Confocal Raman depth profiling of surface-modified polymer films: effects of sample refractive index

A solution-cast poly(vinyl chloride) (PVC) film of thickness 81 μm was modified with aminothiophenol using surface-selective reaction conditions. After the identification of suitable Raman bands characteristic of the modifier (1596 cm-1) and the polymer (1425 cm-1), confocal Raman measurements demonstrated a depth variation of the modifier peak intensity which was consistent with a highly surface-selective reaction. Data also showed features clearly attributable to refractive index effects, principally the lack of symmetry in the modifier depth profile obtained from the raw data and an underestimated film thickness. The film was cross sectioned under liquid nitrogen to allow conventional lateral Raman measurements across its thickness. The depth profile obtained directly in this way was in good agreement with confocal data after normalization using the polymer peak intensity and an expansion of the thickness scale to the value obtained independently with a micrometer. The non-destructive confocal method has clear advantages in analyzing small samples or where spatial mapping is also required. To simulate the effect of sample refractive index, the directly measured cross-sectional depth profile was used to calculate the confocal depth profile, using the method of analysis developed by Everall. The general features of the profile were reproduced, assuming a "fill factor" for the 100× microscope objective of 0.63. However, a broadening of features was apparent in the simulated profile. This suggests that direct conventional Raman measurements of a sectioned film may provide a more sensible approach for the detection of interfaces in polymer films.