Macromolecular NanotechnologyTransition from shish-kebab to fibrillar crystals during ultra-high hot stretching of ultra-high molecular weight polyethylene fibers: In situ small and wide angle X-ray scattering studies
Graphical abstract
Introduction
High performance fibers are considered as those fibers with significant properties in stiffness, strength, conductivity, so on and so forth, designed for specifically required uses. Ultra-high molecular weight polyethylene (UHMWPE) fibers are a kind of high performance fibers in mechanical engineering application, possessing high strength of 3.8 GPa, high tensile modulus of 166 GPa, and low density of 0.98 g/cm3, which continuously attract considerable interests. UHMWPE fibers are a typical kind of semicrystalline polymer materials, that the properties are essentially determined by the structures [1]. The studies about the relationship among the processing, structures and properties of semicrystalline polymers have been widely reported as about the processing or deformation induced the evolution of microstructures like microvoids, long period structures, microfibrils, etc. [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Polyethylene with the concise chemical structure, the vinyl repeat unit, and high-density polyethylene (HDPE) and UHMWPE with even less branched structures are the very ideal candidates for the fundamental study of structure–properties relations and other polymer physics questions. There have already been reports about the macroscopic deformation induced microstructures evolution in HDPE films by Men et al. that lamellar structure could transform into the micro-fibrils during uniaxial stretching through the stress-induced melting and crystallization mechanism [15], [16]. More interestingly, UHMWPE precursor fibers will go through an ultra-high stretching process to achieve the high performance properties. During this process, the formation of shish-kebab and the corresponding transition have been reported early [17], [18], [19], [20]. Our previous article also studied the relatively stretching process of UHMWPE precursor fibers at the temperature of 80 °C, and a lamellae-break induced formation of shish-kebab model was proposed by us [21]. However, the behaviors of semicrystalline polymers strongly depend on the temperature with the possible α relaxation process influenced mechanism [22]. That industrial stretching process of UHMWPE precursor fibers is conducted in a higher stretching ratio and temperature. Therefore it is worthy to further investigate the ultra-high stretching process of UHMWPE precursor fibers under higher temperature that is closer to the real industrial situation.
Small-angle X-ray scattering (SAXS) is the very effective technique for the study of about nanometer range microstructure of polymer materials. And its theory has been fully developed that can be applied for the situations of polymer lamellae, polymer microfibrils and microvoids [23], [24], [25], [26]. Furthermore, with the awareness of the value of synchrotron radiation and continuous development, the in situ X-ray measurements have become available and popular for the various studies [13]. There have been reports about kinds of polymer fibers as poly (vinylidene fluoride) (PVDF), nylon 6 (PA6), polyethylene (PE), poly (oxymethylene), polybutene-1, polypropylene (PP), poly (ethylene terephthalate) (PET), etc.; the in situ measurements during the processing revealed the several significant evolution processes of microstructures as lamellar organization, transition and microvoids development [27], [28], [29], [30]. And also other forms of polymer materials as polymer films or sheets have been used in some in situ studies by Li et al., which also effectively indicated the occurrence of the transient structures during the deformation process of HDPE and PA 12 [31], [32], [33]. As be mentioned above, our group has already established the in situ SAXS measurement technique to investigate the hot stretching process of UHMWPE precursor fibers [21]; moreover in this article based on the well-established technique, we conducted the further in situ small and wide angle X-ray scattering (SAXS/WAXS) studies about pre-stretched UHMWPE fibers at 90 °C and 100 °C temperatures, for the purpose of totally about 20-fold ultra-high stretching ratio to take a deep look into the possible transition from shish-kebab structures to fibrillar crystals.
Section snippets
Materials
A bundle of extracted UHMWPE precursor fibers with 230 filaments fabricated by our collaborator were previously stretched at 90 °C and 100 °C to reach the effective drawing ratio of 400%. The pre-stretched fibers were used in this experiment for further hot stretching, and the main properties are listed in Table 1.
Ultra-high hot stretching and in situ X-ray measurements
Self-developed drawing apparatus was used to conduct ultra-high hot stretching of fibers, which has been described in our previous research [21], here we will not reiterate much but a
Transition from shish-kebab to fibrillar crystals detected by in situ SAXS
Fig. 1, Fig. 2 show the selected SAXS patterns of pre-stretched UHMWPE fibers during ultra-high hot stretching of 90 °C and 100 °C. As labeling in figures, q = (4π/λ)sin θ is scattering vector, qy and qz are two components of the scattering vector, where y and z stand for the equator and meridian directions of scattering pattern respectively.
The SAXS images consist of streak pattern in the meridian direction and two-point pattern along the equator direction. The two-point pattern indicated the
Conclusions
Synchrotron radiation was used for the in situ SAXS/WAXS measurements of pre-stretched UHMWPE precursor fibers ultra-high stretching at 90 °C and 100 °C. The simultaneous records of SAXS confirm the transition from the shish-kebab structure to fibrillar crystals after about total 20-fold ultra-high stretching at 100 °C. The results of WAXS and Raman spectra support the implication of macroscopic deformation on the internal microscopic structures; however also indicate the remaining of the
Acknowledgements
This work was supported by National Natural Science Foundation of China (11179027 and 21304059), 973 Program (2011CB605604), Guangdong Doctor Startup fund (S2013040015706), Natural Science Foundation of Shenzhen University (Grant no. 201204) and the Shenzhen basic research project (JCYJ20120614085820810). The authors would like to thank SSRF, Shanghai, China for provision of the synchrotron facilities at beamline BL16B, as well as Dr. Yang from College of Mechanical Engineering, Donghua
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