Effect of Changes in Absorber Concentration on Infrared Reflection-Absorbance of Polymer Films on Metallic Substrates

This paper examines the relationships between the concentration of functional groups and observable infrared reflection-absorbance (IR-RA) spectral changes and applies the findings to bisphenol-A polycarbonate (BPA-PC) on gold substrates. The paper then notes the importance of studying the mechanisms of photochemically and catalytically enhanced degradation at polymer-111etal (oxide) interfaces for solar energy collection and gives the rationale for using the IR-RA technique for these studies. An apparatus is described which applies Fourier transform spectroscopy (FT-IR) for the IR-RA studies. An expression is developed showing the independence of the real and imaginary parts of the complex refractive index (N = n - ik) at the fundamental oscillator frequency when the frequency distribution of k can be approximated by a Lorentzian distribution. The values of k and n redetermined for the polymer between 3200 and 950 cm using an iterative calculation and experimentally determined reflectance spectra. From the optical constants determined for each functional group in BPA-PC, the sensitivity of the IR-RA measurement to changes in the functional group concentration was calculated for polymer thicknesses ranging from 10 um to 10 um. The optimum sensitivity is generally found at film thickness between 0.1 and 1 um. At greater thicknesses, IR-RA spectra may be insensitive to changes in functional group concentration, and at thicknesses below 10 nm, the sensitivity is limited by instrumental signal-to-noise ratios.