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polybutene-1 blends
Polymer 182 (2019) 121817
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Crystallization of forms I′ and form II of polybutene-1 in stretched polypropylene/polybutene-1 blends
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Yao Xua, Yaping Maa, Chenguang Liua,∗∗, Yongfeng Menb, Aihua Hea,∗ a Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, 130022, Changchun, PR China
H I GH L IG H T S
immiscible PP/PB-1 (50/50) blend was studied by in-situ tensile 2D wide-angle X-ray diffraction. • The crystallization of PB-1 in form II and I′ in stretched PP/PB-1 blend was displayed explicitly. • Orientation • The mechanism of stable PB-1 in form I′ was discussed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Confined crystallization Orientated crystallization Polybutene-1
In this work, the effects of orientated crystal morphology of polypropylene (PP) induced by tensile stretching on the subsequent crystallization behavior of polybutene-1 (PB-1) in the incompatible PP/PB-1 (50/50) blends were studied. When the PP/PB-1 blends were stretched at a temperature above melting temperature (Tm) of PB-1 but below the Tm of PP, strong orientation of PP crystals was observed under stretching. As temperature dropped below Tm of PB-1, orientated form II and I′ of PB-1 appeared. In the framework of PP crystals, form I′ of PB-1 was formed due to the confined effect which suppressed the formation of form II crystals. Interfacial interaction played a role in guiding the nucleation of both form II and I’ crystals resulting in highly oriented structures. The mechanism of PB-1 orientation and confined crystallization under stretching in PP/PB-1 (50/50) blends was discussed.
1. Introduction When PB-1 crystallizes during cooling process, it is an unstable crystalline polymer [1]. The PB-1 metastable form II with tetragonal structure obtained directly from molten state, transforms spontaneously into stable form I with hexagonal structure which can take weeks [2,3]. Crystal form I brings PB-1 many excellent performances such as high toughness, good environmental stress crack resistance and perfect creep resistance at elevated temperature. However, this solid-solid transition causes shrinkage of the materials, which may lead to distortion of the product and limit the application of PB-1 in some aspects [4,5]. Many researches [6–11] have focused on bypassing of the disadvantageous metastable state form II. Form I′ generated directly produced by melt is considered to be importance for industrial application because it avoided the crystal transformation. One of the conditions for producing
∗
form I′ is to blend PB-1 with isotactic polypropylene (PP). Shieh et al. [12] firstly claimed that form I′ directly appeared from the melts of PP/ PB-1 blends. The I′-form PB-1 and α-form PP have similar 31 helix conformation. Ballesteros [13] and De Rosa [14] reported that form I′ can be obtained in butene-propylene or butene-ethylene copolymers, which is attributed to the compact type of ab plane and the c axis of the monoclinic unit cell of PB-1 phase. Men [15–18] clarified that interplay between the dimension of crystallizable chain segmental segregation in heterogeneous melt and corresponding nucleation size of form II and form I determined direct formation of either form II iPB or form I′ iPB. Li [19] claimed that after incorporation of PP, form I′ of PB-1 was found by adjusting its thermal history. And the I′-form PB-1 was attributed to molecular chain interaction. Stolte [20] and our previous work [21] indicated that form I′ was observed when PB-1 was confined in different locations.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected], [email protected] (A. He).
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https://doi.org/10.1016/j.polymer.2019.121817 Received 25 June 2019; Received in revised form 5 September 2019; Accepted 17 September 2019 Available online 17 September 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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min and kept at this temperature for 0.5 h to allow an isothermal crystallization of PB-1. The stretched specimen was firstly stretched to 300% elongation under a crosshead speed of 50 μm/s at 160 °C after being annealed for 5 min and kept for another 5 min. The stretched sample was then cooled down to 90 °C at a cooling rate of 10 °C/min and kept at this temperature for 0.5 h to allow an isothermal crystallization of PB-1. The phase morphologies of samples were observed with thickness about 25 μm by means of an Olympus optical microscope (BX 51-P) with a Linkam hot stage (LNP95) to control the observation temperature. The samples were observed with phase contrast optical microscopy (PCOM) and polarized optical microscopy (POM) modes, respectively. The etched surface of the sample was observed with scanning electron microscope (SEM, JSM-7500F).
If one component with a molten state in the other oriented component matrix with a solid state, in the molten component some unusual oriented structures were obtained, such as orthogonal orientation [22,23] or oblique orientation [24,25]. Different mechanisms were proposed to explain these orientation behaviors, such as epitaxial crystal growth [16–20], heat-shrinking stress [26] or transcrystallization [27,28]. The PB-1 in form I′ is rarely formed directly from the molten state unless under the conditions of ultrathin film [29], selfseeding [30,31], high pressure [32], presence of counits [20] and blends [19,21]. In the PP/PB-1 mixture, the mechanism of formation of I′ PB-1 has been attributed to distinct reasons [33] like melt memory [34], molecular chain interaction [19], interfacial nucleation [35] or confined crystallization [20,21]. The controversy on mechanism of the form of I′ PB-1 in PP/PB-1 blends requires more convincible experiments to support. In this paper, PP/PB-1 blends were 300% stretched at 160 °C and then held during the subsequent cooling process. By means of in-situ 2D wide-angle X-ray diffraction (2D WAXD) experiments, the orientated PB-1 crystallization in the presence of orientated PP crystalline phase has been investigated. It was discovered that pre-orientated PP crystals have a lot of effect on the orientated crystallization and directly lead to the formation of form I′ of PB-1 from molten state.
3. Results and discussion 3.1. In-situ 2D WAXD When the crystallization temperature (Tc) of PP above the melting temperature (Tm) of PB-1, PB-1 still in melting state during the crystallization process of PP. As temperature decreases further, PB-1 crystallizes subsequently within crystalline framework of PP. Fig. 1 shows in-situ 2D WAXD patterns of the PP/PB-1 (50/50) blends. The PP/PB-1 (50/50) blend after annealed at 160 °C for 5 min shows undirected α PP with identified reflections of (110), (130) and (040) planes, as shown in Fig. 1(a). With the further cooling to 90 °C, undirected meta-stable form II crystals of PB-1 with identified reflections of (200) and (213) planes are observed as given in Fig. 1(b). Fig. 1(c) presents WAXD pattern of the sample which was stretched to 300% elongation at 160 °C. Clearly, the stretching yields a highly oriented structure of PP crystals in form α with chain axis along stretching direction with an orientation of −0.5. The degree of orientation in form α of PP, form II and form I/I′ crystals of PB-1 is measured by azimuthal scan of 110 plane of PP, 200 plane of PB-1 form II and 110 plane of PB form I/I' (Fig. S1), respectively. While PB-1 melt shows typical isotropic amorphous scattering on the pattern at this temperature. When this stretched sample with high oriented PP crystals and PB-1 melt was cooled down to 90 °C, oriented crystallization of PB-1 also occurred. WAXD pattern in Fig. 1(d) indicates the existence of PB-1 crystals are oriented form II with the orientation degree of −0.43 and oriented form I′ or/and I with the orientation degree of −0.14. Fig. 2 (a) shows integrated WAXD pattern of PP/PB-1 blend stretched to 300% at 160 °C and then isothermally crystallized at 90 °C for 0.5 h, direct formation of PB-1 crystals in form I′ or/and I were observed in spite of the formation of form II. PB-1 lamellae in form I were thicker than form I′, but they all have the same hexagonal structure with a 31 helix [15]. As shown in Fig. 2(b), the above sample was further annealed at 110 °C for 5min in order to distinguish form I and I′ crystals. Form I′ is less stable at melting temperature below 100 °C whereas form I stabilize when the melting temperature is above 120 °C [36]. Clearly, the disappearance of diffraction peak representing form I′ or/and I in Fig. 2(b) after annealing at 110 °C indicates that the crystals are definitely form I'. Therefore, isothermal crystallization at 90 °C after stretching at 160 °C result in the formation of form II and I′ of PB-1 crystals. The crystallinities of PP and PB-1 in PP/PB-1 (50/50) blends in Fig. 1(b) and (d) were calculated and showed in Table S1.
2. Experimental 2.1. Materials PB-1 with melt flow index (MFI) of 0.4 g/10 min (190 °C, 2.16 kg), isotacticity of 95.6% and weight-average molecular weight (Mw) of 734000, was supplied by DongfangHongye Chemical China. PP with MFI of 0.62 g/10 min (190 °C, 2.16 kg), was supplied by Maoming Petrochemical China, isotacticity of 96.9% and the Mw of 718000. 2.2. Samples preparation PP/PB-1 (50/50) blend was prepared with weight ratio of PP/ PB = 50/50 by using Haake twin-screw extruder at 190 °C for 5 min with shear rate of 50 rps. Recipe: PP/PB-1100 phr, antioxidant 1010 0.1 phr, antioxidant 626 0.3 phr. The non-stretched PP/PB-1 sheet for the WAXD, PCOM and POM measurements: the PP/PB-1 blend was compression molded at 190 °C for 5 min with 10 MPa pressure, then followed by quenching at 0 °C with ice-water mixture. The above prepared non-stretched PP/PB-1 sheet was annealed at 190 °C for 30min, quenched to room temperature and then etched with cyclohexane at 50 °C for 3 h. The obtained sample was coated with platinum for SEM observation. 2.3. Measurements In-situ 2D WAXD experiments were measured in a customized micro-focus X-ray beam delivery system of Xenocs France with wavelength 0.154 nm. The WAXD patterns were recorded by a Pilatus 100 K detector (DECTRIS, Swiss). The X-ray beam out of a micro-focus Cu Kα X-ray tube produced at 50 kV and 0.6 mA was focused at the sample to a size of 40*60 m2. A sample was coated with an aluminum cover, and a portable heating device (TST350, Linkam, UK) controlled the temperature. For the scattering angle 2θ, the effective range between 5° and 25°. The distance between sample and detector was 179 mm. Each pattern was collected within 1 min. The examination proceeded as follows. Samples for WAXD measurement were dumbbell specimens with dimensions of 25 mm (length) × 12 mm (neck width) × 5 mm (neck length) × 0.5 mm (thickness). The unstretched specimen was annealed at 160 °C for 5 min, and then dropped to 90 °C at a cooling rate of 10 °C/
3.2. Phase morphology investigations The LCST-like phase diagram of the PP/PB-1 blends was observed in previous work [21]. Fig. 3(a1-2) shows the initial phase morphologies of PP/PB-1 (50/50) blend at 190 °C for 5 min. The PP/PB-1 (50/50) blend exhibited bicontinuous phase separated morphology with domain size in the range of micron size (Fig. 3 (a1)). The polarizing microscope 2
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Fig. 1. In-situ 2D WAXD patterns of PP/PB-1 (50/50) blend: (a) annealed at 160 °C for 5 min; (b) isothermally crystallized at 90 °C for 0.5 h after annealing at 160 °C for 5 min; (c) stretched to 300% elongation after annealing at 160 °C for 5 min; (d) isothermal crystallized at 90 °C for 0.5 h after stretching to 300% at 160 °C.
bicontinuous phase structure of the blend after annealing at 190 °C for 30 min. The PB-1 phase domain size in Fig. 4 is larger than that Fig. 3 (a1), indicating that a longer annealing time at 190 °C results in more severe phase separation. 3.3. Confined crystallization induced form I′ PB-1 Based on Figs. 1 and 2, both form II and form I′ of PB-1 were formed in pre-stretched PP/PB-1 blend, while only PB-1 in form II crystal was formed in unstretched PP/PB-1 blend, although in both cases PB-1 melts crystallized within α form PP crystalline framework. The orientation crystallization of PB-1 in the form I'/II is influenced by stretching of the PP component. The appearance of PB-1 oriented crystallization can be explained by several theories, such as epitaxial crystallization [37,38], melt memory [38] and surface induced crystallization [29]. The widely believed epitaxial theory of helical conformation matching between α form PP and form I/I′ PB-1 cannot explain the formation of form II and absence of form I′ PB-1 in the unstretched PP/PB-1 blend, where are plenty of surfaces of α PP in contact with PB-1 melt during the crystallization of PB-1. Annealing at 160 °C for 5 min and then stretching lead to a strong orientation of PP phase and deformation of PB-1 phase. PB-1 melts relax to eliminate melt memory during annealing at 160 °C, since melting temperature of PB-1 crystals is much lower than the annealing temperature, presenting typical isotropic amorphous of PB melt after PP stretching. When the temperature was decreased to 90 °C with a rate of 10 °C/min, PB-1 crystallized in the oriented PP crystalline framework. The orientation of PB-1 chains in crystallites of both form I′ and form II is in the same direction along the tensile direction. The orientation both form I′ and form II of PB-1 can be understood as follows: although PP/PB-1 (50/50) blend is phase separated, partial miscibility between PP and PB-1 results in some stretched PB-1 chain segments at the phase boundaries which cannot be relaxed at 160 °C. It is believed that these stretched PB1 chain segments at the phase boundaries induced oriented crystallization of PB-1. Understanding of the polymorphous selection (II or I′) requires a more detailed discussion. Works on crystallization of butene-1/ethylene
Fig. 2. Integrated WAXD curves of PP/PB-1 (50/50) blend: (a) isothermally crystallized at 90 °C for 0.5 h after stretching to 300% elongation at 160 °C; (b) annealing at 110 °C for 5 min of sample (a).
image in Fig. 3 (a2) shows no crystals under this condition. The sample was then quenched to room temperature to mimic the quenching processing at 0 °C of the PP/PB-1 sheet for the WAXD measurement. Both PP and PB-1 crystallized quickly during this quenching process. The obtained PP/PB-1 (50/50) blend was then annealed 5 min at 160 °C and measured using PCOM (Fig. 3 (b1)) and POM (Fig. 3 (b2)). Phase structure of the sample at 160 °C is very similar to that of Fig. 3 (a1). This demonstrates that crystallization of PP does not alter the phase structure formed in molten state. Fig. 3 (b2) demonstrates the crystallization of PP at 160 °C. The thus treated sample was then isothermally crystallized at 90 °C for 0.5 h before taking PCOM and POM images shown in Fig. 3 (c1) and Fig. 3 (c2). More crystals are observed in Fig. 3 (c2), however, the phase morphology of the blend after annealing at 90 °C for 0.5 h hardly changed. The micron domain size determines the crystal size of both PP and PB-1 phases. Fig. 4 shows a distinct 3
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Fig. 3. Phase morphology of non-stretched PP/PB-1 (50/50) blends (a1-2) annealing at 190 °C for 5 min, then (b1-2) quenching to room temperature and annealing at 160 °C for 5 min, then (c1-2) isothermal crystallized at 90 °C for 0.5 h. 1: phase contrast microscope (PCOM) images, 2: polarizing microscope (POM) images.
Fig. 4. SEM images of non-stretched PP/PB-1 (50/50) blends after annealing at 190 °C for 30 min, then quenching to room temperature and etching with cyclohexane at 50 °C for 3 h.
Fig. 5. Schematic representation of the polymorphism crystallization of stretched PP/PB-1 (50/50) blend.
growth of form II crystals in PB-1 domains regardless the seemingly favored epitaxial relation of iPP crystals and form I′ of PB-1. The appearance of both form I′ and II PB-1 crystals during crystallization process of PB-1 in the stretched sample suggests that in some PB-1 domains the formation of form II crystals was strongly suppressed due
copolymer [16,18] showed clearly that the occurrence of stable form I′ requires a suppression of nucleation and growth of metastable form II because the growth rate of form II is much faster than form I'. In the current situation, form II PB-1 develops overwhelming nucleation and growth in the quiescent state PB-1 phase due to the fast nucleation and 4
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to the confinement effect. In the phase separated PP/PB-1 blend, there must be a rather wide distribution of the domain size. After uniaxial stretching, a sharp reduction in the cross-section area of the domains can be achieved. The domain size of PB-1 has a significant effect on the PB-1 form I′ content in PP crystalline framework in the unoriented PP/ PB-1 blends [21] in accordance with the findings on butene-1/ethylene random copolymer [16,18]. Schematic representation of such case is illustrated in Fig. 5. As discussion above, the formation of PB-1 form I′ is due to the confined effect which suppresses the nucleation and growth of form II crystals. Nevertheless, interfacial interaction certainly plays a role in guiding the nucleation of both form II and I′ crystals resulting in highly oriented structures.
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4. Conclusions Crystallization form II of PB-1 in unstretched PP/PB-1 blend and orientation crystallization form II and I′ of PB-1 in stretched PP/PB-1 blend was observed. Stretching on the PP/PB-1 blend results in the orientation of PP crystals and deformation of the PB-1 melt domain. The partial miscibility between PP and PB leads to the some stretched PB-1 chain segments that cannot be relaxed at the phase boundaries and then induced oriented crystallization of PB-1. The subsequent confinement of crystalline PB-1 within the framework of PP crystals occur in two cases including form II formation in larger PB-1 domains and form I′ formation in smaller PB domains. The confined crystallization of PB-1 within the framework of PP crystals results in the form I'. This work provides a possible solution to acquire stable form I′ PB-1 and a phenomenon of interaction between incompatible PP and PB-1. Acknowledgements This work was supported by the Key R&D Program (Major Scientific and Technological Innovation Project) of Shandong Province, Natural Science Foundation of Shandong Province (ZR2019MB072) and Taishan Scholar Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121817. References [1] L. Luciani, J. Seppälä, B. Löfgren, Poly-1-butene: its preparation, properties and challenges, Prog. Polym. Sci. 13 (1) (1988) 37–62 https://doi.org/10.1016/00796700(88)90010-X. [2] R. Androsch, M.L.D. Lorenzo, C. Schick, Mesophases in polyethylene, polypropylene, and poly(1-butene), Polymer 51 (21) (2010) 4639–4662 https://doi. org/10.1016/j.polymer.2010.07.033. [3] B.H. Clampitt, R.H. Hughes, Differential thermal analysis of polybutene-1, J. Polym. Sci. Part C: Polym.Symposia 6 (1) (1964) 43–51 https://doi.org/10.1002/polc. 5070060107. [4] S. Kopp, J.C. Wittmann, B. Lotz, Phase II to phase I crystal transformation in polybutene-1 single crystals: a reinvestigation, J. Mater. Sci. 29 (23) (1994) 6159–6166 https://doi.org/10.1007/BF00354556. [5] B. Lotz, C. Mathieu, A. Thierry, Chirality constraints in crystal-crystal Transformations: isotactic poly(1-butene) versus syndiotactic polypropylene, Macromolecules 31 (26) (1998) 9253–9257 https://doi.org/10.1021/ma9810215. [6] C. Choi, J.L. White, Crystal-crystal transformations in isotactic polybutene-1 oriented filaments and in thick molded rods, Polym. Eng. Sci. 41 (6) (2001) 933–939 https://doi.org/10.1002/pen.10792. [7] H.F. Shao, Y.P. Ma, H.R. Nie, A.H. He, Solvent vapor annealing induced polymorphic transformation of polybutene-1, Chin. J. Polym. Sci. 34 (9) (2016) 1141–1149 https://doi.org/10.1007/s10118-016-1823-3. [8] J. Shi, P. Wu, L. Li, T. Liu, L. Zhao, Crystalline transformation of isotactic polybutene-1 in supercritical CO studied by in-situ fourier transform infrared spectroscopy, Polymer 50 (23) (2009) 5598–5604 https://doi.org/10.1016/j.polymer. 2009.09.078. [9] J. Li, P. Guan, Y. Zhang, F. Xue, C. Zhou, J. Zhao, Y. Shang, M. Xue, D. Yu, S. Jiang, Shear effects on crystallization behaviors and structure transitions of isotactic poly1-butene, J. Polym. Res. 21 (9) (2014) 555 https://doi.org/10.1007/s10965-0140555-8.
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Crystallization of forms I′ and form II of polybutene-1 in stretched polypropylene/polybutene-1 blends
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Yao Xua, Yaping Maa, Chenguang Liua,∗∗, Yongfeng Menb, Aihua Hea,∗ a Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, 130022, Changchun, PR China
H I GH L IG H T S
immiscible PP/PB-1 (50/50) blend was studied by in-situ tensile 2D wide-angle X-ray diffraction. • The crystallization of PB-1 in form II and I′ in stretched PP/PB-1 blend was displayed explicitly. • Orientation • The mechanism of stable PB-1 in form I′ was discussed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Confined crystallization Orientated crystallization Polybutene-1
In this work, the effects of orientated crystal morphology of polypropylene (PP) induced by tensile stretching on the subsequent crystallization behavior of polybutene-1 (PB-1) in the incompatible PP/PB-1 (50/50) blends were studied. When the PP/PB-1 blends were stretched at a temperature above melting temperature (Tm) of PB-1 but below the Tm of PP, strong orientation of PP crystals was observed under stretching. As temperature dropped below Tm of PB-1, orientated form II and I′ of PB-1 appeared. In the framework of PP crystals, form I′ of PB-1 was formed due to the confined effect which suppressed the formation of form II crystals. Interfacial interaction played a role in guiding the nucleation of both form II and I’ crystals resulting in highly oriented structures. The mechanism of PB-1 orientation and confined crystallization under stretching in PP/PB-1 (50/50) blends was discussed.
1. Introduction When PB-1 crystallizes during cooling process, it is an unstable crystalline polymer [1]. The PB-1 metastable form II with tetragonal structure obtained directly from molten state, transforms spontaneously into stable form I with hexagonal structure which can take weeks [2,3]. Crystal form I brings PB-1 many excellent performances such as high toughness, good environmental stress crack resistance and perfect creep resistance at elevated temperature. However, this solid-solid transition causes shrinkage of the materials, which may lead to distortion of the product and limit the application of PB-1 in some aspects [4,5]. Many researches [6–11] have focused on bypassing of the disadvantageous metastable state form II. Form I′ generated directly produced by melt is considered to be importance for industrial application because it avoided the crystal transformation. One of the conditions for producing
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form I′ is to blend PB-1 with isotactic polypropylene (PP). Shieh et al. [12] firstly claimed that form I′ directly appeared from the melts of PP/ PB-1 blends. The I′-form PB-1 and α-form PP have similar 31 helix conformation. Ballesteros [13] and De Rosa [14] reported that form I′ can be obtained in butene-propylene or butene-ethylene copolymers, which is attributed to the compact type of ab plane and the c axis of the monoclinic unit cell of PB-1 phase. Men [15–18] clarified that interplay between the dimension of crystallizable chain segmental segregation in heterogeneous melt and corresponding nucleation size of form II and form I determined direct formation of either form II iPB or form I′ iPB. Li [19] claimed that after incorporation of PP, form I′ of PB-1 was found by adjusting its thermal history. And the I′-form PB-1 was attributed to molecular chain interaction. Stolte [20] and our previous work [21] indicated that form I′ was observed when PB-1 was confined in different locations.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected], [email protected] (A. He).
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https://doi.org/10.1016/j.polymer.2019.121817 Received 25 June 2019; Received in revised form 5 September 2019; Accepted 17 September 2019 Available online 17 September 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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min and kept at this temperature for 0.5 h to allow an isothermal crystallization of PB-1. The stretched specimen was firstly stretched to 300% elongation under a crosshead speed of 50 μm/s at 160 °C after being annealed for 5 min and kept for another 5 min. The stretched sample was then cooled down to 90 °C at a cooling rate of 10 °C/min and kept at this temperature for 0.5 h to allow an isothermal crystallization of PB-1. The phase morphologies of samples were observed with thickness about 25 μm by means of an Olympus optical microscope (BX 51-P) with a Linkam hot stage (LNP95) to control the observation temperature. The samples were observed with phase contrast optical microscopy (PCOM) and polarized optical microscopy (POM) modes, respectively. The etched surface of the sample was observed with scanning electron microscope (SEM, JSM-7500F).
If one component with a molten state in the other oriented component matrix with a solid state, in the molten component some unusual oriented structures were obtained, such as orthogonal orientation [22,23] or oblique orientation [24,25]. Different mechanisms were proposed to explain these orientation behaviors, such as epitaxial crystal growth [16–20], heat-shrinking stress [26] or transcrystallization [27,28]. The PB-1 in form I′ is rarely formed directly from the molten state unless under the conditions of ultrathin film [29], selfseeding [30,31], high pressure [32], presence of counits [20] and blends [19,21]. In the PP/PB-1 mixture, the mechanism of formation of I′ PB-1 has been attributed to distinct reasons [33] like melt memory [34], molecular chain interaction [19], interfacial nucleation [35] or confined crystallization [20,21]. The controversy on mechanism of the form of I′ PB-1 in PP/PB-1 blends requires more convincible experiments to support. In this paper, PP/PB-1 blends were 300% stretched at 160 °C and then held during the subsequent cooling process. By means of in-situ 2D wide-angle X-ray diffraction (2D WAXD) experiments, the orientated PB-1 crystallization in the presence of orientated PP crystalline phase has been investigated. It was discovered that pre-orientated PP crystals have a lot of effect on the orientated crystallization and directly lead to the formation of form I′ of PB-1 from molten state.
3. Results and discussion 3.1. In-situ 2D WAXD When the crystallization temperature (Tc) of PP above the melting temperature (Tm) of PB-1, PB-1 still in melting state during the crystallization process of PP. As temperature decreases further, PB-1 crystallizes subsequently within crystalline framework of PP. Fig. 1 shows in-situ 2D WAXD patterns of the PP/PB-1 (50/50) blends. The PP/PB-1 (50/50) blend after annealed at 160 °C for 5 min shows undirected α PP with identified reflections of (110), (130) and (040) planes, as shown in Fig. 1(a). With the further cooling to 90 °C, undirected meta-stable form II crystals of PB-1 with identified reflections of (200) and (213) planes are observed as given in Fig. 1(b). Fig. 1(c) presents WAXD pattern of the sample which was stretched to 300% elongation at 160 °C. Clearly, the stretching yields a highly oriented structure of PP crystals in form α with chain axis along stretching direction with an orientation of −0.5. The degree of orientation in form α of PP, form II and form I/I′ crystals of PB-1 is measured by azimuthal scan of 110 plane of PP, 200 plane of PB-1 form II and 110 plane of PB form I/I' (Fig. S1), respectively. While PB-1 melt shows typical isotropic amorphous scattering on the pattern at this temperature. When this stretched sample with high oriented PP crystals and PB-1 melt was cooled down to 90 °C, oriented crystallization of PB-1 also occurred. WAXD pattern in Fig. 1(d) indicates the existence of PB-1 crystals are oriented form II with the orientation degree of −0.43 and oriented form I′ or/and I with the orientation degree of −0.14. Fig. 2 (a) shows integrated WAXD pattern of PP/PB-1 blend stretched to 300% at 160 °C and then isothermally crystallized at 90 °C for 0.5 h, direct formation of PB-1 crystals in form I′ or/and I were observed in spite of the formation of form II. PB-1 lamellae in form I were thicker than form I′, but they all have the same hexagonal structure with a 31 helix [15]. As shown in Fig. 2(b), the above sample was further annealed at 110 °C for 5min in order to distinguish form I and I′ crystals. Form I′ is less stable at melting temperature below 100 °C whereas form I stabilize when the melting temperature is above 120 °C [36]. Clearly, the disappearance of diffraction peak representing form I′ or/and I in Fig. 2(b) after annealing at 110 °C indicates that the crystals are definitely form I'. Therefore, isothermal crystallization at 90 °C after stretching at 160 °C result in the formation of form II and I′ of PB-1 crystals. The crystallinities of PP and PB-1 in PP/PB-1 (50/50) blends in Fig. 1(b) and (d) were calculated and showed in Table S1.
2. Experimental 2.1. Materials PB-1 with melt flow index (MFI) of 0.4 g/10 min (190 °C, 2.16 kg), isotacticity of 95.6% and weight-average molecular weight (Mw) of 734000, was supplied by DongfangHongye Chemical China. PP with MFI of 0.62 g/10 min (190 °C, 2.16 kg), was supplied by Maoming Petrochemical China, isotacticity of 96.9% and the Mw of 718000. 2.2. Samples preparation PP/PB-1 (50/50) blend was prepared with weight ratio of PP/ PB = 50/50 by using Haake twin-screw extruder at 190 °C for 5 min with shear rate of 50 rps. Recipe: PP/PB-1100 phr, antioxidant 1010 0.1 phr, antioxidant 626 0.3 phr. The non-stretched PP/PB-1 sheet for the WAXD, PCOM and POM measurements: the PP/PB-1 blend was compression molded at 190 °C for 5 min with 10 MPa pressure, then followed by quenching at 0 °C with ice-water mixture. The above prepared non-stretched PP/PB-1 sheet was annealed at 190 °C for 30min, quenched to room temperature and then etched with cyclohexane at 50 °C for 3 h. The obtained sample was coated with platinum for SEM observation. 2.3. Measurements In-situ 2D WAXD experiments were measured in a customized micro-focus X-ray beam delivery system of Xenocs France with wavelength 0.154 nm. The WAXD patterns were recorded by a Pilatus 100 K detector (DECTRIS, Swiss). The X-ray beam out of a micro-focus Cu Kα X-ray tube produced at 50 kV and 0.6 mA was focused at the sample to a size of 40*60 m2. A sample was coated with an aluminum cover, and a portable heating device (TST350, Linkam, UK) controlled the temperature. For the scattering angle 2θ, the effective range between 5° and 25°. The distance between sample and detector was 179 mm. Each pattern was collected within 1 min. The examination proceeded as follows. Samples for WAXD measurement were dumbbell specimens with dimensions of 25 mm (length) × 12 mm (neck width) × 5 mm (neck length) × 0.5 mm (thickness). The unstretched specimen was annealed at 160 °C for 5 min, and then dropped to 90 °C at a cooling rate of 10 °C/
3.2. Phase morphology investigations The LCST-like phase diagram of the PP/PB-1 blends was observed in previous work [21]. Fig. 3(a1-2) shows the initial phase morphologies of PP/PB-1 (50/50) blend at 190 °C for 5 min. The PP/PB-1 (50/50) blend exhibited bicontinuous phase separated morphology with domain size in the range of micron size (Fig. 3 (a1)). The polarizing microscope 2
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Fig. 1. In-situ 2D WAXD patterns of PP/PB-1 (50/50) blend: (a) annealed at 160 °C for 5 min; (b) isothermally crystallized at 90 °C for 0.5 h after annealing at 160 °C for 5 min; (c) stretched to 300% elongation after annealing at 160 °C for 5 min; (d) isothermal crystallized at 90 °C for 0.5 h after stretching to 300% at 160 °C.
bicontinuous phase structure of the blend after annealing at 190 °C for 30 min. The PB-1 phase domain size in Fig. 4 is larger than that Fig. 3 (a1), indicating that a longer annealing time at 190 °C results in more severe phase separation. 3.3. Confined crystallization induced form I′ PB-1 Based on Figs. 1 and 2, both form II and form I′ of PB-1 were formed in pre-stretched PP/PB-1 blend, while only PB-1 in form II crystal was formed in unstretched PP/PB-1 blend, although in both cases PB-1 melts crystallized within α form PP crystalline framework. The orientation crystallization of PB-1 in the form I'/II is influenced by stretching of the PP component. The appearance of PB-1 oriented crystallization can be explained by several theories, such as epitaxial crystallization [37,38], melt memory [38] and surface induced crystallization [29]. The widely believed epitaxial theory of helical conformation matching between α form PP and form I/I′ PB-1 cannot explain the formation of form II and absence of form I′ PB-1 in the unstretched PP/PB-1 blend, where are plenty of surfaces of α PP in contact with PB-1 melt during the crystallization of PB-1. Annealing at 160 °C for 5 min and then stretching lead to a strong orientation of PP phase and deformation of PB-1 phase. PB-1 melts relax to eliminate melt memory during annealing at 160 °C, since melting temperature of PB-1 crystals is much lower than the annealing temperature, presenting typical isotropic amorphous of PB melt after PP stretching. When the temperature was decreased to 90 °C with a rate of 10 °C/min, PB-1 crystallized in the oriented PP crystalline framework. The orientation of PB-1 chains in crystallites of both form I′ and form II is in the same direction along the tensile direction. The orientation both form I′ and form II of PB-1 can be understood as follows: although PP/PB-1 (50/50) blend is phase separated, partial miscibility between PP and PB-1 results in some stretched PB-1 chain segments at the phase boundaries which cannot be relaxed at 160 °C. It is believed that these stretched PB1 chain segments at the phase boundaries induced oriented crystallization of PB-1. Understanding of the polymorphous selection (II or I′) requires a more detailed discussion. Works on crystallization of butene-1/ethylene
Fig. 2. Integrated WAXD curves of PP/PB-1 (50/50) blend: (a) isothermally crystallized at 90 °C for 0.5 h after stretching to 300% elongation at 160 °C; (b) annealing at 110 °C for 5 min of sample (a).
image in Fig. 3 (a2) shows no crystals under this condition. The sample was then quenched to room temperature to mimic the quenching processing at 0 °C of the PP/PB-1 sheet for the WAXD measurement. Both PP and PB-1 crystallized quickly during this quenching process. The obtained PP/PB-1 (50/50) blend was then annealed 5 min at 160 °C and measured using PCOM (Fig. 3 (b1)) and POM (Fig. 3 (b2)). Phase structure of the sample at 160 °C is very similar to that of Fig. 3 (a1). This demonstrates that crystallization of PP does not alter the phase structure formed in molten state. Fig. 3 (b2) demonstrates the crystallization of PP at 160 °C. The thus treated sample was then isothermally crystallized at 90 °C for 0.5 h before taking PCOM and POM images shown in Fig. 3 (c1) and Fig. 3 (c2). More crystals are observed in Fig. 3 (c2), however, the phase morphology of the blend after annealing at 90 °C for 0.5 h hardly changed. The micron domain size determines the crystal size of both PP and PB-1 phases. Fig. 4 shows a distinct 3
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Fig. 3. Phase morphology of non-stretched PP/PB-1 (50/50) blends (a1-2) annealing at 190 °C for 5 min, then (b1-2) quenching to room temperature and annealing at 160 °C for 5 min, then (c1-2) isothermal crystallized at 90 °C for 0.5 h. 1: phase contrast microscope (PCOM) images, 2: polarizing microscope (POM) images.
Fig. 4. SEM images of non-stretched PP/PB-1 (50/50) blends after annealing at 190 °C for 30 min, then quenching to room temperature and etching with cyclohexane at 50 °C for 3 h.
Fig. 5. Schematic representation of the polymorphism crystallization of stretched PP/PB-1 (50/50) blend.
growth of form II crystals in PB-1 domains regardless the seemingly favored epitaxial relation of iPP crystals and form I′ of PB-1. The appearance of both form I′ and II PB-1 crystals during crystallization process of PB-1 in the stretched sample suggests that in some PB-1 domains the formation of form II crystals was strongly suppressed due
copolymer [16,18] showed clearly that the occurrence of stable form I′ requires a suppression of nucleation and growth of metastable form II because the growth rate of form II is much faster than form I'. In the current situation, form II PB-1 develops overwhelming nucleation and growth in the quiescent state PB-1 phase due to the fast nucleation and 4
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to the confinement effect. In the phase separated PP/PB-1 blend, there must be a rather wide distribution of the domain size. After uniaxial stretching, a sharp reduction in the cross-section area of the domains can be achieved. The domain size of PB-1 has a significant effect on the PB-1 form I′ content in PP crystalline framework in the unoriented PP/ PB-1 blends [21] in accordance with the findings on butene-1/ethylene random copolymer [16,18]. Schematic representation of such case is illustrated in Fig. 5. As discussion above, the formation of PB-1 form I′ is due to the confined effect which suppresses the nucleation and growth of form II crystals. Nevertheless, interfacial interaction certainly plays a role in guiding the nucleation of both form II and I′ crystals resulting in highly oriented structures.
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4. Conclusions Crystallization form II of PB-1 in unstretched PP/PB-1 blend and orientation crystallization form II and I′ of PB-1 in stretched PP/PB-1 blend was observed. Stretching on the PP/PB-1 blend results in the orientation of PP crystals and deformation of the PB-1 melt domain. The partial miscibility between PP and PB leads to the some stretched PB-1 chain segments that cannot be relaxed at the phase boundaries and then induced oriented crystallization of PB-1. The subsequent confinement of crystalline PB-1 within the framework of PP crystals occur in two cases including form II formation in larger PB-1 domains and form I′ formation in smaller PB domains. The confined crystallization of PB-1 within the framework of PP crystals results in the form I'. This work provides a possible solution to acquire stable form I′ PB-1 and a phenomenon of interaction between incompatible PP and PB-1. Acknowledgements This work was supported by the Key R&D Program (Major Scientific and Technological Innovation Project) of Shandong Province, Natural Science Foundation of Shandong Province (ZR2019MB072) and Taishan Scholar Program. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121817. References [1] L. Luciani, J. Seppälä, B. Löfgren, Poly-1-butene: its preparation, properties and challenges, Prog. Polym. Sci. 13 (1) (1988) 37–62 https://doi.org/10.1016/00796700(88)90010-X. [2] R. Androsch, M.L.D. Lorenzo, C. Schick, Mesophases in polyethylene, polypropylene, and poly(1-butene), Polymer 51 (21) (2010) 4639–4662 https://doi. org/10.1016/j.polymer.2010.07.033. [3] B.H. Clampitt, R.H. Hughes, Differential thermal analysis of polybutene-1, J. Polym. Sci. Part C: Polym.Symposia 6 (1) (1964) 43–51 https://doi.org/10.1002/polc. 5070060107. [4] S. Kopp, J.C. Wittmann, B. Lotz, Phase II to phase I crystal transformation in polybutene-1 single crystals: a reinvestigation, J. Mater. Sci. 29 (23) (1994) 6159–6166 https://doi.org/10.1007/BF00354556. [5] B. Lotz, C. Mathieu, A. Thierry, Chirality constraints in crystal-crystal Transformations: isotactic poly(1-butene) versus syndiotactic polypropylene, Macromolecules 31 (26) (1998) 9253–9257 https://doi.org/10.1021/ma9810215. [6] C. Choi, J.L. White, Crystal-crystal transformations in isotactic polybutene-1 oriented filaments and in thick molded rods, Polym. Eng. Sci. 41 (6) (2001) 933–939 https://doi.org/10.1002/pen.10792. [7] H.F. Shao, Y.P. Ma, H.R. Nie, A.H. He, Solvent vapor annealing induced polymorphic transformation of polybutene-1, Chin. J. Polym. Sci. 34 (9) (2016) 1141–1149 https://doi.org/10.1007/s10118-016-1823-3. [8] J. Shi, P. Wu, L. Li, T. Liu, L. Zhao, Crystalline transformation of isotactic polybutene-1 in supercritical CO studied by in-situ fourier transform infrared spectroscopy, Polymer 50 (23) (2009) 5598–5604 https://doi.org/10.1016/j.polymer. 2009.09.078. [9] J. Li, P. Guan, Y. Zhang, F. Xue, C. Zhou, J. Zhao, Y. Shang, M. Xue, D. Yu, S. Jiang, Shear effects on crystallization behaviors and structure transitions of isotactic poly1-butene, J. Polym. Res. 21 (9) (2014) 555 https://doi.org/10.1007/s10965-0140555-8.
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