A series of fibers of polyethylene terephthalate) with different physical structures, varying from amorphous to 36 % crystalline, has been investigated by 13C solid-state NMR measurements. Several relaxation times, i.e., relaxation times in the rotating frame ( / ), Tip(13C)) and -130 cross-polarization transfer times (7ch), have been studied. 13C CP/MAS spectra show that, especially with respect to the ethylene and carbonyl carbons, the chemical shift for carbons in ordered structural surroundings ("NMR crystalline") is higher than that for carbons in unordered surroundings ("NMR amorphous"). Also the width of the distribution of chemical shifts in an ordered structure is smaller than that in a less ordered structure (narrow vs broad resonance line). Analysis of the line-shape changes during the Tip(13C) relaxation process of semicrystalline yarns showed that the NMR amorphous phase relaxes with two time constants. A threeregion model composed of NMR crystalline, rigid NMR amorphous, and mobile NMR amorphous regions is proposed. The sizes of the rigid domains in semicrystalline PET yarns estimated from / ) measurements correspond well with the crystal sizes as determined with X-ray diffraction. In a yarn, which is almost completely amorphous according to X-ray measurements, fast and slow ethylene and aromatic group motions, relative to a correlation time of 5 X 10-6 s, occur. From 1 ) experiments it can be concluded that both mobile and rigid regions in the amorphous yarn have dimensions smaller than 50 A. Tch measurements reveal that ethylene and aromatic groups in the mobile amorphous domains of all yarns undergo large-amplitude motions. The principal elements of the chemical shift tensor of aromatic carbons remain independent of temperature up to at least 327 K, indicating that, in crystalline regions in semicrystalline yarns and in rigid parts of the amorphous yarn, phenyl reorientations have a very small amplitude (not exceeding ~5°).

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A systematic study of the dispersion curves of the refractive indexes of nylon 6 yarns was made. The parameters were the draw ratio and strain. The measurements show that the dispersions of the refractive indexes n∥ and n⊥, parallel and perpendicular to the fiber axis, are equal, independently of draw ratio and strain. The average dispersion equals nF − nC = 109 × 10−4. Consequently, the birefringence is, within experimental accuracy, independent of the wavelength. The refractive indexes and the birefringence show a change in trend at 10–12% strain. This point corresponds to the yield strain in the stress–strain diagrams. The inference is that beyond the yield point the overall molecular orientation must increase less strongly with strain than before. An analysis shows that the Lorentz–Lorenz relation holds for the average refractive index n̄ = ⅓ (n∥ + 2n⊥). So the change in n̄ versus draw ratio is mainly due to the change in density. By applying the Lorentz–Lorenz relation to the change of n̄ on straining, a value of Poisson's ratio (μ) could be derived. The average value found for nylon 6 yarns was μ = 0.48, which means that the density hardly changes with strain.

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