| Ultra-high molecular weight polyethylene (UHMWPE) fibers possess high strength of2.4-3.8GPa, high tensile modulus of88-166GPa, and low density of0.97-0.98g/cm3, which spread the applications in many fields.The commercial available UHMWPE fibers are mainly produced via the method of "gel spinning and ultra-high hot stretching".During the stretching process, the transition from chain-folded crystals to chain-extended crystals will occur which is the key part to achieve the high strength and high modulus of fibers. The strength and modulus of present UHMWPE fibers are far less than the values calculated from the theoretical model of fully extended chains. The properties of materials essentially depend on the structures. Therefore it is worthy to study the microstructure evolution process during stretching to explore the possibility of improvement. In addition, UHMWPE is a typical kind of semicrystalline polymers with the simple chemical structure. The investigationabout the stretching induced microstructure evolution of UHMWPE also contributes to the better understanding of polymer systems. X-ray is an effective method to detect the microstructure of materials. X-raymeasurements using synchrotron radiation can take full advantages of high time-resolution and high space-resolution to achieve the in situ measurements.In this paper, the stretching induced microstructure evolution of UHMWPE fibers was investigated byin situ small-angle X-ray scattering (SAXS) and in situ wide-angle X-ray diffraction (WAXD) measurements,based on Shanghai Synchrotron Radiation Facility (SSRF).The microstructure evolution of UHMWPE precursor fibers during hot stretching, the microstructure evolution of UHMWPE pre-stretched fibersduring ultra-high stretching,themicrostructure evolution of UHMWPEcommercial fibers during breakprocess were studied to explore the microstructure evolution of this typical semicrystalline polymer. The main results are as follows:(1) The methods about the correction of original scattering signals have been discussed and applied to obtain the high quality scattering patterns. The data analyses of SAXS and WAXD have been conducted mainly based on Bragg’s law combining Scherrer equation, crystalline orientation equation, Ruland’s streak method and scattering invariant method. Two sets of in situ stretching devices based on SSRF have been designed and developed including:temperature-controllable symmetrical stretching device, temperature-controllable roller stretching device. The self-designed stretching devices have been applied in SSRF BL16B SAXS beamline to contact in situ measurements.(2) The process of UHMWPE precursor fibers stretched to the strain of-910%at90℃,80℃,70℃and60℃have been investigated by in situSAXS measurement. The precursor fibers which have only been previously extracted and dried without stretching, exhibited isotropic scattering patterns, indicating randomly oriented lamellae structure. The streak in the meridian direction in scattering pattern has been confirmed to come from the chain-extended crystalline structures along the fiber axis.The hot stretching process can be divided into Zone I, Ⅱ and III according to different categories of SAXS patterns. At80℃, the primary lamellae stacking perpendicularly to the fiber axis in Zone Ⅰ went through the tilted state in Zone Ⅱ, then broke and reorganized into novel stacking structures in Zone Ⅲ. At90℃, the enhanced relaxation process effectively released the stress eliminating in the break of lamellae. At70℃and60℃, due to the unobvious relaxation, lamellae reached broken in advance resulting in no tilted state. Accompanied with the break and reorganization of lamellae, the chain-extended crystal was developed to accelerate the formation of shish-kebab structures in Zone Ⅲ.(3) The pre-stretched UHMWPE fibers with the stretching ratio of four-fold have been further stretched to510%at90℃and100℃reaching the final ultra-high stretching of-20times. In situ SAXS and WAXD measurements were conducted to analyze thisevolution process. The SAXS patterns of pre-stretched fibers consist of streak in meridian and point in equator indicating the existence of shish-kebab structures. In further hot stretching, point pattern receded with the the enhancement of streak demonstrating the transition from chain-folded lamellae to chain-extended shish. In the end of ultra-high stretching at100℃, only streak pattern can be observed indicating the original shish-kebab has transformed into fibrillar crystals.The results from WAXD have not exhibited significant changing of crystal lattice and crystalline size, UHMWPE also remained in the orthorhombic phase, which explainsthe still un-fully stretched state of chains.(4) The process of commercial grade UHMWPE fibers stretched to broken at25℃,40℃, 50℃,70℃and100℃has been investigated by in situ SAXS and WAXD measurements. The initial SAXS pattern exhibited only streak in the meridian direction demonstrating the only existence of fibrillar crystals parallel to fiber axis. At25℃, the fibrillar crystal was damaged as the fibers reaching broken. At40℃and50℃, the elongation at break of fibers increased with the decrease of the break of fibrillar crystal. At70℃and100℃, the fibers cannot be broken within the experimental strain range; the fibrillar crystal could also maintain the original state.The results from WAXD have not exhibited significant changing of crystal lattice, crystalline size and orientation, which indicatesthat the break process of fibers is more closely related to fibrillar crystal while not crystal lattice. |
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