Structural Shape of Polymer Molecules

Polymers can be classified, based on structural shape of polymer molecules, as linear, branched, or network (cross-linked). Schematic representations are given in Fig. 1.3. 

Polymers can be classified in many ways, such as by source, method of synthesis, structural shape, thermal processing behavior, and end use of polymers. Some of these classifications have already been considered in earlier sections. Thus, polymers have been classified as natural and synthetic according to source, as condensation and addition (or step and chain) according to the method of synthesis or polymerization mechanism, and as linear, branched, and network according to the structural shape of polymer molecules. According to the thermal processing behavior, polymers are classified as thermoplastics and thermosets, while according to the end use it is convenient to classify polymers as plastics, fibers, and elastomers (Rudin, 1982). 

By collecting the integrated intensities of diffraction peaks and subjecting them to a sequence of analyses, it is possible to determine the positions of the atoms packed in the crystalline unit cell. Such an endeavor constitutes the traditional process of crystal structure analysis, and much of the information that is available today about the shape of polymer molecules and their arrangement in crystals was derived by this method. 

The chemical structures of the repeat units of some common polymers are shown in fig. 1.2, where for simplicity of drawing the backbone bonds are shown as if they were collinear. The real shapes of polymer molecules are considered in section 3.3. Many polymers do not consist of simple linear chains of the type so far considered more complicated structures are introduced in the following section. 

Diffusion selectivity is based on the ability of the polymer matrix to transmit molecules of a certain shape and size. This ability is determined by the structure of the polymer and the rigidity of the macromolecular ensemble as well as by the properties of the penetrant, the size and shape of its molecules. 

In most instances it is the form factor P q) that we wish to determine. The form factor gives information about the average shape of the molecule. Suppose we have a blend of hydrogenous and deuterated polymers, both having the same chemical structure and the same number of segments, Z, per molecule, and their volume fractions are 0h and 0D, respectively (0h + 0d = 1). In the blend there are thus TVh = Ncj>h hydrogenous molecules and N = N0d deuterated molecules, and instead of (6.63) the scattering intensity is now written as... 

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