Polymer membranes, liquid phase

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-film composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has little effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich solid phase that forms the membrane and a polymer-poor liquid phase that forms the membrane pores or void spaces. 

Chemistry on soluble polymer matrices has recently emerged as a viable alternative to solid-phase organic synthesis (SPOS) involving insoluble cross-linked polymer supports. Separation of the functionalized matrix is achieved by solvent or heat precipitation, membrane filtration, or size-exclusion chromatography. Suitable soluble polymers for liquid phase synthesis should be crystalline at room temperature, with functional groups on terminal ends or side chains, but must not be not cross-linked they are therefore soluble in several organic solvents. 

Higher initial flux values for the extracted species are usually obtained when PIMs incorporate high-polarity and low-viscosity plasticizers such as NPOE and NPPE, and this has led to the conclusion that the initial flux values increase with an increasing dielecfiic constant and decreasing viscosity of the plasticizer. However, it should be noted that most of the plasticizers used in PIMs have similar viscosity values (Table 27.4), and moreover, the dielectric constant of the membrane liquid phase is also dependent on the dielectric constants of the carrier and the base polymer [32]. 

The main disadvantage of the SL membrane is the slow leaching of the membrane liquid phase into the receiver and feed solutions, thus reducing its lifetime. Attempts have been made to slow down the loss of the membrane liquid phase by converting it to a polymer gel. The corresponding SL membrane is known as gelled SL membrane. 

To prepare an asymmetric membrane, either the phase-inversion process (skin and support made of the same material) or a two-step process (barrier layer deposited on a porous substructure) is used. In the latter case, the barrier and support structures are usually made from different materials. Symmetric and asymmetric polymeric membranes can be prepared using the phase separation process. ° A precipitation/solidification process is used to transform a polymer solution into two phases (a polymer-rich solid and a polymer-lean liquid phase). The following techniques can be used to solidify the polymer ... 

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

Among all the polymers used in preparing ion-selective membranes, poly(vinylchloride) (PVC) is the most widely used matrix due to its simplicity of membrane preparation [32, 70], In order to ensure the mobility of the trapped ionophore, a large amount of plasticizer (approximately 66%) is used to modify the PVC membrane matrix (approximately 33%). Such a membrane is quite similar to the liquid phase, because diffusion coefficients for dissolved low molecular weight ionophores are high, on the order of 10 7-10 8cm2/s [59],... 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, poly(tetrafluoroethylene), and cellulose acetate, have been used for porous substrates for supported-liquid membranes. The PVdF coated polyolefin-based microporous membranes used in gel—polymer lithium-ion battery fall into this category. Gel polymer... 

Divisek et al. presented a similar two-phase, two-dimensional model of DMFC. Two-phase flow and capillary effects in backing layers were considered using a quantitatively different but qualitatively similar function of capillary pressure vs liquid saturation. In practice, this capillary pressure function must be experimentally obtained for realistic DMFC backing materials in a methanol solution. Note that methanol in the anode solution significantly alters the interfacial tension characteristics. In addition, Divisek et al. developed detailed, multistep reaction models for both ORR and methanol oxidation as well as used the Stefan—Maxwell formulation for gas diffusion. Murgia et al. described a one-dimensional, two-phase, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymer membrane for a liquid-feed DMFC. 

LC techniques are widely diffused for the determination of hydrophilic but not volatile and thermally unstable pesticides. Since the European Community Directive [68] indicates 0.1 pg L" as the concentration threshold level for a single pesticide in waters destined for human consumption, to quantify these concentration levels, suitable pre-concentration and extraction procedures must be generally performed prior to the HPLC determination. The extraction methods are based on LLE, MAE, on-line continuous flow liquid membrane extraction (CFLME), and mainly on SPE and SPME. Many SPE procedures are used the packing materials are graphitized carbon, ODS, styrene-divinylbenzene co-polymers, or selective phases based on immunoafflnity. The extraction can be performed on- and off-line, manually, or in a semi-automated way. 

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

Membranes are thin polymeric films that may permit the faster diffusion of some molecules than of others. Thin films of polymers are widely used for the separation of gases and for liquid-phase separations (dialysis). Because of the ease of property tuning, polyphosphazenes are of great interest for these types of applications, although only a few examples have yet been investigated. 

Liquid-phase separations can be carried out with membranes produced from several different classes of polyphosphazenes. An example membrane prepared from [NP(NHC4H9)2] is shown in Figure 3.13. The polymer, [NP(OCH2CF3)2] , can be employed to concentrate alcohols, because the alcohol diffusion rate is much faster than that of water.156... 

Yang et al. [47,48,53,54] developed a HWG sensing system for liquid and soil analyses based on an extractive polymer membrane coated onto the inside of the HWG. The polymer coating performs a solid-phase microextraction of the analyte from the headspace of the sample and preconcentrates the analyte prior to IR analysis. 

Figure 5.9 A supported liquid membrane (SLM) a porous polymer membrane whose pores are filled with the organic liquid and a carrier, set in between the aqueous source phase and the aqueous receiving phase, which are being gently stirred.

In Figure 3.13 the precipitation pathway enters the two-phase region of the phase diagram above the critical point at which the binodal and spinodal lines intersect. This is important because it means that precipitation will occur as a liquid droplet in a continuous polymer-rich phase. If dilute casting solutions are used, in which the precipitation pathway enters the two-phase region of the phase diagram below the critical point, precipitation produces polymer gel particles in a continuous liquid phase. The membrane that forms is then weak and powdery. 

Table 2.1 Overview of main polymer membrane characteristics and membrane-based processes for molecular separations in liquid phase.

---