From soup to structure: Simulating hydrated semi-crystalline proton exchange membranes

Area of Science:

• Materials Science
• Computational Chemistry
• Polymer Physics

Background:

• Proton exchange membranes (PEMs) are crucial for fuel cells, but simulating their complex polymer structures is computationally challenging.
• Understanding the semi-crystalline morphology of polymers like Nafion is key to optimizing their performance.
• Existing simulation methods often lack the efficiency or accuracy needed for long-timescale crystallization processes.

Purpose of the Study:

• To develop an efficient and accurate computational methodology for simulating high molecular weight polymers, specifically Nafion.
• To investigate the influence of side chain distribution on polymer crystallinity and morphology.
• To provide a predictive tool for understanding structure-property relationships in PEMs.

Main Methods:

• Implemented a novel simulation protocol involving random insertion of long polymer chains and ghost chain randomization for enhanced computational efficiency.
• Utilized dissipative particle dynamics (DPD) to relax overlaps and capture long-timescale crystallization.
• Incorporated a two-stage process: initial relaxation of soft chains (promoting crystallinity) followed by chain stiffening (simulating cooling and crystal growth).

Main Results:

• The simulation methodology accurately reproduced Nafion's semi-crystalline morphology, validated against small-angle X-ray scattering (SAXS) data.
• Demonstrated that side chain distribution significantly impacts polymer crystallinity and the formation of realistic morphologies.
• Resolved key molecular processes including chain ordering, local alignment, and molecular packing.

Conclusions:

• The developed simulation protocol offers strong predictive capability for crystallinity content, crystallite size, and shape in Nafion and related polymer systems.
• This methodology provides a deeper understanding of structure-property relationships in semi-crystalline polymers.
• The approach is applicable to a wide range of polymer systems, offering a valuable tool for materials design and optimization.