AFM/FTIR: A New Technique for Materials Characterization

A new type of infrared spectroscopy for obtaining the molecular composition of the surfaces of materials at ultra-high spatial resolution has been developed by combining atomic force microscopy (AFM) with Fourier-transform infrared spectroscopy (FTIR). This new analytical technique involves the use of an AFM to detect the response of a material to the absorption of modulated infrared radiation from an FTIR spectrometer and is referred to as AFM/FTIR spectroscopy. When the technique of AFM/FTIR spectroscopy is completely developed, we plan to use it to probe the molecular structure of interphases in polymer composites and adhesive bonds. Two approaches have been used to measure the response of polymer systems to infrared absorption. The first involves the use of a contact mode AFM probe to measure the thermal expansion of the polymer; the second involves using a scanning thermal microscopy (SThM) probe to measure the polymer's temperature increase. In either case, the output of the probe resembles an interferogram to which a Fourier-transform can be applied to obtain the infrared absorption spectrum. The first approach was used to obtain excellent quality AFM/FTIR spectra from various neat polymer films, including polystyrene, polycarbonate, and a model adhesive system consisting of an epoxy resin cross-linked with dicyandiamide. Excellent spectra were also obtained from polystyrene beads having a diameter of about 2 µm. The second approach, using an SThM probe to determine the temperature increase that accompanies infrared absorption, was also used to obtain interferograms of polymer samples such as polystyrene. However, the interferograms were noisy and the AFM/FTIR spectra obtained from them had a low signal-to-noise ratio. The present results, thus, show that AFM/FTIR spectroscopy is feasible but the spatial resolution of the technique remains to be shown.