Nafion mechanical relaxations

Because the effect of neutralization by monovalent cations on the mechanical relaxations presents interesting features, it is also of interest to explore the effect of divalent cations. A preliminary study of this kind was conducted by Kyu and coworkers (58). Nafion acid of 1155 EW was neutralized with an appropriate BaCl2 solution to prepare a Ba-salt sample. The torsion pendulum results in the form of G, G" and tan 6 versus temperature are depicted in Figure 32. A peak is evident at approximately -90°C in the tan 6 and G" curves. Judging from the peak temperature, this y relaxation is probably caused by the same mechanism as in the acid and the monovalent salt samples described before. The 6 peak occurs at approximately -2O C and thus overlaps slightly with the y peak. The observation of the 6 region at such a low temperature may be due to the presence of residual water which would reduce ionic interactions within the ionic domains and in turn would enhance backbone mobility. 

The primary objective of this paper is to review the mechanical behavior of dry and hydrated Nafions with an emphasis on the mechanical relaxations. The tentative assignments of the relaxation mechanisms underlying the three mechanical relaxations are discussed in connection with the structure of the ionic aggregates... 

In other words, the counter cation may be isolated from the bound anion by water shielding, resulting in a weakening of the interaction between the cation and anion. Hence, the effect of the counterion on the mechanical relaxation in the under-water state is not significant. It is obvious, therefore, that the enhancement of the primary relaxation upon neutralization of Nafion membranes in the dry state is mainly due to the strong ionic interactions. ... 

Effect of Crystallinity. While neutralization is known to exert a strong influence on mechanical relaxations, it is conceivable that the degree of crystallinity is also significant. To explore this aspect, an amorphous Nafion salt sample was prepared by rapid quenching from the melt (40). The original Nafion salt (EW = 1200) had a degree of crystallinity of ca. 7% as revealed by a wide angle X-ray diffraction study (40). The quenched sample was also found to be completely amorphous by the absence of the crystalline diffraction peak. 

In a subsequent communication, Elliott and coworkers found that uniaxially oriented membranes swollen with ethanol/water mixtures could relax back to an almost isotropic state. In contrast, morphological relaxation was not observed for membranes swollen in water alone. While this relaxation behavior was attributed to the plasticization effect of ethanol on the fluorocarbon matrix of Nafion, no evidence of interaction between ethanol and the fluorocarbon backbone is presented. In light of the previous thermal relaxation studies of Moore and co-workers, an alternative explanation for this solvent induced relaxation may be that ethanol is more effective than water in weakening the electrostatic interactions and mobilizing the side chain elements. Clearly, a more detailed analysis of this phenomenon involving a dynamic mechanical and/ or spectroscopic analysis is needed to gain a detailed molecular level understanding of this relaxation process. 

Paddison et al. performed high frequency (4 dielectric relaxation studies, in the Gig ertz range, of hydrated Nafion 117 for the purpose of understanding fundamental mechanisms, for example, water molecule rotation and other possible processes that are involved in charge transport. Pure, bulk, liquid water is known to exhibit a distinct dielectric relaxation in the range 10—100 GHz in the form of an e" versus /peak and a sharp drop in the real part of the dielectric permittivity at high / A network analyzer was used for data acquisition, and measurements were taken in reflection mode. 

Cable and Moore performed DMA (dynamic mechanical analysis) studies of various Nafion membranes including the acid form. ° A tan <3 peak with maximum at 110 °C, referred to as Tg , was seen, and there is a suggestion of a shoulder on the low temperature side that might arise from another mechanism. As this membrane was dried at only 60 °C, the possibility of residual water incorporation exists. Moore and Cable concluded that the a relaxation was due to chain motions within and/or near the ion-rich domains and that the ji relaxation was... 

The lowest temperature 6 peak appears at approximately -160 C in the vibrating reed measurement whereas only the onset of 6 relaxation can be seen near -18O C in the torsional pendulum test. As discussed below, this peak is not present in the Nafion acid or in its salt membranes in both mechanical and dielectric measurements. 

A low-temperature y relaxation at ca. -160°C at a. 300 Hz has an activation energy of 17 kJ/mol as determined dielectrically. It is mechanically very weak (E"max Vj x 10 N/m, tan 6 vLCT ), but it is dielectrically most intense (e" 10 2). it is due to the -SCLF group and is probably caused by either the rotation about tne C-S bond or by a localized wagging motion of the SO2F group itself. This Y relaxation is not detected in the frequency-temperature range of the torsion pendulum studies and is not found in the dielectric studies of the ionized Nafion membranes. 

A 3 relaxation occurs in the temperature range from -100 to -20°C. At low frequencies, ca. 0.3 Hz, it can be resolved into two components at ca. -130 and -95°C, labeled 3 and 3" respectively. The activation energy for the dielectric 3 relaxation process is ca. 45 kJ/mole. The high temperature 3" process primarily reflects the dielectric 3 relaxation process which likely originates from motions of the fluorinated ether side chains. On the other hand, the low temperature 3 process, which corresponds to the mechanical y relaxation in the ionized Nafions, as well as in PTFE, is attributed to local short-range motions of the fluorocarbon backbone. The mechanism of the 3M process should also be observable in the case of ionized Nafion however, this is hidden by the pronounced water-dependent 3 peak. 

The recent observation of the water sensitivity of the primary relaxation of both Nafion acid and its salts has led to conflicting ideas regarding the assignment of the relaxation mechanisms of the a and 6 peaks. At this time, it should be of interest to reanalyse the phenomena based on the overall evidence available to date. The phenomena which make this assignment so difficult may be summerized as follows. 

The long delayed responses of the fuel cell to changes in load have been attributed to mechanical property changes in the polymer. We have initiated measurements of polymer stress relaxation. The stress relaxation and viscoelastic creep of Nafion is both temperature and water concentration dependent. Response times vary from 1 s to 10 s, which can give a wide range of characteristic response times for PEM fuel cells. 

Di Noto, V., Gliubizzi, R., Negro, E. and Pace, G. 2006. Effect of SiOj on relaxation phenomena and mechanism of ion conductivity of [Nafion/(Si02)j] composite membranes. J. Phvs. Chem. B 110(49) 24972-24986. 

Nicotera et al. investigated the behavior of water confined in recast NAFION and in NAFION-clay hybrids membranes using PFGSE NMR and spin—lattice relaxation time Tj, and concluded that the transport mechanism is influenced from the dimensions of the dispersed platelets and mainly from the type of nanocomposites formed upon mixing the clay particles with the polymer matrix. Compared to pure NAFION, the water uptake and the water diffusion of the hybrid membranes are increased, with the exception of the Kunipia-NAFION composite [83]. 

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