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.