anasys: characterisation of a polymer blend by PFM

Characterisation of a Polymer Blend by Variable Temperature Pulsed Force Mode AFM

For certain polymers, polystyrene being a good example, there is a systematic variation in the magnitude of the adhesion-dependent pull-off force with temperature. Furthermore, a step increase may occur as a material passes through its Tg. For other polymers, the variation in adhesion measurement is relatively insensitive to variations in temperature. Such information on the response of component polymers can be used to characterise a blend. This is accomplished best by scanning above and below the Tg of one or both of the polymers, utilising a variable temperature microscope stage. A good example is the polystyrene-poly(methyl methacrylate) PS-PMMA system, results for which are shown below. Both these polymers have a Tg of approximately 100°C, but display very different responses to PFM adhesion measurements over a temperature range of 60°C - 160°C.

Pull-off force (adhesion) images at different temperatures. Bright means high adhesion. The phase-separated morphology is gradually revealed, but which phase is which?

The increase in adhesion is not produced by a change in topography

Adhesion images of the individual components are shown below. From this we are able to conclude that the occluded phase in the blend must be polystyrene.

The 3-D pull-off force images show that PS (left) undergoes a large increase in adhesion with temperature, whereas the response of PMMA (right) is relatively insensitive to temperature.

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