Morphology of Crystallites

FIGURE 3-12 Morphology of crystallites from polymer melts... 

In polymer regions of PFCM bordering on the filler one typically observes the formation of crystallites as tape-like structures orientated perpendicularly to the filler surface [295, 305, 306]. In real fact these are transcrystallite structures. A morphology of this kind can appear in a mechanical mixture as well [305, 306], but there the layer thickness will be much smaller [306, 307]. 

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

How does a support affect the morphology of a particle on top of it Which surface planes does the metal single crystal expose The thermodynamically most stable configuration of such small crystallites is determined by the free energy of the surface facets and the interface with the support, and can be derived by the so-called Wulff construction, which we demonstrate for a cross section through a particle-support assembly in two dimensions (Fig. 5.13). 

Figure 6.9 The morphology of a commercial mortar, showing well-developed needle-like crystallites. Micrograph span (a) 75 pm, (b) 30 pm (Abdelrazig er at., 1984).

Possible morphologies of partially crystalline polymers are shown in Fig. 18. Figure 18a depicts the case of small crystallites that act as physical crosslinks between polymeric chains, thus connecting those chains into a 3-dimensional network. In the case depicted in Fig. 18b, the material forms ribbon-shaped or needle-shaped crystalline regions in which different segments of a large number of chains are incorporated. This could explain the low degree of crystallinity at the LST as detected for the iPP system [80]. 

Fig. 18a, b. Possible morphologies of partially crystalline polymers. Small crystallites act as crosslinks (a) large ribbon-shaped or needle-shaped crystalline regions connect a large number of polymeric chains (b)... 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

In what follows, we use simple mean-field theories to predict polymer phase diagrams and then use numerical simulations to study the kinetics of polymer crystallization behaviors and the morphologies of the resulting polymer crystals. More specifically, in the molecular driving forces for the crystallization of statistical copolymers, the distinction of comonomer sequences from monomer sequences can be represented by the absence (presence) of parallel attractions. We also devote considerable attention to the study of the free-energy landscape of single-chain homopolymer crystallites. For readers interested in the computational techniques that we used, we provide a detailed description in the Appendix. ... 

The morphology of the crystallite structures present is also important in determining the mechanical properties. 

The crystal structure of the novel chabazite-type material CAL-4 displays a layered organization of the SDAs, which may be responsible for an unusual plate-like particle shape. It is predicted that the morphology of the crystallites can be designed by choosing suitable SDAs. 

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