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

The creep behavior and elastic properties of cold-drawn poly(ethylene terephthalate) (PET) films, annealed in the range 60-240 degreesC have been investigated by means of microindentation testing. Two indentation methods have been used. The imaging method has been employed to examine the viscoplastic properties of the polymer materials while the depth-sensing method was used for the determination of Young's modulus values. The creep behavior (plastic flow) of cold-drawn PET is shown to be intimately correlated to the nanostructural changes occurring upon annealing. The observed decrease in the rate of creep, when the glassy material is annealed at 60 degreesC, has been associated with the emerging smectic structure, which confers to the material a higher mechanical performance. The elastic properties of the smectic phase are found to be comparable to those of the glassy state. Young's modulus 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 crystallinity. Results suggest that Young's modulus values of the amorphous constrained regions within the crystals are higher than the E value of the fully amorphous material. (C) 2001 American Institute of Physics.