Pyrolysis Pathways of Sulfonated Polyethylene, an
Alternative Carbon Fiber Precursor
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Abstract
Polyethylene is an emerging precursor
material for the production
of carbon fibers. Its sulfonated derivative yields ordered carbon
when pyrolyzed under inert atmosphere. Here, we investigate its pyrolysis
pathways by selecting <i>n</i>-heptane-4-sulfonic acid (H4S)
as a model compound. Density functional theory and transition state
theory were used to determine the rate constants of pyrolysis for
H4S from 300 to 1000 K. Multiple reaction channels from two different
mechanisms were explored: (1) internal five-centered elimination (E<sub><i>i</i></sub>5) and (2) radical chain reaction. The pyrolysis
of H4S was simulated with kinetic Monte Carlo (kMC) to obtain thermogravimetric
(TGA) plots that compared favorably to experiment. We observed that
at temperatures <550 K, the radical mechanism was dominant and
yielded the <i>trans</i>-alkene, whereas <i>cis</i>-alkene was formed at higher temperatures from the internal elimination.
The maximum rates of % mass loss became independent of initial ȮH
radical concentration at 440–480 K. Experimentally, the maximum
% mass loss occurred from 440 to 460 K (heating rate dependent). Activation
energies derived from the kMC-simulated TGAs of H4S (26–29
kcal/mol) agreed with experiment for sulfonated polyethylene (∼31
kcal/mol). The simulations revealed that in this region, decomposition
of radical HOSȮ<sub>2</sub> became competitive to α-H
abstraction by HOSȮ<sub>2</sub>, making ȮH the carrying
radical for the reaction chain. The maximum rate of % mass loss for
internal elimination was observed at temperatures >600 K. Low-scale
carbonization utilizes temperatures <620 K; thus, internal elimination
will not be competitive. E<sub><i>i</i></sub>5 elimination
has been studied for sulfoxides and sulfones, but this represents
the first study of internal elimination in sulfonic acids