Structural transformation from shish‐kebab crystals to micro‐fibrils through hot stretching process of gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration solution

Abstract

Structural evolution of gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration solution via hot stretching process was investigated by in situ small‐angle X‐ray scattering, in situ wide‐angle X‐ray diffraction measurements, scanning electron microscopy, and differential scanning calorimetry. With the increase of stretching strain, the long period continuously increases at relative lower stretching temperature, while it first increases and then decreases rapidly at relative higher stretching temperature. The kebab thickness almost keeps constant during the whole hot‐stretching process and the kebab diameter continually decreases for all stretching temperatures. Moreover, the length of shish decreases slightly and the shish quantity increases although there is almost no change in the diameter of shish crystals during the hot stretching process. The degree of crystal orientation at different temperatures is as high as above 0.9 during the whole stretching process. These results indicate that the shish‐kebab crystals in ultra‐high molecular weight polyethylene fibers can transform continuously into the micro‐fibril structure composed mostly of shish crystals through the hot stretching process. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 225–238

Formation and evolution of shish‐kebab structure during hot stretching in gel‐spun ultra‐high molecular weight polyethylene fibers with high concentration gel solution

“…With the increase of strain, more shish crystals form from the transformation of kebab crystals, and the shish crystals can be obviously seen in SEM images. Moreover, it is easy to notice that the kebab crystals are almost connected as shown in Figure , which is similar to our previous work . The statistic results of SEM micrographs listed in Table confirm the calculative results of long period and kebab thickness from SAXS results.…”

“…Furthermore, the final value of kebab lateral size is almost equal to the shish diameter (Table ), which means most of kebab crystals transform to fibrillar crystals. This result is the same as that of prestretched gel‐spun UHMWPE fibers from low and high concentration solutions . The increase of shish lengths (at 100°C, 110°C, and 120°C) is due to the transition of shish‐kebab crystals to shish crystals (fibrillar crystals) via melting recrystallization, and high temperature is good for proceeding melting recrystallization, which means more chain can be disentangled to form the longer shish crystals.…”

“…With further hot stretching (stage III), the slip, local fragmentation, or melting recrystallization of fibrillar crystals (or the few rest kebab crystals) occur until the weakest structure of fibers change to chain entanglement or other structural defection before the fibers stretched to break. In our previous work, the shish‐kebab crystals grown in prestretched gel‐spun UHMWPE fibers from high concentration gel solution from industrial production line have better structures than those in this study, and can smoothly transform to fibrillar crystals, which seems to indicate that the formation of shish‐kebab crystals is conducive to the transition of fiber initial structure to fibrillar crystals.…”

“…Figure shows the SAXS patterns, long periods, and lateral sizes of UHMWPE fibers upon heating from room temperature to 120°C, which is taken as an example to analyze the preheating process. In Figure A, the streak pattern across the beam stop in the meridian direction, suggesting the existence of microfibril structures (mainly consisting of shish crystals), while the two‐point pattern along the equatorial direction is the scattering pattern of highly oriented lamellar structures (ie, kebab structure) . With the increase of temperature, the streak pattern has no obvious change up to 120°C, but the intensity of two‐point pattern strengthens.…”

“…So the transformation of randomly oriented lamellae to shish‐kebab crystal, that is, the formation of shish‐kebab crystal, is the key precondition for the evolution of a shish‐kebab crystal to a fibrillar crystal. Moreover, we also studied the transformation of shish‐kebab crystals of gel‐spun UHMWPE fibers both were obtained from low and high concentration gel solutions, and we found that the shish‐kebab crystals formed under the same conditions could transform to microfibril structures at stretching temperatures ranging from 90 to 120°C, though the transformations of those from high spinning concentration were more difficult than those from low spinning concentration …”