Polypropylene isotactic crystal lamella

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

FIGURE 9.3 Crystallization of isotactic polypropylene chains leads to lamellae and fringed micelles. 

As was shown in Section 9.2.2, the crystalline form of iPP can be correlated to isotactic block length. Based on this, the samples prepared using both MAO and borate activation (Table 9.2) should mainly crystallize in the y-form. To study the presence of the different crystalline forms in these MAO- and borate-activated low-crystalline polypropylene samples, WAXS analysis was performed as shown in Figure 9.15. Both series A and B display WAXS peaks at = 9.3°, characteristic for the presence of the a-modification, and at = 10°, characteristic for the presence of the y-modification, pointing out that lamellae will crystallize in a mixed a/y-form. 

FIGURE 9.27 TM-SFM phase shift images of microtomed, melt pressed polypropylene films derived from lOa/borate having different amounts of isotacticity show only individual lamellae crystallized in the a-form ... 

Small-angle neutron scattering (SANS) studies on melt crystallized linear polyethylene, isotactic polypropylene, and poly(ethylene oxide) have shown that deuteriated molecules in a hydro-chain matrix have the same radius of gyration, and so molecular conformation, as in melt. This is evidently inconsistent with a regular adjacent re-entry folded chain lamellae. Recently Stamm et in comparing the scattering of neutrons at intermediate angles, of... 

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... 

Well defined lath-shaped lamellar crystals of isotactic polypropylene were grown from solution [163]. The thickness of these lamellae, as determined by small-angle X-ray diffraction of crystal aggregates, is 125 A. The crystals were mainly monolayer with the shorter and larger dimensions, respectively, of about 0.2 and 1 pm. Similar lamellar crystals were observed by Sauer et al. [164], Morrow et al. [165], Kojima [166], and Lotz and Wittmann [167]. Fibrils parallel to the longitudinal direction of the lamella were often observed. According to Kojima [166] these fibrils connect the planes of fracture and cleavage that are parallel to the a axis. This is an indication that the 010 planes are the most probable fold planes. Lotz and... 

Lamellar habit is also known for crystallization from the melt under isothermal and nonisothermal conditions. Typical views of lamellae in isotactic polypropylene or isotactic sequences in elastomeric polypropylene are shown in Figure 6.15, also for comparison with data discussed in Chapter 3. The typical cross-hatched morphology with angles of 80° agrees well with the theoretically predicted and experimentally confirmed value [26]. Independent of the AFM mode (contact mode, intermittent contact mode, pulsed force mode) consistent observations were made. 

Figure 3.52 AFM images of an edge-on lamella of imperfectly isotactic polypropylene growing at room temperature. Overall crystallization times are 55 minutes, 73 minutes, 169 minutes, and 190 minutes. The first two images show that the growth rate is about 6 nm/min. In frames (c) and (d) the formation of a screw dislocation with its axis in the plane of the image can be seen. From Franke and Rehse [73] with permission from the American Chemical Society.

Figure 5.14 Time dependence of the various structural parameters estimated in the isothermal crystallization process of isotactic polypropylene. The FTIR data show the growth of regular helical segments. The Rg, and L are the radius of gyroid of the higher-density domains, the correlation distance between the neighboring domains, and the long period of the stacked lamellae, respectively, revealed by the SAXS data analysis. The Q is the invariant and is approximately proportional to the degree of crystallinity Xc, which was evaluated also using the WAXD data [67].

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