Change in the structure of a polyethylene fiber, obtained by the gel-formation method, during its orientational drawing

Abstract

This article examines how the structure of a polyethylene fi lament obtained by the gel method changes during its orientational drawing. It is shown that the length of the crystallites increases and the crystallization front moves deep into the amorphous interlayers of the nanofi brils as the drawing operation proceeds. As a result, the amorphous regions of the fi lament may be almost completely crystallized and a continuous crystalline phase may be formed. The end result of these transformations is the formation of an anisotropic product having excellent elastic and strength characteristics.The oriented drawing of gel fi laments of ultrahigh-molecular-weight polyethylene (UHMWPE) is accompanied by the formation of a fi brillar material that has excellent mechanical properties [1]. Atomic-force microscopy [2] can be used to view different levels of the fi brillar morphology from the micron scale down to atomic dimensions. Bundles of fi brils, microfi brils, and nanofi brils were observed in the study just cited. Nanofi brils with a diameter ~20 nm constitute the main (base) element of the fi brillar morphology and do not undergo subdivision into smaller fi brils. Nanofi brils are formed during the initial stage of drawing of the fi bers, when crystallites begin to break up into folded chains (lamellae) simultaneously with the formation of a large number of randomly distributed micronecks. The nanofi brils are drawn from the thin ends of the lamellae.The structure of nanofi brils that are formed from lamellae is far from perfect. Crystalline and amorphous regions alternate regularly along their axes. Mechanically, the weakest points of such systems are the amorphous layers: there are considerably fewer intact segments of macromolecules in the cross section of these layers than in the crystallites. In addition, due to differences in the lengths of the continuous chains that connect adjacent crystallites with one another, only some of these chains help support the external load. The problem of strengthening the material reduces mainly to increasing the degree of order in the system: increasing the number of continuous chains and reducing the differences in their lengths by straightening them, i.e. by making the structure of the amorphous interlayers more like the structure of the crystallites. It is also necessary to orient the macromolecules more along the tensile axis and increase microfi bril packing density in the larger structures. These phenomena, which are important in a practical sense and take place during the oriented drawing of gel fi laments, are examined in this article.The object of study was a gel fi lament formed from a 3% solution of UHMWPE with MW = 4.3·10 6 g/mole in vaseline oil. Thermal drawing of the gel fi lament was done using a near-optimum temperature-rate regime for tension. The fi rst part of the drawing operation was performed in a solvent of the polymer -vaseline oil. The drawing operation was then continued in hot air after the solvent was washed off the polymer and the latter was drie...

Twist-film gel spinning of large-diameter high-performance ultra-high molecular weight polyethylene monofilaments

“…40 This is consistent with WAXD measurements that the fiber is orientated and crystallized by hot drawing to convert the amorphous regions into laminates. 41 By hot drawing from the first stage to the third stage, the peak around 1060 cm −1 keeps diminishing while the peak around 1416 cm −1 is increasing, which can support the selected orientations at such stages in WAXD measurements. For the band around 1300 cm −1 that is assigned as the interphase between crystalline and amorphous, it shows almost no change in the first stage, while a huge decrease is obtained in the second stage.…”

Gel spinning of UHMWPE fibers with polybutene as a new spin solvent

“…In the hot drawing process, long straightened segments of chains can penetrate the amorphous interlayers and coherently link adjacent crystallites to one another. As drawing continues, both the concentration and degree of orientation of the continuous intrafibrillar chains increase due to the conversion of convoluted conformers into straight conformers . This in turn increases the length of the crystallinites and causes the crystallization front to move deep into the amorphous interlayers.…”