Polymeric membranes electrical properties

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

The function of the polymeric membrane electrolyte is to permit the transfer of protons produced in anodic semi-reaction (3.11) from anode to cathode, where they react with reduced oxygen to give water. This process is of course essential for fuel cell operation, as it allows the electric circuit to be closed inside the cell. On the other hand, the membrane must also hinder the mixing between fuel and oxidant, and exhibit chemical and mechanical properties compatible with operative conditions of the fuel cell (temperature, pressures, and humidity). 

A new area of polymer science termed nano-macromolecular chemistry [Eirich, 1993] also has relevance to future polymer blend technology and application. Langmuir-Blodget techniques allow for the formation of films of one molecule thickness. Utilizing polymerizable molecules for these films, a polymer molecule or network can yield a film with the thickness of several nanometers. Alternating layers comprised of different polymers could be prepared to yield specific optical or electrical properties. Polymerization of calix-arenes to yield molecular sieving membranes for gas separation has been discussed by Conner et al. [1993]. 

Membranes which may be used in the removal of alkali metal ions by electrodialysis are those which are impermeable to anions, but which allow the flow therethrough of cations. Such cation-selective membranes should, of course, possess chemical durability, high resistance to oxidation and low electrical resistance in addition to their ion-exchange properties. Homogeneous-type polymeric membranes are preferred, for example, network polymers such as phenol, phenosulfonic acid, formaldehyde condensation polymers and linear polymers such as sulfonated fluoropolymers and copolymers of styrene, vinyl pyridine and divinylbenzene. Such membranes are well known in the art and their selection for use in the method of the invention is well within the skill of the art. 

The fabrication of PEDOT nanofiber, nanotube and nanowire has been mainly focused on the AAO membrane, because AAO has advantages such as rigid shape, uniform diameter, and various pore sizes [364-368]. Electrochemical polymerization was performed in the pores of AAO membrane using SDS, LiC104 and EDOT solution and measured the resistance of the PEDOT nanofiber with a diameter of 35 nm and 150 nm, respectively. The resistance of PEDOT nanofiber was between 1.5 K and 300 K and the resistance ratio of R(T)/R (300 K) was the relevant term to investigate the intrinsic electrical properties of the conducting polymer. The resistance ratio increased... 

Chapter 4 then expands the diseussion on the use of nanoparticles in membrane modification processes. Materials in the form of nanoparticles have a large surface area to volume ratio, which infers many interesting properties on nanoparticulate systems due to the involved interfaeial properties. As a consequence, nanoparticles are currently receiving a lot of interest in many industries, such as membrane technology where the control of interfacial interactions is important. Nanoparticles affect the permeability, selectivity, hydrophilicity, thermal and electrical conductivities, mechanical strength, thermal stability, and the antiviral and antibacterial properties of the polymeric membranes. Chapter 4 discusses important examples of... 

Polysulfones are materials of great technological interest due to their applications as polymeric membranes, resists for microelectronics, heat-resistant materials in aerospace applications, matrices for composites, etc. It has been shown that ion beam treatment applied in the low-keV range is a suitable way to modify some relevant properties of polysulfones, such as solubility or electrical conductivity (117, 118], XPS is probably the most suitable analytical method for the characterization of ion-beam-induced changes in the composition and chemical state of polysulfone,s, e.g of aromatic polyfether sulfone) (PEiS) [119, 120). 

The miniaturization of traditional solvent polymeric membrane-based potentiometric sensors has obvious limitations in terms of reduced lifetimes and extremely high electrical resistances. Therefore, a completely different construction is needed to overcome these limitations. One approach that takes advantage of the unique opportunities provided by nanostructures and shown to be of perspective is based on the use of nanoporous membranes. The concept behind nanopore potentiometry is based on shrinking the nanopore restriction to approach molecular dimensions so that the physical-chemical properties of the inner wall of the nanopores determine the ion transport through the nanopore. Thus, in this approach, there is no solvent polymeric membrane, but solely the surface functionality of the nanopores is generating the ion-selective behavior. 

Many efforts have been made in the design and fabrication of controlled organic/inorganic composites with novel properties, which include chemical, optical, electrical, biological, and mechanical properties [84-87]. For these hybrid systems, phase separation occurs naturally based on the fact that they are composed of two materials with totally different chemical characteristics [88,89]. MMMs are fabricated from polymer matrix and inorganic particles for improvement polymeric membrane properties. Dispersed particles in polymeric matrix are categorized in two groups porous and dense (non-porous) particles [90]. 

GC material was widely modified with conducting (or nonconducting) polymers in order to obtain an improved surface for DNA adsorption and detection. The initial approaches were performed by the physical attachment of nylon or nitrocellulose membranes on GC electrodes [51]. As explained, these membranes were extensively used in classical DNA analysis due to their well-known adsorption properties [33]. Other approaches were performed by the direct adsorption of the polymeric film on the GC surface. Finally, polymeric films were electrochemically grown on the GC substrate. These conducting polymers are particularly promising for the adsorption, but also for inducing electrical signals obtained from DNA interactions. 

---