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

Abstract

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.

Mechanical and Structural Characterization of Semicrystalline Polyethylene under Tensile Deformation by Molecular Dynamics Simulations

“…Half of the simulation cell remained crystalline while the other half was transformed to a non-crystalline region 12.7 nm thick. Twenty-four bonds were cut in the non-crystalline region, resulting in 48 end-sites in each configuration, and 2136 UA sites were removed in the non-crystalline region to match the density of amorphous PE, which has been estimated to be 0.855 g/cm 3 at 298 K. 17 The resulting semicrystalline configuration contained 26664 UA sites. Then the non-crystalline region was subjected to equilibration by IMC with a set of Monte Carlo (MC) moves that includes both local rearrangements and alterations of chain topology.…”

Structural hierarchy and surface morphology of highly drawn ultra high molecular weight polyethylene fibers studied by atomic force microscopy and wide angle X-ray diffraction

“…The peaks are indexed according to their corresponding scattering planes for orthorhombic and monoclinic crystals in the fiber [35,36]. While there is some amount of monoclinic crystal phase present in each fiber type, it is calculated to be less than 4 percent of the total area under the diffraction curve, with the majority of crystalline material within the fiber being orthorhombic [37]. To calculate effective scattering domain sizes, the Scherrer Equation was used [38].…”

“…To calculate effective scattering domain sizes, the Scherrer Equation was used [38]. [37,39]. Each of these dimensions are perpendicular to the fiber axis, corresponding to microfibril widths in real space.…”

Diffusivity of Water Molecules in Amorphous Phase of Nylon-6 Fibers

“…Domain sizes are obtained by a fit of the entire spin-diffusion curve 33,34 or by using the initial slope of the spindiffusion curve. 36 (Figure 2). Here the signal intensity of the crystalline phase is already relatively large at short mixing times and differs significantly for these three fibers studied.…”