Polymeric membranes crystallinity

Knowledge of crystalline morphology is essential in understanding the permeability and permselectivity of polymer membranes such as dense membrane, dialysis membrane and gas separation membrane. X-ray is very common to study the crystallinity in polymeric membranes. Crystalline structure of a polymer membrane includes dimensions of unit cell, percentage of crystallinity, crystallite size, and orientation. The most generally applicable technique that provides information is the X-ray diffraction method. 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

Many experimental techniques have been used to examine the detailed structure of perfluorinated polymeric membranes. These include transmission electron microscopy [23], small angle X-ray scattering [24], Infra Red spectroscopy [25,26], neutron diffraction [27], Nuclear Magnetic Resonance [26,28], mechanical and dielectric relaxation [25,29], X-ray diffraction, and transport measurements. All these methods show convincing evidence for the existence of two phases in the perfluorosulfonate and perfluorocarboxylate polymers. One phase has crystallinity and a structure close to that of polytetrafluoroethylene (PTFE), and the other is an aqueous phase containing ionic groups. 

A FI multisensor system comprising potentiometric sensors of different types for the determination of free cyanide activity in basic solutions for extraction of noble metals has been developed [35]. Solvent polymeric membrane sensors based on metalloporphyrin and crystalline sensors were combined in the sensor system. The sensors of different types were built into the system to form a multisensor detector. The FI multisensor was also beneficial due to computerizing of measurements, automatic sampling, and sample treatment and also due to minimizing amounts of reagents. Preparation of sensor membranes is described below. 

Partial to complete 3-0-octadecylated polysaccharides exhibited characteristic solution and solid properties based on hydrophilic-hydrophobic structures. These polymers are suggested to form micellar conformations in water and in chloroform polysaccharide-coated liposomes, polymeric membranes, and thermotropic liquid-crystalline mesophase, depending on the octadecyl content. Hydrolysis of 3-deoxygenat-ed, 3-0-methylated, and 3-0-octadecylated dextrans by an endo-acting dextrans is compared. The possibility of a combshaped branched polysaccharide toward cell-specific biomedical materials is discussed. 

Polymeric membranes generally fail to maintain their physical integrity in organic solvents because of their tendency to swell or dissolve. This is a major drawback since nonaqueous processes generally require polymers that are rigid and crystalline, thermally... 

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