Effect of Shear Stress on Crystallization of Isotactic Polypropylene from

Effects of melt structure on non-isothermal crystallization behavior of isotactic polypropylene nucleated with α/β compounded nucleating agents. Qiy...
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Effect of Shear Stress on Crystallization of Isotactic Polypropylene from a Structured Melt Bin Zhang,†,⊥ Jingbo Chen,*,†,‡ Jing Cui,† Hui Zhang,§ Fangfang Ji,† Guoqiang Zheng,† Barbara Heck,§ Günter Reiter,§ and Changyu Shen*,‡ †

School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, People’s Republic of China National Engineering Research Center for Advanced Polymer Processing Technologies, Zhengzhou University, Zhengzhou 450002, People’s Republic of China § Institute of Physics, University of Freiburg, D-79104 Freiburg, Germany ⊥ Hermann Staudinger Graduate School, University of Freiburg, D-79104 Freiburg, Germany ‡

S Supporting Information *

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INTRODUCTION Crystallization behavior and morphology of semicrystalline polymers are both strongly influenced by flow conditions, such as shear strain, shear stress, and shear rate.1−6 Therefore, in order to control the hierarchical structure of semicrystalline polymers, it is important to understand the mechanisms of how shear affects crystallization.7−9 One of the most widely used polymers, isotactic polypropylene (iPP), exhibits pronounced polymorphism and a wide variety of morphologies. Thus, iPP represents a suitable candidate for studying shear-induced polymer crystallization. IPP can crystallize in three crystal forms, known as the monoclinic α-form, the hexagonal β-form, and the orthorhombic γ-form.10 The β-form is a metastable crystalline phase and can only be obtained under special conditions, such as by quenching (fast cooling) of the melt, by using specific nucleating agents,11 or via shear-induced crystallization.12,13 Alfonso and Yan et al.14−17 attributed the ultimate melting temperature of iPP to nucleation precursors still existing at high temperatures in the polymer melt. Previous researchers18−22 also reported that some local order of some polymer chains exists within an otherwise isotropic melt which thus can be regarded as a composite of chemically homogeneous structurally heterogeneous polymer, to be contrasted with the “blank melt” (the isotropic melt without any bundles of partially ordered chain segments) existing at temperatures above T0m (the equilibrium melting point). On the basis of the assumption of an only incompletely molten polymer, we defined for our study the term “structured melt” to account for the contribution of various bundles of partially ordered chain segments which may act as self-nuclei within the quiescent polymer melt. At a given temperature of the melt, such residual structures may differ as a function of thermal and previous mechanical history, i.e., the chosen kinetics of the melting process and original structure of material. The following questions arise: What is the nature and origin of such bundles of partially ordered chain segments in a polymer near melting point melt (denoted as “NMP” melt) at temperatures near but belowT0m? Why can an oriented crystalline structure not be obtained by crystallization from a sheared blank melt?18,23−26 Which role does the partially ordered chain segments play in the formation of cylindrites? Recently, we have demonstrated the existence of partially ordered structures in a NMP structured melt and have investigated, for a © 2012 American Chemical Society

given shear stress, the impact of various melting temperatures on the formation of cylindrites.27 However, until now, very little work has focused on characterizing these bundles of partially ordered chain segments and on the possibility of an influence of shear flow on the nucleation probability related to these structures. In this paper, we want to describe some experimental results about the effect of shear stress generated at the wall of a capillary leading to the formation of iPP cylindrites in connection with bundles of partially ordered chain segments required for the generation of orientation in a NMP structured melt.

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RESULTS AND DISCUSSION The typical crystalline morphology of iPP after shear flow at constant NMP temperature (T*E ) of structured melt and at various shear stresses (σw) is shown in Figure 1. The fan-shaped domains along the thread-like nuclei represent the β-form which, in these micrographs, appears brighter than the α-form of iPP. The β-form of iPP showed interference colors even without the use of a λ plate and exhibited very strong negative birefringence, which disappeared when melting the β-form at about 150 °C. All these features are characteristic for β-iPP. The core layer in the center of the microtomed slices shows a random spherulitic structure, corresponding to crystallization from a melt state which did not contain any oriented partially ordered molecular bundles or self-nuclei. The layer close to the surface, which was touching the wall of the extruder (the surface layer: located in the range of 0 to 600−1100 μm from the wall for the different shear stress), showed oriented crystalline structures representing both α-form and β-from cylindrites.29 It can also be seen that both the width of the surface layer, where crystallization was affected by the flow conditions, and the number of cylindrites30 increased with σw (at a constant temperature of the extruder). It is interesting to note that the number of β-iPP domains31 on a thread-like rows of nuclei α-iPP cylindrites decreased with increasing σw. By heating the sample above 150 °C, the threadlike rows still remain, verifying that these rows cannot be of β-form. There are two possible explanations for this influence of shear stress: (1) Shearing not only increased the number of Received: July 16, 2012 Revised: October 4, 2012 Published: October 16, 2012 8933

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way showed mainly isotropic ring patterns and only rather weak reflexes, indicating that only a minor part or the sample was oriented.33 Figure 2a,b shows the averaged 1D WAXS intensity profiles obtained from the 2D WAXS patterns taken at different positions of the samples. These results suggest that, as σw increased, the reflection signal from β-form iPP crystals (e.g., the peak due to reflections from the β (300) plane located at 2θ = 16°) gradually decreased while the overall crystallinity (obtained from an integration of the whole WAXS spectrum) remained almost invariant. Figure 2c,d shows the one-dimensional correlation function K(z) for the lamellar stacks obtained at different values of σw. At a wall shear stress of 10 kPa, a small peak at about 13.5 nm (the shoulder marked by “α-iPP”) can be identified in the correlation function;34 a second peak was located at about 18.1 nm (marked by “β-iPP”). The first peak at 13.5 nm became stronger with increasing σw. In contrast, the other peak changed in the opposite way. In the core layer, only one peak could be observed. Vleeshouwers et al.35 have reported the origin of the occurrence of two different long periods for crystallized iPP containing α- and β-forms with the one which corresponds to the β-form much larger. The effect of self-nucleation or self-seeding in a partially molten melt on the crystallization process has been well established, and it has been confirmed by Hsiao and Ryan et al.21,22 Naturally, one might ask what role did structured melt play in the formation of nucleation under shear flow? In order to answer this question, an additional experiment was carried out. Two iPP melts with different initial melt structure, i.e., NMP structured melt and supercooled melt, were sheared. Assuming the resulting crystalline structure is mainly depending on the shear conditions, we would not expect to observe any difference between the two samples. Interestingly, the crystalline structures obtained at the same temperature (180 °C) and for the same local value of wall shear stress (20 kPa) were different for these two samples, as can be seen already from the optical signatures. This difference suggests that the partially ordered molecular bundles play a very important role in the formation of shearinduced cylindrites of iPP. However, very little is known about the nature and origin of these residual structures in the melt. In this paper, we provide some analysis of these bundles of partially ordered chain segments in the NMP structured melt. As shown in Figure 3a-1, the SAXS data obtained for a quiescent NMP structured melt monitored at TE* = 180 °C can be fitted by using a form factor for polydisperse cylinders. The scattering amplitude for a cylindrical particle is given by

Figure 1. Optical micrographs obtained under crossed polarizers for samples prepared by shearing iPP melts at TE* = 180 °C through a capillary at various shear stresses σw acting at the extruder wall: (a) 10, (b) 35, and (c) 45 kPa.

⎛ sin(qh /2 cos θ ) J (qrc sin θ ) ⎞2 1 ⎟ φ(q) = ⎜2v qh/2 cos θ qrc sin θ ⎠ ⎝

cylindrites but also increased the nucleation probability of α-iPP at the expense of β-iPP within the sheared sample. The resulting competition of growing α-iPP and β-iPP crystals results in the reduction of the areal fraction of β-iPP. (2) Higher σw leads to short shear time and thus to a shorter residence time of the sample in the region of the exit of the extruder where the temperature is in the range of 100−140 °C32the temperature range where the α to β transformation takes place. In order to determine crystalline structure of the samples obtained from a sheared NMP structured melt, 2D-WAXS/ SAXS measurements were performed for the surface layers and for the core layers. We note that all our samples processed in this

(1)

Here rc is the average radius of the cylinder; h and v denote the height and volume of the cylinder, respectively. J1 is the first-order Bessel function. θ denotes the angle between the scattering vector q⃗ and the cylinder axis. Averaging over all orientations of the cylinder axis has been done numerically. Furthermore, a Gaussian distribution has been introduced to represent the distribution of the cylinder radius. The scattered intensity I(q) then is proportional to φ(q). The experimental curve was fitted by adjusting the values of rc and h (Figure 3a-1). The following values were derived from such fitting: rc ≈ 17 ± 1 nm and h ≈ 40 ± 1 nm. 8934

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Figure 2. One-dimensional WAXS curves of samples prepared by shearing iPP melt at constant T*E = 180 °C and various σw, as indicated by the labels next to the curves: (a) for the surface layer; (b) for the core layer. One-dimensional correlation function K(z) obtained from SAXS measurements from these samples: (c) for the surface layer; (d) for the core layer.

Figure 3. Schematic presentation indicating the correlation between the structured melt and cylindrites at a local wall shear stress. (a)SAXS measurements were performed for NMP structured melt after annealing for 10 min at TE* = 180 °C (a-1: granular iPP was heated directly from room temperature to 180 °C) and supercooled melt at T*E (a-2: granular iPP was heated first from room temperature to 210 °C; kept there for 15 min to ensure complete melting of the crystalline microstructure; followed by cooling at 4 °C/min down to TE* and waiting there for 10 min). (b) A schematic diagram of shear stress distribution in a capillary die. (c) Optical micrographs obtained under crossed polarizers for samples prepared by shearing the melt at constant TE* = 180 °C through a capillary at wall shear stress σw = 20 kPa for (c-1) a NMP structured melt and (c-2) a supercooled melt.

Because of the weak scattered intensity at higher q, it is difficult to determine the precise size and shape of the cylinders or the cylinder-like particles. But in any case, the NMP structured melt

in comparison to the blank melt differed distinctively, confirming the existence of partially ordered molecular bundles which may act as nuclei. Isothermal melting at 180 °C destroyed most of the 8935

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Innovation Scientists and Technicians Troop Construction Projects of Henan Province (114200510018), and The Research Fund for the Doctoral Program of Higher Education (20104101110002).

crystalline structures already present at room temperatures, and even most of those crystalline features formed during the heating process. From the SAXS scattering data, we can estimate that the content of partially ordered structures remaining in the quiescent NMP structured melt has to be less than about 2.5 vol %, since no peak representing an interparticle distance could be observed in the low q range, the lowest accessible q being 0.06 nm−1. The WAXS patterns showed only an amorphous halo,36,37 no crystalline structure, the threshold for detecting crystalline structures being 1%. According to the fringed micelle model38 and the bundle model,39 consistent with experimental results described above, we propose a schematic model (shown in Figure 3) that can explain why cylindrites can be obtained from a NMP structured melt, whereas in our experiment for the sheared surpercooled melt only spherulites have been observed (see Figure 3c-2). In contrast to the experiments done by Alfonso and Peters,40,41 the possible reason for this difference is that weak shear flow applied at supercooled melt at T*E (T*E was higher than the nominal melting temperature) leads to fast relaxation of very faint levels of molecular or segmental orientation.22,42

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CONCLUSION Several new insights can be deduced from studying the combined effect of shear stress and structured melt on the morphology of oriented structures of iPP, which can be summarized as follows. (i) For a given structured melt, the number of cylindrites increased with shear stress. Concomitantly, the nucleation density of α-iPP within a single cylindrite structure increased with shear stress at the expense of β-iPP nucleation density. (ii) In-situ SAXS and WAXS measurements indicate that nanoscale ciliated bundles of partially ordered chain segments (one may speculate that these segments formed a smectic mesophase) were produced by controlling the melt structure in a quiescent near melting point melts of iPP. The SAXS patterns of NMP melt monitored at 180 °C can be fitted by using a form factor for polydisperse cylinders. The size of such ciliated bundles is around rc ≈ 17 ± 1 nm (radius) and h ≈ 40 ± 1 nm (height). (iii) Cylindrites were generated by applying shear stress of different magnitude during the flow of a NMP structured melt. Since at the same temperature no cylindrites were produced by shearing an iPP melt without any obvious ciliated bundles, we conclude that the partially ordered molecular bundles play a very important role in the formation of shear-induced cylindrites of iPP.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.C.); [email protected] (C.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Gert Strobl of University of Freiburg for fruitful discussions. This work was supported by the National Natural Science Foundation of China (No. 11172272, 10772164, 10590352, 50803060, and 51173171), “973 Program” (2012CB025903), 8936

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on October 16, 2012, with the incorrect last name for the sixth author, and a minor text error. The corrected version was reposted on October 19, 2012.

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