Ionomer chemical modification

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]. 

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. 

Stabilizing Low EW Membranes Through Chemical Modification of the Ionomer... 

A commonly used method of inducing compatibility between PE and polyamides is chemical modification of the PE to contain pendant carboxyl groups, which form chemical linkages to the polyamide via the terminal amino groups. The concept has been employed to produce commercial compatibilizers such as MAH grafted PP or PE. Other approaches reported in the literature include the use of ionomers, acrylic acid/butyl acrylate/styrene terpolymers and nylon 6-polybutene multiblock copolymers. 

In summary, design, integration and performance optimization of advanced PEM needs systematic experimental-theoretical efforts to focus on studying the effects of chemical modifications of the base ionomer, thereby, increasing the ion exchange capacity (to improve transport properties without sacrificing stability) and reducing thickness. 

The polysulfone ionomers are obtained by chemical modification of commercial polysulfone or by polymerization of ionic monomers using step-growth polymerization. In order to enhance their performance as a PEM, different approaches (enlisted below) have been reported which will be discussed thoroughly in this chapter. 

Use of sulfonated polymers as the proton-conductive component in the fuel cell membranes at T < 100°C Use of nonfluorinated ionomers physical and/or chemical cross-linking of the fuel cell membranes Use of nonfluorinated ionomers physical and/or chemical cross-linking of the fuel cell membranes Development of organic-inorganic composite membranes, based on our cross-linked ionomer membrane systems, in which the inorganic membrane component serves as water storage or even contributes to H -conduction Use of commercially available polymers for chemical modification and membrane formation, which avoids expensive development of novel polymers... 

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. 

Further modification of polyethylene is possible by chemical substitution of hydrogen atoms this occurs preferentially at the tertiary carbons of a branching point and primarily involves chlorination, sulfonation, phosphorylination, and intermediate combinations. (See chlorinated polyethylene chlorosulfonated polyethylene phosphorylated polyethylene ionomer.)... 

Modification/development of ionomers and ionomeric membranes to obtain enhanced chemical durability tmder low-RH conditions [6, 7] as well as increased mechanical stability during RH-cycles, both of which are frequently occurring conditions under automotive fuel cell operation [2, 8]. 

PFSA ionomers are linear statistical copolymers of hydrophobic polytetrafluo-roethylene backbones with randomly grafted vinyl ether sidechains, terminated by sulfonic acid (-SO3H) head groups. Materials of this type vary in the chemical structure of the pendant sidechains. Other materials modifications are done to reduce membrane thickness and increase the lEC, through increasing the grafting density... 

Simulations of physical properties of realistic Pt/support nanoparticle systems can provide interaction parameters that are used by molecular-level simulations of self-organization in CL inks. Coarse-grained MD studies presented in the section Mesoscale Model of Self-Organization in Catalyst Layer Inks provide vital insights on structure formation. Information on agglomerate formation, pore space morphology, ionomer structure and distribution, and wettability of pores serves as input for parameterizations of structure-dependent physical properties, discussed in the section Effective Catalyst Layer Properties From Percolation Theory. CGMD studies can be applied to study the impact of modifications in chemical properties of materials and ink composition on physical properties and stability of CLs. 

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. 

---