Abstract
Sulfonated polyethylene is an emerging precursor for the production of carbon fibers. Pyrolysis of sulfonated polyethylene was characterized by thermogravimetric analysis (TGA). n-heptane-4-sulfonic acid (H4S) was selected as a model compound for the study of sulfonated
polyethylene. Density functional theory and conventional transition state theory were used to
determine the rate constants of pyrolysis for H4S from 300-1000 K. Multiple reaction channels from two different mechanisms were explored: 1) internal five-centered elimination (Ei 5) and 2) radical chain reaction. The pyrolysis of H4S was simulated with kinetic Monte Carlo (kMC) to obtain TGA plots that compared favorably to experiment. We observed that at tem-
peratures < 550 K, the radical mechanism was dominant and yielded the trans-alkene, whereas
cis-alkene was formed at higher temperatures from the internal elimination. The maximum
rates of % mass loss became independent of initial OH radical concentration at 440-480 K.
Experimentally, the maximum % mass loss occurred from 440-460 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 HOSO2 became competitive to αH abstraction by
HOSO2, making OH 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. Ei5 elimination has been studied for sulfoxides and sulfones, but this represents the first study of internal elimination in sulfonic acids. Nonlinear Arrhenius plots were found for all bimolecular reactions. The most significant nonlinear behavior was observed for reactions where the barrier was small. For reactions with low activation barriers, nonlinearity was traced to conflicting trends between the exponential temperature dependence of the energetic term and the temperature dependence of the vibrational partition function of the transitional modes.
