Morphology, Chain Dynamics, and Domain Sizes in Highly Drawn Gel-Spun Ultrahigh Molecular Weight Polyethylene Fibers at the Final Stages of Drawing by SAXS, WAXS, and 1H Solid-State NMR

Morphology, phase composition, and molecular mobility for a series of semicommercial gel-spun UHMWPE fibers were studied using a combination of WAXS, SAXS, and H-1 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 NIVIR 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 similar to 35-45 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 similar to 89 to similar 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 Xe-129 NMR study on the same fibers. The role of alpha-crystalline relaxation in structural reorganizations during fiber drawing is also discussed.