Interfacial enhancement by shish–calabash crystal structure in polypropylene/inorganic whisker composites
Graphical abstract
Introduction
In recent years, polymer/filler composites have been intensively investigated in both industrial and academic fields, because they exhibit valuable mechanical and thermal properties when compared with pure polymers [1], [2], [3]. Good adhesion results in efficient stress transfers from the continuous polymer matrix to the dispersed filler and can increase the ability of the material to absorb energy, thus improving the mechanical properties of composites [4], [5], [6], [7]. The polymer matrix structure and the interface are strongly influenced by filler in semi-crystalline polymer composites because the fillers have the potential to nucleate the polymer crystallization. The structure of the nucleated crystalline polymer layer on filler is of particular interest. Transcrystallization at interfaces, as a surface-induced crystallization, is an effective and economical method to improve the interfacial adhesion, and thus is attractive for many researchers in the past [8], [9], [10], [11], [12], [13]. The prerequisite for transcrystallization is the presence of a high density of active nuclei on the substrate/fiber surface. The closely packed nuclei hinder the full extension of spherulites, which are then forced to grow in one direction, namely perpendicular to the substrate/fiber [9]. On the other hand, hybrid shish–kebab (HSK) structure, in which filler (carbon nanotubes or whisker) acts as shish and induces polymer (PE or nylon 6) crystal lamellae (kebab) periodically decorating on its surface and aligning approximately perpendicular to its long axis, has been recently reported, and shows great potential to enhance the interfacial interaction for polymer composites [14], [15], [16], [17], [18], [19], [20], [21], [22]. Between transcrystallization and HSK, if a fibrous-shaped filler can only initiate a few nuclei, one expects a formation of separated spherulites on the surface of filler, instead of transcrystal morphology. And if the diameter of the fiber is much smaller than that of formed spherulites, one expects a formation of peculiar shish–calabash structure, with fibrous filler serves as shish and polymer spherulites serves as calabash. This shish–calabash structure is different from the shish–kebab structure, for the polymer crystals in shish–kebab are lamellae, while they are spherulites in shish–calabash. Furthermore, the package density of crystal is much larger in shish–kebab structure than in shish–calabash structure. The schematic representation of the formation process of this shish–calabash crystallization morphology is represented in Fig. 1. Thus depending on the package density of crystallization on fibrous filler, there may be four types of crystal morphology, namely, randomly separated crystals, shish–calabash, shish–kebab, transcrystalline.
Whiskers, with large length/diameter ratio, are fiber-shaped single crystals. Its diameter (0.2–2 μm) is much larger than carbon nanotubes (CNTs) but much smaller than common fibers. Because whiskers could have much higher specific strength than short glass or carbon fibers, a lot of works have been focused on the preparation of various polymer/inorganic whiskers (such as aluminium borate whisker and potassium titanate whisker) composites and it was found that whiskers could reinforce thermoplastics more effectively. Therefore, whiskers are considered to be an attractive alternative to short glass or carbon fibers for reinforcing thermoplastics and have attracted considerable interests of scientists and engineers [23], [24], [25], [26], [27], [28], [29], [30]. In our previous work, a new type of SiO2–MgO–CaO whisker (SMCW) and HDPE composites were prepared and the crystal morphology was investigated. A hybrid shish–kebab (HSK) structure has been observed in the injection molded bar of PE/SMCW composites. In this HSK structure, SMCW acted as shish and induced PE crystal lamellae (kebab) periodically decorating on its surface and aligning approximately perpendicular to its long axis under the function of shear [21], [22].
Due to the scientific importance and technical significance, in this paper, we will explore the possibility of formation of shish–calabash structure using whisker as the fibrous filler. To do this, whisker was first melt blended with polypropylene (PP) with various compositions. Then the isothermal crystallization of PP at the presence of whisker was carried out via Polarized light microscope (PLM). It was shown that only a few nuclei could be induced on the whisker surface, which develop into large PP spherulites without hindrance, and finally stringed by the whisker, forming the shish–calabash structure. More importantly, the shish–calabash structure was also observed in the injection molded bar of PP/whisker composites. As a result of interfacial enhancement via formation of shish–calabash structure, the prepared composites showed a significantly improved tensile property compared with pure PP.
Section snippets
Materials
A commercially available isotactic PP was manufactured by Dushanzi Petroleum Chemical Incorporation (Xinjiang, China), with a melt flow index of 0.96 g/10 min (190 °C, 2.16 kg). The whisker (SMCW) with a density of 3.0 g/cm3 was produced in Mianyang Guangda Company (Sichuan, China). Its diameter ranges from 0.2 to 2 μm (mainly from 0.2 to 0.5 μm), and its length is in the range of 5–50 μm. Silicohydride (KH-550), supplied by Chenguang Research Institute of Chemical Industry (Chengdu, China), was used
Dispersion and orientation of whiskers in PP/SMCW composites
In this work, SEM was employed to assess the whisker orientation and dispersion because the dispersion and orientation of fibrous filler in polymer matrix are very important for the enhancement of mechanical properties of polymer composites. Fig. 2 represents the SEM fractured surface of injection molded bar of PP/SMCW composites viewed parallel to the flow direction. It can be observed that most of the whiskers are aligned parallel to the flow direction for all the composites with different
Conclusion
In this work, whiskers, with a large length/diameter ratio and with a diameter (0.2–2 μm) much larger than that of carbon nanotubes but much smaller than that of common fibers, were used to nucleate crystal morphology in polypropylene (PP)/whisker composites. A kind of peculiar shish–calabash crystallization morphology, with whisker serves as shish and PP spherulites serves as calabash, was observed for the first time in the thin film via PLM and in the injection molded bars by SEM. The
Acknowledgements
We would like to express our sincere thanks to the National Natural Science Foundation of China for Financial Support (50533050, 20874064, 20634050). The authors gratefully acknowledge the help of Ms Xin Yuan Zhang of the Analytical and Testing Center at Sichuan University for the SEM micrographs.
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