In the past two decades, synthesis of responsive membranes with pores that could be opened and closed by changing chemical or physical properties of their environment has been the subject of many publications (see Ulbricht's work for a recent review). [1][2][3][4][5][6][7][8] In most studies the variable pore permeability was attained by the surface modification of commercial microfiltration membranes using polymers that expand or contract in response to external stimuli. Radiation-and plasma-induced graft polymerizations were employed to immobilize a monolayer of surface-attached responsive polymer chains (brushlike layers) or crosslinked polymer networks (gels) on the membrane and/or pore surfaces. Stimuli-induced changes of the conformation of the grafted-polymer chains affected the permeability of nanometer-sized pores in the membranes. Responsive gels were used to fill the interior of larger submicrometer/micrometer pores and regulate the membrane permeability. Membranes sensitive to changes in temperature, pH, ionic strength, light intensity, reduction-oxidation state of functional groups, and concentration of various substances have thus been fabricated based on the above-mentioned principles. The application of such stimulus-responsive membranes or "chemical valves" (functional gates) includes flow control, size-selective filtration, chemical and bioseparation, controlled release of chemical substances and drugs, and chemical sensors.In the present study, we report on a novel method for the fabrication of flexible stimulus-responsive polymer gel (PG) membranes. These membranes are thin porous films made of a crosslinked polyelectrolyte. Past research has focused on porous thin-film membranes made of polyelectrolyte complexes. [9,10] In our approach the porous films are formed via phase separation of a polyelectrolyte and a volatile additive. This approach provides a broad possibility to regulate pore sizes and the membrane responsiveness. In our method, the PG membranes can be prepared on any flat substrate with a low surface roughness (e.g., Si wafer); afterward, the membrane can be transferred (and attached chemically, if necessary) onto various porous or nonporous supports (with flat, profiled, and even curved surfaces, e.g., membrane filter, fabrics, chemical sensor, or human skin). The fabrication of these membranes consists of three very simple steps, described in detail below.The PG membranes operate in a similar manner to those prepared by the grafting of polymers on the surface of porous substrates. That is, 3D swelling of the PG upon an external stimulus leads to shrinkage of the pores and, consequently, to the regulation of the membrane permeability. However, the PG membranes, unlike the membranes reported in literature, respond via swelling and shrinking of the entire body of the membrane. The shrinking allows for the stimuli-responsive mechanism of pore-size regulation in a very broad range, from completely closed pores (pore opening size = 0) to large open pores (pore opening size = 0.3 lm)....
The electrochemical gate based on a chemical signal-responsive membrane was assembled on a Au electrode surface. The polyelectrolyte gel membrane was capable to bind cholesterol because of the hydrogen bonding between cholesterol and the polymer backbone resulting in the gel swelling. The membrane channels were reversibly closed and opened upon addition and washing out cholesterol, respectively. Thus, the electrochemical process of a soluble redox probe, [Fe(CN)(6)](3-/4-), at the membrane-modified electrode was reversibly switched "on-off" by the cyclic addition and washing out cholesterol. The electrochemical reaction was also tuned by the variation of the concentration of the added cholesterol that controlled the extent of the channels closing. The switchable and tuneable operation of the chemically controlled electrochemical gate was characterized by Faradaic impedance spectroscopy and atomic force microscopy, indicating that the extent of the pores opening and closing is controlled by the concentration of the membrane-associated cholesterol. The chemical-responsive electrochemical gate was suggested to be a part of future biochemical/electrochemical systems with logic operations.
We report the fabrication of microporous thin film membranes with two-dimensionally arranged submicron pores whose size can be varied by changing pH of aqueous medium. A solution containing poly(2-vinylpyridine) partially quaternized with 1,4-diiodobutane (qP2VP) and unreacted 1,4-diodobutane (DIB) was used for the formation and deposition of the membranes on solid substrates. The membranes were spin-coated onto solid substrates in a controlled humid environment. The presence of water vapor in air was found to be a necessary condition for the pore formation. We studied the influence of relative humidity on the membrane morphology and proposed a mechanism of pore formation. Cross-linking the qP2VP membranes with DIB made them insoluble (stable) in organic solvents and acidic water. The cross-linked membranes demonstrated pHdependent swelling, which had a strong influence on the pore size. IntroductionMicroporous polymer membranes (pore size 0.01-20 µm) are widely used in industry, medicine, pharmacology, and research for separation and concentration of particles, colloids, proteins, enzymes, and cells. Phase inversion and track etching are well-established, commercially implemented techniques for the fabrication of such membranes. In the phase inversion technique, a solvent for a polymer turns into a nonsolvent causing the polymer precipitation; the nonsolvent serves as a porogen that evaporates after the membrane formation. Phase inversion is usually achieved by immersion of a solution film into a coagulation bath with a nonsolvent (immersion precipitation), by exposure to nonsolvent vapor, or by temperature change (temperature-induced phase separation). Membranes prepared by this technique exhibit a highly porous inner structure represented by a continuous network of interconnected tortuous pores. Furthermore, the structure is usually asymmetric with a thin dense surface layer and a thick spongelike basic layer. The surface layer determines the separation properties and the overall flow resistance of the membrane, while the basic layer acts as a mechanical support. 1 The track etch (TE) membranes are prepared using a two-step fabrication procedure. 2 First, a polymer film (polycarbonate or polyester) is exposed to a collimated beam of heavy ions that produce parallel tracks across the film. The tracks are then chemically etched, forming cylindrical pores. Unlike the phase inversion membranes, the track etch membranes are characterized by uniform pore size and relatively low pore density, resulting in the high flow resistance.In addition to the above-mentioned techniques, various template-assisted methods for the fabrication of microporous polymer membranes have been reported in the literature. Colloidal crystals 3-8 and emulsion droplets 9 have been successfully implemented as templates for the fabrication of selfstanding membranes. Microcontact molding 10 and casting of polymer layers on solid substrates with 2D pillar arrays 11 were found to be feasible for the fabrication of thin film membranes.
A b s t r a c t Comparative experimental study o f t h e gas p y r o l y s i s induced by COP l a s e r r a d i a t i o n a t 10.6 urn and by h e a t i n g o f t h r i c h l o r o e t h y l e n e and oxyaen has been performed. The a n a l y s i s o f t h e r e a c t i o n products does n o t i n d i c a t e any d i f f e r e n c e between these two techniques o f p y r o l y s i s i n t h e range o f pressures 100-760 t o r r . Laser s a f e t y aspect. Since one o f t h e p y r o l i t i c p r o d u c t s i s phosgene t h e use o f t h r i c h l o r o e t h y l e n e i n t h e v i c i n i t y o f unprotected COP l a s e r beam i s v e r y dangerous.
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