Post-deposition ageing reactions differ markedly between plasma polymers deposited from siloxane and silazane monomers

Plasma polymer coatings deposited from hexamethyldisiloxane (HMDSO) and hex amethyldisilazane (HMDSA) were monitored by XPS, FTIR and contact angle (CA ) measurements as they aged in air after fabrication. Of particular interes t was the influence of the monomer structure on the long term properties of the plasma deposited materials: in conventionally synthesized organosilico n materials, the siloxane unit provides very good long-term stability where as the silazane structure is prone to hydrolytic attack. During plasma depo sition of both coatings, abstraction of methyl groups was the major activat ion mechanism and the monomer structure was retained to a substantial exten t. In the case of plasma polymerised (pp) HMDSA, however, other reactions s uch as Si-N bond cleavage resulted in considerably more structural diversit y. During storage in air, the ppHMDSO him underwent minor chemical changes such as incorporation of additional siloxane crosslinks and a small extent of loss of methyl groups. The chemical structure of both the freshly deposi ted material and the aged coating were unusually homogeneous, compared with the broad range of chemical structures typically found in most other plasm a polymers. The structure of the ppHMDSA coating, in contrast, changed dram atically on ageing: almost all silazane moieties were lost after one year a nd substantial amounts of oxygen incorporated, mainly in the form of siloxa ne links. In spite of the initial chemical differences, the two materials b ecame more similar over time, with the final structures of the aged materia ls based on a cross-linked siloxane backbone. The wettability data reflecte d the structural differences between the two materials. However, correlatio ns between structures and surface properties were not predictable. The over all wettability of these surfaces was determined by a complex balance of se veral factors such as chemical structure, topography and mobility. (C) 1999 Elsevier Science Ltd. All rights reserved.