Observation of elastic modulus inhomogeneities in thermosetting epoxies using AFM - Discerning facts and artifacts

Observations of a nanometer-scale nodular morphology on differently prepared surfaces of thermosets have frequently been interpreted as a sign for an inhomogeneous molecular network, which would result in an inhomogeneous modulus distribution within those thermosets. In order to test this hypothesis, the Peak-Force Tapping atomic force microscopy (AFM) mode was used on fracture surfaces and ultramicrotome cuts of epoxy and other polymers using differently sharp AFM probes. The nodular morphology is quite likely caused by an AFM artifact, which also seems to cause an apparently inhomogeneous modulus distribution; a variation in the tip-sample contact area could explain this effect. Smooth surfaces are necessary in order to reduce the contribution from this artifact. Ultramicrotome cutting currently seems to be the most appropriate surface preparation technique for the measurement of modulus distribution at the nanometer scale. All investigated materials seem to be homogeneous on a scale on the order of 10 nm-1000 nm. If modulus inhomogeneities are present, their amount or their lateral size is too small to be unambiguously measurable with this technique. From this data, it seems unlikely that epoxy exhibits an inhomogeneous molecular network. (C) 2014 Elsevier Ltd. All rights reserved.