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2336 SATTERFIELD ET AL. Figure 9. Long time creep rate (defined by eq 4) of Nafion 115 (A) as a function of temperature at a fixed water content of k $ 8 6 2 H2O/SO3 and (B) as a func- tion of the water content at 21–25 8C. greater than 104 s. For fuel cell performance, we are principally interested in the long-time creep, and we have defined the creep rate (eq 4) as the slope of the strain–log(time) response at times greater than 100 min, normalized by the applied stress:44,45 dstrain Creep Rate 1⁄4 stress ð4Þ Figure 9 shows the creep rate of extruded Nafion 115 membranes as a function of the temperature and water content. We determined the water con- tent by weighing the membranes immediately af- ter the creep measurements. At a fixed stress, the creep rate increased with increasing temperature and decreased with increasing water content. The reduced creep rate with an increased water con- tent, shown in Figure 9, would appear to be at odds with Figure 8, in which a higher water content leads to greater total creep. This apparent discrep- ancy is because the creep rate refers to long times. Samples with higher water contents crept faster initially, but the rate of creep slowed more at lon- ger times. The dry samples crept less initially but continued to creep more at longer times. Dynamic Water Uptake Figure 10 shows typical results for the water uptake from saturated vapor for unconstrained samples of extruded and recast Nafion/titania com- posite membranes at various temperatures. The same three materials reported in Table 1 were used for the data shown in Figure 10. The results show that water was absorbed faster at higher temperatures and that recast Nafion membranes, with or without titania, absorbed water faster than extruded Nafion. The results for equilibrated water sorption given in Table 1 show small differ- ences in the water sorption between the three dif- ferent materials. It appears that the kinetics of water sorption is sensitive to the method of prepa- ration. This is not surprising considering because others have reported different microstructural morphologies (based on small-angle X-ray scatter- ing experiments) between extruded Nafion and so- lution-cast Nafion.46 The extrusion process results in orientation of the material’s microstructure, and this leads to differences in the physical proper- ties (viz., electrical conductance and swelling). Because solvent transport is linked to both the degree of swelling and morphology, the kinetics of water absorption should differ for recast and ex- truded Nafion. The water diffusivity (D) was estimated by the fitting of the data at a low water uptake to eq 5:12 where M/M? is the mass uptake at time t with respect to the mass uptake after 24 h and ‘ is the initial membrane thickness. A value of $1 108 cm2/s was obtained for the diffusivity. However, the diffusivities for the recast membranes were lower than those for extruded Nafion. This result was disturbing because the recast membranes sorbed water faster than extruded Nafion. The problem with this analysis for diffusivities is that it fails to account for the energy changes and dimensional changes that accompany water sorption. Values of diffusivities are not reported Journal of Polymer Science: Part B: Polymer Physics d1⁄2logðtimeÞ M 2p‘2 D1⁄4 M1 16t ð5Þ DOI 10.1002/polbPDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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