Stretching temperature and direction dependency of uniaxial deformation mechanism in overstretched polyethylene

Abstract

In order to elucidate microscopic deformation behavior at different locations in isotropic semicrystalline polymers, the structural evolution of a preoriented high‐density polyethylene sample during tensile deformation at different temperatures and along different directions with respect to the preorientation was investigated by means of combined in situ synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques. For samples stretched along preorientation, two situations were found: (1) at 30 °C, the sample broke after a moderate deformation, which is accomplished by the slippage of the microfibrils; (2) at 80 and 100 °C, fragmentation of original lamellae followed by recrystallization process was observed resulting in new lamellar crystals of different thickness depending on stretching temperature. For samples stretched perpendicular or 45° with respect to the preorientation, the samples always end up with a new oriented lamellar structure with the normal along the stretching direction via a stress‐induced fragmentation and recrystallization route. The thickness of the final achieved lamellae depends only on stretching temperature in this case. Compared to samples stretched along the preorientation direction, samples stretched perpendicular and 45° with respect to the preorientation direction showed at least several times of maxima achievable stress before macroscopic failure possibly due to the favorable occurrence and development of microdefects in those lamellar stacks with their normal parallel to the stretching direction. This result might have significant consequence in designing optimal procedure to produce high performance polyethylene products from solid state. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 716–726

Transparent, Lightweight, and High Strength Polyethylene Films by a Scalable Continuous Extrusion and Solid‐State Drawing Process

“…This reduction in ultimate mechanical properties with increasing λ pre is the result of chain alignment along the MD in the melt state during the film extrusion process . A previous study by Fu et al showed that the elongation and ultimate tensile strength of pre‐oriented HDPE films increased when the angle between pre‐orientation direction and stretching direction was altered from 0° to 45° or 90°, which indicated that λ max is also dependent on the solid‐state drawing direction. Thus, the draw down or pre‐orientation ratio ought not to be chosen too high for the purpose of achieving a high ultimate mechanical performance.…”

Microscopic deformation behavior of hard elastic polypropylene during cold-stretching process in fabrication of microporous membrane as revealed by synchrotron X-ray scattering

“…2). It should be mentioned that a stress-induced melting [25][26][27][28] is involved in the fragmentation process as the WAXD intensity of each crystalline peak is significantly decreased in Stage IV (Fig. 3), which may be related to a decrease of the crystallinity.…”

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

“…From the results of long period and thickness mentioned above, it can be easily deduced that the mechanisms before and after the transition point are different. Men and coworkers 32 proposed that the increase of long period at small strain was attributed to the elongation of amorphous layers between lamellar crystals on the basis of the structural results of the stretch of HDPE strip. It can be seen from SEM results that the pre-stretched fibers are almost composed of stacked lamellar crystals (kebab crystals); furthermore, the transition points of long spacing are very close to the yield point in the stretching curves, in addition the increase rates of long period at all temperatures are almost constant before the transition point.…”

“…Moreover, they found that two lamellar to fibrillar transitions in the tensile deformation of high‐density polyethylene, which were followed by stress‐induced fragmentation recrystallization process. They also investigated the effect of stretching temperature on uniaxial deformation of overstretched polyethylene using in situ synchrotron SAXS. It was found that the rapid increase of long spacing of polyethylene at low strain was mainly due to the elongation of amorphous layers between lamellae when samples were stretched along preorientation.…”