Amorphous polymers diffraction from

Unlike simple inorganic compounds (e.g., NaCl or KC1), polymers do not have a perfectly ordered crystal lattice formation and are not completely crystalline. In fact, they contain both crystalline and amorphous regions. Hence, the X-ray diffractions from them are found to be a mixture of sharp as well as diffused patterns. 

For crystalline polymer systems, transition from the crystalline structure to a mesophase structure occurs, whereas for amorphous polymer systems, the mesophase occurs after the Tg has occurred. Some polymer LC systems form several mesophases. Mesophases can be detected using DSC, x-ray diffraction, and polarizing microscopy. 

Many polymeric materials have a fibrous texture in which elongated particles with an ordered internal structure are preferentially aligned parallel to a particular direction termed the fiber axis. Diffraction patterns obtained from such materials contain information about both the particles and the matrix in which they are embedded. This matrix may consist of amorphous polymer of the same or different composition to the particle or may be a liquid. 

If an isotropic polymer is subjected to an imposed external stress it undergoes a structural rearrangement called orientation. In amorphous polymers this is simply a rearrangement of the randomly coiled chain molecules (molecular orientation). In crystalline polymers the phenomenon is more complex. Crystallites may be reoriented or even completely rearranged and oriented recrystallisation may be induced by the stresses applied. The rearrangements in the crystalline material may be read from the X-ray diffraction patterns. 

Many polymers have the capability to crystallize. This capability basically depends on the structure and regularity of the chains and on the interactions between them. The term sernicrystalline state should be used rather than crystalline state, because regions in which the chains or part of them have an ordered and regular spatial arrangement coexist with disordered regions typical of the amorphous state. X-ray diffraction studies of samples of polymers crystallized from the melt reveal diffuse zones, char-... 

Wegner, however, established that radiation-induced solid-state polymerization of BCMO leads to a polymer morphology, which is incompatible with the so-called topochemical polymerization, i.e., a process in which monomer molecules are transformed into polymer without destruction of the crystal lattice 36). Electron microscopy, X-ray analysis and electron diffraction studies, have shown that polymerization starts at the edges and imperfections of the monomer crystals and that amorphous polymer is formed initially. Further transition from the amorphous state leads to the thermodynamically unstable monoclinic p-form. Density measurements indicate that the polymer is only 45-50% crystalline. The density of the amorphous poly-BCMO is 1.368 g/cm3 the density calculated for the crystalline polymer from crystallographic data of the p-form is 1.456 g/cm3. The density of the product of the radiation-induced solid-state polymerization is 1.41 g/cm3 36). 

Figure 8.46 Schematic diffraction pattern from a semicrystalline polymer, showing how both crystalline and amorphous phases may be detected. The amorphous portion results in broad scattering while the crystalline portion shows a typical diffraction peak pattern. A totally amorphous polymer would show no diffraction peaks. (There are no 100% crystalline pol3miers.) The units on the x-axis are degrees 6.

The role chitin as a material of highly ordered crystalline structure has been reported in the study [96]. X-ray diffraction analysis was carried out in order to find the changes of the crystalline structure upon the substitution reaction with NCO terminated prepolymer. The X-ray diffraction studies showed that crystallinity mainly depends on the concentration of chitin in the polyurethane backbone, crystallinity increased as the concentration of chitin into the final PU increased (Fig. 3.22). The crystallinity of some polymers was clearly observed by optical microscopic studies [114]. The results of X-ray diffraction experiments correlate with optical microscopy findings. A crystalline polymer is distinguished from an amorphous polymer by the presence of sharp X-ray Unes superimposed on an amorphous halo. Under an optical microscope, the presence of polycrystalline aggregates appear as spherulites [114]. The spheruhtes are made of small crystallites and grow Irom a nucleus at their centre. They consist of narrow chain folded lamellae growing radially. Since the fibrous crystals are radial, the chains folded with the lamellae are circumferentially oriented. From the evaluation of the X-ray and optical microscopic studies, it has been observed that the involvement of chitin in the PU formulation and have improved crystallinity of the final polyurethane. 

Polymers with side chain structure similar to that of low molecular weight liquid crystalline compounds can achieve various levels of organization in the bulk. These polymers are sometimes formed by polymerization of vinyl monomers that themselves exhibit mesomorphic behavior. In other cases, they can be obtained from monomers that do not form liquid crystalline states. At one extreme the structure of the polymer is highly organized, approaching that of crystalline polymers and giving rise to a number of x-ray diffraction peaks. At the other extreme the polymer chains are disorganized, with x-ray diffraction patterns that resemble those from amorphous polymers. 

---