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Precursor assisted crystallization in cross-linked isotactic polypropylene
Polymer 180 (2019) 121674
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Polymer journal homepage: www.elsevier.com/locate/polymer
Precursor assisted crystallization in cross-linked isotactic polypropylene a
b
a
a,c
Jianzhu Ju , Nan Tian , Zhen Wang , Fengmei Su , Haoran Yang Sarmad Alia, Yuanfei Lina, Liangbin Lia,*
a,d
a
T a
, Jiarui Chang , Xueyu Li ,
a
National Synchrotron Radiation Lab, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China MOE Key Laboratory of Space Applied Physics and Chemistry, Shaanxi Key Laboratory of Macromolecular Science and Technology, School of Science, Northwestern Polytechnical University, Xi'an, 710072, China c National Engineering Research Center for Advanced Polymer Processing Technology, The Key Laboratory of Material Processing and Mold of Ministry of Education, Zhengzhou University, Zhengzhou, 450002, China d State Laboratory of Surface and Interface Science and Technology, School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002, PR China b
H I GH L IG H T S
induced precursor with long lifetime is observed in extended cross-linked iPP. • Flow model of precursor assisted crystallization is proposed based on the results. • The • Daughter lamellar with high self-similarity can be observed under SAXS.
A R T I C LE I N FO
A B S T R A C T
Keywords: Flow induced crystallization Precursor SAXS/WAXS
Combining the extension rheometry and simultaneous in-situ synchrotron radiation small/wide angle X-ray scattering (SAXS/WAXS) measurement, extension induced crystallization of cross-linked isotactic polypropylene (CL-iPP) is studied at different temperature from 130 to 160 °C. After extension at strain of 1.5, precursor is generated before crystallization and the relative stability increases with temperature. Precursor growth is confined due to limited chain diffusion, resulting in a periodic two-dimensional distribution. Crystallization takes place later with the existence of precursor, accompanied with the slowdown of precursor dynamics. Based on the results, a model of precursor assisted crystallization is proposed to illustrate this unique phase behavior.
1. Introduction In flow induced crystallization (FIC) of polymer, the effective extension of molecular chains can dramatically change the structural dynamics and final morphology. Hierarchical structure formation can be controlled by flow parameters, leading to different macroscopic material properties. With complex chain structures such as polydispersity, branching and blending, chain response and structure formation will be more complicated. On the other hand, these unique characters will provide new approaches to material development in polymer industry. It has been widely investigated that existing ordered pre-structure can influence crystallization behavior, such as nucleation rate [1–3], structure orientation [4], crystal modification [5,6], etc. The pre-existing ordering provides a substrate with high affinity and reduces the local surface free energy penalty. By applying external ordered/
*
oriented structures such as nanotubes [7,8] and solid surface [8–10], it is possible to accelerate crystallization and precisely induce the desired structure. On the other hand, nucleation substrate generated from polymer itself has been proved to be the most efficient for its closest morphology [11–13], which is normally described as self-nucleation. Self-nucleation always involves a special thermal history or chain orientation over melting temperature to induce non-crystalline self-seeds [2,4,14]. The influence of the initial condition in self-nucleation on dynamics and morphology have been studied in detail [2,13,15–17]. It has also been reported that self-seeds can be produced under flow field in a successive condition without pretreatment at high temperature [6,18–22]. Instead of preparing self-seeds as an initial condition, successive flow induced self-nucleation under constant supercooling degree is more similar to the practical processing situation. In certain condition such as high temperature and fast flow, non-crystalline precursor with lower energy barrier and high defect tolerance can be
Corresponding author. E-mail address: [email protected] (L. Li).
https://doi.org/10.1016/j.polymer.2019.121674 Received 10 May 2019; Received in revised form 17 July 2019; Accepted 21 July 2019 Available online 22 July 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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favored over crystallization, as a metastable phase with local minimum free energy [23,24]. The flow induced precursor can behave as selfseeds and influence the following crystallization. In this case, one has to consider the interaction between crystal and precursor, concerning the significant thermodynamics difference. Indeed, precursor formation and growth play an integral role in crystallization process. It has been reported that at the late stage of induction period, precursor dynamics and morphology is similar to crystalline one, indicating the coupling between precursor formation and crystallization [25,26]. On the other hand, the generation and growth of the precursor can also compete with crystallization, showing strong dynamics dependence [20,27,28]. Due to the complex and important role of precursor during FIC, in-depth investigation of the dynamics competition and coupling in the hierarchical structure will help to understand this unique mechanism. Even though flow induced precursor has been observed in lots of works, it is mostly treated as only an intermedium phase and the experimental parameter controlling is far from ideal [21]. The understanding of physics mechanism of flow induced precursor and precursor/crystal interaction is still not clear. In our previous work [29], we produced and characterized a unique system of γ-irradiated isotactic polypropylene (iPP). The partially crosslinked iPP shows asymmetric relaxation dynamics under flow and polydisperse systems have been discovered in favor of precursor generation [30,31]. In this paper, we applied extensional flow on the system under different temperature to study precursor generation and crystallization process. Simultaneous SAXS/WAXS measurement is used to investigate the structure morphology and dynamics at different spatial scale. The structural and dynamical difference between crosslinking network and free chains managed to highly enlarge dynamics difference of crystallization and precursor generation. After extension, flow induced precursor can be observed at long time before crystallization. Cross-shaped signal can be observed in SAXS later, indicating a periodic two-dimensional distribution of the precursor. With continuous generation and growth of precursor, crystallization is finally favored over precursor formation. Based on the results, we propose a model of precursor assisted crystallization and provide a microscopic explanation on the structural and dynamical mechanism.
Fig. 1. Illustration of simultaneous SAXS/WAXS testing on iPP. 2D SAXS and WAXS patterns after extension under 160 °C: (a) WAXS, (b) SAXS.
detectable reflections, which are (110), (040), (130) at 11.4°, 13.6° and 14.8°, respectively. The (110) reflection is commonly applied to indicate the starting of crystallization for its strongest intensity. For nucleation at lower strain, daughter crystallization has been reported favored over the parent one [33]. Unfortunately, (110) reflection for daughter crystal orients along the meridian direction, which is missed in our measurement. As a result, here (040) reflection is applied to indicate the onset of crystallization and characterize crystallization kinetic. Different reflections will influence the characterization of crystallization onset time but the difference is acceptable in our experiments with time period over 300 s. Azimuthal integration is performed with the inner and outer boundary indicated in Fig. 1. Zero azimuth has also been indicated and integral is oriented in the counterclockwise. Fig. 2 provides SAXS and WAXS patterns under different temperatures after extension. At 130 °C, crystallization begins right after extension, which can be discovered from crystalline reflections in WAXS (3 s). Meridian oriented signal can also be observed in SAXS right after extension, with intensity increasing rapidly. At higher temperature from 140 to 160 °C, it is clearly observed that SAXS signal shows up after an induction time but much earlier than WAXS crystal reflections. SAXS peaks before Bragg peaks in WAXS corresponds to a periodic distribution of non-crystalline region with higher density, indicating the generation of the precursor structure. Remarkably, precursor under 160 °C lasts around 400 s before crystallization. From 130 to 150 °C, an unusual bifurcated signal can be observed in SAXS along the equation direction at different times after crystallization. The simultaneous measurement technology allows us to investigate the structure morphology and phase transition dynamics in detail. WAXS and SAXS intensity curves under different temperatures are provided in Fig. 3. Induction periods are decided by the intersection of intensity baseline and the slope line of intensity file to avoid error from intensity resolution at earlier stage. Under 130 °C, it can be discovered from Fig. 3(a) that the crystallization kinetics falls behind SAXS ones, although they all start immediately after extension. At around 270 s, both intensities in SAXS begin to decrease due to high crystallinity [34]. Under all experiment temperature, the induction time for three different structures can be distinguished, as indicated in Fig. 3. For 160 °C, since precursor is generated around 300 s before crystallization, more detail transition can be observed. From SAXS intensity, equatorial and meridian oriented structures are generated at around 550 s and 750 s, respectively, corresponding well with 2D patterns. Crystallization starts at around 860 s, after which both intensities in SAXS increase faster. SAXS 2D intensity under 160 °C as a function of q and time along the meridian direction is provided in Fig. 4 (a). Along q axis, a distinct
2. Experiment Pure iPP material was provided by SABIC-Europe with number- and weight-average molecular masses (Mn and Mw) of 150 and 720 kg/mol, respectively. To prepare CL-iPP, Chemical grade trimethylolpropane triacrylate (TMPTA) was applied as a crosslinking agent, with amount of 0.2 mmol in 100 g of iPP. Irradiation dose was set at 10.4 kGy with dose rate of 0.65 kGy/h. Gel content in the CL-iPP is 61.2%. The chain structure and rheological characterization of the CL-iPP has been provided in our previous work [29]. Extension is performed with a home-made rheometer which has been introduced in detail somewhere else [27]. CL-IPP is firstly heated to 210 °C and remains for 10 min to erase the thermal history. After that, sample is cooled to desired temperatures with cooling rate of 2 °C/ min. Then the extension with strain of 1.5 and strain rate of 5 s−1 is applied to the sample. After extension, the structure evolution process is measured by in situ simultaneous 2D SAXS and WAXD measurement with wavelength of 0.124 nm and time resolution of 3 s. The experiments were performed at the beamlines BL16B of Shanghai Synchrotron Radiation Facility (SSRF). 3. Results 2D SAXS and WAXS patterns at 2400 s after extension under 160 °C are provided in Fig. 1, as a demonstration of the SAXS/WAXS simultaneous measurement and data processing. Because of the arrangement of SAXS and WAXS detector [32], only part of WAXS patterns can be detected. For α modification of iPP, there are three 2
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Fig. 2. 2D SAXS and WAXS patterns after extension at different temperatures. The last collected patterns for each temperature are rescaled in intensity to adjust the contract.
network and free chains response differently under extension. Crosslinked network can be effectively extended while shorter free chains are squeezed out. For network dominated domain, effective extension and slow relaxation result in high orientation, while at the same time the squeezing out of the shorter chains reduces the chain density. Inside crosslinking units, chain diffusion is limited by cross-linking points created during irradiation. For free chains dominated domain, chain density and diffusion ability are higher while the orientation degree is reduced due to strong disentanglement and relaxation. Extension of the material results in a heterogeneous system where network and free chains have different orientation degree, diffusion ability and chains density. As a result, in the stretched network region with lower chain density and diffusion ability, periodic precursor is favored over crystallization due to lower energy barrier and high conformational defect tolerance [27]. Note that the deformation response of cross-linked part is almost independent of temperature, and for short chains, relaxation time decreases with temperature. Compared to crystal, precursor is relatively more stable under higher temperature since high temperature will further enlarge the energy barrier difference. The difference of generation time of precursor and crystal, namely precursor period here, reflects the relative stability of precursor structure. Below 160 °C, the generation of different structures cannot be directly distinguished from 2D patterns, but fitted dynamics difference can be observed in 1D intensities, as seen in Fig. 3. After fitting and calculation, precursor period is discovered increasing with temperature, which are 12 s (130 °C), 76 s (140 °C), 89 s (150 °C) and 310 s (160 °C), respectively. The oriented precursor will reduce the nucleating energy barrier and provide nucleating sites, so that crystallization can be more easily
maximum can be observed after 600 s. The peak intensity of the precursor increases with time even before crystallization, indicating the generation and ordering of the non-crystalline periodic structure. Long period of the periodic precursor can be calculated from the intensity maximum, which is shown in Fig. 4(b). The decrease rate (dL/dt) of long period is also calculated. The long period in SAXS continuously decreases, while dL/dt reaches a minimum at around 850 s, which corresponds well to the onset time of crystal nucleation. To investigate the mesoscopic orientation behavior, azimuthal intensity curves of the last pattern of SAXS under different temperature are shown in Fig. 5(a), where starting azimuthal angles and integration direction are indicated in Fig. 1. Intensity along meridian direction has been renormalized to show the comparison of the azimuthal distribution. It can be discovered that the relative intensity of the equator direction peak generally increases with temperature. At 140 °C and 150 °C, two peaks can be observed near the meridian direction, with a deviation of around 9°. From 2D intensity under 150 °C in Fig. 5(b), the azimuthal angle distribution remains the same during crystallization.
4. Discussion The simultaneous SAXS/WAXS measurement with high time resolution provides a convincing result of flow induced phase transition at different temperatures. Mesoscopic/microscopic transition dynamics and their interaction can be investigated from the hierarchical structural information. In our previous work [29], we have reported that relaxation asymmetry can be induced in γ-irradiated cross-linked iPP, where gel 3
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Fig. 3. 1D integration intensity as functions of time for (a) 130 °C, (b) 140 °C, (c) 150 °C and (d) 160 °C. SAXS and WAXS intensities under 160 °C are shown on logarithmic scales for small change at the early stage of the phase transition. Induction periods are decided by the intersection of intensity baseline and the slope line (Fig. 3(a)).
Fig. 4. (a) 2D Intensity as functions of q and time for 90° in SAXS at 160 °C. (b) Meridian long period of SAXS and the decrease rate of the long period (dL/dt).
crystallization. (i) Primary precursor. As discussed, precursor generation can be favored over crystallization due to lower energy barrier and higher defect tolerance. Precursors locate along the flow direction, corresponding to blob signal in the early stage along equator direction in SAXS. Long period increases with temperature, indicating lower
induced. As seen in Fig. 2, at later stage under different temperature, crystallization takes place with the existence of precursor. The kinetics and morphology of precursor and crystal can be related by simultaneous SAXS/WAXS testing. We propose here a schematic diagram in Fig. 6 to demonstrate the mechanism of precursor assisted 4
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Fig. 5. (a) Azimuth intensity at the end of the measurement at different temperatures. Meridian direction has been indicated and meridian intensity has been normalized. (b) Azimuth intensity as the function of time at 150 °C.
Fig. 6. Structure transition and corresponding SAXS/WAXS patterns at different stages after extension.
structure in our results is non-crystalline. The non-crystalline block here provides nucleating sites, so that crystallization can be more easily induced in these regions. During the growth of the block precursor, the polymer chains between precursor domains are arranged into precursor. (iii) Inner crystallization of precursor. With the continuous decrease of the free volume between precursors, the deformation of random coils toward the conformation in precursor gets more difficult due to the increase of energy barrier. Gradually, crystallization is dynamically favored and replace the generation of precursor. From long period results in Fig. 4(b), the reduction kinetic of long period slows down after the emerging of crystal signal at 850 s, indicating that the generation and growth of precursor have been suppressed due to crystallization. With the existence of precursor, crystallization takes place in precursor domain more easily, since chains are more oriented and quasi-ordered. After that, SAXS signal is contributed to density differences from both precursor/melt and crystal/melt. From Fig. 3, SAXS intensities increase much faster after crystallization while the generation and growth of precursor are slower here. It can be inferred that the rapid intensity increase in SAXS after crystallization is mainly contributed to the averaged density increase of precursor/crystal composite due to inner crystallization. As seen in Fig. 4, from 130 °C to 150 °C, bifurcated signals can be
precursor density at high temperature. Under the remaining stress conserved by the extended cross-linked network, precursor can be continuously generated. With thickening and continuous generation of the precursor, the average distance between the domains decreases, resulting in the reduction of long period, as seen in Fig. 4. This is different from the widely-accepted result of phase separation by spinodal decomposition where long period increases with time, since the precursor growth in those static situation follows self-similarity [35–38]. In this case of flow induced phase transition, even at early stage, the concentration of quasi-ordered domain is much higher than that in static situation. The interpolation of precursor will be highly favored over epitaxial growth. (ii) Confined growth of precursor. In crosslinked domain, chain diffusion is limited due to the lower motion ability. With the interpolation and growth of the precursor, free volume decreases rapidly. As a result, the precursor is difficult to grow and the size is highly limited. The confined growth of precursor leads to a periodic two-dimensional distribution of the precursor and it can be defined from the cross-shaped signal in SAXS. The cross-shaped signal has been observed in FIC of cross-linking PEO [39] and quenched iPP [40]. Strobl's model is normally applied to describe the block-like structure, which regards it as an intermediate structure [41–44]. From the SAXS and WAXS structural dynamics, we can confirm that the block 5
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observed after crystallization along equator direction. The azimuthal intensity distribution remains constant during crystallization with two peaks located at 9° away from the equator direction. The azimuthal distribution corresponds well with “cross-hatched” lamellar morphology in parent/daughter crystal [45]. It can be concluded that there are two branches of daughter lamellar with a deflection angle of 9° from parent lamellar, as chirality coupling in microscopic scale. In the SAXS characterization of iPP, it has always been hard to distinguish block structure and cross-hatched structure [40,46]. Here, with simultaneous SAXS/WAXS measurement, equator intensity increases before the onset of crystallization and bifurcated signal shows up in the late stage of crystallization. These two kind of structures can be clearly distinguished. Also for the first time, ordered daughter lamellar growth has been observed in SAXS, following highly self-similarity.
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5. Conclusion In summary, precursor assisted crystallization in cross-linked iPP is observed with simultaneous SAXS/WAXS measurement. Different relaxation dynamics of cross-linked and free chains under flow result in the heterogeneous distribution of chain density, diffusion and orientation. Precursors can be induced before crystallization and the relative stability increases with temperature. Precursor shows a periodic twodimensional distribution due to the confined growth limited by low chain diffusion ability. Eventually, crystallization can be induced due to the lower energy barrier with the assistance of existing precursor. Acknowledgements The authors would like to thank the assistance of Dr. Feng Tian in synchrotron measurement. This work is supported financially by the National Natural Science Foundation of China (Grant Nos. 51227801, 51325301, 21504069). References [1] F. Su, Y. Ji, L. Meng, J. Chang, L. Chen, L. Li, Shear-induced precursors in polyethylene: an in-situ synchrotron radiation scanning X-ray microdiffraction study, Polymer 135 (2018) 61–68. [2] F. Su, W. Zhou, X. Li, Y. Ji, K. Cui, Z. Qi, L. Li, Flow-induced precursors of isotactic polypropylene: an in situ time and space resolved study with synchrotron radiation scanning X-ray microdiffraction, Macromolecules 47 (13) (2014) 4408–4416. [3] K. Cui, L. Meng, N. Tian, W. Zhou, Y. Liu, Z. Wang, J. He, L. Li, Self-acceleration of nucleation and formation of shish in extension-induced crystallization with strain beyond fracture, Macromolecules 45 (13) (2012) 5477–5486. [4] Y. Hayashi, G. Matsuba, Y. Zhao, K. Nishida, T. Kanaya, Precursor of shish–kebab in isotactic polystyrene under shear flow, Polymer 50 (9) (2009) 2095–2103. [5] R.H. Somani, B.S. Hsiao, A. Nogales, H. Fruitwala, S. Srinivas, A.H. Tsou, Structure development during shear flow induced crystallization of i-PP: in situ wide-angle Xray diffraction study, Macromolecules 34 (17) (2001) 5902–5909. [6] B. Zhang, J. Chen, F. Ji, X. Zhang, G. Zheng, C. Shen, Effects of melt structure on shear-induced β-cylindrites of isotactic polypropylene, Polymer 53 (8) (2012) 1791–1800. [7] X. Hu, H. An, Z.-M. Li, Y. Geng, L. Li, C. Yang, Origin of carbon nanotubes induced poly(L-lactide) crystallization: surface induced conformational order, Macromolecules 42 (8) (2009) 3215–3218. [8] Y.-H. Chen, G.-J. Zhong, J. Lei, Z.-M. Li, B.S. Hsiao, In situ synchrotron X-ray scattering study on isotactic polypropylene crystallization under the coexistence of shear flow and carbon nanotubes, Macromolecules 44 (20) (2011) 8080–8092. [9] H. Li, S. Jiang, J. Wang, D. Wang, S. Yan, Optical microscopic study on the morphologies of isotactic polypropylene induced by its homogeneity fibers, Macromolecules 36 (8) (2003) 2802–2807. [10] H. Ishida, P. Bussi, Surface induced crystallization in ultrahigh-modulus polyethylene fiber-reinforced polyethylene composites, Macromolecules 24 (12) (1991) 3569–3577. [11] W. Banks, M. Gordon, A. Sharples, The crystallization of polyethylene after partial melting, Polymer 4 (1963) 289–302. [12] D. Blundell, A. Keller, A. Kovacs, A new self‐nucleation phenomenon and its application to the growing of polymer crystals from solution, J. Polym. Sci. B Polym. Lett. 4 (7) (1966) 481–486. [13] A.T. Lorenzo, M.L. Arnal, J.J. Sanchez, A.J. Müller, Effect of annealing time on the self‐nucleation behavior of semicrystalline polymers, J. Polym. Sci. B Polym. Phys. 44 (12) (2006) 1738–1750. [14] R. Michell, A. Mugica, M. Zubitur, A. Müller, Self-nucleation of crystalline phases within homopolymers, polymer blends, copolymers, and nanocomposites, Polymer Crystallization I, Springer, 2015, pp. 215–256.
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Precursor assisted crystallization in cross-linked isotactic polypropylene a
b
a
a,c
Jianzhu Ju , Nan Tian , Zhen Wang , Fengmei Su , Haoran Yang Sarmad Alia, Yuanfei Lina, Liangbin Lia,*
a,d
a
T a
, Jiarui Chang , Xueyu Li ,
a
National Synchrotron Radiation Lab, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China MOE Key Laboratory of Space Applied Physics and Chemistry, Shaanxi Key Laboratory of Macromolecular Science and Technology, School of Science, Northwestern Polytechnical University, Xi'an, 710072, China c National Engineering Research Center for Advanced Polymer Processing Technology, The Key Laboratory of Material Processing and Mold of Ministry of Education, Zhengzhou University, Zhengzhou, 450002, China d State Laboratory of Surface and Interface Science and Technology, School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou, 450002, PR China b
H I GH L IG H T S
induced precursor with long lifetime is observed in extended cross-linked iPP. • Flow model of precursor assisted crystallization is proposed based on the results. • The • Daughter lamellar with high self-similarity can be observed under SAXS.
A R T I C LE I N FO
A B S T R A C T
Keywords: Flow induced crystallization Precursor SAXS/WAXS
Combining the extension rheometry and simultaneous in-situ synchrotron radiation small/wide angle X-ray scattering (SAXS/WAXS) measurement, extension induced crystallization of cross-linked isotactic polypropylene (CL-iPP) is studied at different temperature from 130 to 160 °C. After extension at strain of 1.5, precursor is generated before crystallization and the relative stability increases with temperature. Precursor growth is confined due to limited chain diffusion, resulting in a periodic two-dimensional distribution. Crystallization takes place later with the existence of precursor, accompanied with the slowdown of precursor dynamics. Based on the results, a model of precursor assisted crystallization is proposed to illustrate this unique phase behavior.
1. Introduction In flow induced crystallization (FIC) of polymer, the effective extension of molecular chains can dramatically change the structural dynamics and final morphology. Hierarchical structure formation can be controlled by flow parameters, leading to different macroscopic material properties. With complex chain structures such as polydispersity, branching and blending, chain response and structure formation will be more complicated. On the other hand, these unique characters will provide new approaches to material development in polymer industry. It has been widely investigated that existing ordered pre-structure can influence crystallization behavior, such as nucleation rate [1–3], structure orientation [4], crystal modification [5,6], etc. The pre-existing ordering provides a substrate with high affinity and reduces the local surface free energy penalty. By applying external ordered/
*
oriented structures such as nanotubes [7,8] and solid surface [8–10], it is possible to accelerate crystallization and precisely induce the desired structure. On the other hand, nucleation substrate generated from polymer itself has been proved to be the most efficient for its closest morphology [11–13], which is normally described as self-nucleation. Self-nucleation always involves a special thermal history or chain orientation over melting temperature to induce non-crystalline self-seeds [2,4,14]. The influence of the initial condition in self-nucleation on dynamics and morphology have been studied in detail [2,13,15–17]. It has also been reported that self-seeds can be produced under flow field in a successive condition without pretreatment at high temperature [6,18–22]. Instead of preparing self-seeds as an initial condition, successive flow induced self-nucleation under constant supercooling degree is more similar to the practical processing situation. In certain condition such as high temperature and fast flow, non-crystalline precursor with lower energy barrier and high defect tolerance can be
Corresponding author. E-mail address: [email protected] (L. Li).
https://doi.org/10.1016/j.polymer.2019.121674 Received 10 May 2019; Received in revised form 17 July 2019; Accepted 21 July 2019 Available online 22 July 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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favored over crystallization, as a metastable phase with local minimum free energy [23,24]. The flow induced precursor can behave as selfseeds and influence the following crystallization. In this case, one has to consider the interaction between crystal and precursor, concerning the significant thermodynamics difference. Indeed, precursor formation and growth play an integral role in crystallization process. It has been reported that at the late stage of induction period, precursor dynamics and morphology is similar to crystalline one, indicating the coupling between precursor formation and crystallization [25,26]. On the other hand, the generation and growth of the precursor can also compete with crystallization, showing strong dynamics dependence [20,27,28]. Due to the complex and important role of precursor during FIC, in-depth investigation of the dynamics competition and coupling in the hierarchical structure will help to understand this unique mechanism. Even though flow induced precursor has been observed in lots of works, it is mostly treated as only an intermedium phase and the experimental parameter controlling is far from ideal [21]. The understanding of physics mechanism of flow induced precursor and precursor/crystal interaction is still not clear. In our previous work [29], we produced and characterized a unique system of γ-irradiated isotactic polypropylene (iPP). The partially crosslinked iPP shows asymmetric relaxation dynamics under flow and polydisperse systems have been discovered in favor of precursor generation [30,31]. In this paper, we applied extensional flow on the system under different temperature to study precursor generation and crystallization process. Simultaneous SAXS/WAXS measurement is used to investigate the structure morphology and dynamics at different spatial scale. The structural and dynamical difference between crosslinking network and free chains managed to highly enlarge dynamics difference of crystallization and precursor generation. After extension, flow induced precursor can be observed at long time before crystallization. Cross-shaped signal can be observed in SAXS later, indicating a periodic two-dimensional distribution of the precursor. With continuous generation and growth of precursor, crystallization is finally favored over precursor formation. Based on the results, we propose a model of precursor assisted crystallization and provide a microscopic explanation on the structural and dynamical mechanism.
Fig. 1. Illustration of simultaneous SAXS/WAXS testing on iPP. 2D SAXS and WAXS patterns after extension under 160 °C: (a) WAXS, (b) SAXS.
detectable reflections, which are (110), (040), (130) at 11.4°, 13.6° and 14.8°, respectively. The (110) reflection is commonly applied to indicate the starting of crystallization for its strongest intensity. For nucleation at lower strain, daughter crystallization has been reported favored over the parent one [33]. Unfortunately, (110) reflection for daughter crystal orients along the meridian direction, which is missed in our measurement. As a result, here (040) reflection is applied to indicate the onset of crystallization and characterize crystallization kinetic. Different reflections will influence the characterization of crystallization onset time but the difference is acceptable in our experiments with time period over 300 s. Azimuthal integration is performed with the inner and outer boundary indicated in Fig. 1. Zero azimuth has also been indicated and integral is oriented in the counterclockwise. Fig. 2 provides SAXS and WAXS patterns under different temperatures after extension. At 130 °C, crystallization begins right after extension, which can be discovered from crystalline reflections in WAXS (3 s). Meridian oriented signal can also be observed in SAXS right after extension, with intensity increasing rapidly. At higher temperature from 140 to 160 °C, it is clearly observed that SAXS signal shows up after an induction time but much earlier than WAXS crystal reflections. SAXS peaks before Bragg peaks in WAXS corresponds to a periodic distribution of non-crystalline region with higher density, indicating the generation of the precursor structure. Remarkably, precursor under 160 °C lasts around 400 s before crystallization. From 130 to 150 °C, an unusual bifurcated signal can be observed in SAXS along the equation direction at different times after crystallization. The simultaneous measurement technology allows us to investigate the structure morphology and phase transition dynamics in detail. WAXS and SAXS intensity curves under different temperatures are provided in Fig. 3. Induction periods are decided by the intersection of intensity baseline and the slope line of intensity file to avoid error from intensity resolution at earlier stage. Under 130 °C, it can be discovered from Fig. 3(a) that the crystallization kinetics falls behind SAXS ones, although they all start immediately after extension. At around 270 s, both intensities in SAXS begin to decrease due to high crystallinity [34]. Under all experiment temperature, the induction time for three different structures can be distinguished, as indicated in Fig. 3. For 160 °C, since precursor is generated around 300 s before crystallization, more detail transition can be observed. From SAXS intensity, equatorial and meridian oriented structures are generated at around 550 s and 750 s, respectively, corresponding well with 2D patterns. Crystallization starts at around 860 s, after which both intensities in SAXS increase faster. SAXS 2D intensity under 160 °C as a function of q and time along the meridian direction is provided in Fig. 4 (a). Along q axis, a distinct
2. Experiment Pure iPP material was provided by SABIC-Europe with number- and weight-average molecular masses (Mn and Mw) of 150 and 720 kg/mol, respectively. To prepare CL-iPP, Chemical grade trimethylolpropane triacrylate (TMPTA) was applied as a crosslinking agent, with amount of 0.2 mmol in 100 g of iPP. Irradiation dose was set at 10.4 kGy with dose rate of 0.65 kGy/h. Gel content in the CL-iPP is 61.2%. The chain structure and rheological characterization of the CL-iPP has been provided in our previous work [29]. Extension is performed with a home-made rheometer which has been introduced in detail somewhere else [27]. CL-IPP is firstly heated to 210 °C and remains for 10 min to erase the thermal history. After that, sample is cooled to desired temperatures with cooling rate of 2 °C/ min. Then the extension with strain of 1.5 and strain rate of 5 s−1 is applied to the sample. After extension, the structure evolution process is measured by in situ simultaneous 2D SAXS and WAXD measurement with wavelength of 0.124 nm and time resolution of 3 s. The experiments were performed at the beamlines BL16B of Shanghai Synchrotron Radiation Facility (SSRF). 3. Results 2D SAXS and WAXS patterns at 2400 s after extension under 160 °C are provided in Fig. 1, as a demonstration of the SAXS/WAXS simultaneous measurement and data processing. Because of the arrangement of SAXS and WAXS detector [32], only part of WAXS patterns can be detected. For α modification of iPP, there are three 2
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Fig. 2. 2D SAXS and WAXS patterns after extension at different temperatures. The last collected patterns for each temperature are rescaled in intensity to adjust the contract.
network and free chains response differently under extension. Crosslinked network can be effectively extended while shorter free chains are squeezed out. For network dominated domain, effective extension and slow relaxation result in high orientation, while at the same time the squeezing out of the shorter chains reduces the chain density. Inside crosslinking units, chain diffusion is limited by cross-linking points created during irradiation. For free chains dominated domain, chain density and diffusion ability are higher while the orientation degree is reduced due to strong disentanglement and relaxation. Extension of the material results in a heterogeneous system where network and free chains have different orientation degree, diffusion ability and chains density. As a result, in the stretched network region with lower chain density and diffusion ability, periodic precursor is favored over crystallization due to lower energy barrier and high conformational defect tolerance [27]. Note that the deformation response of cross-linked part is almost independent of temperature, and for short chains, relaxation time decreases with temperature. Compared to crystal, precursor is relatively more stable under higher temperature since high temperature will further enlarge the energy barrier difference. The difference of generation time of precursor and crystal, namely precursor period here, reflects the relative stability of precursor structure. Below 160 °C, the generation of different structures cannot be directly distinguished from 2D patterns, but fitted dynamics difference can be observed in 1D intensities, as seen in Fig. 3. After fitting and calculation, precursor period is discovered increasing with temperature, which are 12 s (130 °C), 76 s (140 °C), 89 s (150 °C) and 310 s (160 °C), respectively. The oriented precursor will reduce the nucleating energy barrier and provide nucleating sites, so that crystallization can be more easily
maximum can be observed after 600 s. The peak intensity of the precursor increases with time even before crystallization, indicating the generation and ordering of the non-crystalline periodic structure. Long period of the periodic precursor can be calculated from the intensity maximum, which is shown in Fig. 4(b). The decrease rate (dL/dt) of long period is also calculated. The long period in SAXS continuously decreases, while dL/dt reaches a minimum at around 850 s, which corresponds well to the onset time of crystal nucleation. To investigate the mesoscopic orientation behavior, azimuthal intensity curves of the last pattern of SAXS under different temperature are shown in Fig. 5(a), where starting azimuthal angles and integration direction are indicated in Fig. 1. Intensity along meridian direction has been renormalized to show the comparison of the azimuthal distribution. It can be discovered that the relative intensity of the equator direction peak generally increases with temperature. At 140 °C and 150 °C, two peaks can be observed near the meridian direction, with a deviation of around 9°. From 2D intensity under 150 °C in Fig. 5(b), the azimuthal angle distribution remains the same during crystallization.
4. Discussion The simultaneous SAXS/WAXS measurement with high time resolution provides a convincing result of flow induced phase transition at different temperatures. Mesoscopic/microscopic transition dynamics and their interaction can be investigated from the hierarchical structural information. In our previous work [29], we have reported that relaxation asymmetry can be induced in γ-irradiated cross-linked iPP, where gel 3
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Fig. 3. 1D integration intensity as functions of time for (a) 130 °C, (b) 140 °C, (c) 150 °C and (d) 160 °C. SAXS and WAXS intensities under 160 °C are shown on logarithmic scales for small change at the early stage of the phase transition. Induction periods are decided by the intersection of intensity baseline and the slope line (Fig. 3(a)).
Fig. 4. (a) 2D Intensity as functions of q and time for 90° in SAXS at 160 °C. (b) Meridian long period of SAXS and the decrease rate of the long period (dL/dt).
crystallization. (i) Primary precursor. As discussed, precursor generation can be favored over crystallization due to lower energy barrier and higher defect tolerance. Precursors locate along the flow direction, corresponding to blob signal in the early stage along equator direction in SAXS. Long period increases with temperature, indicating lower
induced. As seen in Fig. 2, at later stage under different temperature, crystallization takes place with the existence of precursor. The kinetics and morphology of precursor and crystal can be related by simultaneous SAXS/WAXS testing. We propose here a schematic diagram in Fig. 6 to demonstrate the mechanism of precursor assisted 4
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Fig. 5. (a) Azimuth intensity at the end of the measurement at different temperatures. Meridian direction has been indicated and meridian intensity has been normalized. (b) Azimuth intensity as the function of time at 150 °C.
Fig. 6. Structure transition and corresponding SAXS/WAXS patterns at different stages after extension.
structure in our results is non-crystalline. The non-crystalline block here provides nucleating sites, so that crystallization can be more easily induced in these regions. During the growth of the block precursor, the polymer chains between precursor domains are arranged into precursor. (iii) Inner crystallization of precursor. With the continuous decrease of the free volume between precursors, the deformation of random coils toward the conformation in precursor gets more difficult due to the increase of energy barrier. Gradually, crystallization is dynamically favored and replace the generation of precursor. From long period results in Fig. 4(b), the reduction kinetic of long period slows down after the emerging of crystal signal at 850 s, indicating that the generation and growth of precursor have been suppressed due to crystallization. With the existence of precursor, crystallization takes place in precursor domain more easily, since chains are more oriented and quasi-ordered. After that, SAXS signal is contributed to density differences from both precursor/melt and crystal/melt. From Fig. 3, SAXS intensities increase much faster after crystallization while the generation and growth of precursor are slower here. It can be inferred that the rapid intensity increase in SAXS after crystallization is mainly contributed to the averaged density increase of precursor/crystal composite due to inner crystallization. As seen in Fig. 4, from 130 °C to 150 °C, bifurcated signals can be
precursor density at high temperature. Under the remaining stress conserved by the extended cross-linked network, precursor can be continuously generated. With thickening and continuous generation of the precursor, the average distance between the domains decreases, resulting in the reduction of long period, as seen in Fig. 4. This is different from the widely-accepted result of phase separation by spinodal decomposition where long period increases with time, since the precursor growth in those static situation follows self-similarity [35–38]. In this case of flow induced phase transition, even at early stage, the concentration of quasi-ordered domain is much higher than that in static situation. The interpolation of precursor will be highly favored over epitaxial growth. (ii) Confined growth of precursor. In crosslinked domain, chain diffusion is limited due to the lower motion ability. With the interpolation and growth of the precursor, free volume decreases rapidly. As a result, the precursor is difficult to grow and the size is highly limited. The confined growth of precursor leads to a periodic two-dimensional distribution of the precursor and it can be defined from the cross-shaped signal in SAXS. The cross-shaped signal has been observed in FIC of cross-linking PEO [39] and quenched iPP [40]. Strobl's model is normally applied to describe the block-like structure, which regards it as an intermediate structure [41–44]. From the SAXS and WAXS structural dynamics, we can confirm that the block 5
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observed after crystallization along equator direction. The azimuthal intensity distribution remains constant during crystallization with two peaks located at 9° away from the equator direction. The azimuthal distribution corresponds well with “cross-hatched” lamellar morphology in parent/daughter crystal [45]. It can be concluded that there are two branches of daughter lamellar with a deflection angle of 9° from parent lamellar, as chirality coupling in microscopic scale. In the SAXS characterization of iPP, it has always been hard to distinguish block structure and cross-hatched structure [40,46]. Here, with simultaneous SAXS/WAXS measurement, equator intensity increases before the onset of crystallization and bifurcated signal shows up in the late stage of crystallization. These two kind of structures can be clearly distinguished. Also for the first time, ordered daughter lamellar growth has been observed in SAXS, following highly self-similarity.
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5. Conclusion In summary, precursor assisted crystallization in cross-linked iPP is observed with simultaneous SAXS/WAXS measurement. Different relaxation dynamics of cross-linked and free chains under flow result in the heterogeneous distribution of chain density, diffusion and orientation. Precursors can be induced before crystallization and the relative stability increases with temperature. Precursor shows a periodic twodimensional distribution due to the confined growth limited by low chain diffusion ability. Eventually, crystallization can be induced due to the lower energy barrier with the assistance of existing precursor. Acknowledgements The authors would like to thank the assistance of Dr. Feng Tian in synchrotron measurement. This work is supported financially by the National Natural Science Foundation of China (Grant Nos. 51227801, 51325301, 21504069). References [1] F. Su, Y. Ji, L. Meng, J. Chang, L. Chen, L. Li, Shear-induced precursors in polyethylene: an in-situ synchrotron radiation scanning X-ray microdiffraction study, Polymer 135 (2018) 61–68. [2] F. Su, W. Zhou, X. Li, Y. Ji, K. Cui, Z. Qi, L. Li, Flow-induced precursors of isotactic polypropylene: an in situ time and space resolved study with synchrotron radiation scanning X-ray microdiffraction, Macromolecules 47 (13) (2014) 4408–4416. [3] K. Cui, L. Meng, N. Tian, W. Zhou, Y. Liu, Z. Wang, J. He, L. Li, Self-acceleration of nucleation and formation of shish in extension-induced crystallization with strain beyond fracture, Macromolecules 45 (13) (2012) 5477–5486. [4] Y. Hayashi, G. Matsuba, Y. Zhao, K. Nishida, T. Kanaya, Precursor of shish–kebab in isotactic polystyrene under shear flow, Polymer 50 (9) (2009) 2095–2103. [5] R.H. Somani, B.S. Hsiao, A. Nogales, H. Fruitwala, S. Srinivas, A.H. Tsou, Structure development during shear flow induced crystallization of i-PP: in situ wide-angle Xray diffraction study, Macromolecules 34 (17) (2001) 5902–5909. [6] B. Zhang, J. Chen, F. Ji, X. Zhang, G. Zheng, C. Shen, Effects of melt structure on shear-induced β-cylindrites of isotactic polypropylene, Polymer 53 (8) (2012) 1791–1800. [7] X. Hu, H. An, Z.-M. Li, Y. Geng, L. Li, C. Yang, Origin of carbon nanotubes induced poly(L-lactide) crystallization: surface induced conformational order, Macromolecules 42 (8) (2009) 3215–3218. [8] Y.-H. Chen, G.-J. Zhong, J. Lei, Z.-M. Li, B.S. Hsiao, In situ synchrotron X-ray scattering study on isotactic polypropylene crystallization under the coexistence of shear flow and carbon nanotubes, Macromolecules 44 (20) (2011) 8080–8092. [9] H. Li, S. Jiang, J. Wang, D. Wang, S. Yan, Optical microscopic study on the morphologies of isotactic polypropylene induced by its homogeneity fibers, Macromolecules 36 (8) (2003) 2802–2807. [10] H. Ishida, P. Bussi, Surface induced crystallization in ultrahigh-modulus polyethylene fiber-reinforced polyethylene composites, Macromolecules 24 (12) (1991) 3569–3577. [11] W. Banks, M. Gordon, A. Sharples, The crystallization of polyethylene after partial melting, Polymer 4 (1963) 289–302. [12] D. Blundell, A. Keller, A. Kovacs, A new self‐nucleation phenomenon and its application to the growing of polymer crystals from solution, J. Polym. Sci. B Polym. Lett. 4 (7) (1966) 481–486. [13] A.T. Lorenzo, M.L. Arnal, J.J. Sanchez, A.J. Müller, Effect of annealing time on the self‐nucleation behavior of semicrystalline polymers, J. Polym. Sci. B Polym. Phys. 44 (12) (2006) 1738–1750. [14] R. Michell, A. Mugica, M. Zubitur, A. Müller, Self-nucleation of crystalline phases within homopolymers, polymer blends, copolymers, and nanocomposites, Polymer Crystallization I, Springer, 2015, pp. 215–256.
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