Polybenzimidazole drying

To devise polymeric systems that can conduct protons with little or no water, in addition to possessing good chemical and electrochemical stability above 100 °C, is very challenging and has been the focus of intensive research over the last decade. One of the most successful strategies has been to replace the water with less volatile compounds. For example, strong 0x0 acids, e.g. phosphoric acid or sulfuric acid, have been complexed with basic polymers such as polybenzimidazole (PBI). PEMs based on PBIs doped with phosphoric acid have been used successfully at temperatures well above 100 °C under very dry conditions. However, there is still a risk that the acid will be partly leached out, especially at low temperature and high humidity. 

The resulting polymer has high temperature stability, good chemical resistance, and nonflammability. The polymer releases very little toxic gas and does not melt when exposed to pyrolysis conditions. The polymer can be formed into fibers by dry-spinning processes. Polybenzimidazole is usually amorphous, with a Tg near 430°C. ... 

One of the frequently advertised advantages of the phosphoric acid imbibed polybenzimidazole systems is their zero water drag coefficient and their possibihty to operate with dry hydrogen and oxygen. However, a vast literature has been devoted to the study of the proton conduction and the effect of relative humidity on the conductivity of the PBl-phosphoric acid system. The promoting effect and the physicochemical interactions of water vapors with the polymer electrolyte and on the fuel cell performance have been explicitly shown for the PBl/PPy(50)coPSF 50/50 polymer blend imbibed with phosphoric acid under fuel cell conditions. ... 

C. After 16 h the temperature is raised to 230°C for a further 4 h. The cooled suspension is neutralised with sodium bicarbonate, then the polybenzimidazole particulates are collected, washed and dried in the usual way (Figure 6.16). 

Typically the surface area of the product is very low 2 m and further work is underway to improve the porosity of these species. In an alternative procedure a pre-formed polybenzimidazole prepared in the presence of stabilising polymer to provide aliphatic hydrocarbon graft chains can be dissolved in cone. H2SO4 and the acid solution dispersed, without additional stabiliser, in paraffin oil. Slow addition of methanol to the suspension causes precipitation of the polybenzimidazole within the droplets and eventually solid particulates are formed. These can be collected, washed and dried in the usual way. Materials prepared by this method have better porosity and a surface area of 30 m g ... 

Asymmetric polybenzimidazole membranes have been developed for RO applications, in the form of hollow fibers [154] and flat film membranes [155] for water transport. By comparison with cellulose acetate, FBI has very attractive chemical, flammability and thermal properties. There are two problems encountered in attempting the preparation of such membranes for TEM (1) deformation during drying and (2) lack of contrast. Often specific methods must be developed for each membrane type, although method development is quite time consuming. 

The screen printing process is the most used process to manufacture gas diffusion electrodes for HT- and LT-PEM. This process works in a very efficient way. Here the membrane of a fuel cell, for HT-PEM it s usually a polybenzimidazole and for LT-PEM a Nafion-based membrane, is fixed in its undoped form in a holder. After that a catalyst slurry, what is an electrode ink mixed with catalyst, is getting sprayed on the membrane and afterwards dried. The slurry is usually consisting of binder materials like poly (1,1,2,2-tetrafluoroethylene) and newest more often poly-l,l-difluoroethene, catalyst-loaded support material what is most likely carbon, for example, and dispersing agents like water and alcohols. 

The copolymer with STA DCP = 50 50 showed good solubihty in N-MP, but only similar level of conductivity to the sulfonated polymer. When the content of DCP was increased to STA/DCP = 30 70, the conductivity jumped to similar level of phosphonated polymer, while maintaining the solubihty in N-MP. Further increase of DCP content to STA/DCP = 15 85 induced the decrease in solubihty. There is a narrow region where good conductivity and solubihty coexist in these copolymers. Ion-containing polybenzimidazoles showed exceUent stabihty against hydrolysis, radical attack and dimensional change in wet/dry cycles. 

Dry spinning of particular fiber types acrylics, cellulose acetate and triacetate, polybenzimidazole (PBl), spandex and polyvinyl chloride (PVC)... 

Dry spinning is also the method of choice for several other polymers including spandex, polyvinyl chloride (PVC) and polybenzimidazole (PBI). These polymers share the characteristic of thermally degrading prior to/during melting and solubility in high volatility solvents. In all cases it is the cost-effectiveness of dry spinning that leads to its adoption as the fiber formation method of choice. 

As mentioned earlier, a variety of membrane materials have been investigated for possible use in hydrogen/alr fuel cells and DMFCs, including sulfonated polyimides, sulfonated poly(arylene ether)s, sulfonated poly(ether ether ketone)s, and phosphoric acid-doped polybenzimidazole [5,321-331]. Many of these membrane materials suffer from poor oxidative stability, excessive swelling when wet and brittleness when dry, and only moderately high proton conductivity. Thus, none of the polymers has yet emerged as the preferred proton exchange membrane material for fuel cell... 

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