  During the carbonization process of raw polymer precursors, graphitic
structures can evolve. The presence of these graphitic structures affects
mechanical properties of the carbonized carbon fibers. To gain a better
understanding of the chemistry behind the evolution of these structures, we
performed atomistic scale simulations using the ReaxFF reactive force field.
Three different polymers were considered as a precursor: idealized ladder PAN
(polyacrylonitrile), a proposed oxidized PAN and PBO
(poly(p-phenylene-2,6-benzobisoxazole)). We determined the underlying molecular
details of polymers conversion into a carbon fiber structure. Since these are
C/H/O/N-based polymers, we first developed an improved force field for C/H/O/N
chemistry based on the Density Functional Theory (DFT) data with a particular
focus on N2 formation kinetics and its interactions with polymer-associated
radicals formed during the carbonization process. Then, using this improved
force field, we performed atomistic scale simulations of the initial stage of
the carbonization process for the considered polymers. Based on our simulation
data we determined the molecular pathways for the formation of low-molecular
weight gas-species, all-carbon rings crucial for further graphitic structures
evolution and possible alignment of the evolved all-carbon 6-membered rings
clusters.
