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Recrystallization of biaxially oriented polyethylene film from partially melted state within crystallite networks
Polymer 191 (2020) 122291
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Recrystallization of biaxially oriented polyethylene film from partially melted state within crystallite networks Minqiao Ren , Yujing Tang , Dali Gao , Yueming Ren , Xuerong Yao , Hongwei Shi , Taoyi Zhang , Changjiang Wu * SINOPEC Beijing Research Institute of Chemical Industry, No. 14 Beisanhuan Donglu, Chao Yang District, Beijing, 100013, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Partially melted state Orientation Recrystallization Biaxially oriented polyethylene (BOPE) film
Recrystallization behavior and the final crystallite orientation of biaxially oriented polyethylene (BOPE) film from different melted states have been studied by using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. WAXD result shows that two kinds of crystallites (1 and 2) with different orientations are aligned around the transverse direction (TD) of the film plane. When BOPE film is annealed in the temperature range of 110~125 � C, the residual crystallites 1 are still aligned along the TD, while the residual crystallites 2 are gradually rotated and aligned along the machine direction (MD). Recrystallization of BOPE film from the partially melted states is enhanced and the maximum crystallization peak shifts to higher temperature by 7 � C. The newly formed crystallites after recrystallization are aligned along the TD and MD, respectively. When annealing temperature attains to 128 � C, a trace amount of residual crystallites 1 persists along the TD. The newly formed crystallites after recrystallization are also aligned the TD. When the storing temperature range is in 130–150 � C, the melt memory effect occurs in BOPE film, and it influences crystallization kinetics and the crystallite orientation.
1. Introduction Biaxial stretching of polymer sheets is a common processing tech nology for the production of products in packaging films [1,2]. Semi crystalline polyolefin films represent a large segment of the plastics industry. The most widely used biaxial processes for films are blowing process for various polyethylene (PE) [3–5], double-bubble process for polypropylene (PP) [6] and linear low-density PE (LLDPE) [7], and cast film biaxial orientation or tentering for high-density PE (HDPE) [8] and PP [9]. All these stretching processes impart certain degree of orienta tion to polyolefins, which enhances their mechanical, impact, barrier, and optical properties. Biaxial orientation has the advantage of allowing these enhancements in both machine direction (MD) and transverse direction (TD). Conventional LLDPE resins are used in both blowing and doublebubble processes, while these resins are seldom used in tenter process through PP line due to their poor stretchability [10–12]. However, compared to double-bubble process, LLDPE films produced by tenter process exhibit many advantages such as high puncture strength and shrink force in both MD and TD [12]. So specific LLDPE resins for biaxial
orientation are needed to further develop and improve. Recently, Dow has developed PE resins which could be stretched with stretching ratio 5 in MD and 9 in TD in a wide temperature window [13,14]. The biaxially oriented polyethylene (BOPE) films via the commercial scale tenter frame line exhibit higher modulus, higher dart and puncture impact strength, easier tear, and better optical properties than the blown films. SINOPEC has also developed the high speed BOPE resins which have excellent processing properties by tenter processing. The BOPE films obtained from these resins have high tensile strength, good trans parency, high puncture and impact resistance. These BOPE films can be used for agricultural film, multi-layer film, frozen food packaging and other special applications [15–17]. Though tenter process BOPE films have been developed successfully, there are only a few reports published in the literature. Wilkes et al. discussed the tentering process of HDPE [8]. Their studies focused on the change in the crystalline morphology from spherulitic lamellae into lamellar stacks that resulted from the MD stretching at temperatures near the HDPE melting temperature. Owing to the fact that the overall stretching process is done sequentially, the information provided the basis from which the TD stretch was undertaken, but no TD stretching
* Corresponding author. E-mail address: [email protected] (C. Wu). https://doi.org/10.1016/j.polymer.2020.122291 Received 9 December 2019; Received in revised form 10 February 2020; Accepted 13 February 2020 Available online 14 February 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.
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data were presented. Ajji et al. investigated the biaxial stretchability, orientation structure, and shrinkage of LLDPEs biaxially stretched by using a laboratory biaxial stretcher [12]. The results indicated that the high molecular weight tail and comonomer content played important roles in orientation of the BOPE films. Chen et al. found that only LLDPE which increased the lateral connection structure was beneficial for biaxial stretching [18]. Li et al. studied the crystal structure evolution of PE gel films containing paraffin oil during sequentially biaxial stretching at the temperature around the melting point [19]. They found that the morphologies before TD stretching were mainly composed of fibrillary crystals along MD with a small amount of oriented lamellar stacks. During subsequent TD stretching, the lamellar stacks vanished gradually together with the tilting of fibril along TD. Finally, formation of fibril lary crystal along TD was observed, forming an interconnected network with that in MD. During tenter frame process, crystallite orientation occurs in semi-solid state and has a significant effect on the film prop erties. For the BOPE films with sequential stretching process, the results showed that the c-axis of crystallites was oriented mainly along the last processing direction (TD) in the film plane, while a-axis or b-axis was strongly oriented along the ND of the film [20]. We studied the orien tation of BOPE film produced by the Karo IV unit from Brückner in the laboratory with stretching ratio of 4 in MD and 5 in TD, and found that the c-axis orientation of crystalites in BOPE film was along TD. In addition, there was a small fraction of crystallites with c-axis randomly distributed within the MD-TD plane [16]. Ajji et al. investigated the biaxial orientation of BOPE film produced by Brückner biaxial stretcher with simultaneous equibiaxially stretching process and found the different orientation of crystallites in MD and TD [21]. Sealing of polyolefin films by application of heat is widely used in the packaging industry to join film. The seal strength is related to the structure formed during sealing process [22–24]. The film ranges from partially to fully melted states when they are heat sealed. As most of the crystallites are formed during subsequent cooling process, and the final properties of film are determined by the morphology and structure formed from this process. So the study of oriented recrystallization from different molten states in polyolefin films is very important. Owing to the change of the temperature, the reorientation of the residual crys tallites under the two-dimensional (2D) residual stress conditions will occur, which will influence the subsequent crystallization behavior. Up to date, the oriented recrystallization has been studied mainly in uni axially oriented PE samples. For example, Zheng et al. studied the crystallization kinetics of partially melting extruded HDPE sheets [25]. An enhanced crystallization and an increment in tensile strength was achieved in the sheets extruded, which was mainly attributed to the self-nucleating effect with increased crystallinity, long period, and lamellar thickness. Stribeck et al. studied the oriented quiescent crys tallization of injection molded PE rods by ultra small-angle X-ray scat tering [26]. They found that the orientation of the crystallites could be controlled by choice of the melt annealing temperature. A three-stage model of crystallization was developed. This model comprised row structure nucleation, the almost statistical insertion of extended lamellae, and finally the insertion of blocky crystallites. Vasileko et al. studied the PE monofilaments crystallized in matrix from a partially oriented melt containing a highly oriented crystalline phase [27]. Electron microscopic examination showed that the structure of speci mens formed by such crystallization was heterogeneous and contained differing long periods. Humbert et al. studied memory effect of the molecular topology of lamellar PE on the strain-induced fibrillar struc ture and found that the diameter of the fibrils kept the memory of the chain topology to which a major role was ascribed in the strain-induced fragmentation of the crystalline lamellae [28]. Though the oriented recrystallization has been studied in uniaxially oriented PE samples, the oriented recrystallization in BOPE film has not been investigated. In this work, the differential scanning calorimetry (DSC) and wideangle X-ray diffraction (WAXD) methods have been used to investigate the recrystallization of BOPE film with different thermal conditions. The
Fig. 1. Pole figure scheme of multiple scans for PE (200) crystal plane: circle A: 2θ ¼ 24� , tilt angle ¼ 20� , 360� rotation; circle B: 2θ ¼ 24� , tilt angle ¼ 65� , 360� rotation.
aim is to study the effect of original crystallites orientation and annealing on the course of the recrystallization processes of BOPE film from partial melt within complex crystallite network or complete melt. The crystallites structure and morphology formed in biaxial orientation condition can provide the guidance for choosing the optimal processing window of BOPE film during heat sealing with desired structure and properties in industry. 2. Experiment 2.1. Materials The PE resin for biaxial stretching was provided by Sinopec. The melt index (MI) is 1.7 g/10 min (190 � C/2.16 kg), and the density is 0.918 g/ cm3. The weight-average molecular weight is 110,000 with a poly dispersity index of 4.47 obtained by gel permeation chromatography (GPC). BOPE film was produced in a Brückner tenter-frame line of sequen tial stretching with the width of 4.6 m and the manufacture speed of 100 m/min. The stretching ratio in MD was 4 and the stretching ratio in TD was 8, respectively. The final BOPE film was 50 μm in thickness. 2.2. Methods and techniques The melting and crystallization behavior of BOPE film were followed by DSC using a TA Q100 instrument. The temperatures and heat flows were calibrated using material standards at a cooling and heating rate of � 10 C/min. The sample was heated to a pre-set storing temperature � (denoted as Ts) for 5 min and then cooled to 25 � C at rate of 10 C/min. � After that, the sample was heated again at rate of 10 C/min. WAXD experiment was carried out on the Bruker D8 Discover X-ray diffractometer. The X-ray was generated using IμS microfocus X-ray source incorporating a 50 W sealed-tube X-ray generator with Cu target. The wavelength of X-ray was 0.1542 nm. The power of generator used for measurement was 45 kV and 900 μA. The X-ray intensity was recorded on VÅNTEC-500 2D detector with a pixel size of 136 � 136 μm2. The distance from the sample to detector was 98.6 mm for trans mission mode and 198 mm for reflection mode. The spot size of the beam 2
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crystallites are annealed and under metastable states. So the melting peak temperatures of crystalline polymers represent the melting points of metastable polymers crystals which have broad melting region [33, 34]. Fast enough heating could suppress crystal annealing during the heating process [35]. However, with the increase of heating rate, the melting peaks shift to higher temperature, as shown in Fig. 2. The result is different to that observed in the presence of annealing and recrys tallization effects, which should involve a shift of melting peak to higher temperatures with decreasing heating rate [33]. So the observed dif ference between DSC melting curves at different heating rate is not due to annealing and recrystallization effect and could be attributed to the thermal lag which is in fact a general problem observed when using the traditional calorimeters [36,37]. In order to gain information about the crystalline structure and orientation of BOPE film, the WAXD and SAXS experiments are con ducted in transmission mode. X-rays pass along the normal direction (ND) of BOPE film. The 2D WAXD image of BOPE film is shown in Fig. 3 (a). The heterogeneous (110) crystal plane diffraction ring is observed in BOPE film. The strongly intense meridianal diffraction arcs are coupled with moderate intensity diffraction arcs of (110) crystal planes which are deviated from the meridianal direction of ca. 48� . This indicates that two kinds of crystallites with different orientations exist in BOPE film. The diffraction intensity of (110) crystal plane in equatorial direction is weak. For quantitative analysis, 2D WAXD images are reduced to onedimensional (1D) WAXD intensity profiles over the azimuthal angle range of 0–360� as a function of 2θ, as shown in Fig. 3(b). The strong diffraction intensity of (110) crystal plane shows at ca. 21.4� and a very weak diffraction intensity of (200) crystal plane shows at ca. 24� . The diffraction of (200) crystal plane almost disappears when the WAXD experiments is conducted in transmission mode. This means that the orientation of a-axis is strongly along the ND of BOPE film. This is also reported in other BOPE films [16,17,20]. Thus the strong orientation of a-axis along the ND confines the b-axis and c-axis of crystallites to the film plane. The corresponding azimuthal intensity distribution curves of (110) and (200) crystal planes are displayed in Fig. 3(c), where the azimuthal angle of 180� represents the meridianal direction. The intense and defined diffractions of (110) and (200) crystal planes of BOPE film at the maximum both located on the meridian, which indicates that lamellae are not twisted and belong to Keller-Machin typeIIwhich is formed under high stress orientation [38]. From Fig. 3(c), two kinds of crystallites (1 and 2) with different orientation account for 37% and 63% of all the crystallites, respectively. As the weak (200) crystal planes appear on the meridian, it indicates that besides some crystallites with a-axis are along the ND, there are still some crystallites with a-axis along the MD in MD-TD plane. This is also confirmed by the X-ray pole figure of (200) crystal plane of BOPE film shown in Fig. 4. The strong orien tation of a-axis along the ND plane confines the b-axis and c-axis to the film plane, (110) and (200) crystal planes are mainly located on the meridian, so b-axis of the two kinds of crystallites are both oriented mainly along MD, while c-axis which represents the direction of the molecular chain is along the last processing direction (TD) of the film plane. There are several types of small-scale laboratory pieces of equipment which have been used as a screening tool to establish initial stretching conditions for additional scale-up investigation of biaxial stretching processing on larger equipment. However, the remaining challenge concerning the laboratory studies mainly lie in their correla tion with larger-scale tenter frame. For example, we have studied the orientation of BOPE produced by the Karo IV unit from Brückner in the laboratory and found that only one kind of crystallite orientation exists [17]. The main difference of orientation between laboratory and com mercial BOPE film may be attributed to the certain processing param eters or the design difference for commercial sized equipment and the small scale laboratory evaluation. For example, owing to high strains rate used in commercial tenter equipment, it is not clear how to precisely use the laboratory experiments to help define commercial processes. In addition, the small scale laboratory tests are in the intermittent mode of
Fig. 2. DSC melting curves of BOPE film with different heating rates.
was 0.5 mm. The exposure time was 5 min. The in-situ experiment was conducted on the Linkam FTIR600 hot stage. The BOPE film was wrapped up with aluminum foil and heated to a pre-set storing tem � perature Ts at the rate of 10 C/min for 2 min before WAXD image was � collected. After that, the sample was cooled to 25 � C at rate of 10 C/min, and WAXD image was collected accordingly. The temperature of sample in chamber was calibrated by using the thermocouple. In order to have the optimum pole figure coverage, a data collection strategy was laid out first [29]. Fig. 1 shows the pole figure scheme of multiple scans for PE (200) crystal plane which was generated by GADDS software. By using this scheme, we got reasonable data point on the pole figure that covered about 70� of the stereographic projection. After the raw data was collected, the pole figure was obtained by using Multex software. Small-angle X-ray scattering (SAXS) experiment was carried out on the Xenocs XEUSS SAXS instrument. The wavelength of X-ray was 0.1542 nm. The power of generator used for measurement was 50 kV and 600 μA. The X-ray intensity was recorded on a 2D detector with a pixel size of 172 � 172 μm2. The distance from the sample to detector was 1161 mm. In order to obtain the relatively strong SAXS intensity, multi-layer BOPE film stack with the whole thickness of ca. 400 μm were used to test. The exposure time was 20 min. Temperature rising elution fractionation (TREF) was carried out in a TREF300 instrument commercialized by Polymer Char (Spain) using 1,2,4-trimethylbenzene as the solvent. The crystallization rate was 0.1 � C/min from the stabilization temperature of 95 � C to 35 � C, and the subsequent solvent elution rate was 0.5 mL/min from 35 � C to 130 � C at � a rate of 1 C/min. 3. Results and discussion 3.1. Thermal behavior and crystallite orientation of BOPE film A set of normalized DSC melting curves of BOPE film at different � heating rates (10–80 C/min) are shown in Fig. 2. The melting of BOPE film occurred within a temperature range of ca. 40–130 � C yields broad melting curves. This may be attributed to a broad thickness distribution of lamellae formed during stretching and subsequent recrystallization in processing process [30]. The unstable melting peak at ca. 50 � C for BOPE film may be caused by some ordered structure formed during the stretching process [31]. The melting peak temperature of BOPE film � with 10 C/min heating rate is ca. 124 � C. The degree of crystallinity of BOPE film is ca. 44% if the value of equilibrium fusion enthalpy of PE is chosen 293 J/g [32]. It is worthy to note that the DSC heating curves does not represent the original melting of crystalline polymers because during the heating, the molecules are continuously moving, the 3
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(a)
(b)
(c) Fig. 3. WAXD of BOPE film: (a) 2D WAXD image, (b) 1D WAXD intensity profile as a function of 2θ, and (c) azimuthal intensity distribution of (110) and (200) crystal planes. 4
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Fig. 4. X-ray pole figure of (200) crystal plane of BOPE film.
production, while commercial process lines are in the continuous mode of production. 2D SAXS image of BOPE film is shown in Fig. 5(a). The better defined scattering in TD implies that much better lamellar packing is formed in TD. The integrated 1D SAXS profiles obtained by averaging the scat tering intensity within 10� on both TD and MD are shown in Fig. 5(b). The scattering profiles show well-defined scattering peaks in BOPE film. This indicates that well-organized lamellar packing structures are formed in BOPE film. An important structural parameter on the lamellar stacks is the average long period, meaning the average total thickness of crystalline region and amorphous region or the average inter-distance of neighboring crystalline lamellae. For BOPE film, the calculated long periods in TD and MD are 22.7 nm and 21.8 nm, respectively. The long period in TD is slightly greater than that in MD in BOPE film. The broad distribution of the lamellae formed during stretching and subsequent recrystallization in both TD and MD correlates with the broad melting curves of BOPE film shown in Fig. 2. The azimuthal scan of long period of lamellae in BOPE film is shown in Fig. 5(c). The majority of lamellar stacks are along TD, while a minority of lamellar stacks are along MD. As the normal direction of the lamellar stack approximately represents molecular chain direction (c-axis), so the PE molecular chains are preferably oriented along the TD. This is in accordance with that ob tained from WAXD results.
(a)
(b)
3.2. Recrystallization behavior of BOPE film from different melted states What’s the recrystallization behavior of BOPE film from partially or fully melted states? In order to address this question, BOPE film is heated to different storing temperature Ts for 5 min. Different Ts are chosen based on the first DSC melting curve of BOPE film in Fig. 2. The cooling and subsequent melting curves of the BOPE film after storing at different Ts are shown in Fig. 6. The thermal parameters such as crystallization peak temperature (Tc,peak), crystallization enthalpy (ΔHc), melting peak temperature (Tm), fusion enthalpy (ΔHf), and the degree of crystallinity (Wc,h) of BOPE are shown in Table 1. The Tc,peak as a function of Ts is plotted and shown in Fig. 7. Three domains are used to describe the different recrystallization behaviors of BOPE film according to selfnucleation theory [39]. In Domain I(complete melting domain), the PE melts completely after thermal conditioning at Ts. The chains adopt the random coil conformation characteristic of the homogenous melt. This state is found to persist down to 165 � C, because no difference is detected in Tc,peak. The crystallization peak position shows the lowest value at ca. 111 � C, indicating recrystallization occurs from homogeneous melt. In Domain II (melt-memory and liquid-liquid phase separation (LLPS) domain), the melt is heterogeneous and retains some residual
(c) Fig. 5. SAXS of BOPE film: (a) 2D SAXS image, (b) 1D SAXS profiles, and (c) azimuthal intensity distribution of long period of lamellae.
5
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T (
)
T
T T
T
T T
T Fig. 7. Variation of the crystallization peak temperature (Tc,peak) as a function of the storing temperature (Ts) for BOPE film.
(a)
T (
)
Fig. 8. TREF profile of BOPE film.
segmental orientation. Self-nuclei remain in the melt as a memory of the sequence selection and self-assembly of long ethylene sequences during the initial crystallization path. The surviving self-nuclei are associated with molten ethylene sequences that remain in close proximity and are unable to diffuse quickly to the randomized melt state. A complex to pology of knots, loops, ties and other types of entanglements builds up in the interlamellar region of random copolymers, especially at high levels of transformation. Upon melting, diffusion of crystalline sequences through this complex topology back to the randomized melt requires temperatures well above the equilibrium melting point. Recrystalliza tion is accelerated in this temperature range of heterogeneous melts [40]. So thermal conditioning at 163 � C is able to self-nucleate PE, as Tc, peak shifts to higher temperatures. However, an unexpected retraction of Tc,peak with further decreasing Ts below 155 � C, which can be explained by the interplay between the strong memory of melt and LLPS [41,42]. As the commercial LLDPEs often broadly distributed in molar mass and inter-chain comonomer content, different fractions are not always miscible as a single phase. The miscibility of the different fractions is critically dependent on molecular characteristics such as molecular weight, short-chain branch concentration, branch type, composition, and processing conditions although some of the results reported in those studies are still being debated. It is known from early works that PE blends with a difference in comonomer content >8 mol% phase separate in the melt [43–47]. Most PE blends exhibit an upper critical solution
(b) Fig. 6. DSC curves of BOPE film during (a) recrystallization from different Ts, � and (b) subsequent melting (both the heating and cooling rate is 10 C/min). Table 1 Thermal parameters of BOPE film recrystallized from different Ts. Ts (oC)
Tc,peak (oC)
ΔHc(J/g)
Tm (oC)
ΔHf (J/g)
Wc,h (%)
First melting 110 120 125 128 130 135 140 145 150 155 160 163 165 170 180 200
– – 115.3 117.9 112.2 111.9 112.8 112.7 113.5 113.2 113.4 112.4 112.0 110.8 111.0 110.6 110.8
– – 40.8 107.9 111.1 111.8 112.6 109.4 113.6 108.6 111.3 102.6 112.0 109.5 110.0 111.3 112.0
123.9 124.0 124.5 125.8 124.7 125.1 124.5 125.3 124.6 125.1 124.1 124.5 124.2 124.8 123.7 124.6 124.2
129.3 129.3 136.5 108.1 90.4 104.9 111.4 105.8 110.3 104.3 108.4 106.6 111.0 101.5 106.8 106.6 111.3
44.1 44.1 46.6 36.9 30.9 35.8 38.0 36.1 37.6 35.6 37.0 36.4 37.9 34.6 36.5 36.4 38.0
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T
Ts: 110 oC
Ts: 120 oC
25 oC
25 oC
(a) T
Ts: 125 oC
25 oC
Ts: 130 oC
25 oC
Figure 9. 2D WAXD images of BOPE film annealed at different storing tem perature Ts and then cooled down to 25 � C.
(b)
temperature (UCST) at ca. 150 � C, which is in the temperature range of Domain II in Fig. 7. The difference in inter-chain comonomer content of BOPE film is more clearly resolved by the TREF profile in Fig. 8. It can be seen that about 50% of the copolymer molecules elute in a temperature range between 30 and ~90 � C, while the other 50% elutes in a very narrower range, between ~90 and 104 � C. So this large difference in comonomer content in BOPE could lead to LLPS. When the crystallites of BOPE film are brought to the temperature below the UCST, self-seeds do not survive as the strong thermodynamic drive for LLPS dissolves these seeds. So the recrystallization rate decreases in this region because the drive to phase separation is stronger. In Domain III (self-nucleation and annealing), small crystal frag ments survive melting and constitute the self-seeds (Ts below 130 � C). Partial melting is produced and the unmelted crystals can experience annealing during the 5 min thermal conditioning at Ts. When Ts is 125 � C, a broad crystallization peak appears at ca. 118 � C in BOPE film. Tc, � peak shifts to higher temperature by 7 C. This indicates that the residual crystallites can enhance the recrystallization rate of BOPE film. The subsequent melting endotherm shows a small high-temperature sharp peak, which is due to the melting of the annealed crystallites. When Ts is at 110 � C, there is almost no exothermal peak, indicating that the
Fig. 10. 1D WAXD intensity profiles of BOPE film annealed at different storing temperature Ts (a), and then cooled down to 25 � C (b).
annealing effect is dominant in this condition. 3.3. Crystallite orientation of BOPE film after recrystallization from different melted states In accordance with the DSC experiments, the BOPE film is heated to different Ts, and then cooled at 25 � C and these processes are observed in-situ by WAXD. Some representative 2D WAXD images of BOPE film annealed at different Ts and then cooled down to 25 � C are shown in Fig. 9, respectively. With the increase of Ts, the intensity of the diffraction arcs of BOPE film becomes weak while the amorphous halos show up clearly. This means that the crystallites melt gradually. The crystallites in BOPE film reach the complete melt at ca. 130 � C. The orientation of the residual crystallites also changes gradually with in crease of Ts. However, when recrystallization occurs within existing oriented crystallite network, the new crystallites almost grow along the orientation direction of the residual crystallites. 7
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Table 2 The degree of crystallinity of BOPE film annealed at different storing tempera ture Ts and then cooled down to 25 � C. Ts (oC)
Wc,x (%) (at Ts)
Wc,x (%) (at 25 � C)
Original film 110 120 125 128 130 140 150 160
41.4 16.2 9.3 1.1 0.4 0 0 0 0
41.4 34.1 34.2 31.7 32.6 33.0 32.0 31.7 31.4
T
(a)
T
Fig. 11. X-ray pole figure of (200) crystal plane of BOPE film annealed at 125 � C for 5 min and then cooled down to 25 � C.
For quantitative analysis, 2D WAXD images are reduced to 1D WAXD intensity profiles over the azimuthal angle range of 0–360� as a function of 2θ, as shown in Fig. 10(a) and 10(b), respectively. The degrees of crystallinity (Wc,x) obtained by WAXD method are also summarized in Table 2. With the increase of Ts, the residual degree of crystallinity at Ts decreases until all crystallites in BOPE film completely melts at ca. 130 � C. During the recrystallization of BOPE film, the degrees of crystallinity of BOPE film varies with the change of Ts, with the maximum value occurs when Ts is 120 � C. As the residual crystallites are annealed before the new crystallites are formed in the network of the residual crystallite skeleton, the whole degree of crystallinity of BOPE attains to the maximum value when Ts is 120 � C. Compared with the original BOPE film, the degree of crystallinity obtained from recrystallization of BOPE decreases. In addition, the diffraction intensity of (200) crystal plane increases during recrystallization of BOPE. This means that a-axis is no long strongly oriented along the film thickness or the orientation degree becomes weak under relatively relaxed stress conditions during the recrystallization process. This is confirmed by the X-ray pole figure of (200) crystal plane of BOPE film after recrystallization, as shown in Fig. 11. It can be seen the (200) crystal plane is randomly distributed in the MD-TD plane of BOPE film. The azimuthal intensity distribution of (110) diffraction plane of original BOPE film and BOPE film annealed at different Ts (110–128 � C) and then cooled to 25 � C are shown in Fig. 12(a) and 12(b) respectively. The azimuthal angle of 180� represents the meridianal direction. In Ts range of 110–125 � C, the BOPE film is in the partially melted state. With the increase of Ts, residual crystallites 1 are still along the TD, while the residual crystallites 2 are gradually rotated and aligned along the MD. This may be related to change of the complex 2D residual stress distri bution during the partial melting process. When BOPE film is cooled
(b) Fig. 12. Azimuthal intensity distribution of (110) crystal plane of BOPE film at different Ts (110–128 � C) (a), and then cooled down to 25 � C (b).
from this interconnected crystallite network, it still exhibits two kinds of crystallites (1 and 2) with c-axis orientation of crystallites 1 and 2 lie along the TD and MD, respectively. However, crystallites 2 are pre dominant in this case. When Ts attains to 128 � C, a trace amount of residual crystallites 1 still persist along TD. The newly formed crystal lites after recrystallization are also aligned along the orientation direc tion of residual crystallites 1. These results indicate that the orientation of the newly formed crystallites lies along the orientation direction of the residual crystallites when recrystallization of BOPE film from the partially melted states. The azimuthal intensity distribution of amorphous region at different Ts (130–160 � C), and the (110) crystal plane of BOPE film recrystallized from different Ts are shown in Fig. 13(a) and 13(b) respectively. In this Ts range, the BOPE film reaches the complete melt but is in different melt states. With the increase of Ts, the orientation of the molecular segments in the melt of BOPE still persists until Ts attains to ca. 160 � C. During recrystallization from these melt states, the (110) crystal planes of new 8
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4. Conclusions
T
Two kinds of crystallites (1 and 2) with different orientations are aligned around the TD of BOPE film. When BOPE film is annealed in the temperature range of 110~125 � C, the residual crystallites 1 are still aligned along the TD, while the residual crystallites 2 are gradually rotated and aligned in the MD. Recrystallization of BOPE film from the partially melted states is enhanced and the maximum crystallization peak shifts to higher temperature by 7 � C. The newly formed crystallites after recrystallization are aligned along TD and MD, respectively. However, crystallites 2 are predominant in this case. When annealing temperature attains to 128 � C, a trace amount of residual crystallites 1 persists along the TD. The newly formed crystallites after recrystalliza tion are also aligned along the orientation direction of residual crystal lites 1. When the storing temperature is in the temperature range of 130–150 � C, the retained orientation memory of BOPE melt influences both crystallization kinetics and crystallite orientation. The new crys tallites are aligned along the direction of the pre-orientation amorphous segments. Declaration of competing interest
(a)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement
T
Minqiao Ren: Data curation, Writing - original draft, Methodology. Yujing Tang: Conceptualization, Methodology. Dali Gao: Resources. Yueming Ren: Data curation. Xuerong Yao: Writing - review & editing. Hongwei Shi: Writing - review & editing. Taoyi Zhang: Resources. Changjiang Wu: Supervision. Acknowledgements This work is supported by the SINOPEC project (#217009). References [1] M.T. DeMeuse, Biaxial Stretching of Film: Principles and Applications, Woodhead Publishing Limited, Oxford, 2011. [2] D. Langhe, M. Ponting, Manufacturing and Novel Applications of Multilayer Polymer Films, Elsevier, Amsterdam, 2016. [3] T.H. Kwack, C.D. Han, M.E. Vickers, Development of crystalline structure during tubular film blowing of low-density polyethylene, J. Appl. Polym. Sci. 35 (1988) 363–389. [4] K. Choi, J.E. Spruiell, J.L. White, Orientation and morphology of high-density polyethylene film produced by the tubular blowing method and its relationship to process conditions, J. Polym. Sci., Polym. Phys. Ed. 20 (1982) 27–47. [5] D. Duraccio, A. Mauriello, S. Cimmino, C. Silvestre, F. Auriemma, C.D. Rosa, B. Pirozzi, G.R. Mitchell, Structure-property relationships in polyethylene based films obtained by blow molding as model system of industrial relevance, Eur. Polym. J. 62 (2015) 97–107. [6] H. Benkreira, M.C. Talford, Biaxially Oriented PP films using the double bubble process, Int. Polym. Process. 17 (2002) 228–232. [7] A.L. Bobovitch, R. Tkach, A. Ajji, S. Elkoun, Y. Nir, Y. Unigovski, E.M. Gutman, Mechanical properties, stress-relaxation, and orientation of double bubble biaxially oriented polyethylene films, J. Appl. Polym. Sci. 100 (2006) 3545–3553. [8] V. Ratta, G.L. Wilkes, T.K. Su, Structure-property-processing investigations of the tenter-frame process for making biaxially oriented HDPE film. I. Base sheet and draw along the MD, Polymer 42 (2001) 9059–9071. [9] Th Lüpke, S. Dunger, J. S€ anze, H.J. Radusch, Sequential biaxial drawing of polypropylene films, Polymer 45 (2004) 6861–6872. [10] H. Uehara, K. Sakauchi, T. Kanai, T. Yamada, Deformation behavior, processability and physical properties for biaxially oriented film of LLDPE, Int. Polym. Process. 19 (2004) 155–162. [11] H. Uehara, K. Sakauchi, T. Kanai, T. Yamada, Stretchability and properties of LLDPE blends for biaxially oriented film, Int. Polym. Process. 19 (2004) 163–171. [12] A. Ajji, J. Auger, J. Huang, L. Kale, Biaxial stretching and structure of various LLDPE resins, Polym. Eng. Sci. 44 (2004) 252–260. [13] Dow TF-BOPE sustainable packaging solutions and ENGAGETM PV POE Win 2018 R&D 100 Awards, China Chem. Rep. 30 (2019) 6.
(b) Fig. 13. Azimuthal intensity distribution of amorphous region at different Ts (130–160 � C) (a), and (110) crystal plane of BOPE film cooled down to 25 � C (b).
crystallites are still oriented along TD until Ts attains to ca. 160 � C. This temperature is very close to the onset temperature of complete melting domain obtained by DSC (165 � C). This means that retained orientation memory of BOPE melt influences both crystallization kinetics and crystallite orientation. The new crystallites are aligned along the di rection of the pre-orientation amorphous segments. During the recrys tallization process, a-axis is no long strongly oriented along the film thickness or the orientation is weak under relatively relaxed stress. The maximum intensity of both (200) and (110) crystal planes are both on the equator (Ts: 130–150 � C). This indicates that the new crystallites formed still exhibit the Keller-Machin Type II morphology (lamellae is not twisted) which is typical morphology of crystallization from high stress under constrained condition [38].
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Polymer 191 (2020) 122291 [31] Y. Xie, J. Kong, X. Fan, W. Qiao, Effects of heating, tensile and high pressure treatments on aggregate structure of low density polyethylene, China Plast. 18 (2004) 18–23. [32] B. Wunderlich, Thermal Analysis, Acedemic Press, Boston, 1990. [33] B. Wunderlich, Macromolecular Physics, vol. 3, Academic press, New York, 1976. [34] S.Z.D. Cheng, Phase Transitions in Polymers-The Role of Metastable States, Elsevier, Amsterdam, 2008. [35] D. Kalapat, Q. Tang, X. Zhang, W. Hu, Comparing crystallization kinetics among two G-resin samples and iPP via flash DSC measurement, J. Therm. Anal. Calorim. 128 (2017) 1859–1866. [36] T.F.J. Pijpers, V.B.F. Mathot, B. Goderis, R.L. Scherrenberg, E.W. van der Vegte, High-speed calorimetry for the study of the kinetics of (de)vitrification, crystallization, and melting of macromolecules, Macromolecules 35 (2002) 3601–3613. [37] A. Greco, A. Maffezzoli, Statistical and kinetic approaches for linear low-density polyethylene melting modeling, J. Appl. Polym. Sci. 89 (2003) 289–295. [38] A. Keller, M.J. Machin, Oriented crystallization in polymers, J. Macromol. Sci. Part B B1 (1967) 41–91. [39] A.T. Lorenzo, M.L. Arnal, J.J. S� anchez, A.J. Müller, Effect of annealing time on the self-nucleation behavior of semicrystalline polymers, J. Polym. Sci., Polym. Phys. Ed. 44 (2006) 1738–1750. [40] B.O. Reid, M. Vadlamudi, A. Mamun, H. Janani, H. Gao, W. Hu, R.G. Alamo, Strong memory effect of crystallization above the equilibrium melting point of random copolymers, Macromolecules 46 (2013) 6485–6497. [41] A. Mamun, X. Chen, R.G. Alamo, Interplay between a strong memory effect of crystallization and liquid–liquid phase separation in melts of broadly distributed ethylene–1-alkene copolymers, Macromolecules 47 (2014) 7958–7970. [42] M. Ren, X. Chen, Y. Sang, R.G. Alamo, Effect of heterogeneous short chain branching distribution on acceleration or retardation of the rate of crystallization from melts of ethylene copolymers synthesized with Ziegler-Natta catalysts, Macromol. Symp. 356 (2015) 131–141. [43] G.D. Wignall, R.G. Alamo, E.J. Ritchson, L. Mandelkern, D. Schwahn, SANS Studies of liquid liquid phase separation in heterogeneous and metallocene-based linear low-density polyethylenes, Macromolecules 34 (2001) 8160–8165. [44] R.G. Alamo, W.W. Graessley, R. Krishnamoorti, D.J. Lohse, J.D. Londono, L. Mandelkern, F.C. Stehling, G.D. Wignall, Small angle neutron scattering investigations of melt miscibility and phase segregation in blends of linear and branched polyethylenes as a function of the branch content, Macromolecules 30 (1997) 561–566. [45] B. Crist, M.J. Hill, Recent developments in phase separation of polyolefin melt blends, J. Polym. Sci., Polym. Phys. Ed. 35 (1997) 2329–2353. [46] H. Wang, K. Shimizu, E.K. Hobbie, G.Z. Wang, J.C. Meredith, A. Karim, E.J. Amis, B.S. Hsiao, E.T. Hsieh, C.C. Han, Phase diagram of a nearly isorefractive polyolefin blend, Macromolecules 35 (2002) 1072–1078. [47] S. Wang, C. Wu, M. Ren, R.M. Van Horn, M.J. Graham, C.C. Han, E. Chen, S.Z. D. Cheng, Liquid-liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies, Polymer 50 (2009) 1025–1033.
[14] Y. Lin, J. Xu, J. Pan, X.B. Yun, M. Demirors, Biaxially oriented polyethylene (BOPE) films fabricated via tenter frame process and applications thereof, in: Annual Technical Conference - ANTEC, Conf. Proc., 2018. [15] D. Xu, BOPE special materials and film development, Packaging Forefront 3 (2018) 26–30. [16] Y. Tang, M. Ren, H. Shi, D. Gao, B.B. He, X-ray pole figure analysis on biaxially oriented polyethylene films with sequential biaxial drawing, Adv. X Ray Anal. 61 (2018) 38–45. [17] M. Ren, Y. Tang, H. Shi, D. Gao, T. Zhang, Characterization on the crystalline orientation of uniaxially and biaxially oriented polyethylene films, Petrochem. Technol. 47 (2018) 872–878. [18] Q. Chen, D. Chen, J. Kang, Y. Cao, J. Chen, Structure evolution of polyethylene in sequential biaxial stretching along the first tensile direction, Ind. Eng. Chem. Res. 58 (2019) 12419–12430. [19] C. Wan, X. Chen, F. Lv, X. Chen, L. Meng, L. Li, Biaxial stretch-induced structural evolution of polyethylene gel films: crystal melting recrystallization and tilting, Polymer 164 (2019) 59–66. [20] R.J. Pazur, R.E. Prud’homme, An X-ray pole figure analysis on biaxially deformed polyethylene film, J. Polym. Sci., Polym. Phys. Ed. 32 (1994) 1475–1484. [21] A. Ajji, X. Zhang, S. Elkoun, Biaxial orientation in LLDPE films: comparison of infrared spectroscopy, X-ray pole figures, and birefringence techniques, Polym. Eng. Sci. 46 (2006) 1182–1189. [22] Z. Najarzadeh, A. Ajji, A novel approach toward the effect of seal process parameters on final seal strength and microstructure of LLDPE, J. Adhes. Sci. Technol. 28 (2014) 1592–1609. [23] K. Yamada, K. Miyata, R. Konishi, K. Okada, T. Tsujii, Molecular orientation effect of heat-sealed PP film on peel strength and structure, Adv. Mater. Phys. Chem. 5 (2015) 439–446. [24] P. Meka, F.C. Stehling, Heat sealing of semicrystalline polymer films. I. Calculation and measurement of interfacial temperatures: effect of process variables on seal properties, J. Appl. Polym. Sci. 51 (1994) 89–103. [25] H. Zheng, B. Wang, G. Zheng, Z. Wang, K. Dai, C. Liu, C. Shen, Study on crystallization kinetics of partially melting polyethylene aiming to improve mechanical properties, Ind. Eng. Chem. Res. 53 (2014) 6211–6220. [26] N. Stribeck, A.A. Camarillo, S. Cunis, R.K. Bayer, R. Gehrke, Oriented quiescent crystallization of polyethylene studied by USAXS Part 1: observations of nanostructure evolution, Macromol. Chem. Phys. 205 (2004) 1445–1454. [27] Zh G. Vasilenko, V.G. Brusentsova, I.I. Petrova, V.I. Gerasimo, A. Ye Chalykh, N. F. Bakeyev, Morphology of specimens crystallized from partially melted oriented polyethylene, Polym. Sci. 16 (1974) 1595–1599. [28] S. Humbert, O. Lame, J.M. Chenal, R. Seguela, G. Vigier, Memory effect of the molecular topology of lamellar polyethylene on the strain-induced fibrillar structure, Eur. Polym. J. 48 (2012) 1093–1100. [29] B.B. He, Two-dimensional X-Ray Diffraction, second ed., Wiley, Hoboken, 2018. [30] T. Tsujii, U.S. Ishiaku, K. Kitagawa, Y. Hashimoto, M. Mizoguchi, H. Hamada, Characterisation of heat-sealing part of laminated oriented nylon and polyethylene films, Plast., Rubber Compos. 34 (2005) 189–195.
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Recrystallization of biaxially oriented polyethylene film from partially melted state within crystallite networks Minqiao Ren , Yujing Tang , Dali Gao , Yueming Ren , Xuerong Yao , Hongwei Shi , Taoyi Zhang , Changjiang Wu * SINOPEC Beijing Research Institute of Chemical Industry, No. 14 Beisanhuan Donglu, Chao Yang District, Beijing, 100013, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Partially melted state Orientation Recrystallization Biaxially oriented polyethylene (BOPE) film
Recrystallization behavior and the final crystallite orientation of biaxially oriented polyethylene (BOPE) film from different melted states have been studied by using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. WAXD result shows that two kinds of crystallites (1 and 2) with different orientations are aligned around the transverse direction (TD) of the film plane. When BOPE film is annealed in the temperature range of 110~125 � C, the residual crystallites 1 are still aligned along the TD, while the residual crystallites 2 are gradually rotated and aligned along the machine direction (MD). Recrystallization of BOPE film from the partially melted states is enhanced and the maximum crystallization peak shifts to higher temperature by 7 � C. The newly formed crystallites after recrystallization are aligned along the TD and MD, respectively. When annealing temperature attains to 128 � C, a trace amount of residual crystallites 1 persists along the TD. The newly formed crystallites after recrystallization are also aligned the TD. When the storing temperature range is in 130–150 � C, the melt memory effect occurs in BOPE film, and it influences crystallization kinetics and the crystallite orientation.
1. Introduction Biaxial stretching of polymer sheets is a common processing tech nology for the production of products in packaging films [1,2]. Semi crystalline polyolefin films represent a large segment of the plastics industry. The most widely used biaxial processes for films are blowing process for various polyethylene (PE) [3–5], double-bubble process for polypropylene (PP) [6] and linear low-density PE (LLDPE) [7], and cast film biaxial orientation or tentering for high-density PE (HDPE) [8] and PP [9]. All these stretching processes impart certain degree of orienta tion to polyolefins, which enhances their mechanical, impact, barrier, and optical properties. Biaxial orientation has the advantage of allowing these enhancements in both machine direction (MD) and transverse direction (TD). Conventional LLDPE resins are used in both blowing and doublebubble processes, while these resins are seldom used in tenter process through PP line due to their poor stretchability [10–12]. However, compared to double-bubble process, LLDPE films produced by tenter process exhibit many advantages such as high puncture strength and shrink force in both MD and TD [12]. So specific LLDPE resins for biaxial
orientation are needed to further develop and improve. Recently, Dow has developed PE resins which could be stretched with stretching ratio 5 in MD and 9 in TD in a wide temperature window [13,14]. The biaxially oriented polyethylene (BOPE) films via the commercial scale tenter frame line exhibit higher modulus, higher dart and puncture impact strength, easier tear, and better optical properties than the blown films. SINOPEC has also developed the high speed BOPE resins which have excellent processing properties by tenter processing. The BOPE films obtained from these resins have high tensile strength, good trans parency, high puncture and impact resistance. These BOPE films can be used for agricultural film, multi-layer film, frozen food packaging and other special applications [15–17]. Though tenter process BOPE films have been developed successfully, there are only a few reports published in the literature. Wilkes et al. discussed the tentering process of HDPE [8]. Their studies focused on the change in the crystalline morphology from spherulitic lamellae into lamellar stacks that resulted from the MD stretching at temperatures near the HDPE melting temperature. Owing to the fact that the overall stretching process is done sequentially, the information provided the basis from which the TD stretch was undertaken, but no TD stretching
* Corresponding author. E-mail address: [email protected] (C. Wu). https://doi.org/10.1016/j.polymer.2020.122291 Received 9 December 2019; Received in revised form 10 February 2020; Accepted 13 February 2020 Available online 14 February 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.
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data were presented. Ajji et al. investigated the biaxial stretchability, orientation structure, and shrinkage of LLDPEs biaxially stretched by using a laboratory biaxial stretcher [12]. The results indicated that the high molecular weight tail and comonomer content played important roles in orientation of the BOPE films. Chen et al. found that only LLDPE which increased the lateral connection structure was beneficial for biaxial stretching [18]. Li et al. studied the crystal structure evolution of PE gel films containing paraffin oil during sequentially biaxial stretching at the temperature around the melting point [19]. They found that the morphologies before TD stretching were mainly composed of fibrillary crystals along MD with a small amount of oriented lamellar stacks. During subsequent TD stretching, the lamellar stacks vanished gradually together with the tilting of fibril along TD. Finally, formation of fibril lary crystal along TD was observed, forming an interconnected network with that in MD. During tenter frame process, crystallite orientation occurs in semi-solid state and has a significant effect on the film prop erties. For the BOPE films with sequential stretching process, the results showed that the c-axis of crystallites was oriented mainly along the last processing direction (TD) in the film plane, while a-axis or b-axis was strongly oriented along the ND of the film [20]. We studied the orien tation of BOPE film produced by the Karo IV unit from Brückner in the laboratory with stretching ratio of 4 in MD and 5 in TD, and found that the c-axis orientation of crystalites in BOPE film was along TD. In addition, there was a small fraction of crystallites with c-axis randomly distributed within the MD-TD plane [16]. Ajji et al. investigated the biaxial orientation of BOPE film produced by Brückner biaxial stretcher with simultaneous equibiaxially stretching process and found the different orientation of crystallites in MD and TD [21]. Sealing of polyolefin films by application of heat is widely used in the packaging industry to join film. The seal strength is related to the structure formed during sealing process [22–24]. The film ranges from partially to fully melted states when they are heat sealed. As most of the crystallites are formed during subsequent cooling process, and the final properties of film are determined by the morphology and structure formed from this process. So the study of oriented recrystallization from different molten states in polyolefin films is very important. Owing to the change of the temperature, the reorientation of the residual crys tallites under the two-dimensional (2D) residual stress conditions will occur, which will influence the subsequent crystallization behavior. Up to date, the oriented recrystallization has been studied mainly in uni axially oriented PE samples. For example, Zheng et al. studied the crystallization kinetics of partially melting extruded HDPE sheets [25]. An enhanced crystallization and an increment in tensile strength was achieved in the sheets extruded, which was mainly attributed to the self-nucleating effect with increased crystallinity, long period, and lamellar thickness. Stribeck et al. studied the oriented quiescent crys tallization of injection molded PE rods by ultra small-angle X-ray scat tering [26]. They found that the orientation of the crystallites could be controlled by choice of the melt annealing temperature. A three-stage model of crystallization was developed. This model comprised row structure nucleation, the almost statistical insertion of extended lamellae, and finally the insertion of blocky crystallites. Vasileko et al. studied the PE monofilaments crystallized in matrix from a partially oriented melt containing a highly oriented crystalline phase [27]. Electron microscopic examination showed that the structure of speci mens formed by such crystallization was heterogeneous and contained differing long periods. Humbert et al. studied memory effect of the molecular topology of lamellar PE on the strain-induced fibrillar struc ture and found that the diameter of the fibrils kept the memory of the chain topology to which a major role was ascribed in the strain-induced fragmentation of the crystalline lamellae [28]. Though the oriented recrystallization has been studied in uniaxially oriented PE samples, the oriented recrystallization in BOPE film has not been investigated. In this work, the differential scanning calorimetry (DSC) and wideangle X-ray diffraction (WAXD) methods have been used to investigate the recrystallization of BOPE film with different thermal conditions. The
Fig. 1. Pole figure scheme of multiple scans for PE (200) crystal plane: circle A: 2θ ¼ 24� , tilt angle ¼ 20� , 360� rotation; circle B: 2θ ¼ 24� , tilt angle ¼ 65� , 360� rotation.
aim is to study the effect of original crystallites orientation and annealing on the course of the recrystallization processes of BOPE film from partial melt within complex crystallite network or complete melt. The crystallites structure and morphology formed in biaxial orientation condition can provide the guidance for choosing the optimal processing window of BOPE film during heat sealing with desired structure and properties in industry. 2. Experiment 2.1. Materials The PE resin for biaxial stretching was provided by Sinopec. The melt index (MI) is 1.7 g/10 min (190 � C/2.16 kg), and the density is 0.918 g/ cm3. The weight-average molecular weight is 110,000 with a poly dispersity index of 4.47 obtained by gel permeation chromatography (GPC). BOPE film was produced in a Brückner tenter-frame line of sequen tial stretching with the width of 4.6 m and the manufacture speed of 100 m/min. The stretching ratio in MD was 4 and the stretching ratio in TD was 8, respectively. The final BOPE film was 50 μm in thickness. 2.2. Methods and techniques The melting and crystallization behavior of BOPE film were followed by DSC using a TA Q100 instrument. The temperatures and heat flows were calibrated using material standards at a cooling and heating rate of � 10 C/min. The sample was heated to a pre-set storing temperature � (denoted as Ts) for 5 min and then cooled to 25 � C at rate of 10 C/min. � After that, the sample was heated again at rate of 10 C/min. WAXD experiment was carried out on the Bruker D8 Discover X-ray diffractometer. The X-ray was generated using IμS microfocus X-ray source incorporating a 50 W sealed-tube X-ray generator with Cu target. The wavelength of X-ray was 0.1542 nm. The power of generator used for measurement was 45 kV and 900 μA. The X-ray intensity was recorded on VÅNTEC-500 2D detector with a pixel size of 136 � 136 μm2. The distance from the sample to detector was 98.6 mm for trans mission mode and 198 mm for reflection mode. The spot size of the beam 2
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crystallites are annealed and under metastable states. So the melting peak temperatures of crystalline polymers represent the melting points of metastable polymers crystals which have broad melting region [33, 34]. Fast enough heating could suppress crystal annealing during the heating process [35]. However, with the increase of heating rate, the melting peaks shift to higher temperature, as shown in Fig. 2. The result is different to that observed in the presence of annealing and recrys tallization effects, which should involve a shift of melting peak to higher temperatures with decreasing heating rate [33]. So the observed dif ference between DSC melting curves at different heating rate is not due to annealing and recrystallization effect and could be attributed to the thermal lag which is in fact a general problem observed when using the traditional calorimeters [36,37]. In order to gain information about the crystalline structure and orientation of BOPE film, the WAXD and SAXS experiments are con ducted in transmission mode. X-rays pass along the normal direction (ND) of BOPE film. The 2D WAXD image of BOPE film is shown in Fig. 3 (a). The heterogeneous (110) crystal plane diffraction ring is observed in BOPE film. The strongly intense meridianal diffraction arcs are coupled with moderate intensity diffraction arcs of (110) crystal planes which are deviated from the meridianal direction of ca. 48� . This indicates that two kinds of crystallites with different orientations exist in BOPE film. The diffraction intensity of (110) crystal plane in equatorial direction is weak. For quantitative analysis, 2D WAXD images are reduced to onedimensional (1D) WAXD intensity profiles over the azimuthal angle range of 0–360� as a function of 2θ, as shown in Fig. 3(b). The strong diffraction intensity of (110) crystal plane shows at ca. 21.4� and a very weak diffraction intensity of (200) crystal plane shows at ca. 24� . The diffraction of (200) crystal plane almost disappears when the WAXD experiments is conducted in transmission mode. This means that the orientation of a-axis is strongly along the ND of BOPE film. This is also reported in other BOPE films [16,17,20]. Thus the strong orientation of a-axis along the ND confines the b-axis and c-axis of crystallites to the film plane. The corresponding azimuthal intensity distribution curves of (110) and (200) crystal planes are displayed in Fig. 3(c), where the azimuthal angle of 180� represents the meridianal direction. The intense and defined diffractions of (110) and (200) crystal planes of BOPE film at the maximum both located on the meridian, which indicates that lamellae are not twisted and belong to Keller-Machin typeIIwhich is formed under high stress orientation [38]. From Fig. 3(c), two kinds of crystallites (1 and 2) with different orientation account for 37% and 63% of all the crystallites, respectively. As the weak (200) crystal planes appear on the meridian, it indicates that besides some crystallites with a-axis are along the ND, there are still some crystallites with a-axis along the MD in MD-TD plane. This is also confirmed by the X-ray pole figure of (200) crystal plane of BOPE film shown in Fig. 4. The strong orien tation of a-axis along the ND plane confines the b-axis and c-axis to the film plane, (110) and (200) crystal planes are mainly located on the meridian, so b-axis of the two kinds of crystallites are both oriented mainly along MD, while c-axis which represents the direction of the molecular chain is along the last processing direction (TD) of the film plane. There are several types of small-scale laboratory pieces of equipment which have been used as a screening tool to establish initial stretching conditions for additional scale-up investigation of biaxial stretching processing on larger equipment. However, the remaining challenge concerning the laboratory studies mainly lie in their correla tion with larger-scale tenter frame. For example, we have studied the orientation of BOPE produced by the Karo IV unit from Brückner in the laboratory and found that only one kind of crystallite orientation exists [17]. The main difference of orientation between laboratory and com mercial BOPE film may be attributed to the certain processing param eters or the design difference for commercial sized equipment and the small scale laboratory evaluation. For example, owing to high strains rate used in commercial tenter equipment, it is not clear how to precisely use the laboratory experiments to help define commercial processes. In addition, the small scale laboratory tests are in the intermittent mode of
Fig. 2. DSC melting curves of BOPE film with different heating rates.
was 0.5 mm. The exposure time was 5 min. The in-situ experiment was conducted on the Linkam FTIR600 hot stage. The BOPE film was wrapped up with aluminum foil and heated to a pre-set storing tem � perature Ts at the rate of 10 C/min for 2 min before WAXD image was � collected. After that, the sample was cooled to 25 � C at rate of 10 C/min, and WAXD image was collected accordingly. The temperature of sample in chamber was calibrated by using the thermocouple. In order to have the optimum pole figure coverage, a data collection strategy was laid out first [29]. Fig. 1 shows the pole figure scheme of multiple scans for PE (200) crystal plane which was generated by GADDS software. By using this scheme, we got reasonable data point on the pole figure that covered about 70� of the stereographic projection. After the raw data was collected, the pole figure was obtained by using Multex software. Small-angle X-ray scattering (SAXS) experiment was carried out on the Xenocs XEUSS SAXS instrument. The wavelength of X-ray was 0.1542 nm. The power of generator used for measurement was 50 kV and 600 μA. The X-ray intensity was recorded on a 2D detector with a pixel size of 172 � 172 μm2. The distance from the sample to detector was 1161 mm. In order to obtain the relatively strong SAXS intensity, multi-layer BOPE film stack with the whole thickness of ca. 400 μm were used to test. The exposure time was 20 min. Temperature rising elution fractionation (TREF) was carried out in a TREF300 instrument commercialized by Polymer Char (Spain) using 1,2,4-trimethylbenzene as the solvent. The crystallization rate was 0.1 � C/min from the stabilization temperature of 95 � C to 35 � C, and the subsequent solvent elution rate was 0.5 mL/min from 35 � C to 130 � C at � a rate of 1 C/min. 3. Results and discussion 3.1. Thermal behavior and crystallite orientation of BOPE film A set of normalized DSC melting curves of BOPE film at different � heating rates (10–80 C/min) are shown in Fig. 2. The melting of BOPE film occurred within a temperature range of ca. 40–130 � C yields broad melting curves. This may be attributed to a broad thickness distribution of lamellae formed during stretching and subsequent recrystallization in processing process [30]. The unstable melting peak at ca. 50 � C for BOPE film may be caused by some ordered structure formed during the stretching process [31]. The melting peak temperature of BOPE film � with 10 C/min heating rate is ca. 124 � C. The degree of crystallinity of BOPE film is ca. 44% if the value of equilibrium fusion enthalpy of PE is chosen 293 J/g [32]. It is worthy to note that the DSC heating curves does not represent the original melting of crystalline polymers because during the heating, the molecules are continuously moving, the 3
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(a)
(b)
(c) Fig. 3. WAXD of BOPE film: (a) 2D WAXD image, (b) 1D WAXD intensity profile as a function of 2θ, and (c) azimuthal intensity distribution of (110) and (200) crystal planes. 4
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Fig. 4. X-ray pole figure of (200) crystal plane of BOPE film.
production, while commercial process lines are in the continuous mode of production. 2D SAXS image of BOPE film is shown in Fig. 5(a). The better defined scattering in TD implies that much better lamellar packing is formed in TD. The integrated 1D SAXS profiles obtained by averaging the scat tering intensity within 10� on both TD and MD are shown in Fig. 5(b). The scattering profiles show well-defined scattering peaks in BOPE film. This indicates that well-organized lamellar packing structures are formed in BOPE film. An important structural parameter on the lamellar stacks is the average long period, meaning the average total thickness of crystalline region and amorphous region or the average inter-distance of neighboring crystalline lamellae. For BOPE film, the calculated long periods in TD and MD are 22.7 nm and 21.8 nm, respectively. The long period in TD is slightly greater than that in MD in BOPE film. The broad distribution of the lamellae formed during stretching and subsequent recrystallization in both TD and MD correlates with the broad melting curves of BOPE film shown in Fig. 2. The azimuthal scan of long period of lamellae in BOPE film is shown in Fig. 5(c). The majority of lamellar stacks are along TD, while a minority of lamellar stacks are along MD. As the normal direction of the lamellar stack approximately represents molecular chain direction (c-axis), so the PE molecular chains are preferably oriented along the TD. This is in accordance with that ob tained from WAXD results.
(a)
(b)
3.2. Recrystallization behavior of BOPE film from different melted states What’s the recrystallization behavior of BOPE film from partially or fully melted states? In order to address this question, BOPE film is heated to different storing temperature Ts for 5 min. Different Ts are chosen based on the first DSC melting curve of BOPE film in Fig. 2. The cooling and subsequent melting curves of the BOPE film after storing at different Ts are shown in Fig. 6. The thermal parameters such as crystallization peak temperature (Tc,peak), crystallization enthalpy (ΔHc), melting peak temperature (Tm), fusion enthalpy (ΔHf), and the degree of crystallinity (Wc,h) of BOPE are shown in Table 1. The Tc,peak as a function of Ts is plotted and shown in Fig. 7. Three domains are used to describe the different recrystallization behaviors of BOPE film according to selfnucleation theory [39]. In Domain I(complete melting domain), the PE melts completely after thermal conditioning at Ts. The chains adopt the random coil conformation characteristic of the homogenous melt. This state is found to persist down to 165 � C, because no difference is detected in Tc,peak. The crystallization peak position shows the lowest value at ca. 111 � C, indicating recrystallization occurs from homogeneous melt. In Domain II (melt-memory and liquid-liquid phase separation (LLPS) domain), the melt is heterogeneous and retains some residual
(c) Fig. 5. SAXS of BOPE film: (a) 2D SAXS image, (b) 1D SAXS profiles, and (c) azimuthal intensity distribution of long period of lamellae.
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T (
)
T
T T
T
T T
T Fig. 7. Variation of the crystallization peak temperature (Tc,peak) as a function of the storing temperature (Ts) for BOPE film.
(a)
T (
)
Fig. 8. TREF profile of BOPE film.
segmental orientation. Self-nuclei remain in the melt as a memory of the sequence selection and self-assembly of long ethylene sequences during the initial crystallization path. The surviving self-nuclei are associated with molten ethylene sequences that remain in close proximity and are unable to diffuse quickly to the randomized melt state. A complex to pology of knots, loops, ties and other types of entanglements builds up in the interlamellar region of random copolymers, especially at high levels of transformation. Upon melting, diffusion of crystalline sequences through this complex topology back to the randomized melt requires temperatures well above the equilibrium melting point. Recrystalliza tion is accelerated in this temperature range of heterogeneous melts [40]. So thermal conditioning at 163 � C is able to self-nucleate PE, as Tc, peak shifts to higher temperatures. However, an unexpected retraction of Tc,peak with further decreasing Ts below 155 � C, which can be explained by the interplay between the strong memory of melt and LLPS [41,42]. As the commercial LLDPEs often broadly distributed in molar mass and inter-chain comonomer content, different fractions are not always miscible as a single phase. The miscibility of the different fractions is critically dependent on molecular characteristics such as molecular weight, short-chain branch concentration, branch type, composition, and processing conditions although some of the results reported in those studies are still being debated. It is known from early works that PE blends with a difference in comonomer content >8 mol% phase separate in the melt [43–47]. Most PE blends exhibit an upper critical solution
(b) Fig. 6. DSC curves of BOPE film during (a) recrystallization from different Ts, � and (b) subsequent melting (both the heating and cooling rate is 10 C/min). Table 1 Thermal parameters of BOPE film recrystallized from different Ts. Ts (oC)
Tc,peak (oC)
ΔHc(J/g)
Tm (oC)
ΔHf (J/g)
Wc,h (%)
First melting 110 120 125 128 130 135 140 145 150 155 160 163 165 170 180 200
– – 115.3 117.9 112.2 111.9 112.8 112.7 113.5 113.2 113.4 112.4 112.0 110.8 111.0 110.6 110.8
– – 40.8 107.9 111.1 111.8 112.6 109.4 113.6 108.6 111.3 102.6 112.0 109.5 110.0 111.3 112.0
123.9 124.0 124.5 125.8 124.7 125.1 124.5 125.3 124.6 125.1 124.1 124.5 124.2 124.8 123.7 124.6 124.2
129.3 129.3 136.5 108.1 90.4 104.9 111.4 105.8 110.3 104.3 108.4 106.6 111.0 101.5 106.8 106.6 111.3
44.1 44.1 46.6 36.9 30.9 35.8 38.0 36.1 37.6 35.6 37.0 36.4 37.9 34.6 36.5 36.4 38.0
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T
Ts: 110 oC
Ts: 120 oC
25 oC
25 oC
(a) T
Ts: 125 oC
25 oC
Ts: 130 oC
25 oC
Figure 9. 2D WAXD images of BOPE film annealed at different storing tem perature Ts and then cooled down to 25 � C.
(b)
temperature (UCST) at ca. 150 � C, which is in the temperature range of Domain II in Fig. 7. The difference in inter-chain comonomer content of BOPE film is more clearly resolved by the TREF profile in Fig. 8. It can be seen that about 50% of the copolymer molecules elute in a temperature range between 30 and ~90 � C, while the other 50% elutes in a very narrower range, between ~90 and 104 � C. So this large difference in comonomer content in BOPE could lead to LLPS. When the crystallites of BOPE film are brought to the temperature below the UCST, self-seeds do not survive as the strong thermodynamic drive for LLPS dissolves these seeds. So the recrystallization rate decreases in this region because the drive to phase separation is stronger. In Domain III (self-nucleation and annealing), small crystal frag ments survive melting and constitute the self-seeds (Ts below 130 � C). Partial melting is produced and the unmelted crystals can experience annealing during the 5 min thermal conditioning at Ts. When Ts is 125 � C, a broad crystallization peak appears at ca. 118 � C in BOPE film. Tc, � peak shifts to higher temperature by 7 C. This indicates that the residual crystallites can enhance the recrystallization rate of BOPE film. The subsequent melting endotherm shows a small high-temperature sharp peak, which is due to the melting of the annealed crystallites. When Ts is at 110 � C, there is almost no exothermal peak, indicating that the
Fig. 10. 1D WAXD intensity profiles of BOPE film annealed at different storing temperature Ts (a), and then cooled down to 25 � C (b).
annealing effect is dominant in this condition. 3.3. Crystallite orientation of BOPE film after recrystallization from different melted states In accordance with the DSC experiments, the BOPE film is heated to different Ts, and then cooled at 25 � C and these processes are observed in-situ by WAXD. Some representative 2D WAXD images of BOPE film annealed at different Ts and then cooled down to 25 � C are shown in Fig. 9, respectively. With the increase of Ts, the intensity of the diffraction arcs of BOPE film becomes weak while the amorphous halos show up clearly. This means that the crystallites melt gradually. The crystallites in BOPE film reach the complete melt at ca. 130 � C. The orientation of the residual crystallites also changes gradually with in crease of Ts. However, when recrystallization occurs within existing oriented crystallite network, the new crystallites almost grow along the orientation direction of the residual crystallites. 7
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Table 2 The degree of crystallinity of BOPE film annealed at different storing tempera ture Ts and then cooled down to 25 � C. Ts (oC)
Wc,x (%) (at Ts)
Wc,x (%) (at 25 � C)
Original film 110 120 125 128 130 140 150 160
41.4 16.2 9.3 1.1 0.4 0 0 0 0
41.4 34.1 34.2 31.7 32.6 33.0 32.0 31.7 31.4
T
(a)
T
Fig. 11. X-ray pole figure of (200) crystal plane of BOPE film annealed at 125 � C for 5 min and then cooled down to 25 � C.
For quantitative analysis, 2D WAXD images are reduced to 1D WAXD intensity profiles over the azimuthal angle range of 0–360� as a function of 2θ, as shown in Fig. 10(a) and 10(b), respectively. The degrees of crystallinity (Wc,x) obtained by WAXD method are also summarized in Table 2. With the increase of Ts, the residual degree of crystallinity at Ts decreases until all crystallites in BOPE film completely melts at ca. 130 � C. During the recrystallization of BOPE film, the degrees of crystallinity of BOPE film varies with the change of Ts, with the maximum value occurs when Ts is 120 � C. As the residual crystallites are annealed before the new crystallites are formed in the network of the residual crystallite skeleton, the whole degree of crystallinity of BOPE attains to the maximum value when Ts is 120 � C. Compared with the original BOPE film, the degree of crystallinity obtained from recrystallization of BOPE decreases. In addition, the diffraction intensity of (200) crystal plane increases during recrystallization of BOPE. This means that a-axis is no long strongly oriented along the film thickness or the orientation degree becomes weak under relatively relaxed stress conditions during the recrystallization process. This is confirmed by the X-ray pole figure of (200) crystal plane of BOPE film after recrystallization, as shown in Fig. 11. It can be seen the (200) crystal plane is randomly distributed in the MD-TD plane of BOPE film. The azimuthal intensity distribution of (110) diffraction plane of original BOPE film and BOPE film annealed at different Ts (110–128 � C) and then cooled to 25 � C are shown in Fig. 12(a) and 12(b) respectively. The azimuthal angle of 180� represents the meridianal direction. In Ts range of 110–125 � C, the BOPE film is in the partially melted state. With the increase of Ts, residual crystallites 1 are still along the TD, while the residual crystallites 2 are gradually rotated and aligned along the MD. This may be related to change of the complex 2D residual stress distri bution during the partial melting process. When BOPE film is cooled
(b) Fig. 12. Azimuthal intensity distribution of (110) crystal plane of BOPE film at different Ts (110–128 � C) (a), and then cooled down to 25 � C (b).
from this interconnected crystallite network, it still exhibits two kinds of crystallites (1 and 2) with c-axis orientation of crystallites 1 and 2 lie along the TD and MD, respectively. However, crystallites 2 are pre dominant in this case. When Ts attains to 128 � C, a trace amount of residual crystallites 1 still persist along TD. The newly formed crystal lites after recrystallization are also aligned along the orientation direc tion of residual crystallites 1. These results indicate that the orientation of the newly formed crystallites lies along the orientation direction of the residual crystallites when recrystallization of BOPE film from the partially melted states. The azimuthal intensity distribution of amorphous region at different Ts (130–160 � C), and the (110) crystal plane of BOPE film recrystallized from different Ts are shown in Fig. 13(a) and 13(b) respectively. In this Ts range, the BOPE film reaches the complete melt but is in different melt states. With the increase of Ts, the orientation of the molecular segments in the melt of BOPE still persists until Ts attains to ca. 160 � C. During recrystallization from these melt states, the (110) crystal planes of new 8
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Polymer 191 (2020) 122291
4. Conclusions
T
Two kinds of crystallites (1 and 2) with different orientations are aligned around the TD of BOPE film. When BOPE film is annealed in the temperature range of 110~125 � C, the residual crystallites 1 are still aligned along the TD, while the residual crystallites 2 are gradually rotated and aligned in the MD. Recrystallization of BOPE film from the partially melted states is enhanced and the maximum crystallization peak shifts to higher temperature by 7 � C. The newly formed crystallites after recrystallization are aligned along TD and MD, respectively. However, crystallites 2 are predominant in this case. When annealing temperature attains to 128 � C, a trace amount of residual crystallites 1 persists along the TD. The newly formed crystallites after recrystalliza tion are also aligned along the orientation direction of residual crystal lites 1. When the storing temperature is in the temperature range of 130–150 � C, the retained orientation memory of BOPE melt influences both crystallization kinetics and crystallite orientation. The new crys tallites are aligned along the direction of the pre-orientation amorphous segments. Declaration of competing interest
(a)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement
T
Minqiao Ren: Data curation, Writing - original draft, Methodology. Yujing Tang: Conceptualization, Methodology. Dali Gao: Resources. Yueming Ren: Data curation. Xuerong Yao: Writing - review & editing. Hongwei Shi: Writing - review & editing. Taoyi Zhang: Resources. Changjiang Wu: Supervision. Acknowledgements This work is supported by the SINOPEC project (#217009). References [1] M.T. DeMeuse, Biaxial Stretching of Film: Principles and Applications, Woodhead Publishing Limited, Oxford, 2011. [2] D. Langhe, M. Ponting, Manufacturing and Novel Applications of Multilayer Polymer Films, Elsevier, Amsterdam, 2016. [3] T.H. Kwack, C.D. Han, M.E. Vickers, Development of crystalline structure during tubular film blowing of low-density polyethylene, J. Appl. Polym. Sci. 35 (1988) 363–389. [4] K. Choi, J.E. Spruiell, J.L. White, Orientation and morphology of high-density polyethylene film produced by the tubular blowing method and its relationship to process conditions, J. Polym. Sci., Polym. Phys. Ed. 20 (1982) 27–47. [5] D. Duraccio, A. Mauriello, S. Cimmino, C. Silvestre, F. Auriemma, C.D. Rosa, B. Pirozzi, G.R. Mitchell, Structure-property relationships in polyethylene based films obtained by blow molding as model system of industrial relevance, Eur. Polym. J. 62 (2015) 97–107. [6] H. Benkreira, M.C. Talford, Biaxially Oriented PP films using the double bubble process, Int. Polym. Process. 17 (2002) 228–232. [7] A.L. Bobovitch, R. Tkach, A. Ajji, S. Elkoun, Y. Nir, Y. Unigovski, E.M. Gutman, Mechanical properties, stress-relaxation, and orientation of double bubble biaxially oriented polyethylene films, J. Appl. Polym. Sci. 100 (2006) 3545–3553. [8] V. Ratta, G.L. Wilkes, T.K. Su, Structure-property-processing investigations of the tenter-frame process for making biaxially oriented HDPE film. I. Base sheet and draw along the MD, Polymer 42 (2001) 9059–9071. [9] Th Lüpke, S. Dunger, J. S€ anze, H.J. Radusch, Sequential biaxial drawing of polypropylene films, Polymer 45 (2004) 6861–6872. [10] H. Uehara, K. Sakauchi, T. Kanai, T. Yamada, Deformation behavior, processability and physical properties for biaxially oriented film of LLDPE, Int. Polym. Process. 19 (2004) 155–162. [11] H. Uehara, K. Sakauchi, T. Kanai, T. Yamada, Stretchability and properties of LLDPE blends for biaxially oriented film, Int. Polym. Process. 19 (2004) 163–171. [12] A. Ajji, J. Auger, J. Huang, L. Kale, Biaxial stretching and structure of various LLDPE resins, Polym. Eng. Sci. 44 (2004) 252–260. [13] Dow TF-BOPE sustainable packaging solutions and ENGAGETM PV POE Win 2018 R&D 100 Awards, China Chem. Rep. 30 (2019) 6.
(b) Fig. 13. Azimuthal intensity distribution of amorphous region at different Ts (130–160 � C) (a), and (110) crystal plane of BOPE film cooled down to 25 � C (b).
crystallites are still oriented along TD until Ts attains to ca. 160 � C. This temperature is very close to the onset temperature of complete melting domain obtained by DSC (165 � C). This means that retained orientation memory of BOPE melt influences both crystallization kinetics and crystallite orientation. The new crystallites are aligned along the di rection of the pre-orientation amorphous segments. During the recrys tallization process, a-axis is no long strongly oriented along the film thickness or the orientation is weak under relatively relaxed stress. The maximum intensity of both (200) and (110) crystal planes are both on the equator (Ts: 130–150 � C). This indicates that the new crystallites formed still exhibit the Keller-Machin Type II morphology (lamellae is not twisted) which is typical morphology of crystallization from high stress under constrained condition [38].
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M. Ren et al.
Polymer 191 (2020) 122291 [31] Y. Xie, J. Kong, X. Fan, W. Qiao, Effects of heating, tensile and high pressure treatments on aggregate structure of low density polyethylene, China Plast. 18 (2004) 18–23. [32] B. Wunderlich, Thermal Analysis, Acedemic Press, Boston, 1990. [33] B. Wunderlich, Macromolecular Physics, vol. 3, Academic press, New York, 1976. [34] S.Z.D. Cheng, Phase Transitions in Polymers-The Role of Metastable States, Elsevier, Amsterdam, 2008. [35] D. Kalapat, Q. Tang, X. Zhang, W. Hu, Comparing crystallization kinetics among two G-resin samples and iPP via flash DSC measurement, J. Therm. Anal. Calorim. 128 (2017) 1859–1866. [36] T.F.J. Pijpers, V.B.F. Mathot, B. Goderis, R.L. Scherrenberg, E.W. van der Vegte, High-speed calorimetry for the study of the kinetics of (de)vitrification, crystallization, and melting of macromolecules, Macromolecules 35 (2002) 3601–3613. [37] A. Greco, A. Maffezzoli, Statistical and kinetic approaches for linear low-density polyethylene melting modeling, J. Appl. Polym. Sci. 89 (2003) 289–295. [38] A. Keller, M.J. Machin, Oriented crystallization in polymers, J. Macromol. Sci. Part B B1 (1967) 41–91. [39] A.T. Lorenzo, M.L. Arnal, J.J. S� anchez, A.J. Müller, Effect of annealing time on the self-nucleation behavior of semicrystalline polymers, J. Polym. Sci., Polym. Phys. Ed. 44 (2006) 1738–1750. [40] B.O. Reid, M. Vadlamudi, A. Mamun, H. Janani, H. Gao, W. Hu, R.G. Alamo, Strong memory effect of crystallization above the equilibrium melting point of random copolymers, Macromolecules 46 (2013) 6485–6497. [41] A. Mamun, X. Chen, R.G. Alamo, Interplay between a strong memory effect of crystallization and liquid–liquid phase separation in melts of broadly distributed ethylene–1-alkene copolymers, Macromolecules 47 (2014) 7958–7970. [42] M. Ren, X. Chen, Y. Sang, R.G. Alamo, Effect of heterogeneous short chain branching distribution on acceleration or retardation of the rate of crystallization from melts of ethylene copolymers synthesized with Ziegler-Natta catalysts, Macromol. Symp. 356 (2015) 131–141. [43] G.D. Wignall, R.G. Alamo, E.J. Ritchson, L. Mandelkern, D. Schwahn, SANS Studies of liquid liquid phase separation in heterogeneous and metallocene-based linear low-density polyethylenes, Macromolecules 34 (2001) 8160–8165. [44] R.G. Alamo, W.W. Graessley, R. Krishnamoorti, D.J. Lohse, J.D. Londono, L. Mandelkern, F.C. Stehling, G.D. Wignall, Small angle neutron scattering investigations of melt miscibility and phase segregation in blends of linear and branched polyethylenes as a function of the branch content, Macromolecules 30 (1997) 561–566. [45] B. Crist, M.J. Hill, Recent developments in phase separation of polyolefin melt blends, J. Polym. Sci., Polym. Phys. Ed. 35 (1997) 2329–2353. [46] H. Wang, K. Shimizu, E.K. Hobbie, G.Z. Wang, J.C. Meredith, A. Karim, E.J. Amis, B.S. Hsiao, E.T. Hsieh, C.C. Han, Phase diagram of a nearly isorefractive polyolefin blend, Macromolecules 35 (2002) 1072–1078. [47] S. Wang, C. Wu, M. Ren, R.M. Van Horn, M.J. Graham, C.C. Han, E. Chen, S.Z. D. Cheng, Liquid-liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies, Polymer 50 (2009) 1025–1033.
[14] Y. Lin, J. Xu, J. Pan, X.B. Yun, M. Demirors, Biaxially oriented polyethylene (BOPE) films fabricated via tenter frame process and applications thereof, in: Annual Technical Conference - ANTEC, Conf. Proc., 2018. [15] D. Xu, BOPE special materials and film development, Packaging Forefront 3 (2018) 26–30. [16] Y. Tang, M. Ren, H. Shi, D. Gao, B.B. He, X-ray pole figure analysis on biaxially oriented polyethylene films with sequential biaxial drawing, Adv. X Ray Anal. 61 (2018) 38–45. [17] M. Ren, Y. Tang, H. Shi, D. Gao, T. Zhang, Characterization on the crystalline orientation of uniaxially and biaxially oriented polyethylene films, Petrochem. Technol. 47 (2018) 872–878. [18] Q. Chen, D. Chen, J. Kang, Y. Cao, J. Chen, Structure evolution of polyethylene in sequential biaxial stretching along the first tensile direction, Ind. Eng. Chem. Res. 58 (2019) 12419–12430. [19] C. Wan, X. Chen, F. Lv, X. Chen, L. Meng, L. Li, Biaxial stretch-induced structural evolution of polyethylene gel films: crystal melting recrystallization and tilting, Polymer 164 (2019) 59–66. [20] R.J. Pazur, R.E. Prud’homme, An X-ray pole figure analysis on biaxially deformed polyethylene film, J. Polym. Sci., Polym. Phys. Ed. 32 (1994) 1475–1484. [21] A. Ajji, X. Zhang, S. Elkoun, Biaxial orientation in LLDPE films: comparison of infrared spectroscopy, X-ray pole figures, and birefringence techniques, Polym. Eng. Sci. 46 (2006) 1182–1189. [22] Z. Najarzadeh, A. Ajji, A novel approach toward the effect of seal process parameters on final seal strength and microstructure of LLDPE, J. Adhes. Sci. Technol. 28 (2014) 1592–1609. [23] K. Yamada, K. Miyata, R. Konishi, K. Okada, T. Tsujii, Molecular orientation effect of heat-sealed PP film on peel strength and structure, Adv. Mater. Phys. Chem. 5 (2015) 439–446. [24] P. Meka, F.C. Stehling, Heat sealing of semicrystalline polymer films. I. Calculation and measurement of interfacial temperatures: effect of process variables on seal properties, J. Appl. Polym. Sci. 51 (1994) 89–103. [25] H. Zheng, B. Wang, G. Zheng, Z. Wang, K. Dai, C. Liu, C. Shen, Study on crystallization kinetics of partially melting polyethylene aiming to improve mechanical properties, Ind. Eng. Chem. Res. 53 (2014) 6211–6220. [26] N. Stribeck, A.A. Camarillo, S. Cunis, R.K. Bayer, R. Gehrke, Oriented quiescent crystallization of polyethylene studied by USAXS Part 1: observations of nanostructure evolution, Macromol. Chem. Phys. 205 (2004) 1445–1454. [27] Zh G. Vasilenko, V.G. Brusentsova, I.I. Petrova, V.I. Gerasimo, A. Ye Chalykh, N. F. Bakeyev, Morphology of specimens crystallized from partially melted oriented polyethylene, Polym. Sci. 16 (1974) 1595–1599. [28] S. Humbert, O. Lame, J.M. Chenal, R. Seguela, G. Vigier, Memory effect of the molecular topology of lamellar polyethylene on the strain-induced fibrillar structure, Eur. Polym. J. 48 (2012) 1093–1100. [29] B.B. He, Two-dimensional X-Ray Diffraction, second ed., Wiley, Hoboken, 2018. [30] T. Tsujii, U.S. Ishiaku, K. Kitagawa, Y. Hashimoto, M. Mizoguchi, H. Hamada, Characterisation of heat-sealing part of laminated oriented nylon and polyethylene films, Plast., Rubber Compos. 34 (2005) 189–195.
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