Siloxane Decomposition and Clustering in Atmospheric Pressure Non-Thermal Plasma Discharge

Abstract

The plasma-assisted decomposition and gas-phase polymerization of hexamethyldisiloxane [HMDSO, (CH3)3SiOSi(CH3)3] were investigated in an atmospheric-pressure dielectric barrier discharge (DBD) operated with a He/HMDSO mixture containing ~150 ppm HMDSO. Plasma-formed species were analyzed by molecular-beam mass spectrometry with high mass resolution, enabling detailed identification of reactive intermediates and oligomeric products. The plasma power was varied between 0.5 and 15 W to elucidate its influence on fragmentation and clustering kinetics. Increasing power significantly enhances the formation of higher-mass species, revealing a repetitive mass increment of 74.019 amu assigned to C2H6SiO (CH3SiOCH3) units. These observations indicate stepwise gas-phase clustering processes leading to oligomeric structures that represent fundamental building blocks of polydimethylsiloxane (PDMS). At lower power, mass growth is suppressed, while small silicon-containing fragments (e.g., SiH3O, CH5Si) and hydrocarbon radicals (CH, CH2, CH3, C2H5) dominate. Stable hydrocarbons such as CH4, C2H2, C2H4, and C2H6 are primarily detected at higher power, reflecting enhanced fragmentation and recombination pathways. A chemical kinetic model incorporating a simplified four-step polymerization sequence of dimethyloxosilane (CH3SiOCH3) up to C8H24Si4O4 reproduces the experimentally observed oligomer growth trends. The combined experimental and modeling approach provides mechanistic insight into plasma-induced precursor reformation and the early stages of PDMS formation under atmospheric-pressure conditions.