Surface of poly(ethylene terephthalate/isophthalate) copolyesters studied by atomic force microscopy

An atomic force microscope (AFM) operating in Tapping and contact modes has been used to study the surface topography and the molecular organization of poly(ethylene terephthalate) (PET) films containing 2% (PET-2I) and 10% (PET-10I) isophthalate, and of injection/blow molded bottles containing 2.6% (PET-2.6I) and 10% isophthalate. Large-scale (15-μm × 15-μm) AFM images have shown that both surfaces are fairly flat and heterogeneous in nature, often containing inclusions. Whereas the PET surface appears to be formed mainly by microfibrils, isophthalic acid (IPA) incorporation at the 2-10 mol % level gives the surface a granular appearance. The IPA-containing PET surfaces are frequently coated by a lacelike film consisting of submicron beads joined together by filaments. These strings of beads form bundles and can also connect bundles. AFM images of PET-2I closely resemble those generated for PET films. By contrast, the lacelike structure becomes a dominant feature of the PET-10I surface. The level of inclusions observed on film surfaces appears to correlate with the levels of extractable oligomers present in the polymers. Nanometer-scale AFM images of PET-10I exhibit surfaces composed of short stacks of plates or rods, with 30-50-nm voids or pores between these stacks. Whereas surface deposits of what we suggest is most likely an oligomer correlates with isophthalate concentration, we see no gross structural features in PET-2I and PET-10I that explain the observed improvement in gas diffusion barrier in these polymers.