Correlating Surface and Buried Interfacial Structures to Polymer Adhesion.

Abstract

Robust adhesion of polymers is necessary for applications in many industries; however, adhesion is poorly understood on a molecular level. Here, a surface and interface-sensitive nonlinear optical technique, sum frequency generation (SFG) vibrational spectroscopy was used to study polymer adhesive surfaces and polymer/adhesive interfaces to gain a better understanding of polymer adhesion mechanisms.

Silane adhesion promoters are often used to enhance the adhesion of elastomeric materials to polymer substrates. The interfacial structures of a known silane adhesion promoting mixture at polymer/liquid, silicone elastomer/liquid and polymer/silicone elastomer buried interfaces were studied with SFG and correlated to adhesion testing results. The silane adhesion promoters exhibited orientational order at all buried interfaces, implying that interfacial ordering of silane adhesion promoters may be necessary to enhance adhesion.

Silane adhesion promoters cannot be used in microfluidics fabrication. In these applications, surface treatments are used to oxidize the silicone surface prior to adhesion, but due to hydrophohbic recovery these effects are temporary. Hydrophobic recovery can be slowed by removing extractable short chain oligomers from the silicone bulk. SFG analysis of silicone surfaces before and after extractable materials were removed showed that the silicone surface methyl group orientation changed after extractions. Also, the surface flattened after extractable materials were removed. These changes in surface structure may slow hydrophobic recovery, and therefore improve adhesion enhancement by oxidative surface treatments.

Lastly, the adhesion of a different class of polymer adhesives, epoxy resins, was investigated. Epoxy resins are used as underfills in flip-chip devices, and adhesion failure of underfills can cause the flip-chip device to fail. Here, SFG was used to study the surface structures of model epoxy compounds before cure, after cure and after moisture exposure. Surface structural changes were observed for all three conditions, which may affect adhesion of epoxy underfills. Further, the buried interfacial structures of the cured epoxies and a model polymer were investigated before and after moisture exposure and correlated to adhesion testing results.

These studies showed that SFG is a powerful technique to study polymer adhesion mechanisms and the surface and interfacial structures required for adhesion.