Blends covalent cross-linking

ESR techniques use nitroxyl radicals either dispersed in polymer matrix (spin probe) or covalently bonded to polymer chain (spin label) which are sensitive to the environment allowing molecular motion and microstructure of polymers to be identified from spectra (103). Quantitative methods of heterogeneous ESR spectra are divided into (J) outer hyperfine etrema, (2) signal intensities related to the relative concentration of the probe in different phases, and (3) simulation of the spectra. The presence of two well-separated outer maxima above the glass-transition temperature could be ascribed to two phases in natural rubber (104), miscible blends (105), immiscible blends (106), cross-linked polymers (107), and polyurethanes (108). ESR has used the measurement of the oxidation product to monitor the consumption of stabilizer in polypropylene (109). 

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. 

The preparation (Fig. 4.1) and structure (Fig. 4.3) of an HsPOa-doped ionically cross-linked acid-base blend membrane (membranes 1927A and 1940 in Table 4.7) was already depicted schematically in the introduction. Figure 4.11 presents the preparation and the structure of a covalently cross-linked blend membrane (membranes 1921C, 1925C, and 1938), and Fig. 4.12 the preparation and structure of a covalent-ionically cross-linked blend membrane (membrane 1943). 

Fig. 4.11 Preparation and structure of covalently cross-linked intermediate-T blend membranes...

However, contrary to the expectation, both covalently cross-linked blend membranes... 

From Fig. 4.16 can be read that both covalently cross-linked blend membranes expose nearly similar thermal stabilities, while the thermal stability of pure partially fluorinated B2 is slightly better than that of the pure fluorine-free B4 only above 450 °C which is however not relevant for the intended intermediate-T fuel cell application of these membranes. Therefore, covalent cross-linking can be regarded as a good method to improve thermal (and chemical, see below) stabilities of PBI-type membranes. 

Kerres J, Zhang W, Haring T (2004) Covalently cross-linked ionomer (blend) membranes for fuel cells. J New Mater Electrochem Syst 7 299-309... 

One disadvantage of the ionicaUy cross-linked (blend) membranes from polysulfonates and polybases is that the hydrogen bridges and electrostatic interactions break in aqueous enviromnent when the temperature is raised to T > 70-90°C, leading to unacceptable swelling in water and therefore to mechanical instability, which could lead to destruction of the membrane in the fuel cell. To overcome this instability, covalently cross-linked (blend) membranes also have been developed. 

Fig. 8.3 Scheme of covalently cross-linked membranes type (i) ionomer blend type (ii) ionomer... 

Both ionically cross-linked membranes (splitting-off of the ionic bonds at T = 70-90°C) and covalently cross-linked membranes (bleeding-out of sulfonated macromolecules from covalently cross-linked blend membranes, brittleness of dry membranes) show disadvantages. To overcome these disadvantages, we started the development of covalent-ionically cross-linked membranes [60] ... 

Table 8.3 Covalently cross-linked blend membranes with different cross-linking density...

Dependence of the membrane properties from the type of sulfmated poly(ethersnlfone). The two different poly(ethersulfones), PSU and PPSU, were used as the cross-linking component in the covalently cross-linked blend membranes. The membranes prepared from these polymers showed comparable properties. There are some indications that the thermal and mechanical stability of the membranes from sulfmated PPSU is slightly better than the thermal and mechanical stability of membranes using sulfmated PSU [88]. 

From Table 8.8 it can be concluded that the covalently cross-linked membrane 1,398 clearly showed the best properties in this series This membrane has the lowest resistance and is the only one which does not dissolve when inunersed in 90°C hot water. In the other membranes of this series, the interactions between the acidic blend component and other components are not strong enough to prevent dissolution in 90°C hot water. Particularly, the membrane 1,397 shows a low lEC and a high ff-resistance, indicating that already a considerable amount of sulfonated macromolecules had diffused out from the blend membrane matrix. Membrane 1,397 is not transparent, indicating a microphase-separated morphology, which facilitates leaching out of the sulfonated blend membrane component. 

At the covalently cross-linked (blend) membranes, also a wide variability in properties could be obtained. This membrane type showed good H+-conductivities and stabilities. Feaching-out of the sulfonated component could be avoided by... 

Covalently cross-linked ionomer membranes or blend membranes are expected to have dimensional and chemical stability to reduce methanol crossover. Cross-linking agents such as divinylbenzene, sulfonyl -imidazolide, 4,4 -diaminodiphenylsulfone can imbibe in the polymer main chain during polymerizations. Cross-linked polymer membranes showed reduced methanol crossover however, it is questionable whether the cross-linking bridges are stable in the strongly acidic environment of the fuel cell. 

Flexible ionomer networks can be built up via mixing polymeric acids and polymeric bases, obtaining networks that contain ionic cross-links formed by proton-transfer from the polymeric acid onto the polymeric base. It was observed that upon drying, acid-base blend membranes show reduced brittleness compared with un— cross-linked or covalently cross-linked ionomer membranes, which is possibly caused by the flexibility of ionic network. The developed acid-base blend membranes show outstanding thermal stabilities determined by DSC and TGA. 

The idea described above for glassy amorphous homopolymers can be extended to include miscible amorphous polymer blends, such as PS/PPO. Furthermore, a low degree of covalent cross-links can be considered as equivalent to entanglements for controlling the deformation mode. The strand density of cross-linked polymers is defined as the sum of the entanglement density and the covalent cross-link density [18] as... 

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