Vapor-Phase Silanation of Plasma-Polymerized Silica-Like Films by γ-Aminopropyltriethoxysilane

Abstract

ABSTRACT Silica-like films were deposited onto aluminum substrates by plasma polymerization of hexamethyldisiloxane (HMDSO) in the presence of oxygen using a capacitively coupled Radio-Frequency (RF)-powered plasma reactor. Two types of films, referred to as low hydroxyl and high hydroxyl, were obtained by varying parameters such as the power and the flow rates of oxygen and HMDSO. Two bands that were assigned to hydrogen-bonded silanol groups were observed in reflection–absorption infrared (RAIR) spectra of high-hydroxyl films. The first band was near 3400 cm−1 and was very broad; this band was assigned to the OH-stretching mode of silanol groups in which the hydrogen atom was hydrogen bonded to the oxygen atom of an adjacent silanol group. The second band was near 3650 cm−1 and was assigned to OH stretching in isolated silanol groups in which the hydrogen atom was hydrogen bonded to the oxygen atom of an Si‒O‒Si group. Only the band near 3650 cm−1 was observed in RAIR spectra of low-hydroxyl films. RAIR spectra of the high-hydroxyl films were characterized by a band near 934 cm−1 assigned to the Si‒O-stretching mode of silanol groups. This band was not observed in spectra of the low-hydroxyl films. γ-Aminopropyltriethoxysilane (γ-APS) was adsorbed onto the silica-like films from the vapor phase at a substrate temperature of 100°C. Results obtained from X-ray photoelectron spectroscopy (XPS) showed that some of the nitrogen atoms in the adsorbed γ-APS films were protonated, probably by strong hydrogen bonding with silanol groups on the surface of the silica-like films. However, most of the amino groups were not protonated, indicating that adsorption in that case involved condensation of ethoxy groups with surface silanol groups. Thus, a band observed near 1100 cm−1in RAIR spectra of γ-APS that adsorbed onto silica-like films was tentatively assigned to a vibrational mode characteristic of Si‒O‒Sisubstrate bonds. Lap joints were prepared using a two-part epoxy adhesive and aluminum substrates that were primed with silica-like films. Joints prepared from substrates that were primed with low-hydroxyl films had an average strength of about 23.4 MPa whereas those prepared from substrates primed with high-hydroxyl films had an average strength of about 14.3 MPa. Vapor-phase silanation of the primer films did not have a significant effect on the initial strength of the joints. However, it did have an effect on the locus of failure as determined by XPS. When no silane was used, joints prepared using substrates primed with the low-hydroxyl films failed near the adhesive–primer interface. Joints prepared using substrates primed with the low-hydroxyl films and coated with γ-APS failed cohesively, within the adhesive.