Role of Controlled Diameter of Polyamide 6 (PA6) Fibers on the

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Materials and Interfaces

Role of controlled diameter of PA6 fibers on formation of interfacial hybrid crystal morphology in HDPE/PA6 microfibril blend Lei Liu, Yan-Hao Huang, Dan-Dan Xie, Li-Bo Chen, Rui Chen, Zhengying Liu, Wei Yang, and Ming-Bo Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01045 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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Industrial & Engineering Chemistry Research

Role of controlled diameter of PA6 fibers on formation of interfacial hybrid crystal morphology in HDPE/PA6 microfibril blend Lei Liu, Yan-Hao Huang, Dan-Dan Xie, Li-Bo Chen, Rui Chen, Zheng-Ying Liu, Wei Yang, Ming-Bo Yang* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, China. Corresponding Author Tel.: +86-28-85401988; Fax: +86-28-85401988 Email: [email protected]

ABSTRACT:

In this work, PA6 (polyamide 6) fibers with various diameters distribution (0.18 m -5.6 m) were initially prepared by using different hot drawing rates during the in-situ fiber forming process. The results showed that the PA6 fibers with smaller diameters well distributed in the HDPE matrix as the stretching rate increased. Hybrid shish-kebab (HSK) structures could be observed in a large scale when the diameters of fibril PA6 fillers were from 0.18 to 2.2 m, and more oriented structures were formed and retained with the decreasing of microfiber diameter.

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Interestingly, the surface of PA6 microfibers with a larger diameter (about 2.2 m), can even induce hybrid structures without obeying the classical size-dependent soft epitaxy (SSE) mechanism absolutely. This work provides a new insight to tailor the crystalline morphology of the polymer during melt processing.

1.Introduction For semicrystalline polymers, one of the important effect factors determining the properties of them is the morphology tailoring, i.e., to transform the crystals form from an isotropic, regular spherulitic structure to a highly oriented crystal structure under flow field. For instance, it has been reported that the shish-kebab structure, can lead to increased rigidity, toughness and modulus of products.1-2 However, obtaining large-area oriented structures in practical polymer processing techniques is not an easy task because of the inadequate shear flow and limited cooling rate.3-4 Aiming at structuring a full-scale orientation crystalline morphology, hybrid shish-kebab structures induced by various rigid fibers have been explored.5-11 Our team also conducted much work in the control of the crystalline structures in fiber reinforced polyolefin blends.3-4, 12-13 The classic shish-kebab crystal structure was firstly observed in polyethylene (PE) solution in 1960s by Pennings et al. and Keller et al.5, 14 In recent years, the hybrid shish-kebab (HSK) crystal structure has attracted considerable interest because of its geometry similarity with the classic shish-kebab superstructure and the improved mechanical properties of articles compared with their counterpart with classic shish-kebab structures. In 1990, Thierry et al. first discovered hybrid shish-kebab structure in PE solution induced by dibenzylidene sorbitol (DBS) fibers, in which the polyethylene folded chain lamellae were epitaxially located on the surface of DBS fibers.15 Recently, Li et al. observed the nano hybrid


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shish-kebab (NHSK) structure in PE and Nylon66 solution by introducing carbon nanotubes (CNT) and carbon nanofibers (CNF). They found that PE single-crystal lamellae periodically grew on the CNT to form the NHSK structure.16-17 This NHSK superstructure can be also obtained by solvent evaporating, thin film crystallizing and supercritical carbon dioxide-induced epitaxial growth method in PE/CNTs system.18-20 Interestingly, a kind of new hierarchical shishkebab structures in multiwall carbon nanotubes (MWCNT)/poly(-caprolactone) (PCL) system have been created via the introduction of double levels of shish-kebab structures, in which the first NHSK of multiwall MWCNT/PCL was created by electrospinning and the second nanofiber shish-kebab structures were induced in a supersaturated PCL solution.21 This fascinating hierarchical crystal structure also was shown in the work of Xia et al. through the introduction of PE-g-CNF in the lightly cross-linked HDPE melt under intense shear.22 Although the HSK structure was first observed in 1990, the mechanism of HSK formation is still under heated discussion in recent years.16, 23-31 Recently, Li et al. 2, 16-17 brought up the “sizedependent soft epitaxy” (SSE) mechanism, where the growth mechanism of HSK was “soft epitaxy” instead of the lattice matching. For CNTs with small diameters, their sizes were similar to the gyro radius of PE so that the kebab could periodically align on the CNT surface and was perpendicular to the CNT axis while strict lattice matching was not available. If the dimension of fiber was large enough to make a flat surface for molecular chains, the crystalline growth would obey lattice matching and the epitaxy growth would simultaneously occur. It can be seen that the fiber diameter is extremely important for the formation of HSK structure in SSE theory. Besides, Ning et al.

32

studied the interfacial crystallization of polyethylene/whiskers composites and

found that SiO2MgOCaO whisker (SMCW) and borate whisker (BW) with different diameters showed different effects on the nucleation. The heterogeneous nucleation ability of small


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diameter whisker (SMCW) was obviously stronger than that of large diameter whisker (BW) under oscillatory shear. Xu et al. 33reported large diameter ramie fiber can induce PLA to form a transcrystalline in ramie fiber/PLA composite, while the nano-sized ramie fiber tended to absorb and stabilize the row-nuclei, and then the hybrid shish-kebab structure grew on the surface of the fiber. Similar work has been reported by Xia et al.3 In our previous works, the HSK structure can be observed in in-situ microfibril reinforced semicrystalline polymer composites under secondary melt flow. The HSK structure is comprised of shish (microfiber i.e., nylon(PA), polycarbonate (PC)) and kebabs (semicrystalline polymer, i.e., PE,iPP) and can be utilized to reinforce the polymer matrix.3-4, 12, 34-35 In the preparation of in-situ microfibril blend, the microfibril precursor firstly should disperse well in the matrix under the strong shear field, and then transform from droplets to fibers via the “extrusion-hot stretchquenching” technique. The morphology of the microfibrils in the blend can be tailored by controlling the viscosity ratio, stretch ratio, and composition to meet various requirements. Carbon fibers of different diameters have been used to examine the SSE mechanism of the growth of the HSK superstructures. However, carbon fibers with the diameter from a few hundred nanometers to 7 μm are currently unavailable, and other kinds of fibers also fail to meet the size requirements from nano to micron scales. To solve this problem, in this paper, PA6 fibers with various diameters were prepared by varying the rate of hot drawing in the in-situ fiber formation process in PA6/HDPE blends. The thermodynamic properties, rheological properties, orientation of HDPE phase were investigated under the same tensile flow field. The effect of fiber size on the formation of hybrid crystallites had been found out according to SSE theory and the formation mechanism of unique HSK superstructure in HDPE/PA6 microfibrillar blend was discussed.


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2.Experiment 2.1 Materials A commercial HDPE with a trade name of 6098, was supplied by Qilu Petrochemical Co., China. Its weight average molecular weight (Mw) is about 5.09 x 105 g/mol and its molecular weight distribution index is 2.1. It has a density of 0.96 g/cm3 and a melting point of 132 oC. The melt flow rate (MFR) of HDPE is 0.1 g/10 min (196 oC, 2.16 kg, ASTM D1238). PA6 (Grade M2800) with a melt flow rate of 11 g/10 min (190 oC, 2.16 kg, ASTM D1238) was produced by Guangdong Xinhui MeiDa Nylon Co., China. It has a melting temperature of 220 oC and a density of 1.5 g/cm3. 2.2 Sample Preparation PA6 granules were fully dried at 80 oC over 12 h and then were blended with HDPE granules. The blend mixing was conducted on a twin-screw extruder at a screw speed of 180 rpm, and the volume ratio of PE/PA was 85/15. The temperature distribution from the barrel to the die is set as 140, 180, 230, 250, 250 and 245 oC. From ordinary blends to HDPE/PA6 microfiber blends, the morphological change of the PA6 dispersed phase is realized by an “extrusion-hot stretchquenching” process in a single-screw extruder with a slit die possessing a dimension of 1.0 mm × 100 mm (thickness × width).35 The extrusion process has a temperature profile of 180, 230, 250, and 250 oC from hopper to die. The hot stretching and cooling processing were simultaneously applied to the extrudate through a take-up equipment with three pinching rolls and an air knife separately to prepare HDPE/PA6 microfibril blends. The HSR (the ratio of the area of the transverse section of the die to the area of the transverse section of the extrudate) was adjusted to control microfibrils into different diameters. After drying, each sample experienced a “secondary


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hot stretching” under the same external field conditions on an Instron universal tensile testing machine. The process is implemented as follow: the sample was initially heated to 135 oC, and kept at 135 oC for 10 minutes to preheat for uniform stretching, and then it was stretched at a tensile rate of 60 mm/min until the strain of the sample along the "extrusion - hot stretching" direction reached 150%, followed by the procedure of cooling to room temperature (20 oC) at a rate of 25 oC/min. In the abbreviation for A1, ‘A’ represents the microfibrillar blends of HDPE/PA6, and ‘1’ corresponds its HSR of 1.18, the others, A2, A3, A4, A5 have their HSR of 3.23, 5.58, 8.40, 9.65, respectively. 2.3 Scanning Electron Microscopy (SEM) observations In order to better observe the morphology, and to estimate the size of the fibers in the HDPE/PA6 microfibril blends, the samples were brittly fractured at the central zone along the machine direction in liquid nitrogen. Then three etching reagents were used for the observation of different targets: formic acid for better dimensional statistics, mixed acid (chromium trioxide and concentrated sulfuric acid) and xylene for the observation of crystal morphology, respectively. After coating with a thin layer of gold, the fracture surface in different direction was analyzed by a SEM (JSM-5900LV, JEOL, Japan) operating at a voltage of 20 kV. 2.4 Differential scanning calorimetry (DSC) The nucleation effect of PA6 microfiber on the crystallization of HDPE was examined using a differential scanning calorimetry (DSC, TA-Q20). The specimen with a weight of 5-8 mg was heated to 200 oC for 3 minutes at 50 oC/min and cool to 40 oC at a cooling rate of 10 oC/min under a nitrogen atmosphere, for the analysis of non-isothermal crystallization behavior of HDPE. A second heating process was conducted at 10 oC /min. The whole process was


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conducted under a nitrogen atmosphere. The crystallization onset temperature is got by “onset point” function in Universal Analysis 2000 software. 2.5 Rheological Experiments Rheological test was carried out using a stress-controlled dynamic rheometer AR2000-EX (TA, USA) with a 25 mm-diameter parallel plate clamp. The measuring temperature was 200 °C and nitrogen gas was utilized to retard the degradation of samples. The frequency sweep test was conducted from 0.01 to 100 Hz at a strain of 1.0% to ensure that the test was performed in a linear viscoelastic region. 2.6 Two-Dimensional Small-angle X-ray Scattering (2D-SAXS) Characterization The effect of PA6 fibers of different diameters on the crystalline morphologies and the orientation of HDPE was studied at room temperature using Rigaku Denki RAD-B diffraction meter in the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The wavelength of the monochromatic X-ray is 0.124 nm, and the distance between the sample and detector was 2150 mm. The flow direction of placed sample is perpendicular to the X-ray beam along vertical direction. The orientation analysis of HDPE is determined according to the Herman’s orientation formula, defined as follows: f=

3cos2φ ― 1 2

(1)

in which (cos 2) is the orientation factor defined as 2

𝜋 2

2

𝜋 2

𝑐𝑜𝑠 φ = ∫0𝐼(𝜑)𝑐𝑜𝑠 φsin 𝜑𝑑𝜑/∫0𝐼(𝜑)sin 𝜑𝑑𝜑 (2) Where  is the azimuth angle, and I(φ) is the scattering intensity corresponding to the azimuth angle . The f was analyzed mathematically using Picken’s method.


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From the measurements of 2D SAXS, we can get structural information of HDPE/PA6 microfibril blends by calculating the electron density correlation function K(z). The onedimensional electron density correlation function analysis was used to know K(z) by the following equation: ∞

K(𝑧) =

∫0 𝐼(𝑞)𝑞2𝑐𝑜𝑠(𝑞𝑧)𝑑𝑞

(3)

∞

∫0 𝐼(𝑞)𝑞2𝑑𝑞

Where Z indicates the location along a direction normal to the lamellar surface, multiplication of I (q) and q2 (Lorentz correction) was conducted due to the isotope distribution stacking of parallel layered crystallites in the sample. 3. Results and discussion 3.1 Different diameter fibers in HDPE/PA6 microfibrillar blends By changing the hot stretching rate during the “extrusion-hot stretch” process, HDPE/PA6 microfibrillar blend samples with different microfiber sizes were prepared. As shown in Figure 1a, it is obvious that the PA6 microfibers are arranged parallel to the hot stretching direction, and PA6 fibers were formed at various hot stretching rates. To assess the diameter distribution of microfiber, the samples were etched by formic acid along the direction of the PA6 fiber cross section. Figure 1b shows the average diameter and diameter distribution of the microfiber in the composites which was fitted by GaussAmp. The formula of Gauss Amp is shown as follows y = 𝑦0 + 𝐴𝑒

―

(𝑥 ― 𝑥𝑐)2 2𝑤2

(4)


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Figure 1 (a) Phase morphologies of HDPE/PA6 microfibril blends with different hot stretching rates, and (b) diameter distributions of PA6 microfibrils fitted by GaussAmp. The flow direction is horizontal.

As shown in Table 1, the average diameter of PA6 microfiber is about 1.85 μm for A1, and 1.42 μm for A2. For the sample with the largest HSR, A5, the average diameter is 0.83 μm, which is the submicron size. From the fitted results, the peak center position of the fitted curve (Xc) is also gradually decreasing from 1.75 μm to 0.75 μm with the increasing HSR. Moreover, HDPE/PA6 microfibril blends with different HSR possess various diameter distribution. For the larger HSR system, the sample A4 and A5 have a narrower distribution with a small FWHM, while for A1, A2, A3, they show a wide diameter distribution with a large FWHM. The decreasing standard deviation from 0.68 (A1) to 0.35 (A5) can also indicate such a result. As the HSR increases, the diameter of PA6 microfiber gradually decreases and the diameter distribution becomes more uniform. This result may be caused by two factors: firstly, the viscosity ratio becomes smaller when the HSR increases, which is beneficial for the formation of microfiber.3637

Secondly, when the dispersed phase concentration is higher than the critical concentration of

dispersed phase for coalescence (in PA6/HDPE blends, it is >5 vol %), coalescing and dropletdroplet interplay also begins to work on overall deformation and final fiber formation, and the


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change is associated with the dispersed phase deformation from the single particle deformation to particle-particle coalescence.38 So, the dispersed phase concentration in this paper (15 vol %) is good for particle-particle coalescence to form PA6 fibers of various diameters with different HSR. Table 1 The statistical results of PA6 fiber diameter fitted by GaussAmp sample

dav(μm)



Diameter

GaussAmp fit to d

distribution(m) Xc

FWHM

A1

1.85

0.68

0.54-5.60

1.75

1.14

A2

1.42

0.60

1.26-4.14

1.25

1.52

A3

1.26

0.57

0.36-4.33

1.09

1.28

A4

0.95

0.36

0.36-2.69

0.90

0.48

A5

0.83

0.35

0.18-2.16

0.75

0.73

 represents the standard deviation of the average fiber diameter, Xc is the peak center position of the fitted curve, FWHM is the full width of the fitted curve at half maxima, and the number of counted fibers is more than 500. 3.2 Nucleation Effect of PA6 Microfibrillar on HDPE with Various Diameters For the semicrystalline polymer/filler composites, the PA6 microfiber was carried out as nucleating agent to enhance the onset crystallization temperature and the crystallization rate.35 By analyzing the crystallization curves of pure HDPE and HDPE/PA6 microfiber blends, the effects of different diameters of PA6 microfibers on the crystallization behavior of HDPE were explored. As shown in Figure 2a, compared with pure HDPE, the onset crystallization temperature (To) of HDPE/PA6 microfiber blends was increased from 117.8 oC to a higher temperature (118.8-119.8 oC). However, the To of the samples from A1 to A5 showed small difference, indicating that the PA6 microfibers with various diameters had a similar heterogeneous nucleation ability. In addition, in Figure 2b, the t1/2 (the time for completing 50%


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of the crystallization process) of each sample of HDPE/PA6 microfiber blend is significantly shorter than the pure HDPE. It is consistent with the results of the increasing To of the microfiber blends. Similarly, the fibers with various diameters had little effect on the crystallization rate. These results indicate that the heterogeneous ability of the fibers with different diameters has no significant difference. It may be caused that the PA6 microfibers with several micrometers do not lose the ability to heterogeneously nucleate for HDPE matrix. In addition, the difference in fiber size is not as obvious as the carbon fiber system.39

Figure 2 (a) DSC cooling curves of pure HDPE and HDPE/PA6 microfibril blends. (b) The relative crystallinity (Xt) versus time (t) for pure HDPE and HDPE/PA6 microfibril blends.

3.3 Rheological behavior of HDPE/PA6 Microfibrillar Blends As shown in Figure 3a, each sample shows a distinct shear thinning phenomenon with the increasing shear frequency. And the complex viscosity (*) of HDPE/PA6 microfiber blends is significantly higher than that of pure HDPE under test frequency range (0.01-100 Hz). This is because the rheological test temperature is lower than the melting temperature of PA6, and the solid-like PA6 microfiber can maintain its microfibril morphology better. The introduction of PA6 microfibers will effectively hinder the mobile ability of PE molecular chain, leading to a


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significant increase in the melt viscosity of the blends.3, 35 With the decrease of the diameter of PA6 fibers, the PA6 microfibers have stronger restrictions on the movement of PE molecule chains, and thus the melt viscosity of blends increases.

Figure 3 Rheological and relaxation behaviors, (a) dependence of complex viscosity under different frequency at 200 oC; (b) H*τ versus τ curves for characterization time. Furthermore, Figure 3b showed the plot of H*t versus t, which presented the continuous relaxation spectra of PE and blends using nonlinear regularization program developed by Weese and Honerkamp.40-41 From the maximum peak value of τ shown in Figure 3b, the approximating relaxation behavior of the sample can be clarified. Compared with pure HDPE, HDPE/PA6 microfiber blends show longer characteristic relaxation time. Moreover, the relaxation time increased noticeably with the decreasing diameter of PA6 microfiber. This is due to the strong interaction between PA6 microfibers and PE molecular chain will limit the movement of HDPE chains. The effect of relaxation resistance is involved with diameter variation of the microfibrils introduced in system, that is, the “slimmer” of the micro-fibril is, the longer relaxation time of the attached orientation PE molecular chains will have, owing to the straighten maintenance of the micro-fibril surface. As a result, highly oriented crystalline structure can be formed and maintained well on the surface of the microfibrils.


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3.4 Hybrid Shish-Kebab Structures in the composites under the Secondary Hot Stretching Recently, the PA6 microfibrils-reinforced HDPE composites have been studied by SEM and non-isothermal crystallization techniques.35 It has been found that t1/2 decreases after adding PA6 microfibers in composites, indicating that PA6 microfibers could induce polymer crystallization. Simultaneously, the clear SEM pictures of typical HSK structure were demonstrated that PA6 microfibers can act as the “shish” to induce the growth of PE kebab. Figure 4 shows the crystallization structures of the HDPE/PA6 microfiber blends subjected to secondary hot stretching. As is shown in Figure 4f, fibrous cavities are considered to be the PA6 microfibers, because PA can be easily etched by mixed acid while PE cannot. Figure 4a clearly shows that PE lamellae still grow on the surface of large diameter PA6 microfiber. It is significantly different from the case where PE lamellae cannot be periodically decorated on the surface of micron-sized carbon fibers.17 Moreover, the PE lamellae in sample A1 is perpendicular or oblique to the PA6 axis. On the other hand, the SEM images of PE/PA6 microfiber blend from A1 to A5 show that more PE kebabs are almost perpendicular to the surface of PA6 fibers. In Figure 4e, the kebabs of PE have an orthogonal orientation to form uniform and perfect crystals.16-17 Interestingly, both shish-kebab (yellow arrows in Figure 4e) and hybrid shish-kebab structures are observed from our results for A5 sample. This observation indicates that small PA6 fibers also are conducive to the formation of classical shish-kebab structures. The subsequent analysis of 2D-SAXS can also prove the formation of hybrid structures.


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Figure 4 Crystalline morphologies of second hot-stretching HDPE/PA6 microfibril blends: (a)-(e) are samples from A1 to A5, respectively. Schematic representations of (f) PE/PA6 HSK crystals. The flow direction is horizontal.

Figure 5 Crystalline morphologies of (a), (b) second hot-stretching HDPE/PA6 microfibril blend etched by xylene solution. (c) Higher magnification image of (b). The flow direction is indicated by the arrow.

Combining the results of the previous DSC and rheological tests, one can make a conclusion that the PA6 fibers have a better nucleation effect than pure PE. Thence, the more regular PE crystal lamellae is periodically decorated on the surface of PA6 fibers to form the HSK structures. This is because that the PE molecular chains and the PA6 microfibers are aligned along the flow direction under the flow field, the adjacent two PA6 microfibers resemble solid walls in the PE melt, so that the flow field is redistributed and further amplified between these “walls” .42-43 For PA6 microfiber samples with larger diameter, the growth of PE lamellae


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possess multiple orientations, and less oriented or stretched molecular chains are retained in the cooling crystallization stage because of the short relaxation time of PE molecular chains. Based on the results above, it can be found that hybrid shish-kebab crystals can be induced by PA6 fibers from nano-scale to micro-scale, which is different in the PE/CNTs system. In order to clearly investigate the effect of PA6 size on inducing PE interfacial crystallization, A1 sample with larger average diameter (1.85 m) and large distribution (0.54-5.60 m) was further studied as presented in Figure 4a. We thus conducted etching experiment of PE amorphous chains to get the integral PA6 microfibers by xylene. In this case, when the PA6 fiber diameter is about 2.2 m, PE lamellae still attaches to the surface of the PA6 microfibers regularly, which is perpendicular to the PA6 microfiber axis, as shown in Figure 5a In the early stage of PE crystallization of Reduced Graphene Oxide (RGO)/PE system,44 discrete nuclei and small rodlike crystals result that PE crystals are preferentially growing at multiple directions. However, small rod-like PE nuclei are uniformly distributed on the surface of PA6 microfibers, and then grow into larger lamellae. When the diameter of microfiber is as large as 2.2 μm, which is much larger than the radius of gyration of PE molecular chain (about 13.8 nm assumed at the  condition ),45 a large amount of polyethylene lamellae is still decorated on the surface of the PA6 microfiber. This indicates that the large diameter PA6 fibers still locally have regular nucleation sites to induce the growth of PE lamellae. This is also the reason for the little difference in heterogeneous nucleation ability of fiber systems with different diameters, as shown in Figure 2. 3.5 The 2D-SAXS Analysis of HDPE/PA6 Microfibrillar Blends with different Diameter PA6 Microfibers The oriented crystal structure of the secondary hot drawn HDPE/PA6 microfiber blends were investigated by 2D-SAXS, as shown in Figure 6. The appearance of a two-blade and triangular


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scattering signal manifests the formation of a highly oriented hybrid shish-kebab structure in the sample.46-48 In Figure 6, the scattering signal of the A1 to A5 sample gradually changes from a uniform distributed ring pattern to a double-blade shape pattern, and the signal is concentrated in the meridian direction indicating the formation of an oriented platelet structure, which is perpendicular to the flow direction. This proves that the orientation of the PE lamella grown on the surface of PA6 fibers is more stable instead of having multiple orientations as the diameter of fiber decreases. Simultaneously, the triangular scattering signal appearing in the equatorial direction represents that the orientation structures arranged in the flow direction are formed. The scattering signal gradually changes from a circular distribution to a double-leaf shape, indicating that the orientation of the crystalline structure gradually increases as the diameter of the PA6 microfiber decreases.

Figure. 6 Representative 2D-SAXS patterns of second hot-stretching HDPE/PA6 microfibril blends. a-e are A1 to A5 samples, respectively. To quantitatively describe the effect of fibers of different diameters on the orientation structure of PE, one-dimensional integration was used to integrate the scattering along the azimuthal angle in Figure 7a. In the HDPE/PA6 microfibre samples, A3 to A5 shows high degree of orientation (all reach 0.7 or more), and the degree of orientation decrease from A3 to A1, as shown in the Figure 7b). This corresponding to the previous rheological results: small fibers increase the relaxation time of the polymer chains, which helps to grow regular PE lamellae and retain more oriented molecular chains during cooling and crystallization, forming more highly oriented crystal structures. As the diameter increases, the effect of the fibers on the formation and


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Figure 7(a) Evolution of azimuthal intensity and (b) orientation parameters of second hot-stretching HDPE/PA6 microfibril blends.

retention of the orientation structure of the molecular chain becomes weaker, and the growth direction of the PE lamellae is more likely to be distorted. Figure 8a shows the relationship between the one-dimensional scattering intensity along the meridional direction and the vector, and the long-period (L), amorphous and lamellar thicknesses of the five samples are calculated by the correlation function method,49-50 as shown in Figure 8c. The thickness of the amorphous layers (da), lamella (dc) and L of PE lamellar structure are 6.7 nm,13.4 nm, 20.1 nm respectively, which indicates the crystal size of PE is not unaffected by the diameter of the PA6 microfiber. The crystal size does not change significantly as the diameter decreases, and it is consistent with the results of the similar nucleation effect of the different diameter fiber we mentioned earlier. The crystallization of the PE on the surface of the PA6

Figure 8 (a) 1D-SAXS curves of five samples; (b) The resultant correlation function curves via inverse Fourier transformation, (c)the result obtained from (b) of long spacing dac and the lamellar thickness dc, (dc =dac- da).


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fibers is not only related to the size of the fiber, but also affected by factors such as the physical and chemical properties of the fiber surface, the type of the external field, and the cooling rate, etc.17, 26, 44 As is shown in Figure 9, two distinct melting peaks can be observed on the heating curves in samples A1. A2. A3. A4, yet A5 only shows a single melting peak. The peak with the lower melting temperature (Tm) gradually shifts to the peak with higher Tm as the PA6 fiber diameter decreases, which is regarded as the result of formation and retention of more oriented structures for small diameter PA6 fiber. PE crystals with more perfect crystalline structures are responsible for the higher intensity of the melting peak, shown in Figure 9.

Figure. 9 DSC molten curves of second hotstretching HDPE/PC microfibril blends. 3.6 Size Dependence of Hybrid Shish-Kebab Structure on PA6 Microfiber In the formation mechanism of HSK, many investigators are agreed on the theory of Li, who proposed that CNTs with small diameter can induce polyethylene to form HSK crystallites by soft epitaxy due to “size-dependent soft epitaxy” mechanism on small surface curvature.16-17 For nanoscale fibers, the SSE mechanism is often used to explain the growth of hybrid shish-kebab structures, but for our HDPE/PA6 systems, it is clearly not sufficient to explain the formation of


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hybrid structures like this as shown in Figure 5. Here, the effect of fiber diameter on the interfacial crystal morphology between HDPE and PA6 microfibrillar blends should be explained more clearly. Under the dynamic external field, both the polyethylene molecular chain and the PA6 microfiber are first stretched or oriented along the flow direction. Subsequently, the aligned molecular chains in the vicinity of the PA6 microfiber are adsorbed to form PE coating (hybrid “shish”) on the surface of fiber, and form crystal nucleus, proved by previous work.2 The surface of PA6 fibers with small-diameter has more nucleation sites for a strong nucleating ability, which is favorable for inducing the formation of coating and then the growth of the lamellae. The enlargement of the fiber diameter results in a decrease in the nucleation sites of the surface, and there is no longer a coating formation, but a local growth of the lamellae directly, which is mainly due to the reduction of fiber surface energy.33,

51

Further, as the PA6 fiber

diameter increases, the direction of lamella growth begins to change from a single direction, which is perpendicular to the fiber surface, to multi-direction. This is because the van der Waals interaction between PA6 microfibers and PE decreases with microfiber diameter increasing, the interaction with the polyethylene molecular chain is insufficient to drive PA6 microfiber adsorption to form a complete polyethylene coating as the microfiber diameter is to a critical value (about 2.2 m). Only the strongly interacting local surface can adsorb a small amount of oriented molecular chains to form a local polyethylene base layer and crystal nucleus, thereby forming a local hybrid structure. The specific formation mechanism is shown in Figure 10.


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Figure 10 Schematic representation of the formation mechanism of hybrid shish-kebab structures induced by PA6 microfibrils with different diameter under the intense flow field. 4.Conclusions In this study, by introducing PA6 microfibrils with different diameters (from 0.18 m to 5.6 m) into HDPE matrix, the effect of PA6 microfiber diameter on the formation of hybrid shishkebab crystal structure was investigated under the same secondary hot stretching filed, and the formation mechanism was further discussed. The introduction of PA6 microfibers has a heterogeneous nucleation effect for PE crystallization, which improves the crystallization ability of polyethylene (higher onset crystallization temperature and faster crystallization rate than pure PE). As the diameter of the microfibers decreases, amplification effects on the stretch field are also more significant. The further enlargement of the stretch field and the increase of the relaxation time of the system enable more oriented molecular chains to form and be retained, thereby more highly oriented crystal structures can be acquired as a result. Under the action of dynamic stretch field, PA6 microfibrils of different diameters can induce polyethylene to form


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hybrid crystal structure, showing size dependence evidently, that is, the PA6 microfiber with a small diameter has more nucleation sites to induce the polyethylene crystal to form a complete hybrid shish-kebab structure. When the diameter of PA6 fibers increases, PE lamellae with the same crystal size start to grow towards multiple orientations. While the fiber diameter reaches a certain value (about 2.2 m), a local periodical hybrid crystal structure is formed on the local surface of the PA6 microfiber. This leads to the little difference in heterogeneous nucleation of PA6 microfiber and small change in the crystal size of PE between the HDPE/PA6 microfibril blends. The gradual transition of the Tm from bimodal to monomodal was attributed to the stronger templating effect of the small diameter fibers. ACKNOWLEDGMENTS: This work was supported by the National Natural Science Foundation of China (Grant number 51473105 、 21674069 and 51721091). The authors also express sincere thanks to Chao-liang Zhang for the assistance of SEM observations and the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) for the kind help on 2D-SAXS measurement. REFERENCES: (1) Kimata, S.; Sakurai, T.; Nozue, Y.; Kasahara, T.; Yamaguchi, N.; Karino, T.; Shibayama, M.; Kornfield, J. A., Molecular Basis of the Shish-Kebab Morphology in Polymer Crystallization. Science 2007, 316, 1014. (2) Li, L.; Li, B.; Yang, G.; Li, C. Y., Polymer decoration on carbon nanotubes via physical vapor deposition. Langmuir. 2007, 23, 8522. (3) Xia, X.-C.; Yang, W.; He, S.; Xie, D.-D.; Zhang, R.-Y.; Tian, F.; Yang, M.-B., Formation of various crystalline structures in a polypropylene/polycarbonate in situ microfibrillar blend during the melt second flow. Phys. Chem. Chem. Phys. 2016, 18, 14030. (4) Xia, X.-C.; Yang, W.; Zhang, Q.-P.; Wang, L.; He, S.; Yang, M.-B., Large scale formation of various highly oriented structures in polyethylene/polycarbonate microfibril blends subjected to secondary melt flow. Polymer 2014, 55, 6399. (5) Keller, A., Polymer crystals. Rep. Prog. Phys. 1968, 31, 623. (6) Le, T.; Collazos, N.; Simoneaux, A.; Murru, S.; Depan, D.; Subramaniam, R., Statistical modelling and simulation of nanohybrid shish-kebab architecture of PE-b-PEG copolymers and


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25

Role of Controlled Diameter of Polyamide 6 (PA6) Fibers on the

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