ADHESION MECHANISMS OF SILICA LAYERS ON PLASMA-TREATED POLYMERS .1. POLYCARBONATE

We have studied the influence of plasma treatment in various gases (Ar , NH3, N-2) on the surface modification of polycarbonate (PC) and on t he adhesion of plasma-deposited silica layers to PC. Surface modificat ion was investigated using in situ IR ellipsometry, X-ray photoelectro n spectroscopy, and contact angle measurements interpreted in terms of electron-acceptor/donor groups. In addition, in situ UV-visible ellip sometry enabled crosslinking analysis. Due to UV photon emission, Ar p lasma treatment induced crosslinking as well as photo-Fries rearrangem ents and related reactions, thus creating phenolic groups. These group s are acidic sites and are likely to react with the electronegative ox ygen atoms of silica. Adhesion of silica to PC, as measured by the mic ro-scratch test, was enhanced by Ar plasma treatment. The improvement is attributed to the crosslinking of PC, on the one hand, and to acid- base interactions and/or covalent bonding between PC and silica, on th e other hand. A further improvement in adhesion was achieved using N-2 plasma treatments (pure or Ar-diluted). These treatments also induced crosslinking and phenolic group formation and, in addition, nitrogen grafting at the surface. Adhesion enhancement from Ar to N-2 treatment s is thus attributed to nitrogen-containing groups, which are likely t o promote covalent bonding between silica and the treated polymer surf ace. In contrast, Ar-diluted NH3 plasma treatment following Ar treatme nt resulted in decreased adhesion, which is attributed to reduced acid ity and low nitrogen grafting.