(180am) Simulations of the Pyrolysis of Hydridopolycarbosilane (HPCS) Polymer Using the Reaxff Reactive Force Field

Silicon carbide (SiC) membranes have great advantages for separation processes in harsh and corrosive conditions, due to their high corrosion resistance, large thermal conductivity, high thermal shock resistance, and excellent chemical and mechanical stability. They are fabricated by the pyrolysis of polycarbosilane (PCS) polymer precursors, such as HPCS. The polymer coats the pore space of the membrane's support, the pore space of which consists of interconnected pores of irregular shapes and sizes. Pyrolysis is then used to generate a layer of SiC that coats the pores' surface. Thus, understanding how the pyrolysis occurs is of paramount importance. A key step for developing a model of the membrane is understanding of the pyrolysis of the polymeric precursor.

We use quantum-chemical calculations to develop a force field, the ReaxFF, to study the pyrolysis of HPCS at high temperatures. The force field parameters are optimized for bonds, angles, charges, and equation of state of silicon carbide. Based on molecular dynamics (MD) simulations using ReaxFF we determine the thermal decomposition products of the HPCS to be the hydrogen radicals and the associated polymer radicals, indicating that the decomposition and subsequent crosslinking of the polymer is initiated by Si-H and C-H bond cleavage, in agreement with experimental observations. Secondary reactions involving the H radicals lead primarily to formation of hydrogen gas, the major product of the pyrolysis process. The simulations also indicate that the presence of methane, silane, and methylsilane gases in traces amount, also in agreement with experimental observations. We also study the temperature-dependence of the HPCS pyrolysis by following the bond population and the rate of production of the various radicals and molecules. The temperature dependence of the production of the H₂ molecules was studied, in order to extract the Arrhenius parameters for the failure modes of the HPCS.