Side chain flexibility in perfluorosulfonic acid ionomers: an ab initio study

Area of Science:

• Materials Science
• Computational Chemistry
• Polymer Science

Background:

• Perfluorosulfonic acid (PFSA) ionomers are crucial for electrochemical applications.
• Understanding side chain flexibility is key to optimizing ionomer performance.
• Previous studies have not fully explored the conformational dynamics of different PFSA side chains.

Purpose of the Study:

• To investigate the side chain flexibility of three distinct PFSA ionomers: Nafion, Aquivion (short side chain - SSC), and 3M PFSA.
• To determine the influence of solvent effects on the rotational potential energy surfaces of these side chains.
• To compare the conformational freedom and energetic landscapes of different PFSA side chain structures.

Main Methods:

• Ab initio electronic structure calculations using density functional theory (DFT).
• Employed B3LYP and B97D functionals, with and without a solvation model.
• Calculated rotational potential energy surfaces for bonds along the PFSA side chains.

Main Results:

• The carbon-sulfur bond exhibits the highest flexibility, further enhanced by solvent inclusion.
• Nafion side chains show substantial conformational freedom despite some high-energy barriers.
• The 3M PFSA side chain is the most rigid, with significant rotational barriers around its central carbon-carbon bonds.

Conclusions:

• Side chain length and chemical structure significantly influence rotational potential energy surfaces in PFSA ionomers.
• Solvent effects are more pronounced near the sulfonic acid group than the backbone.
• Conformational flexibility, particularly of the C-S bond, is a critical factor in PFSA ionomer behavior.