Alignment of crystallites

The properties of a semicrystalline polymer are controlled by its degree of crystallinity, the alignment of crystallites relative to one another, the number and type of links between the crystallites and amorphous regions, and the overall orientation of molecules within the material. 

Figure 1.15 shows the orientation function vs. elongation for seven of the absorption bands measured. Each molecular motion in its own way responds to the applied strain. The three NXL bands (720, 740, 780 cm i) increase steadily with stretching but each exhibits a different slope. Two of the XL bands (625 and 633 cm ) increase in nearly parallel fashion but the 553 and 516 cm bands exhibit a plateau at elongations X > 1.6. The orientation function rises more steeply for the XL phase than for the NXL, indicating pronounced alignment of crystallites into the draw direction. The plateau indicates that a maximum of orientation is reached that is specific to the XL phase. NoncrystaUine material... 

The structure of metallic deposits is determined primarily by the size, shape (faceting), type of arrangement, and mutual orientation of the crystallites. Two factors may influence the orientation and spatial alignment of the microcrystals in electrocrystallization the field direction (or direction of the electric current) and the nature of the substrate. The deposits are said to have texture when the crystallites are highly oriented in certain directions. Epitaxy implies that the lattice is altered under the influence of the substrate. 

Figure 23,8 Planar crystallite structure of nylon 6, showing the anti-parallel alignment of neighboring chains...

Fig. 10 Snapshot of a shish-kebab crystallite induced by a pre-aligned single chain (drawn much thicker than other chains for better visibility) in a solution. The chain length is 32 units and the thickness of crystallites is about 7 units. The bonds are drawn in solid cylinders [58]... 

Through the use of multiple experimental techniques, we have shown how both the NXL and XL phases of PILE interact and respond to applied tensile deformation. Strains transmitted to PILE crystals lead to two distinct slip modes and, at higher strains, to the breakup and alignment of lamellar fragments. In our experiments, crystallites in PTFE orient fuUy with respect to the draw direction at strains between 70 to 200%. With increasing strain, some chains originally in the XL phase are transformed to NXL material. Noncrystalline chains continue to orient until macroscopic failure is reached. This could be a fairly general microstructural response for semicrystalline polymers. 

Early on, before the existence of macromolecules had been recognized, the presence of highly crystalline structures had been suspected. Such structures were discovered when undercooling or when stretching cellulose and natural rubber. Later, it was found that a crystalline order also existed in synthetic macromolecular materials such as polyamides, polyethylenes, and polyvinyls. Because of the polymolecularity of macromolecular materials, a 100% degree of crystallization cannot be achieved. Hence, these polymers are referred to as semi-crystalline. It is common to assume that the semi-crystalline structures are formed by small regions of alignment or crystallites connected by random or amorphous polymer molecules. 

FIGURE 5.17 Cellulose diffraction patterns. Top left synchrotron radiation x-ray diffraction pattern for cotton fiber bundle. The fiber was vertical and the white circle and line correspond to a shadow from the main beam catcher and its support. (Credit to Zakhia Ford.) Top right electron diffraction pattern of fragments of cotton secondary wall. The much shorter arcs in the top right figure are due to the good alignment and small number of crystallites in the electron beam. (Credit to Richard J. Schmidt.) Bottom a synthesized powder pattern for cellulose, based on the unit cell dimensions and crystalline coordinates of Nishiyama et al. [209]. (Credit to Zakhia Ford.) Also shown are the hkl values for the Miller indices. The 2-theta values are for molybdenum radiation instead of the more commonly used copper radiation. 

Crystallite morphology prediction and morphology control are important for several kinds of applications. Zeolite membranes can require alignment of many distinct crystallites, this being facilitated by having uniformly shaped... 

---