PBI blends

Polymer blending allows the opportunity to lower costs without sacrificing the desired properties, the ability to tailor properties without creating 

Polymer blends may be miscible, immiscible or partial (a combination of the two). Miscible blends offer additional desirable and, often needed, end-use properties intermediate to the individual components across the miscible composition range. Typically, these have uniform physical properties. As stated by Flory ° It is well known that, regarding the mixing of thermoplastic polymers, incompatibility is the rule and miscibility or even partial miscibility is the exception. Since most thermoplastic polymers are immiscible with other thermoplastic polymers, the discovery of a homogeneous mixture or partially miscible mixture of two or more thermoplastic polymers is, indeed, usually unpredictable with any degree of certainty . And, it is generally known or deduced that most pairs of polymers tend to 

Typical blend polymers for PBI include poly (aryl ether ketone) (PARK), pofyetherimide (PEI), poly (aryl ether sulfone), poly (phenylene sulfide), and mixtures thereof. Different types of PAEK resins (Fig. 7.5) include PEK, PEEK, PEKEKK, PEKK, PEEKK, etc., several of which have melting points over 300°C. PAEK resins with relatively low Tg values can be further improved by the addition of reinforcement fillers, such as fiberglass, carbon fibers, and ceramic or mineral fillers. Such additions, however, can 

5 Polylaryletherketone structures. In panel (a), X,Y and N are positive integers.There are several variants of PAEK, for example (b) polyetherketone (PEK) (c) polyetheretherketone (PEEK) (d) polyetherketoneketone (PEKK). 

Due to its very unique and high thermal stability characteristics, PBI blends were studied and developed early on for aerospace and composite materials. The discovery of new polymeric alloys by blending commercially available polymers was the trend in materials development. A key reason for growth in this area was the ability to tailor existing materials to a new/unique set of property-performance-price specifications through combinations of materials. For example, a miscible blend based on PBI and polyimide (PI) was discovered and, after thorough investigations, PBI was found not only to have superior thermal/chemical properties, but to also 

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PBI blends (with polyimides, polyarylates, poly (4-vinyl pyridine)) Semi-interpenetrating Polyimide Network ... 

Mechanical property studies of the 10% 6F-PAI and PBI blend films further confirmed the existence of synergistic miscibility between the blend component since the tensile modulus and strength of each corresponding file was either equivalent to or exceeded the simple rule of mixtures (Fig. 11). 

Fig. 11a. Tensile strength of 10% 6F-PAI/PBI blend film, b Modulus of 10% 6F-PAI/PBI blend...

Sulfonated polybenzoimidazole (sPBI) membranes have also been prepared by poly condensation of 2-sulphoterephthalic acid with 1,2,4,5-tetraaminobenzene tetrahydrochloride [193] or by grafting alkyl or aryl sulfonates on to the imidazole ring [194]. ABPBI was sulphonated by immersion in sulfuric acid followed by heat treatment at 450 °C [195]. Other methods of sulphonation have been reviewed by Asensio et al. [196], along with a detailed description of the properties of PBI blends with sulfonated and non-sulfonated polymers, and block PBI copolymers. 

Several PBI blends have been investigated in relation to their DMFC performance. PBI layered Nafion and PBI/Nafion composite have been discussed in Sect. 6.5.7. PBI composites with sPOP [415], sPPO [435], and sPEEK [436] exhibit... 

The most extensively studied PBI blend system that has been studied is PBI blended with a polyetherimide originally available firom GE, Ultem (Takekoshi 1980). The chemical stmcture of Ultem is shown in Fig. 16.4. 

In further investigations of PBI blends, Jaffe et al. (1992a, b) showed that PBI is miscible with certain polyimides that contain the hexafluoroisopropylidene or 6F moiety, shown in Fig. 16.6. The miscibility and phase behavior in these blend systems is dependent on the -CF3-C-CF3 content and the overall structure of the polyimide. In addition, the amount of the 6F chemical group affects the Tg of the polyimide itself as well as the Tg of any miscible blends. 

Thus, we have two different types of behavior in blends of high temperature polymers. The first situation is exemphfied by the PBI blends in which specific interactions through the N-H group of the PBI are responsible for the observed miscibility. The other situation is displayed by the mixtures of two LCP s in which entropic effects are important for the miscibility. These types of behavior are the extreme case of behavior. This also suggests that there are blend systems of high temperature polymers in which both enthalpic and entropic effects should be important factors in the miscibihty. 

Multitudes of other organically modified PBI membranes exist that include, but are not limited to, fluorinated PBI, ionicaUy and covalently cross-linked PBI, PBI blends, and a wide variety of PBI copolymers. Because there are far too many to describe, this subsection wiU highlight select PBI membranes that have not been included in the prior subsections. 

From the abovementioned results, it can be concluded that the combination of the partially lluorinated sulfonated ionomer M with the partially fluorinated PBI B2 is of advantage in terms of chemical stability and compatibility, which can be explained with chemical similarity, since both blend components consist of electron-deficient aromatic ring systems. Good PBI blend membrane stabilities can be achieved by good compatibility between the blend components, electron-deficiency of the aromatic ring systems, or both. To get hints of which is the... 

Apart from sulfonated polymers, it could recently be shown that phosphonated polymers can also be used advantageously as macromolecular ionical cross-linkers for PBI blend membranes poly(tetrafluorostyrene-4-phosphonic acid) (PWN2010, Sll, Fig. 4.5) was synthesized via nucleophilic aromatic... 

From the reviewed work, it can be concluded that obviously the use of partially fluorinated aromatic cationomers as ionical cross-linkers leads in most cases to better chemical and thermal stabilities of the blend membranes than if nonfluorinated cationomers would be applied as acidic blend components. Among aU acidic cross-linkers, the sulfonated and partially fluorinated ionomer S9 (Fig. 4.5) leads to the best chemical stability of the referring base-excess PBI blend membranes. 

From the comparative study covering different ionically, covalently, and covalent-ionicaUy cross-linked PBI blend membrane types, it can be concluded that within the investigated membrane types none of the cross-linking types has principal advantages over the other ones in terms of chemical stability or proton conductivity in PA-doped state. 

Future work in the field of PBI blend membranes for use in intermediate-T fuel cells will incorporate ... 

Increase of molecular weight of acidic crosslinkers (e.g., of ) to further enhance the chemical, thermal, and mechanical stability of the referring PBI blend membranes... 

Li QF, Rudbeck HC, Chromik A et al (2010) Properties, degradation and high temperatine fuel cell test of different types of PBI and PBI blend membranes. J Membr Sci 347 260-270... 

Arrieta, C., David, E., Dolez, P., Vu-Khanh, T. Hydrolytic and photochemical aging studies of a Kevlar -PBI blend. Polym. Degrad. Stab. 96. 1411-1419 (2011)... 

Arrieta et al. [47] conducted hydrolytic and photochemical aging studies for a Kevlar-Polybenzimidazole (PBI) blend in yam form. UV irradiation was conducted at X, = 340 nm and four different temperatures (50, 60, 70 and 80 °C) between 4 and 31 days. The PBI stracture is given in Fig. 5 [47] (reproduced with kind permission from Elsevier—License no. 3890161358622). 

Fig. 12 Properties of SPBPP-PBI blended membranes as a function of PBI content a proton conductivity in water at room temperature, b equilibrium water swelling at room temperature, and c methanol permeability at 60 for 1.0 M methanol...

Table 12.4 Tribological Properties of Victrex pic Victrex TL-60 (30% Carbon Fiber Reinforcement, Unspecified Lubricant Filled) PEEK/PBI Blend...

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