Toward a Force Spectroscopy of Polymer Surfaces

The adhesional forces between a series of polymer film surfaces and chemically well-defined atomic force microscopy tips have been measured and found to depend strongly on the chemical nature of both probe and sample surfaces. For a given series of polymers, the ranking in adhesion strength was markedly different for polar and nonpolar probes, irrespective of the precise chemical composition of those probes. In the case of nonpolar polymers, a correlation of adhesion force with calculations based on the Lifshitz theory of Van der Waals interactions was found. In the case of polar polymers, a reasonable correlation with water-contact angle was observed. The adhesional differences between different probe tips translate into reversals of chemical contrast in high-spatial-resolution lateral force images, when examining polymer blends using chemically different tips, demonstrating the potential of this approach for the nanometer-scale, friction-mediated surface-chemical imaging of polymers. Central to these experiments has been the use of perfluorodecalin as a medium for measuring interactions. Employment of this liquid greatly facilitates measurement of the forces between the probe tip and the polymer surface.