The
gelation behavior of Nafion dispersions was investigated using
small-angle neutron scattering to better understand the mechanical
toughness of dispersion-cast Nafion membranes. Three types of gelation
were observed, depending on dispersing fluids: (i) homogeneous, thermally
reversible gelation that was present in most aprotic polar dispersing
fluids; (ii) inhomogeneous, thermally irreversible gelation as films,
found in alcohols; and (iii) inhomogeneous, thermally irreversible
gelation which precipitates in water/monohydric alcohol mixtures.
The mechanical toughness of dispersion-cast Nafion membranes depends
on the dispersing fluid, varying by more than 4 orders of magnitude.
Excellent correlation between the critical gelation concentration
and mechanical toughness was demonstrated with the Nafion membranes
cast at 140 °C. Additional thermal effects among Nafion membranes
cast at 190 °C were qualitatively related to the boiling point
of dispersing fluids. Little correlation between mechanical toughness
and percent crystalline area of Nafion was observed, suggesting that
the origin of mechanical toughness of dispersion-cast Nafion membranes
is due to chain entanglements rather than crystallinity. The correlation
between critical gelation concentration and mechanical toughness is
a new way of predicting mechanical behavior in dispersion-cast polymer
systems in which both polymer-dispersing fluid and polymer–polymer
interactions play a significant role in the formation of polymer chain
entanglements.
Syndiotactic polystyrene was lightly sulfonated in 1,2,4-trichlorobenzene using acyl sulfate complexes. The sulfonation efficiency of the acyl sulfate increased significantly when anhydrides containing long aliphatic groups were used to complex sulfuric acid. The high sulfonation efficiencies, relative to acetyl sulfate, were attributed to the increased solubility of the longer hydrocarbon complexes in trichlorobenzene. The incorporation of small quantities (less 3.4 mol %) of sulfonic acid groups onto the syndiotactic polystyrene backbone was found to have little effect on the glass transition temperatures of these new materials. However, a much more pronounced effect of sulfonation was observed in the ionomer crystallization. Increasing the level of sulfonation decreased the melting point, degree of crystallinity, and apparent rate of crystallization due to a rejection of sulfonated styrene units from the crystalline domains.
A major, unprecedented improvement in the durability of polymer electrolyte membrane fuel cells is obtained by tuning the properties of the interface between the catalyst and the ionomer by choosing the appropriate dispersing medium. While a fuel cell cathode prepared from aqueous dispersion showed 90 mV loss at 0.8 A cm(-2) after 30,000 potential cycles (0.6-1.0 V), a fuel cell cathode prepared from glycerol dispersion exhibited only 20 mV loss after 70,000 cycles. This minimum performance loss occurs even though there was an over 80% reduction of electrochemical surface area of the Pt catalyst. These findings indicate that a proper understanding and control of the catalyst-water-ionomer (three-phase) interfaces is even more important for maintaining fuel cell durability in typical electrodes than catalyst agglomeration, and this opens up a novel path for tailoring the functional properties of electrified interfaces.
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