Isotactic polypropylene epitaxial crystallization

R. Su, K. Wang, Q. Zhang, F. Chen, Q. Fu, N. Hu, E, Chen, Epitaxial crystallization and oriented structure of linear low-density polyethylene/isotactic polypropylene blends obtained via dynamic packing injection molding. Polym. Adv. Technol. 22, 225-231 (2011)... 

F. Rybnikar, M. Kaszonyiova, Epitaxial crystallization of linear polyethylene in blends with Isotactic polypropylene. J. Macromol. Sci. Part B Phys. 53, 217-232 (2014)... 

PBT fibers nucleate fiber surface-induced crystallization of isotactic polypropylene. PBT fiber has the highest nucleating abihty of the three polyesters studied (PET, PTT, and PBT). Remarkable nucleating ability of PBT fibers is associated with epitaxial crystalU-zation. ... 

Keywords epitaxy, isotactic polypropylene, syndiotactic polypropylene, crystal structure, crystal polymorphism, helical hand, nucleating agents, AFM, a-phase, 3-phase, lamellae. 

Stocker, W Magonov, S. N. Cantow, H. J. Wittmann, J. C. Lotz, B. Contact faces of epitaxially crystallized a- and y-phase isotactic polypropylene observed by atomic force microscopy. Macromolecules 1993, 26, 5915-5923. 

Somani RH, Yang L, Zhu L, Hsiao BS (2005) Flow-induced shish-kebab precursor structures in entangled polymer melts. Polymer 46 8587-8623 Sorrentino A, De Santis F, Titomanlio G (2007) Polymer crystallization under high cooling rate and pressure a step towards polymer processing conditions. Lect Notes Phys 714 329-344 Stocker W, Magonov SN, Cantow HI, Wittmann JC, Lotz B (1993) Contact faces of epitaxially crystallized a- and y-phase isotactic polypropylene observed by atomic force microscopy. Macromolecules 26 5915-5923... 

Secondary crystallization in isotactic polypropylene occurs in a peculiar way. On cooling additional crystallites are formed, however, many of them are oriented oblique to the primary lamellae rather than p lrallel as is normally observed. As explained by Lotz and Wittmann [8], this cross-hatching is caused by an epitaxial growth mechanism. If cross-hatching occurs the one-dimensional model used so far in the analysis of SAXS data becomes invalid. There is a simple check which shows the deviations. For a stack of crystallites one expects... 

Isotactic polypropylene displays a highly unusual ability to induce epitaxial crystallization of a number of different polymers with their chain axes tilted at large angles, 40—80°, relative to the helix axis direction of the polypropylene substrate. The same polymer shows an epitaxy, i.e. homoepitaxy, at an angle of 80° (Fig. 7.30). The homoepitaxy is responsible for crosshatching, a structure typical of the monoclinic a structure. This feature causes the lamellar branching and the optical complexity, with both positive and negative spherulites, typical of isotactic polypropylene (section 7.4). 

Sun Y, Li H, Huang Y, Chen E, Gan Z, Yan S. Epitaxial crystallization of poly (butylene adipate) on highly oriented isotactic polypropylene thin film. Polymer 2006 47 2455-2459. 

Figure 8.4 The packing of chains in the monochnic, a phase of isotactic polypropylene (odPP). Left as seen in chain axis projection, successive layers parallel to the ac plane (horizontal trace) are made of alternatively right-handed and left-handed helices. Middle and right the succession of different ac faces of odPP encountered as one travels along the +i>-axis direction. The helical hand (curved arrows) as weU as the pattern of methyl groups differs in the two distinct faces. Travelling along the -fo-axis direction, the mirror images of these faces would be observed. The less densely populated face (middle) is involved in epitaxial crystallization. Reproduced from Reference [27].

Figure 8.8 Schematic illustration of the /phase crystal structure of isotactic polypropylene and its structural filiation with the aiPP homoepitaxy. The threefold helices are represented as triangular bars. The systematic repetition of the self-epitaxial packing illustrated in Figure 8.6 (indicated here with arrows) generates the first and so far the only polymer crystal structure with nonparallel chain axes. Reproduced from Reference [47] with permission.

In isotactic polypropylene [73,76], the smectic phase or the monoclinic a phase can be obtained according to cooling conditions. Furthermore, the a phase may, in certain conditions, exhibit a bimodal crystalline texture, that is, two populations of crystals with their c or their a-Sixis along the fiber axis, respectively (a is the axis of the reciprocal lattice related to a). This is revealed by additional arcs or spots on the 110 and 130 reflections, while the 040 reflections remain unchanged [79] (Fig. 15.21). This can be interpreted in terms of a cylindritic morphology with two types of lamellae classical radial lamellae and tangential ones obtained by epitaxial growth, which is a particular feature of polypropylene crystallization (see Chapter 8). 

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