DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings arestudied by means of differential scanningcalorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in colddrawn,un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence ofcrystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystalthickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to thebasal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 8C) annealed samples have a glassdispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possiblemechanism able to limit lamellar thickening during the annealing process at these low temperatures.