| Summary: | The surface crystallization behaviour of PET (poly(ethylene terephthalate)) and PEN (poly(ethylene 2,6-naphthalate)) spin-coated thin films was compared by means of AFM (atomic force microscopy) with an in-situ heating stage. As the films were heated up stepwise, characteristic surface crystals appeared at a crystallization temperature (Tc) in the near-surface region which is about 15 ºC under the bulk Tc, and were replaced by bulk crystals when the temperature was increased to the bulk Tc. In the case of films whose thickness is less than 70 nm (PET) and 60 nm (PEN), significant increases in the bulk Tc were observed. SFM (scanning force microscopy) force-distance curve measurements showed that the glass transition temperature (Tg) of the near-surface region of PET and PEN were 22.0 and 26.6 ºC below their bulk Tg (obtained by DSC). After the onset of surface crystallization, edge-on and flat-on crystals appeared at the free surface of PET and PEN thin films, whose morphologies are very different to those of the bulk crystals. Although the same general behaviour was observed for both polyesters, there are significant differences both the influence of the surface and substrate on the transition temperatures, and in morphology of the surface crystals. These phenomena are discussed in terms of the differences in the mobility of polymer chains near the surface.
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