Nick Evan Shephard

Headgroup effect on silane structures at buried polymer/silane and polymer/polymer interfaces and their relations to adhesion.

Sum frequency generation (SFG) vibrational spectroscopy was used to study the effect of silane headgroups on the molecular interactions that occur between poly(ethylene terephthalate) (PET) and various epoxy silanes at the PET/silane and PET/silicone interfaces. Three different silanes were investigated: (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-glycidoxypropyl) methyl-dimethoxysilane (γ-GPMS), and (3-glycidoxypropyl) dimethyl-methoxysilane (γ-GPDMS). These silanes share the same backbone and epoxy end group but have different headgroups. SFG was used to examine the interfaces between PET and each of these silanes, as well as silanes mixed with methylvinylsiloxanol (MVS). We also examined the interfaces between PET and uncured or cured silicone with silanes or silane-MVS mixtures. Silanes with different headgroups were found to exhibit a variety of methoxy group interfacial segregation and ordering behaviors at various interfaces. The effect of MVS was also dependent upon silane headgroup choice, and the interfacial molecular structures of silane methoxy headgroups were found to differ at PET/silane and PET/silicone interfaces. Epoxy silanes have been widely used as adhesion promoters for polymer adhesives; therefore, the molecular structures probed using SFG were correlated to adhesion testing results to understand the molecular mechanisms of silicone-polymer adhesion. Our results demonstrated that silane methoxy headgroups play important roles in adhesion at the PET/silicone interfaces. The presence of MVS can change interfacial methoxy segregation and ordering, leading to different adhesion strengths.

Phenolic Resin Surface Restructuring upon Exposure to Humid Air: A Sum Frequency Generation Vibrational Spectroscopic Study

Epoxy and phenolic resins are extensively used for modern microelectronics, for example, as packaging materials. Humidity may greatly alter or degrade their function and application, leading to failure of the device. A nonlinear optical laser technique, sum frequency generation (SFG) vibrational spectroscopy, was used to investigate the molecular surface structures of the epoxy and phenolic resins after exposure to humid air. It was found that the adsorbed water molecules at the phenolic resin surface can induce substantial surface restructuring. The surface phenyl groups were reoriented closer to a perpendicular position to the surface after exposure to humid air from a more parallel position in air. Epoxide group surface restructuring was not observed.