Mechanical properties, poly phosphazene

The inorganic poly(phosphazene) backbone has received attention as a PEM candidate. This is an attractive system for study due to its ease of synthesis and subsequent modification by many functional groups. However, these membranes generally show low glass transition temperatures and somewhat poor mechanical properties, and they require cross-linking to enhance their performance in hydrated environments. 

Poly[(trifluoroethoxy) (octafluoropentoxy) (X)phosphazene] (X — cross-link site) PFAP(II), undergoes a loss in molecular weight at elevated temperatures (135°-200°C) that is detrimental to mechanical properties. The mechanism of molecular weight loss was found to be random chain scission at weak links in the PFAP(II) chain. These weak links are postulated to be phosphazane moieties which would be attacked by water, acids, and POH to produce PN chain scission. 

Poiyphosphazene Biends Blends of sulfonated polyphosphazene, for example, sulfonated poly[bis(3-methylphenoxy)phosphazene] or poly[(bisphenoxy)phosphazene] (see Fig. 29.14) with either an inert organic polymer such as poly(vinylidene fluoride) (Pintauro and Wycisk, 2004 Wycisk et al., 2002) or polyacrylonitrile (Carter et al., 2002) or a reactive polymer (e.g., polybenzimidazole) (Wycisk et al., 2005) have been investigated. The resultant membranes had conductivities of 0.01-0.06 S/cm (in water at 25°C) and equilibrium water swelling from 20 to 60% (at 25°C). Blends of poly[(bisphe-noxy)phosphazene] and polybenzimidazole (where acid-base complexation occurred between the sulfonic acid and the imidazole nitrogen) exhibited good mechanical properties and low methanol permeability. MEAs with this membrane material outperformed Nafion 117 in a DMFC at 60°C with concentrated (5-10 M) methanol feeds. With 1.0 M methanol and 0.5 L/min ambient air at 60°C, the maximum power density was 97 mW/cm and the methanol crossover was 2.5 times lower than that with Nafion 117 (Wycisk et al., 2005). 

Metal alkoxides of silicon, titanium, zirconium, and aluminum have been used to prepare optically transparent composite films with the etheric phosphazene polymer poly[bis(2-(2-methoxyethoxy)ethoxy)-phosphazene] (MEEP). Dynamic mechanical and stress-strain techniques are used to evaluate and compare the mechanical properties of the different composites. Despite strong interactions with the inorganic oxide network, the glass transition temperature (Tg) of MEEP does not shift to higher temperature upon addition of the inorganics, but the polymer is reinforced by the oxides. The addition of potassium triflate to the composites resulted in materials with both excellent mechanical properties and increased conductivity. 

We reported previously that the mechanical properties of the amorphous etheric poly[bis(methoxyethoxyethoxy)-phosphazene] (MEEP) were significantly improved by in situ polymerization of tetraethoxysilane (TEOS) (4). This work on etheric phosphazene composites was consistent with reports from the research groups of Mark (5) and Wilkes (6), where metal alkoxides were added to poly(dimethylsiloxane) and poly(tetramethylene oxide). We now extend the phosphazene sol-gel composites to include the more reactive metal alkoxides of titanium, zirconium, and aluminum, and we report that when polymerized in situ, each is compatible with the etheric phosphazene polymer. Dynamic mechanical and stress-strain techniques are utilized to evaluate and compare the mechanical properties of the different composites. Thermal and... 

Oxygen Gas Permeability and the Mechanical Properties of Poly(n-butylamino)(di- -hexylamino)phosphazene Membranes... 

Figure 4. Mechanical properties of poly(n-butylamino)(di-n-hexylaniino)-phosphazene films cured with 0.05 wt% of NPG. 0 50 "C, 100 C.

Furthermore, despite still being the route able to prepare the highest Mjy, ROP inherently produces polymers with broad polydispersities (Mjy/M >2) due to its initiation mechanism, in which the formation of new chains can occur throughout. Although such polydispersity is perfectly tolerable for many medical applications, for example, as inert biomaterials, the method is less suitable for some biomedical applications, in which precise molecular size is often an essential property. Furthermore, advanced polymer architectures and macromolecular constructs cannot be readily attained via this method, due to the absence of end-group control, and hence the development of poly(dichloro)phosphazene with controlled... 

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