Creep behavior and elastic properties of annealed cold-drawn poly(ethyleneterephthalate): The role of the smectic structure as a precursor of crystallization

The creep behavior and elastic properties of cold-drawn poly(ethylene terep hthalate) (PET) films, annealed in the range 60-240 degreesC have been inve stigated by means of microindentation testing. Two indentation methods have been used. The imaging method has been employed to examine the viscoplasti c properties of the polymer materials while the depth-sensing method was us ed for the determination of Young's modulus values. The creep behavior (pla stic flow) of cold-drawn PET is shown to be intimately correlated to the na nostructural changes occurring upon annealing. The observed decrease in the rate of creep, when the glassy material is annealed at 60 degreesC, has be en associated with the emerging smectic structure, which confers to the mat erial a higher mechanical performance. The elastic properties of the smecti c phase are found to be comparable to those of the glassy state. Young's mo dulus E values of the semicrystalline samples are discussed in light of the parallel model of crystalline and amorphous layers. E values are shown to depend on the crystalline lamellar thickness and the degree of crystallinit y. Results suggest that Young's modulus values of the amorphous constrained regions within the crystals are higher than the E value of the fully amorp hous material. (C) 2001 American Institute of Physics.