Chain‐folding measurements in annealed poly(ethylene terephthalate)

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Abstract
Chain folding in unoriented poly(ethylene terephthalate) (PET) films has been investigated as a function of annealing time and temperature. To meet this objective dynamic mechanical, infrared, and molecular weight measurements were used, together with selective chemical degradation to remove chain folds and amorphous regions. The β dispersion in the dynamic mechanical spectrum of PET is here tentatively associated with motions of methylene and/or carboxyl groups in irregular chain folds; the β dispersion is not found in quenched amorphous polymer, in polymer where amorphous regions and chain folds have been removed, or in highly annealed PET where the irregular folds have regularized. Upon mild crystallization and annealing (30 min at 110°C) of initially amorphous film a large β dispersion appears and then diminishes upon further annealing at 220°C. As the β dispersion diminishes, the infrared regular fold band increases more than the crystallinity band, indicating regularization of folds. The molecular weight of the degraded residue corresponds approximately to typical fold‐period dimensions (∼130 Å), and increases on annealing as expected from lamellar thickening. The degradation process has, by fold removal, reduced the chains in the crystals to a very short, uniform length.