Scanning Force Microscopy of Polyester Film Materials

In many applications the performance of a polymeric material is determined by its surface structure and interfacial interactions. The emergence of scanning force microscopy (SFM) has led to rapid advances in our capability to study polymer surface structures and properties during the last ten years. Work at UMIST has been exploring the use of methods based on SFM for the characterisation of complex heterogeneous materials. A particular focus of our work has been the investigation of biaxially oriented polyester film materials. These are manufactured by extruding molten poly(ethylene terephthalate) (PET) onto a roller which draws the material to approximately three times extension in the forwards direction, usually referred to as the machine direction. The material is then subjected to a second drawing process, perpendicular to the machine direction. Each drawing operation causes crystallisation of polymer chains, and, following heat treatment to induce further crystallisation, the result is a highly oriented material with a crystallinity of approximately 50%. Often, additive particles are incorporated into the film, either in the bulk or at the surface, to prevent adjacent film surfaces from sticking together (known as blocking).

Figure 1 shows a contact mode image of a sample of Mylar D, a material that incorporates additive particles at the surface. The additives may be seen as pronounced bumps in the surface. Friction force microscopy reveals that the additive particles have lower frictional contrast than the surrounding polymer. This is because the hard silicate of which they are composed is less readily deformed than the surrounding polymer, leading to a smaller tip-sample contact area.

Figure 1

Figure 2 shows a tapping mode topographical image of a sample of Mylar D. The image closely resembles the contact mode images. However, the phase image recorded simultaneously with the topographical image shows a wealth of detail. The additive particles, which appear to be hemispherical in the topographical images, clearly have an irregular morphology. More significantly still, the surface is covered densely with small elongated features.

Figure 2

These details may be more clearly observed in the high resolution phase image shown in figure 3. The Surface of the additive appears to be pitted, and in places it seems to be covered over with polymeric material. The small elongated features, resembling grains of rice scattered across the surrounding surface, exhibit orientation around the additive.

Figure 3

It is believed that the elongated features observed in figures 2 and 3 are crystallites. Many semi-crystalline thermoplastics contain spherulites which are microns or tens of microns in size. The crystallites in biaxially oriented PET films, in contrast, have been shown by X-ray diffraction to be much smaller. They are thought to be plate-like structures, a few nm thick and a few tens of nm in length. This is consistent with the dimensions of the features observed in figure 3, given that some broadening of such small features would be expected due to convolution of the profiles of the tip and the surface feature.

Similar features were also observed in phase images of Melinex O, an additive-free material. Figure 4 shows a topographical image of a large region of the film surface. The surface is generally quite flat but there are several raised fibrillar structures, some running almost the entire length of the imaged area. One of these is identified in figure 4 by a white arrow placed alongside it. Careful examination reveals that elongated features - the putative crystallites - run along its length, oriented perpendicular to its central axis. Analysis of the other fibrillar structures indicates that this is a general characteristic. It is consistent with our identification of these small elongated features as crystallites. Presumably the directions of the fibrillar structures are determined by stresses induced in the film during drawing and the elongated features result from crstallisation during which alignment of the polymer molecular axes occurs parallel to the stress.

Previously, the crystallites in PET films have not been resolved by microscopy; these data illustrate the power of SFM, and tapping mode in particular, for polymer characterisation.

Figure 4

Further information

  • B. D. Beake, G. J. Leggett and P. H. Shipway, Surf. Interface Anal. 31 (2001) 39-45.
  • B. D. Beake, J. S. G. Ling and G. J. Leggett, Polymer 41 (2000) 2241-2248.
  • B. D. Beake, G. J. Leggett and P. H. Shipway, Surf. Interface Anal. 27 (1999) 1084-1091.
  • B. D. Beake and G. J. Leggett, Polymer 40 (1999) 5973-5976.

Material Copyright © 2002 Graham Leggett