Synergistic and Competitive Effects of Temperature and Flow on

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The Synergistic and Competitive Effects of Temperature and Flow on Crystallization of Polyethylene during Film Blowing Haoyuan Zhao, Qianlei Zhang, Lifu Li, Wei Chen, Daoliang Wang, Lingpu Meng, and Liangbin Li ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00391 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Polymer Materials

The Synergistic and Competitive Effects of Temperature and Flow on Crystallization of Polyethylene during Film Blowing Haoyuan Zhao, Qianlei Zhang, Lifu Li, Wei Chen*, Daoliang Wang, Lingpu Meng, Liangbin Li National Synchrotron Radiation Laboratory, Anhui Provincial Engineering Laboratory of Advanced Functional Polymer Film, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China

Abstract The influences of both temperature and external flow field on film blowing have been studied with a combination of a custom-built film blowing device and in-situ synchrotron radiation small- and wide-angle X-ray scattering techniques (SRSAXS/WAXS). Polyethylene (PE) with different structural topologies, namely linear (MPE) and long-chain branched polyethylene (LPE), were used here with different responses to temperature and flow. The MPE film shows a spherulite-like superstructure with low orientation independent of take-up ratio (TUR), while the LPE has a typical row-nucleated structure at high TUR. But for LPE film obtained at low TUR, it exhibits the combination of both crystal morphologies. Further analysis of the microscopic structural evolution of PE during film blowing by synchrotron X-ray scattering reveals three different types of network evolution: i) the temperature-induced crystallization (TIC) dominated process (MPE); ii) the flow-induced crystallization

*correspondence

author: [email protected] 1

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(FIC) dominated process (LPE with high TUR); iii) nucleation and growth are determined by the coupling effects of both temperature and flow (LPE with low TUR). The current results are expected to guide the processing of film blowing as well as provide a new viewpoint for FIC study under far-from-equilibrium conditions.

Keywords: Film blowing; Temperature-induced crystallization; Flow-induced crystallization; Synchrotron radiation X-ray scattering; Polyethylene

1. Introduction Film blowing is widely used in the production of agricultural and packaging films on account of its high productivity and capability to produce films with excellent properties achieved by biaxial stretching.1, 2 It is a rather complicated process especially for the very beginning from the extrusion of polymer melt to the formation of the final tubular film. This process is influenced by various processing parameters, i.e. rapid cooling (ca. 50 K/s) and non-uniform flow filed, which is far away from equilibrium state.3, 4 Besides these external factors, the film blowing is also highly related to the intrinsic structural parameters of polymeric material. For instance, branched lowdensity polyethylene (LDPE) and linear low-density polyethylene (LLDPE) exhibit different bubble stabilities during processing and different optical and mechanical properties of the film after processing.4-6 Therefore, both external processing and intrinsic structural parameters could significantly influence the film blowing process, which is closely related to the ultimate properties of films. Understanding the 2


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relationship of structure-process-property during film blowing is crucial for industrial application to optimize current blowing procedure and molecular characteristics of polymeric materials.7-12 However, the coupling influences of various intrinsic and extrinsic parameters complicate the analysis, and investigations successfully decoupling these effects have been rarely reported. The influence of temperature on polymer crystallization from bulk melt state can be traced back to almost six decades ago when the theory of polymer crystallization just began.13-15 The general physical description of polymer crystallization can be described as follows: cooling temperature below the equilibrium melting temperature (𝑇0𝑚) would lead to the formation of the nucleus, and afterwards, the growth of crystal starts followed by perfection and formation of spherulite.16 Under quiescent crystallization condition, the polymer crystallization is mostly driven by supercooling (∆𝑇), namely temperature-induced crystallization (TIC).16,

17

The apparent crystal

growth rate G is highly dependent on the crystallization temperature (𝑇𝑐) and three different regimes are widely observed and well explained by Hoffmann-Lauritzen theory.18, 19 Besides the thermodynamics factors, the polymer crystallization process is highly determined by kinetics factors, such as cooling rate.17, 20 During slow cooling, crystallization usually begins at a lower supercooling (the melt of 10-40 K below 𝑇0𝑚), where the nucleation rate is the rate limit step.16, 21 Further decreasing temperature leads to the increment of nucleus density. At a sufficiently fast cooling rate, the maximum nucleation rate of the polymer is found to be near the glass transition temperature (𝑇𝑔).16 Compared with TIC, polymer crystallization under the external flow field, namely 3



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flow-induced crystallization (FIC) shows quite different characteristics: i) leading to the thermodynamic state of the system far away from the equilibrium;22 ii) enhancing the crystallization kinetics by up to several orders of magnitude;23, 24 iii) inducing the formation of highly oriented nuclei such as row-nuclei or shish.25,

26

Besides these

differences, TIC and FIC share some intrinsic similarities. For instance, according to the entropy-reduction model (ERM) proposed by Flory, stretching leads to the reduction of the conformational entropy of the molecular chain, which equals to increasing the supercooling.27, 28 It is worth noting that the precursors or crystal nuclei induced by the flow promote the system to become inhomogeneous, thereby amplifying the local flow field response and increasing the crystallization rate.29,

30

Therefore,

although the physical entanglement induced by the entangled polymer chain is dynamic and transient in nature, the crystallization rate can be enhanced when the initially formed crystals act as the new cross-linked points to form a crystal-based network.22, 31 Recently, Wang et al.32 used a two-step flow protocol to study the effect of ordered structures formed in isotactic polypropylene on subsequent nucleation and crystallization. The existence of a transition from a chain network to a crystal network was confirmed with an accompanying increase in crystallization kinetics. For a strong flow applied, a nearly temperature-independent nucleation rate occurs.33 If the response of the melt to the flow is weak, a change in the nucleation mechanism controlled by the supercooling may be recognized.16 With respect to the film blowing, TIC and FIC coexist during the whole process.3, 34

Due to the synergy and competition of these external factors, such coupling 4


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complicates the analysis of the nucleation and growth of polymeric materials.35, 36 The effects of supercooling on shear-induced crystallization were probed by Rhoades et al.17, and it points out that the crystallization under high supercooling was mainly driven by homogeneous nucleation rather than previous shear flow. For the crystallization of polyethylene, the onset temperature of homogeneous nucleation or the fastest crystallization temperature is considered to be ca. 50 oC, which is close to the crystallization temperature of polyethylene during film blowing.36,

37

Meanwhile, a

certain increment in supercooling could result in the reduction of segmental mobility promoting the response of the molecular chains to the external flow.16 Generally, two different superstructures are displayed in film blowing: the row-nucleated structure with good optical properties, i.e. LDPE and HDPE, and the spherulite-like structure with excellent mechanical properties, i.e. LLDPE.5, 9 Even LDPE exhibits so-called interlocked twisted lamellae and non-interlocked lamellae at different take-up ratio (TUR), as well as large processing stability differences exhibited by different materials, whose origin is still unclear.6,

38, 39

Therefore, some models based on computer

simulation techniques are proposed, such as the two-phase microstructural constitutive model,40 thin-shell model,41 quasi-cylindrical model,3 and used to reconstruct the film blowing process to study the interrelationship of various structural parameters and external field parameters. Nevertheless, there is still a big gap between the theoretical simulation and the actual film blowing. In-situ tracking of the structural evolution process during film blowing is considered to be more effective in establishing the relationship of processing-structure5



property.42 Ogale and Peters et al.42, 43 first conducted such measurement of the film blowing process using in-situ small- and wide-angle X-ray scattering techniques (SAXS and WAXS). It was found that the bubble is crystallized before it is close to the frost line and shows a significantly different crystal structure in the frost line for materials with different structures and different processing conditions.42, 43 However, the structural information and detection range obtained by them are relatively limited, which cannot characterize the structural evolution right above the die. Following the similar strategy, our group designed a film blowing machine which can be coupled with the synchrotron facility, and capture the structural evolution of film right from the die to frost line for the first time.36 Meanwhile, the temperature gradient and the external flow filed information during processing were characterized by infrared probe and particle tracing techniques. The structural evolution model was proposed from the melt chain entanglement to the crystal entanglement network and then to the crystal scaffold during film blowing.36 Nevertheless, the coupling of TIC and FIC is always encountered in the above studies without clear decoupling. Meanwhile, the different responses caused by different molecular architectures as mentioned above are still unexplored. The present work focuses on the influence of temperature and flow field on nucleation and crystallization behavior of polyethylene during film blowing. For this purpose, a combination of a custom-built film blowing device and in-situ synchrotron radiation X-ray scattering techniques (SR-SAXS/WAXS) was used to investigate the structural evolution of two different polyethylene, linear (MPE) and long-chain 6


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branched polyethylene (LPE), at different TURs during film blowing. The primary chemical structures of MPE/LPE were first characterized by NMR, and their different responses to external flow filed were checked by rheological measurement. Later on, the detailed structural parameters, i.e. crystallinity, long period and orientation were obtained by SR-SAXS/WAXS. Finally, the structures and properties of the final films, i.e. the crystal morphology and mechanical property, were characterized. Based on these experimental results, the structural evolution process can be generally divided into three types: i) temperature gradient dominates, ii) flow field dominates, and iii) the synergistic control of temperature and flow field. The results make insight into the role of various parameters in film blowing and provide more meaningful guidance for theoretical research and industry of film processing.

2. Experiments 2.1 Materials Two different types of polyethylene were used in this study. A hexene base polyethylene sample catalyzed by metallocene catalysts was manufactured by ExxonMobil (MPE), and a low-density polyethylene (LPE) sample was produced by Dow Chemical. The molecular weight distribution was measured at 150 oC in trichlorobenzene by gel permeation chromatography (GPC) analysis (PL-GPC220) using monodisperse polystyrene (PS) standards. The resultant average molecular weight and polydispersity index are summarized in Table 1. Table 1. Structural parameters of PE samples 7



Density

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Melt index Mw (g/mol)

Mn (g/mol)

Mw/Mn

1.0

127,000

33,000

3.85

0.75

178,000

17,000

10.47

(g/cm3)

(g/10 min)

MPE

0.920

LPE

0.923

2.2 Film Preparation The blown film was prepared in a custom-built film blowing machine equipped with the die diameter and the gap of 30 mm and 1mm, respectively. The detailed description of this equipment has been reported previously,44 and herein only key processing parameters are mentioned. Processing parameters include the die temperature (Tdie), the frost line height (FLH, i.e. the position where the diameter of the bubble does not change), the blow-up ratio (BUR, i.e. the ratio of bubble diameter to die diameter) and the take-up ratio (TUR, i.e. the ratio of take-up speed to extrusion speed). In this work, TUR was changed from 8 to 24, while other parameters were fixed: Tdie was 190 oC, FLH was 60 mm, and BUR was 2. The change of TUR was realized by changing the take-up speed and the extrusion speed was 2.39 mm/s while the takeup speed was changed from 19.12 mm/s to 57.26 mm/s.

2.3 Characterization Methods 2.3.1

13C

NMR. Branched architectures of polyethylene were determined by

solution 13C NMR analysis. The samples were dissolved at 1, 1, 2, 2-tetrachloroethaned2 at 120 oC. All spectra were recorded using an Avance III HD NanoBay at 120 oC. 8


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2.3.2 Rheological measurement. The linear viscoelastic data were obtained by DHR-2 dynamic shear rheometer (TA instruments) in parallel plate (25 mm diameter) configuration, with a gap of 1mm. Dynamic frequency sweep measurements were carried out with frequency ranging from 620 to 0.02 rad/s. A strain amplitude of 1% was used, which was in the linear viscoelastic regime for two samples. Measurements were performed at 130, 150, 170, and 190 oC under a nitrogen atmosphere, and resin stability under testing conditions was verified. 2.3.3 SR-SAXS and WAXS measurement. In-situ SAXS and WAXS measurements were carried out on synchrotron radiation X-ray scattering station (BL16B1) with a radiation wavelength (λ) of 0.124 nm in the Shanghai Synchrotron Radiation Facility (SSRF). Two dimensional (2D) SAXS and WAXS patterns were collected using X-ray detector of Mar165 CCD detector (2048×2048 pixels with a pixel size of 80 μm) and Pilatus 300K detector (487×619 pixels with a pixel size of 172 μm), respectively. The SAXS and WAXS pattern of each data frame was acquired with an exposure time of 15 s. The film-to-detector distances were 2230 mm and 150 mm for SAXS and WAXS, respectively. The detection range was changed by the lifting platform, with distance ranging from 13 mm to 165 mm above the die. The temperature of the film was obtained by an infrared temperature probe. 2.3.4 Strain and strain rates calculation. Real-time velocity measurements of the bubble at different positions were performed by particle tracing techniques (see Figure S1 in Supporting Information).45 Movies of the bubble were collected for each sample and processing condition by means of a CCD camera and analyzed using the 9



AOS-TEMA Starter software. All videos were 50 frames per second and a scale was placed at the parallel position of the bubble for scale checking. 2.3.5 Scanning electron microscopy (SEM) measurement. Crystal morphology of films was examined using Gemini-SEM 500 with an accelerating voltage of 1 kV. The etched films were prepared at etchant for 10 min to remove amorphous phase. Each etching solution was prepared by thoroughly mixing 25 ml of concentrated nitric acid, 25 ml of concentrated sulfuric acid and 0.4 g of potassium permanganate by careful and rapid stirring.9 2.3.6 Mechanical and optical properties measurement. The mechanical properties of films were characterized using a Tensile Testing Machine, model HDB604-S, equipped with 100 N load cell at room temperature. The dumbbell-shaped film samples were used for tensile tests, with an initial length of 25 mm and the width of the film was 5.5 mm. The mechanical properties tests were carried out at a tensile speed of 200 mm/min in both Machine Direction (MD, i.e. take-up direction) and Transverse Direction (TD, i.e. vertical to the take-up direction). Light transmittance/haze meter (SGW-820) was used to obtain for the optical properties of films, following GB/T24102008 standard test. All tensile and optical tests were repeated five times.

3. Results 3.1 Architecture of PE with Different Topologies The solution 13C NMR spectra for LPE and MPE are presented in Figure 1a (the nomenclature used to describe the various carbons are given in Figure S3). The spectra 10


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indicate that LPE possesses many branches of different lengths, i.e. ethyl, butyl, pentyl, and longer side groups, and the total concentration of branches is 2.43 %. MPE has a relatively simple type of branching and contains only butyl branches with a total amount of 1.11 %.

Figure 1. (a) The

13C

NMR spectra of LPE and MPE together with the corresponding peak

assignments. (b) Storage modulus G' and loss modulus G'' as functions of angular frequency ω for samples measured at 170 oC.

Different architectures usually lead to different rheological responses.46 According to the classical linear viscoelastic theory, the storage modulus G' with a linear macromolecular structure grows with the square of ω in the terminal region (i.e. G' ∝ ω2 at low frequencies), where only the longest relaxation time contributes to the viscoelastic behavior.47 As shown in Figure 1b, the G' values in the low frequencies zone increase pronouncedly for LPE with slope of 0.95 while the value for MPE is 1.76 close to the theoretical value of 2.47, 48 The terminal relaxation time (τd) can be obtained from the cross-over points of G' and G" (values of τd at different temperatures are given in Table S1).49, 50 The τd of LPE (1.575 s at 170 oC) is about two orders of magnitude 11



longer than that of MPE (0.012 s at 170 oC), which should be attributed to the entanglement of its long chain branching and long chain tails. This significant difference in relaxation behavior will profoundly affect their response to the processing flow field.

3.2 Temperature Gradient and Flow Field Parameters during Film Blowing Since the film blowing is a complicated process with the coupling of multiple processing parameters, it is essential to ascertain its external field parameters, i.e. temperature gradient and flow filed.51, 52 The temperature gradient and the evolution of strain and strain rate of the bubble within an X-ray detection window are shown in Figure 2. The TUR-12 data is presented here as a typical example (other TUR data are shown in Figure S4). The temperature gradients for LPE and MPE are shown in Figure 2a. Both samples show similar tendency: the temperature decreases rapidly at the very beginning with a cooling rate of ca. 35 oC/s, whereas beyond a certain distance (D = 60 mm) above the die (frost line), the decreasing rate slows down with the corresponding temperature of ca. 60 oC. With higher positions (D  60 mm), the temperature continues to drop until ca. 45 oC at the maximum height of D = 165 mm. Although the initial setting conditions of the die are the same for MPE and LPE, apparent temperature gap is observed in which the temperature of LPE is lower than that of MPE before the frost line. Such a gap is attributed to the different flow fields of the two samples as shown in 12


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Figure 2b. Below the frost line, the strain of the LPE bubble is always larger than that of MPE until both films’ strains reach 2.63 at the frost line. With respect to the evolution of strain rates, both LPE and MPE increase rapidly to the maximum value of 2.6 s-1, whereas LPE arrives earlier, and afterward, both gradually decrease to 0 s-1 at the frost line. Such delay in the build-up curve of the strain rate is well consistent with the gap of temperature gradients. In other words, after extrusion from the same die, the thickness of LPE film decreases faster as compared with that of MPE film. As a result, the temperature of LPE bubble becomes lower than that of MPE at the same positions.

Figure 2. (a) The temperature gradient of bubble as a function of the distance from the die exit for MPE (black square) and LPE (blue triangle) at TUR-12, and inset in (a) shows the photograph of the bubble in the experiment; (b) Strain (open) and strain rate (dot center) of bubble along the machine direction (take-up direction) as a function of the distance from the die exit for MPE and LPE at TUR-12. (c) Enlarged view of (b) before the frost line.

3.3 In-situ SR-SAXS/WAXS The detailed structural evolution of PE during blowing is in-situ captured by SRSAXS and WAXS. As shown in Figure 3, a series of 2D WAXS and SAXS patterns at different positions of (a) MPE and (b) LPE are presented. The take-up direction is vertical as shown in Figure 3, defined as the meridional direction. The data obtained 13



under TUR-12 are used here as a typical example. At a position close to the die exit, such as D = 40 mm, only two amorphous halos exit in the WAXS patterns for both samples, which are resulted from the melt of the double layer films (Figure S5). So are to the SAXS patterns without apparent scattering streaks. Further increasing the distance leads to the appearance of two meridional streaks in the SAXS pattern at D = 46 mm (MPE) and 44 mm (LPE), while the corresponding WAXS images still show two amorphous halos. These streaks indicate the appearance of domains with density contrast. When D = 47 mm (MPE) and 46 mm (LPE), the crystal diffraction rings begin to appear in the WAXS patterns, which are assigned to (110) and (200) planes of PE orthorhombic structure. At higher positions, such as D = 55 mm, the crystalline diffraction of WAXS patterns are gradually enhanced, and corresponding SAXS patterns also exhibit strong meridional streaks. The anisotropy of the SAXS patterns of the MPE sample is lower than that of the LPE sample. The later shows a tear drop pattern. This suggests the emergence of the oriented crystal in LPE, while those in MPE film own lower orientation. It is further confirmed by the corresponding WAXS patterns, where MPE shows isotropic ring, and LPE shows a clear two-point pattern of (110) plane. Meanwhile, (020) plane appears in the equatorial direction for WAXS patterns of LPE bubble, which does not appear in the MPE bubble. Moreover, the initial streaks signal in the SAXS patterns of the LPE is wider along the meridian direction as compared with that of MPE, together with narrower width along the equatorial direction. Such phenomenon holds at the frost line (D = 60 mm). Above 60 mm, for WAXS patterns, the diffraction patterns of the crystalline signal remain almost unchanged for 14


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both samples, except for the enhancement of intensities. For SAXS patterns, the isotropic ring superimposed by a crescentic signal is observed for MPE. While for LPE, a meridional lobes pattern appears at 90 mm, which is slightly different from that at 60 mm.

Figure 3. The in-situ SR-WAXS and SAXS patterns at different positions for different PE samples under TUR-12 during film blowing. (a) MPE and (b) LPE. The red arrows within the purple rectangle represent the onset of scattering signal of the crystallization.

The above 2D SAXS and WAXS patterns reveal the different structural evolution of different PE samples during film blowing. The quantitative description of their difference can be obtained through the 1D integration of 2D WAXS and SAXS patterns. The contour plots of MPE and LPE consisting of 1D WAXS integral curves at different positions are shown in Figure S6 (a, b), respectively. An apparent wing of the WAXS diffraction peaks is observed for MPE, whereas those of LPE remains almost 15



unchanged. This phenomenon is originated from the difference of bubble stability during film blowing: the bubble of LPE owns higher stability than that of MPE. This is well consistent with previously reported works of literature.7, 10

Figure 4. (a) The crystallinity (χc, open graphics) and the growth rate of crystallinity (dχc / dD, dot center graphics) as a function of distance from the exit of the die for MPE and LPE. (b) The crystallinity as functions of bubble temperature for MPE and LPE. (c) The relative crystallinity (χ'c) in the equatorial direction as functions of distance from the exit of the die exit for MPE and LPE. 16


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(d) The azimuth integral curve of the 2D WAXS pattern at 60 mm from the die and the azimuth integral curves at a higher position are subtracted from the curve at 60 mm for LPE. (e) The long period (L) of lamella as functions of distance for MPE and LPE. (f) The orientation (f) of lamella as functions of distance for MPE and LPE at TUR-12.

The crystallinity (χc) can be obtained through the multi-peak deconvolution of 1D WAXS integral curves (Figure S2). Figure 4a summarizes the evolution of χc as a function of distance D referring to the die exit, together with the corresponding growth rate (dχc / dD). The evolution curves of both samples are almost overlapped with each other, and the general evolutionary trend of the crystal content is well consistent with the previous experimental results.36 The green line in Figure 4 represents the position of the frost line. Due to the existence of different temperature gradients (Figure 2a), the bubble temperature T rather than distance D is a more direct physical parameter for crystallization. The evolution of χc as a function of temperature is shown in Figure 4b. A significant turning point appears in the crystallinity curve, where c = 5.2 % for MPE and 3.7 % for LPE. Previous experimental results show that the evolution of crystallinity as a function of temperature before the frost line has a turning point during the film blowing, which corresponds to the formation of the crystal-based network, and the corresponding crystallinity is the intrinsic parameter of the molecular structure of the material.35, 36 Since the crystal diffraction ring of LPE significantly splits in the 2D WAXS patterns while the MPE does not, the relative crystallinity (χ'c) within a small angle along 17



the equatorial direction is calculated to evaluate the content of crystals oriented in the take-up direction (the c-axis is parallel to the stretching direction) as shown in Figure 4c. Interestingly, the χ'c of LPE and its bulk crystallinity are significantly different. The content of crystals regularly arranged along stretching direction before the frost line is almost negligible, and thereafter the χ'c rises rapidly. In order to rule out the possibility that the increase in χ'c is caused by the ring broadening, the diffraction ring of the (110) plane is azimuthally integrated as presented in Figure 4d and the curve for MPE also displayed in Figure S7. The curve formed by the black hollow box is the azimuth integral curve at D = 60 mm, and the other curves are obtained at D = 65, 75, and 85 mm after subtraction of the curve at 60 mm, respectively. Obviously, the curve obtained at each position agrees well with that at D = 60 mm, except that the intensity increases gradually with increasing distance around 0 o. This indicates that a new crystal signal appears along the equatorial direction. Besides the crystallinity, the long period (L) of lamellae is another crucial structural parameter to delve the structural evolution as summarized in Figure 4e. Before the frost line (D  60 mm), L remains almost constant: L = ~28 and ~30 nm for MPE and LPE, respectively. Subsequently (D  60 mm), L of the lamella decreases rapidly, and the evolution curves of L of MPE and LPE are basically overlapped. As D  100 mm, the decreasing rate of MPE is significantly smaller than that of LPE. Finally, L of the MPE film tends to reach a plateau value of 19 nm, whereas that of the LPE is close to 17 nm. The orientation (f) of the lamellae is one of the most direct manifestations of the 18


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crystal affected by the external field. f of lamellae at each position of the bubble is quantitatively calculated by Hermans' method53 (see Equation (3) in Supporting Information), and is shown in Figure 4f. Prior to the frost line (D  60 mm), f of both MPE and LPE film decreases monotonically. With D increases close to 60 mm (frost line), f of MPE and LPE reaches minimum values of 0.17 and 0.39, respectively. Afterward (D  60 mm), f of the LPE first increases slightly to 0.40 and then decreases to a constant value of 0.37, while that of the MPE monotonously increases to a plateau of 0.20.

3.4 Crystal Morphologies and Mechanical/Optical Properties Besides the microscopic parameters, the difference induced by different polymer architectures and processing parameters can be furthered elucidated by the crystal morphology. In order to clearly detect the crystal morphology, the amorphous component is removed through chemical etching.9 Figure 5 (a, b) are the SEM images of the film for MPE and LPE at TUR-12, respectively. MPE forms a bundle-like spherulite structure without clear oriented lamellae, while LPE forms oriented lamellae with an orientation perpendicular to MD, coexisting with a small number of disordered crystals. Such a result is well consistent with the above X-ray scattering analysis.

19



Figure 5. (a-b) SEM micrographs show the final surface morphology of film after etching at TUR12 for MPE and LPE, respectively. (c) Stress-strain (-) curves of films along MD and TD for MPE and LPE at TUR-12. (d) Transmittance and haze of films for MPE and LPE at TUR-8, 12, 16, 20, 24.

The crystal anisotropy is closely related to the macroscopic mechanical properties. Figure 5c shows the tensile stress-strain (-) curves along MD (i.e. take-up direction) and TD (i.e. vertical to the take-up direction) for MPE/LPE obtained at TUR-12. The mechanical curves for MPE along two vertical directions are quite similar suggesting the low anisotropy of film. Along the MD, the yield strength (yield), the ultimate tensile strength (max), and the elongation at break (εmax) of the MPE film are 10.0 MPa, 55.1 MPa, and 1328 %, respectively, while those along the TD are 8.7 MPa, 41.5 MPa, and 1173 %, respectively. In contrast, LPE shows a dramatic difference along two directions. 20


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Along the MD, the LPE film appears two separate yield points with max of 32.2 MPa and εmax of only 340 %. However, along the TD, the tensile behavior is similar to that of MPE films, and max and εmax are 26.8 MPa and 1020 %, respectively. The significant difference in the stretching behavior in both directions is due to the anisotropic alignment of the crystal, and the sharp drop in elongation at break along the MD is related to the alignment of the molecular chain oriented along the take-up direction. Besides orientation dependent mechanical properties, the anisotropy of the PE lamellae could also significantly influence corresponding optical properties. The optical properties of final films at different TUR are shown in Figure 5d. The optical properties of LPE films are generally better than MPE films, i.e. higher transmittance and lower haze.54 This ascribes the large-scale spherulite superstructure in the MPE films compared to LPE films. With the increase of TUR, the transmittance of the film increases first and then maintains unchanged (even decreased slightly), and the overall change range is small. In addition, the haze of the LPE films monotonically increases with TUR.

3.5 Effect of Take-up Ratio In order to explore the effect of processing parameters on the film blowing, five different sets of TUR experiments were carried out without changing other processing parameters. The 2D SAXS patterns of the bubble at the beginning of nucleation and growth for MPE/LPE are shown in Figure 6 (a, b). For MPE, the scattering signal is essentially independent with the increasing of TUR, except for a slight elongation along 21



the meridional direction. For LPE at high TUR (16, 20 and 24), the streak signal appears in the equatorial direction, indicating that highly oriented ordered structure oriented along the take-up direction is formed early.55 Since the in-situ measurements did not acquire complete 2D WAXS patterns, Figure 6 (c, d) show the complete 2D WAXS patterns of the final films. Consistent with above 2D SAXS patterns obtained from insitu measurement, the MPE film exhibits only (110) and (200) diffraction rings, while the LPE has (020) crystal plane (not shown in the figure).5 It is obvious that the (200) crystal plane is oriented in two-point patterns in the meridional direction at low TUR, and split into four-point patterns at high TUR, as presented in Figure 6d, and the (020) crystal plane is always oriented in the equatorial direction. The two-point diffraction of (200) plane means that the crystallization under the low stress, while the four-point represent the crystallization under higher stress level, i.e. intermediate stress.56

Figure 6. The 2D SAXS patterns of the initial bubble after the crystal signal appears during film blowing at different TUR for (a) MPE and (b) LPE. The 2D WAXS patterns of the final films at different TUR for (c) MPE and (d) LPE.

22


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In addition to the qualitative analysis of the scattering patterns, quantitative information for each structural parameter at different TURs is given in Figure 7. The green line in Figure 7 represents the position of the frost line. As displayed in Figure 7 (a, b), the evolution of crystallinity (χc) of neither MPE nor LPE changes significantly with various TUR. Nevertheless, since the increase of the TUR speeds up the movement of the bubble resulting in a larger cooling rate, the structural evolution interval of the film moves toward the low-temperature region. Furthermore, consistent with the foregoing analysis, MPE and LPE have a turning point with χc of 5.2 % and 3.7 %, respectively.36

23



Figure 7. (a, b) The crystallinity (χc) as a function of bubble temperature at different TUR. (c, d) The orientation (f) of lamellae as a function of bubble temperature at different TUR. (e, f) The relative crystallinity (χ'c) as a function of bubble temperature at different TUR.

The influence of TUR on the orientation (f) of lamellae are shown in Figure 7 (c, d). For MPE, the general tendency of the evolution of f is almost independent of TUR, 24


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where f decreases before the frost line (green line) and later increases to a plateau value. However, the evolution of f of LPE films presents two distinct trends at low and high TURs, although the final orientation of the film still increases with increasing TUR. At low TUR (8 and 12), f of LPE initially drops rapidly to a minimum, then increases slightly, and finally falls again until reaching the plateau value. But for high TUR conditions (16, 20 and 24), f exhibits a tendency to increase rapidly and then gradually decrease. This indicates that LPE may have different structural evolution processes at low and high TUR. In order to further explore the above differences, the relative crystallinity (χ'c) along the equatorial direction at different TURs for MPE and LPE are shown in Figure 7 (e, f). Similar to the evolution tendency of f, the evolution of χ'c for MPE films is almost independent of TUR, while that of LPE films is different at low and high TUR. For LPE at low TUR (8 and 12), no crystal appears along the equatorial direction before the frost line, and χ'c increases rapidly near the frost line. At high TUR (16, 20 and 24), crystal apparently exist before the frost line, and there is no obvious sudden change point or turning point along the evolution of χ'c, where χ'c increases almost linearly with D. Such difference is consistent with the evolution of f, and the evolution of structural parameters during film blowing of MPE and LPE show three different trends. The origin of such phenomenon is discussed in detail in the discussion part.

25



Figure 8. SEM images of the etched surfaces of (a, b) MPE and (c, d) LPE blown film with different TUR. The TUR is located in the left bottom corner, (a, c) TUR = 12 and (b, d) TUR = 20. The red dashed lines are used to outline crystals with different morphologies.

The influence of TUR on crystal morphology is characterized by SEM images as shown in Figure 8. For MPE, the bundle-like spherulite is observed and changes slightly with variable TUR except for the decrement of the spherulite size with increasing TUR. For LPE, the typical row-nucleated structure is formed, especially at high TUR (TUR = 20 here), in which the oriented lamellae are stacked parallelly. But for LPE at low TUR, as shown in Figure 8c, there are numerous randomly oriented lamellae (marked in the red oval frame) between the tightly stacked lamellae (marked in the red square frame). Furthermore, the thickness of the tightly stacked lamellae is thinner than that of the unoriented lamellae. Concerns related to the morphology change induced by etching can be removed by the comparison between SEM images of original and etched samples 26


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(Figure S7).

4. Discussion Before clarifying the influences of temperature and flow field on film blowing, the general network formation process is briefly described as follows, which has been reported in our previous works.36 The polymer is sufficiently melt-plasticized by the screw before extruding from the die, where the initial network is formed by the entanglement of molecular chains. As the bubble just exits to the die, the crystal is formed from the chain entanglement network, which leads to the formation of a crystal based network. With the increment of crystallinity, the modulus of the crystal network eventually balances with the external field. The network later translates into a crystal scaffold, which macroscopically appears as the frost line. After the frost line, the stretching effect almost disappears, and the growth and perfection of the crystal finally solidify the film. The rapid cooling and complex flow fields before the frost line will have different effects on the formation of crystal network and crystal scaffold of polymers with different topologies, which finally results in films with different macroscopic properties. With the assistance of two different kinds of PE (MPE/LPE), we are able to decouple the influence of temperature and flow during film blowing. The in-situ SAXS and WAXS measurements during film blowing classify the structural evolution of these two PE into three types: temperature gradient dominates crystallization (MPE); flow field dominates crystallization (LPE with high TUR); the synergistic influence of 27



temperature and flow field (LPE with low TUR). Corresponding to different structural evolution processes, distinct crystal morphologies and the anisotropy of mechanical properties are observed. The relationship of the structure-process-property during film blowing mainly results from the synergistic and competition effects of the temperature gradient (TIC) and the external flow filed (FIC) on film blowing, which will be discussed in the following.

4.1 Competition and Synergy of Temperature and Flow Field during Film Blowing The coupling influence of both temperature and flow filed results in three different crystal morphologies (Figure 8): the spherulite-like superstructure with lower orientation (MPE), the row-nucleated structure with the normal direction of lamellae preferred orientation along the take-up direction (LPE with high TUR) and the coexistence of these two crystal morphologies (LPE with low TUR). Admittedly, the complete decoupling of TIC and FIC cannot be achieved, but the selective changes of processing parameters and polymer systems could enhance the influence of a single factor. The distinct influence of TIC and FIC on film blowing can be well resolved in 2D SAXS patterns as summarized in Figure 9. Figure 9 (a, b and c) show the 2D SAXS patterns of the bubble around the frost line under different conditions, together with corresponding contour plots of 1D integral curves at different positions. The three systems here display dramatic different evolution processes of lamellae orientation: for LPE-12, two significantly different scattering signals appear simultaneously, while 28


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MPE-12 shows single broad scattering ring and LPE-20 displays single tear drop pattern. For MPE, due to the fast relaxation behavior (Table S1) of the linear structure, the chain entanglement network of MPE melt follows a viscoelastic deformation, and the extent of molecular orientation induced by the flow field can be qualitatively evaluated based on the dimensionless number 𝐷𝑒 = 𝜀 × 𝜏𝑑 < 1.57 At 𝐷𝑒 > 1, the molecular chain untangles and orients along the take-up direction. Here, the deformation rate induced by the flow filed is not sufficiently fast as compared with the relaxation of the molecular chain. As a result, it is difficult to produce highly oriented crystal nuclei.58 More importantly, the temperature has dropped to around 60 oC with a cooling rate of ca. 35 oC/s

(Figure 2) at the beginning of crystallization. Considering the 𝑇0𝑚 of 141 oC for

PE, the bubble has a high supercooling ΔT of at least 80 oC.59 Therefore, the initial crystallite with low orientation and parse distribution is formed by TIC, resulting in a widely distributed scattering signal near the beamstop as shown in Figure 9a.38 Subsequently, the crystals grow radially along the nucleus, which reduces the orientation (Figure 4f) with nearly isotropic lamellae. Finally, the ellipsoidal crystal is generated in the final film, corresponding to the SEM image as shown in Figures 8a.5, 38

Therefore, the network evolution of MPE is assigned to the TIC dominated process,

which is independent of TUR in current experimental parameter space.

29



Figure 9. The selected 2D SAXS patterns at different position and the contour maps of 1D integrated SAXS curves for (a) MPE-12, (b) LPE-12 and (c) LPE-20. (d) The azimuthal integrated curves for scattering signals at different positions for LPE-12. (e) The full width at half maximum (FWHM) of the azimuth integral curve of the scattering signal for LPE-12 as a function of distance from the die exit. (f) Evolution of 1D SAXS integrated intensity and the first derivative of intensity with distance from the die exit.

For LPE, once increasing the TUR ≥ 16, highly oriented crystal rather than isotropic one appears in LPE. As shown in Figure 9c, the single tear drop pattern appears before and after the frost line. This indicates at high TUR conditions, the 30


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external field is strong enough to dominate the initial network formation, namely 𝐷𝑒 > 1. The film blowing of the LPE with high TUR is thus assigned to FIC dominating process. With respect to LPE with low TUR (TUR=12), the two distinct signals of LPE12 appear simultaneously (Figure 9b) representing the formation of two domains with different length scale: the inner pattern (q  0.22 nm-1) represents domain with lager size and lower orientation, whereas the outer one (q  0.22 nm-1) represents the highly oriented domain. The difference of anisotropy is further quantitatively compared by the azimuthal integrated curve (Figure 9d), together with the FWHM (Figure 9e). The inner signal has a larger FWHM (36-76 o) with lower orientation as compared with the outer one (26-42 o), which suggests that the domains are more sparsely distributed in the bubble.60,

61

Meanwhile, the inner signal has a similar shape and distribution as

compared with that of MPE-12, while the outer signal is similar to that of LPE-20. The scattering signal near the beamstop as shown by MPE-12 (Figure 9a) is generally considered to be a low orientated crystal generated by the TIC,55, 62 whereas that of LPE-20 far away from the beamstop (Figure 9c) with higher orientation has been known to be produced by the FIC.34, 63 Regarding these features, the network evolution of LPE12 is expected to be influenced by the synergistic effects of both TIC and FIC starting from the very beginning. Such phenomenon differs from the double periodicity phenomena of PE found in injection molding by Baltá Calleja et al.64

31



Figure 10. (a) The schematic illustrations of crystal scaffold formation during film blowing around the frost line for LPE. (b-d) The schematics of crystal morphology of final films.

The structural evolution of the crystal network of LPE is schematically illustrated in Figure 10a. At low TUR (8 and 12), due to the weak flow, only polymer chains with the long relaxation time (𝐷𝑒 > 1) can be strongly stretched, while others are weakly stretched. In the early stage of crystallization, the crystallization of the polymer is dominated by TIC under high supercooling. The low orientated crystals will be formed. Accompanying the formation of the crystal-based network, the response of the polymer chain to the flow field is enhanced, and the molecular chains on the stretched crystal network are induced to produce oriented crystals.30, 32 The orientation of those two parts are different as depicted in Figure 10a, which are reflected by two different SAXS 32


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scattering signals (Figure 9b). Also, the shift of the initial nuclei along the flow direction will also leads to the highly oriented crystal structure, which is known as the ghost nuclei.30 Above the frost line, the crystal network is continuously filled, and the highly oriented crystals form a relatively rigid crystal scaffold, which also makes other domains more difficult to deform. The crystal morphology as shown in Figure 10c is finally formed, as evidenced by the SEM image (Figure 8c). Such structure also helps to explain the different mechanical behaviors of the LPE film in the take-up direction and its vertical direction (Figure 5c). The stretching along the take-up direction (MD) is carried by the crystal skeleton formed by the tightly arranged lamellae, while that along the vertical direction (TD), the regular lamellae and the randomly oriented crystals are forced in-serial. During actual film blowing, the orientation of lamellae gradually decreased by the influence of TIC (Figure 4f), and then the highly oriented crystal nuclei will be generated before the frost line, thereby producing regularly stacked lamellae.26, 30, 65 Correspondingly, the SAXS scattering intensity together with the changing rate along the meridional direction increase again after the frost line, which further confirms the formation of crystals (Figure 9f). Because of this, the stretching effect almost disappears above the frost line, but the overall orientation of lamellae tends to increase (Figure 4f), which is attributed to the formation of the orientated nucleus prior to the frost line. More interestingly, the sudden increment in the relative crystallinity of crystals is observed (Figure 7f), which further confirms the transition from the randomly oriented crystals induced by temperature (TIC) to those induced by flow (FIC) to form oriented lamellae. 33



For LPE at high TUR (16, 20 and 24), with 𝐷𝑒 > 1 for most of the polymer chains, the external flow field is strong enough to stretch the polymer chains and maintain its orientation during the whole blowing process.34, 66 The highly oriented crystal nuclei is induced as shown in Figure 10a (high TUR), followed by uniform filling into a crystal scaffold. During film blowing, the fibrillar nuclei, which is important for the rownucleated structure, is generated at first. It is confirmed by the presence of a streak-like signal in the equatorial direction on the 2D SAXS patterns of the bubble in the early stage of crystallization (Figure 6b).34, 38 With the continuous extension imposed by the taking up roller before the frost line, the orientation of lamellae gradually increases. Thus, a typically scattering signal induced by the flow field is generated in the meridional direction as described by Hsiao et al.60, 63, and is also similar to the outer signal of Figure 9b. Therefore, for LPE at high TUR, FIC dominates the film blowing process while TIC is suppressed. Consequently, the tightly stacked lamellae is generated as shown in Figure 10d and further confirmed by the SEM image in Figure 8d, which is also consistent with previous studies.38

4.2 Manipulation of Structure-Process-Property in Film Blowing The row-nucleated model proposed by Keller and Machin has been widely used to understand the orientation and crystal morphology of PE blown film.67 However, for different molecular structures or processing parameters, the difference in specific crystal structure such as Keller/Machin I or II is still difficult to understand via post mortem on the processed films.38, 68 The suspicion raised by Garth L. Wilkes et al.69 34


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regarding the necessity of fibril nuclei may be due to the fact that highly oriented nuclei are sometimes not formed by molecular chains stretched directly from the melt state. It is possible that the synergy of TIC and FIC induces stretching of the crystal based network32, 36 or secondary nucleation namely the ghost nuclei30 during film blowing. Different coupling levels of temperature gradient and flow field lead to a different orientation and crystal morphologies, such as spherulite-like superstructure, interlocked twisted lamellae or non-twisted lamellae.5, 38 Furthermore, in the establishment of a more accurate theoretical model of film blowing, the two-phase model from the melt to the semicrystalline phase, which is currently widely used, can consider semicrystalline phases of different modulus or form factor rather than simple rigid-rod.3, 41, 70 Whether it is a microstructure model or a continuum model, the evolution of stress after the appearance of the semicrystalline phase needs to be adjusted according to the coupling extent of the external field, thereby affecting the determination of the orientation tensor in film blowing.70 The nucleation and growth process dominated by TIC shows that the nucleation density is largely related to temperature, and the nucleation density will be limited during the actual film blowing process because the temperature is far from the fastest nucleation temperature range (near 𝑇𝑔).16, 36 On the other hand, during extrusion, the entangled network deforms inhomogeneously. The entanglements with fast relaxation time τd can be easily migrated, while those with larger τd maintains during deformation.30, 31, 58 The unevenly distribution accompanied with the anisotropy of the structure produced by FIC and the lower crystal crosslink density brought by the low 35



nucleation density of TIC need to be comprehensively considered for the film blowing stability.35,

39

Furthermore, the film with the spherical crystal (Figure 8a and 10b)

formed by TIC has more excellent and balanced mechanical properties, while the film having the row-nucleated structure (Figure 8d and 10d) formed by FIC has more excellent optical properties. These specific results can be applied to design an optimized film blowing process that includes regulating chain structure of raw materials, flow strength, and cooling conditions in order to produce good specific microstructures, process stability and product property.5, 38

5. Conclusions In this study, the film blowing of two polyethylene samples with different topological structures under different TUR were investigated by in-situ measurements of SR-SAXS/WAXS. Combining with the characteristics results of structural evolution and crystal morphological of film, three types of structural evolutions are proposed. For MPE, the structural evolution process is dominated by TIC at all TUR to form an ellipsoidal crystal superstructure, so that the film has excellent mechanical properties. For LPE at high TUR, the effect of TIC is almost negligible before the frost line and the high orientation of the row-nucleated superstructure generated by FIC. For LPE at low TUR, the temperature and flow field synergistically affect the crystallization of the film blowing, and two crystal scattering signals appear simultaneously near the frost line. The crystal produced by the TIC promotes the response to the flow field to produce orientated crystals that further enhance the modulus of the crystal network, improve 36


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film blowing stability, and achieve films with balanced mechanical and optical properties. Such phenomenological models are important toward decoupling the influences of FIC and TIC on film blowing, which is of great significance for the production and performance control of the blown film.

Acknowledgments This work is supported by the National Key Research and Development Program of China (2016YFB0302500 and 2018YFB0704200) and the National Natural Science Foundation of China (51703217 and 51633009). The SR-WAXS/SAXS experiments are carried out at the beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF).

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For Table of Contents use only

The Synergistic and Competitive Effects of Temperature and Flow on Crystallization of Polyethylene during Film Blowing Haoyuan Zhao, Qianlei Zhang, Lifu Li, Wei Chen*, Daoliang Wang, Lingpu Meng, Liangbin Li National Synchrotron Radiation Laboratory, Anhui Provincial Engineering Laboratory of Advanced Functional Polymer Film, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China

*correspondence

author: [email protected] 42


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Synergistic and Competitive Effects of Temperature and Flow on

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