Morphology Variations of Polypropylene - Formatexformatex.org/microscopy3/pdf/pp704-712.pdf · Morphology Variations of Polypropylene J. Výchopňová1, R. Čermák1,1 and M. Obadal2

Morphology Variations of Polypropylene

J. Výchopňová1, R. Čermák1,1 and M. Obadal2 1Department of Polymer Engineering, Faculty of Technology, Tomas Bata University in Zlín, TGM 275,

762 72 Zlín, Czech Republic 2Borealis Polyolefine GmbH, St.-Peter-Strasse 25, 4021 Linz, Austria The morphology of isotactic polypropylene is investigated using scanning electron microscopy. The structure is modified by the addition of specific nucleating agent, special processing conditions or blending with polyethylene. The addition of β-nucleating agent into polypropylene induces the crystallization into fine β-spherulitic structure on the contrary to relatively large α-spherulites observed in neat polypropylene. Extrusion of polypropylene with special extrusion line equipped by semi-hyperbolic extrusion die causes the formation of fibrillar structure. Such a self-reinforced material possesses significantly improved strength and modulus. The extrusion of polypropylene/polyethylene blend using the same extrusion line leads to the creation of microfibrillar-reinforced material where the polypropylene serves as a stiffening rod and polyethylene as a matrix.

Keywords Scanning Electron Microscopy; Polymorphism; Structure; Self-reinforcement; Blends

1. Introduction

Macroscopic behaviour of polymers reflects their structure. The structure, however, can be followed in several hierarchical levels starting from chemical nature of macromolecules established already during the polymerization process. The crystalline structure of semi-crystalline polymers represents the next level of the hierarchy. This level, however, depends on the molecular structure. In polymer blends and composites an even higher level of hierarchical structure can be recognized. It is important to note that interrelation exists between individual structural levels [1]. The morphology can be controlled by several physical methods including thermal history, mixing conditions and compatibilizations of the blends, shear fields and temperature gradients during solidification, orientation processes and the nucleation. Resulting morphology can be then unambiguously assessed by combination of both – indirect techniques, particularly scattering methods (scattering of X-rays, neutrons and light), spectroscopy or thermoanalysis and direct methods, in particular light microscopy, electron microscopy or atomic force microscopy. Namely, scanning electron microscopy offers advantageous rapid observation and invaluable data acquisition of polymer morphology. The addition of nucleating agents is one of the most important methods to modify morphology and is widely applied in the plastic industry. The heterogeneous nuclei influence crystallization kinetics, size of spherulites and, consequently, resulting properties [2, 3]. In particular, the formation of specific crystalline form in polymorphic polymers can influence the macroscopic behaviour quite dramatically [4−6]. Isotactic polypropylene (PP) represents clear example. Commonly, PP crystallizes into monoclinic α-phase [7, 8]. However, specific α-nucleating agents are often added into the material primarily to improve the transparency, whilst the shortening of processing time and some improvement of mechanical properties are bonuses [2, 3, 9, 10]. The second crystalline phase of PP, trigonal β-phase, can become predominant in the presence of specific β-nucleating agent [11−14]. Such a β-nucelated material shows strong differences in mechanical properties as compared to common α-polypropylene, namely higher toughness and drawability, but lower stiffness and strength [4−6]. Basically, the addition of nucleating agents significantly decreases a spherulitic size as a number of (heterogeneous)

1 Corresponding author: e-mail: [email protected], Phone: +420 576 031 384, Fax: +420 576 032 733

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crystallization nuclei rises. This make an assessment of the morphology by light microscopy rather complicated, frequently reaching its resolution limit. A use of electron microscopy is even more justified. During the orientation of the polymer melt in one direction the fibrillar structure can be formed and locked by subsequent cooling under pressure [15, 16]. It is generally believed that such systems with extended-chain crystals cause the high strength and modulus of elasticity [17]. This way of property modification, i.e. self-reinforcement, is appropriate namely for commodity polymers, such as polypropylene. It has low costs but unfortunately usually rather low mechanical properties and therefore, any improvement is desirable. On the other hand, these polymers possess high flexibility of the molecular chains allowing consequently to orient them quite easily. The self-reinforcement of polymers in molten state can be carried out using two techniques: discontinuously, by means of a capillary viscosimeter fitted with a conical die [e.g. 18, 19], and continuously, using conventional extrusion equipment with a converging die [15, 16, 20]. Studies of properties and morphology of polyolefin blends is recently one of the greatest research interests. This is not only due to wide industrial applicability of these materials but also due to their rich and fascinated morphology depending on molecular structure, thermal history, and external stress fields [21]. Polymer compounding leading to special morphology governed by processing conditions (commonly known as in-situ composites preparation) offers extra possibilities to improve the blend properties [22–23]. Relatively successful in-situ composites preparation was achieved via blending of common thermoplastic resins with liquid crystalline polymers. Liquid crystalline polymers represent special macromolecular material containing rigid mesogenic units that in molten state keep defined conformation [24]. During processing these rigid domains can be oriented along the flow, which results in a fine fibril structure reinforcing the thermoplastic matrix [25]. Besides this approach, in-situ composites were effectively prepared also from common polymeric materials several times cheaper than liquid crystalline polymers. One of such example is the combination of polypropylene with polyamide 6 [26]. Nevertheless, nowadays when plastic waste recycling is one of the main issues of environmental concerns, studies of polypropylene and polyethylene (PE) blends play particularly important role, due to their consumed amount and difficult separation of these materials apart. Thus, the polypropylene microfibrillar-phase reinforcement of polyethylene matrix has a high practical importance. In this work, the utilization of scanning electron microscopy for the assessment of PP morphology strictly rising from the real industrial processes is discussed. Thus, three methods of PP modification are applied and the resulting morphology is presented. First, the specific β-nucleating agent is added into the material. Second, the continuous self-reinforcement in molten state is carried out and finally, the microfibrillar composites are prepared by a blending of PP and high-density PE. It is worth noting that the morphology variations in isotactic polypropylene possess a wide range of different forms reflecting individual structural level; present contribution is thus a small fragment in this fascinating area.

2. Experimental

2.1 Materials and specimens

To study the effect of β-nucleating agent on morphology of PP a common isotactic polypropylene was used. The material is characterized by a melt flow index of 3.2 g/10 min (2.16 kg, 230 °C, ISO 1133), a weight-average molecular weight of 360 000 (GPC) and an isotacticity index of 98 % (ISO 9113). To enhance the β-phase content, the material was modified by a β-nucleating agent (mixture of lanthanum stearate and stearic acid in molar ration of 60:40 [27]) in the concentration of 0.10 wt.%. To facilitate a homogeneous distribution of the β-nucleating agent in a given sample, PP pellets were first mixed with 0.30 wt.% of paraffin oil and subsequently the β-nucleating agent was added. Finally, this mixture was processed by a Brabender twin-screw extruder. From the prepared blend and also from neat PP, standard impact testing bars 4x10x80 mm were injection-moulded using a Demag NC 4 injection-moulding machine. The processing conditions are listed in Table 1.

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Table 1 Processing parameters for injection-moulding of neat and β-nucleated polypropylene.

Temperature under hopper (°C) 50 Injection speed (mm/s) 50Input zone (°C) 200 Holding pressure (MPa) 400Transition zone (°C) 220 Holding time (s) 25Output zone (°C) 240 Mould temperature (°C) 60Nozzle temperature (°C) 250 Cooling time (s) 25

The material intended for self-reinforcement was commercially available isotactic polypropylene possessing similar characteristics as previous material. Self-reinforced rods with diameter approx. 5 mm were extruded using an extrusion line containing a Brabender single screw extruder 30/25D, Zenite PEP II gear pump with 1.2 cc/rev, extrusion die with semi-hyperbolic convergent channel and two heating/cooling zones [25]. The gear pump was used to achieve high extrusion pressures which were necessary for manufacturing of self-reinforced extrudates. The semi-hyperbolic channel ensured an almost pure elongational melt flow and cooling/heating zones fixed resulting article shape. The processing conditions for extrusion of self-reinforced PP rods are summarized in Table 2.

Table 2 Processing parameters for production of self-reinforced polypropylene.

Temperatures of heating zones (°C) Temperature on gear pump (°C) 1801. zone 140 Temperature in die channel (°C) 1602. zone 180 Pressure on gear pump (MPa) 403. zone 210 Temperature in 1. fixing zone (°C) 1354. zone 220 Temperature in 2. fixing zone (°C) 130

Table 3 Processing conditions for microfibrillar-reinforced composites preparation.

Temperatures of heating zones (°C) Temperatures in the die (°C) 1. zone 150 Cylinder zone 1852. zone 170 Converging zone 1703. zone 190 Calibration zone 1704. zone 210 Revolutions of gear pump 20Temperature on gear pump (°C) 200 (rev/min)

A similar concept of extrusion line was used for in-situ microfibrillar-reinforced composite preparation. The PP serving as a microfibrillar reinforcing part was extruded with high-density PE copolymer (melt flow index 0.55 g/10 min; 2.16 kg, 230 °C, ISO 1133). The PP characteristics were the same as in previous cases. PE/PP blends were prepared using counter-rotating twin-screw extruder. The extruded filaments of blends with 20, 30 and 40 wt.% of PP were quenched and granulated. In addition, for determination of the effect of compounding on resulting morphology of microfibrillar-reinforced composites (MFC) products, blends were also prepared by hand-mixing of polymers (approx. 3 min). As already mentioned, the MFC were prepared using the same extrusion line as for production of self-reinforced PP described above. However, instead of round outlet of semi-hyperbolic die the rectangular 2x20 mm outlet was employed. The fixing cooling/heating zones at the end of extrusion line were not used. The polymer melt passing through die was firstly accelerated in convergent section with semi-hyperbolic convergency and subsequently cooled in calibration section. The temperature profile within the extrusion line is listed in Table 3. The prepared specimens were labelled by 3 characters indicating the content of PP in blend (20, 30 and 40) and compounding process (c − compounded, n − hand-mixed).


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2.2 Scanning electron microscopy

The scanning electron microscopy is excellent an observation method suitable for most types of surface morphology examination. Basically, the surface for SEM observations can be prepared using three possible ways – i) using fracture surface (at low temperature – cryofracture or at higher temperatures – soft-matrix fracture [28]), ii) polishing or microtoming and iii) direct use of the surface after processing or after free-surface crystallization. Within present work, the morphology of cryo-fracture surfaces of all specimens was observed by a Jeol JSM-6460 scanning electron microscope using secondary electron detector and accelerated voltage of 20 kV. Specimens were prepared according to following description: As for injection-moulded neat and β-nucleated PP the specimens were inserted into liquid nitrogen for 30 s and broken. Subsequently, the surfaces were etched for highlighting the crystal regions. The selective permanganic etching of polymers can be taken as one of the essential inventions regarding specimen preparation for electron microscopy. The technique was invented and further developed by Olley and Basset group [e.g. 29 and 30]; presently, several modifications of the technique are used. In our case, the etching was performed by 1% solution of KMnO4 in 85% H3PO4 at 25 °C for 25 min. Finally, the surfaces of the broken specimens were coated by a thin gold layer in a sputter device. The specimens of self-reinforced rods and microfibrillar-reinforced composite tapes were prepared by breaking the extrudates along the extrusion direction in liquid nitrogen. The etching and coating of the surfaces of the specimens were performed using the same conditions as in the case of injection-moulded samples as mentioned above.

3. Results and Discussion

3.1 Morphology of neat and β-nucleated polypropylene

Generally, in injection-moulded polypropylene a sandwich-like structure is formed, usually referred to a skin-core morphology. The skin layer which is close to the surface of the mould is composed of a various kinds of fibrous structure oriented in the flow direction. The core layer is composed of almost unoriented spherulites. The maximum of molecular orientation can be found in the layer in direct contact with a mould, sometimes it is taken as an individual – shear layer. The formation of the layers is caused by the temperature gradient and shear applied during the mould filling: the outer portion cools more rapidly than the innermost parts. In this work, all SEM micrographs are taken from the core of the specimens. In a questient melt, neat isotactic polypropylene crystallizes into α-spherulitic structures [31]. Figure 1 shows the SEM micrographs of injection moulded neat polypropylene scanned in two different magnitudes. In the micrograph with smaller magnification, individual α-spherulites of dimensions approx. 60 µm with distinct boundaries can be observed. These boundaries are weak sites in the polymer as failure of the PP is often initiated at these places [32]. The second micrograph in Figure 1 shows the detail of the boundaries between individual spherulites. The scanning electron micrographs of injection-moulded β-nucleated polypropylene are displayed in Figure 2. The micrographs revealed that the addition of β-nucleating agent into polypropylene leads to a substantial decrease in the size of the spherulites to approx. 5 µm. Moreover, the spherulites are distinctly different from the α-spherulites in non-nucleated PP. It is evident from the Figure 2 that β-spherulites exhibit a sheaf-like morphology, and the boundaries between them are hardly distinguishable. It can be also seen that the bundles of lamellae of neighbouring spherulites tend to cross each other. Indeed, the higher number of bridges connecting individual β-crystallites and higher continuity of the amorphous phase as compared to the material containing solely α-crystallites are one of the reasons of [4, 5] the superiour toughness of β-nucleated PP. In this case, the notch impact strength of β-nucleated PP is approx. 4 times higher as compared to non-nucleated material (2.3±0.1 kJm-2 for neat PP and 10.4±0.5 kJm-2 for β-nucleated material).


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Fig. 1 Morphology of neat polypropylene scanned with different magnification.

Fig. 2 Morphology of β-nucleated polypropylene scanned with different magnification.

3.2 Morphology of self-reinforced polypropylene

The self-reinforcement of polymeric materials can be achieved using specific flow conditions, temperatures and high pressure in the specifically designed extrusion die. Under these conditions, the prepared self-reinforced extrudates can be easily identified even by naked eye because of their smoothness and transparency. The extrudates also show an extreme stiffness, a low degree of deformability and an ultra high strength. Conventionally extruded rods, on the other hand, are hazy and significantly softer than the self-reinforced ones [15, 25]. As mentioned, scanning electron microscopy is an excellent tool to verify the assumption of fibrillar morphology of self-reinforced extrudates. The morphology of crack surfaces of self-reinforced polypropylene rods scanned at various magnifications is shown in Figure 3. It can be seen that the morphology is unambiguously fibrous: no spherulites, common supermolecular chain-folded aggregates, can be recognized; compare with Figure 1. Uniform and regular microfibrils with thickness more than 500 nm can be identified. It is evident that these fibrils are oriented in the extrusion direction which is favourable for the improvement of strength and stiffness in the longitudinal direction. Indeed, the fibrils might behave similarly to common continual heterogeneous fibres and macroscopically cause a reinforcing effect. The elastic modulus of such self-reinforced material is then 4 600 MPa on the contrary to 1 700 MPa for common material consisting of usual chain-folded morphology [33].


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Fig. 3 Morphology of self-reinforced polypropylene rod scanned with different magnification (extrusion direction is horizontal).

3.3 Morphology of self-reinforced polypropylene

The SEM micrographs of crack surfaces of extruded tapes are shown in Figure 4. While the micrographs of pure extrudates show isotropic structure (bottom pictures), the morphology of the tapes extruded from blends is oriented along the flow direction. This result is the first prerequisite to rate these blends as microfibrillar-phase composites. As can be seen, the structure of compounded-blend extrudates gradually changes from fine-fibril morphology to coarse co-continuous phase structure, in respect of the PP content increase. On the other hand, this structure gradation is not clearly manifested in the extrudates prepared from hand-mixed materials. Although the transition between fibrillar and co-continuous morphology can be recognized, the diameter of PP domains is not directly controlled by the PP content.


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4. Conclusions

A scanning electron microscopy was used to observe the morphology of PP subjected to various structure-modifying methods. The spherulitic structures of neat and β-nucleated injection-moulded PP were compared. These two morphologies differ in both the magnitude, appearance of spherulites and lamellar arrangement. The structure of self-reinforced PP was found to be highly oriented in the direction of extrusion. The fibrils were easily recognizable in the micrographs. The microfibrillar-phase morphology in the extruded PP/PE blends with PP as a minority component was observed. An employment of scanning electron microscopy is proved to be one of the essential techniques during overall assessment of polymer morphology.


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Fig. 4 Morphology of extrudates prepared from the PE/PP blends and pure materials (extrusion direction is vertical).

Acknowledgements The authors kindly acknowledge the support provided by the Czech Science Foundation, GAČR (project 106/07/P262) and the Ministry of Education, Youth and Sport of the Czech Republic (project MSM7088352101). In addition, we thank to Dr. Jiachun Feng for kind providing of β-nucleating agent.


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References

[1] M. Raab, J. Kotek, Polimeri 26, 4, 172 (2005) [2] C.Y. Kim, Y.C. Kim, S.C. Kim, Polym Eng Sci, 33,1445 (1993) [3] Y.C. Kim, C.Y. Kim, Polym Eng Sci, 31, 1009(1991) [4] M. Raab, J. Kotek, J. Baldrian, W. Grellmann, J Appl Polym Sci, 69, 2255 (1998) [5] J. Kotek, M. Raab, J. Baldrian, W. Grellmann, J Appl Polym Sci, 85, 1174 (2002) [6] M. Obadal, R. Čermák, N. Baran, K. Stoklasa, J. Šimoník, Int Polym Process, 19, 35 (2004) [7] F.J. Padden, H.D. Keith, J Appl Phys, 30, 1479 (1959) [8] A. Turner-Jones, J.M. Aizlewood, D.R. Beckett, Makromol Chem, 75, 134 (1964) [9] Q.D. Gui, Z. Xin, W.P. Zhu, G.C. Dai, J Appl Polym Sci, 88, 297 (2002) [10] P.M. Kristiansen, A. Gress, P. Smith, D. Hanft, H.W. Schmidt, Polymer, 47, 249 (2006) [11] J. Varga, J Macromol Sci Phys, 41, 1121 (2002) [12] J. Varga, Crystallization, Melting and Supermolecular Structure of Isotactic Polypropylene. In: Karger-Koc

sis J, editor. Polypropylene: Structure Blends and Composites, London, Chapman & Hall, 1995, pp. 56-115 [13] A. Menyhárd, J. Varga, G. Molnár, J Therm Anal Calorim, 83, 625 (2006) [14] M. Fujiyama, Int Polym Process, 11, 271 (1996) [15] B. Pornimit, G.W. Ehrenstein, Adv Polym Tech, 11, 91 (1992) [16] H.X. Huang, J Appl Polym Sci, 67, 2111 (1998) [17] G.K. Elyashevich, E.A. Karpov, O.V. Kudasheva, E. Rosova, Mech Time-Depend Mat, 3, 319 (1999) [18] J.H. Southern, R.S. Porter, J Macromol Sci Phys, 4, 541 (1970) [19] J.A. Odell, D.T. Grubb, A. Keller, Polymer, 19, 617 (1978) [20] A.J. McHugh, D.A. Tree, B. Pornimit, G.W. Ehrenstein, Int Polym Process, 6, 208 (1991) [21] Utracki, L. A., Two-phase polymer systems. Oxford Univ Press, New York (1991) [22] S. Fakirov, M. Evstatiev, Adv Mater, 6, 395 (1994) [23] S. Fakirov, A. Apostolov, Z. Denchev, N. Stribeck, M. Evstatiev, K. Friedrich, J Macromol Sci, 40, 935

(2001) [24] S.C. Tjong, Mater Sci Eng, 41, 1 (2003) [25] M. Obadal, R. Čermák, K. Stoklasa, V. Habrová, Plast Rubber Compos, 33, 295 (2004) [26] J. Varga, A. Breining, G. W. Ehrenstein, Int Polym Process, 15, 53 (2000) [27] J. Feng, M. Chen, Polym Int, 52, 42 (2003) [28] F. Lednický, G.H. Michler, J Mat Sci, 25, 4549 (1990) [29] R.H. Olley, A.M. Hodge, D.C. Bassett, J Polym Sci Polym Phys, 17, 627 (1979) [30] R.H. Olley, D.C. Bassett, Polymer, 23, 1707 (1982) [31] J. Varga, Spherulitic Crystallization and Structure, in: J. Karger-Kocsis, Polypropylene – An A-Z Reference,

Springer-Verlag, 1999, pp. 759-768 [32] S.C. Tjong, J.S. Shen, R.K.Y. Li, Polymer, 37, 12, 2309 (1996) [33] R. Čermák, M. Obadal, V. Habrová, K. Stoklasa, V. Verney, S. Commereuc, F. Fraïsse, Rheol Acta, 4, 45,

366 (2006)


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Morphology Variations of Polypropylene - Formatexformatex.org/microscopy3/pdf/pp704-712.pdf · Morphology Variations of Polypropylene J. Výchopňová1, R. Čermák1,1 and M. Obadal2