Polypropylene/Montmorillonite Nanocomposites: Continuous Stretching and Load-Cycling
In the last two decades there have been a lot of efforts to improve mechanical properties of polypropylene [1] via compounding it with layered silicates (clay) [2–7]. For instance, enhancements of storage modulus [8–10], Young’s modulus [11, 12], impact s
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Polypropylene/Montmorillonite Nanocomposites: Continuous Stretching and Load-Cycling
In the last two decades there have been a lot of efforts to improve mechanical properties of polypropylene [1] via compounding it with layered silicates (clay) [2–7]. For instance, enhancements of storage modulus [8–10], Young’s modulus [11, 12], impact strength [13, 14], and tensile strength [14, 15] have been reported. Layered silicates can be mixed with polypropylene in the melt state using conventional polymer processing machinery [3, 4, 6, 16]. Nevertheless, incompatibility of layered silicates with hydrophobic polypropylene chains provokes problems with dispersing them inside the matrix [17, 18]. The problem of dispersion has partly been solved by chemical treatment [4, 6, 7, 14, 16, 19–21] of clay surfaces, addition and tailoring of compatibilizers [4, 6, 16, 18, 22, 23] and modification of mixing methods [4, 6, 24–26]. In spite of relatively good dispersion (intercalation and exfoliation) [14, 16, 27–29] the obtained improvement in some mechanical properties of PP/clay (e.g. tensile strength) is still modest compared to other thermoplastic nanocomposites such as nylon 6/clay [6]. Hence dispersion of clay particles appears not to be the only determining parameter. Several papers [11, 30–38] report alteration of semi-crystalline structure of polypropylene in the presence of layered silicates. Two important effects have been observed; firstly polypropylene chains crystallize at temperatures higher than the crystallization temperature Tc of the neat polymer [10, 15, 30, 32–34], secondly clay exfoliation enhances the shear-induced nucleation and the overall crystallization rate [35, 38]. Microstructure alteration of the matrix (PP) influences in turn the properties of the nanocomposite [6]. Such effects may be investigated by small-angle X-ray scattering (SAXS). In particular results from SAXS-monitoring of structure evolution under load may advance the understanding of the relation between the composite’s morphology and its practicality in a load-bearing application.
5.1 Mechanical Data The self-made [39] tensile tester performs symmetric drawing. Thus the same spot of the sample is monitored by the X-ray, as long as the sample is homogeneously extended. If the tested material starts to neck, a peculiar problem is encountered that A. Zeinolebadi, In-situ Small-Angle X-ray Scattering Investigation of Transient Nanostructure of Multi-phase Polymer Materials Under Mechanical Deformation, Springer Theses, DOI: 10.1007/978-3-642-35413-7_5, © Springer-Verlag Berlin Heidelberg 2013
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5 Polypropylene/Montmorillonite Nanocomposites
σ(ε) [MPa]
30
e
(c)
20
(d)
(b) 10
beam position
(a) 0
0
0.1
0.2
ε
0.3
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Fig. 5.1 Stress relaxation and spot translation caused by necking. True stress σ (ε) as function of local strain ε. a Start of test. The X-ray beam spot is indicated by a cross. b Homogeneous stretching. Spot does not move. c Necking has started (see ellipse). Material shows stress relaxation. d Spot moving tow
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