From Molecular Entanglement Network to Crystal-Cross-Linked

Macromolecules , Article ASAP. DOI: 10.1021/acs.macromol.8b00346. Publication Date (Web): May 31, 2018. Copyright © 2018 American Chemical Society...
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From Molecular Entanglement Network to Crystal-Cross-Linked Network and Crystal Scaffold during Film Blowing of Polyethylene: An in Situ Synchrotron Radiation Small- and Wide-Angle X‑ray Scattering Study Qianlei Zhang, Lifu Li, Fengmei Su, Youxin Ji,* Sarmad Ali, Haoyuan Zhao, Lingpu Meng, and Liangbin Li* National Synchrotron Radiation Lab, CAS Key Laboratory of Soft Matter Chemistry, Anhui Provincial Engineering Laboratory of Advanced Functional Polymer Film, University of Science and Technology of China, Hefei, China S Supporting Information *

ABSTRACT: Combining a homemade film blowing machine and an in situ synchrotron radiation source with small- and wide-angle X-ray scattering (SAXS and WAXS) capability, an investigation of film blowing of polyethylene (PE) has been studied. From the die exit to the positions above the frost line, four zones defined with different structural features are observed with SAXS and WAXS measurements. In zone I, precursor and crystal structures emerge from the polymer entanglement network during cooling and extension, which lead to the formation of a deformable crystal-cross-linked network at the boundary between zones I and II. The occurrence of the crystal-cross-linked network enhances the effective chain stretching during further deformation in zone II. Crystallization is largely accelerated, which generates crystals with high orientation. Further increasing the crystallinity results in the deformable crystal-cross-linked network transforming into a nondeformable crystal scaffold at the frost line (the boundary between zones II and III), which stabilizes the bubble and prevents further deformation. In zones III and IV, the scaffold and the entire sample are gradually filled up by crystals, respectively. Interestingly, increasing the take-up ratio (TUR) does not influence the critical crystallinity (χI−II) for the formation of the deformable crystal-cross-linked network, while the crystallinity (χf) at the frost line or for the formation of nondeformable scaffold does vary with TUR. This suggests that the former (χI−II) is mainly controlled by molecular parameters, while the latter (χf) is determined by both processing and molecular parameters of PE material.

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INTRODUCTION Film blowing is one of the most important polymer processing technologies and is extensively used to produce packaging films, protective films, and agriculture films. The world production of agriculture films was nearly 3.82 million tons in the year of 2016.1−3 During film blowing, the polymer melt is extruded from an annular die and then blown up to form a tubular film or bubble, during which the film is biaxially stretched with large deformation and cooled by an air ring.4−8 For semicrystalline polymers like polyethylene (PE), both cooling and stretching promote crystallization,9−13 which eventually results in a stable tubular bubble without further plastic deformation. The position where no further deformation occurs to the bubble is defined as the frost line position.5,12 Below the frost line position, complex flow and temperature field are imposed on the melt or semisolid polymer. As the flow and the cooling rate are intimately coupled with each other,5,13 it is a challenge to elucidate their specific roles on crystallization, film stability, and the final properties of the film. Flow-induced crystallization (FIC) is an important factor for the stability of film blowing and the product properties. For decades, FIC has been widely studied with the focus mainly on flow induced nucleation, during which the experimental © XXXX American Chemical Society

temperature is generally near the melting point to suppress spontaneous nucleation induced by thermal fluctuations.14−25 The so-called “short-term flow” protocol is also widely adopted in FIC experiments to separate nucleation and subsequent growth processes, where crystal nuclei are assumed to not be formed during flow.26−30 On the basis of these experiments, it is widely accepted that flow can enhance crystallization rate by orders of magnitude31−34 and vary the morphologies of crystal nuclei according to flow intensities.28,31,35−44 At the small strain region, point nuclei are generated while shish nuclei are formed at the high strain region after “strain hardening point” for isotactic polypropylene (iPP) and some other polymers.18,23,45−47 Based on Flory’s entropic reduction model (ERM),48−51 a modified ERM model is recently proposed to account for the enhancement of nucleation and the formations of new crystal forms and morphologies. However, this modified ERM model still describes a one-step transition from the polymer melt with reduced entropy to crystal.52 Actually, even for the “short-term flow”, there is still enough time for Received: February 13, 2018 Revised: May 22, 2018

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DOI: 10.1021/acs.macromol.8b00346 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules generation of crystals during flow (shorter than 1 s for PE and iPP).53−55 These flow-induced crystals can act as the physical cross-links, forming the crystal-cross-linked network.56−62 Then, the deformation of the crystal-cross-linked network and the relative displacement between nuclei and their surrounding chains may happen during flow, which complicate the mechanism of FIC. With the presence of crystal-crosslinked network, polymer chains are expected to deform more easily, which further accelerates nucleation.20 Recently, a “ghost nucleation” model has been proposed by Cui et al. to account for the relative displacement between crystal nuclei and their surrounding matrix, which creates extra surfaces to nucleate and increases the crystallization rate as large as nearly 3 orders of magnitude.11 These behaviors and mechanisms are closely related to the crystallization in the practical processing, like film blowing, during which the film suffers a large deformation and a continuous cooling. During film blowing, the intimate coupling between flow and temperature makes FIC much more complicated than that in the model experiments under isothermal condition with the experimental temperature close to the melting point. From the die exit to the frost line, the temperature of film bubble drops quickly down to a value far below the melting point. Therefore, unlike the model FIC experiments, the crystallization during film blowing is not only promoted by flow but also strongly influenced by temperature.63−69 Moreover, before reaching the frost line position, the film thickness decreases continuously during film blowing, which consequently further accelerates cooling. Thus, the influences of flow and temperature on crystallization kinetics are coupled with each other, which are difficult to be studied separately.5,13 Since the strain imposed on the melt is rather high during film blowing, the formation and the subsequent deformation of crystal-cross-linked network57,70 are expected to occur during film blowing, which further increase the complication of crystallization kinetics. Considering such complexity, in situ SAXS and WAXS measurements are suggested to explore structural evolution during film blowing as studied by the groups of Gerrit W. M. Peters and others.1,5,12,13 It was found that both crystallinity and lamellar orientation of the blown film are influenced by the material properties and the processing parameters.5,12 Modeling is also considered by Lee and others to study this complex process, by which the stress at the frost line position is calculated and suggested to determine the crystalline morphology and the product properties.71−75 Nevertheless, although experiments and modeling have been carried out to study the relationship between processing−structure−property during film blowing, the structural evolution below the frost line position and the stabilization mechanism of the bubble remain unclear. In this work, focusing on the structural evolution below the frost line position, a combination of an homemade film blowing machine and the in situ simultaneous synchrotron radiation WAXS and SAXS measurements is employed to investigate structural evolution of PE at different take-up ratios (TUR) during film blowing. The evolutions of crystallinity, long period, and orientation of lamellar crystals are studied at different positions of PE bubble. From the die exit to the positions above the frost line position, the structural evolution can be divided into four zones. In zone I, cooling and extension of entangled polymer network induce crystallization and a deformable crystal-cross-linked network is generated at the boundary between zones I and II. After that, the crystal-cross-linked

network is further deformed and gradually evolves into a nondeformable crystal scaffold to stabilize the bubble at the boundary between zones II and III (the frost line position). As the position goes into zones III and IV, the scaffold and the entire sample are further filled up by crystals, accompanied by reduced crystallization kinetics. The crystallinity for the formation of crystal-cross-linked network keeps constant for different TUR, while the formation of crystal scaffold requires different crystallinity at different TUR. This work provides a guidance for improving the bubble stability and the properties of the film.

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EXPERIMENTAL SECTION

Materials. The material used in this study is the PE blend, which is mixed by two kinds of linear low-density polyethylenes: LLDPE 1 (the ethylene−octene copolymer, XUS61530, DOW), LLDPE 2 (the ethylene−butene copolymer, 1001AV, ExxonMobil) and one lowdensity polyethylene:LDPE (0725N, TASNEE) with a weight ratio of 2:1:1. Since it costs almost 1 h to finish the in situ film blowing experiment for one sample, the stable blowing process and good property of the film are necessary during the experiment. Therefore, these three materials are blended, and after trying many different blend ratios, the best blend ratio (2:1:1) is found and used to improve the stability of the bubble and the property of the film in this work. In addition, it is found that only one peak appears in the flash DSC cooling curve of this blend, which indicates that these polyethylenes do not crystallize separately or at different temperatures (see Figure S1 in the Supporting Information). The molecular parameters measured by gel permeation chromatography (GPC) and rheological parameters of these polymers are listed in Table 1, including polymer density (ρ),

Table 1. Molecular and Rheological Parameters of the Raw Polymers LLDPE 1 LLDPE 2 LDPE

ρ (g/cm3)

MFI (g/10 min)

Mw (g/mol)

PDI

0.917 0.919 0.923

0.8 1.0 0.75

133000 151000 109000

4.2 4.9 5.1

melt flow index (MFI), weight-average molecular weight (Mw), and polydispersity (PDI). MFI is defined as the weight of the polymer extruded in 10 min through a capillary by pressure applied through dead weight. The information on short chain branch are measured using 13C NMR spectra. These spectra are obtained at 120 °C for all samples. The equipment used is an Advance III HD NanoBay. Polymer solutions are prepared with tetrachloroethane-d2 as solvent. From the 13 C NMR spectra, LDPE consists of 0.55 mol % butyl branches, 0.17 mol % amyl branches, and 0.43 mol % long branches (n ≥ 6). The total amount of branching is 1.15 mol %. Since LLDPE 1 is the ethylene−octene copolymer, LLDPE 1 mainly consists of long branches with the total amount of branching of 2.17 mol %, while LLDPE 2 mainly consists of ethyl branches with the total amount of branching of 2.30 mol % as LLDPE 2 is the ethylene−butene copolymer. Film Blowing. A homemade film blowing machine with the die diameter and the gap of 30 and 1 mm, respectively, is used to blow film in this work, as shown in Figure 1. The details of this apparatus have been described elsewhere.76 A special air ring is designed. Its top surface is set with the same height as the die exit, which allows SAXS/ WAXS to characterize the structure of polymer melt close to the die exit. By two adjustable autolifters, the film blowing machine can go up and down with a speed of 0.05 mm/s, during which the temperatures (as shown in Figure 2a) and the structures of the bubble from 0 to 160 mm away from the die exit can be detected with an infrared thermometer and the synchrotron radiation SAXS/WAXS. For industrial processing, the PE films are produced at TUR of 15 and B

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Figure 1. Photograph of film blowing apparatus assembled in BL16B1, SSRF.

Figure 2. (a) Temperatures of bubble (T) as a function of the distance from the die exit (D) for TUR-7 (square), TUR-10 (cycle), TUR-15 (up triangle), TUR-20 (down triangle), and TUR-25 (diamond). (b) Image of bubble for TUR-15 sample. (c) The frost line positions as a function of TUR. blow-up ratio (BUR) of 2 for the high optical properties. In order to investigate the effects of TUR on the structural evolution, a wide range of TUR is adopted in this study. Considering that the bubble becomes unstable at TUR of 30, TUR are set from 7 to 25 with a proper interval in the film blowing experiment, during which other processing parameters keep constant, including die temperature (220 °C), BUR (2), extrusion speed (2.37 mm/s), and air flux for cooling. A diagram of the bubble and the blowing process is presented in Figure 3. As shown in Figure 3, the TUR and BUR are defined as follows: TUR =

Vt Ve

(1)

BUR =

W W0

(2)

2b. With this approach, the frost line positions (Df) for different samples are obtained and presented in Figure 2c, which are confirmed by the particle tracing experiments (see Figure S2). Considering the definition of the frost line, no further deformation occurs to the bubble above the frost line position for the following reasons. (i) Since the width of bubble remains constant above the frost line position, no further deformation occurs in radial direction of bubble above the frost line position. (ii) By the method of the particle tracing, the velocity component along the axial direction is also identified at a certain distance from the die exit by a CCD camera (see Supporting Information). As shown in Figure S2, the velocity component along the axial direction first increases with increasing distance and then keeps constant above the frost line position (65 mm from the die exit), which demonstrates that no further deformation occurs in axial direction of bubble above the frost line position as reported in the previous work.12 X-ray Scattering. The in situ SAXS and WAXS measurements are carried out to monitor the evolutions of structure and morphology during film blowing at the beamline BL16B of the Shanghai Synchrotron Radiation Facility (SSRF). The X-ray wavelength (λ) is 0.124 nm. A Mar 165 CCD detector (2048 × 2048 pixels with pixel size of 80 μm) and a Pilatus 300 k detector (487 × 619 pixels with

where Vt and Ve are the taking up speed and the extrusion speed and W and W0 are the bubble width and the die diameter, respectively. Frost Line. To identify the frost line position of the bubble, images of bubbles are taken with a CCD camera and analyzed using an imageprocessing software. As an example, the image of TUR-15 sample is shown in Figure 2b. We define the frost line at the position where the width of bubble does not change, as indicated by the red line in Figure C

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where q is the module of the scattering vector, λ is the X-ray wavelength, and 2θ is the scattering angle. The long period (L) of lamella is calculated from the peak position (qmax) of one-dimensional SAXS curves according to Bragg’s law: L=

2π qmax

(4)

The orientation parameter of lamella (f) is estimated from the full width at half-maximum (fwhm) of the azimuthal intensity distribution of the 2D SAXS patterns according to Hermans’ orientation parameter, which is defined as

f=

3⟨cos φ2⟩ − 1 2

(5)

where φ is the angle between the reference direction (extensional direction) and the normal direction of the lamellae. The ⟨cos φ2⟩ is defined as follows: 2

⟨cos φ ⟩ =

TUR-7

TUR-10

TUR-15

TUR-20

TUR-25

16.6 7

23.9 10

35.4 15

47.3 20

59.3 25

χc =

pixel size of 172 μm) are employed to collect 2D SAXS and WAXS patterns, respectively. As shown in Figures 2b and 3, since the X-ray transmits two layers of film (front face and back face) and consequently generates two sets of diffraction signals for each crystal plane, the distances between the back face of the film and the detectors are measured as the sample-to-detector distances. In this case, the sample-to-detector distances are 2250 and 152.4 mm for SAXS and WAXS, respectively. The data acquisition time is 20 s per frame with an exposure time of 15 s. Fit2D software from the European Synchrotron Radiation Facility is used to analyze the data. In addition, considering the small angle of scattering signals and the short sampleto-detector distance (2250 mm), two scattering layers of SAXS signals are all collected by the detector and almost reflected on the same pixel position as discussed in the previous work.76 Both SAXS and WAXS data are corrected for background scattering through air. The 2D SAXS patterns are first integrated azimuthally to obtain the 1D profiles as a function of

4π sin θ λ

I(φ) cos φ2 sin φ dφ π /2

I(φ) sin φ dφ

(6)

where I(φ) is the 1-D intensity distribution along with the azimuthal angle after the subtraction of the background intensity. Thus, f equals to 1 when all lamellae are perpendicular to the flow direction, while 0 for no preferred orientation. To get more insights into crystallization, WAXD peak-fitting is performed to get the bulk crystallinity (χc) of samples, according to the method of Turner−Jones:77

Table 2. Processing Parameters of Blown Films (Vt Is the Taking-Up Speed)

q=

π /2

∫0

Figure 3. A diagram of the bubble and the blowing process.

Vt (mm/s) TUR

∫0

∑ Ac × 100% ∑ Ac + ∑ A a

(7)

where Ac and Aa are the fitting areas of crystalline and amorphous regions, respectively. Here both sets of WAXS peaks from front and back films are used to estimate crystallinity.

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RESULTS SAXS Results. Several representative 2D SAXS and WAXS patterns for TUR-15 during film blowing are shown in Figure 4, where the meridional direction is defined as the vertical direction (take-up direction). At the position near the die exit, 2D SAXS and WAXS patterns of the bubble show typical amorphous scattering without lamellar scattering or crystal diffractions. The scattering intensity decreases with increasing distance away from the die exit due to the decrease of film thickness. When the detection position increases to around 51 mm away from the die exit, two meridional streaks are observed in the SAXS pattern while no crystal diffraction can be observed in the WAXS pattern yet, indicating that precursors appear

(3)

Figure 4. Representative SAXS and WAXS patterns collected during film blowing. The two diffraction rings at low 2θ are from (110) and (200) planes for the back face of the bubble, while the other two diffraction rings at high 2θ are from (110) and (200) planes for the front face of the bubble, as shown in the WAXS pattern at 160 mm. D

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Figure 6a shows Lorentz-corrected 1D SAXS intensity curves during the film blowing with TUR of 15.78,79 As shown in Figure 6a, the scattering intensity starts increasing at 51 mm away from the die exit, which corresponds to the emergence of the scattering streaks in 2D SAXS patterns (Figure 4) and indicates the formation of precursors. After that, the scattering maximum appears, which becomes more and more obvious and shifts to high q with increasing distance, suggesting that the long period of lamellar crystal decreases with increasing distance away from the die exit (as highlighted with black dot line in Figure 6a). As presented in Figure 6b, the evolution of long period of lamella during film blowing is further analyzed quantitatively with the peak position shown in Figure 6a and the Bragg law (eq 4). Note that as the distance away from the die exit has no direct physical meaning correlating with crystallization, the long period is also plotted versus temperature in Figure 6c through the conversion with temperature−distance relation (Figure 2a). The evolution of long period with temperature shows almost the same tendency as that with distance. As shown in Figures 6b and 6c, the long period is about 35 nm at the beginning and remains nearly constant before reaching the position or temperature at the frost line (65 mm, 51.8 °C), which follows a nearly linear decrease from 35 to 22 nm at higher distance or lower temperature. The long period reaches 16.10 nm after the sample is completely cooled down. The fluctuation in the L−T curve is due to the temperature fluctuation as shown in Figure 2. To investigate the orientation of lamella during the film blowing, Figures 7a,b display the azimuthal intensity distribution of the 2D SAXS patterns for TUR-15 sample, where 0° represents the horizontal or equatorial direction. With the fwhm of azimuthal distribution of SAXS, we analyze the lamellar orientation ( f) quantitatively with eq 5, which are plotted as a function of distance and temperature in Figures 7c and 7d, respectively.

before the onset of crystallization. Further increasing the distance from the die exit, the SAXS signal becomes dumbbellshaped along with the appearance of scattering maximum, which stands for periodically aligned lamellar stacks. The occurrence of crystallization is evidenced by the two crystal diffraction peaks of (110) and (200) planes (as denoted in the pattern) at about 58 mm away from the die exit, which then become more and more pronounced as the distance increases. The splitting of scattering from (110) crystal plane is due to the formation of twisted lamellae. Note that the other two diffraction rings in WAXS patterns at the high 2θ are from the front face of the bubble. Both crystal diffractions in WAXS and scattering streaks in SAXS patterns appear at the positions below the frost line position as shown in Figure 4, which indicates that crystallization occurs in the bubble below the frost line position. Similar to TUR-15, precursors, crystals and frost line appear sequentially for all TUR from low to high distance away from the die exit. The onset distances for precursors (Ds), crystals (Dw), and frost line (Df) at different TUR are summarized in Figure 5. With the increase of TUR, Ds, Dw, and Df all follow a nonmonotonic tend from initial increase to later decrease.

Figure 5. Positions of frost line, appearance of scattering streaks, and crystal diffractions for samples with different TUR.

Figure 6. (a) Contour map of one-dimensional SAXS curves for TUR-15 sample with the X and Y axes referred to q and distance (D); the colors refer to the intensity multiplied by q2. The long period of lamella as a function of distance from die exit (b) and temperature (c). The dotted lines represent the frost line position. E

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Figure 7. (a, b) Contour map of azimuthal-integrated intensity distribution of the 2D SAXS patterns for TUR-15 sample with the X and Y axes referring to azimuthal angle (φ) and distance (D); the colors refer to the intensity, and 0° represents the horizontal direction. (b) is the enlarged figure. (c, d) Evolution of orientation parameter of lamella with distance and temperature.

Figure 8. (a) Contour maps of the one-dimensional WAXS curves for TUR-15 sample with the X and Y axes referred to 2θ and distance (D); the colors refer to the intensity. (b) A typical example of the peak decomposition of the one-dimensional WAXS curves. Points are experimental data. The line is the best fitting curve. Peaks 1, 2, and 3 are from the amorphous halo and (110) and (200) planes of the back face of the bubble, respectively, while peaks 4, 5, and 6 are from the amorphous halo and (110) and (200) planes of the front face of the bubble, respectively. (c, d) The bulk crystallinity (χc) and the growth rate of crystallinity (dχc/dD) as a function of distance from die exit and temperature, respectively.

then almost keeps constant with a value of around 0.70 in zone IV (from 92 to 160 mm). Note that the decrease of lamellar orientation parameter in zone I may be due to the broad distributions of lamellar periodicity and lateral size, according to Hsiao’s lamellae model.80,81 WAXS Results. To evaluate the evolution of crystallinity during film blowing, we integrate the 2D WAXS patterns into 1D diffraction curves at different positions, which are shown in Figure 8a. Clearly, the (110) crystal plane is observed at around 58 mm away from the die exit, indicating the generation of

According to the evolution of lamellar orientation, the curves in Figures 7c and 7d can be divided into four zones with different features. In zone I (from 51 to 61 mm), the orientation parameter decreases to 0.74 with increasing distance or decreasing temperature. After that, an increase occurs in zone II (from 61 to 65 mm). Interestingly, this increasing trend of orientation parameter stops precisely at the frost line (the boundary between zones II and III), as shown in Figure 7c. Further increasing the distance into zone III (from 65 to 92 mm), the orientation parameter starts to decrease again, which F

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Figure 9. (a) Long period, (b) lamellar orientation parameter, and (d) crystallinity as a function of temperature for TUR-7 (square), TUR-10 (cycle), TUR-15 (up triangle), TUR-20 (down triangle), and TUR-25 (diamond). (c) The crystallinity is also plotted versus time (from the beginning of crystallization) with an enlarged figure. The normalized temperature (T′) with value of 1 referred to the highest temperature and 0 referred to the lowest temperature for each TUR is used in (b).

TUR Effects. To investigate the effects of TUR on structural evolution of PE during film blowing, the 2D SAXS and WAXS patterns are also collected during film blowing with different TUR. Following the same approaches as presented above for TUR-15 sample, the long period, orientation parameter of lamella, and crystallinity are obtained and plotted versus temperature for different TUR, as shown in Figures 9a, 9b, and 9d, respectively. The overall trend of long period (L) does not vary with TUR, except that the temperatures for the appearance of the lamellar structures decrease with increasing TUR due to the increase of cooling rate (Figure 9a). The evolutions of orientation parameter (f) of lamella at different TUR are compared in Figure 9b. It can be first found that the final orientation parameter increases with increasing TUR, indicating that the higher TUR results in the higher orientation of lamella. Since the temperatures for the appearance of lamellae decrease with the increase of TUR, it is hard to compare the evolution of orientation parameters at different TUR. Thus, the normalized temperature (T′) with value of 1 referred to the highest temperature and 0 referred to the lowest temperature for each TUR is used in Figure 9b. With the same approach in Figure 7d, these curves are also divided into four zones. For TUR-10, -15 and -20, the curves follow the similar change tendency, which may be due to the similar molecular stretching level. For TUR-7 sample, orientation parameter decreases in all zones, while for TUR-25 sample, a nearly constant orientation parameter appears in zone II. This may be due to that the stretching ratio of TUR-7 sample is too low to bring in the increase of orientation in zone II, while for the TUR-25 sample, stretching is already too strong in zone I, and it is hard to further increase the lamellar orientation in zone II. Figures 9c and 9d present the evolutions of crystallinity as a function of crystallization time and temperature for different TUR during film blowing, respectively. Obviously, with the increase of TUR, the curves in Figure 9c shift to shorter times

crystals. Considering that the precursors found in SAXS appear at 51 mm away from the die exit, precursors, crystals, and the frost line appear sequentially as mentioned in Figure 5, which is general for different TUR. Through peak-fitting (as shown in Figure 8b), crystallinity is calculated with eq 7. Since the X-ray transmits two layers of film (front face and back face) as shown in Figure 3 and both sets of WAXS peaks from front and back films are used to estimate crystallinity, the crystallinity calculated by the peak decomposition in this work may be different from the absolute crystallinity. However, the calculated crystallinity can still reflect the contents of crystals to a certain degree, which can be used to compare the crystallization kinetics for different samples. The calculated crystallinity is plotted versus distance and temperature in Figures 8c and 8d, respectively. The crystallinity (χc) follows a monotonic increasing trend from 0 to 23% and does not show obvious inflection at the boundaries between different zones defined above. However, its derivative curve dχc/dD (reflecting the rate of crystallization) does provide clear evidence for the division of the four zones as shown in Figure 8c. As the distance increases to zone II, dχc/dD shows a turn-up, while a transition from increasing to decreasing trends occurs precisely at the boundary between zones II and III (the frost line position). These results indicate that the crystallization rate increases continuously until the end of zone II and reaches the highest value at the frost line position. Corresponding to the decrease of orientation parameter, further increasing the distance in zone III causes a steep reduction of crystallization rate. In zone IV crystallinity increases with a continuous weak reduction of crystallization rate. The evolution of crystallinity as a function of temperature also follows the same tendency as shown in Figure 8d, which, however, is hard to get the derivative curve due to the fluctuation introduced during temperature−distance conversion. The crystallinity of the bubble reaches 32.13% after the sample is completely cooled down. G

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extracted. (i) After extruding out from the die exit, polymer melt is cooled and stretched, which leads to the formation of precursors and crystals in zone I. With the increase of crystallinity, a transition occurs with an accelerated crystallization rate and an increased crystal orientation, which is defined as the boundary between zones I and II. (ii) The frost line position is defined as the boundary between zones II and III, where crystallization rate reaches the maximum and crystal orientation turns from increasing to decreasing trend. In crystallinity−temperature plots, the boundary between zones III and IV is indicated with a sharp slowdown of crystallization rate at all TUR. (iii) The crystallinities (χI−II and χIII−IV) at the boundary between zones I/II and zones III/IV are about 2.5 and 14.5%, respectively, which are not affected by the TUR we studied, while χf at the frost line position (the boundary between zones II and III) varies with TUR. On the basis of these findings, the two key questions about film blowing aforementioned in the introduction will be discussed. Structural Evolution of PE during Film Blowing. Based on the above experimental results, a schematic illustration on the structural evolution of PE during film blowing is proposed in Figure 11, which is divided into four zones according to the

and show faster crystallization kinetics, which can be attributed to the increase of flow intensity and cooling rate. The evolutions of crystallinity with temperature at different TUR are divided into four zones according to Figure 8d. With the increase of TUR, the onset temperature of crystallization decreases, while crystallinities follow almost the same evolution trend with the decrease of temperature for all TUR. As the temperature windows of crystallization vary in a large range for different TUR while the overall shapes of the crystallinity versus temperature curves keep nearly the same, the different evolution trends of crystallinity in different zones should not be mainly caused by the decrease of temperature but be resulted from different chain deformations imposed by flow in different zones. In other words, varying TUR or flow intensity shifts the crystallization temperature window, but temperature does not obviously affect the transitions of different zones under a specific TUR. Thus, the transitions of different zones should be mainly attributed to the different chain deformations in different stages of film blowing. This point will be discussed in the Discussion section. As shown in Figure 9d, except at the boundary between zones II and III (the frost line position), the crystallinities at the boundaries between different zones are almost the same; e.g., the crystallinities are about 2.5% (χI−II) at the boundary between zones I and II and 14.5% (χIII−IV) at the boundary between zones III and IV. Figure 10 summarizes the

Figure 10. Crystallinities at the boundaries between different zones for different TUR. Figure 11. Structural evolution of PE during film blowing.

crystallinities at these boundaries for different TUR. χI−II and χIII−IV are indeed the same for different TUR while the crystallinity at the frost line position (χf) is influenced by TUR. χf decreases from 7.4% to 4.9% with the increase of TUR from 7 to 20 and then increases to 6.5% for TUR-25 sample. The crystallinities at the transition points of different zones give two interesting indications. (i) TUR has no effect on χI−II and χIII−IV, suggesting that crystallinities at these transition points are not determined by processing parameters. Instead, χI−II and χIII−IV may be controlled by the material itself, which reflect the intrinsic molecular properties of the material. (ii) χf is affected by TUR, which is a processing controlled structural parameter. Indeed, the frost line position is a key parameter to control the processing stability and the properties of the final product in industrial film blowing, during which χf may be tuned by industrial engineers.

structural behaviors. In zone I, the chain network is gradually stretched to induce precursors and crystals, which further leads to the establishment of a crystal-cross-linked network spanning the entire sample. In zone II, the deformation of crystal-crosslinked network causes the self-acceleration of crystallization, which enhances the modulus of bubble and the strength of crystal-cross-linked network. Once the strength of crystal-crosslinked network is in balance with the stretching force, a nondeformable crystal scaffold forms, and the frost line emerges (at the boundary between zones II and III). In zones III and IV, because of the nondeformable nature of crystal scaffold, the scaffold and the entire sample are gradually filled up by crystals accompanied by the slowdown of crystallization and the decrease of lamellar orientation. In zone I, the chain network is gradually stretched due to the stretching force imposed by the taking up roller. According to the entropic reduction model (ERM), stretching of chain network reduces the entropy of polymer melt and lowers nucleation barrier, which equivalently increases equilibrium melting temperature and accelerates crystallization rate in zone I as shown in Figure 8c. Besides, the cooling of the bubble by

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DISCUSSION Combining the results of in situ SAXS and WAXS measurements during film blowing of PE, four zones with specific structural features are defined from the die exit to the position above the frost line, and some interesting findings can be H

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keep constant for different TUR, which is an unexpected result. As TUR varies from 7 to 25, the temperatures at the boundary between zones I/II decrease from 65 to 48 °C, which cover a large temperature range. An intuitive thought is that χI−II is a molecule controlled rather than a processing controlled parameter, as variations of processing parameters including flow field and temperature do not affect the value of χI−II. (We have also done the film blowing experiment with different BUR. The χI−II and χIII−IV are also 2.5 and 14.5%, respectively, as shown in Figure S3.) If we consider that the crystallinity of 2.5% is sufficient to build a crystal-cross-linked network spanning the entire sample, the high nucleation rate with the homogeneous distribution is required. This requirement is fortunately satisfied during the blowing process of PE film. The temperature window from 65 to 48 °C is around the maximum crystallization temperature of PE (about 50 °C), and the nucleation rates are also high around these temperatures. Taking into account the flow-induced nucleation effect, high nucleation rate does happen during film blowing of PE. Note that no shish steak sign appears in SAXS patterns, indicating that the distribution of nuclei is rather homogeneous in the entire sample. If high nucleation rate and homogeneous distribution occur in all TUR conditions, the value of χI−II would not be affected by processing parameters significantly, which is mainly controlled by molecular parameters of material. As reported in previous works,90 the length of chains and the degree of branching affect the relaxation of polymer chains and the generation of entanglement network, which play important roles in determining the nucleation kinetics during flow. Since high nucleation kinetics promotes the generation of crystalcross-linked network as indicated before, the molecular parameters (the length of chains and degree of branching) are proposed to affect the value of χI−II. We also investigate another pure PE with different molecular parameters (the total amount of branching is 2.30 mol %). For that material, the χI−II also remains unchanged with varying processing parameters, though the value is different from that in present work. This indicates that the independence of χI−II on processing parameters is available for all polyethylenes and the value of χI−II is indeed determined by the molecular characteristics. Thus, with in situ SAXS/WAXS measurements, following the values of χI−II may provide an indicator to track the processing and the final properties of PE film materials. The constant χIII−IV at different TUR is also controlled by material parameter, which may be related to the χI−II. As the crystal-cross-linked network with χI−II already determines the overall structure of the later formed scaffold, the constant χI−II naturally leads to constant χIII−IV at different TUR, if we consider that the crystallization in zone III is a spatial filling process of the scaffold. Although the variation of TUR does not affect the values of χI−II and χIII−IV, it does influence the crystallization rate. Larger TUR leads to faster crystallization, which is attributed to both strong flow field and fast cooling rate. As discussed above, the effect of flow field on crystallization is more pronounced in zone II after the formation of crystal-linked network, which can largely amplify the effect of external flow field and result in a sharp increase of crystallization rate. The structural evolution in zone II is very similar to that in the processing of biaxially stretched film, where the deformation is imposed on a semisolid polymer rather than polymer melt.91,92 Considering the super mechanical and optical properties of biaxially stretched film, fine-tuning and controlling the structural

air ring also promotes the crystallization rate. As mentioned above, a constant crystallinity of 2.5% (χI−II) at the boundary between zones I and II is observed for all TUR, at which we speculate that the crystal-cross-linked network is formed. According to the literature, during isothermal crystallization of PE and iPP, the dynamic moduli show abrupt increases at the early stage of crystallization with a crystallinity of about 2.0%, which is attributed to the formation of crystal-crosslinked gel network spanning the entire sample.56−61 This critical crystallinity (2.0%) is almost on the same level as that at the boundary between zones I and II (χI−II, 2.5%) in the present work, which implies that a crystal-cross-linked network with critical crystallinity of 2.5% may also form here. As the blowing process goes into zone II, the results that both crystallization rate and crystal orientation increase fast also support the speculation of the formation of crystal-cross-linked network. Since the frost line is observed at the end of zone II, stretching is still imposed on the film in zone II after the formation of crystal-cross-linked network, which leads to the further deformation of crystal-cross-linked network and the selfacceleration of crystallization.11 Based on the dimensionless parameter Weissenberg number Wi = ε̇τd > 1, which can be employed as a qualitative criterion for FIC,82−85 the slower relaxation time due to the formation of crystal-cross-linked network and the fast cooling rate of bubble cause a stronger flow field imposed on the chains. Thus, the crystal-cross-linked chains prefer to be oriented or stretched, becoming more prone to form crystals due to lower nucleation barrier. As a result, the crystallization rate and the lamellar orientation increase fast in zone II. The stretched chains between crystals in Figure 11 are sketched to demonstrate this effect. A similar physical picture was suggested before by Zuidema, Cui, and Seki.86,87 The influence of cross-link on the rheological relaxation time is emphasized in Zuidema’s work,86 while the deformation of physical network and ghost nucleation mechanism are proposed to be the reasons of accelerated crystallization in Cui’s work.11 On the basis of the self-acceleration of crystallization, a large amount of crystals and oriented chains form in zone II. Considering the linear relationship between the logarithm of modulus and crystallinity,70,88,89 increasing the volume fraction of crystals will enhance the strength of crystal-cross-linked network and the bubble modulus. Once the balance between the stretching force and the crystal-cross-linked network strength is established at the crystallinity of 5.3% for TUR-15 (χf), no further deformation occurs to the bubble, resulting in the formation of a nondeformable crystal scaffold to stabilize the bubble at the frost line position. Here the nondeformable crystal scaffold is essentially a crystal-cross-linked network with high modulus, which is too strong to be further deformed. The decrease of crystal orientation, slowdown of crystallization rate (see Figure 9), and constant bubble width (see Figure 2) also indicate that the crystal scaffold cannot be further deformed above the frost line position (zones III and IV). Note that even though no further deformation is imposed on polymer chains in zone III, the oriented crystal scaffold and polymer chains can still result in a high crystallization rate though it gradually slows down with the reduced crystal orientation. We speculate that spatial filling processes of the scaffold and the entire sample happen in zones III and IV, respectively. Effects of TUR on the Formation of Crystal-CrossLinked Network. The crystallinities at the boundaries between zones I/II (χI−II, 2.5%) and III/IV (χIII−IV, 14.5%) I

DOI: 10.1021/acs.macromol.8b00346 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

independent of processing parameters and is determined by molecular parameters of the PE material. The crystallinity at the frost line position χf decreases with the increase of TUR from 7 to 20, which is affected by both molecular and processing parameters, as the external force is balanced with the elastic force sustained by the nondeformable crystal scaffold at the frost line position. The deformation behavior of crystal-crosslinked network in zone II is similar to that in the biaxially stretched film where the semisolid sample rather than melt is deformed. Fine tuning the structural evolution in zone II may enhance the mechanical and optical properties of blown films. This work supplies a guidance for improving the stability of bubble and the properties of products.

evolution in zone II may be an effective approach to improve the properties of blown film. Effects of TUR on the Formation of Crystal Scaffold. Different from χI−II and χIII−IV, which mainly come from the molecular parameters of PE material, the crystallinity at the frost line (χf) increases with the increase of TUR from 7 to 20, which is not only controlled by molecular parameters but also affected by processing parameters. The frost line defines the formation of nondeformable crystal scaffold, where the external force is balanced with the elastic force sustained by the crystal scaffold and no further deformation occurs anymore. The elastic force sustained by the crystal scaffold is determined by its modulus and elastic strain. As increasing TUR increases both strain rate and strain, it is reasonable that larger elastic strain exists at larger TUR. Correspondingly, a lower χf can be sufficient to sustain the external force and prevent from further deformation. On the other hand, increasing TUR enhances crystallization kinetics or nucleation rate, which may generate more crystallites even with the same total crystallinity. In other words, the higher nucleation rate at the larger TUR creates more crystal-cross-linking points or the higher cross-linking density, which results in the crystal scaffold with higher modulus. Combining these two points related to elastic strain and modulus of the crystal scaffold, it is possible to understand the reduction of χf with the increase of TUR from 7 to 20 qualitatively. However, as the TUR increases to 25, χf increases to 6.5%. This may be due to that the stretching force or strain imposed on PE film is so high at TUR 25 that the crystal-crosslinked network is disrupted61 or inhomogeneous structures (e.g., rodlike nuclei) are generated,35 which leads to the occurrence of stress concentration and makes the bubble unstable. As a result, more crystals are required to stabilize the bubble, causing the increase of χf. Indeed, we also performed the experiments with TUR of 30 and found that the bubble is not stable anymore. This suggests that TUR of 25 is already close to the unstable region of film blowing at current conditions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00346. Flash DSC cooling curve of the blend, the results of the particle tracing experiments, and the effects of BUR on the formation of crystal-cross-linked network (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.J.). *E-mail: [email protected] (L.L.). ORCID

Liangbin Li: 0000-0002-1887-9856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China (2016YFB0302500) and the National Natural Science Foundation of China (51633009 and 51703217). The experiments are carried out in Shanghai Synchrotron Radiation Facility (SSRF).

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CONCLUSION We combine a homemade film blowing machine and the simultaneous in situ synchrotron radiation WAXS and SAXS measurements to investigate structural evolution of PE during film blowing. On the basis of the WAXS and SAXS results, the structural evolution is defined into four zones with different features. In zone I, the molecular entanglement network is cooled and extended simultaneously under flow field, resulting in the formation of precursors and crystals. When enough crystals form at the boundary between zones I and II, a deformable crystal-cross-linked network is established, which determines the structures and the properties of products. In zone II, flow further deforms the crystal-cross-linked network, which accelerates crystallization greatly because of the stronger effective stretching of chains and the corresponding lower nucleation barrier. With the further increase of crystallinity in zone II, the crystal-cross-linked network gradually evolves into a nondeformable crystal scaffold with sufficient strength to stabilize the bubble at the frost line position (the boundary between zones II and III). In zones III and IV, the scaffold and entire sample are gradually filled up by crystals with a continuous reduction of crystallization rate. Interestingly, our results demonstrate that increasing TUR does not influence the critical crystallinity (χI−II, 2.5%) for the formation of deformable crystal-cross-linked network, which seems to be

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REFERENCES

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DOI: 10.1021/acs.macromol.8b00346 Macromolecules XXXX, XXX, XXX−XXX