Morphology, phase composition, and molecular mobility for a series of semicommercial gel-spun UHMWPE fibers were studied using a combination of WAXS, SAXS, and 1 H solid-state NMR methods. The fibers show uncommon for this type of fibers decrease in the break load with increasing draw ratio, whereas their modulus and the tenacity reach very high ultimate values. The X-ray and NMR methods have provided complementary information about the fiber morphology and structural reorganizations occurring at the final stage of the fiber drawing. The results suggest that the fiber morphology can be described by a mixture of crystalline fibrils with long period of ∼35À45 nm, as shown by SAXS, and large, so-called, chain-extended crystals. The presence of large crystals with embedded defects is shown by NMR. The drawing causes increase in the crystallinity from ∼89 to ∼96 wt % and in chain orientation, while the long period of fibrils and the break load of fibers surprisingly decrease. The decrease in the long period with the drawing could indicate a partial reorganization of the amorphous phase and/or some fragmentation of the fibrils, while the decrease in the break load could correspond to a decrease in number of load-bearing chains. A disorder of the crystals and a small increase in chain mobility in the constrained amorphous fraction is also observed with increasing the drawing. Approximately 1 wt % of the chain fragments in the amorphous fraction has a high molecular mobility. It is assumed that these chain fragments reside in nanovoids, the presence of which was shown previously by a 129 Xe NMR study on the same fibers. The role of α-crystalline relaxation in structural reorganizations during fiber drawing is also discussed.

Changes in morphology and chain dynamics of a series of drawn gel‐spun UHMWPE fibers were investigated by 1D and 2D 1H, 13C and 129Xe NMR spectroscopy. The dependence of the rms oscillation angle for the internuclear proton spin pairs around the chain axis can be evaluated from 13C/1H WISE and correlated with the Young modulus of UHMWPE fibers. Based on the novel 13C QUCPMAS method it was shown that the relative amount of orthorhombic, monoclinic and intermediate fractions increase with the Young modulus as opposed to the highly mobile amorphous phase. The chain dynamics of this last fraction becomes less hindered. 129Xe NMR spectroscopy shows that the volume‐average diameter of voids increases with the Young modulus and the production of nanovoids is accelerated in drawn UHMWPE fibers. magnified image