Analyzing polymer membranes by attenuated total reflection infrared spectroscopy (ATR-IR) can lead to irreversible damage to the material and induces systematic errors in the data. Attenuated total reflection infrared spectroscopy is a common tool for analyzing the surface of polymer membranes. In order to provide sufficient contact between the membrane and the internal reflection element (i.e., the ATR crystal), pressure is applied via a metal stamp. This procedure, however, can lead to mechanical damage. In this work, we study this damage using the example of a polyethersulfone (PES) membrane for water filtration and we show how the damage can be avoided. Attenuated total reflection infrared spectroscopy, laser-scanning microscopy (LSM), and atomic force microscopy (AFM) are employed to understand the mechanically-induced phenomena at the molecular and macroscopic scales. The data reveal that the mechanical impact does not only result in a compressed membrane structure with smaller pores, but it also leads to deformations at the molecular level. Moreover, in light of the mechanical damage, a detailed analysis of the PES IR spectrum indicates that several previous vibrational assignments of peaks may be incorrect and that many published results may be biased and should be revisited.
Keywords: FT-IR; Fourier transform infrared spectroscopy; compression; polymer membrane; porosity.