Surface nanochemical studies of polymers and other organic surfaces by scanning force microscopy

The nanochemical characterization of organic/polymer surfaces is an increasing technological need that cannot be met by current ultrahigh vacuum, charged particle analytical approaches. The high spatial resolution of scanning force microscopy offers the possibility of making measurements on a nanometer scale relatively simply. In the case of species displaying different pKa values that are within a reasonable range, electrostatic contrast may be used in order to distinguish between surface species (such as COOH and CH₃, for example), when the sample is examined by lateral force microscopy. In the case of most polymers, however, a more promising approach is to obtain contrast by measuring a combination of van der Waals forces and (for polar polymers) H-bonding and polar effects. This can be used to distinguish components of certain polymer blends, for example. In the case of distinguishing between non-polar polymers, the pull-off forces measured by AFM can be calculated by applying the appropriate contact mechanical model, provided that this series of polymers exhibits similar mechanical properties. Under this condition, the pull-off force is proportional to the work of adhesion, which can also be calculated from Israelachvili's approximation to the Lifshitz theory of van der Waals interactions, leading to the observation that pull-off force increases with refractive index of the polymer. Chemical differences between organic surfaces can frequently be distinguished by AFM approaches, provided that the situation is not dominated by the effects of mechanical properties.