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Figure 8. Creep data for Nafion 115 and Nafion/3 wt % titania 115 at 23 8C for both 0% RH (dry) and 100% RH with an applied engineering stress of 7.5 MPa. modulus is $300 MPa, and it decreases to $100 MPa at 80 8C (the normal operating temper- ature for PEM fuel cells). The elastic modulus drops precipitously to less than 10 MPa above 100 8C. The large decrease in the elastic modulus coincides with Tg, which has been reported to be 110 8C.17,20,43 Absorbed water also reduces the elas- tic modulus; the elastic modulus at 25 8C decreases from 300 to 50 MPa as the water content in the membrane increases. The plastic modulus of Nafion (Fig. 7) shows a weaker dependence on the temperature and water content than the elastic modulus. The plastic modulus is almost independent of the temperature below Tg (Tg $ 110 8C) with a value of $8–10 MPa, and it decreases to $2 MPa above Tg. At room temperature, the water content in the membrane has little measurable effect on the plastic modulus. We have found two key results when compar- ing the properties of Nafion and Nafion/titania composites: (1) water sorption reduces the elastic modulus of Nafion more than that of Nafion/tita- nia composites and (2) water sorption reduces the plastic modulus of Nafion/titania composites more than that of Nafion. The temperature affects the mechanical properties of Nafion and Nafion/titania composite membranes compara- bly. Composite materials generally have higher Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb elastic moduli, but at the low loading of 3 wt % ($1.5 vol %), we did not see much effect in the dry materials. The effects of adding the metal ox- ide particles were manifested at higher mem- brane water contents. Creep Tests During the normal operation of a fuel cell, electro- des are pressed against the polymer electrolyte membrane, and this puts the polymer under com- pressive stress. The effect of constant tensile stress on two different membrane materials was eval- uated by the measurement of their creep. Figure 8 compares the creep responses of an extruded Nafion 115 membrane and a Nafion/3 wt % TiO2 115 membrane. Nafion 115 crept nearly 40% more than the composite membrane over a period of 3000 min (>2 days) when subjected to the same stress. Both the polymer and composites stretched rap- idly over the first several minutes (the short-time response is blown up in the inset in Fig. 8). After $1–10 min, the polymer membrane strains much more slowly. Polymer creep data can be fit to vari- ous functional forms, including stretched exponen- tials derived from spring-dashpot models. The data shown in Figure 8 are linear when plotted as the strain versus log(time) for times from 10 s to TITANIA/NAFION COMPOSITE MEMBRANES 2335PDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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