Formation and Alignment of Hybrid Shish-Kebab Morphology with

Research Note pubs.acs.org/IECR

Formation and Alignment of Hybrid Shish-Kebab Morphology with Rich Beta Crystals in an Isotactic Polypropylene Pipe Min Nie, Rui Han, and Qi Wang* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China ABSTRACT: In this work we successfully controlled formation and alignment of a crystal superstructure in an isotactic polypropylene (iPP) pipe via self-assembly of a β nucleating agent and rotation extrusion to tailor the performances of the iPP pipe. The results showed that during iPP pipe extrusion, β nucleating agents first self-assembled into the fibrous structure, and then were aligned off the axial direction through rotation of the mandrel and die in the self-designed rotation extrusion device. Accordingly, fibrils, as oriented templates, directed the epitaxial growth of iPP to yield hybrid shish-kebabs off the axial direction which was composed of β crystals, so as to achieve simultaneous hoop reinforcement and toughening of the iPP pipe.

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INTRODUCTION Polypropylene (PP) has become a key material for pipeline construction owing to its comprehensive advantages, such as easy processing and good resistance to chemicals and thermal distortion. Now, PP pipes are widely used as drinking-water pipes or underground drain pipes. However, during conventional extrusion, PP pipes mainly involved with α spherulites exhibit low impact toughness.1,2 More importantly, the polymer melt is extruded and drawn along the axial direction to have molecular chains oriented accordingly,3 but an internally pressurized pipe bears a hoop stress twice as high as the axial stress in its application, which requires a high hoop orientation degree in the PP pipe. Therefore, a PP pipe of the typical structures has low toughness and hoop strength. Isotactic polypropylene (iPP) is a typical polymorphic polymer with α-form, β-form, γ-form, and mesomorphic smectic form.4 It is generally believed that when subjected to external shocks, the β-form is transformed into the α-form, causing a large number of crazes and energy absorption in the iPP matrix, and hence it has superior toughness and impact resistance to the α-form.5,6 However, the β-form is a metastable phase and only generated under some specific conditions. The addition of the β-nucleating agent is considered as a most effective and accessible method to raise the β-form content in the iPP matrix. Bernreitner increased the content of β-form crystals by adding an amide-based β nucleating agent to improve the iPP pipe’s toughness.7 Unfortunately, the toughness improvement of the iPP pipe was accompanied by low hoop strength. It is difficult to simultaneously increase toughness and hoop strength of the PP pipe by directly adding the β nucleating agent. The properties of the polymers depend on their polymorphic composition as well as the morphology. Spherulite and shishkebab occur most frequently in polymer products.8 Compared to isotropic spherulite, products of shish-kebab morphology demonstrate high strength in the orientation direction.8,9 Obviously, the anisotropic morphology off the axial direction can effectively allievate the negative influence of the β-form on hoop strength of the PP pipe. Shish-kebab is composed of the stretched chain fibrils (shish) and the oriented chain-folded lamellae (kebabs) perpendicularly growing on the shish,10 so its © 2014 American Chemical Society

formation is considered as the prerequisite for anistropic morphology development.11,12 However, the shish is quite unstable and may easily relax back into the random state due to thermal motions.13 Therefore, only under special processing conditions, such as high-speed injection or dynamic packing molding, can the shish kebab morphology be available. Apparently, it is a great practical challenge to prepare the PP pipes with excellent hoop strength and impact strength. The morphology of the semicrystalline polymer is closely related to the shape of the nuclei during initial crystallization.14 It is evident that introduction of fibrous nuclei can induce the formation of the hybrid shish kebab through the growth of folded chain lamellae perpendicular to its surface and improve the mechanical performance.15,16 Therefore, it is very important for PP pipe to manipulate the morphology of the nuclei and its aligment off the axial direction during the extrusion processing. In this paper, based on supramolecular scientific principles, TMB-5, an efficient β nucleating agent of aryl amides compound, was driven by intermolecular hydrogen bonding to self-assemble into a fibrous structure. Then through the rotation of the mandrel and the die in a self-designed rotation extrusion device, the hoop movement was superimposed on the axial movement to deviate the alignment of the fibrous nucleating agent from the axial direction, and accordingly polymer epitaxially grew on the surface of the nucleating agent to form the hybrid shish kebab composed of β-form crystals off the axial direction. This paper also presented the relationship between the structure and performance for PP pipes with considerably increased hoop strength and toughness.

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EXPERIMENTAL SECTION Materials. A commercially available isotactic polypropylene resin T30S provided by DuShanZi Petrochemical Co., Ltd. (Xinjiang, China), was used in this study. Its melt flow index, measured at 230 °C under 2.16 kg, was 2.5g/10 min, and the Received: Revised: Accepted: Published: 4142

November 20, 2013 February 23, 2014 February 27, 2014 February 27, 2014 dx.doi.org/10.1021/ie403944k | Ind. Eng. Chem. Res. 2014, 53, 4142−4146

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weight average molecular weight was 587 000 g/mol. β nucleating agent (TMB-5) supplied by Shanxi Provincial Institute of Chemical Industry was aryl amide-based compound. Its molecular structure was N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide.17 Sample Preparation. IPP containing 0.1 wt % β nucleating agent (TMB-5) was first premixed in a high-speed mixer and extruded at the range of 170−200 °C from hopper to die with a screw rotation speed of 100 rpm in a TSSJ-25/33 corotating twin-screw extruder (Chenguang Research Institute of Chemical Industry, China). The extruded pellets were subsequently extruded into an iPP pipe by using the selfdesigned rotation extrusion system, whose schematic representation and more details were described elsewhere.18 In this paper, during the extrusion process, when the mandrel and die rotated in the same direction at the speed of 4r/min, iPP pipes with a diameter of 32 mm and a wall thickness of 3 mm were prepared, which were named RTPP. The processing temperature was set at 230 °C to achieve the dissolution of TMB-5 and the self-assembly into the fibrls in the polymer melts.19 For comparison, rotation extruded iPP pipes without β nucleating agent and conventionally extruded iPP pipes with and without nucleating agent were also prepared and named RPP, CTPP, and CPP, respectively. Characterization. As to the hoop strength test, the specimens were cut from the extruded pipe in the hoop direction into the 10 mm-wide rings, and then measured by a universal testing machine (model RG L-10, Shenzhen Reger Instrument Co., Ltd.) with the cross-head speed of 20 mm/min (see the previous study for more details20). The iPP pipe’s impact strength was evaluated according to ISO 3127, in a drop-hammer impact test instrument (Chengde Precision Testing Machine Co., Ltd., China). In this test, iPP pipes were subjected to the impact from a 1 kg striker falling freely at different heights. One pipe was only subjected to one impact. If not broken upon the impact, it would be abandoned and a new one would be adopted with the adjusted falling height of the striker until the pipe was broken. The impact energy was then calculated by the product of the weight and falling height of the striker. The crystal structure of the iPP pipes was observed by an Inspect F (FEI) SEM instrument at 0.5 Torr and 20 kV. Prior to the observation, the SEM samples were cut from the pipes and etched chemically by permanganic etchant. The samples were then gold-sputtered. Two-dimensional small-angle X-ray scattering (2D-SAXS) experiments were performed on the BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. The wavelength generated by the synchrotron light source was 0.124 nm and the sample-to-detector distance was 5 m. The samples were placed with the axial direction perpendicular to the beams. Scattering patterns were recorded. To investigate the crystalline structure in the iPP pipes, onedimensional WAXD (1D-WAXD) was performed on a DX1000 diffractometer (Dandong Fangyuan Instrument Co., Ltd., China) with the Cu Kα generator system operated at 40 kV and 25 mA. The diffraction patterns were recorded for 2θ values ranging from 10° to 40° at a scanning rate of 0.06°/s. The relative amount of β-form crystals in the prepared iPP pipe was calculated according to the Tuner−Jones equation:21 Kß

where Iß(300) was the peak intensity of the (300) plane of the βform, and Iα(110), Iα(040), Iα(130) were the peak intensities of the (110), (040), (130) planes of the α-form.

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RESULTS AND DISCUSSION WAXD examinations were carried out to obtain information about the crystalline modifications in the prepared iPP pipes, and the typical results were shown in Figure 1. Clearly, CPP

Figure 1. WAXD patterns for the iPP pipes prepared at the different processing conditions.

showed the five typical diffraction peaks at 2θ = 13.9, 16.7, 18.3, 20.9, and 21.6°, corresponding to the (110), (040), and (130), overlapping (111) and (131) reflections of the α-form crystal, respectively. For RPP, an additional peak at diffraction angle 2θ = 16.0° was identified corresponding to the primary reflection of the (300) plane of the β-form crystal, but its intensity was very weak; that is, a small number of β-form crystals were generated. When β nucleating agents were introduced into the iPP matrix, the diffraction intensity of the β-form crystal increased significantly, even greater than any characteristic peak of the α-form crystal. The relative content of β-form crystals of CTPP and RTPP reached 70% and 73%, respectively. These results showed α-form crystals mainly existed in the pure PP pipe, whereas the addition of TMB-5 indeed induced the majority of iPP to crystallize into β-form crystals in CTPP and RTPP. Moreover, the calculated crystalline degree of all the samples showed that the nucleating agent and rotation of mandrel and die had small effects on their crystallinity degrees. Important morphological information was available via SEM examinations of the iPP pipes’ etched surface. As shown in Figure 2, CPP and RPP were only covered by small α spherulites with the lamella growing radially from the nucleus center. In contrast, the different morphologies formed in the TMB modified iPP pipes. An array of grooves in Figure 2 represented the nucleating agents which were first removed by the etchant. Clearly, the nucleating agents appeared in the form of the fibril structure, and the fine and parallel lamellae were stacked on both sides of the fibril, which was a typical hybrid shish-kebab structure where the TMB fibril served as “shish” and the polymer lamellae acted as “kebab”. Interestingly, the alignment of the anisotropic structures in the iPP pipes was controlled by extrusion modes. The hybrid shish-kebab

Iß(300) Iß(300) + Iα(110) + Iα(040) + Iα(130) 4143

dx.doi.org/10.1021/ie403944k | Ind. Eng. Chem. Res. 2014, 53, 4142−4146

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Figure 2. SEM photos of the iPP pipes prepared at the different processing conditions. The direction of arrow showed the axial direction.

Figure 3. SAXS patterns of CPP (a) and RTPP (b).

The topology structure of nuclei in the early stages of crystallization determined the final morphology formed in the iPP pipes. Under flow, the chains underwent a transition from a coiled conformation to a highly oriented state. The oriented molecules came together to form the threadlike shish which triggered the epitaxial growth of lamellae, and then were transformed into the anisotropic shish kebab crystal.11 However, the shish was unstable and easily dissolved back into its random state.24 Therefore, during the convention and rotation extrusion of pure iPP pipes, only isotropic spherulites could form due to sufficient relaxation caused by high temperature. However, the molecular chains in the amorphous region could orient whose dirction depended on the extrusion modes; that is, the orientation of CPP was parallel to the axial while that of the RPP apparently deviated from the axial direction.2,20 As a result, the hoop strength of RPP was a little higher, as shown in Figure 4, but the impact strengths of both were low because CPP and RPP were inundated with α-form crystals.

structure in CTPP preferentially aligned along the axial direction, while that in RTPP tilted at an angle of 40°. To get a comprehensive insight into the lamellar arrangement in the prepared PP pipes, SAXS measurement was also conducted and the typical patterns were seen in Figure 3. Obviously, pure iPP pipe prepared by conventional extrusion exhibited a strong isotropic scattering ring referred to randomly oriented lamellae. However, two pairs of scattering reflections off the meridian directions were clearly observed in the TMBmodified iPP pipe obtained via rotation extrusion, indicating the presence of the orientation morphology. The “crosshatched” morphology should be ascribed to epitaxial growth of the secondary or daughter lamellae on the parent lamellae.22,23 On the basis of the alignment of the nucleating agent in Figure 2b, the two scattering spots at 43° and 223° off the meridian should result from the parent lamellae, which were preferentially oriented perpendicular to the fiber direction, while the other two spots were attributed to daughter lamellae with the aaxis oriented along the fiber. 4144

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higher than those of the conventionally extruded pipes without nucleating agent, respectively.

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CONCLUSION A small quantity of TMB-5 dispersing in polymer matrix provided an oriented fibrous template via self-assembly, and its alignment deviated from the axial direction under a helical flow caused by rotation of mandrel and die. Subsequently, the fibrils served as shish to direct the epitaxial crystallization of polymer and the epitaxial crystals were mainly the β-form crystals due to the selective nucleating effect of TMB-5. As a result, hybrid shish-kebab morphology composed of β-form crystals off the axial direction was obtained to successfully achieve the simultaneous reinforcement and toughening of the iPP pipes.

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Figure 4. Comparison of the hoop tensile strength and impact strength for the iPP pipes prepared at the different processing conditions.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-28-85402465. Tel: +86-28-85405133.

It was evident that TMB-5 dissolved in the iPP melt at high temperature and recrystallized during cooling.19 Two phase transitions occurred for TMB-5 modified iPP, that is, the crystallization of TMB-5 at high temperature and that of iPP at low temperature. Upon initial cooling, the dissolved TMB-5 in the polymer melt crystallized prior to iPP and self-assembled into high aspect ratio fibrils through intramolecular hydrogen bonding due to the presence of amino groups.25,26 Compared to the oriented molecules, little relaxation of the fibrils kept them intact without significant changes over the whole processing. TMB-5 was a good nucleating agent for iPP and had a favorable lattice matching with iPP, so the surface of these fibrils offered a large number of nucleation sites for iPP. The energy barrier for heterogeneous nucleation on the surface of the TMB-5 fibrils was lower than that for homogeneous nucleation of iPP itself.27 Most of the polymer “preferred” to nucleate and grow vertically on the surface of the fibril and into a oriented shish-kebab morphology, where the core was made of TMB-5 fibril and the kebabs were made by iPP. Moreover, the epitaxial crystals were mainly the β-form crystals which were selectively nucleated by TMB-5. As a result, anisotropic hybrid shish-kebab morphology with rich β-form crystal formed. The fibrils in the flow tended to align parallel to the direction of the flow. With this templating mechanism, the alignment of fibrils directed the orientation of the hybrid shish-kebab morphology in the iPP matrix. During the convention extrusion, the polymer melts flowed along the axial and thus the alignment of the fiber paralleled the axial direction, which guided the subsequent crystallization to yield the hybid shish kebab crystals along the axial. Therefore, although the impact strength of CTPP was high due to rich β-form crystals, its hoop strength was low. During rotation extrusion, with the superposition of the axial flow caused by extrusion and the hoop drag flow by mandrel and die rotation, the polymer melts went through a helical flow deviating from the axial direction, and thus the alignment of the fiber and the subsequent hybrid shish-kebab morphology with β-form crystals were off the axial direction. Therefore, the anisotropic morphology exhibited excellent properties of the β-form crystal and shish-kebab morphology to improve the hoop strength and toughness of the iPP pipe. As shown in Figure 4, its hoop strength and toughness reached 33.1 MPa and 13.3 kJ, 70.6% and 47.8%

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financed by the National Natural Science Foundation of China (Grant Nos. 51127003, 51303114 and 51121001). Synchrotron 2D-SAXS experiments were supported by the Shanghai Synchrotron Radiation Facility.

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REFERENCES

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