Crystallization of oriented amorphous poly(ethylene terephthalate) as revealed by X-ray diffraction and microhardness

Abstract

The structural changes occurring when amorphous cold-drawn poly(ethylene terephthalate) films are annealed at different temperatures (50°C-240°C) for different annealing times (10-104 s) were investigated by means of X-ray diffraction and microhardness techniques. The X-ray results reveal the appearance of smectic order at 60°C with a period of 10.7 Å. At 70°C, a layer structure in the scale of 110 Å emerges. Finally, triclinic order is observed above 80°C. The appearance of a layer structure prior to the development of triclinic crystals is associated with a density difference along the molecular direction produced by a molecular tilting mechanism. The microhardness behaviour of annealed cold-drawn PET films is correlated to the developing morphologies. At high annealing temperatures (>100°C), the plastic component of hardness is shown to vary with the occurring microstructural changes. Results indicate that the hardness of the amorphous intrafibrilar regions is higher than that of a fully amorphous material. The indentation anisotropy, ΔH, which is related to the elastic recovery of the material shows a conspicuous decrease at T(a) ~ 70°C, which is explained in terms of a relaxation of the fibrils in the chain direction. | The structural changes occurring when amorphous cold-drawn poly(ethylene terephthalate) films are annealed at different temperatures (50 °C-240 °C) for different annealing times (10-104 s) were investigated by means of X-ray diffraction and microhardness techniques. The X-ray results reveal the appearance of smectic order at 60 °C with a period of 10.7 angstroms. At 70 °C, a layer structure in the scale of 110 angstroms emerges. Finally, triclinic order is observed above 80 °C. The appearance of a layer structure prior to the development of triclinic crystals is associated with a density difference along the molecular direction produced by a molecular tilting mechanism. The microhardness behaviour of annealed cold-drawn PET films is correlated to the developing morphologies. At high annealing temperatures (>100 °C), the plastic component of hardness is shown to vary with the occurring microstructural changes. Results indicate that the hardness of the amorphous intrafibrilar regions is higher than that of a fully amorphous material. The indentation anisotropy, ΔH, which is related to the elastic recovery of the material shows a conspicuous decrease at Ta to approximately 70 °C, which is explained in terms of a relaxation of the fibrils in the chain direction.