Step-scan FT-IR photothermal spectral depth profiling of polymer films

Abstract

Abstract In step-scan Fourier transform infrared (FT-IR) spectroscopy, a constant modulation frequency can be applied to the entire spectral range. This is particularly useful for photoacoustic (PA) detection since, in the absence of saturation, the PA response at all wavelengths in step-scan FT-IR PA spectroscopy will correspond to the same depth in the sample, whereas in a typical continuous-scan FT-IR PA spectrum each wavelength corresponds to a different sampling depth. A step-scan FT-IR PA spectral depth profile can be obtained by changing the modulation frequency for different scans. Alternatively, depth information can be obtained from the phase of the PA signal. The step-scan mode of data collection permits easier access to the PA phase than does the continuous-scan mode. The combined use of modulation frequency variation and analysis of the phase allows a wider range of depths to be probed. This type of spectrally resolved depth information is particularly useful when applied to polymeric materials with special surface properties. This includes not only characterization of surface-coated and laminar materials, but also studies of weathering, aging, curing and the diffusion of species into or out of a polymer matrix. Step-scan FT-IR PA spectral depth profiling of some of these types of samples is illustrated and the theory and techniques involved are discussed.