Pyrolysis pathways of sulfonated polyethylene, an alternative carbon fiber precursor

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Polyethylene derivatives are promising precursors for carbon fiber production.
• Understanding pyrolysis pathways is crucial for optimizing carbon material synthesis.

Purpose of the Study:

• To investigate the pyrolysis mechanisms of sulfonated polyethylene using n-heptane-4-sulfonic acid (H4S) as a model compound.
• To determine the rate constants and dominant reaction channels for H4S pyrolysis across a wide temperature range.

Main Methods:

• Density Functional Theory (DFT) and Transition State Theory (TST) were employed to calculate reaction rate constants.
• Kinetic Monte Carlo (kMC) simulations were used to model pyrolysis and generate thermogravimetric analysis (TGA) plots.
• Comparison of simulated TGA data with experimental results for validation.

Main Results:

• Radical chain reactions dominate H4S pyrolysis below 550 K, yielding trans-alkenes.
• Internal five-centered elimination (Ei5) becomes significant at higher temperatures (>600 K), producing cis-alkenes.
• Simulated TGA plots showed good agreement with experimental data, with activation energies of 26-29 kcal/mol.
• The radical mechanism's maximum mass loss rate is independent of initial hydroxyl radical concentration between 440-480 K.

Conclusions:

• The study elucidates the competing pyrolysis mechanisms of sulfonated sulfonic acids.
• Radical pathways are dominant at temperatures relevant for low-scale carbonization (<620 K).
• This work provides the first investigation into internal elimination mechanisms in sulfonic acids.