AFM-FTIR: A new technique for materials characterization

A new type of infrared spectroscopy for obtaining the molecular composition of the surfaces of materials has been developed by combining the inherently high spatial resolution of the atomic force microscope (AFM) with the powerful chemical characterization capabilities of Fourier transform infrared (FTIR) spectroscopy. When the new technique is fully developed, we plan to use it to determine the molecular structure of polymer systems on a sub-micrometer scale, or about several orders of magnitude greater than currently possible using "conventional" infrared spectroscopy techniques. We are especially interested in probing the molecular structure of "interphases," the transition regions between an adhesive and a substrate in polymer composites and adhesive bonds. We have used both contact mode AFM probes and thermal mode AFM probes to measure the response of polymer systems to infrared absorption. In either case, when the probe is engaged with a sample that is absorbing infrared radiation from the FTIR spectrometer, the output from the AFM probe resembles an interferogram to which a Fourier-transform can be applied to obtain the infrared spectrum. The contact mode approach was successfully used when various neat polymer samples, including polystyrene, polycarbonate and a model epoxy adhesive system, were characterized by AFM-FTIR techniques. Excellent spectra were also obtained from polystyrene beads having a diameter of about 2 mum. AFM thermal mode probes have been used to obtain preliminary interferograms and infrared spectra from neat polystyrene samples, but the signal-to-noise ratio was low. AFM-FTIR spectroscopy results to this point show that this new method is feasible, but the spatial resolution of the technique remains to be shown.