An XPS and ToF-SIMS investigation of the outermost nanometres of a poly(vinylidene difluoride) coating

Abstract

The outermost nanometres of a poly(vinylidene difluoride) (PVdF)-based coil coating have been investigated using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). A reference PVdF-based coating formulation and three variations of this formulation were characterised by XPS. The addition of flow agent and/or acrylic co-polymers induced significant changes in the elemental and chemical composition of the coating's air/coating surface. The XPS results indicate that both the flow agent and acrylic co-polymers segregate towards the coating's air/coating surface. The XPS results also suggest that in the fully formulated coating, segregation of the flow agent and co-polymers results in the formation of a surface/sub-surface acrylic layer of sufficient thickness to mask the fluorine signal originating from PVdF. Using a Buckminsterfullerene (C 60) ion source as an etch source for polymers, depth profiles were obtained of the outermost nanometres of the fully formulated PVdF coating. Fluorine and oxygen depth profiles revealed compositional changes in the coating with respect to depth. The oxygen depth profile revealed the presence of an oxygen-rich sub-surface layer within the coating. Molecular depth profiles, acquired for fragment ions diagnostic of the PVdF and acrylic co-polymer components of the coating formulation, revealed the presence of an acrylic co-polymers-rich layer in the coating's sub-surface region. All depth profiles suggest that the PVdF coating bulk possesses a homogeneous material composition. The XPS and SIMS results suggest the fully formulated coating is composed of three distinct layers: a thin flow agent layer at the air/coating surface, an acrylic co-polymer-rich sub-surface layer and the coating bulk.