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Metrology & Instrumentation: Polymer Imaging


Sample Preparation

Practically, AFM can be applied to samples of any kind and allows the examination of surfaces with corrugations of several microns, yet it is rather difficult to get high-quality information about sample composition and fine morphology from AFM images of the corrugated surfaces. Imaging of polymer materials brings best results when applied to samples with flat surfaces (with corrugations below 100 nm). Industrial polymer films satisfy this requirement. In other cases, polymer samples with flat surfaces should be prepared by various techniques. When a polymer layer of interest is on a curved surface (fiber, cylinder, etc), then only a top part of such sample is accessible for AFM imaging.


A choice of preparation technique is also related to the particular goal a researcher pursues in an AFM study. AFM studies of thin and ultrathin (thickness below 100 nm) polymer films are straightforward for samples prepared by spin casting of their solution on a flat substrate or by dipping the substrate into the polymer solution. Among commonly used substrates are polycrystalline silicon, glass, gold films with large crystal terraces, atomically flat surfaces of mica, graphite or inorganic layered materials. When polymer concentration in solution approaches 0.001 mg/ml, isolated individual macromolecules can be obtained on a substrate by spin casting or dipping. With increase of the polymer concentration, such deposition leads to layers of different thickness and these layers are convenient samples for AFM studies of macromolecular self-organization and crystallization.


For many samples, the structures of interest are hidden inside the bulk of the polymer. For example, the impact strength of a polymer blend is determined mainly by the bulk microstructure, not the surface structure. Usually an ultramicrotome, preferably equipped with a diamond knife, should be used to section the sample. For AFM analysis one can use either thin slices or flat faced-off blocks. The preparation of the block is much easier. It is important to know that the preparation of flat surfaces of polymer samples, by cutting with ultramicrotome, requires optimization of temperature. Often, this procedure is performed at temperatures below glass transition of a polymer sample or its components. There is also no need in staining of the sample for AFM examination whereas this is a must procedure for compositional imaging with TEM. For some samples, it might be helpful to use different etching procedures (which are common for sample preparation for TEM studies) for a removal of non-specific surface layers or amorphous material of semi-crystalline polymers.


There are some specifics of sample preparation for high-temperature AFM measurements. For example, the polymer sample should be fixed to a sample puck with glue, which does not contain traces of solvents. Also one can foresee difficulties with measurements of polymer samples, which contain low molecular species and other volatile components. At elevated temperatures, they will exude from the sample during heating and can contaminate an optically transparent cell or the probe.