Ionomers modification

Those critical functions of membrane for DMFC are simple but most important. Required functions are ionic conductivity, electrical insulation, gas and liquid (especially methanol) tightness, and chemical and mechanical stability. As indicated in Fig. 13.2, ohmic polarization is mainly due to the ionic resistance of membranes, but the low open circuit potential of cathode is also mainly coming from the voltage drop by mixed potential made of fuel crossover through the membrane. The low cost of material and process is also another factor in terms of commercialization. Especially for mobile applications, membranes have the additional function for mass balance of liquid fuel and water products circulated out of or through the membrane. In this manner, alternative membranes are under development and researchers are focused on four types perfluorinated and partially fluorinated membranes hydrocarbon and composite and other ionomer modifications inorganic materials. The current state of the art and technical approaches to these materials are discussed in detail elsewhere in this volume. 

Initial materials of this super-tough type were blends of nylon 66 with an ionomer resin (see Chapter 11). More recent materials are understood to be blends of nylon 66 with a modified ethylene-propylene-diene terpolymer rubber (EPDM rubber—also see Chapter 11). One such modification involves treatment of the rubber with maleic anhydride, this reacting by a Diels—Alder or other... 

Crisp, S., Merson, S. A. Wilson, A. D. (1980). Modification of ionomer cements by the addition of simple metal salts. Industrial Engineering Chemistry, Product Research Development, 19, 403-8. 

In addition, the polymer modification reactions leading to acidic and ionomeric functionalities are described in detail. The derived ion-containing block copolymers may aid in the correlation of chemical architecture with ionomer morphology and properties. 

Several modifications of commercial silane coupling agents have been evaluated to search for improved bonding at the interface. These approaches attempt to use modifiers to counteract basic faults of the individual silanes. Some of these modifications include increased hydrophobic character, increased crosslinking of the siloxane structure, increased thermal stability, and ionomer bond formation to reduce shear degradation at the interface. 

Sulfonation is very useful chemical modification of polymer, as it induces high polarity in the polymer changing its chemical as well as physical properties. Sulfonated polymers are also important precursors for ionomer formation [75]. There are reports of sulfonation of ethylene-propylene diene terpolymer (EPDM) [76, 77], polyarylene-ether-sulfone [78], polyaromatic ether ketone [79], polyether ether ketone (PEEK) [80], styrene-ethylene-butylene-styrene block copolymer, (SEBS) [81]. Poly [bis(3-methyl phenoxy) phosphozene] [82], Sulfonated polymers show a distinct peak at 1176 cm"1 due to stretching vibration of 0=S=0 in the -S03H group. Another peak appears at 881 cm 1 due to stretching vibration of S-OH bond. However, the position of different vibrational bands due to sulfonation depends on the nature of the cations as well as types of solvents [75, 76]. 

A wide range of conventional organic type polymers has been used for modification with the ferrocene functional group. Treatment of polystyrene with lithioferrocene or dilithioferrocene led to incorporation of the ferrocene as a pendant group [59]. Another interesting example is the preparation of a ferrocene-containing ionomer... 

The second route to ionomers Involves modification of a preformed polymer. Sulfonation of EPDM, for example, permits the preparation of sulfonated-EPDM with a level of sulfonate groups in proportion to the amount of sulfonating agent(27). These reactions are conducted in homogeneous solutions permitting the direct neutralization of the acid functionality to the desired level. Isolation of the neutralized ionomer is effected by conventional polymer isolation techniques, such as coagulation in a nonsolvent or solvent flashing. These procedures are detailed in several patents and publications(28-31). 

Duvdevani(40) have been directed at modification of ionomer properties by employing polar additives to specifically interact or plasticize the ionic interactions. This plasticization process is necessary to achieve the processability of thermoplastic elastomers based on S-EPDM. Crystalline polar plasticizers such as zinc stearate can markedly affect ionic associations in S-EPDM. For example, low levels of metal stearate can enhance the melt flow of S-EPDM at elevated temperatures and yet improve the tensile properties of this ionomer at ambient temperatures. Above its crystalline melting point, ca. 120°C, zinc stearate is effective at solvating the ionic groups, thus lowering the melt viscosity of the ionomer. At ambient temperatures the crystalline additive acts as a reinforcing filler. 

The novel reaction scheme which we had proposed for "blocky" ionomers through emulsion polymerization prompted us to prepare very well defined ion containing block copolymers through anionic polymerization. The initial system chosen for synthesis, modification, and characterization was the poly-styrene-poly(isobutyl methacrylate) diblock (PS-PIBM DB) copolymer. This particular system was chosen for the following reasons ... 

For high temperature membranes, three avenues are being pursued a) synthesis of new ionomers, b) covalent modification of Nafion , and c) pol mier-inorganic composites. Some of the approaches aim to increase the attractive forces holding water within the membrane, enabling operation at low RH. Others seek to embody Bronstead bases as an alternative to water, which allow for proton mobility in the absence of water. This mechanism is exemplified by proton conductivity in e.g. H3PO4 or CSHSO4, in which the protons hop and the protonated base need not move macroscopic distances. 

Chemical modification of polyolefins is a broad and rapidly growing field of science. Such modification, often times, is done to introduce either subtle or gross changes that enhance the attributes of the original polymer. For example, introduction of ionic interactions in polymers provides a means of controlling polymer structure and properties. As would be expected, ion-containing polymers, otherwise known as ionomers , display properties which are dramatically different from those of the parent polymer. Therefore, a broad spectrum of material properties may be created by varying the ion content, type of counter ion, and extent of neutralization. 

Modification of Engineering Resins Specific interaction of the phosphonium ionomer from Exxpro elastomer with selected engineering resins such as Polycarbonates(PC), Polyesters(PET), Polyacrylates(PAE), Polyamides(PA), Polyphenylene Oxide(PPO), and Acetals(PAc) can be utilized to compatibilize, impact modify or nucleate the above resin in blends with similar polymers. Typical examples are ... 

The use of low-cost basic polymers instead of Nation is an interesting alternative [19,20].The development of new polymers for ionomer membranes including perfluorinated ionomers, partially fluorinated ionomers, nonfluorinated ionomers, high-molecular/low-molecular composite membranes as well as novel polymer modification processes and novel membrane materials is summarised in [21]. 

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