Conductivity model and photoacoustic FT-IR surface depth profiling of heterogeneous polymers

A novel thermal model is developed for surface depth profiling of heterogeneous polymeric surfaces using step-scan photoacoustic Fourier transform (SS PA FT-IR) spectroscopy. This approach is based on the propagation of thermal waves generated during the photoacoustic effect which travel to the film–air (F–A) interface, thus generating acoustic signals above the surface, which upon Fourier transform, result in infrared spectra. The developed model volumetrically slices the surface into finite homogeneous layers parallel to the film–air (F–A) interface and a composition of each ith layer is assumed to be the same, but the layers among themselves (ith+1 and ith−1) may or may not exhibit compositional changes. Overall thermal properties of the multi-layered surface consist of the sum of in-series connected thermal conductor layers. The proposed model can be utilized to polymeric films containing the following parametrically analyzed inclusions: (1) inclusions with no interphase between the matrix polymeric and (2) inclusions with a finite interphase. This model is flexible, allowing variations of the particle size, shape, and surface/interfacial microstructural changes. It was tested for depths of penetrations in the range of 5–50 μm for carbon black inclusions imbedded into a two-component (2K) polyurethane (PUR) film deposited on acrylonitrile–butadiene–styrene (ABS) substrate. These studies show that the experimental results are consistent with the proposed model, allowing predictions of interphase layers on particles; for example, a 10 nm water layer adsorbed on carbon black particle surfaces can be detected.