Phase composition and molecular mobility in nylon 6 fibers as studied by proton NMR transverse magnetization relaxation

The phase composition and molecular mobility of Nylon 6 fibers has been studied using H-1 solid-state NMR transverse magnetization relaxation (T-2 relaxation) spectroscopy. One of the objectives of this work is to determine the usefulness of the NMR relaxation method to accurately determine the crystallinity of Nylon 6 fibers. The NMR relaxation results have been interpreted using a three-phase model, which is proposed on the basis of distinct differences in chain mobility in the crystalline phase, the semi-rigid crystalamorphous interface, and the soft amorphous phase. In order to obtain accurate data on the phase composition, the effects of absorbed water and annealing of fibers at elevated temperatures were studied in detail. It was shown that water mainly plasticizes the soft fraction of the amorphous phase. Changes in the phase composition and molecular mobility caused by fiber annealing at elevated temperatures were studied with real-time NMR T2 relaxation experiments. The most pronounced annealing effects (changes in the phase composition) were observed for undrawn fibers with low molecular orientation. Annealing slows down for fibers produced at larger winding speed and especially those with a high draw ratio. Fiber processing conditions, i.e., winding speed and draw ratio, are found to affect the phase composition and molecular mobility in different phases. It was shown that the present NMR relaxation method provides a fast and accurate technique to analyze the phase composition of Nylon 6 fibers. The amount of rigid phase determined from NMR is in good agreement with the crystallinity of the fiber obtained using the traditional method for Nylon 6 fibers, which involves a combination of density measurement, wide-angle X-ray diffraction and C-13 NMR spectroscopy. Hence, the present NMR relaxation method greatly simplifies measurement of crystallinity in Nylon 6 fibers.