes: An experimental and theoretical study

A modified gas‐translation (GT) model based on a GT mechanism was successfully applied to the pore‐size evaluation and gas transport behavior analysis of microporous membranes with different pore‐size distributions. Based on the gas permeation results of three microporous membranes derived from different alkoxides, the effects of activation energy and the selection of a standard gas on the pore‐size evaluation were discussed in a comparative study. The presence of nano‐sized defects had an important influence on the gas permeation performance of microporous membranes, depending largely on the original pore size of the membrane in question. Moreover, the gas‐separation effect of the pore‐size distribution in a silica membrane was theoretically studied and revealed a significant increase in gas permeance for relatively large gas species but not for small ones. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2268–2279, 2015     

Effect of the exposure time on the structure and performance of hydrophobic polydimethylsiloxane–poly(vinylidene fluoride) membranes via a non‐solvent‐induced phase separation process in a clean room

The objective of this study was to investigate the effects of the exposure time on the properties and permeability of polydimethylsiloxane (PDMS)–poly(vinylidene fluoride) (PVDF) blend hydrophobic microporous membranes, which were fabricated via a non‐solvent‐induced phase separation process at 25 °C and 60% relative humidity in a clean‐room circumstance. For the prepared PDMS–PVDF membranes, the membrane morphologies were observed by scanning electron microscopy. Crystalline structures were observed by X‐ray diffraction. Pore structures were analyzed by membrane porosity and mean pore size. Hydrophobicity was measured by contact angle measurement, and the mechanical properties were characterized by tensile strength testing. Our study results show that with increasing exposure time from 10 to 110 s, all of the membranes showed a similar pore structure: a spongelike substrate layer with a thin realm of fingerlike structures under the top surface. Phase separation between PDMS and PVDF occurred. The membrane porosity and mean pore radius decreased, and the membrane thickness increased. The membrane hydrophobicity decreased, and the mechanical properties first increased and then decreased. In addition, vacuum membrane distillation experiments were conducted. With the increase in the exposure time from 10 to 110 s, the membrane permeate flux decreased from 16.54 to 6.65 kg m⁻²·h⁻¹, and the salt rejection was higher than 99.9%. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43842.     

Numerical calculations of the thermal conductivity of porous ceramics based on micrographs

The overall thermal conductivity of a porous material is strongly sensitive to the volume fraction and spatial distribution of the pores. For this second aspect analytical models predicting thermal conductivity as a function of pore volume fraction are obliged to make a simplifying assumption concerning the pore shape. In order to describe the effects of the microstructure on heat transfer in greater detail, we have developed a method involving 2D finite element calculations based on real micrographs of the porous solid. The approach was tested on micrographs of tin oxide samples with pore contents from 10% to 50%. Quantitative results obtained for pore contents up to 20% give very good agreement to Rayleigh's model. Higher pore contents lead to a number of difficulties but the qualitative results are used to support the choice of Landauer's effective medium expression as an appropriate general analytical model for the thermal conductivity of a porous ceramic material.     

Modelling relationships between permeability and cement paste pore microstructures

Pore size distribution and the interconnectivity of cement paste pores have a strong influence on permeability. A computer network model has been developed to simulate flow through the pore microstructure cement paste. Network models have been successfully applied to predicting permeabilities of geological materials including rocks and soils but have not been specifically developed in the area of cement and concrete. The network model presented in this paper is used to obtain further insight into how pore size distributions of cement paste microstructures influences permeability. The model accounts for variations in pore microstructure by using a two-dimensional network of tubes with different log-normal size distributions.     

STRUCTURAL MODELS RELATED TO TRANSPORT PROPERTIES FOR THE DRIED LAYER OF FOOD MATERIALS UNDERGOING FREEZE-DRYING

The values of thermal conductivity and permeability have been presented for the dried layer of raw beef, coffee solutions and sliced as well as mashed apples. A structural model for beef, coffee solution and mashed apple was developed for predicting the permeability of water-vapor flowing through the dried layer. In modeling the porous dried layer was assumed to be a bundle of capillary tubes with the pore space having an equivalent pore radius, porosity, and tortuosity factor. These results indicated that the structure for the solution systems was controllable by both solute concentration and freezing manners.

Another cellular structural model has been presented for predicting the permeability of a sliced apple. The resistance of a cell membrane to the molecular transfer of water vapor was determined from the value of permeability and the size as well as number of the cells existed in the dried layer. Since the drying rate of cellular materials was limited by the mass flux across the dried layer, the model was considered to play an important role in predicting the optimum heating program for the surface temperature of cellular food materials such as fresh fruits and vegetables.     

Fluid inhomogeneity within nanoslits and deviation from Hagen–Poiseuille flow

Recently, the deviation of nano‐confined flows from classical hydrodynamic theories has been frequently reported. In this work, such a flow is theoretically investigated by means of dissipative particle dynamics (DPD) simulation. The simulation results show that the density and viscosity inhomogeneities near solid/fluid interfaces depends on the slit wettability only. Flow enhancement relative to the Hagen–Poiseuille (H‐P) flow occurs together with the flow inhomogeneity. Combining of flow inhomogeneity and the Stokes equation, a theoretical model for flux calculation is established. As the slit being widened, the model can be simplified by gradually eliminating the higher order traces, and be simplified into the model with Navier's slip condition and the well‐known H‐P relation at last. The theoretical results of flux are in good agreement with the simulations by DPD. © 2016 American Institute of Chemical Engineers AIChE J, 63: 834–842, 2017     

Membranes as fractals: Implication and consequences

The fractal nature of synthetic membranes suggests a new approach to implement pore size distributions in membrane characterization. The fractal definition of membrane morphology implies that the pore size distribution of a synthetic membrane can be described by a set of three parameters such as the minimum and maximum pore radius and the fractal dimension D. The effect of such a pore size distribution on solute sieving curves has been determined parametrically for the permeation of dilute aqueous solutions of polyethylene glycols through ultrafiltration membranes. These results have been compared to sieving curves obtained from the normal and log-normal pore size distributions which represent the conventional approach in defining the porosity of synthetic membranes.     

Interfacial formation of porous membranes with poly(ethylene glycol) in a microfluidic environment

In a microfluidic environment, the liquid–liquid interface, formed by laminar flows of immiscible solutions, can be used to generate thin membranes via interfacial polymerization. Because these thin nylon membranes have a very small pore size or lack porosity entirely, their utilization in some biological applications is greatly limited. We introduce an in situ fabrication method using the interfacial reaction of a two‐phase system to generate a porous nylon membrane. The membranes were characterized with scanning electron microscopy and fluorescent beads. Scanning electron microscopy micrographs verified the asymmetrical structure of the porous membrane, and the membrane pore sizes ranged from 0.1 to 1 μm. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

Comparative study of hydrophilic modification of polyacrylonitrile membranes by nitrogen and carbon dioxide RF plasma

Low temperature plasma treatment using radio frequency (RF) discharge of nitrogen and carbon dioxide gases was employed to enhance hydrophilicity of the polyacrylonitrile copolymer membrane surface. Influence of various plasma operating conditions, namely, power and exposure time on improvement of surface energy, permeability, and hydrophilicity of the membrane was investigated. Surface energy of RF nitrogen plasma‐treated membrane (70 W, 8 min) was enhanced by 70%. Surface etching due to plasma treatment was confirmed by weight loss of the treated membranes. About 78% increase in average pore size was obtained using RF carbon dioxide plasma treatment due to surface etching. Hydrophilicity of nitrogen plasma modified membrane was enhanced by 32% and it was maintained up to 100 days. The pore enlargement due to plasma etching is more effective compared to surface energy in enhancing permeability (70%) of RF carbon dioxide modified (70 W, 6 min) membrane throughout the aging period. The permeability of nitrogen RF plasma‐treated membrane is affected by surface energy and pore enlargement for initial 20 days of aging. After that, the permeability of treated PAN only depends on pore enlargement due to plasma etching. The nitrogen plasma modified surfaces appear to retain their functionality better than carbon dioxide plasma‐treated samples. Oxygen and nitrogen functional groups were identified to be responsible for surface hydrophilicity. POLYM. ENG. SCI., 59:2148–2158, 2019. © 2019 Society of Plastics Engineers