Polyimide Based Polymer Blends

Trogadas and Ramani summarized the modification of PEM membranes, including Nafion modified by zirconium phosphates, heteropolyacids, hydrogen sulfates, metal oxides, and silica. Membranes with sulfonated non-fluorinated backbones were also described. The base polymers polysulfone, poly(ether sulfone), poly(ether ether ketone), polybenzimidazole, and polyimide. Another interesting category is acid-base polymer blend membranes. This review also paid special attention to electrode designs based on catalyst particles bound by a hydrophobic poly-tetrafluoroethylene (PTFE) structure or hydrophilic Nafion, vacuum deposition, and electrodeposition method. Issues related to the MEA were presented. In then-study on composite membranes, the effects of particle sizes, cation sizes, number of protons, etc., of HPA were correlated with the fuel cell performance. To promote stability of the PTA within the membrane matrix, the investigators have employed PTA supported on metal oxides such as silicon dioxide as additives to Nafion. 

Alkali-Developable Base Polymers. Since the dissolution rate of PAAs is too fast in 2.38% TMAH solution, the photosensitive system composed of PAA and a DNQ compound suffers from low contrast of the difference in dissolution rate between exposed and unexposed area. It implies that the optimization of the dissolution rate of base polyimide precursor in the developer is needed. PAA/PAE blends, PAA/PAE copolymer, and PAE with pendant carboxylic acid (PAE-COOH) were evaluated as base polymers for photosensitive polyimides. 

Okazaki,Y. Nagaoka,S. Kawakami,H., Proton-conductive membranes based on blends of polyimides,/. Polym. Sci. Part B Polym. Phys. 45,1325-1332 (2007). 

Commercial membranes for CO2 removal are polymer based, and the materials of choice are cellulose acetate, polyimides, polyamides, polysulfone, polycarbonates, and polyeth-erimide [12]. The most tested and used material is cellulose acetate, although polyimide has also some potential in certain CO2 removal applications. The properties of polyimides and other polymers can be modified to enhance the performance of the membrane. For instance, polyimide membranes were initially used for hydrogen recovery, but they were then modified for CO2 removal [13]. Cellulose acetate membranes were initially developed for reverse osmosis [14], and now they are the most popular CO2 removal membrane. To overcome state-of-the-art membranes for CO2 separation, new polymers, copolymers, block copolymers, blends and nanocomposites (mixed matrix membranes) have been developed [15-22]. However, many of them have failed during application because of different reasons (expensive materials, weak mechanical and chemical stability, etc.). 

To aid dispersion of the carbon fiber in the polymer matrix, it is usual to apply a polymer compatible size, normally a lower molecular weight version of the polymer can be used, preferably in a water base (e.g. a polyurethane). If the fiber has an epoxy size, this must be removed by solvent extraction in a solvent degreasing plant. Under some conditions, it is possible to blend 10% epoxy sized fiber. If the thermoplastic polymer is to be used at high temperatures (e.g. PEEK), then the size must be temperature resistant, such as a polyimide. 

Herrero and Acosta (80) investigated the microstmcture of poly(ethylene oxide)-poly[(octafluoropentoxy)(trifluoroethoxy)phosphazene] blends. Limited miscibility of both components was inferred, based on the observed shift of the components glass-transition temperatures. Wycisk and co-workers (81) prepared membranes from blends of sulfonated poly[bis(3-methylphenoxy)phosphazene] with polyimides, polyacrylonitrile, and Kynar FLEX PVDF. Morphology, electrochemical performance, and methanol permeabilities of the membranes were then evaluated as part of a program to investigate such blends in direct methanol fuel cells. The polymers were immiscible and a domain-type structure was observed. The best compatibility resulted when the tetrabutylammonium or sodium salt of the polyphosphazene was used (82). 

The first experimental material based on this type of model comprised a blend of two polymers—a low-MW polyimide (7) containing multiple 7r-electron-poor diimide units and a low-MW polysiloxane (8) containing tv-electron-rich pyrenyl end-groups (Figure 14). ... 

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