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2342 SATTERFIELD ET AL. data show that the elastic modulus is less than that of a dry membrane at those temperatures or that of a wet membrane at 25 8C. There are not yet sufficient data to identify how Tg changes with the water content. Adding TiO2 to make a composite membrane increased the elastic modulus of the membrane at increased water content. This improvement is high- lighted in Figure 6. The addition of TiO2 did not have a significant effect on the temperature depend- ence of the elastic modulus. A surprising result obtained for the Nafion/titania composite materials was that the plastic modulus decreased. The com- posite membranes showed less strain hardening than Nafion, especially at high water contents. Figure 7 shows that the plastic modulus of the com- posite membrane decreased compared with that of Nafion at high water contents and 25 8C. Some Nafion/Zirconia composite membranes that we tested showed a plastic modulus of zero; there was no change in the stress as they were strained. The results show that modifying membranes with titania causes the mechanical properties of the membranes to be altered more by water than temperature. Because the metal oxide particles are hydrophilic, we believe that they interact strongly with the hydrophilic (sulfonic acid) domains of Nafion. Water sorption alters the interaction between the metal oxide surfaces and the hydro- philic domains of Nafion, and this results in changes in the mechanical properties. We do not yet have sufficient experimental information to quantify the magnitude of the particle/Nafion/ water interaction. The creep experiments showed that the mem- branes crept over a long time period when placed under moderate stress. It was surprising that Nafion/TiO2 composite membranes crept much less than Nafion when placed under a constant stress, as shown in Figure 8. It was anticipated that the composite membranes would creep more than Nafion because they had a lower plastic mod- ulus, which indicated less strain hardening; this should have permitted the composite materials to flow more readily. However, most of the creep experiments were performed at stresses less than the yield stress, so there was not a direct connec- tion between the creep and the plastic modulus. Creep does increase with increasing temperature and decreasing water content, as shown in Figure 9(A,B). Creep may play an important role in fuel cell failure; the creep from high stress points may thin out the membrane over time, eventually caus- ing pinholes in the membrane. The measurements shown in Figures 12 and 13 are some of the first data showing how the com- pressive forces can alter the resistance of polymer electrolytes. These results also show substantial hysteresis in the membrane resistivity from com- pression. Membrane resistivity increased by 10– 15% for an applied pressure of 7.25 MPa. This increase in resistivity appears to be the result of simple mechanical compression of the membrane and not actually a change in the resistivity. The re- sistance of the central section of the dog bone is measured lengthwise along the neck of the dog bone. The resistance (Rmembrane) is given by eq 8: Rmembrane 1⁄4 q L ð8Þ Wt where q is the resistivity, t is the membrane thick- ness, W is the width, and L is the length. When the membrane is compressed, t will decrease by Dt with the applied stress (Papplied): t 1⁄4 Papplied t ð9Þ E where E is the elastic modulus. A wet membrane at 25 8C has an elastic modulus of $50 MPa (see Fig. 6), so an applied stress of 7 MPa will reduce the membrane thickness by 14% and hence in- crease the membrane resistance by 14%. The data shown in Figure 12 assume a constant membrane thickness, so the changes in the resistivity may represent dimensional changes in the membrane due to applied stresses. At room temperature, the elastic modulus is large, so dimensional changes of the membrane by compression produce only mod- est changes in the membrane resistance. However, at higher temperatures of 80–100 8C, the modulus is less than 10 MPa, so the applied compression sealing the fuel cell could increase the membrane resistance by factors of 2 or more. If the hysteresis in the resistivity shown in Fig- ures 12 results from dimensional changes of the membrane, the results from Figure 13 suggest substantial hysteresis in the dimensions of the membrane upon compression and relaxation. Acc- ording to the compression and relaxation experi- ment shown in Figure 13, a membrane that is com- pressed may be frozen into a partially compressed state for extremely long periods of time. The resist- ance measurements indicate a change of 5% existed between compressing a membrane to 3.6 MPa from 0 MPa and decompressing a membrane to 3.6 MPa from 7.2 MPa. Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polbPDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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