Molecular Modeling of Proton Transport in the Short-Side-Chain Perfluorosulfonic Acid Ionomer
Bibliographic record
Abstract
An explanation for the superior proton conductivity of low equivalent weight (EW) short-side-chain (SSC) perfluorosulfonic acid membranes is pursued through the determination of hydrated morphology and hydronium ion diffusion coefficients using classical molecular dynamics (MD) simulations. A unique force field set for the SSC ionomer was derived from torsion profiles determined from ab initio electronic structure calculations of an oligomeric fragment consisting of two side chains. MD simulations were performed on a system consisting of a single macromolecule of the polymer (EW of 580) with the general formula F3C-[CF(OCF2CF2SO3H)-(CF2)7]40-CF3 at hydration levels corresponding to 3, 6, and 13 water molecules per sulfonic acid group. Examination of the hydrated morphology indicates the formation of hydrogen bond "bridges" between distant sulfonate groups without significant bending of the polytetrafluoroethylene backbone. Pair correlation functions of the system identify the presence of ion cages consisting of hydronium ions hydrogen-bonded to three sulfonate groups at the lowest water content. Such structures exhibit very low S-OH3+ separations, well below 4 A and severely inhibit vehicular diffusion of the protons. The number of sulfonate groups in the first solvation shell of a given hydronium ion correlates well with the differences between Nafion and the SSC polymer (Hyflon). The calculated hydronium ion diffusion coefficients of 2.84 x 10-7, 1.36 x 10-6, and 3.47 x 10-6 cm2/s for water contents of 3, 6, and 13, respectively, show only good agreement to experimentally measured values at the lowest water content, underscoring the increasing contribution of proton shuttling or hopping at the higher hydration levels. At the highest water content, the vehicular diffusion accounts for only about 1/5 of the total proton transport similar to that observed in Nafion.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".