Preparation of composite membranes by means of plasma polymerization of tiophene

The structure and the charge transport properties of poly(ethylene terephthalate) track membranes modified in a thiophene plasma were studied. It was found that polymer deposition on the surface of a track membrane via the plasma polymerization of thiophene results in composite membranes that, in the case of the formation of a semipermeable layer, exhibit conductivity asymmetry—rectifying effect—in electrolyte solutions. It was shown that chemical doping with iodine or photo-oxidation of the polymer layer produced in plasma leads to alteration in the electrochemical properties of plasma-modified membranes.     

Structure and electrochemical properties of polymeric composite membranes with a selective layer

The structure and electrochemical properties of polyethylene terephthalate track-etched membranes were studied. The membranes were covered with a solution of a styrene-butylmethacrylate copolymer on one side. The formation of a selective layer of copolymer in the pores of the starting membranes led to composite membranes characterized by asymmetric conductivity in electrolyte solutions—a rectification effect similar to the p-n transition in semiconductors. The asymmetry resulted from a considerable decrease in the pore diameter in the deposited copolymer layer, changing the pore geometry, and was also due to the existence of an interface in the pores between the starting membrane and the copolymer layer, having different levels of hydrophilicity.     

Radiofrequency-discharge plasma treatment of heavy ion-irradiated poly(ethylene terephthalate) films

The effects of treatment in a radiofrequency (RF) discharge plasma on the rate of chemical etching of the tracks made by xenon ions (with an energy of ~1 MeV/nucleon) in poly(ethylene terephthalate) (PETP) films were investigated. The influence of plasma treatment conditions on the structure and properties of nuclear track membranes formed by etching was studied. It was found that the RF plasma treatment of heavy ion-bombarded PETP films leads to a decrease in etchability of both tracks and the starting polymer matrix. The changes in track and matrix etchability due to crosslinking of the polymer surface layer were shown to be responsible for the asymmetry of the track membrane structure.     

Structure of Conducting Polymer Films Prepared on the Surface and in the Pores of Poly(ethylene terephthalate) Track Membranes by Template Synthesis

The results of a structural study of conducting polymer coatings deposited onto poly(ethylene terephthalate) (PET) track membranes by template synthesis are reported. The following aspects of the quality of polymer coatings were studied: the ratio between film and granular polymers, the polymer distribution over the surface of track membranes, and the thickness of polymer layers on the opposite sides of track membranes. The fraction of granular polypyrrole (PPy) on the surface and in the pores of a film increased with pore diameter. A decrease in the polymerization temperature decreased the amount of granular PPy on the surface of membranes, whereas the effect of granular PPy on the water permeability of track membranes remained unchanged. A more homogeneous distribution of PPy over the surface of track membranes can be obtained by density equalization of reacting solutions; however, the fraction of granular PPy on the membrane surface increased in this case. It was found that polymer coatings on the two sides of the surface of a membrane template had different thicknesses. Poly(N-methylpyrrole) completely covered only one side of a track membrane facing a monomer solution.     

Etching of tracks of accelerated heavy ions in poly(ethylene terephthalate) and some physicochemical properties of track membranes

The features of etching of latent tracks of heavy ions in poly(ethylene terephthalate) up to pore formation were investigated. It was found that the etching process included the following stages: (1) electrolyte penetration into the pores due to capillary forces and removal of monomeric radiolysis products; (2) swelling of the cross-linked (due to secondary electrons) polymer areas around the tracks to give the gel layer; (3) removal of the gel layer and the formation of track membranes with a pore radius of 40–50 Å. The radiation-chemical processes in polymers influence the physicochemical properties of the obtained membranes. The microrelief of the membrane surface was studied by atomic-force microscopy.     

Improvement of pore geometry and performances of poly(ethylene terephthalate) track membranes by a protective layer method using plasma-induced graft polymerization of 1H,1H,2H-perfluoro-1-octene monomer

The process of geometrical modification of pores in poly(ethylene terephthalate) track-etched membranes (TM) by use of plasma deposition of a fluorine-containing polymer protective layer on one membrane surface and alkali etching of the other surface has been studied in order to produce membranes with improved performance characteristics. Samples of membranes with conical pores have been obtained which have better filtration efficiency compared with initial TM with cylindric pores. Plasma polymerization of 1H,1H,2H-perfluoro-1-octene at the membrane surface was used to produce the protective layer resistant to alkali solutions. The occurrence of plasma modification and changing of pore geometry have been verified by X-ray photoelectron spectroscopy and scanning electron microscopy studies. The filtration efficiency and selectivity of the modified membranes have been studied.     

Surface and electrochemical properties of polypropylene track membrane modified by plasma of non-polymerizing gases

The surface and electrochemical properties of polypropylene track membrane treated by plasma of nitrogen, air, and oxygen are studied. The effect of the plasma-forming gas composition on the surface morphology is considered. The membrane surface microrelief formed during the gas-discharge etching is found to change. Moreover, the non-polymerizing gas plasma treatment induces oxidation of the membrane surface layer and generates oxygen-containing functional groups, mostly carbonyl and carboxyl. The higher membrane roughness and its hydrophilization is shown to lead to its better wettability. In addition, the presence of polar groups in the membrane surface layer modifies its hydrodynamic and electrochemical properties so that water permeability and conductivity of modified membranes increase.     

Use of poly(ethylene terephthalate) track membranes as template matrices for the preparation of polymeric nanomaterials based on polypyrrole and poly(<Emphasis Type="Italic">N</Emphasis>-methylpyrrole)

The oxidative polymerization of pyrrole and N-methylpyrrole with the use of template synthesis on poly(ethylene terephthalate) (PET) track membranes was studied. By scanning electron microscopy (SEM), it was found that a polymer film was formed on the surface of membranes and on the walls of membrane pores. The rates of polymerization on the surface of membranes and pore walls, as well as the yield and structure of the polymer film, depend on the membrane parameters (pore length and diameter and membrane porosity), the chemical structure of the monomer, the diffusion of polymerized solutions through template pores, and the temperature.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 15–20.Original Russian Text Copyright © 2005 by Dobretsova, Ermolaev, Jitariouk, Milinchuk.     

Electrokinetic study of etching latent tracks of accelerated heavy ions in poly(ethylene terephthalate) and polyimide

The etching of latent tracks and pore formation in track membranes are studied. It is shown that the incorporation of K⁺ and Ba²⁺ ions into alkaline solutions accelerates the etching of poly(ethylene terephthalate) (PET) and latent tracks in it. The etching is accelerated due to a decrease in the negative surface charge of PET surface and the pores in track membranes. Isoelectric points are determined for PET and polyimide track membranes and it is established that they depend on pore diameter. As the pore diameter is enlarged from 30 to 70 nm, the magnitude of the negative surface charge rises seemingly due to the increase in the concentration of carboxyl groups on the pore surface. It is assumed that this effect is due to either a high mobility of carboxyl groups in a gel, which is formed on pore walls as a result of etching latent tracks, or a displacement of the slipping plane caused by a decrease in the thickness of the gel layer.     

Investigation of electrotransport properties of poly(ethylene terephtalate) track membranes modified by plasma of aniline

The pore walls, and one of the surfaces of poly(ethylene terephtalate) track membrane were modified by plasma of aniline, which resulted in formation of the polyelectrolyte transient cationite-anionite layers. It is shown that the membrane produced possesses asymmetry of conductivity, i.e., the effect of the current rectification similar to p-n junction in semiconductors is observed.     

Direct Current Conductometry of Track Membranes

The electrical conductivity measured for a KCl solution in pores of poly(ethylene terephthalate) track membranes has been studied as depending on electrolyte concentration and pore diameter with the use of a direct-current source. The difference between the experimentally determined conductivity and the standard value has been shown to decrease with increasing electrolyte concentration and pore diameter. At the same time, its value is significantly lower than that determined by impedance spectroscopy. This result is related to a decrease in the contribution of a gel layer formed on the pore surface upon coming into the contact with the electrolyte to the electrical resistance of a membrane.     

Proton-conducting polymer membranes for direct methanol fuel cells

Three methods to block the methanol transport through proton-conducting polymer membranes while maintaining the proton conductivity unchanged have been conducted; 1) selective layer formation on the surface of the membrane, 2) prearation of nanoclay composite membrane providing tortuous pathway of methanol, 3) control and fixation of the proton transport channels. The methanol permeability through the membranes decreased significantly at the expense of the small decrease in the proton conductivity. It is thus concluded that both the structure and the fixation of the proton transport channels are crucial in optimizinging proton conducting membranes for direct methanol fuel cells.     

TFC polyamide membranes modified by grafting of hydrophilic polymers: an FT-IR/AFM/TEM study

Surface modification using grafting of a hydrophilic polymer onto the membrane surface is a possible route to improving the fouling properties of polyamide thin-film composite membranes. The structure of nanofiltration (NF) and reverse osmosis (RO) membranes modified using graft polymerization of acrylic (AA) monomers was visualized and analyzed using attenuated total reflection–Fourier transform infrared spectroscopy, atomic force microscopy and transmission electron microscopy. The results show that a layer of AA polymer is indeed formed on the polyamide surface, which could be accompanied by a change of the surface morphology. It was observed that for the NF membranes studied polymerization could also take place inside the pores of the support as a result of penetration of the monomer through the active layer, particularly for high degrees of grafting. It suggests that the modification procedures should be optimized so that the latter effect is minimized.     

Computer Simulations on the Channel Membrane Formation by Nonsolvent Induced Phase Separation

The formation of channel membrane of polystyrene‐block‐poly(4‐vinyl pyridine) block copolymer is studied by computer simulations with the nonsolvent induced phase separation (SNIPS) method. Dissipative particle dynamics is employed to study the microphase separation process and the SNIPS mechanism. Simulation results indicate that polymer concentration has a significant effect on the membrane structure. Channel membranes form in the copolymer concentration range of 44–58%. Block ratio plays an important role in shaping the membrane structure. Solvent exchange rate also affects the degree of microphase separation at each evolution stage of simulation. The time evolution of morphologies shows that the microphase separation processes happen with the following sequences: the polymer self‐assembled and many small pores appear, then they form irregular cavities and cross‐link gradually, finally the channel membrane forms. These results throw light on the formation mechanism of polymer membranes and provide insightful guidance for future membrane design and preparation.     

Electrotransport properties and morphology of MF-4SK membranes after surface modification with polyaniline

The method of template synthesis is used for the surface modification of MF-4SK membranes with polyaniline. The influence of the time of polyaniline synthesis in the surface layer of a perfluorinated MF-4SK membrane on its morphology and electrotransport properties is investigated. It is established that under the synthesis conditions, a gradient distribution of polyaniline develops across the thickness of the membrane, and as a result of this, an anisotropic composite structure is formed. It is shown that the specific electrical conductivity and the electroosmotic and diffusion permeability exhibit an extremal character as functions of the time of polyaniline synthesis. When the orientation of these composite membranes is changed with respect to electrolyte flow, an asymmetry effect in their diffusion characteristics is found. With the application of the bilayer fine porous membrane model, the modified-layer thickness is estimated, and the determining influence of the difference in absolute values of effective fixed-charge volume densities on the development of the asymmetry effect is found.     

Theoretical study of the electrokinetic and electrochemical behaviors of two-layer composite membranes

A theoretical study concerning the effect of structure (porosity, pore radius and layer thickness) and surface characteristics (zeta potential) of two-layer composite membranes on global streaming potential (SP_g), membrane potential (Em_g) and membrane conductivity (λ_g) is presented. To this end, each layer of the composite membrane (composed of a support layer and a filtering layer) was modeled as a bundle of identical capillary tubes with connections between pores of the two layers (the pores in the filtering layer being smaller than those of the support layer). The global parameters SP_g, Em_g and λ_g were calculated by using the theory of thermodynamics of irreversible processes and a space charge model. SP_g, Em_g and λ_g were expressed as a function of the individual parameters of each layer SP⁽ⁱ⁾, Em⁽ⁱ⁾ and λ⁽ⁱ⁾, respectively, the length fraction of the support layer, the porosity and pore radius ratios. It was shown that the electrokinetic (streaming potential and membrane conductivity) and electrochemical (membrane potential) behaviors of such composite membranes vary between that of single layers. For streaming potential, the results indicate that the contribution of the filtering layer to the global streaming potential is very little influenced by zeta potentials of both types of pores. It appears that the individual streaming potential of the filtering layer greatly dominates the global streaming potential. This is due to the fact that the streaming potential of the filtering layer is weighted by the pore radius ratio which is a predominant parameter in determining the global streaming potential. In contrast to the streaming potential, the contribution of the filtering layer to the global membrane potential (Em_g) or membrane conductivity (λ_g) depends more or less on the zeta potentials of both kinds of pores and the corresponding electrokinetic radii as well. As to the membrane potential, the contribution of the filtering layer to Em_g is all the more sensitive to the zeta potentials than the electrokinetic radii are small. The filtering layer greatly dominates the global membrane potential when its pores are narrow (with regard to the Debye length) and strongly charged. For the electrolyte conductivity inside pores, the smaller pores (inside the filtering layer) have an effect all the more dominant on the apparent membrane conductivity than their zeta potential is low and that of larger pores (inside the support layer) is high.     

Formation of composite membranes containing hydrophobic polymer layers by electron-beam sputter deposition

The chemical structure and the contact and morphological properties of composite membranes prepared by electron-beam sputter deposition of a polytetrafluoroethylene layer on the surface of track-etched polypropylene membrane have been studied. It has been found that the application of such layers results in bilayer composite membranes with both the layers having hydrophobic properties. It has been shown that an increase in the thickness of the deposited polytetrafluoroethylene layer leads to development of its roughness, resulting in the formation of a polymer with superhydrophobic properties on the surface of the initial membrane.     

Determining the Zeta Potential of Porous Membranes Using Electrolyte Conductivity inside Pores

The zeta potential is an important and reliable indicator of the surface charge of membranes, and knowledge of it is essential for the design and operation of membrane processes. The zeta potential cannot be measured directly, but must be deduced from experiments by means of a model. The possibility of determining the zeta potential of porous membranes from measurements of the electrolyte conductivity inside pores (lambda(pore)) is investigated in the case of a ceramic microfiltration membrane. To this end, experimental measurements of the electrical resistance in pores are performed with the membrane filled with KCl solutions of various pHs and concentrations. lambda(pore) is deduced from these experiments. The farther the pH is from the isoelectric point and/or the lower the salt concentration is, the higher the ratio of the electrolyte conductivity inside pores to the bulk conductivity is, due to a more important contribution of the surface conduction. Zeta potentials are calculated from lambda(pore) values by means of a space charge model and compared to those calculated from streaming potential measurements. It is found that the isoelectric points are very close and that zeta potential values for both methods are in quite good agreement. The differences observed in zeta potentials could be due to the fact that the space charge model does not consider the surface conductivity in the inner part of the double layer. Measurements of the electrolyte conductivity within the membrane pores are proved to be a well-adapted procedure for the determination of the zeta potential in situations where the contribution of the surface conduction is significant, i.e., for small and charged pores. Copyright 2001 Academic Press.     

Composite ultrafiltration membranes with tunable properties based on a self‐assembling block copolymer/homopolymer system

Composite ultrafiltration membranes were fabricated by coating a thin film of self‐assembling polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) block copolymers and poly(acrylic acid) homopolymers on top of a support membrane. Block copolymers self‐assembled into a nanostructure where the minority component forms cylinders, whereas homopolymers reside in the core of the cylinders. Selective removal of the homopolymers led to the formation of pores. The morphology of the polymer layer was controlled by varying the content of homopolymers or polymer concentration of the coating solution, which led to membranes with different molecular weight cutoffs (MWCOs) and permeabilities. Uniform pores were obtained using low homopolymer contents, whereas high homopolymer contents caused macrophase separation and resulted in large polydisperse pores or craters at the surface. The thickness of the block copolymer film also influenced the structure and performance of the membranes, where a thicker film results in a strong decrease in permeability but a lower MWCO. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1546–1558     

Electrochemistry of capillary systems with narrow pores. I. Overview

In capillary systems with narrow pores the Helmholtz electrochemical double layer located at the pore wall extends over the entire cross section of the pores. It loses its character as the “charge on the wall”. It will be shown that not only the electrokinetic phenomena but also the electrical conductivity and the dialysis potential of membranes with narrow pores can be understood from the same point of view, namely: the electrolyte solution in the pores of a membrane with narrow pores is considered to be an approximately homogeneous solution in contact with immobilised charges located at the pore wall. In this case the electrochemical equations contain the fixed ion concentration as a parameter instead of the ζ potential. This makes it possible to describe quantitatively to a good approximation data on the electroosmosis, the electrical conductivity, the streaming potential and the dialysis potential taken from the literature, as well as results of our own measurements, by using a single membrane constant.