Thickness Dependence of Reflection-Absorption Infrared Spectra of Supported Thin Polymer Films

Reflection-absorption infrared spectroscopy (RAIRS) analysis of polymer films with thickness values ranging from 1 to ≮500 nm was performed to show that film thickness has a systematic influence on the spectral band intensities, a factor that must be considered when quantifying chemical changes in films characterized by RAIRS. Amorphous cellulose films were prepared by regenerating spin-coated films of trimethylsilyl cellulose (TMSC) on silicon wafers into cellulose by exposing the films to the vapor of aqueous 2 M HCl solution for 2 min as described in ref 11. The results show that the films are in the ultrathin regime and the lack of band shift is a clear indication that the films stay amorphous to as thick as 539 nm. Below 100 nm the spectroscopic approach does not seem to be sufficiently sensitive for thickness determination.