Abstract
We present a comparative theoretical study of the differences in rotational degrees of freedom of the various C−C, C−O, C−S, and S−O bonds in oligomeric sequences of the short-side-chain (SSC) perfluorosulfonic acid (PFSA) membrane for the purposes of understanding the flexibility of pendant side chain and the ability of the sulfonic acid group to aggregate in the polymer. Potential energy profiles were determined at the B3LYP/6-31G** level in the CF3CF(-O(CF2)2SO3H)-(CF2)7-CF(-O(CF2)2SO3H)CF3 two pendant side chain fragment of the PFSA polymer. The rotational barrier of the CF2−CF2 bonds along the backbone was found to be nearly 7.0 kcal/mol and is the energy difference between the staggered trans and planar cis conformations. Furthermore, the calculations revealed that the stiffest portion of the side chain is near its attachment to the backbone, with the CF−O and O−CF2 barriers of 9.1 and 8.0 kcal/mol, respectively. The most flexible portion of the side chain is the carbon-sulfur bond with a barrier of only 2.1 kcal/mol.
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