Crystallisation of PET from the Amorphous State: Observation of Different Rates for Surface and Bulk Using XPS and FTIR

Crystalline regions of poly(ethylene terephthalate) (PET) are associated with the trans glycol conformation of the polymer repeat unit. Amorphous regions are largely associated with the gauche conformation. We describe experiments which follow the development of trans conformers at the surface and in the bulk of amorphous PET films (∽150 nm thick) annealed above the glass transition temperature Tg. X-ray photoelectron spectroscopy was used to probe the surface region to a depth of ∽5 nm and Fourier transform infrared spectroscopy (FTIR) to examine changes in the bulk. For PET samples annealed at 90 °C no conformational change was observed either at the surface or in the bulk after 120 min. At 95 and 110 °C the gauche to trans conformational change was over in ∽30 min (surface) and >120 min (bulk) and ∽15 minutes (surface) and ∽30 min (bulk), respectively. Hence the surface rearranges faster than the bulk, consistent with increased mobility at the surface due to the polymer chains being less constrained. This is analogous with the phenomenon of surface premelting in crystalline solids. A comparison of reflection–absorption infrared spectroscopy and transmission FTIR data demonstrates the occurrence of molecular reorientation in the bulk in addition to conformational change.