Polydimethylsiloxane surface irradiated by nitrogen ions: Influence of low molecular fractions

Polydimethylsiloxane (PDMS) surface was treated by nitrogen ions of 20 keV energy with 1016 ions/cm2 fluence. The surface of modified PDMS is characterized by ceramiclike structures. The treated PDMS contains free radicals. The wettability and surface energy change significantly immediately after the treatment and recover in a few days to the values of silicon dioxide or glass. Complex kinetics of growing carbonyl and hydroxyl groups and decaying silane groups is observed. The cage structure of the silicon oxide in the surface ceramiclike layer remains stable, while network structures of the Si-O increase and suboxides of the silicon decrease with storage time. The presence of not-cross-linked low molecular fractions in PDMS gives a fast recovery of the wettability and surface energy to untreated PDMS values in a few hours. This effect of low molecular fractions on the wettability and surface energy was avoided by washing out the cured PDMS in a solvent before the treatment.