Solid-state NMR relaxation and spectral data were able to reveal the molecular nature of β and γ relaxations in ultra-high molecular weight polyethylene (UHMWPE) fibers, which are not amenable to traditional dynamic mechanical analysis methods. It was found that the amorphous chain segments in polyethylenes have two superposing motional modes: a localized, anisotropic, fast (108 s-1) chain rotation (γ motion), which corresponds to the dynamic mechanical γ relaxation, and a cooperative, isotropic, and slow (105 s-1) motion (β motion), which corresponds to the β relaxation. In the amorphous domains of ultra-drawn UHMWPE fibers, the segments undergo a full-scale γ motion and a mostly-suppressed β motion, while both motional modes are active in lower crystallinity polyethylenes. In UHMWPE fibers, the β motion is suppressed until 75°C, indicating better thermal stability than common polyethylenes. The strong γ motion in UHMWPE fibers allows the development of two new NMR methods for quick and precise determinations of crystallinity in the fibers. The 13C method is based on amorphous spin counting, while the 1H method is based on a measurement of 1H T1 relaxation time. The 1H method is very fast and highly sensitive to small variations of amorphous phase content.

Dynamic mechanical relaxations in ultrahigh molecular weight polyethylene fibers probed by solid-state NMR

249th National Meeting (2015)

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