Haojun Xu

Ultra-strong gel-spun ultra-high molecular weight polyethylene fibers filled with chitin nanocrystals

Ultra-high molecular weight polyethylene (UHMWPE)/chitin nanocrystals (CNC) and UHMWPE/acetylated chitin nanocrystals (ACNC) fibers were prepared. The addition of CNC and ACNC significantly enhanced the ultimate tensile strength and Youngs modulus of the UHMWPE fibers matrix. Compared with that of pure UHMWPE fibers, the ultimate tensile strength and Youngs modulus of UHMWPE/CNC fibers are increased by about 14.5% and 17.0%, respectively, with the incorporation of 1.0 wt% CNC. Furthermore, with the addition of 1.0 wt% ACNC, the ultimate tensile strength and Youngs modulus of UHMWPE/ACNC fibers are improved by 15.8% and 21.3%, respectively. To understand the mechanism of CNC and ACNC reinforcing UHMWPE fibers, the thermal, crystallinity, orientation and shish structure of pure UHMWPE fibers, UHMWPE/CNC fibers, and UHMWPE/ACNC fibers were determined by employing a differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) analyses.

The influence of epitaxial crystallization on the mechanical properties of a high density polyethylene/reduced graphene oxide nanocomposite injection bar

High density polyethylene (HDPE)/reduced graphene oxide (RGO) nanocomposite bars were prepared by injection molding and the effects of RGO on the HDPE matrix were investigated. Differential scanning calorimetry results demonstrated that RGO was an effective nucleation agent for HDPE. Two dimensional wide angle X-ray diffraction (2D WAXD) results showed that the incorporation of RGO enhanced the degree of orientation of HDPE crystals in the flow direction but had no influence on the crystal structure of HDPE. Two dimensional small angle X-ray scattering (2D SAXS) results confirmed that the orientation of HDPE chains in the flow direction was enhanced with the increase of RGO content, which was attributed to the fact that RGO obstructed the motion of polymer chains. These results indicated that the incorporation of RGO can enhance the crystallization and orientation of the HDPE matrix, resulting in the improvement of mechanical properties.

An in situ small-angle X-ray scattering study of the structural effects of temperature and draw ratio of the hot-drawing process on ultra-high molecular weight polyethylene fibers

An in situ small-angle X-ray scattering (SAXS) study of the structural effects of temperature and draw ratio (DR1) of the hot-drawing process on ultra-high molecular weight polyethylene (UHMWPE) gel fibers was performed with equipment simulating the hot-drawing process on an industrial production line. The UHMWPE gel fibers were prepared from the industrial production line. The results show that the increase of hot-drawing temperature has a significant effect on the kebab but no obvious effect on shish length and misorientation. The increase of temperature is beneficial to the formation of the shish in a suitable temperature range of 124–130 °C, while the formation of the shish at higher temperature, 140 °C, needs higher DR1. Moreover, the increase of DR1 is beneficial to the formation of shish at all experimental hot-drawing temperatures, while the kebab formation mainly occurs at low DR1 and the kebab transformation mainly happens at high DR1. The shish length and misorientation decreases with the increase of DR1.

Characterization of structural knot distributions in UHMWPE fibers

Microbeam wide-angle X-ray diffraction (WAXD) experiments were carried out at different structural knot positions of SIOC and M4 fibers of ultra-high molecular weight polyethylene (UHMWPE). The optical microscope images revealed that SIOC fiber had bamboo-like structural knots, and M4 fiber had chaotic distribution of structural knots. WAXD patterns showed the monoclinic unit cell in the whole M4 fiber, but different lamellar orientations in the bamboo joint of SIOC fiber. In addition, small-angle X-ray scattering (SAXS) patterns confirmed that the SIOC fiber contained uniform distribution of shish structures, and differential scanning calorimetry (DSC) measurements showed that its less branched and short chains benefited the orthorhombic-hexagonal phase transformation.

Multiple endothermic peaks resulted from different crystal structures in an isomorphous copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The multiple endothermic peaks of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HB-co-HV)) in differential scanning calorimetry (DSC) results, as one representative phenomenon of polymer with unique cocrystallization behavior, were generally considered as the results of melting/recrystallization. In this study, wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) experiments were conducted to analyze the phenomena of multiple endothermic peaks in DSC results. The results of these analyses indicated that the multiple endotherms were mainly caused by different lamellae structures. For P(HB-co-HV) with lower HV content, it was comprised of two structures of HV total exclusion and HV partial inclusion in the crystal lamellae. For P(HB-co-HV) with higher HV content, it was also comprised of two structures of HV total inclusion and HV partial inclusion in the crystal lamellae. However, only structure with HV partial inclusion in the crystal lamellae remained existing after first melting peak for all samples.

Structural difference of gel-spun ultra-high molecular weight polyethylene fibers affected by cold drawing process

The UHMWPE fibers with different cold drawing ratio (DR0) were obtained from the industrial UHMWPE fibers production line. The effect of cold drawing before the extraction of paraffin oil process on final fibers was investigated by tensile testing, small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). The tensile strength and modulus with 5.0 DR0 were 2.99 and 151.5 GPa, respectively, which were 13.3 % and 41.9 % higher than those with 1.5 DR0. With the increase of DR0, the values of average shish length decreased obviously, while the shish orientation increased and the apparent crystal size along two lattice directions ((110)o and (200)o) in UHMWPE fibers decreased. The increase of degree of orientation and crystallization were verified that better folded chains and amorphous chains were involved in forming shorter and better oriented shish.

Structure and properties of gel-spun ultra-high molecular weight polyethylene fibers with high gel solution concentration

The gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibers were prepared at the industrial production line with different gel solution concentrations of 15 wt%, 18 wt% and 24 wt%. The difference in ultimate structure and mechanical properties of UHMWPE fibers for different gel solution concentrations were analyzed by tensile testing, differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). With the increase of gel solution concentration, the ultimate mechanical properties of UHMWPE fibers were decreased and the crystallization and orientation of UHMWPE fibers became inferior. Besides, both the average shish length () and shish misorientation (Bϕ) of UHMWPE fibers were decreased with the increase of gel solution concentration. In addition, the appropriate increase of spinning temperature led to the further optimization of the ultimate structure and mechanical properties of UHMWPE fibers.

The influence of chitin nanocrystals on structural evolution of ultra-high molecular weight polyethylene/chitin nanocrystal fibers in hot-drawing process

Ultra-high molecular weight polyethylene (UHMWPE)/chitin nanocrystal (CNC) fibers were prepared. Compared with the pure UHMWPE fibers, the ultimate tensile strength and Young’s modulus of UHMWPE/CNC fibers are improved by 15.7% and 49.6%, respectively, with the addition of chitin nanocrystals (CNCs) of 1 wt%. The melting temperature (Tm) of UHMWPE/CNC fibers was higher than that of pure UHMWPE fibers. Pure UHMWPE fibers and UHMWPE/CNC fibers were characterized with respect to crystallinity, orientation and kebab structure by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). It is found that the CNCs act as the shish structure in UHMWPE/CNC fibers and the kebab crystals are grown around the CNCs. There was almost no difference between pure UHMWPE fibers and UHMWPE/CNC fibers in orientation. But the degree of crystallinity of various stages of UHMWPE/CNC fibers was respectively higher than the corresponding stage of pure UHMWPE fibers. Moreover, the addition of 1 wt% CNCs improved the thickness of kebab crystals and accelerated the transformation of kebab to shish.