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.

Precisely substituted polyethylenes have well-defined primary structures and aggregation architecture. Herein, precisely bromine-substituted polyethylene (PE21Br) was chosen as an ideal model to investigate the substituent impact on epitaxial crystallization upon one-dimensional carbon nanotubes (CNT) and two-dimensional reduced graphene oxide (RGO) via solution crystallization. The abilities of different dimensional nanofillers to induce ordered chain packing structures were compared. Transmission electron microscopy (TEM) images showed that kebab-like and rod-like nanofiller-induced crystals were separately observed on the surfaces of CNT and graphene, and selected area electron diffraction (SAED) pattern revealed that the c-axis of the polymer chain was parallel to the surface of RGO. Fast-scan differential scanning calorimetry (Flash DSC) revealed that the melting points of the crystals grown on CNT and graphene were increased by 19 and 99 °C, respectively. More importantly, X-ray diffraction (XRD) suggested that CNT and RGO induced the transition of the crystal structure of PE21Br from the triclinic to orthorhombic form, but with different orderness. More ordered lattice structures and higher melting temperatures of PE21Br/RGO nanocomposites are ascribed to the perfect lattice matching between PE21Br and RGO. This study not only provides a method for fabricating bromine-functionalized polyolefin nanocomposites, but is also anticipated to open up a new opportunity for improving the service temperature of substituted polyethylene by means of epitaxial crystallization.

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.

•The pre-stretched UHMWPE fibers with low concentration solution were prepared.•The structural transition of shish-kebab crystal via hot-stretching was studied.•The fragmentation recrystallization occurs at 90, 100, and 110°C.•The stress-induced melting recrystallization happens at 120 °C.•The shish-kebab crystals can smoothly transform to fibrillar crystals via hot-stretching.The pre-stretched gel-spinning ultra-high molecular weight polyethylene fibers with low concentration solution obtained from industrial production line were used to study the structural transition from shish-kebab to fibrillar crystals during hot-stretching process via in-situ small-angle X-ray scattering, wide-angle X-ray diffraction measurements, and scanning electron microscopy. With the increase of hot-stretching strain, the kebab crystals (lamellae) undergo stress-induced fragmentation and recrystallization at 90, 100, and 110°C and melting and recrystallization at 120°C to transform to fibrillar crystals. Furthermore, the shish length of pre-stretched fibers at all stretching temperatures first increase and then gradually decrease with the strain, but the number of shish continuously increase and the shish diameter increase firstly and finally keep almost constant. The degree of crystal orientation for all stretching temperatures is as high as above 0.9 during the whole stretching process. These results indicate that the shish-kebab crystals of ultra-high molecular weight polyethylene fibers prepared from low concentration solution can smoothly transform to fibrillar crystals through the hot stretching process.Download high-res image (325KB)Download full-size image

•The pre-stretched UHMWPE/Chitin nanocrystals (CNC) composite fibers were prepared.•The effect of CNC on the structural transition of UHMWPE/CNC fibers was studied.•The addition of CNC can improve the stretchability of pre-stretched UHMWPE fibers.•CNC was conducive to accelerate the fragmentation and melting process of lamellae.•CNC can promote the formation of fibrillar crystals via recrystallization process.The pre-stretched ultra-high molecular weight polyethylene/chitin nanocrystal composite fibers after first hot-stretching stage were prepared. The effect of chitin nanocrystal on the structural evolution of shish-kebab to fibrillar crystals of ultra-high molecular weight polyethylene/chitin nanocrystal fibers during hot-stretching process was studied by using in-situ small-angle X-ray scattering, wide-angle X-ray diffraction measurements, and scanning electron microscopy. The results showed that the addition of chitin nanocrystal can improve the stretchability of pre-stretched fibers, and makes the elongation of amorphous layers between lamellae more uniform. The chitin nanocrystal accelerates the fragmentation recrystallization process of kebab crystals (lamellae) at 90, 100, and 110 °C and the melting recrystallization at 120 °C during hot-stretching process. Moreover, more fibrillar crystals form in UHMWPE/CNC fibers comparing with pure UHMWPE fibers at the same strain and stretching temperatures. These results suggest that the addition of chitin nanocrystal is conducive to the structural evolution of shish-kebab to fibrillar crystals of ultra-high molecular weight polyethylene fibers during hot-stretching process.Download high-res image (132KB)Download full-size image

Besides the rather poor and irregular bands with large periods formed at high temperatures, banded spherulites were seldom spotted in pure poly(ε-caprolactone) (PCL). Herein, well-ordered extinction banded spherulites with the smallest spacing less than 10 μm are encountered in solution-cast PCL films via evaporative crystallization, and structural analyses unveil that such narrow bands suggest strong twisting of achiral orthorhombic PCL lamellae. It is proposed that solvent evaporation can enhance dramatically the disorder on opposite lamellar surfaces that stems from a slightly asymmetrical structural feature from the disposition of ester groups within a PCL unit cell, which gives rise to a striking enhancement of the unbalanced surface stresses resulting in the intensification of twisting frequency. The dependence of band spacing on crystallization conditions is also investigated to verify this intensifying effect. The present findings are envisaged to improve our understanding of the mechanical origin of unequal surface stresses for periodic twisting of polymer lamellar crystals.

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.

The ultrahigh molecular weight polyethylene (UHMWPE) fibers were obtained directly from the industrial production line. Two-step industrial hot-drawing-to-specific-drawing ratios were carried out at the temperature of 120 and 130 °C, respectively. Small-angle X-ray scattering (SAXS) measurements using synchrotron radiation were applied to study the evolution of kebab structure and the formation of shish structure. The slight increase of long period and the rapid decrease of lateral sizes indicated the destruction of original lamellae which was accomplished by chain slip resulted in the orientation of lamellae to form shish structure. The decrease of average shish length was explained that the formed new shish structure had shorter shish length than the original shish at the early stage with the high concentration of spinning solution. Wide-angle X-ray diffraction (WAXD) measurements were performed to explore the changes of the degree of orientation of the crystals. It was found that the elevated drawing temperature was benefited to the evolution of the orientational order. The DSC result confirmed the evolution of shish–kebab structure through the melting behavior.

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.

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 Young's modulus of the UHMWPE fibers matrix. Compared with that of pure UHMWPE fibers, the ultimate tensile strength and Young's 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 Young's 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 gel-spun ultrahigh molecular weight polyethylene (UHMWPE) fibers were prepared at the industrial production line with different gel solution concentration of 10–18 wt %. The effect of gel solution concentration on the structure and performance of UHMWPE fibers was studied 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 decreased while the melting temperature increased. And the increase of gel solution concentration made the crystallization and orientation of UHMWPE fibers become worse. Furthermore, the average length (⟨Lshish⟩) and misorientation (Bϕ) of shish structure in UHMWPE fibers decreased with the increase of gel solution concentration.

•PE21Cl was chosen to study the impact of substituent on epitaxial crystallization.•Kebab-like and rod-like PE21Cl thinner crystals formed on CNT and RGO, respectively, by traditional and supercritical CO2 assisted solution crystallization.•The melting point of rod-like crystal on RGO is 75 °C higher owing to structural transformation induced by epitaxial crystallization.We report morphology, thermal analysis and beam diffraction of polyethylene chlorine-substituted precisely on every 21st backbone carbon, after solution crystallization induced by low-dimensional carbonaceous nanofillers (carbon nanotube and graphene). Kebab-like and rod-like nanofiller-induced crystals were separately observed on the surfaces of carbon nanotube and graphene. Fast-scan differential scanning calorimetry revealed that while the melting point of crystals grown on carbon nanotube remains close to that in the bulk phase, the melting point of crystals grown on graphene is 75 °C higher. X-ray diffraction and selected area electron diffraction suggested that graphene induces the formation of orthorhombic form, which is of higher density than the triclinic form in the bulk.

•PE21F was chosen to study the impact of substituent on epitaxial crystallization.•Kebab-like and rod-like thinner crystals formed on CNT and RGO, respectively.•Crystallizable sequence length and crystal structure of PE21F remained unchanged.•SC CO2 can accelerate the lateral growth of lamellae formed on nanofillers.Crystallization of well-defined precision polyethylene with fluorine substituent on every 21st backbone carbon (PE21F) induced by low-dimensional carbonaceous nanofillers (carbon nanotube (CNT) and reduced graphene oxide (RGO)) via solution crystallization and supercritical CO2 assisted solution crystallization were investigated. Transmission electron microscopy was used to investigate the morphology of carbonaceous nanofiller-induced PE21F crystals. The kebab-like and rod-like crystals formed on the CNT and RGO, respectively. Selected area electron diffraction (SAED) pattern revealed that the c-axis of polymer chain is parallel to the surface of the RGO. Differential scanning calorimetry (DSC) revealed the melting temperatures (Tm) of PE21F lamellae nanocomposites increased with crystallization temperature increasing. The X-ray diffraction (XRD) results showed that the incorporation of nanofillers did not influence the crystal structure of PE21F. The chemical composition of the PE21F nanocomposites measured by X-ray photoelectron spectra (XPS) confirmed substituent F as a defect of chain was accommodated into the crystal lattice.

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.

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.

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.

Poly(ε-caprolactone) (PCL)/reduced graphene oxide (RGO) nanocomposite bars were prepared by injection molding, and the effects of RGO on the PCL matrix were investigated. Differential scanning calorimeter and polarized optical microscopy results demonstrated that RGO was an effective nucleation agent for PCL. Two-dimensional wide angle X-ray diffraction results showed that the incorporation of RGO can enhance the orientation degree of PCL crystals in the flow direction and did not influence the crystal structure of PCL. Two-dimensional small angle X-ray scattering results confirmed that the orientation of PCL chains in the flow direction was enhanced with the increase of RGO content, which was attributed to that RGO obstructed the motion of polymer chains. These results indicated that the incorporation of RGO can enhance the crystallization and orientation of PCL matrix, which was the major factor for the improvement of mechanical properties.

Spherulite morphologies of a highly asymmetrical double crystallizable poly(ε-caprolactone-b-ethylene oxide) diblock copolymer in solution-cast films were explored from a unified standpoint of tuning radial lamellar organization via controlled evaporation. Besides Maltese cross spherulites, three kinds of ring-banded spherulites that display non- and half-birefringent concentric ringed features as well as extinction banding were first encountered in the same polymer. Structural analyses based on atomic force microscopy, transmission electron microscopy, and grazing incidence X-ray technique revealed that concentric ringed spherulites possess the nature of a rhythmic variation of the radial lamellar packing and extinction banded spherulites have the origin of a periodic change of the radial lamellar orientation. Morphological transitions among different kinds of spherulites were achieved by altering the drying condition. PEO segment crystallized at low temperatures even if it is being confined by PCL lamellae. Combined with poly(ε-caprolactone) and poly(ethylene adipate), the influences of structural feature and crystallization condition on radial lamellar organization of polymer spherulites were discussed. These present findings are encouraged to enhance our understanding and then governing generation of expected polymer crystal morphology for special material performance.

Electrically conductive ultrahigh molecular weight polyethylene (UHMWPE)/graphene nanosheets composites with a segregated structure were prepared by electrostatic adsorption method. It was found that UHMWPE powders could generate static electricity after high-speed mechanical friction which was beneficial to adsorb fluffy graphene on the surface of polymer powders. UHMWPE/graphene nanosheets composites with a segregated network produced by hot-pressing exhibited a dramatic enhancement in electrical conductivity with the percolation threshold of 0.1 vol.%.

Epitaxial crystallization of poly(ε-caprolactone) (PCL) on reduced graphene oxide (RGO) was investigated by melt and solution crystallization. RGO and graphene oxide (GO) provided an opportunity to investigate the influence of surface functional groups on epitaxial crystallization with lattice matching. After annealing treatment, PCL/RGO composites showed an extra melting peak, which implied that epitaxial crystallization of PCL on RGO could form thicker lamellae. An analogous procedure of solution crystallization was used to confirm the epitaxy morphology and orientation of polymer chains. The results showed that PCL chains existed along the ⟨2110⟩ direction of the RGO (0001) plane forming edge-on lamellae, which implied strong interaction between PCL crystals and the RGO surface. Tensile test results showed that the yield strength and Young’s modulus of PCL/RGO composites with epitaxial interaction were improved by about 34.2 and 53.2%, respectively, compared with neat PCL.

Despite the extensive study of periodic precipitation and rhythmic crystal growth into ringed patterns, the detail of the evolution process remains unclear, and thus the explanation is rather elusive. Herein, we focus on monitoring the detailed growth process and dynamic, elucidating the underlying mechanism, and exploring key factors for the generation of poly(ε-caprolactone) (PCL) concentric ringed spherulites in evaporating droplets. In situ observation exhibits that accompanying the rhythmic evolution of the crystal, the region ahead the growth face changes periodically and the radial growth within each period is non-linear. It shows that having an evaporation-driven convection that carries liquid to the growth face drives the periodic dimple generation and rupture that leads to the rhythmic growth into discrete ringed spherulites. The non-linear growth is attributed to the coupling of evaporation and crystallization. We find that there are two key factors in the drying process that ensure the occurrence of the evaporation-driven flow and then the periodic crystal pattern. The structure formation reveals a complex interplay among solvent extraction, solution flow, solute diffusion, and crystal growth.