Wetting of Ion Containg Polymers: Micelles Versus Molecular Absorption
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189 Mat. Res. Soc. Symp. Proc. Vol. 543 01999 Materials Research Society
A large effort is underway to understand the structure and dynamics of ion containing polymers in the bulk [9,10,11]. Combination of small, wide, and anomalous x-ray scattering, carried out on the bulk, have shown that for most common ionomers such as Nafion derivatives, the polar functional groups on the backbone, aggregate in clusters. These clusters, which incorporate the counter ions, and are randomly distributed in a hydrophobic matrix. Their dimensions vary between 3-5nm, depending on the side chain length and counter-ion. The hydrophobic backbone consists of both amorphous and crystalline domains where the ratio between the two micro phases depends on the molecular weight, the nature of the hydrophobic backbone and the counter-ion. The ionic clusters act as physical cross-links but in contrast to chemical cross-links they allow rearrangements. In the bulk, these Coulombic associations form a dynamic network in which ion pairs can be transferred between different aggregates. Crystallinity in the system locks the configuration in and does not allow further rearrangements. Sample preparation, humidity as well as annealing have significant effects on the resulting properties, indicating that in many cases the system is kinetically trapped in a meta-stable state [11,12]. When the polymer is dissolved in polar solvents, a micellar solution, which consists of cylindrical aggregates was observed. The ionic groups either form internal clusters, similar to the melt or lie towards the periphery of the aggregate [13]. The studies of melts as well as of solutions provided an immense amount of knowledge on the nature and behavior of these physical networks. This work focuses on interfacial behavior in ultra thin films and its correlation to the properties of the polymer in the mother solution. Two different polymers with a perfluoro carbon backbone were investigated: (I) [(CF 2CF 2)n(CF 2CFR)]m
where R= [OCF 2CF 2SO 3 (X)] (II) [(CF 2)n (CF 2CFR)]m
where R= [OCF 2 CF(CF 3)OCF 2CF2SO2N(X)SO 2CF3] and X- correspond to a counter-ions These polymers differ from each other in the polarity of their side chain. Polymer (I) is currently used as one of the materials that form the solid polymeric membranes in fuel cells and polymer (II) is a novel material recently developed [14, 15] with extremely high temperature stability and unique potential as a novel fuel cell membrane. EXPERIMENTAL Polymer (I) was obtained from Dow Chemical and is 800 equivalent weight. Polymer (II) was made by D. Desmarteau. The details of the synthesis are given in ref [15]. Light scattering measurements suggested that the polymer is of -1.6 million, molecular weight and n varies between 4 and 5. The polymers were cast on a 2mm thick, single crystal, silicon wafers. The wafers were first oxidized and treated with a 5Wt% solution of I-F. The ionomers were dissolved in a water/ethanol mixtures with small amounts of t-butanol with concentrations indicated for the specific solutions.
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