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V bimetallic catalysts
Composites Science and Technology 175 (2019) 46–54
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Hierarchical structure manipulation of UHMWPE/HDPE fibers through inreactor blending with Cr/V bimetallic catalysts
T
Xuefeng Pana, Jiaji Shia,1, Yulong Jinb, Boping Liub, Liangbin Lic, Xuelian Hea,∗ a
Shanghai Key Laboratory of Multiphase Material Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China College of Materials and Energy, South China Agricultural University, Guangzhou, 510642, China c National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, 230026, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: High density polyethylene Ultra-high molecular weight polyethylene A. Fibres In-reactor polymerization Crystallization
A dramatically improved, uniform distribution of commercial ultrahigh molecular weight polyethylene (UHMWPE) with high-density polyethylene (HDPE) via solution blending, rather than that of melt blending, is often apparent in experimental investigation. Recently, we demonstrated that a UHMWPE/HDPE blend created by in-reactor polymerization with a Cr/V bimetallic catalyst supported on modified silica can obtain an even better molecular chain distribution while saving energy and solution costs, even more so than experiments using a solution blend. Furthermore, by gel-spinning processing, an abundance of hierarchical, interlocking shishkebab crystals were generated in the in-reactor polymerized blend fibers (IRBFs), exhibiting better mechanical properties than those of solution blend fibers (SBFs). The tensile strength of the IRBFs increased by about 20% under the same draw ratio, since the interlocking shish-kebab structures contributed to homogenous deformation. The two kinds of fibers exhibit high crystallinity and have a highly oriented shish-kebab structure. Compared with the SBFs, the IRBFs inclined to form longer shish features and smaller kebab sizes, and the draw process was more conducive to the transformation of kebab to shish structures. Thus, the IRBFs can obtain an excellent hierarchal network structure, benefiting more from the hot-stretching process. Our work lays a solid foundation for the efficient fabrication of high-performance UHMWPE/HDPE fibers as well as the optimization of an original polyethylene catalyst system.
1. Introduction Tensile strength and the elastic modulus of fibers are commonly referred to as being high-performance above 17.6 cN/dtex and 440 cN/ dtex, respectively [1]. Ultra-high molecular weight polyethylene fibers (UHMWPE) are one of the typical high-performance fibers and have been widely used in space exploration, marine development, and military industry due to their excellent mechanical performance [2,3]. The high orientation and crystallinity of the molecular chain structure in the fibers determine their superb mechanical properties. However, the maxima tensile strength and elastic modulus of commercial UHMWPE fibers are still far below theoretical values due to unsatisfactorily oriented structures in industrial production [4]. The macroscopic tensile property involved in stress distribution has a strong connection with the architecture of inner structures at the microscale. Commonly, the fundamental microstructures of UHMWPE
fibers are composed of extended chain microfibers (shish), interlocking and standalone kebabs epitaxially crystallized on shish, standalone shish-kebabs, voids, or interfaces between the microfibers and tie chains across voids [5]. In the drawing process, stress is transferred both along the fiber axis and through shear between adjacent microfibers. The shish structures are the main architecture components to bear the extended stress, providing the extremely high strength and elastic modulus of UHMWPE fibers [6]. The strong interconnection of adjacent epitaxial kebabs could also enhance the cohesion between the microfibers under tension, further improving the mechanical properties. On the contrary, stress concentration usually happens due to the existence of interior void spaces and microfibril misplacement in the fiber, impairing the strength and tenacity [4,7]. Not confined to the structure and relationship of polyethylene fibers, the reinforcement and dissipation of high-density polyethylene composites is also strongly influenced by filler-polymer interactions and filler topology [8].
∗
Corresponding author. E-mail address: [email protected] (X. He). 1 Co-first author: Jiaji Shi. https://doi.org/10.1016/j.compscitech.2019.03.004 Received 29 December 2018; Received in revised form 25 February 2019; Accepted 1 March 2019 Available online 04 March 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
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combining Ziegler-Natta and metallocene or different metallocene catalysts in a single reactor were successfully developed to produce the in-reactor bimodal polyethylene [27–29]. Commonly, one catalyst was mainly used for producing high molecular weight fractions, and another catalyst was used for the low ones, providing a broad molecular weight distribution. By controlling different molar ratios of catalyst species, desirable mass fractions of UHMWPE and HDPE could be obtained. According to our previous work [30], UHMWPE could be synthesized in a wide range of polymerization temperatures by vanadiumoxide-based catalysts supported on modified silica, and HDPEs were produced by silylchromate-based catalysts at the same time. More interestingly, a series of silylchromate(Cr)/vanadium-oxide(V) bimetallic catalysts supported on aluminum-modified silica (SiO2-Al2O3) were synthesized for controllable UHMWPE/HDPE IRBs with high polymerization activity. Herein, simply choosing the appropriate ratios of Cr/V were required to obtain desirable UHMWPE fractions in the blends; thus the interlocking shish-kebab structures in the blend fibers were effectively manipulated to maintain a relatively balanced mechanical performance from UHMWPE and processability from HDPE. Solution blends (SBs) are frequently used to obtain a far better dispersion of molecular chains than that of melt blends (MBs) and are mainly applied in academic research that is limited by the massive solution waste. In view of the predominant distribution of UHMWPE in HDPE by solution blending and in-reactor blending, it is highly desirable to compare the final properties of UHMWPE/HDPE gel-spun fibers prepared by SBs with IRBs. Through the characterization of crystal morphology and structures combining with rheological behaviors, the evolution mechanism of the shish-kebab structure was proposed, and the excellent mechanical properties of IRBFs were demonstrated by efficient structure manipulation.
Furthermore, numerous works have focused on the evolution of epitaxial kebab crystals in the drawing process. Interestingly, assisted by transmission electron microscopy (TEM), Ohta [9] suggested that the folded chains in kebabs can be transformed into the extended shish structures through inter-chain slippage, which promotes longitudinal extension but induces little change in the diameter of shish structures. According atomic force microscopy (AFM) results, Kenneth [5] also demonstrated that extended stress forced the epitaxial kebabs to combine and overlap with each other, and then the stress is gradually shared by adjacent shish. Another work [10] suggested that shish-kebabs of UHMWPE fibers prepared from low-concentration solution can smoothly transform to fibrillar crystals at high stretching temperatures (above 110 °C) due to stress-induced melt recrystallization. Closely related to the evolution process of epitaxial kebabs in fibers, the formation of shish-kebab and even its precursor in the melt is also significant and worthy of further to elucidation. In the early flow stage, the coiled chains in enriched molecular regions gradually turn into bunches of stretched strands, which usually is referred to as the coilstretch transition [11]. While in the late flow stage, the oriented molecular chains in enriched regions form a stretched shish structure and epitaxial kebabs due to a strong flow field [12,13]. Furthermore, Li reported that all of the stretched long-chains have equal opportunity to form uniformly distributed nuclei in bimodal blends with a high longchain concentration. Thus, the high density of stress-induced nuclei was shown to be compactly connected with each other, which constructed a long-chain entanglement network leading to homogenous deformation [14]. In this scenario, the molecular distribution state in the melt significantly affected the evolution of the oriented crystal structure and determined what kind of shish-kebabs would come into being. Therefore, to obtain high tensile strength and elastic modulus of UHMWPE fibers that are as close to the theoretical values as possible, the construction of interlocking shish-kebabs is essential for the uniform network structure, further leading to the effective stress distribution under tension. However, despite the excellent performance of UHMWPE fibers, the UHMWPE matrix has rather poor processability due to the high melt viscosity and high production cost [15]. Thus, the advantages of a high strength and high elastic modulus cannot be utilized to their maximum extent, limiting the application of UHMWPE fibers. Conventional physical blending for the modification of matrix resin is usually adopted by mixing low molecular weight polyethylene for good processability and high molecular weight components for mechanical properties. The introduction of relatively low molecular weight polyethylene, sharing the same chemical structure, is a reasonable method to improve the UHMWPE processing performance and save production costs. Among them, blending HDPE with UHMWPE is the common modification method to improve the processing properties and maintain the materials’ original toughness, impact resistance, and corrosion resistance at the same time [16–18]. Also, assisted by some special processing techniques, oscillating shear fields and extensional flow have both been imposed on the fabrication of self-reinforced polyethylene composites [19–22]. However, micrometer-sized UHMWPE particles, which do not completely melt, would usually damage the dispersion of UHMWPE in HDPE matrix through conventional injection molding and extrusion. Also, high temperatures and severe shear field during the melt blending process could lead to molecular chain scission and oxidation [23]. Too high of HDPE content in the blend would increase the difficulty of gel spinning and weaken the final performance of the fibers. In-reactor blends (IRBs) of UHMWPE and HDPE synthesized during an ethylene polymerization process provides another novel approach to avoid the problems in physical blending processes and obtain a good control of UHMWPE/HDPE distribution in the blend at the molecular level. A small amount of UHMWPE could be incorporated into HDPE using multiple-step polymerization in a cascade reactor [24–26], which is strongly dependent on high equipment investments and the precise control of the polymerization conditions. Recently, multisite catalysts
2. Experiment 2.1. Materials UHMWPE was purchased from Shanghai Lianle Chemical Technology Co., Ltd., China, with a weight-average molecular weight of 4000 kg/mol and a density of 0.943 g/cm3. HDPE was purchased from the Daqing Petrochemical Branch of China National Petroleum Corporation, with a molecular weight of 318 kg/mol and a melt index 10 g/10 min (190 °C, 21.6 kg). 2.2. In-reactor blends preparation The UHMWPE/HDPE IRBs were synthesized according to our previous work [30], using a Cr/V bimetallic catalyst supported on SiO2Al2O3 with an ethylene pressure of 1 MPa at 80 °C. By controlling the molar ratio of Cr to V as 1:1, 1:2, and 1:4, IRBs with approximate molar ratios of HDPE to UHMWPE (1:1, 1:2, and 1:4) were obtained, respectively. 2.3. Solution blends preparation The corresponding molar ratios of HDPE to UHMWPE of SBs were also selected as 1:1, 1:2, and 1:4 (see Table S1). Desired amounts of UHMWPE and HDPE were dissolved in hot xylene with 3 wt% antioxidants (Irganox 1010) to form a homogeneous solution under a nitrogen atmosphere. After keeping the solution at 160 °C for 4 h, it was quickly precipitated into cold methanol, filtered from xylene, and washed with methanol several times. Finally, the SBs were dried at 80 °C under vacuum for several days to fully remove the remaining solvent, and then ground into powder form as thoroughly as possible. 2.4. Blend fibers preparation The detailed procedures of the gel-spun processes are shown in 47
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Fig. 1. Schematic of gel spun of process of blend fibers.
230 °C in a nitrogen atmosphere. Dynamic frequency sweep measurements were carried out with a strain of 1% in the linear viscoelastic regime at frequencies between 100 and 0.1 Hz. In preparation for the rheological detection of all of the samples, the polymer blends were stabilized by the addition of 0.25 wt% Irganox 1010 to avoid thermaloxidative degradation.
Fig. 1. First, moderate amounts of SBs or IRBs and antioxidant Irganox 1010 (0.5 wt%) and Irgafos 168 (0.5 wt%) were fully mixed with decaline under a nitrogen atmosphere. To obtain a well-dissolved gel solution, a step-by-step heating procedure was carried out by first heating to 125 °C at 1 °C/min, holding this temperature for 1 h, and then heating to 180 °C at the same speed and in the same heating time. Meanwhile, the blend solution required to be set in a counter-stirred mode while the Weissenberg-effect occurred. Subsequently, the obtained blend gel with a concentration of 20 g PE/L was cooled down to room temperature. After the dry gel was obtained, the mass of exuded decaline was deducted so that the gel spun concentration was maintained at about 7 wt%. Second, the yarn was extruded and spun through a spinning machine at 180 °C followed by being quenched in cool water. The drawn process was precisely controlled by a running roller at a speed of 5 cm/s with an initial draw ratio (DR0) of 3.6. Third, after drying at room temperature in a cooling chimney to fully remove the remaining solvent, the gel-spun fibers with an initial length of 50 mm produced above were continuously drawn at a speed of 20 mm/min in a uniaxial multiplied tensile tester (STM-H-500BP, Toyo Baldwin). Finally, in order to investigate structural evolution during the stretching process, two-stage hot drawing was carried out at 110 °C with a first draw ratio (DR1) of 4 at 135 °C and a second draw ratio (DR2) of either 2 or 3. The diameter and linear density is shown in Table S2. The fibers prepared from the IRBs, synthesized by the bimetallic catalysts with Cr/V ratios of 1:1, 1:2, and 1:4 (1Cr1V/SiO2-Al2O3, 1Cr2V/SiO2-Al2O3, and 1Cr4V/SiO2-Al2O3), were denoted as IRBF1-1, IRBF1-2, and IRBF1-4, respectively, while the fibers prepared from SBs in which the molar ratio of HDPE to UHMWPE were approximately controlled at 1:1.01, 1:2.03, and 1:4.07, were denoted as SBF1-1, SBF12, and SBF1-4, respectively.
2.6. Scanning electron microscopy observation The blend fibers were carefully etched with hot n-octane to remove the amorphous phase at 115–120 °C [10]. Then, the surfaces of all of the samples were sputtered with thin layers of gold and observed by a field-emission Scanning electron microscopy (FE-SEM, NOVA NanoSEM450, FEI, USA) operating at 3 kV. Additional samples of IRBF1-2 and SBF1-2 were chosen to retain the original surface morphology for SEM observation without etching.
2.7. Tensile properties measurement The tensile strength of the blend fibers was measured with an electronic single fiber strength tester (LLY-06E, Laizhou Electron Instrument Co., Ltd., China) at room temperature. The testing speed was 20 mm/min, and the gap between the clamps was 20 mm. Each sample was tested for 30 times to obtain the average value. Additionally, a single yarn with a length of 40 mm after the drying process was drawn at 110 °C until its failure, and the maximum hot draw ratio (DRm) of the single yarn was calculated as,
DRm = 2.5. Rheological measurement
L2 , L1
(1)
where L1 and L2 was the length of single yarn before and after stretching, respectively. Each yarn sample was repeated 20 times to obtain the average value.
Rheological properties of the polymer blends were characterized on a rotational rheometer (Hakke MARSΙΙΙ, Thermo Scientific, USA) with a parallel plate geometry of 25 mm in diameter and a gap of 0.7 mm at 48
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observation. As shown in Fig. 3(e), large quantities of folded-chain lamellae entities stacked along the stretching direction with periodic gaps observed, which were usually identified as typical shish-kebab structures, as indicated with crossed, red, dotted lines. It is clear that both IRBFs and SBFs possessed a large number of shish-kebab crystals. In the IRBF samples, there are many uniform shish-kebabs and few impurities on the surface from amorphous regions whether etched or not, as shown in Fig. 3(a–d). More strikingly, the shish-kebabs were interconnected, and the adjacent kebabs were also interpenetrated with each other, exhibiting a homogenous interlocking network structure, as indicated by a red, dotted rectangle in Fig. 3(b). With the increase of UHMWPE content, the network structure became more compact, which narrowed the space between shish-kebabs. Furthermore, this network structure could maintain a relatively stable state during the drawing process, rather than being easily destroyed under tension. Whereas with the SBF samples, clear surface defects, such as voids and crystal misalignments, were observed, indicating a loosely aligned structure, as shown in Fig. 3(e–h). Additionally, the standalone shish-kebabs, indicated by a red, dotted rectangle in Fig. 3(e), were independently arranged and showed a clear boundary, leaving large gaps between each other. The interlocking shish-kebabs could also be found in the blend fibers with high contents of UHMWPE, as shown in Fig. 3(g). These results imply that some single shish-kebabs could also connect with each other and evolve into an interlocking structure.
2.8. Small angle X-ray scattering and wide angle X-ray diffraction measurements X-ray scattering measurements were carried out using an in-house setup with a 30 W micro X-ray source (Incoatec, GmbH) in Hefei, providing a highly parallel beam (divergence about 1 mrad) of monochromatized Cu Kα radiation with a wavelength of 0.154 nm [31]. A bundle of parallel blend fibers was used for small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXD) measurements. The distance from the fibers to the detector was 3725 mm for SAXS and 235 mm for WAXD. A Mar345 CCD detector (3450 × 3450 pixels in an area of 150 × 150 μm2) and a Pilatus 300 k detector (487 × 619 pixels in an area of 172 × 172 μm2) was used to collect the SAXS and WAXD signal patterns, respectively. All of the X-ray images were corrected for background scattering by subtracting contributions from air and the sample holder, and the image acquisition time of each data frame was 30 min. The SAXS and WAXD measurement data analyses were carried out by the Fit2D software package. 3. Results and discussion 3.1. Miscibility of HDPE and UHMWPE blends Miscibility of HDPE and UHMWPE in the blends affects the formation of crystal structures and the final tensile performance of the blend fibers. As shown in Fig. 2, the rheological behaviors of SBs and IRBs were characterized. Regarding the miscibility of the blends, the ColeCole curves showed no drift or semicircles with different radii, implying that the two blends were rheologically miscible and showed no phase separation behavior. This phenomenon is also in good agreement with the sole melt peak shown in the differential scanning calorimetry (DSC) results, indicating good co-crystallization [30]. Furthermore, each Han curve of G′ vs G″ was displayed and linearly fitted nearly indicated with its slope. The slopes of the IRBs were 1.24, 1.33 and 1.62, respectively. It is worth noting that there was overlap between IRB1-1 and IRB1-4. Furthermore, the slopes of SBs were 0.81, 1.14, and 1.52, respectively, showing a relatively large variation. These results indicate that the IRBs have an even better dispersion of HDPE and UHMWPE than that of SBs due to the relatively smaller deviation of slope value and the overlap of the Han curves [32,33]. Hence, in-reactor blending method has an advantage over solution blending in the mixing effect of the two polyethylene components, and even more so when compared to melt blending.
3.3. Tensile properties The tensile performances of the two kinds of blend fibers are shown in Fig. 4. It can be clearly seen that the tensile strength of IRBFs ranged from 2.1 to 2.7 GPa, much better than that of the common fibers (below 1 GPa) and close to the commercial UHMWPE fibers (2–4 GPa), which meets the requirement of high-performance fibers [1,34]. The tensile strength of both IRBFs and SBFs was enhanced with the increase of UHMWPE content and draw ratio due to their highly oriented structures. As can be seen in Fig. 4(a), the tensile strength of the IRBFs is about 20% higher under the same draw ratio, while the DRm is slightly lower than that of the SBFs in Fig. 4(b). The relatively better tensile strength and worse drawability suggest that a considerable tensile property of blend fibers could be obtained at a low draw ratio. Normally, through the method of solution blending, HDPE and UHMWPE could attain a well-miscible state; however, the rheological results from Fig. 2 imply that the blends of HDPE and UHMWPE synthesized by inreactor blending achieved an even better dispersion than that of solution blending at the molecular scale. In that way, the uniform molecule distribution improved the formation of interlocking shish-kebab structures during the stretching process, as shown in Fig. 3. More importantly, the interpenetrated kebabs in the network structures could serve as physical junctions between the adjacent shish-kebabs. In that
3.2. Hierarchical crystalline morphology To obtain visual information of the crystal structure of blend fibers, some samples were carefully treated with hot n-octane prior to SEM
Fig. 2. (a) Cole-Cole curves and (b) Han curves and the fitted slopes of IRBs and SBs. 49
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Fig. 3. SEM micrographs of surface morphology of different blend fibers at the DR2 of 3: (a)∼(c) IRBFs after etching, (d) IRBF1-2 before etching, (e)∼(g) SBFs after etching and (h) SBF1-2 before etching. The fiber axis is vertical.
the two kinds of fibers, it is shown that, different from the large-size microcrystals of the SBFs, the IRBFs tend to form smaller size microcrystal structures, which were closely arranged and preferred to be oriented under the draw process. However, the f110 of the IRBFs was slightly higher than that of SBFs with the corresponding UHMWPE content. It is suggested that the smaller size of crystallite in IRBFs contributes to the orientation degree due to a uniform molecular chain distribution. In addition, the f110 of both two kinds of fibers is higher than that of f200, indicating a preferable orientation along the normal direction of the (110) plane. 2-D SAXS patterns are displayed in Fig. 7 to gain more detailed information about the orientation structures. The 2-D SAXS patterns were integrated to obtain 1D scattering profile as a function of the scattering vector,
scenario, the network structures, with few void spaces and defects in fibers, significantly affected the tensile load of the fibers, leading to a homogenous deformation through the whole network. Thus, the regular molecular chain distribution dramatically improved the effective tensile rate of the gel-spun fibers, and it is reasonable to expect that the mechanical properties of the IRBFs are better of that of SBFs under the same tensile ratio.
3.4. Crystal orientation and lamellar structure The crystal size and orientation of the blend fibers were obtained from WAXD detection. The two-dimensional (2-D) diffraction patterns are shown in Fig. 5. The two arc diffraction profiles are assigned to the (110) and (200) planes of orthorhombic crystals, as indicated with the red arrow. The relative crystallinity, XC, crystal size, Lhkl, and orientation degree, fhkl, were calculated in the Supporting Information (see Eqs. S(1)–(4)). It is clear all the fibers’ patterns mainly displayed two concentrated diffraction patterns related to the (110) and (200) planes along the fiber axis, indicating a highly orientated structure due to the draw field. As shown in Fig. 6, XC was not significantly altered between the two blends fibers and remained around 93–95%, suggesting that despite the increasing molar ratio of UHMWPE to HDPE under the low draw ratio, the overall crystallinity did not drastically change. Regarding the crystal size, as shown in Table 1, the L110 and L200 of the IRBFs increased by about 1.5 nm on average compared with the SBFs. Based on the insignificant difference in relative crystallinity between
q=
4πsinθ , λ
(2)
where q is the module of the scattering vector, 2θ is the scattering angle, and λ is the X-ray wavelength as mentioned above. The scattering vectors parallel and perpendicular to the fiber axis direction are defined as q1 and q2, respectively. It is interesting to note that there is a strong streak signal passing through beam stop in meridian direction along q2, indicating the shish crystal structure growing along the axis. At the same time, the scattering patterns also show periodic signals in equatorial direction along q1, referring to the kebab crystal structure perpendicular to the fiber axial direction, which is consistent with SEM findings (Fig. 3). Compared with SBFs, IRBFs have a relatively stronger
Fig. 4. (a) Tensile strength at two DR2 and (b) DRm of IRBFs and SBFs. 50
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Fig. 5. 2D-WAXD patterns of IRBFs and SBFs at two DR2.
the increase of molecular weight of the UHMWPE and draw ratio, the Lshish changed little, suggesting that the strong entanglement of molecular chains prevents shish from growing further. The orientation error of both two fibers decreases with the increase of molecular weight of UHMWPE and draw ratio. When it comes to the low content of UHMWPE in blends, the blend fibers tend to form loose shish structures and folded lamellae leading to relatively high macroscopic orientation. When the fiber was stretched further, however, the loose shish was straightened resulting in the decrease of orientation error. Two significant parameters, long-period, L, and lateral size of kebab, LSAXS, related to the kebab on shish crystals can also be obtained from the 2-D SAXS patterns, which are shown in Fig. S3. Fig. 9 depicts the LSAXS, L, and Δq1 of kebabs in the IRBFs and SBFs. Both the two kinds of fibers have obviously longer L and smaller LSAXS values at higher draw ratios. It is likely that the stretching flow improved the molecular orientation and the transformation of kebabs into shish. With the increase of UHMWPE content, the L and LSAXS of the fibers increased slightly. The long molecular chain of UHMWPE has a long relaxing time and moves slowly, which will attract surrounding molecules crystallizing on its surface and further promote the growth of epitaxial kebabs. However, the IRBFs possess a smaller L on average than that of SBFs and a sharp decrease in distribution of long-period, Δq1, during the drawing process, which might result from the compactly stacked lamellae. However, the small LSAXS together with crystal size calculated from WAXD also demonstrated that IRBFs are much more sensitive to the draw process compared to SBFs.
Fig. 6. Relative crystallinity of IRBFs and SBFs at two DR2. Table 1 Crystal size and orientation degree of IRBFs and SBFs. Samples
IRBF1-1 IRBF1-2 IRBF1-4 SBF1-1 SBF1-2 SBF1-4
DR2
2 3 2 3 2 3 2 3 2 3 2 3
(110) plane
(200) plane
L110 (nm)
f110
L200 (nm)
f200
14.5 14.5 15.5 14.6 15.3 15.4 16.2 16.4 16.8 16.9 16.0 16.9
0.789 0.793 0.843 0.877 0.806 0.816 0.771 0.798 0.861 0.869 0.825 0.832
14.4 14.5 15.6 14.5 15.3 15.4 16.1 16.3 16.8 16.9 15.9 16.8
0.770 0.781 0.826 0.870 0.797 0.812 0.775 0.788 0.859 0.866 0.816 0.829
3.5. Evolution mechanism of the shish-kebab structure According to the calculation results of the shish and kebab structures above, the evolution mechanism of the shish-kebab structure in HDPE/UHMWPE SBFs and RBFs can be inferred from Fig. 10. There still exists some local enrichment of UHMWPE molecules in SBs (A1, B1) due to the relatively unevenly distribution of the two polyethylene molecules. However, through in-reactor blending with Cr/V bimetallic catalysts, the molecular chains of HDPE and UHMWPE in IRBs can attain a uniform distribution state (A2), which is consistent with the rheological results in Fig. 2. In other words, the long UHMWPE chains are more likely to be liberally surrounded by short HDPE chains evenly distributed in the blends (B2). Generally, the long chains of bimodal blends play a crucial role in catalyzing the formation of shish structures under follow field [35]. The substantial presence of nearby HDPE chains were involved in the formation of shish structure together with UHMWPE chains. In the meantime, the remaining parts of HDPE molecules that absorbed on shish surface gradually evolved into compactly
meridian signals of shish and weaker equational signals of kebab. Besides, with the draw ratio increased, the IRBFs kebab signal decreased quickly. The average length of shish, Lshish, and misorientation of shish, Bφ, were calculated from the SAXS meridional streak feature, where the details were shown in Fig. S2. As shown in Fig. 8, the Lshish of the IRBFs is about 50 nm higher than that of the SBFs. During the stretching process of fibers, uniform molecular chain distribution of IRBs contributed to the formation of oriented shish under stretching filed. With 51
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Fig. 7. 2D-SAXS patterns of IRBFs and SBFs at two DR2.
Fig. 8. (a) Average length and (b) misorientation of shish of IRBFs and SBFs at two DR2.
kebabs undergo stress-induced fragmentation and recrystallization to form shish structures [10]. When the kebabs were deformed into small pieces, their molecule chains were not totally relaxed and maintained their original trans-conformation until the broken pieces were adsorbed on adjacent shish crystals [37]. For IRBFs, compact and interpenetrating kebabs narrow the distance between kababs and adjacent shish. Subsequently, more and more kebabs gradually incorporated into shish with continuous stretching, and the remaining kebabs on shish contribute to the increase in the long period and the wide long-period distribution. Hence, the network crystal structures significantly improved the transformation of shish-kebabs during the drawing process. Also, large size of shish-kebabs in SBFs were independently distributed with each other. In this way, the extensional stress imposed on fiber axis forced the kebabs to transform into shish, which in turn led to large voids and gaps between individual shish-kebabs, impairing their mechanical properties. The effective transformation of kebab into shish structures during the drawing process of fiber fabrication provides a plausible foundation for generating superb mechanical properties.
stacked kebab structures during the yarn gel spinning process. Thus, the stretching field further improves the overstocked kebabs that penetrate each other, which creates the possibility to form network-like interlocking shish-kebab crystals (C2) [36]. In addition, the hierarchical network was gradually improved with the increase of UHMWPE content, which agrees well with the SEM results in Fig. 3(a–c). Upon undergoing the stretching process, the closely packed shish-kebabs significantly increased fibrillary adhesion. Apart from the stretching stress imposed on the fiber axis, the lateral interlocking kebab could also offset some portion of the shear stress. Hence, the interlocking shishkebab made a great contribution to the homogenous deformation of the hot-stretching process, illustrating the excellent tensile performance of IRBFs, as shown in Fig. 4. In addition, the enriched UHMWPE region in SBFs resulting from their uneven distribution of molecule chains was hard to be disentangled by the drawing filed. Thus, these uncoiled parts were gradually evolved into a main structure of standalone shish-kebabs (C1). The HDPE close to UHMWPE molecule chains in SBFs involved the formation of kebab, resulting in the increase of the longperiod and lateral size of the kebabs. Also, with the increase of UHMWPE content, the standalone shish-kebabs could also transform into portions of interlocking structure during tension, as shown in Fig. 3(g). During the hot-stretching stage, it is worth noting that both the two fibers show an obvious increase in long-period and a decrease in the lateral size of the kebabs (Fig. 9). We believe that some parts of the
4. Conclusion UHMWPE/HDPE blend fibers were prepared from SBs and IRBs during a hot-stretching process. An improved uniform distribution of UHMWPE with HDPE, compared to that of solution blending, were obtained via in-reactor blending. The hierarchical interlocking shish52
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Fig. 9. (a) Long period, (b) long period distribution and (c) lateral size of kebab of IRBFs and SBFs at two DR2.
Fig. 10. Schematic of the evolution mechanism of shish-kebab structures of IRBFs and SBFs. 53
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kebab network structures were induced by drawing flow, providing a feasible approach for structure manipulation. The efficient transformation of kebab into shish structures due to the homogenous stress distribution significantly improved the mechanical performance of the HDPE/UHMWPE blend fibers in the hot-stretching process. Through the method of in-reactor blending, significant energy and solution costs could be saved during the fiber manufacturing process, as well as solving the problem of dispersion of HDPE and UHWMPE in the melt blending process. It is reasonable to believe that the in-reactor blending has great potential in the industrial fabrication of HDPE/UHMWPE blend fibers and catalyst development.
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Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 51573048). We also appreciate the great help for SAXS/WAXD data analysis form Postdoctor Xiaowei Chen in University of Science and Technology of China and Professor Jinyou Lin in Shanghai Synchrotron Radiation Facility (SSRF), Beamline BL16B1. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compscitech.2019.03.004. References [1] J.W.S. Hearle, High-performance Fiber, Wood head Publishing Limited, Cambridge, 2001. [2] N.D. Jordan, R.H. Olley, D.C. Bassetta, P.J. Hineb, I.M. Ward, The development of morphology during hot compaction of tensylon high-modulus polyethylene tapes and woven cloths, Polymer 43 (12) (2002) 3397–3404. [3] D.T. Grubb, K. Prasad, High-modulus polyethylene fiber structure: as shown by Xray diffraction, Macromolecules 25 (18) (1992) 4575–4582. [4] J. Smook, W. Hamersma, A.J. Pennings, The fracture process of ultra-high strength polyethylene fibres, J. Mater. Sci. 19 (4) (1984) 1359–1373. [5] K.E. Strawhecker, E.J. Sandoz-Rosado, T.A. Stockdale, E.D. Laird, Interior morphology of high-performance polyethylene fibers revealed by modulus mapping, Polymer 103 (2016) 224–232. [6] D.T. Grubb, A structural model for high-modulus polyethylene derived from entanglement concepts, J. Polym. Sci. Polym. Phys. Ed 21 (2) (1983) 165–188. [7] W.G. Hu, K. Schmidt-Rohr, Characterization of ultradrawn polyethylene fibers by NMR: crystallinity, domain sizes and a highly mobile second amorphous phase, Polymer 41 (8) (2000) 2979–2987. [8] Y.H. Song, A.Z. Guan, L.B. Zeng, Q. Zheng, Time-concentration superpositioning principle accounting for the reinforcement and dissipation of high-density polyethylene composites melts, Compos. Sci. Technol. 151 (2017) 104–108. [9] Y. Ohta, H. Murase, T. Hashimoto, Effects of spinning conditions on the mechanical properties of ultrahigh-molecular-weight polyethylene fibers, J. Polym. Sci., Part B: Polym. Phys. 43 (19) (2010) 2639–2652. [10] M.F. An, Y. Lv, H.J. Xu, B.J. Wang, Y.X. Wang, Q. Gu, Z.B. Wang, Effect of gel solution concentration on the structure and properties of gel-spun ultrahigh molecular weight polyethylene fibers, Ind. Eng. Chem. Res. 55 (30) (2016) 8357–8363. [11] P.G.D. Gennes, Coil-stretch transition of dilute flexible polymers under ultrahigh velocity gradients, J. Chem. Phys. 60 (12) (1974) 5030–5042. [12] H. Murase, Y. Ohta, T. Hashimoto, A new scenario of shish-kebab formation from homogeneous solutions of entangled polymers: visualization of structure evolution along the fiber spinning line, J. Polym. Sci., Part A: Polym. Chem. 44 (18) (2011) 7335–7350. [13] T. Hashimoto, H. Murase, Y. Ohta, A New Scenario of flow-induced shish-kebab formation in entangled polymer solutions, Macromolecules 43 (16) (2010) 87–96. [14] K.P. Cui, L.P. Meng, Y.X. Jin, S.S. Zhu, X.Y. Li, N. Tian, D. Liu, L.B. Li, Extensioninduced crystallization of poly (ethylene oxide) bidisperse blends: an entanglement network perspective, Macromolecules 47 (2) (2014) 677–686. [15] S. Rastogi, Y. Yao, S. Ronca, J. Bos, J. van der Eem, Unprecedented high-modulus high-strength tapes and films of ultrahigh molecular weight polyethylene via
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Hierarchical structure manipulation of UHMWPE/HDPE fibers through inreactor blending with Cr/V bimetallic catalysts
T
Xuefeng Pana, Jiaji Shia,1, Yulong Jinb, Boping Liub, Liangbin Lic, Xuelian Hea,∗ a
Shanghai Key Laboratory of Multiphase Material Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China College of Materials and Energy, South China Agricultural University, Guangzhou, 510642, China c National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, 230026, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: High density polyethylene Ultra-high molecular weight polyethylene A. Fibres In-reactor polymerization Crystallization
A dramatically improved, uniform distribution of commercial ultrahigh molecular weight polyethylene (UHMWPE) with high-density polyethylene (HDPE) via solution blending, rather than that of melt blending, is often apparent in experimental investigation. Recently, we demonstrated that a UHMWPE/HDPE blend created by in-reactor polymerization with a Cr/V bimetallic catalyst supported on modified silica can obtain an even better molecular chain distribution while saving energy and solution costs, even more so than experiments using a solution blend. Furthermore, by gel-spinning processing, an abundance of hierarchical, interlocking shishkebab crystals were generated in the in-reactor polymerized blend fibers (IRBFs), exhibiting better mechanical properties than those of solution blend fibers (SBFs). The tensile strength of the IRBFs increased by about 20% under the same draw ratio, since the interlocking shish-kebab structures contributed to homogenous deformation. The two kinds of fibers exhibit high crystallinity and have a highly oriented shish-kebab structure. Compared with the SBFs, the IRBFs inclined to form longer shish features and smaller kebab sizes, and the draw process was more conducive to the transformation of kebab to shish structures. Thus, the IRBFs can obtain an excellent hierarchal network structure, benefiting more from the hot-stretching process. Our work lays a solid foundation for the efficient fabrication of high-performance UHMWPE/HDPE fibers as well as the optimization of an original polyethylene catalyst system.
1. Introduction Tensile strength and the elastic modulus of fibers are commonly referred to as being high-performance above 17.6 cN/dtex and 440 cN/ dtex, respectively [1]. Ultra-high molecular weight polyethylene fibers (UHMWPE) are one of the typical high-performance fibers and have been widely used in space exploration, marine development, and military industry due to their excellent mechanical performance [2,3]. The high orientation and crystallinity of the molecular chain structure in the fibers determine their superb mechanical properties. However, the maxima tensile strength and elastic modulus of commercial UHMWPE fibers are still far below theoretical values due to unsatisfactorily oriented structures in industrial production [4]. The macroscopic tensile property involved in stress distribution has a strong connection with the architecture of inner structures at the microscale. Commonly, the fundamental microstructures of UHMWPE
fibers are composed of extended chain microfibers (shish), interlocking and standalone kebabs epitaxially crystallized on shish, standalone shish-kebabs, voids, or interfaces between the microfibers and tie chains across voids [5]. In the drawing process, stress is transferred both along the fiber axis and through shear between adjacent microfibers. The shish structures are the main architecture components to bear the extended stress, providing the extremely high strength and elastic modulus of UHMWPE fibers [6]. The strong interconnection of adjacent epitaxial kebabs could also enhance the cohesion between the microfibers under tension, further improving the mechanical properties. On the contrary, stress concentration usually happens due to the existence of interior void spaces and microfibril misplacement in the fiber, impairing the strength and tenacity [4,7]. Not confined to the structure and relationship of polyethylene fibers, the reinforcement and dissipation of high-density polyethylene composites is also strongly influenced by filler-polymer interactions and filler topology [8].
∗
Corresponding author. E-mail address: [email protected] (X. He). 1 Co-first author: Jiaji Shi. https://doi.org/10.1016/j.compscitech.2019.03.004 Received 29 December 2018; Received in revised form 25 February 2019; Accepted 1 March 2019 Available online 04 March 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
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combining Ziegler-Natta and metallocene or different metallocene catalysts in a single reactor were successfully developed to produce the in-reactor bimodal polyethylene [27–29]. Commonly, one catalyst was mainly used for producing high molecular weight fractions, and another catalyst was used for the low ones, providing a broad molecular weight distribution. By controlling different molar ratios of catalyst species, desirable mass fractions of UHMWPE and HDPE could be obtained. According to our previous work [30], UHMWPE could be synthesized in a wide range of polymerization temperatures by vanadiumoxide-based catalysts supported on modified silica, and HDPEs were produced by silylchromate-based catalysts at the same time. More interestingly, a series of silylchromate(Cr)/vanadium-oxide(V) bimetallic catalysts supported on aluminum-modified silica (SiO2-Al2O3) were synthesized for controllable UHMWPE/HDPE IRBs with high polymerization activity. Herein, simply choosing the appropriate ratios of Cr/V were required to obtain desirable UHMWPE fractions in the blends; thus the interlocking shish-kebab structures in the blend fibers were effectively manipulated to maintain a relatively balanced mechanical performance from UHMWPE and processability from HDPE. Solution blends (SBs) are frequently used to obtain a far better dispersion of molecular chains than that of melt blends (MBs) and are mainly applied in academic research that is limited by the massive solution waste. In view of the predominant distribution of UHMWPE in HDPE by solution blending and in-reactor blending, it is highly desirable to compare the final properties of UHMWPE/HDPE gel-spun fibers prepared by SBs with IRBs. Through the characterization of crystal morphology and structures combining with rheological behaviors, the evolution mechanism of the shish-kebab structure was proposed, and the excellent mechanical properties of IRBFs were demonstrated by efficient structure manipulation.
Furthermore, numerous works have focused on the evolution of epitaxial kebab crystals in the drawing process. Interestingly, assisted by transmission electron microscopy (TEM), Ohta [9] suggested that the folded chains in kebabs can be transformed into the extended shish structures through inter-chain slippage, which promotes longitudinal extension but induces little change in the diameter of shish structures. According atomic force microscopy (AFM) results, Kenneth [5] also demonstrated that extended stress forced the epitaxial kebabs to combine and overlap with each other, and then the stress is gradually shared by adjacent shish. Another work [10] suggested that shish-kebabs of UHMWPE fibers prepared from low-concentration solution can smoothly transform to fibrillar crystals at high stretching temperatures (above 110 °C) due to stress-induced melt recrystallization. Closely related to the evolution process of epitaxial kebabs in fibers, the formation of shish-kebab and even its precursor in the melt is also significant and worthy of further to elucidation. In the early flow stage, the coiled chains in enriched molecular regions gradually turn into bunches of stretched strands, which usually is referred to as the coilstretch transition [11]. While in the late flow stage, the oriented molecular chains in enriched regions form a stretched shish structure and epitaxial kebabs due to a strong flow field [12,13]. Furthermore, Li reported that all of the stretched long-chains have equal opportunity to form uniformly distributed nuclei in bimodal blends with a high longchain concentration. Thus, the high density of stress-induced nuclei was shown to be compactly connected with each other, which constructed a long-chain entanglement network leading to homogenous deformation [14]. In this scenario, the molecular distribution state in the melt significantly affected the evolution of the oriented crystal structure and determined what kind of shish-kebabs would come into being. Therefore, to obtain high tensile strength and elastic modulus of UHMWPE fibers that are as close to the theoretical values as possible, the construction of interlocking shish-kebabs is essential for the uniform network structure, further leading to the effective stress distribution under tension. However, despite the excellent performance of UHMWPE fibers, the UHMWPE matrix has rather poor processability due to the high melt viscosity and high production cost [15]. Thus, the advantages of a high strength and high elastic modulus cannot be utilized to their maximum extent, limiting the application of UHMWPE fibers. Conventional physical blending for the modification of matrix resin is usually adopted by mixing low molecular weight polyethylene for good processability and high molecular weight components for mechanical properties. The introduction of relatively low molecular weight polyethylene, sharing the same chemical structure, is a reasonable method to improve the UHMWPE processing performance and save production costs. Among them, blending HDPE with UHMWPE is the common modification method to improve the processing properties and maintain the materials’ original toughness, impact resistance, and corrosion resistance at the same time [16–18]. Also, assisted by some special processing techniques, oscillating shear fields and extensional flow have both been imposed on the fabrication of self-reinforced polyethylene composites [19–22]. However, micrometer-sized UHMWPE particles, which do not completely melt, would usually damage the dispersion of UHMWPE in HDPE matrix through conventional injection molding and extrusion. Also, high temperatures and severe shear field during the melt blending process could lead to molecular chain scission and oxidation [23]. Too high of HDPE content in the blend would increase the difficulty of gel spinning and weaken the final performance of the fibers. In-reactor blends (IRBs) of UHMWPE and HDPE synthesized during an ethylene polymerization process provides another novel approach to avoid the problems in physical blending processes and obtain a good control of UHMWPE/HDPE distribution in the blend at the molecular level. A small amount of UHMWPE could be incorporated into HDPE using multiple-step polymerization in a cascade reactor [24–26], which is strongly dependent on high equipment investments and the precise control of the polymerization conditions. Recently, multisite catalysts
2. Experiment 2.1. Materials UHMWPE was purchased from Shanghai Lianle Chemical Technology Co., Ltd., China, with a weight-average molecular weight of 4000 kg/mol and a density of 0.943 g/cm3. HDPE was purchased from the Daqing Petrochemical Branch of China National Petroleum Corporation, with a molecular weight of 318 kg/mol and a melt index 10 g/10 min (190 °C, 21.6 kg). 2.2. In-reactor blends preparation The UHMWPE/HDPE IRBs were synthesized according to our previous work [30], using a Cr/V bimetallic catalyst supported on SiO2Al2O3 with an ethylene pressure of 1 MPa at 80 °C. By controlling the molar ratio of Cr to V as 1:1, 1:2, and 1:4, IRBs with approximate molar ratios of HDPE to UHMWPE (1:1, 1:2, and 1:4) were obtained, respectively. 2.3. Solution blends preparation The corresponding molar ratios of HDPE to UHMWPE of SBs were also selected as 1:1, 1:2, and 1:4 (see Table S1). Desired amounts of UHMWPE and HDPE were dissolved in hot xylene with 3 wt% antioxidants (Irganox 1010) to form a homogeneous solution under a nitrogen atmosphere. After keeping the solution at 160 °C for 4 h, it was quickly precipitated into cold methanol, filtered from xylene, and washed with methanol several times. Finally, the SBs were dried at 80 °C under vacuum for several days to fully remove the remaining solvent, and then ground into powder form as thoroughly as possible. 2.4. Blend fibers preparation The detailed procedures of the gel-spun processes are shown in 47
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Fig. 1. Schematic of gel spun of process of blend fibers.
230 °C in a nitrogen atmosphere. Dynamic frequency sweep measurements were carried out with a strain of 1% in the linear viscoelastic regime at frequencies between 100 and 0.1 Hz. In preparation for the rheological detection of all of the samples, the polymer blends were stabilized by the addition of 0.25 wt% Irganox 1010 to avoid thermaloxidative degradation.
Fig. 1. First, moderate amounts of SBs or IRBs and antioxidant Irganox 1010 (0.5 wt%) and Irgafos 168 (0.5 wt%) were fully mixed with decaline under a nitrogen atmosphere. To obtain a well-dissolved gel solution, a step-by-step heating procedure was carried out by first heating to 125 °C at 1 °C/min, holding this temperature for 1 h, and then heating to 180 °C at the same speed and in the same heating time. Meanwhile, the blend solution required to be set in a counter-stirred mode while the Weissenberg-effect occurred. Subsequently, the obtained blend gel with a concentration of 20 g PE/L was cooled down to room temperature. After the dry gel was obtained, the mass of exuded decaline was deducted so that the gel spun concentration was maintained at about 7 wt%. Second, the yarn was extruded and spun through a spinning machine at 180 °C followed by being quenched in cool water. The drawn process was precisely controlled by a running roller at a speed of 5 cm/s with an initial draw ratio (DR0) of 3.6. Third, after drying at room temperature in a cooling chimney to fully remove the remaining solvent, the gel-spun fibers with an initial length of 50 mm produced above were continuously drawn at a speed of 20 mm/min in a uniaxial multiplied tensile tester (STM-H-500BP, Toyo Baldwin). Finally, in order to investigate structural evolution during the stretching process, two-stage hot drawing was carried out at 110 °C with a first draw ratio (DR1) of 4 at 135 °C and a second draw ratio (DR2) of either 2 or 3. The diameter and linear density is shown in Table S2. The fibers prepared from the IRBs, synthesized by the bimetallic catalysts with Cr/V ratios of 1:1, 1:2, and 1:4 (1Cr1V/SiO2-Al2O3, 1Cr2V/SiO2-Al2O3, and 1Cr4V/SiO2-Al2O3), were denoted as IRBF1-1, IRBF1-2, and IRBF1-4, respectively, while the fibers prepared from SBs in which the molar ratio of HDPE to UHMWPE were approximately controlled at 1:1.01, 1:2.03, and 1:4.07, were denoted as SBF1-1, SBF12, and SBF1-4, respectively.
2.6. Scanning electron microscopy observation The blend fibers were carefully etched with hot n-octane to remove the amorphous phase at 115–120 °C [10]. Then, the surfaces of all of the samples were sputtered with thin layers of gold and observed by a field-emission Scanning electron microscopy (FE-SEM, NOVA NanoSEM450, FEI, USA) operating at 3 kV. Additional samples of IRBF1-2 and SBF1-2 were chosen to retain the original surface morphology for SEM observation without etching.
2.7. Tensile properties measurement The tensile strength of the blend fibers was measured with an electronic single fiber strength tester (LLY-06E, Laizhou Electron Instrument Co., Ltd., China) at room temperature. The testing speed was 20 mm/min, and the gap between the clamps was 20 mm. Each sample was tested for 30 times to obtain the average value. Additionally, a single yarn with a length of 40 mm after the drying process was drawn at 110 °C until its failure, and the maximum hot draw ratio (DRm) of the single yarn was calculated as,
DRm = 2.5. Rheological measurement
L2 , L1
(1)
where L1 and L2 was the length of single yarn before and after stretching, respectively. Each yarn sample was repeated 20 times to obtain the average value.
Rheological properties of the polymer blends were characterized on a rotational rheometer (Hakke MARSΙΙΙ, Thermo Scientific, USA) with a parallel plate geometry of 25 mm in diameter and a gap of 0.7 mm at 48
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observation. As shown in Fig. 3(e), large quantities of folded-chain lamellae entities stacked along the stretching direction with periodic gaps observed, which were usually identified as typical shish-kebab structures, as indicated with crossed, red, dotted lines. It is clear that both IRBFs and SBFs possessed a large number of shish-kebab crystals. In the IRBF samples, there are many uniform shish-kebabs and few impurities on the surface from amorphous regions whether etched or not, as shown in Fig. 3(a–d). More strikingly, the shish-kebabs were interconnected, and the adjacent kebabs were also interpenetrated with each other, exhibiting a homogenous interlocking network structure, as indicated by a red, dotted rectangle in Fig. 3(b). With the increase of UHMWPE content, the network structure became more compact, which narrowed the space between shish-kebabs. Furthermore, this network structure could maintain a relatively stable state during the drawing process, rather than being easily destroyed under tension. Whereas with the SBF samples, clear surface defects, such as voids and crystal misalignments, were observed, indicating a loosely aligned structure, as shown in Fig. 3(e–h). Additionally, the standalone shish-kebabs, indicated by a red, dotted rectangle in Fig. 3(e), were independently arranged and showed a clear boundary, leaving large gaps between each other. The interlocking shish-kebabs could also be found in the blend fibers with high contents of UHMWPE, as shown in Fig. 3(g). These results imply that some single shish-kebabs could also connect with each other and evolve into an interlocking structure.
2.8. Small angle X-ray scattering and wide angle X-ray diffraction measurements X-ray scattering measurements were carried out using an in-house setup with a 30 W micro X-ray source (Incoatec, GmbH) in Hefei, providing a highly parallel beam (divergence about 1 mrad) of monochromatized Cu Kα radiation with a wavelength of 0.154 nm [31]. A bundle of parallel blend fibers was used for small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXD) measurements. The distance from the fibers to the detector was 3725 mm for SAXS and 235 mm for WAXD. A Mar345 CCD detector (3450 × 3450 pixels in an area of 150 × 150 μm2) and a Pilatus 300 k detector (487 × 619 pixels in an area of 172 × 172 μm2) was used to collect the SAXS and WAXD signal patterns, respectively. All of the X-ray images were corrected for background scattering by subtracting contributions from air and the sample holder, and the image acquisition time of each data frame was 30 min. The SAXS and WAXD measurement data analyses were carried out by the Fit2D software package. 3. Results and discussion 3.1. Miscibility of HDPE and UHMWPE blends Miscibility of HDPE and UHMWPE in the blends affects the formation of crystal structures and the final tensile performance of the blend fibers. As shown in Fig. 2, the rheological behaviors of SBs and IRBs were characterized. Regarding the miscibility of the blends, the ColeCole curves showed no drift or semicircles with different radii, implying that the two blends were rheologically miscible and showed no phase separation behavior. This phenomenon is also in good agreement with the sole melt peak shown in the differential scanning calorimetry (DSC) results, indicating good co-crystallization [30]. Furthermore, each Han curve of G′ vs G″ was displayed and linearly fitted nearly indicated with its slope. The slopes of the IRBs were 1.24, 1.33 and 1.62, respectively. It is worth noting that there was overlap between IRB1-1 and IRB1-4. Furthermore, the slopes of SBs were 0.81, 1.14, and 1.52, respectively, showing a relatively large variation. These results indicate that the IRBs have an even better dispersion of HDPE and UHMWPE than that of SBs due to the relatively smaller deviation of slope value and the overlap of the Han curves [32,33]. Hence, in-reactor blending method has an advantage over solution blending in the mixing effect of the two polyethylene components, and even more so when compared to melt blending.
3.3. Tensile properties The tensile performances of the two kinds of blend fibers are shown in Fig. 4. It can be clearly seen that the tensile strength of IRBFs ranged from 2.1 to 2.7 GPa, much better than that of the common fibers (below 1 GPa) and close to the commercial UHMWPE fibers (2–4 GPa), which meets the requirement of high-performance fibers [1,34]. The tensile strength of both IRBFs and SBFs was enhanced with the increase of UHMWPE content and draw ratio due to their highly oriented structures. As can be seen in Fig. 4(a), the tensile strength of the IRBFs is about 20% higher under the same draw ratio, while the DRm is slightly lower than that of the SBFs in Fig. 4(b). The relatively better tensile strength and worse drawability suggest that a considerable tensile property of blend fibers could be obtained at a low draw ratio. Normally, through the method of solution blending, HDPE and UHMWPE could attain a well-miscible state; however, the rheological results from Fig. 2 imply that the blends of HDPE and UHMWPE synthesized by inreactor blending achieved an even better dispersion than that of solution blending at the molecular scale. In that way, the uniform molecule distribution improved the formation of interlocking shish-kebab structures during the stretching process, as shown in Fig. 3. More importantly, the interpenetrated kebabs in the network structures could serve as physical junctions between the adjacent shish-kebabs. In that
3.2. Hierarchical crystalline morphology To obtain visual information of the crystal structure of blend fibers, some samples were carefully treated with hot n-octane prior to SEM
Fig. 2. (a) Cole-Cole curves and (b) Han curves and the fitted slopes of IRBs and SBs. 49
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Fig. 3. SEM micrographs of surface morphology of different blend fibers at the DR2 of 3: (a)∼(c) IRBFs after etching, (d) IRBF1-2 before etching, (e)∼(g) SBFs after etching and (h) SBF1-2 before etching. The fiber axis is vertical.
the two kinds of fibers, it is shown that, different from the large-size microcrystals of the SBFs, the IRBFs tend to form smaller size microcrystal structures, which were closely arranged and preferred to be oriented under the draw process. However, the f110 of the IRBFs was slightly higher than that of SBFs with the corresponding UHMWPE content. It is suggested that the smaller size of crystallite in IRBFs contributes to the orientation degree due to a uniform molecular chain distribution. In addition, the f110 of both two kinds of fibers is higher than that of f200, indicating a preferable orientation along the normal direction of the (110) plane. 2-D SAXS patterns are displayed in Fig. 7 to gain more detailed information about the orientation structures. The 2-D SAXS patterns were integrated to obtain 1D scattering profile as a function of the scattering vector,
scenario, the network structures, with few void spaces and defects in fibers, significantly affected the tensile load of the fibers, leading to a homogenous deformation through the whole network. Thus, the regular molecular chain distribution dramatically improved the effective tensile rate of the gel-spun fibers, and it is reasonable to expect that the mechanical properties of the IRBFs are better of that of SBFs under the same tensile ratio.
3.4. Crystal orientation and lamellar structure The crystal size and orientation of the blend fibers were obtained from WAXD detection. The two-dimensional (2-D) diffraction patterns are shown in Fig. 5. The two arc diffraction profiles are assigned to the (110) and (200) planes of orthorhombic crystals, as indicated with the red arrow. The relative crystallinity, XC, crystal size, Lhkl, and orientation degree, fhkl, were calculated in the Supporting Information (see Eqs. S(1)–(4)). It is clear all the fibers’ patterns mainly displayed two concentrated diffraction patterns related to the (110) and (200) planes along the fiber axis, indicating a highly orientated structure due to the draw field. As shown in Fig. 6, XC was not significantly altered between the two blends fibers and remained around 93–95%, suggesting that despite the increasing molar ratio of UHMWPE to HDPE under the low draw ratio, the overall crystallinity did not drastically change. Regarding the crystal size, as shown in Table 1, the L110 and L200 of the IRBFs increased by about 1.5 nm on average compared with the SBFs. Based on the insignificant difference in relative crystallinity between
q=
4πsinθ , λ
(2)
where q is the module of the scattering vector, 2θ is the scattering angle, and λ is the X-ray wavelength as mentioned above. The scattering vectors parallel and perpendicular to the fiber axis direction are defined as q1 and q2, respectively. It is interesting to note that there is a strong streak signal passing through beam stop in meridian direction along q2, indicating the shish crystal structure growing along the axis. At the same time, the scattering patterns also show periodic signals in equatorial direction along q1, referring to the kebab crystal structure perpendicular to the fiber axial direction, which is consistent with SEM findings (Fig. 3). Compared with SBFs, IRBFs have a relatively stronger
Fig. 4. (a) Tensile strength at two DR2 and (b) DRm of IRBFs and SBFs. 50
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Fig. 5. 2D-WAXD patterns of IRBFs and SBFs at two DR2.
the increase of molecular weight of the UHMWPE and draw ratio, the Lshish changed little, suggesting that the strong entanglement of molecular chains prevents shish from growing further. The orientation error of both two fibers decreases with the increase of molecular weight of UHMWPE and draw ratio. When it comes to the low content of UHMWPE in blends, the blend fibers tend to form loose shish structures and folded lamellae leading to relatively high macroscopic orientation. When the fiber was stretched further, however, the loose shish was straightened resulting in the decrease of orientation error. Two significant parameters, long-period, L, and lateral size of kebab, LSAXS, related to the kebab on shish crystals can also be obtained from the 2-D SAXS patterns, which are shown in Fig. S3. Fig. 9 depicts the LSAXS, L, and Δq1 of kebabs in the IRBFs and SBFs. Both the two kinds of fibers have obviously longer L and smaller LSAXS values at higher draw ratios. It is likely that the stretching flow improved the molecular orientation and the transformation of kebabs into shish. With the increase of UHMWPE content, the L and LSAXS of the fibers increased slightly. The long molecular chain of UHMWPE has a long relaxing time and moves slowly, which will attract surrounding molecules crystallizing on its surface and further promote the growth of epitaxial kebabs. However, the IRBFs possess a smaller L on average than that of SBFs and a sharp decrease in distribution of long-period, Δq1, during the drawing process, which might result from the compactly stacked lamellae. However, the small LSAXS together with crystal size calculated from WAXD also demonstrated that IRBFs are much more sensitive to the draw process compared to SBFs.
Fig. 6. Relative crystallinity of IRBFs and SBFs at two DR2. Table 1 Crystal size and orientation degree of IRBFs and SBFs. Samples
IRBF1-1 IRBF1-2 IRBF1-4 SBF1-1 SBF1-2 SBF1-4
DR2
2 3 2 3 2 3 2 3 2 3 2 3
(110) plane
(200) plane
L110 (nm)
f110
L200 (nm)
f200
14.5 14.5 15.5 14.6 15.3 15.4 16.2 16.4 16.8 16.9 16.0 16.9
0.789 0.793 0.843 0.877 0.806 0.816 0.771 0.798 0.861 0.869 0.825 0.832
14.4 14.5 15.6 14.5 15.3 15.4 16.1 16.3 16.8 16.9 15.9 16.8
0.770 0.781 0.826 0.870 0.797 0.812 0.775 0.788 0.859 0.866 0.816 0.829
3.5. Evolution mechanism of the shish-kebab structure According to the calculation results of the shish and kebab structures above, the evolution mechanism of the shish-kebab structure in HDPE/UHMWPE SBFs and RBFs can be inferred from Fig. 10. There still exists some local enrichment of UHMWPE molecules in SBs (A1, B1) due to the relatively unevenly distribution of the two polyethylene molecules. However, through in-reactor blending with Cr/V bimetallic catalysts, the molecular chains of HDPE and UHMWPE in IRBs can attain a uniform distribution state (A2), which is consistent with the rheological results in Fig. 2. In other words, the long UHMWPE chains are more likely to be liberally surrounded by short HDPE chains evenly distributed in the blends (B2). Generally, the long chains of bimodal blends play a crucial role in catalyzing the formation of shish structures under follow field [35]. The substantial presence of nearby HDPE chains were involved in the formation of shish structure together with UHMWPE chains. In the meantime, the remaining parts of HDPE molecules that absorbed on shish surface gradually evolved into compactly
meridian signals of shish and weaker equational signals of kebab. Besides, with the draw ratio increased, the IRBFs kebab signal decreased quickly. The average length of shish, Lshish, and misorientation of shish, Bφ, were calculated from the SAXS meridional streak feature, where the details were shown in Fig. S2. As shown in Fig. 8, the Lshish of the IRBFs is about 50 nm higher than that of the SBFs. During the stretching process of fibers, uniform molecular chain distribution of IRBs contributed to the formation of oriented shish under stretching filed. With 51
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Fig. 7. 2D-SAXS patterns of IRBFs and SBFs at two DR2.
Fig. 8. (a) Average length and (b) misorientation of shish of IRBFs and SBFs at two DR2.
kebabs undergo stress-induced fragmentation and recrystallization to form shish structures [10]. When the kebabs were deformed into small pieces, their molecule chains were not totally relaxed and maintained their original trans-conformation until the broken pieces were adsorbed on adjacent shish crystals [37]. For IRBFs, compact and interpenetrating kebabs narrow the distance between kababs and adjacent shish. Subsequently, more and more kebabs gradually incorporated into shish with continuous stretching, and the remaining kebabs on shish contribute to the increase in the long period and the wide long-period distribution. Hence, the network crystal structures significantly improved the transformation of shish-kebabs during the drawing process. Also, large size of shish-kebabs in SBFs were independently distributed with each other. In this way, the extensional stress imposed on fiber axis forced the kebabs to transform into shish, which in turn led to large voids and gaps between individual shish-kebabs, impairing their mechanical properties. The effective transformation of kebab into shish structures during the drawing process of fiber fabrication provides a plausible foundation for generating superb mechanical properties.
stacked kebab structures during the yarn gel spinning process. Thus, the stretching field further improves the overstocked kebabs that penetrate each other, which creates the possibility to form network-like interlocking shish-kebab crystals (C2) [36]. In addition, the hierarchical network was gradually improved with the increase of UHMWPE content, which agrees well with the SEM results in Fig. 3(a–c). Upon undergoing the stretching process, the closely packed shish-kebabs significantly increased fibrillary adhesion. Apart from the stretching stress imposed on the fiber axis, the lateral interlocking kebab could also offset some portion of the shear stress. Hence, the interlocking shishkebab made a great contribution to the homogenous deformation of the hot-stretching process, illustrating the excellent tensile performance of IRBFs, as shown in Fig. 4. In addition, the enriched UHMWPE region in SBFs resulting from their uneven distribution of molecule chains was hard to be disentangled by the drawing filed. Thus, these uncoiled parts were gradually evolved into a main structure of standalone shish-kebabs (C1). The HDPE close to UHMWPE molecule chains in SBFs involved the formation of kebab, resulting in the increase of the longperiod and lateral size of the kebabs. Also, with the increase of UHMWPE content, the standalone shish-kebabs could also transform into portions of interlocking structure during tension, as shown in Fig. 3(g). During the hot-stretching stage, it is worth noting that both the two fibers show an obvious increase in long-period and a decrease in the lateral size of the kebabs (Fig. 9). We believe that some parts of the
4. Conclusion UHMWPE/HDPE blend fibers were prepared from SBs and IRBs during a hot-stretching process. An improved uniform distribution of UHMWPE with HDPE, compared to that of solution blending, were obtained via in-reactor blending. The hierarchical interlocking shish52
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Fig. 9. (a) Long period, (b) long period distribution and (c) lateral size of kebab of IRBFs and SBFs at two DR2.
Fig. 10. Schematic of the evolution mechanism of shish-kebab structures of IRBFs and SBFs. 53
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kebab network structures were induced by drawing flow, providing a feasible approach for structure manipulation. The efficient transformation of kebab into shish structures due to the homogenous stress distribution significantly improved the mechanical performance of the HDPE/UHMWPE blend fibers in the hot-stretching process. Through the method of in-reactor blending, significant energy and solution costs could be saved during the fiber manufacturing process, as well as solving the problem of dispersion of HDPE and UHWMPE in the melt blending process. It is reasonable to believe that the in-reactor blending has great potential in the industrial fabrication of HDPE/UHMWPE blend fibers and catalyst development.
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